IEC Electrical-Installation Guide

Foreword This book, "Electrical Installation Guide according to IEC International Standards", which is compiled and printed in English by Schneider Electric, facilitates utilization of the IEC 60364 series of international standards concerned with safety, guarding, control, performance and protection of circuits, together with fundamentals and rules of electrical installation design. Moreover, the book contains topics of extreme importance that cover wide fields of electric power systems and their installations in different facilities. This renders the book a useful reference to each engineer and specialist in the field and an easy guide to such international standards and their application. SASO, in recognition of the guide's important and comprehensive electric installation content, has translated it into Arabic to enable researchers and specialists to benefit from it in Arabic, if they desire. In its capacity as the body entrusted with issuing and approving Saudi standards, SASO attaches particular importance to verification of safety in electrical installation in buildings. SASO is also acting assiduously to complete work on the national regulations of electrical installation based upon IEC 60364 series of international standards which are adopted as Saudi standards. In view of the divergence of the items of such standards and the many technical options offered, issuing an application guide to these standards is extremely useful. To this effect, the Saudi national regulations of electrical installation in buildings will be used. The guide being introduced here, will be an important reference for "the electrical installation guide according to Saudi standards" as part of the Saudi national regulations of electrical installation in buildings.

Dr. Khaled Y. Al-Khalaf Vice Chairman, Board of Directors and Director General, SASO

contents

A A. contents

A1

B. general - installed power 1. methodology

B1

2. rules and statutory regulations

B3

2.1 definition of voltage ranges

B3

table B1 standard voltages between 100 V and 1000 V (IEC 38-1983) table B2 standard voltages above 1 kV and not exceeding 35 kV (IEC 38-1983)

2.2 regulations 2.3 standards 2.4 quality and safety of an electrical installation 2.5 initial testing of an installation 2.6 periodic check-testing of an installation

B3 B3 B4 B4 B5 B6 B6

table B3 frequency of check-tests commonly recommended for an electrical installation

B6

2.7 conformity (with standards and specifications) of equipment used in the installation

B7

3. motor, heating and lighting loads

B8

3.1 induction motors

B8

table B4 power and current values for typical induction motors

B9

3.2 direct-current motors

B10

table B6 progressive starters with voltage ramp table B7 progressive starters with current limitation

B10 B10

3.3 resistive-type heating appliances and incandescent lamps (conventional or halogen)

B11 table B8 current demands of resistive heating and incandescent lighting (conventional or halogen) appliances B11

3.4 fluorescent lamps and related equipment table B10 current demands and power consumption of commonly-dimensioned fluorescent lighting tubes (at 220 V/240 V - 50 Hz) table B11 current demands and power consumption of compact fluorescent lamps (at 220 V/240 V - 50 Hz)

B11 B12 B12

3.5 discharge lamps

B13

table B12 current demands of discharge lamps

B13

4. power loading of an installation

B14

4.1 installed power (kW) 4.2 installed apparent power (kVA)

B14 B15

table B13 estimation of installed apparent power

B15

4.3 estimation of actual maximum kVA demand

B16

table B14 simultaneity factors in an apartment block table B16 factor of simultaneity for distribution boards (IEC 439) table B17 factor of simultaneity according to circuit function

B16 B17 B17

4.4 example of application of factors ku and ks

B17

table B18 an example in estimating the maximum predicted loading of an installation (the factor values used are for demonstration purposes only)

B17

contents - A1

contents (continued)

A B. general - installed power (continued) 4. power loading of an installation (continued) 4.5 diversity factor 4.6 choice of transformer rating

B18 B18

table B19 IEC-standardized kVA ratings of HV/LV 3-phase distribution transformers and corresponding nominal full-load current values

B18

4.7 choice of power-supply sources

B19

C. HV/LV distribution substations 1. supply of power at high voltage

C1

1.1 power-supply characteristics of high voltage distribution networks

C1

table C1 relating nominal system voltages with corresponding rated system voltages (r.m.s. values) table C2 switchgear rated insulation levels table C3A transformers rated insulation levels in series I (based on current practice other than in the United States of America and some other countries) table C3B transformers rated insulation levels in series II (based on current practice in the United States of America and some other countries) table C4 standard short-circuit current-breaking ratings extracted from table X IEC 56

C3 C3 C4 C4

1.2 different HV service connections 1.3 some operational aspects of HV distribution networks

C11

2. consumers HV substations

C15

2.1 procedures for the establishment of a new substation

C15

3. substation protection schemes

C17

3.1 protection against electric shocks and overvoltages 3.2 electrical protection

C17

table C18 power limits of transformers with a maximum primary current not exceeding 45 A table C19 rated current (A) of HV fuses for transformer protection according to IEC 282-1 table C20 3-phase short-circuit currents of typical distribution transformers

C13

C22 C25 C26 C27

3.3 protection against thermal effects 3.4 interlocks and conditioned manœuvres

C31

4. the consumer substation with LV metering

C34

4.1 general 4.2 choice of panels

C34

table C27 standard short-circuit MVA and current ratings at different levels of nominal voltage

4.3 choice of HV switchgear panel for a transformer circuit 4.4 choice of HV/LV transformer

A2 - contents

C2

C31

C36 C37 C38 C38

table C31 categories of dielectric fluids table C32 safety measures recommended in electrical installations using dielectric liquids of classes 01, K1, K2 or K3

C42

5. a consumer substation with HV metering

C44

5.1 general 5.2 choice of panels 5.3 parallel operation of transformers

C44

6. constitution of HV/LV distribution substations

C49

6.1 different types of substation 6.2 indoor substations equipped with metal-enclosed switchgear 6.3 outdoor substations

C49

C41

C46 C48

C49 C52

A 7. appendix 1␣ : example in coordination of the characteristics of an HV switch-fuse combination protecting an HV/LV transformer

App C1-1

7.1 transfert current and take-over current

App C1-2

7.2 types of faults involved in the transfer region

App C1-3

8. appendix 2␣ : ground-surface potential gradients due to earth-fault currents

App C2-1

9. appendix 3␣ : vector diagram of ferro-resonance at 50Hz (or 60 Hz)

App C3-1

D. low-voltage service connections 1. low-voltage public distribution networks

D1

1.1 low-voltage consumers

D1

table D1 survey of electricity supplies in various countries around the world. table D2

D1 D6

1.2 LV distribution networks 1.3 the consumer-service connection 1.4 quality of supply voltage

D7

2. tariffs and metering

D14

D10 D13

E. power factor improvement and harmonic filtering 1. power factor improvement

E1

1.1 the nature of reactive energy 1.2 plant and appliances requiring reactive current 1.3 the power factor 1.4 tan ϕ 1.5 practical measurement of power factor 1.6 practical values of power factor

E1

table E5 example in the calculation of active and reactive power table E7 values of cos ϕ and tan ϕ for commonly-used plant and equipment

E2 E2 E3 E4 E4 E4 E4

2. why improve the power factor?

E5

2.1 reduction in the cost of electricity 2.2 technical/economic optimization

E5 E5

table E8 multiplying factor for cable size as a function of cos ϕ

E5

3. how to improve the power factor

E6

3.1 theoretical principles 3.2 by using what equipment? 3.3 the choice between a fixed or automatically-regulated bank of capacitors

E6

4. where to install correction capacitors

E9

4.1 global compensation 4.2 compensation by sector 4.3 individual compensation

E9

E7 E8

E9 E10

contents - A3


A E. power factor improvement and harmonic filtering (continued) 5. how to decide the optimum level of compensation

E11

5.1 general method 5.2 simplified method

E11 E11

table E17 kvar to be installed per kW of load, to improve the power factor of an installation

E12

5.3 method based on the avoidance of tariff penalties 5.4 method based on reduction of declared maximum apparent power (kVA)

E13

6. compensation at the terminals of a transformer

E14

6.1 compensation to increase the available active power output

E14

table E20 active-power capability of fully-loaded transformers, when supplying loads at different values of power factor

E14

6.2 compensation of reactive energy absorbed by the transformer

E15

table E24 reactive power consumption of distribution transformers with 20 kV primary windings

E16

E13

7. compensation at the terminals of an induction motor E17 7.1 connection of a capacitor bank and protection settings

E17

table E26 reduction factor for overcurrent protection after compensation

E17

7.2 how self-excitation of an induction motor can be avoided

E18

table E28 maximum kvar of P.F. correction applicable to motor terminals without risk of self-excitation

E19

8. example of an installation before and after power-factor correction

E20

9. the effect of harmonics on the rating of a capacitor bank

E21

9.1 problems arising from power-system harmonics 9.2 possible solutions 9.3 choosing the optimum solution

E21 E21 E22

table E30 choice of solutions for limiting harmonics associated with a LV capacitor bank

E22

9.4 possible effects of power-factor-correction capacitors on the power-supply system

E23

10. implementation of capacitor banks

E24

10.1 capacitor elements 10.2 choice of protection, control devices, and connecting cables

E24 E25

11. appendix 1␣ : elementary harmonic filters

App E3-1

12. appendix 2␣ : harmonic suppression reactor for a single (power factor correction) capacitor bank

App E4-1

F. distribution within a low-voltage installation

A4 - contents

1. general

F1

1.1 the principal schemes of LV distribution 1.2 the main LV distribution board 1.3 transition from IT to TN

F1 F4 F4

A 2. essential services standby supplies

F5

2.1 continuity of electric-power supply 2.2 quality of electric-power supply

F5 F6

table F10 assumed levels of transient overvoltage possible at different points of a typical installation table F12 typical levels of impulse withstand voltage of industrial circuit breakers labelled Uimp = 8 kV table F18 compatibility levels for installation materials

F8 F8 F13

3. safety and emergency-services installations, and standby power supplies

F15

3.1 safety installations 3.2 standby reserve-power supplies 3.3 choice and characteristics of reserve-power supplies

F15 F15 F16

table F21 table showing the choice of reserve-power supply types according to application requirements and acceptable supply-interruption times

F16

3.4 choice and characteristics of different sources

F17

table F22 table of characteristics of different sources

F17

3.5 local generating sets

F18

4. earthing schemes

F19

4.1 earthing connections

F19

table F25 list of exposed-conductive-parts and extraneous-conductive-parts

F20 F21 F23 F29 F30 F31 F32

4.2 definition of standardized earthing schemes 4.3 earthing schemes characteristics 4.4.1 choice criteria 4.4.2 comparison for each criterion 4.5 choice of earthing method - implementation 4.6 installation and measurements of earth electrodes table F47 resistivity (Ω-m) for different kinds of terrain table F48 mean values of resistivity (Ω-m) for an approximate estimation of an earth-electrode resistance with respect to zero-potential earth

F33 F33

5. distribution boards

F36

5.1 types of distribution board 5.2 the technologies of functional distribution boards 5.3 standards 5.4 centralized control

F36 F37 F38 F38

6. distributors

F39

6.1 description and choice 6.2 conduits, conductors and cables

F39 F41

table F60 selection of wiring systems table F61 erection of wiring systems table F62 some examples of installation methods table F63 designation code for conduits according to the most recent IEC publications table F64 designation of conductors and cables according to CENELEC code for harmonized cables table F66 commonly used conductors and cables

F41 F41 F43 F44 F45 F46

contents - A5


A F. distribution within a low-voltage installation (continued) 7. external influences

F47

7.1 classification

F47

table F67 concise list of important external influences (taken from Appendix A of IEC 364-3)

7.2 protection by enclosures: IP code

F48 F49

G. protection against electric shocks 1. general

G1

1.1 electric shock 1.2 direct and indirect contact

G1 G1

2. protection against direct contact

G2

2.1 measures of protection against direct contact 2.2 additional measure of protection against direct contact

G2 G3

3. protection against indirect contact

G4

3.1 measure of protection by automatic disconnection of the supply

G4

table G8 maximum safe duration of the assumed values of touch voltage in conditions where UL = 50 V table G9 maximum safe duration of the assumed values of touch voltage in conditions where UL = 25 V

3.2 automatic disconnection for a TT-earthed installation table G11 maximum operating times of RCCBs (IEC 1008)

3.3 automatic disconnection for a TN-earthed installation

G4 G4 G5 G6 G6

table G13 maximum disconnection times specified for TN earthing schemes (IEC 364-4-41)

G7

3.4 automatic disconnection on a second earth fault in an IT-earthed system

G8

table G18 maximum disconnection times specified for an IT-earthed installation (IEC 364-4-41)

G9

3.5 measures of protection against direct or indirect contact without circuit disconnection

G10

4. implementation of the TT system

G13

4.1 protective measures

G13

table G26 the upper limit of resistance for an installation earthing electrode which must not be exceeded, for given sensitivity levels of RCDs at UL voltage limits of 50 V and 25 V

G13

4.2 types of RCD

G14

4.3 coordination of differential protective devices

G15

5. implementation of the TN system

G18

5.1 preliminary conditions 5.2 protection against indirect contact

G18

table G42 correction factor to apply to the lengths given in tables G43 to G46 for TN systems table G43 maximum circuit lengths for different sizes of conductor and instantaneous-tripping-current settings for general-purpose circuit breakers table G44 maximum circuit lengths for different sizes of conductor and rated currents for type B circuit breakers table G45 maximum circuit lengths for different conductor sizes and for rated currents of circuit breakers of type C table G46 maximum circuit lengths for different conductor sizes and for rated currents of circuit breakers of type D or MA Merlin Gerin

5.3 high-sensitivity RCDs 5.4 protection in high fire-risk locations 5.5 when the fault-current-loop impedance is particularly high A6 - contents

G18 G20 G20 G20 G21 G21 G22 G22 G23

A 6. implementation of the IT system

G24

6.1 preliminary conditions

G24

table G53 essential functions in IT schemes

G24

6.2 protection against indirect contact

G25

table G59 correction factors, for IT-earthed systems, to apply to the circuit lengths given in tables G43 to G46

G28

6.3 high-sensitivity RCDs 6.4 in areas of high fire-risk 6.5 when the fault-current-loop impedance is particularly high

G29

7. residual current differential devices (RCDs)

G31

7.1 description 7.2 application of RCDs

G31

G29 G30

G31

table G70 electromagnetic compatibility withstand-level tests for RCDs table G72 means of reducing the ratio I∆n/lph (max.)

G32 G33

7.3 choice of characteristics of a residual-current circuit breaker (RCCB - IEC 1008)

G34

table G74 typical manufacturers coordination table for RCCBs, circuit breakers, and fuses

G34

H. the protection of circuits and the switchgear H1. the protection of circuits 1. general

H1-1

1.1 methodology and definitions

H1-1

table H1-1 logigram for the selection of cable size and protective-device rating for a given circuit

H1-1

1.2 overcurrent protection principles 1.3 practical values for a protection scheme 1.4 location of protective devices

H1-3 H1-4 H1-5

table H1-7 general rules and exceptions concerning the location of protective devices

H1-5

1.5 cables in parallel 1.6 worked example of cable calculations

H1-5 H1-6

table H1-9 calculations carried out with ECODIAL software (Merlin Gerin) table H1-10 example of short-circuit current evaluation

H1-8 H1-9

2. practical method for determining the smallest allowable cross-sectional-area of circuit conductors

H1-10

2.1 general

H1-10

table H1-11 logigram for the determination of minimum conductor size for a circuit

H1-10

2.2 determination of conductor size for unburied circuits

H1-10

table H1-12 code-letter reference, depending on type of conductor and method of installation table H1-13 factor K1 according to method of circuit installation (for further examples refer to IEC 364-5-52 table 52H) table H1-14 correction factor K2 for a group of conductors in a single layer table H1-15 correction factor K3 for ambient temperature other than 30 °C table H1-17 case of an unburied circuit: determination of the minimum cable size (c.s.a.), derived from the code letter; conductor material; insulation material and the fictitious current I'z

H1-10 H1-11 H1-11 H1-12 H1-13

contents - A7


A H. the protection of circuits and the switchgear (continued) H1. the protection of circuits (continued) 2. practical method for determining the smallest allowable cross-sectional-area of circuit conductors (continued) 2.3 determination of conductor size for buried circuits table H1-19 correction factor K4 related to the method of installation table H1-20 correction factor K5 for the grouping of several circuits in one layer table H1-21 correction factor K6 for the nature of the soil table H1-22 correction factor K7 for soil temperatures different than 20 °C table H1-24 case of a buried circuit: minimum c.s.a. in terms of type of conductor; type of insulation; and value of fictitious current I'z (I'z = Iz) K

H1-14 H1-14 H1-15 H1-15 H1-15

3. determination of voltage drop

H1-17

3.1 maximum voltage-drop limit

H1-17

table H1-26 maximum voltage-drop limits

H1-17

3.2 calculation of voltage drops in steady load conditions

H1-18

table H1-28 voltage-drop formulae table H1-29

H1-18

phase-to-phase voltage drop ∆U for a circuit, in volts per ampere per km

H1-18

4. short-circuit current calculations

H1-20

4.1 short-circuit current at the secondary terminals of a HV/LV distribution transformer

H1-20

table H1-32 typical values of Usc for different kVA ratings of transformers with HV windings i 20 kV table H1-33 Isc at the LV terminals of 3-phase HV/LV transformers supplied from a HV system with a 3-phase fault level of 500 MVA, or 250 MVA

4.2 3-phase short-circuit current (Isc) at any point within a LV installation table H1-36 the impedance of the HV network referred to the LV side of the HV/LV transformer table H1-37 resistance, reactance and impedance values for typical distribution transformers with HV windings i 20 kV table H1-38 recapitulation table of impedances for different parts of a power-supply system table H1-39 example of short-circuit current calculations for a LV installation supplied at 400 V (nominal) from a 1,000 kVA HV/LV transformer

H1-20 H1-20 H1-21 H1-21 H1-22 H1-23 H1-23

4.3 Isc at the receiving end of a feeder in terms of the Isc at its sending end

H1-23

table H1-40 Isc at a point downstream, in terms of a known upstream fault-current value and the length and c.s.a. of the intervening conductors, in a 230/400 V 3-phase system

H1-24

4.4 short-circuit current supplied by an alternator or an inverter

H1-25

5. particular cases of short-circuit current

H1-26

5.1 calculation of minimum levels of short-circuit current

H1-26

table H1-49 maximum circuit lengths in metres for copper conductors (for aluminium, the lengths must be multiplied by 0.62) table H1-50 maximum length of copper-conductored circuits in metres protected by B-type circuit breakers table H1-51 maximum length of copper-conductored circuits in metres protected by C-type circuit breakers table H1-52 maximum length of copper-conductored circuits in metres protected by D-type circuit breakers table H1-53 correction factors to apply to lengths obtained from tables H1-49 to H1-52 A8 - contents

H1-14

H1-28 H1-29 H1-29 H1-29 H1-30

A 5.2 verification of the withstand capabilities of cables under short-circuit conditions table H1-54 value of the constant k2 table H1-55 maximum allowable thermal stress for cables (expressed in amperes2 x seconds x 106)

H1-31 H1-31 H1-31

6. protective earthing conductors (PE)

H1-32

6.1 connection and choice

H1-32

table H1-59 choice of protective conductors (PE)

H1-33

6.2 conductor dimensioning

H1-33

table H1-60 minimum c.s.a.'s for PE conductors and earthing conductors (to the installation earth electrode) table H1-61 k factor values for LV PE conductors, commonly used in national standards and complying with IEC 724

H1-34 H1-34

6.3 protective conductor between the HV/LV transformer and the main general distribution board (MGDB)

H1-35 table H1-63 c.s.a. of PE conductor between the HV/LV transformer and the MGDB, in terms of transformer ratings and fault-clearance times used in France H1-35

6.4 equipotential conductor

H1-35

7. the neutral conductor

H1-36

7.1 dimensioning the neutral conductor

H1-36

7.2 protection of the neutral conductor

H1-36

table H1-65 table of protection schemes for neutral conductors in different earthing systems

H1-37

H2. the switchgear 1. the basic functions of LV switchgear

H2-1

table H2-1 basic functions of LV switchgear

H2-1

1.1 electrical protection 1.2 isolation

H2-1 H2-1

table H2-2 peak value of impulse voltage according to normal service voltage of test specimen

H2-2

1.3 switchgear control

H2-2

2. the switchgear and fusegear

H2-4

2.1 elementary switching devices

H2-4

table H2-7 utilization categories of LV a.c. switches according to IEC 947-3 table H2-8 factor "n" used for peak-to-rms value (IEC 947-part 1) table H2-13 zones of fusing and non-fusing for LV types gG and gM class fuses (IEC 269-1 and 269-2-1)

H2-5 H2-5 H2-7

2.2 combined switchgear elements

H2-9

3. choice of switchgear

H2-11

3.1 tabulated functional capabilities

H2-11

table H2-19 functions fulfilled by different items of switchgear

H2-11

3.2 switchgear selection

H2-11

contents - A9


A H2. the switchgear (continued) 4. circuit breakers

H2-12

table H2-20 functions performed by a circuit breaker/disconnector

H2-12

4.1 standards and descriptions 4.2 fundamental characteristics of a circuit breaker table H2-28 tripping-current ranges of overload and short-circuit protective devices for LV circuit breakers table H2-31 Icu related to power factor (cos ϕ) of fault-current circuit (IEC 947-2)

H2-12 H2-15 H2-16 H2-17

4.3 other characteristics of a circuit breaker

H2-18 table H2-34 relation between rated breaking capacity Icu and rated making capacity Icm at different power-factor values of short-circuit current, as standardized in IEC 947-2 H2-19

4.4 selection of a circuit breaker table H2-38 examples of tables for the determination of derating/uprating factors to apply to CBs with uncompensated thermal tripping units, according to temperature table H2-40 different tripping units, instantaneous or short-time delayed table H2-43 maximum values of short-circuit current to be interrupted by main and principal circuit breakers (CBM and CBP respectively), for several transformers in parallel

4.5 coordination between circuit breakers table H2-45 example of cascading possibilities on a 230/400 V or 240/415 V 3-phase installation table H2-49 summary of methods and components used in order to achieve discriminative tripping

4.6 discrimination HV/LV in a consumer's substation

H2-20 H2-21 H2-23 H2-25 H2-27 H2-28 H2-29 H2-32

J. particular supply sources and loads 1. protection of circuits supplied by an alternator

J1

1.1 an alternator on short-circuit 1.2 protection of essential services circuits supplied in emergencies from an alternator 1.3 choice of tripping units

J1

1.4 methods of approximate calculation

J6

table J1-7 procedure for the calculation of 3-phase short-circuit current table J1-8 procedure for the calculation of 1-phase to neutral short-circuit current

J4 J5

J6 J7

1.5 the protection of standby and mobile a.c. generating sets

J9

2. inverters and UPS (Uninterruptible Power Supply units)

J10

2.1 what is an inverter? 2.2 types of UPS system

J10

2.3 standards 2.4 choice of a UPS system

J11

2.5 UPS systems and their environment

J14

2.6 putting into service and technology of UPS systems 2.7 earthing schemes

J15

J10 table J2-4 examples of different possibilities and applications of inverters, in decontamination of supplies and in UPS schemes J11

A10 - contents

J12

J17

A 2.8 choice of main-supply and circuit cables, and cables for the battery connection table J2-21 voltage drop in % of 324 V d.c. for a copper-cored cable table J2-22 currents and c.s.a. of copper-cored cables feeding the rectifier, and supplying the load for UPS system Maxipac (cable lengths < 100 m) table J2-23 currents and c.s.a. of copper-cored cables feeding the rectifier, and supplying the load for UPS system EPS 2000 (cable lengths < 100 m). Battery cable data are also included table J2-24 input, output and battery currents for UPS system EPS 5000 (Merlin Gerin)

J20 J21 J21 J21 J22

2.9 choice of protection schemes

J23

2.10 complementary equipments

J24

3. protection of LV/LV transformers

J25

3.1 transformer-energizing in-rush current 3.2 protection for the supply circuit of a LV/LV transformer

J25

3.3 typical electrical characteristics of LV/LV 50 Hz transformers

J26

table J3-5 typical electrical characteristics of LV/LV 50 Hz transformers

J26

3.4 protection of transformers with characteristics as tabled in J3-5 above, using Merlin Gerin circuit breakers

J26

J25

table J3-6 protection of 3-phase LV/LV transformers with 400 V primary windings table J3-7 protection of 3-phase LV/LV transformers with 230 V primary windings table J3-8 protection of 1-phase LV/LV transformers with 400 V primary windings table J3-9

J26 J27 J27

protection of 1-phase LV/LV transformers with 230 V primary windings

J28

4. lighting circuits

J29

4.1 service continuity

J29

4.2 lamps and accessories (luminaires)

J30

table J4-1 analysis of disturbances in fluorescent-lighting circuits

J30

4.3 the circuit and its protection

J31

4.4 determination of the rated current of the circuit breaker

J31

table J4-2 protective circuit breaker ratings for incandescent lamps and resistive-type heating circuits table J4-3 maximum limit of rated current per outgoing lighting circuit, for high-pressure discharge lamps table J4-4 current ratings of circuit breakers related to the number of fluorescent luminaires to be protected

J31 J32 J32

4.5 choice of control-switching devices

J33

table J4-5 types of remote control

J33

4.6 protection of ELV lighting circuits

J34

4.7 supply sources for emergency lighting

J35

5. asynchronous motors

J36

5.1 protective and control functions required

J36

table J5-2 commonly-used types of LV motor-supply circuits

J37

5.2 standards

J38

5.3 basic protection schemes: circuit breaker / contactor / thermal relay

J38

table J5-4 utilization categories for contactors (IEC 947-4)

J39

5.4 preventive or limitative protection

J41 contents - A11


A J. particular supply sources and loads (continued) 5. asynchronous motors (continued) 5.5 maximum rating of motors installed for consumers supplied at LV table J5-12 maximum permitted values of starting current for direct-on-line LV motors (230/400 V) table J5-13 maximum permitted power ratings for LV direct-on-line-starting motors

J43 J43 J43

5.6 reactive-energy compensation (power-factor correction)

J43

6. protection of direct-current installations

J44

6.1 short-circuit currents 6.2 characteristics of faults due to insulation failure, and of protective switchgear

J44 J45

table J6-4 characteristics of protective switchgear according to type of d.c. system earthing

J45

6.3 choice of protective device

J45

table J6-5 choice of d.c. circuit breakers manufactured by Merlin Gerin

J46

6.4 examples

J46

6.5 protection of persons

J47

7. Appendix␣ : Short-circuit characteristics of an alternator

App J1-1

L. domestic and similar premises and special locations

A12 - contents

1. domestic and similar premises

L1

1.1 general 1.2 distribution-board components 1.3 protection of persons 1.4 circuits

L1 L2 L4 L6

table L1-9 recommended minimum number of lighting and power points in domestic premises table L1-11 c.s.a. of conductors and current rating of the protective devices in domestic installations (the c.s.a. of aluminium conductors are shown in brackets)

L7

2. bathrooms and showers

L8

2.1 classification of zones 2.2 equipotential bonding

L8

2.3 requirements prescribed for each zone

L10

3. recommendations applicable to special installations and locations

L11

L6

L10

1. methodology

B the study of an electrical installation by means of this guide requires the reading of the entire text in the order in which the chapters are presented.

listing of power demands The study of a proposed electrical installation necessitates an adequate understanding of all governing rules and regulations. A knowledge of the operating modes of power-consuming appliances, i.e. "loads" (steady-state demand, starting conditions, non-simultaneous operation, etc.) together with the location and magnitude of each load shown on a building plan, allow a listing of power demands to be compiled. The list will include the total power of the loads installed as well as an estimation of the actual loads to be supplied, as deduced from the operating modes. From these data the power required from the supply source and (where appropriate) the number of sources necessary for an adequate supply to the installation, are readily obtained. Local information regarding tariff structures is also required to permit the best choice of connection arrangement to the power-supply network, e.g. at high voltage or low voltage.

corresponding chapter B - general - installed power

service connection This connection can be made at: c High Voltage: a consumer-type substation will then have to be studied, built and equipped. This substation may be an outdoor or indoor installation conforming to relevant standards and regulations (the low-voltage section may be studied separately if necessary). Metering at high-voltage or low-voltage is possible in this case c Low Voltage: the installation will be connected to the local power network and will (necessarily) be metered according to LV tariffs.

C - HV/LV distribution substations

D - low-voltage service connections

reactive energy The compensation of reactive energy within electrical installations normally concerns only power factor improvement, and is carried out locally, globally or as a combination of both methods.

E - power factor improvement

LV distribution The whole of the installation distribution network is studied as a complete system. The number and characteristics of standby emergency-supply sources are defined. Earth-bonding connections and neutralearthing arrangements are chosen according to local regulations, constraints related to the power-supply, and to the nature of the installation loads. The hardware components of distribution, together with distribution boards and cableways, are determined from building plans and from the location and grouping of loads. The kinds of location, and activities practised in them, can affect their level of resistance to external influences.

F - distribution within a low-voltage installation

protection against electric shock The system of earthing (TT, IT or TN) having been previously determined, it remains, in order to achieve protection of persons against the hazards of direct and indirect contact, to choose an appropriate scheme of protection.

G - protection against electric shock

general - installed power - B1

1. methodology (continued)

B circuits and switchgear Each circuit is then studied in detail. From the rated currents of the loads; the level of short-circuit current; and the type of protective device, the cross-sectional area of circuit conductors can be determined, taking into account the nature of the cableways and their influence on the current rating of conductors. Before adopting the conductor size indicated above, the following requirements must be satisfied: c the voltage drop complies with the relevant standard, c motor starting is satisfactory, c protection against electric shock is assured. The short-circuit current Isc is then determined, and the Isc thermal and electrodynamic withstand capability of the circuit is checked. These calculations may indicate that a different conductor size than that originally chosen is necessary. The performance required by the switchgear will determine its type and characteristics. The use of cascading techniques and the discriminative operation of fuses and tripping of circuit breakers are examined.

H1 - the protection of circuits

H2 - the switchgear

particular supply sources and loads Particular items of plant and equipment are studied: c specific sources such as alternators or inverters, c specific loads with special characteristics, such as induction motors, lighting circuits or LV/LV transformers, or c specific systems, such as direct-current networks.

J - particular supply sources and loads

domestic and similar premises and special locations Certain premises and locations are subject to particularly strict regulations: the most common example being domestic dwellings.

L - domestic and similar premises and special locations

Ecodial 2.2 software Ecodial 2.2 software* provides a complete conception and design package for LV installations, in accordance with IEC standards and recommendations. The following features are included: c construction of one-line diagrams, c calculation of short-circuit currents, c calculation of voltage drops, c optimization of cable sizes, c required ratings of switchgear and fusegear, c discrimination of protective devices, c recommendations for cascading schemes, c verification of the protection of persons, c comprehensive print-out of the foregoing calculated design data. * Ecodial 2.2 is a Merlin Gerin product and is available in French and English versions.

B2 - general - installed power

2. rules and statutory regulations

B Low-voltage installations are governed by a number of regulatory and advisory texts, which may be classified as follows: c statutory regulations (decrees, factory acts, etc.), c codes of practice, regulations issued by professional institutions, job specifications, c national and international standards for installations, c national and international standards for products.

2.1 definition of voltage ranges IEC voltage standards and recommendations three phase, four wire or three wire systems nominal voltage (V) 230/400(1) 277/480(2) 400/690(1) 1000

single phase, three wire systems nominal voltage (V) 120/240 -

table B1: standard voltages between 100 V and 1000 V (IEC 38-1983). 1) The nominal voltage of existing 220/380 V and 240/415 V systems shall evolve towards the recommended value of 230/400 V. The transition period should be as short as possible, and should not exceed 20 years after the issue of this IEC publication. During this period, as a first step, the electricity supply authorities of countries having 220/380 V systems should bring the voltage within the range 230/400 V +6% -10% and those of countries having 240/415 V systems should bring the voltage within the range 230/400 V +10% -6%. At the end of this transition period the tolerance of 230/400 V ±10% should have been achieved; after this the reduction of this range will be considered. All the above considerations apply also to the present 380/660 V value with respect to the recommended value 400/690 V. 2) Not to be utilized together with 230/400 V or 400/690 V.

50 Hz and 60 Hz systems series I highest voltage nominal system for equipment (kV) voltage (kV) 3.6(1) 3.3(1) 3((1) 7.2(1) 6.6(1) 6(1) 12 11 10 (17.5) (15) 24 22 20 36(3) 33(3) 40.5(3) 35(3)

60 Hz systems series II (North American practice) highest voltage nominal system for equipment (kV) voltage (kV) 4.40(1) 4.16(1) 13.2(2) 12.47(2) 13.97(2) 13.2(2) 14.52(1) 13.8(1) 26.4(2) 24.94(2) 36.5(2) 34.5(2) -

table B2: standard voltages above 1 kV and not exceeding 35 kV (IEC 38-1983). * These systems are generally three-wire systems unless otherwise indicated. The values indicated are voltages between phases. The values indicated in parentheses should be considered as non-preferred values. It is recommended that these values should not be used for new systems to be constructed in future. 1) These values should not be used for public distribution systems. 2) These systems are generally four-wire systems. 3) The unification of these values is under consideration.


2. rules and statutory regulations (continued)

B 2.2 regulations In most countries, electrical installations shall comply with more than one set of regulations, issued by National Authorities or by recognised private bodies. It is essential to take into account these local constraints before starting the design.

2.3 standards This Guide is based on relevant IEC standards, in particular IEC 364. IEC 364 has been established by medical and engineering experts of all countries in the world comparing their experience at an international level. Currently, the safety principles of IEC 364 and 479-1 are the fundamentals of most electrical standards in the world. IEC - 38 IEC - 56 IEC - 76-2 IEC - 76-3 IEC - 129 IEC - 146 IEC - 146-4

Standard voltages High-voltage alternating-current circuit breakers Power transformer - Part 2: Temperature rise Power transformer - Part 3: Insulation levels and dielectric tests Alternating current disconnectors and earthing switches General requirements and line commutated converters General requirements and line commutated converters - Part 4: Method of specifying the performance and test requirements of uninterruptible power systems IEC - 265-1 High-voltage switches - Part 1: High-voltage switches for rated voltages above 1 kV and less than 52 kV IEC - 269-1 Low-voltage fuses - Part 1: General requirements IEC - 269-3 Low-voltage fuses - Part 3: Supplementary requirements for fuses for use by unskilled persons (fuses mainly for household and similar applications) IEC - 282-1 High-voltage fuses - Part 1: Current limiting fuses IEC - 287 Calculation of the continuous current rating of cables (100% load factor) IEC - 298 AC metal-enclosed switchgear and controlgear for rated voltages above 1kV and up to and including 52 kV IEC - 364 Electrical installations of buildings IEC - 364-3 Electrical installations of buildings - Part 3: Assessment of general characteristics IEC - 364-4-41 Electrical installations of buildings - Part 4: Protection of safety - Section 41: Protection against electrical shock IEC - 364-4-42 Electrical installations of buildings - Part 4: Protection of safety - Section 42: Protection against thermal effects IEC - 364-4-43 Electrical installations of buildings - Part 4: Protection of safety - Section 43: Protection against overcurrent IEC - 364-4-47 Electrical installations of buildings - Part 4: Application of protective measures for safety - Section 47: Measures of protection against electrical shock IEC - 364-5-51 Electrical installations of buildings - Part 5: Selection and erection of electrical equipment - Section 51: Common rules IEC - 364-5-52 Electrical installations of buildings - Part 5: Selection and erection of electrical equipment - Section 52: Wiring systems IEC - 364-5-53 Electrical installations of buildings - Part 5: Selection and erection of electrical equipment - Section 53: Switchgear and controlgear IEC - 364-6 Electrical installations of buildings - Part 6: Verification IEC - 364-7-701 Electrical installations of buildings - Part 7: Requirements for special installations or locations - Section 701: Electrical installations in bathrooms IEC - 364-7-706 Electrical installations of buildings - Part 7: Requirements for special installations or locations - Section 706: Restrictive conductive locations IEC - 364-7-710 Electrical installations of buildings - Part 7: Requirements for special installations or locations - Section 710: Installation in exhibitions, shows, stands and funfairs IEC - 420 High-voltage alternating current switch-fuse combinations IEC - 439-1 Low-voltage switchgear and controlgear assemblies - Part 1: Types-tested and partially type-tested assemblies IEC - 439-2 Low-voltage switchgear and controlgear assemblies - Part 2: Particular requirements for busbar trunking systems (busways) IEC - 439-3 Low-voltage switchgear and controlgear assemblies - Part 3: Particular requirements for low-voltage switchgear and controlgear assemblies intended to be installed in places where unskilled persons have access for their use Distribution boards IEC - 446 Identification of conductors by colours or numerals IEC - 479-1 Effects of current on human beings and livestock - Part 1: General aspects IEC - 479-2 Effects of current on human beings and livestock - Part 2: Special aspects IEC - 529 Degrees of protection provided by enclosures (IP code) IEC - 644 Specification for high-voltage fuse-links for motor circuit applications


B IEC - 664 IEC - 694 IEC - 724 IEC - 742 IEC - 755 IEC - 787 IEC - 831-1

Insulation coordination for equipment within low-voltage systems Common clauses for high-voltage switchgear and controlgear standards Guide to the short-circuit temperature limits of electrical cables with a rated voltage not exceeding 0.6/1.0 kV Isolation transformer and safety isolation transformer. Requirements General requirements for residual current operated protective devices Application guide for selection for fuse-links of high-voltage fuses for transformer circuit application Shunt power capacitors of the self-healing type for a.c. systems having a rated voltage up to and including 660 V. - Part 1: General - Performance, testing and rating - Safety requirements - Guide for installation and operation

2.4 quality and safety of an electrical installation Only by c the initial checking of the conformity of the electrical installation, c the verification of the conformity of electrical equipment, c and periodic checking can the permanent safety of persons and security of supply to equipment be achieved.


2. rules and statutory regulations (continued)

B 2.5 initial testing of an installation Before a power-supply authority will connect an installation to its supply network, strict pre-commissioning electrical tests and visual inspections by the authority, or by its appointed agent, must be satisfied. These tests are made according to local (governmental and/or institutional) regulations, which may differ slightly from one country to another. The principles of all such regulations however, are common, and are based on the observance of rigorous safety rules in the design and realization of the installation. IEC 364 and related standards included in this guide are based on an international consensus for such tests, intended to cover all the safety measures and approved installation practices normally required for domestic, commercial and (the majority of) industrial buildings. Many industries however have additional regulations related to a particular product (petroleum, coal, natural gas, etc.). Such additional requirements are beyond the scope of this guide. The pre-commissioning electrical tests and visual-inspection checks for installations in buildings include, typically, all of the following: c insulation tests of all cable and wiring conductors of the fixed installation, between phases and between phases and earth, c continuity and conductivity tests of protective, equipotential and earth-bonding conductors, c resistance tests of earthing electrodes with respect to remote earth, c allowable number of socket-outlets per circuit check, c cross-sectional-area check of all conductors for adequacy at the short-circuit levels prevailing, taking account of the associated protective devices, materials and installation conditions (in air, conduit, etc.), c verification that all exposed- and extraneous metallic parts are properly earthed (where appropriate), c check of clearance distances in bathrooms, etc.

These tests and checks are basic (but not exhaustive) to the majority of installations, while numerous other tests and rules are included in the regulations to cover particular cases, for example: TN-, TT- or IT-earthed installations, installations based on class 2 insulation, SELV circuits, and special locations, etc. The aim of this guide is to draw attention to the particular features of different types of installation, and to indicate the essential rules to be observed in order to achieve a satisfactory level of quality, which will ensure safe and trouble-free performance. The methods recommended in this guide, modified if necessary to comply with any possible variation imposed by a local supply authority, are intended to satisfy all precommissioning test and inspection requirements.

2.6 periodic check-testing of an installation In many countries, all industrial and commercial-building installations, together with installations in buildings used for public gatherings, must be re-tested periodically by authorized agents. Table B3 shows the frequency of testing commonly prescribed according to the kind of installation concerned. installations which require the protection of employees

installations in buildings used for public gatherings, where protection against the risks of fire and panic are required residential

c locations at which a risk of degradation, annually fire or explosion exists c temporary installations at worksites c locations at which HV installations exist c restrictive conducting locations where mobile equipment is used other cases every 3 years according to the type of establishment and its capacity for receiving the public, the re-testing period will vary from one to three years according to local regulations

table B3: frequency of check-tests commonly recommended for an electrical installation.


B 2.7 conformity (with standards and specifications) of equipment used in the installation conformity of equipment with the relevant standards can be attested in several ways.

attestation of conformity The conformity of equipment with the relevant standards can be attested: c by an official conformity mark granted by the standards organization concerned, or c by a certificate of conformity issued by a laboratory, or c by a declaration of conformity from the manufacturer.

declaration of conformity In cases where the equipment in question is to be used by qualified or experienced persons, the declaration of conformity provided by the manufacturer (included in the technical documentation) together with a conformity mark on the equipment concerned, are generally recognized as a valid attestation. Where the competence of the manufacturer is in doubt, a certificate of conformity can be obtained from an independent accredited laboratory.

the standards define several methods of quality assurance which correspond to different situations rather than to different levels of quality.

mark of conformity Conformity marks are inscribed on appliances and equipment which are generally used by technically inexperienced persons (for example, domestic appliances) and for whom the standards have been established which permit the attribution, by the standardization authority, of a mark of conformity (commonly referred to as a conformity mark).

certification of Quality Assurance A laboratory for testing samples cannot certify the conformity of an entire production run: these tests are called type tests. In some tests for conformity to standards, the samples are destroyed (tests on fuses, for example). Only the manufacturer can certify that the fabricated products have, in fact, the characteristics stated. Quality assurance certification is intended to complete the initial declaration or certification of conformity. As proof that all the necessary measures have been taken for assuring the quality of production, the manufacturer obtains certification of the quality control system which monitors the fabrication of the product concerned. These certificates are issued by organizations specializing in quality control, and are based on the international standard ISO 9000, the equivalent European standard being EN 29000. These standards define three model systems of quality assurance control corresponding to different situations rather than to different levels of quality: c model 3 defines assurance of quality by inspection and checking of final products, c model 2 includes, in addition to checking of the final product, verification of the manufacturing process. This method applies, for example, to the manufacture of fuses where performance characteristics cannot be checked without destroying the fuse, c model 1 corresponds to model 2, but with the additional requirement that the quality of the design process must be rigorously scrutinized; for example, where it is not intended to fabricate and test a prototype (case of a custom-built product made to specification). general - installed power - B7

3. motor, heating and lighting loads

B an examination of the actual apparent-power demands of different loads: a necessary preliminary step in the design of a LV installation.

The examination of actual values of apparent-power required by each load enables the establishment of: c a declared power demand which determines the contract for the supply of energy, c the rating of the HV/LV transformer, where applicable (allowing for expected increases in load), c levels of load current at each distribution board.

3.1 induction motors the nominal power in kW (Pn) of a motor indicates its rated equivalent mechanical power output. The apparent power in kVA (Pa) supplied to the motor is a function of the output, the motor efficiency and the power factor. Pa = Pn η cos ϕ

current demand The full-load current Ia supplied to the motor is given by the following formulae: Pn x 1,000 3-phase motor: Ia = ex U x η x cos ϕ 1-phase motor: Ia = Pn x 1,000 U x η x cos ϕ where Ia: current demand (in amps) Pn: nominal power (in kW of active power) U: voltage between phases for 3-phase motors and voltage between the terminals for single-phase motors (in volts). A single-phase motor may be connected phase-to-neutral or phase-to-phase. η: per-unit efficiency, i.e. output kW input kW cos ϕ: power factor, i.e. kW input kVA input

motor-starting current Starting current (Id) for 3-phase induction motors, according to motor type, will be: c for direct-on-line starting of squirrel-cage motors: v Id = 4.2 to 9 In for 2-pole motors v Id = 4.2 to 7 In for motors with more than 2 poles (mean value = 6 In), where In = nominal full-load current of the motor, c for wound-rotor motors (with slip-rings), and for D.C. motors:

it is generally advantageous for technical and financial reasons to reduce the current supplied to induction motors. This can be achieved by using capacitors without affecting the power output of the motors.

Id depends on the value of starting resistances in the rotor circuits: Id = 1.5 to 3 In (mean value = 2.5 In). c for induction motors controlled by speedchanging variable-frequency devices (for example: Altivar Telemecanique), assume that the control device has the effect of increasing the power (kW) supplied to the circuit motor (i.e. device plus) by 10%.

compensation of reactive-power (kvar) supplied to induction motors The application of this principle to the operation of induction motors is generally referred to as "power-factor improvement" or "power-factor correction". As discussed in chapter E, the apparentpower (kVA) supplied to an induction motor can be significantly reduced by the use of shunt-connected capacitors. Reduction of input kVA means a corresponding reduction of input current (since the voltage remains constant). Compensation of reactive-power is particularly advised for motors that operate for long periods at reduced power.

As noted above cos ϕ = kW input so that a kVA input reduction in kVA input will increase (i.e. improve) the value of cos ϕ. The current supplied to the motor, after power-factor correction, is given by: Ia x cos ϕ cos ϕ' where cos ϕ is the power factor before compensation and cos ϕ' is the power factor after compensation, Ia being the original current.

table of typical values Table B4 shows, as a function of the rated nominal power of motors, the current supplied to them at different voltage levels under normal uncompensated conditions, and the same motors under the same conditions, but compensated to operate at a power factor of 0.93 (tan ϕ = 0.4). These values are averages and will differ to some extent according to the type of motor and the manufacturer concerned. B8 - general - installed power

Note: the rated voltages of certain loads listed in table B4 are still based on 220/380 V. The international standard is now (since 1983) 230/400 V. To convert the current values indicated for a given motor rating in the 220 V and 380 V columns to the currents taken by 230 V and 400 V motors of the same rating, multiply by a factor of 0.95.

B 3.1 induction motors (continued) nominal power Pn kW HP 0.37 0.5 0.55 0.75 0.75 1 1.1 1.5 1.5 2 2.2 3 3 4 3.7 5 4 5.5 5.5 7.5 7.5 10 9 12 10 13.5 11 15 15 20 18.5 25 22 30 25 35 30 40 33 45 37 50 40 54 45 60 51 70 55 75 59 80 63 85 75 100 80 110 90 125 100 136 110 150 129 175 132 180 140 190 147 200 150 205 160 220 180 245 185 250 200 270 220 300 250 340 257 350 280 380 295 400 300 410 315 430 335 450 355 480 375 500 400 545 425 580 445 600 450 610 475 645 500 680 530 720 560 760 600 810 630 855 670 910 710 965 750 1020 800 1090 900 1220 1100 1500

η

% 64 68 72 75 78 79 81 82 82 84 85 86 86 87 88 89 89 89 89 90 90 91 91 91 92 92 92 92 92 92 92 93 93 94 94 94 94 94 94 94 94 94 94 94 95 95 95 95 95 95 95 95 95 95 95 95 95 95 95 95 95 95 95 95 95 95 95

without compensation cos ϕ Pa current at different voltages at Pn 1-PH 3-PH 220 V 220 V 380 V 440 V kVA A A A A 0.73 0.79 3.6 1.8 1.03 0.99 0.75 1.1 4.7 2.75 1.6 1.36 0.75 1.4 6 3.5 2 1.68 0.79 1.9 8.5 4.4 2.6 2.37 0.80 2.4 12 6.1 3.5 3.06 0.80 3.5 16 8.7 5 4.42 0.80 4.6 21 11.5 6.6 5.77 0.80 5.6 25 13.5 7.7 7.1 0.80 6.1 26 14.5 8.5 7.9 0.83 7.9 35 20 11.5 10.4 0.83 10.6 47 27 15.5 13.7 0.85 12.3 32 18.5 16.9 0.85 13.7 35 20 17.9 0.86 14.7 39 22 20.1 0.86 19.8 52 30 26.5 0.86 24.2 64 37 32.8 0.86 28.7 75 44 39 0.86 33 85 52 45.3 0.86 39 103 60 51.5 0.86 43 113 68 58 0.86 48 126 72 64 0.86 51 134 79 67 0.86 57 150 85 76 0.86 65 170 98 83 0.86 70 182 105 90 0.87 74 195 112 97 0.87 79 203 117 109 0.87 94 240 138 125 0.87 100 260 147 131 0.87 112 295 170 146 0.87 125 325 188 162 0.87 136 356 205 178 0.87 159 420 242 209 0.87 161 425 245 215 0.87 171 450 260 227 0.87 180 472 273 236 0.87 183 483 280 246 0.87 196 520 300 256 0.87 220 578 333 289 0.87 226 595 342 295 0.88 242 626 370 321 0.88 266 700 408 353 0.88 302 800 460 401 0.88 311 826 475 412 0.88 335 900 510 450 0.88 353 948 546 473 0.88 359 980 565 481 0.88 377 990 584 505 0.88 401 1100 620 518 0.88 425 1150 636 549 0.88 449 1180 670 575 0.88 478 1250 710 611 0.88 508 1330 760 650 0.88 532 1400 790 680 0.88 538 1410 800 690 0.88 568 1490 850 730 0.88 598 1570 900 780 0.88 634 1660 950 825 0.88 670 1760 1000 870 0.88 718 1880 1090 920 0.88 754 1980 1100 965 0.88 801 2100 1200 1020 0.88 849 1260 1075 0.88 897 1350 1160 0.88 957 1450 1250 0.88 1076 1610 1390 0.88 1316 1980 1700

500 V A 0.91 1.21 1.5 2 2.6 3.8 5 5.9 6.5 9 12 13.9 15 18.4 23 28.5 33 39.4 45 50 55 60 65 75 80 85 89 105 112 129 143 156 184 187 200 207 210 220 254 263 281 310 360 365 400 416 420 445 472 500 527 540 574 595 608 645 680 720 760 830 850 910 960 1020 1100 1220 1500

660 V A 0.6 0.9 1.1 1.5 2 2.8 3.8 4.4 4.9 6.6 8.9 10.6 11.5 14 17.3 21.3 25.4 30.3 34.6 39 42 44 49 57 61 66 69 82 86 98 107 118 135 140 145 152 159 170 190 200 215 235 274 280 305 320 325 337 365 370 395 410 445 455 460 485 515 545 575 630 645 690 725 770 830 925 1140

with compensation cos ϕ capa- Pa at Pn citor rating kvar kVA 0.93 0.31 0.62 0.93 0.39 0.87 0.93 0.48 1.1 0.93 0.53 1.6 0.93 0.67 2.1 0.93 0.99 3 0.93 1.31 4 0.93 1.59 4.8 0.93 1.74 5.2 0.93 1.80 7 0.93 2.44 9.5 0.93 2.4 11.3 0.93 2.6 12.5 0.93 2.50 13.6 0.93 3.37 18.3 0.93 4.12 22.4 0.93 4.89 26.6 0.93 5.57 30 0.93 6.68 36 0.93 7.25 39 0.93 8.12 44 0.93 8.72 47 0.93 9.71 53 0.93 11.10 60 0.93 11.89 64 0.93 10.98 69 0.93 11.66 74 0.93 13.89 88 0.93 14.92 93 0.93 16.80 105 0.93 18.69 117 0.93 20.24 127 0.93 23.84 149 0.93 24 151 0.93 25.55 160 0.93 26.75 168 0.93 27.26 172 0.93 29.15 183 0.93 32.76 206 0.93 33.79 212 0.93 30.78 229 0.93 33.81 252 0.93 38.44 286 0.93 39.45 294 0.93 42.63 317 0.93 44.80 334 0.93 45.66 339 0.93 47.98 356 0.93 51 379 0.93 54 402 0.93 57.1 424 0.93 60.84 453 0.93 64.60 481 0.93 67.63 504 0.93 68.50 509 0.93 70.40 538 0.93 72.26 566 0.93 80.64 600 0.93 85.12 634 0.93 91.33 679 0.93 95.81 713 0.93 101.88 758 0.93 107.95 804 0.93 114 849 0.93 121.68 905 0.93 136.86 1019 0.93 167.35 1245

current at different voltages 1-PH 3-PH 220 V 220 V 380 V 440 V A A A A 2.8 1.4 0.8 0.77 3.8 2.2 1.3 1.1 4.8 2.8 1.6 1.3 7.2 3.7 2.2 2 10.3 5.2 3 2.6 13.7 7.5 4.3 3.8 18 9.9 5.7 5 22 11.6 6.6 6.1 22 12.5 7.3 6.8 31 17.8 10.3 9.3 42 24 13.8 12.2 29 16.9 15.4 32 18 16.4 36 20 19 48 28 25 59 34 30 69 41 36 79 48 42 95 55 48 104 63 54 117 67 59 124 73 62 139 79 70 157 91 77 168 97 83 182 105 91 190 109 102 225 129 117 243 138 123 276 159 137 304 176 152 333 192 167 393 226 196 398 229 201 421 243 212 442 255 221 452 262 230 486 281 239 541 312 270 557 320 276 592 350 304 662 386 334 757 435 379 782 449 390 852 483 426 897 517 448 927 535 455 937 553 478 1041 587 490 1088 602 519 1117 634 544 1183 672 578 1258 719 615 1325 748 643 1334 757 653 1410 804 691 1486 852 738 1571 899 781 1665 946 823 1779 1031 871 1874 1041 913 1987 1135 965 1192 1017 1277 1098 1372 1183 1523 1315 1874 1609

500 V A 0.71 1 1.2 1.7 2.2 3.3 4.3 5.1 5.6 8 10.7 12.7 13.7 17 21 26 31 36 42 46 51 55 60 69 74 80 83 98 105 121 134 146 172 175 187 194 196 206 238 246 266 293 341 345 378 394 397 421 447 473 499 511 543 563 575 610 643 681 719 785 804 861 908 965 1041 1154 1419

660 V A 0.47 0.72 0.88 1.3 1.7 2.4 3.3 3.8 4.2 5.9 7.9 9.7 10.5 13 16 20 23 28 32 36 39 41 45 53 56 62 65 77 80 92 100 110 126 131 136 142 149 159 178 187 203 222 259 265 289 303 306 319 336 350 374 388 420 431 435 459 487 516 544 596 610 653 686 729 785 875 1079

table B4: power and current values for typical induction motors. Reminder: some columns refer to 220 and 380 V motors. The international (IEC 38) standard of 230/400 V has been in force since 1983. The conversion factor for current values

for 230 V and 400 V motors is 0.95, as noted on the previous page.


3. motor, heating and lighting loads (continued)

B 3.2. direct-current motors D.C. motors are mainly used for specific applications which require very high torques and/or variable speed control (for example machine tools and crushers, etc.). Power to these motors is provided via speedcontrol converters, fed from 230/400 V 3-phase a.c. sources; for example, Rectivar 4 (Telemecanique). The operating principle of the converter does not allow heavy overloading. The speed controller, the supply line and the protection are therefore based on the duty cycle of the motor (e.g. frequent starting-current peaks) rather than on the steady-state full-load current. For powers i 40 kW, this solution is progressively replaced with a speedchanging variable-frequency device and an asynchronous motor. It is still used for gradual starters and/or retarders. Im

M V power-supply network

In

fig. B5: diagram of a low-power speed controller. motor maximum power 220 V 380 V 415 V kW kW kW 1.5 3 3.3 4 5.5 6 5.5 7.5 8 11 18.5 20 18.5 30 33 22 37 40 55 60 -

440 V (60 Hz) kW 3.5 6.5 8.5 21.5 35 42 63

motor In A 7 7 12 12 16 16 37 37 60 60 72 72 105 105

GRADIVAR Ith A 10 10 20 20 30 30 60 60 100 100 130 130 200 200

catalogue number weight kg VR2-SA2121 VR2-SA2123 VR2-SA2171 VR2-SA2173 VR2-SA2211 VR2-SA2213 VR2-SA2281 VR2-SA2283 VR2-SA2361 VR2-SA2363 VR2-SA2401 VR2-SA2403 VR2-SA2441 VR2-SA2443

1.95 1.95 3.10 3.10 4.90 4.90 5.30 5.30 5.30 5.30 5.40 5.40 10.00 10.00

table B6: progressive starters with voltage ramp. motor maximum power 220 V 380 V 415 V kW kW kW 4 5.5 6 5.5 7.5 8 11 18.5 20 18.5 30 33 22 37 40 55 60 75 80 132 140 -

440 V (60 Hz) kW 6.5 8.5 21.5 35 42 63 90 147

motor In A 12 12 16 16 37 37 60 60 72 72 105 105 140 140 245 245

GRADIVAR Ith A 20 20 30 30 60 60 100 100 130 130 200 200 350 350 530 530

table B7: progressive starters with current limitation. B10 - general - installed power

catalogue number weight kg VR2-SA3171 VR2-SA3173 VR2-SA3211 VR2-SA3213 VR2-SA3281 VR2-SA3283 VR2-SA3361 VR2-SA3363 VR2-SA3401 VR2-SA3403 VR2-SA3441 VR2-SA3443 VR2-SA3481 VR2-SA3483 VR2-SA3521 VR2-SA3523

3.30 3.30 5.10 5.10 5.50 5.50 5.50 5.50 5.60 5.60 11.00 11.00 45.00 45.00 45.00 45.00

B 3.3. resistive-type heating appliances and incandescent lamps (conventional or halogen) the power consumed by a heating appliance or an incandescent lamp is equal to the nominal power Pn quoted by the manufacturer (i.e. cos ø = 1). the currents are given by: c 3-phase case: Ia = Pn* ex U c 1-phase case: Ia = Pn* U where U is the voltage between the terminals of the equipment.

The power consumed by a heating appliance or an incandescent lamp is equal to the nominal power Pn quoted by the manufacturer (i.e. cos ø = 1). The currents are given by: c 3-phase case: Ia = Pn* ex U c 1-phase case: Ia = Pn* U where U is the voltage between the terminals of the equipment. nominal power kW 0.1 0.2 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 6 7 8 9 10

For an incandescent lamp, the use of halogen gas allows a more concentrated light source. The light output is superior and the life of the lamp is doubled. Note: at the instant of switching on, the cold filament gives rise to a very brief but intense peak of current. * Ia in amps; U in volts. Pn is in watts. If Pn is in kW, then multiply the equation by 1,000.

current demand 1-phase 1-phase 127 V 230 V 0.79 0.43 1.58 0.87 3.94 2.17 7.9 4.35 11.8 6.52 15.8 8.70 19.7 10.9 23.6 13 27.6 15.2 31.5 17.4 35.4 19.6 39.4 21.7 47.2 26.1 55.1 30.4 63 34.8 71 39.1 79 43.5

3-phase 230 V 0.25 0.50 1.26 2.51 3.77 5.02 6.28 7.53 8.72 10 11.3 12.6 15.1 17.6 20.1 22.6 25.1

3-phase 400 V 0.14 0.29 0.72 1.44 2.17 2.89 3.61 4.33 5.05 5.77 6.5 7.22 8.66 10.1 11.5 13 14.4

table B8: current demands of resistive heating and incandescent lighting (conventional or halogen) appliances.

3.4. fluorescent lamps and related equipment the power in watts indicated on the tube of a fluorescent lamp does not include the power dissipated in the ballast. the current is given by: Pballast + Pn Ia = U x cos ø If no power-loss value is indicated for the ballast, a figure of 25% of Pn may be used.

standard tubular fluorescent lamps The power Pn (watts) indicated on the tube of a fluorescent lamp does not include the power dissipated in the ballast. The current taken by the complete circuit is given by: Ia = Pballast + Pn U x cos ø where U = the voltage applied to the lamp, complete with its related equipment. with (unless otherwise indicated): c cos ø = 0.6 with no power factor (PF) correction* capacitor, c cos ø = 0.86 with PF correction* (single or twin tubes), c cos ø = 0.96 for electronic ballast. If no power-loss value is indicated for the ballast, a figure of 25% of Pn may be used. Table B8 gives these values for different arrangements of ballast. * "Power-factor correction" is often referred to as "compensation" in discharge-lighting-tube terminology.


3. motor, heating and lighting loads (continued)

B 3.4. fluorescent lamps and related equipment (continued) arrangement of lamps, starters and ballasts single tube with starter

tube power (W) (1) 18 36 58 single tube without 20 starter (2) with 40 external starting strip 65 twin tubes with starter 2 x 18 2 x 36 2 x 58 twin tubes without starter 2 x 40 single tube with 32 high frequency ballast 50 cos ø = 0.96 twin tubes with high2 x 32 frequency ballast 2 x 50 cos ø = 0.96

power consumed (W) 27 45 69 33 54 81 55 90 138 108 36 56

current (A) at 220V/240 V PF not PF electronic corrected corrected ballast 0.37 0.19 0.43 0.24 0.67 0.37 0.41 0.21 0.45 0.26 0.80 0.41 0.27 0.46 0.72 0.49 0.16 0.25

72 112

0.33 0.50

tube length (cm) 60 120 150 60 120 150 60 120 150 120 120 150 120 150

(1) Power in watts marked on tube. (2) Used exclusively during maintenance operations.

table B10: current demands and power consumption of commonly-dimensioned fluorescent lighting tubes (at 220 V/240 V - 50 Hz).

compact fluorescent tubes Compact fluorescent tubes have the same characteristics of economy and long life as classical tubes. They are commonly used in public places which are permanently illuminated (for example: corridors, hallways, bars, etc.) and can be mounted in situations otherwise illuminated by incandescent lamps. type of lamp

lamp power

globe lamps with integral ballast cos ø = 0.5 (1)

9 13 18 25 9 11 15 20 5 7 9 11 10 13 18 26

electronic lamps cos ø = 0.95 (1)

lamps with starter only incorporated (no ballast)

type single "U" form cos ø ≈ 0.35 type double "U" form cos ø ≈ 0.45

power consumed (W) 9 13 18 25 9 11 15 20 10 11 13 15 15 18 23 31

current at 220/240 V (A) 0.090 0.115 0.160 0.205 0.070 0.090 0.135 0.155 0.185 0.175 0.170 0.155 0.190 0.165 0.220 0.315

(1) Cos ø is approximately 0.95 (the zero values of V and I are almost in phase) but the power factor is 0.5 due to the impulsive form of the current, the peak of which occurs "late" in each half cycle.

table B11: current demands and power consumption of compact fluorescent lamps (at 220 V/240 V - 50 Hz).


B 3.5. discharge lamps the power in watts indicated on the tube of a discharge lamp does not include the power dissipated in the ballast.

These lamps depend on the luminous electrical discharge through a gas or vapour of a metallic compound, which is contained in a hermetically-sealed transparent envelope at a pre-determined pressure. These lamps have a long start-up time, during which the current Ia is greater than the nominal current In. Power and current demands are given for different types of lamp in table B12 (typical average values which may differ slightly from one manufacturer to another). The power in watts indicated on the tube of a discharge lamp does not include the power dissipated in the ballast.

table B12 gives the current taken by a complete unit, including all associated ancillary equipment.

type of power current In(A) starting lamp demand PF not PF Ia/In period (W) at corrected corrected (W) 230V 400V 230V 400V 230V 400V (mins) high-pressure sodium vapour lamps 50 60 0.76 0.3 1.4 4 to 6 to 1.6 70 80 1 0.45 100 115 1.2 0.65 150 168 1.8 0.85 250 274 3 1.4 400 431 4.4 2.2 1000 1055 10.45 4.9 low-pressure sodium vapour lamps standard lamp 18 26.5 0.14 1.1 7 to 15 to 1.3 35 43.5 0.62 0.24 55 72 0.34 90 112 0.84 0.50 135 159 0.73 180 216 0.98 economy lamps 26 34.5 0.45 0.17 1.1 7 to 15 to 1.3 36 46.5 0.22 66 80.5 0.39 91 105.5 0.49 131 154 0.69 mercury vapour + metal halide (also called metaliodide) 70 80.5 1 0.40 1.7 3 to 5 150 172 1.80 0.88 250 276 2.10 1.35 400 425 3.40 2.15 1000 1046 8.25 5.30 2000 2092 2052 16.50 8.60 10.50 6 mercury vapour + fluorescent substance (fluorescent bulb) 50 57 0.6 0.30 1.7 3 to 6 80 90 0.8 0.45 to 2 125 141 1.15 0.70 250 268 2.15 1.35 400 421 3.25 2.15 700 731 5.4 3.85 1000 1046 8.25 5.30 2000 2140 2080 15 11 6.1

luminous efficiency lumens (per watt)

average utilization life of lamp (h)

80 to 120

9000

- lighting of large halls - outdoor spaces - public lighting

100 to 200 8000 to - lighting of 12000 autoroutes - security lighting, station platform, stockage areas

100 to 200 8000 to - new types 12000 more efficient same utilization 70 to 90

6000 6000 6000 6000 6000 2000

- lighting of very large areas by projectors (for example:sports stadiums, etc)

40 to 60

8000 to - workshops 12000 with very high ceilings (halls, hangars) - outdoor lighting - low light output (1)

(1) replaced by sodium vapour lamps. Note: these lamps are sensitive to voltage dips. They extinguish if the voltage falls to less than 50% of their nominal voltage, and will not re-ignite before cooling for approximately 4 minutes. Note: Sodium vapour low-pressure lamps have a light-output efficiency which is superior to that of all other sources. However, use of these lamps is restricted by the fact that the yellow-orange colour emitted makes colour recognition practically impossible.

table B12: current demands of discharge lamps.


4. power* loading of an installation

B In order to design an installation, the actual maximum load demand likely to be imposed on the power-supply system must be assessed. To base the design simply on the arithmetic sum of all the loads existing in the installation would be extravagantly uneconomical, and bad engineering practice. The aim of this chapter is to show how all existing and projected loads can be assigned various factors to account for diversity (nonsimultaneous operation of all appliances of a given group) and utilization (e.g. an electric motor is not generally operated at its full-load capability, etc.). The values given are based on experience and on records taken from actual installations. In addition to providing basic installation-design data on individual circuits, the results will provide a global value

4.1 installed power (kW) the installed power is the sum of the nominal powers of all powerconsuming devices in the installation. This is not the power to be actually supplied in practice.


Most electrical appliances and equipments are marked to indicate their nominal power rating (Pn). The installed power is the sum of the nominal powers of all power-consuming devices in the installation. This is not the power to be actually supplied in practice. This is the case for electric motors, where the power rating refers to the output power at its driving shaft. The input power consumption will evidently be greater (See 3.1). Fluorescent and discharge lamps associated with stabilizing ballasts, are other cases in which the nominal power indicated on the lamp is less than the power consumed by the lamp and its ballast (See 3.4). Methods of assessing the actual power consumption of motors and lighting appliances are given in Section 3 of this Chapter. The power demand (kW) is necessary to choose the rated power of a generating set or battery, and where the requirements of a prime mover have to be considered. For a power supply from a LV public-supply network, or through a HV/LV transformer, the significant quantity is the apparent power in kVA.

for the installation, from which the requirements of a supply system (distribution network, HV/LV transformer, or generating set) can be specified.

*power: the word "power" in the title has been used in a general sense, covering active power (kW) apparent power (kVA) and reactive power (kvar). Where the word power is used without further qualification in the rest of the text, it means active power (kW). The magnitude of the load is adequately specified by two quantities, viz: c power, c apparent power. power The ratio = power factor apparent power

B 4.2 installed apparent power (kVA) the installed apparent power is commonly assumed to be the arithmetical sum of the kVA of individual loads. The maximum estimated kVA to be supplied however is not equal to the total installed kVA.

The installed apparent power is commonly assumed to be the arithmetical sum of the kVA of individual loads. The maximum estimated kVA to be supplied however is not equal to the total installed kVA. The apparent-power demand of a load (which might be a single appliance) is obtained from its nominal power rating (corrected if necessary, as noted above for motors, etc.) and the application of the following coefficients: output kW η = the per-unit efficiency = input kW kW cos ø = the power factor = kVA The apparent-power kVA demand of the load Pn Pa = η x cos ø

From this value, the full-load current Ia (amps)* taken by the load will be: Pa 103 for single phase-to-neutral c Ia = V connected load Pa 103 c Ia = for three-phase balanced load ex U where: V = phase-to-neutral voltage (volts) U = phase-to-phase voltage (volts) It may be noted that, strictly speaking, the total kVA of apparent power is not the arithmetical sum of the calculated kVA ratings of individual loads (unless all loads are at the same power factor). It is common practice however, to make a simple arithmetical summation, the result of which will give a kVA value that exceeds the true value by an acceptable "design margin". * For greater precision, account must be taken of the factor of maximum utilization as explained below in 4-3.

When some or all of the load characteristics are not known, the values shown in table B13 may be used to give a very approximate estimate of VA demands (individual loads are

generally too small to be expressed in kVA or kW). The estimates for lighting loads are based on floor areas of 500 sq-metres.

fluorescent lighting (corrected to cos ø = 0.86) type of application estimated (VA/m2) fluorescent tube with industrial reflector (1) roads and highways 7 stockage areas, intermittent work heavy-duty works: fabrication and 14 assembly of very large work pieces day-to-day work: 24 office work fine work: 41 drawing offices high-precision assembly workshops power circuits type of application estimated (VA/m2) pumping station compressed air 3 to 6 ventilation of premises 23 electrical convection heaters: private houses 115 to 146 flats and apartments 90 offices 25 dispatching workshop 50 assembly workshop 70 machine shop 300 painting workshop 350 heat-treatment plant 700

average lighting level (lux = Im/m2) 150 300 500 800

(1) example: 65 W tube (ballast not included), flux 5,100 lumens (lm), luminous efficiency of the tube = 78.5 lm/W.

table B13: estimation of installed apparent power.


4. power* loading of an installation (continued)

B 4.3 estimation of actual maximum kVA demand all individual loads are not necessarily operating at full rated nominal power nor necessarily at the same time. Factors ku and ks allow the determination of the maximum power and apparent-power demands actually required to dimension the installation.

All individual loads are not necessarily operating at full rated nominal power nor necessarily at the same time. Factors ku and ks allow the determination of the maximum power and apparent-power demands actually required to dimension the installation.

factor of maximum utilization (ku) In normal operating conditions the power consumption of a load is sometimes less than that indicated as its nominal power rating, a fairly common occurrence that justifies the application of an utilization factor (ku) in the estimation of realistic values. This factor must be applied to each individual load, with particular attention to electric motors, which are very rarely operated at full load.

In an industrial installation this factor may be estimated on an average at 0.75 for motors. For incandescent-lighting loads, the factor always equals 1. For socket-outlet circuits, the factors depend entirely on the type of appliances being supplied from the sockets concerned.

factor of simultaneity (ks) It is a matter of common experience that the simultaneous operation of all installed loads of a given installation never occurs in practice, i.e. there is always some degree of diversity and this fact is taken into account for estimating purposes by the use of a simultaneity factor (ks). The factor ks is applied to each group of loads (e.g. being supplied from a distribution or sub-distribution board). The determination of these factors is the responsibility of the designer, since it requires a detailed knowledge of the installation and the conditions in which the individual circuits are to be exploited. For this reason, it is not possible to give precise values for general application. Factor of simultaneity for an apartment block Some typical values for this case are given in table B14, and are applicable to domestic consumers supplied at 230/400 V (3-phase 4-wires). In the case of consumers using electrical heat-storage units for space heating, a factor of 0.8 is recommended, regardless of the number of consumers. Example: 5 storeys apartment building with 25 consumers, each having 6 kVA of installed load. The total installed load for the building = 36 + 24 + 30 + 36 + 24 = 150 kVA The apparent-power supply required for the building = 150 x 0.46 = 69 kVA From table B 14, it is possible to determine the magnitude of currents in different sections of the common main feeder supplying all floors. For vertical rising mains fed at ground level, the cross-sectional area of the conductors can evidently be progressively reduced from the lower floors towards the upper floors. These changes of conductor size are conventionally spaced by at least 3-floor intervals. In the example, the current entering the rising main at ground level is 150 x 0.46 x 103 = 100 A 400 x e The current entering the third floor is: (36+24) x 0.63 x 103 = 55 A 400 x e


number of downstream consumers 2 to 4 5 to 9 10 to 14 15 to 19 20 to 24 25 to 29 30 to 34 35 to 39 40 to 49 50 and more

factor of simultaneity (ks) 1 0.78 0.63 0.53 0.49 0.46 0.44 0.42 0.41 0.40

table B14: simultaneity factors in an apartment block.

4th floor

6 consumers 36 kVA

3rd floor

4 consumers 24 kVA

2nd floor

5 consumers 30 kVA

1st floor

6 consumers 36 kVA

ground floor

4 consumers 24 kVA

0.78

0.63

0.53

0.49

0.46

fig. B15: application of the factor of simultaneity (ks) to an apartment block of 5 storeys.

B Factor of simultaneity for distribution boards Table B16 shows hypothetical values of ks for a distribution board supplying a number of circuits for which there is no indication of the manner in which the total load divides between them. If the circuits are mainly for lighting loads, it is prudent to adopt ks values close to unity. number of circuits assemblies entirely tested 2 and 3 4 and 5 6 to 9 10 and more assemblies partially tested in every case choose

factor of simultaneity (ks) 0.9 0.8 0.7 0.6 1.0

Factor of simultaneity according to circuit function ks factors which may be used for circuits supplying commonly-occurring loads, are shown in table B17. circuit function lighting heating and air conditioning socket-outlets lifts and catering hoists (2) - for the most powerful motor - for the second most powerful motor - for all other motors

factor of simultaneity (ks) 1 1 0.1 to 0.2 (1)

1 0.75 0.60

(1) In certain cases, notably in industrial installations, this factor can be higher. (2) The current to take into consideration is equal to the nominal current of the motor, increased by a third of its starting current.

table B16: factor of simultaneity for distribution boards (IEC 439).

table B17: factor of simultaneity according to circuit function.

4.4 example of application of factors ku and ks an example in the estimation of actual maximum kVA demands at all levels of an installation, from each load position to the point of supply.

In this example, the total installed apparent power is 126.6 kVA, which corresponds to an actual (estimated) maximum value at the LV terminals of the HV/LV transformer of 65 kVA only. Note: in order to select cable sizes for the distribution circuits of an installation, the current I (in amps) through a circuit is determined from the equation

kVA x 103 Ue where kVA is the actual maximum 3-phase apparent-power value shown on the diagram for the circuit concerned, and U is the phaseto-phase voltage (in volts). I=

level 1 utilization

workshop A

lathe

pedestaldrill

apparentpower (Pa) kVA

utilization factor max.

apparentsimultaneity power demand factor max. kVA

n°1

5

0.8

4

n°2

5

0.8

4

n°3

5

0,8

4

workshop C

apparentsimultaneity power demand factor kVA

0.75

power circuit

14.4

5

0.8

4

n°1

2

0.8

1.6

2

0.8

1.6

18

1

18

0.2

3

1

3

1

3

0.8

12

1

12 socketoutlets 4,3 lighting circuit

compressor 15 3 socket- 10/16 A 10.6 outlets 1 10 fluorescent lamps

1

10.6

0.4

1

1

1

socketoutlets

apparentpower demand kVA

1

1

2.5

2,5

1

2.5

n°1

15

1

15

n°2

15

1

15

1

18

0.28

5

1

2

1

2

distribution box

1

workshop A distribution board

0,9

18.9

circuit

power circuit

2,5

5 socket10/16 A 18 outlets 20 fluorescent 2 lamps

apparentsimultaneity power demand factor kVA

3,6 lighting

ventilation n°1 fan n°2 oven

level 3

distribution box

n°4

n°2 5 socket10/16 A outlets 30 fluorescent lamps workshop B

level 2

35

power circuit

workshop B distribution board

main general distribution board MGDB

LV/HV

15.6

0.9

65

0.9

workshop C distribution board

0.9

37.8

socketoutlets lighting circuit

table B18: an example in estimating the maximum predicted loading of an installation (the factor values used are for demonstration purposes only). general - installed power - B17

4. power loading of an installation (continued)

B 4.5 diversity factor The term DIVERSITY FACTOR, as defined in IEC standards, is identical to the factor of simultaneity (ks) used in this guide, as described in 4.3. In some English-speaking countries however (at the time of writing) DIVERSITY FACTOR is the inverse of ks i.e. it is always u 1.

4.6 choice of transformer rating When an installation is to be supplied directly from a HV/LV transformer and the maximum apparent-power loading of the installation has been determined, a suitable rating for the transformer can be decided, taking due account of the following considerations: voltage (at no load) rated power (kVA) 50 100 160 250 315 400 500 630 800 1000 1250 1600 2000 2500

c the possibility of improving the power factor of the installation (see chapter E), c anticipated extensions to the installation, c installation constraints (temperature...) standard transformer ratings.

In (A) 400 V

420 V

433 V

480 V

72 144 231 361 455 577 722 909 1155 1443 1804 2309 2887 3608

69 137 220 344 433 550 687 866 1100 1375 1718 2199 2749 3437

67 133 213 333 420 533 667 840 1067 1333 1667 2133 2667 3333

60 120 192 301 379 481 601 758 962 1203 1504 1925 2406 3007

table B19: IEC-standardized kVA ratings of HV/LV 3-phase distribution transformers and corresponding nominal full-load current values.

The nominal full-load current In on the LV side of a 3-phase transformer is given by: Pa 103 In = where Ue Pa = kVA rating of the transformer U = phase-to-phase voltage at no-load* (in volts) In is in amperes. For a single-phase transformer: 3 In = Pa 10 where V V = voltage between LV terminals at no-load* (in volts).


Simplified equation for 400 V (3-phase load) In = kVA x 1.4 The IEC standard for power transformers is IEC 76. * as given on the transformer-rating nameplate. For table B19 the no-load voltage used is 420 V for the nominal 400 V winding.

B 4.7 choice of power-supply sources The study developed in F2 on the importance of maintaining a continuous supply raises the question of the use of standby-power plant. The choice and characteristics of these alternative sources are described in F3-3. For the main source of supply the choice is generally between a connection to the HV or the LV network of the public power-supply authority. In practice, connection to a HV source may be necessary where the load exceeds (or is planned eventually to exceed) a certain level - generally of the order of 250 kVA, or if the quality of service required is greater than that normally available from a LV network. Moreover, if the installation is likely to cause disturbance to neighbouring consumers, when connected to a LV network, the supply authorities may propose a HV service. Supplies at HV can have certain advantages: in fact, a HV consumer: c is not disturbed by other consumers, which could be the case at LV, c is free to choose any type of LV earthing system, c has a wider choice of economic tariffs, c can accept very large increases in load.

It should be noted, however, that: c the consumer is the proprietor of the HV/LV substation and, in some countries, he must build and equip it at his own expense. The power authority can, in certain circumstances, participate in the investment, at the level of the HV line for example, c a part of the connection costs can, for instance, often be recovered if a second consumer is connected to the HV line within a certain time following the original consumer's own connection, c the consumer has access only to the LV part of the installation, access to the HV part being reserved to the supply-authority personnel (meter reading, operational manœuvres, etc.). However, in certain countries, the HV protective circuit breaker (or fused load-break switch) can be operated by the consumer, c the type and location of the substation are agreed between the consumer and the supply authority.


1. supply of power at high voltage

C At present there is no international agreement on precise limits to define “High” voltage. Voltage levels which are designated as “high” in some countries are referred to as “medium” in others. In this chapter, distribution networks which operate at voltages of 1,000 V or less are referred to as Low-Voltage systems, while systems of power distribution which require

one stage of stepdown voltage transformation, in order to feed into lowvoltage networks, will be referred to as HighVoltage systems. For economic and technical reasons the upper nominal voltage limit of high-voltage distribution systems, as defined above, seldom exceeds 36.5 kV.

1.1 power-supply characteristics of high voltage distribution networks the main features which characterize a power-supply system include: c the nominal voltage and related insulation levels, c the short-circuit current, c the rated normal current of items of plant and equipment, c the method of earthing. Note: All voltages and currents are r.m.s. values, unless otherwise stated. in this document, the word “nominal” voltage is used for the network and the word “rated” voltage is used for the equipment.

nominal voltage and related insulation levels The nominal voltage of a system or of an equipment is defined in IEC 38 as “the voltage by which a system or equipment is designated and to which certain operating characteristics are referred”. Closely related to the nominal voltage is the “highest voltage for equipment” which concerns the level of insulation at normal working frequency, and to which other characteristics may be referred in relevant equipment recommendations. The “highest voltage for equipment” is defined in IEC 38 as: “the maximum value of voltage for which the equipment may be used, that occurs under normal operating conditions at any time and at any point on the system. It excludes voltage transients, such as those due to system switching, and temporary voltage variations”. Notes: 1.- The highest voltage for equipment is indicated for nominal system voltages higher than 1,000 V only. It is understood that, particularly for certain nominal system voltages, normal operation of equipment cannot be ensured up to this highest voltage for equipment, having regard to voltage sensitive characteristics such as losses of capacitors, magnetizing current of transformers, etc. In such cases, the relevant recommendations must specify the limit to which the normal operation of this equipment can be ensured. 2.- It is understood that the equipment to be used in systems having nominal voltage not exceeding 1,000 V should be specified with reference to the nominal system voltage only, both for operation and for insulation. 3.- The definition for “highest voltage for equipment” given in IEC 38 is identical to that given in IEC 694 for “rated voltage”. IEC 694 concerns switchgear for nominal voltages exceeding 1,000 V.

HV/LV distribution substations - C1

1. supply of power at high voltage (continued)

C 1.1 power-supply characteristics of high voltage distribution networks (continued) The following Table C1, taken from IEC 38, lists the most-commonly used standard levels of high-voltage distribution, and relates the nominal voltages to corresponding standard values of “Highest Voltage for Equipment”. Two series of highest voltages for equipment are given below, one for 50 Hz and 60 Hz systems (Series I), the other for 60 Hz systems (Series II - North American practice). It is recommended that only one of these series should be used in any one country. It is also recommended that only one of the two series of nominal voltages given for Series I should be used in any one country. These systems are generally three-wire systems unless otherwise indicated. The values shown are voltages between phases. The values indicated in parentheses should be considered as non-preferred values. It is recommended that these values should not be used for new systems to be constructed in future. series I highest voltage for equipement (kV) 3.6 (1) 7.2 (1) 12 (17.5) 24 36 (3) 40.5 (3)

nominal system voltage (kV) 3.3 (1) 3 (1) 6.6 (1) 6 (1) 11 10 (15) 22 20 33 (3) 35 (3)

Notes: 1 - It is recommended that in any one country the ratio between two adjacent nominal voltages should be not less than two. 2 - In a normal system of Series I, the highest voltage and the lowest voltage do not differ by more than approximately ± 10% from the nominal voltage of the system. In a normal system of Series II, the highest voltage does not differ by more than + 5% and the lowest voltage by more than - 10% from the nominal voltage of the system.

series II highest voltage for equipment (kV) 4.40 (1) 13.2 (2) 13.97 (2) 14.52 (1) 26.4 (2) 36.5 (2) -

nominal system voltage (kV) 4.16 (1) 12.47 (2) 13.2 (2) 13.8 (1) 24.94 (2) 34.5 (2) -

1) These values should not be used for public distribution systems. 2) These systems are generally four-wire systems. 3) The unification of these values is under consideration.

table C1: relating nominal system voltages with corresponding rated system voltages (r.m.s. values). In order to ensure adequate protection of equipment against abnormally-high shortterm power-frequency overvoltages, and transient overvoltages caused by lightning, switching, and system fault conditions, etc. all HV equipment must be specified to have appropriate Rated Insulation Levels. Switchgear Table C2 shown below, is extracted from IEC 694 and lists standard values of “withstand” voltage requirements. The choice between List 1 and List 2 values of table C2 depends on the degree of exposure to lightning and switching overvoltages*, the type of neutral earthing, and the type of overvoltage protection devices, etc. (for further guidance reference should be made to IEC 71). * This means basically that List 1 generally applies to switchgear to be used on underground-cable systems while List 2 is chosen for switchgear to be used on overhead-line systems.

C2 - HV/LV distribution substations

C Based on current practice in most European and several other countries rated voltage U (r.m.s. value)

(kV) 3.6 7.2 12 17.5 24 36 52 72.5

rated lightning impulse withstand voltage (peak value) list 1 list 2 to earth, across the to earth, between poles isolating between poles and across distance and across open open switching switching device device (kV) (kV) (kV) 20 23 40 40 46 60 60 70 75 75 85 95 95 110 125 145 165 170 250 325

rated I min powerfrequency withstand voltage (r.m.s. value) across the to earth, across the isolating between poles isolating distance and across distance open switching device (kV) (kV) (kV) 46 10 12 70 20 23 85 28 32 110 38 45 145 50 60 195 70 80 290 95 110 375 140 160

Note: The withstand voltage values “across the isolating distance” are valid only for the switching devices where the clearance between open contacts is designed to meet safety requirements specified for disconnectors (isolators). table C2: switchgear rated insulation levels. It should be noted that, at the voltage levels in question, no switching overvoltage ratings are mentioned. This is because overvoltages due to switching transients are less severe at these voltage levels than those due to lightning. Transformers The two tables C3A and C3B shown below have been extracted from IEC 76-3, and refer to the current practices in countries other than those of North America (Series I) and to those of North America and some other countries (Series II). The significance of list 1 and list 2 in Series I is the same as that for the switchgear table, i.e. the choice depends on the degree of exposure to lightning, etc. highest voltage for equipment Um (r.m.s.) (kV) i 1.1 3.6 7.2 12 17.5 24 36 52 72.5

rated short duration power frequency withstand voltage (r.m.s.) (kV) 3 10 20 28 38 50 70 95 140

rated lightning impulse withstand voltage (peak) list 1 list 2 (kV) (kV) 20 40 40 60 60 75 75 95 95 125 145 170 250 325

table C3A: transformers rated insulation levels in series I (based on current practice other than in the United States of America and some other countries).



C 1.1 power-supply characteristics of high voltage distribution networks (continued) highest voltage for equipment Um (r.m.s.)

rated short duration power frequency withstand voltage (r.m.s.)

(kV) 4.40 13.20 13.97 14.52 26.4 36.5 72.5

(kV) 19

rated lightning impulse withstand voltage (peak) distribution other transformers transformers (kV) (kV) 60 75

34

95

}

110

50 70 140

150 200 350

table C3B: transformers rated insulation levels in series II (based on current practice in the United States of America and some other countries). Other components It is evident that the insulation performance of other HV components associated with these major items, e.g. porcelain or glass insulators, HV cables, instrument transformers, etc. must be compatible with that of the switchgear and transformers noted above. Test schedules for these items are given in appropriate IEC publications.

the national standards of any particular country are normally rationalized to include one or two levels only of voltage, current, and fault-levels, etc.

General note The IEC standards are intended for worldwide application and consequently embrace an extensive range of voltage and current levels. These reflect the diverse practices adopted in countries of different meteorologic, geographic and economic constraints. The national standards of any particular country are normally rationalized to include one or two levels only of voltage, current, and fault-levels, etc.

a circuit breaker (or fuse switch, over a limited voltage range) is the only form of switchgear capable of safely breaking the very high levels of current associated with short-circuit faults occurring on a power system.

short-circuit current A circuit breaker (or fuse switch, over a limited voltage range) is the only form of switchgear capable of safely breaking the very high levels of current associated with short-circuit faults occurring on a power system. Standard values of circuit breaker shortcircuit current-breaking capability are normally given in kilo-amps. These values refer to a 3-phase short-circuit condition, and are expressed as the average of the r.m.s. values of the a.c. component of current in each of the three phases. Short-circuit current-breaking ratings For circuit breakers in the rated voltage ranges being considered in this chapter, IEC 56 gives standard short-circuit currentbreaking ratings as follows. kV kA (r.m.s.)

3.6 10 16 25 40

7.2 8 12.5 16 25 40

12 8 12.5 16 25 40 50

17.5 8 12.5 16 25 40

24 8 12.5 16 25 40

36 8 12.5 16 25 40

52 8 12.5 20

table C4: standard short-circuit current-breaking ratings extracted from table X IEC 56.


C Where the installation of a circuit breaker is electrically remote from a power source, it is only necessary to check that the power factor of the faulty circuit is not less than 0.07 and that the minimum operating time of protective relaying is not less than a half cycle of the power-supply frequency (i.e. 10 ms at 50 Hz). In the great majority of cases, these conditions will be satisfied in a conventional HV distribution network. In such circumstances, it is then only necessary to ensure that the IEC-rated shortcircuit current-breaking capability of the circuit breaker exceeds the r.m.s. value of 3-phase short-circuit current at the point of installation. Where circuit breakers are to be installed close to generating plant, the a.c. component of short-circuit current will diminish rapidly from its initial value (i.e. the a.c. decrement) and the power factor of the fault circuit may be less than 0.07. Such a case would need further investigation along the lines indicated in IEC 56, since the result could lead to an absence of current zeros for several initial cycles*. Maximum peak of current Another aspect of short-circuit current stresses that may be imposed on the component parts of a power system, concerns the maximum possible peak of current which can occur if a circuit breaker is closed on to a dead circuit which is shortcircuited. For such a possibility, circuit breakers have a short-circuit current-making rating, expressed in kA of peak current. The numerical value of this rating is 2.5 times the short-circuit current-breaking rating of the circuit breaker. Explanation The value 2.5 is derived as follows: shortcircuit current is normally highly inductive so that at least two of the phases will contain a transient d.c. component. In the worst possible case, the value of the d.c. component in one of the phases will be equal

to the peak value of the a.c. component, producing the so-called “doubling effect”. However, the d.c. transient diminishes rapidly from the instant of fault, while the peak current occurs a half cycle after that instant. Allowance is made for the diminution in the d.c. component by reducing the doubling factor (2) to a value of 1.8. In IEC 56 this reduction is based on an inductive d.c. time constant value which is representative of average HV distribution systems. The peak current value is therefore rIrms x 1.8 = 2.54 Irms which is rounded off for standardization purposes to 2.5 Irms. The form of the fully-offset short-circuit current is shown in figure C5, reproduced from IEC 56. Note: When a short-circuit (s.c.) occurs on a power system, all electric motors act for a very brief period (1-2 cycles) as generators, and feed current (typically 50 % - 80 % of the motorstarting current) into the fault. This is due to the collapsing magnetic flux in each motor and is generally significant only for the first power-frequency cycle from the moment of s.c. For the latter reason, apart from very exceptional cases, it is not necessary to take account of its effect on the s.c. currentbreaking rating of a circuit breaker (CB). It cannot be neglected however, in the case of s.c. current-making rating. If there are large concentrations of motors near the point of installation of a CB, the s.c. current-making level will be greater than 2.5 times the s.c. current-breaking level at the same location. In order to ensure an adequate s.c. currentmaking capacity, therefore, a CB having an oversized s.c. current-breaking capacity is necessary in such circumstances. * A "natural" current zero is essential for the correct functioning of a CB, unless especially designed for the purpose.

I A E D

A’

C IMC

B

IAC

IDC

D’

C’ X t

B’

E’

AA' = envelope of current-wave BB’ BX = normal zero line CC’ = displacement of current-wave zeroline at any instant DD’ = r.m.s. value of the a.c. component of current at any instant, measured from CC’ EE’ = instant of contact separation (initiation of the arc)

IMC = making current = (A-C) 1.8 where A and C are measured at t = 0 IAC = peak value of a.c. component of current at instant EE’ IAC = r.m.s. value of the a.c. component r of current at instant EE’ IDC = d.c. component of current at instant EE’ IDC x 100 = percentage value of the d.c. IAC component

fig. C5: determination of short-circuit making and breaking currents, and of percentage d.c. component. HV/LV distribution substations - C5


C 1.1 power-supply characteristics of high voltage distribution networks (continued) the short-circuit current level of a HV distribution system is frequently limited by design techniques to a predetermined maximum value typically in the range of 12.5 kA to 25 kA. All HV equipments connected to the system must be capable of withstanding, without damage, the thermal and mechanical stresses of the maximum short-circuit current for 1 second, or in particular cases (depending on equipment specifications) for 3 seconds.

In such a case, a CB having a s.c. currentbreaking rating sufficiently high to ensure an adequate s.c. current-making performance must be installed. The short-circuit current level of a HV distribution system is frequently limited by design techniques to a pre-determined maximum value typically in the range of 12.5 kA to 25 kA. All HV equipments connected to the system must be capable of withstanding, without damage, the thermal and mechanical stresses of the maximum short-circuit current for 1 second, or in particular cases (depending on equipment specifications) for 3 seconds.

the most common normal current rating for general-purpose HV distribution switchgear is 400 A.

Rated normal current The rated normal current is defined as “the r.m.s. value of the current which can be carried continuously at rated frequency with a temperature rise not exceeding that specified by the relevant product standard”. The rated normal current requirements for switchgear are decided at the substation design stage. The most common normal current rating for general-purpose HV distribution switchgear is 400 A. In industrial areas and high-load-density urban districts, circuits rated at 630 A are sometimes required, while at bulk-supply substations which feed into HV networks, 800 A; 1,250 A; 1,600 A; 2,500 A and 4,000 A circuit breakers are listed in IEC 56 as standard ratings for incoming-transformer circuits, bus-section and bus-coupler CBs, etc. At HV/LV substations which include one (or more) transformer(s) with a normal primary current of less than 45 A, a HV switch associated with a set of 3 fuses (or a combination switch-fuse) is generally used to control and protect the transformer, as a more economic alternative to a CB. There are no IEC-recommended normalcurrent rating tables for the combination in these cases. The actual rating will be given by the switch-fuse manufacturer, according to the fuse characteristics, and details of the transformer, such as: c normal current at HV, c permissible overcurrent and its duration, c max. peak and duration of the transformer energization inrush magnetizing current, c off-circuit tapping-switch position, etc. as shown in the example given in Appendix A of IEC 420, and summarized in Appendix C1 of this guide.


The IEC recommends that the normal-current rating value, assigned to the combination by the manufacturer, be one of the “R10” series of (ISO) preferred numbers, viz: 10, 12.5, 16, 20, 25, 31.5, 40, 50, 63, 80 with multiples (or sub-multiples) of 10 as required. In such a scheme, the load-break switch must be suitably rated to trip automatically, e.g. by relays, at low fault-current levels which must cover (by an appropriate margin) the rated minimum breaking current of the HV fuses. In this way, high values of fault current which are beyond the breaking capability of the load-break switch will be cleared by the fuses, while low fault-current values, that cannot be correctly broken by the fuses, will be cleared by the relay-operated load-break switch. Appendix C1 gives further information on this arrangement, as applied to HV switch-fuse combination units.

C Influence of the ambient temperature and altitude on the rated current Normal-current ratings are assigned to all current-carrying electrical appliances, and upper limits are decided by the acceptable temperature rise caused by the I2R (watts) dissipated in the conductors, (where I = r.m.s. current in amperes and R = the resistance of the conductor in ohms), together with the heat produced by magnetic-hysteresis and eddy-current losses in motors, transformers, etc. and dielectric losses in cables and capacitors, where appropriate. The temperature rise above the ambient temperature will depend mainly on the rate at which the heat is removed. For example, large currents can be passed through electric motor windings without causing them to overheat, simply because a cooling fan fixed to the shaft of the motor removes the heat at the same rate as it is produced, and so the temperature reaches a stable value below that which could damage the insulation and

earth faults on high-voltage systems can produce dangerous voltage levels on LV installations. LV consumers (and substation operating personnel) can be safeguarded against this danger by: c restricting the magnitude of HV earth-fault currents, c reducing the substation earthing resistance to the lowest possible value, c creating equipotential conditions at the substation and at the consumer's installation.

result in a burnt-out motor. Oil- and/or air-cooled transformers are among the most widely known examples of such “forced-cooling” techniques. The normal-current values recommended by IEC are based on ambient-air temperatures common to temperate climates at altitudes not exceeding 1,000 metres, so that items which depend on natural cooling by radiation and air-convection will overheat if operated at rated normal current in a tropical climate and/ or at altitudes exceeding 1,000 metres. In such cases, the equipment has to be derated, i.e. be assigned a lower value of normalcurrent rating according to IEC 76-2. In the case of force-cooled transformers it is generally sufficient to provide sun shields, and increase the oil-cooling radiator surfaces, the amount of cooling oil, the power of the circulating-oil pumps, and the size of the aircirculating fans, to maintain the original IEC rating.

earthing connections Earth faults on high-voltage systems can produce dangerous voltage levels on LV installations. LV consumers (and substation operating personnel) can be safeguarded against this danger by: c restricting the magnitude of HV earth-fault currents, c reducing the substation earthing resistance to the lowest possible value, c creating equipotential conditions at the substation and at the consumer's installation. Earthing and equipment-bonding earth connections require careful consideration, particularly regarding safety of the LV consumer during the occurrence of a shortcircuit to earth on the HV system. Earth electrodes In general, it is preferable, where physically possible, to separate the electrode provided for earthing exposed conductive parts of HV equipment from the electrode intended for earthing the LV neutral conductor. This is commonly practised in rural systems where the LV neutral-conductor earth electrode is installed at one or two spans of LV distribution line away from the substation. In most cases, the limited space available in urban substations precludes this practice, i.e. there is no possibility of separating a HV electrode sufficiently from a LV electrode to avoid the transference of (possibly dangerous) voltages to the LV system. Earth-fault current Earth-fault current levels at high voltage are generally (unless deliberately restricted) comparable to those of a 3-phase shortcircuit. Such currents passing through an earth electrode will raise its voltage to a high value with respect to “remote earth” (the earth surrounding the electrode will be raised to a high potential; “remote earth” is at zero potential). For example, 10,000 A of earth-fault current passing through an electrode with an (unusually low) resistance of 0.5 ohms will raise its voltage to 5,000 V. Providing that all exposed metal in the substation is “bonded” (connected together) and then connected to the earth electrode,

and the electrode is in the form of (or is connected to) a grid of conductors under the floor of the substation, then there is no danger to personnel, since this arrangement forms an equipotential “cage” in which all conductive material, including personnel, is raised to the same potential. Transferred potential A danger exists however from the problem known as Transferred Potential. It will be seen in figure C6 that the neutral point of the LV winding of the HV/LV transformer is also connected to the common substation earth electrode, so that the neutral conductor, the LV phase windings and all phase conductors are also raised to the electrode potential. Low-voltage distribution cables leaving the substation will transfer this potential to consumers installations. It may be noted that there will be no LV insulation failure between phases or from phase to neutral since they are all at the same potential. It is probable, however, that the insulation between phase and earth of a cable or some part of an installation would fail. Solutions A first step in minimizing the obvious dangers of transferred potentials is to reduce the magnitude of HV earth-fault currents. This is commonly achieved by earthing the HV system through resistors or reactors at the star points of selected transformers*, located at bulk-supply substations. A relatively high transferred potential cannot be entirely avoided by this means, however, and so the following strategy has been adopted in some countries. The equipotential earthing installation at a consumer's premises represents a remote earth, i.e. at zero potential. However, if this earthing installation were to be connected by a low-impedance conductor to the earthelectrode at the substation, then the equipotential conditions existing in the substation would also exist at the consumer's installation. * the others being unearthed. A particular case of earth-fault current limitation, namely, by means of a Petersen coil, is discussed at the end of Sub-clause 3.2.



C 1.1 power-supply characteristics of high voltage distribution networks (continued) HV

LV 1 2 3 N

fault If consumer If

V=IfRs

Rs

fig. C6: transferred potential. Low-impedance interconnection This low-impedance interconnection is achieved simply by connecting the neutral conductor to the consumer's equipotential installation, and the result is recognized as the TN system of earthing (IEC 364-3) as shown in diagram A of figure C7. The TN system is generally associated with a Protective Multiple Earthing (PME) scheme, in which the neutral conductor is earthed at intervals along its length (every 3rd or 4th pole on a LV overhead-line distributor) and at each consumer's service position. It can be seen that the network of neutral conductors radiating from a substation, each of which is earthed at regular intervals, constitutes, together with the substation earthing, a very effective low-resistance earth electrode. The combination of restricted earth-fault currents, equipotential installations and lowresistance substation earthing, results in greatly reduced levels of overvoltage and limited stressing of phase-to-earth insulation during the type of HV earth-fault situation described above. Limitation of the HV earth-fault current and earth resistance of the substation Another widely-used system of earthing is shown in diagram C of figure C7. It will be seen that in the TT system, the consumer's earthing installation (being isolated from that of the substation) constitutes a remote earth. This means that, although the transferred potential will not stress the phase-to-phase insulation of the consumer's equipment, the phase-to-earth insulation of all three phases will be subjected to overvoltage. The strategy in this case, is to: c restrict the value of HV earth-fault currents, as previously discussed, c reduce the resistance of the substation earth electrode, such that the standard value of 5-second withstand-voltage-to-earth for LV equipment and appliances will not be exceeded.


C HV

HV

LV

LV 1

2

2

3

3

N

N

RS

A LV

HV

B cases C and D

LV

1

1

2

2

3

3

N

N

RS

Where Uw = the rated normal-frequency withstand voltage for low-voltage equipment at consumer installations Uo = phase to neutral voltage at consumer's installations Im = maximum value of HV earth-fault current

D HV

LV

cases E and F

LV

1

1

2

2

3

3

N

N

IT(S)

TT(S) RN

Uw - Uo Im

RS

C

RS

Rs i

IT(N)

TT(N)

HV

No particular resistance value for Rs is imposed in these cases.

IT(R)

TN(R)

HV

cases A and B

1

E

RS

RN

Rs i

Uws - U Im

Where Uws = the normal-frequency withstand voltage for low-voltage equipments in the substation (since the exposed conductive parts of these equipments are earthed via Rs) U = phase to neutral voltage at the substation for the TT(s) system, but the phase-tophase voltage for the IT(s) system Im = maximum value of HV earth-fault current

F

In cases E and F the LV protective conductors (bonding exposed conductive parts) in the substation are earthed via the substation earth electrode, and it is therefore the substation LV equipment (only) that could be subjected to overvoltage. Notes: (R) signifies that the HV and LV exposed conductive parts at the substation and those at the consumer's installations, together with the LV neutral point of the transformer, are all earthed via the substation electrode system. (N) signifies that the HV and LV exposed conductive parts at the substation, together with the LV neutral point of the transformer are earthed via the substation electrode system. (S) signifies that the LV neutral point of the transformer is separately earthed outside of the area of influence of the substation earth electrode. Uw and Uws are commonly given the (IEC 644) value 1.5 Uo + 750 V, where Uo is the nominal phase-to-neutral voltage of the LV system concerned. fig. C7: maximum earthing resistance Rs at a HV/LV substation to ensure safety during a short-circuit to earth fault on the high-voltage equipment for different systems of earthing.



C 1.1 power-supply characteristics of high voltage distribution networks (continued) Practical values adopted by one national electrical power-supply authority, on its 20 kV distribution systems, are as follows: c maximum earth-fault current on overheadline distribution systems, or mixed (O/H line and U/G cable) systems, is 300 A, c maximum earth-fault current on underground systems is 1,000 A. The formula required to determine the maximum value of earthing resistance Rs at the substation, to ensure that the LV withstand voltage will not be exceeded, is: Uw - Uo Rs = in ohms Im (see cases C and D in figure C7). Where Uw = the lowest standard value (in volts) of short-term (5 s) withstand voltage for the consumer's installation and appliances = 1.5 Uo + 750 V (IEC 644 (1991)) Uo = phase to neutral voltage (in volts) at the consumer's LV service position Im = maximum earth-fault current on the HV system (in amps). A third form of system earthing referred to as the “IT” system in IEC 364 is commonly used where continuity of supply is essential, e.g. in hospitals, continuous-process manufacturing, etc. The principle depends on taking a supply from an unearthed source, usually a transformer, the secondary winding of which is unearthed, or earthed through a high impedance (u 1,000 ohms). In these cases, an insulation failure to earth in the low-voltage circuits supplied from the secondary windings will result in zero or negligible fault-current flow, which can be allowed to persist until it is convenient to shut-down the affected circuit to carry out repair work. Diagrams B, D and F of figure C7 show IT systems in which resistors (of approximately 1,000 ohms) are included in the neutralearthing lead. If however, these resistors were removed, so that the system is unearthed, the following notes apply. Diagram B. All phase wires and the neutral conductor are “floating” with respect to earth, to which they are “connected” via the (normally very high) insulation resistances and (very small) capacitances between the live conductors and earthed metal (conduits, etc.).


Assuming perfect insulation, all LV phase and neutral conductors will be raised by electrostatic induction to a potential approaching that of the equipotential conductors. In practice, it is more likely, because of the numerous earth-leakage paths of all live conductors in a number of installations acting in parallel, that the system will behave similarly to the case where a neutral earthing resistor is present, i.e. all conductors will be raised to the potential of the substation earth. In these cases, the overvoltage stresses on the LV insulation are small or non-existent. Diagrams D and F. In these cases, the high potential of the substation (S/S) earthing system acts on the isolated LV phase and neutral conductors: c through the capacitance between the LV windings of the transformer and the transformer tank, c through capacitance between the equipotential conductors in the S/S and the cores of LV distribution cables leaving the S/S, c through current leakage paths in the insulation, in each case. At positions outside the area of influence of the S/S earthing, system capacitances exist between the conductors and earth at zero potential (capacitances between cores are irrelevant - all cores being raised to the same potential). The result is essentially a capacitive voltage divider, where each “capacitor” is shunted by (leakage path) resistances. In general, LV cable and installation wiring capacitances to earth are much larger, and the insulation resistances to earth are much smaller than those of the corresponding parameters at the S/S, so that most of the voltage stresses appear at the substation between the transformer tank and the LV winding. The rise in potential at consumers’ installations is not likely therefore to be a problem where the HV earth-fault current level is restricted as previously mentioned. All IT-earthed transformers, whether the neutral point is isolated or earthed through a high impedance, are routinely provided with an overvoltage limiting device which will automatically connect the neutral point directly to earth if an overvoltage condition approaches the insulation-withstand level of the LV system. In addition to the possibilities mentioned above, several other ways in which these overvoltages can occur are described in Clause 3.1.

C This kind of earth-fault is very rare, and when it does occur is quickly detected and cleared by the automatic tripping of a circuit breaker in a properly designed and constructed installation. Safety in situations of elevated potentials depends entirely on the provision of properly arranged equipotential areas, the basis of which is generally in the form of a widemeshed grid of interconnected bare copper conductors connected to vertically-driven copper-clad* steel rods. The equipotential criterion to be respected is that which is mentioned in Chapter G dealing with protection against electric shock by indirect contact, namely: that the potential between any two exposed metal parts which can be touched simultaneously by any parts of the body must never, under any circumstances, exceed 50 V in dry conditions, or 25 V in wet conditions. Special care should be taken at the boundaries of equipotential areas to avoid steep potential gradients on the surface of the ground which give rise to dangerous “step potentials”. This question is closely related to the safe earthing of boundary fences and is further discussed in Sub-clause 3.1 and in Appendix C2. * Copper is cathodic to most other metals and therefore resists corrosion.

1.2 different HV service connections According to the type of high-voltage network, the following supply arrangements are commonly adopted.

single-line service The substation is supplied by a single circuit tee-off from a HV distributor (cable or line). In general, the HV service is connected into a panel containing a load-break/isolating switch with series protective fuses and earthing switches, as shown in figure C8. In some countries a pole-mounted transformer with no HV switchgear or fuses (at the pole) constitutes the “substation”. Up to transformer ratings of 160 kVA this type of HV service is very common in rural areas. Protection and switching devices are remote from the transformer, and generally control a main overhead-line, from which a number of these elementary service lines are tapped.

overhead line

fig. C8: single-line service.



C 1.2 different HV service connections (continued) ring-main principle Ring-main units (RMU) are normally connected to form a HV ring main* or interconnector-distributor*, such that the RMU busbars carry the full ring-main or interconnector current (figure C9). The RMU consists of three compartments, integrated to form a single assembly, viz: c 2 incoming compartments, each containing a load-break/isolating switch and a circuit earthing switch, c 1 outgoing and general protection compartment, containing a load-break switch and HV fuses, or a combined load-break/fuse switch, or a circuit breaker and isolating switch, together with a circuit-earthing switch in each case. All load-break switches and earthing switches are fully rated for short-circuit current-making duty. This arrangement provides the user with a two-source supply, thereby reducing considerably any interruption of service due to system faults or operational manœuvres by the supply authority, etc. The main application for RMUs is in publicsupply HV underground-cable networks in urban areas. * A ring main is a continuous distributor in the form of a closed loop, which originates and terminates on one set of busbars. Each end of the loop is controlled by its own circuit breaker. In order to improve operational flexibility the busbars are often divided into two sections by a normallyclosed bus-section circuit breaker, and each end of the ring is connected to a different section. An interconnector is a continuous untapped feeder connecting the busbars of two substations. Each end of the interconnector is usually controlled by a circuit beaker. An interconnector-distributor is an interconnector which supplies one or more distribution substations along its length.

underground cable ring main

fig. C9: ring-main service.

parallel feeders Where a HV supply connection to two lines or cables originating from the same busbar of a substation is possible, a similar HV switchboard to that of a RMU is commonly used (figure C10). The main operational difference between this arrangement and that of a RMU is that the two incoming panels are mutually interlocked, such that one incoming switch only can be closed at a time, i.e. its closure prevents the closure of the other. On the loss of power supply, the closed incoming switch must be opened and the (formerly open) switch can then be closed. The sequence may be carried out manually or automatically. This type of switchboard is used particularly in networks of high-load density and in rapidly-expanding urban areas supplied by HV underground cable systems.


paralleled underground-cable distributors

fig. C10: duplicated supply service.

C 1.3 some operational aspects of HV distribution networks overhead lines High winds, ice formation, etc., can cause the conductors of overhead lines to touch each other, thereby causing a momentary (i.e. not permanent) short-circuit fault. Insulation failure due to broken ceramic or glass insulators, caused by air-borne debris; careless use of shot-guns, etc., or again, heavily polluted insulator surfaces, can result in a short-circuit to earth. Many of these faults are self-clearing. For example, in dry conditions, broken insulators can very often remain in service undetected, but are likely to flashover to earth (e.g. to a metal supporting structure) during a rainstorm. Moreover, polluted surfaces generally cause a flashover to earth only in damp conditions. The passage of fault current almost invariably takes the form of an electric arc, the intense heat of which dries the current path, and to some extent, re-establishes its insulating properties. In the meantime, protective devices have usually operated to clear the fault, i.e. fuses have blown or a circuit breaker has tripped. Experience has shown that in the large majority of cases, restoration of supply by replacing fuses or by re-closing a circuit breaker will be successful. For this reason it has been possible to considerably improve the continuity of service on HV overhead-line distribution networks by the application of automatic circuit breaker reclosing schemes at the origin of the circuits concerned. These automatic schemes permit a number of reclosing operations if a first attempt fails, with adjustable time delays between successive attempts (to allow de-ionization of the air at the fault) before a final lock-out of the circuit breaker occurs, after all (generally three) attempts fail.

Other improvements in service continuity are achieved by the use of remotely-controlled section switches and by automatic isolating switches which operate in conjunction with an auto-reclosing circuit breaker. This last scheme is exemplified by the final sequence shown in figure C11, where the isolating switch is referred to as IACT* (voltage-drop-operated outdoor switch). The principle is as follows: If, after two reclosing attempts, the circuit breaker trips, the fault is assumed to be permanent, and, while the distributor is dead, the IACT opens to isolate a section of the network, before the third (and final) reclosure takes place. There are then two possibilities: 1) the fault is on the section which is isolated by IACT, and supply is restored to those consumers connected to the remaining section, or 2) the fault is on the section upstream of IACT and the circuit breaker will trip and lock out. The IACT scheme, therefore, provides the possibility of restoration of supplies to some consumers in the event of a permanent fault. While these measures have greatly improved the reliability of supplies from HV overhead line systems, the consumers must, where considered necessary, make their own arrangements to counter the effects of momentary interruptions to supply (between reclosures), for example: c uninterruptible standby emergency power, c lighting that requires no cooling down before re-striking. (See Chapter F section 2) * Interrupteur Aérien à ouverture dans le Creux de Tension (used by EDF, the French supply authority).

1-cycle RR + 1SR O1

If

RR

SR

O2

O3

In 15 to 30s

Io fault

permanent fault 0.3s

0.4s

2-cycle 2SR a-fault on main distributor If

O1

RR

SR2 O4

SR1 O3

O2

In 15 to 30s

15 to 30s

Io fault 0.3s

0.4s

permanent fault 0.45s

0.4s

b-fault on section supplied through IACT O1

If

RR

SR1 O3

O2

In

SR2 15 to 30s

Io

15 to 30s

fault

opening of IACT 0.3s

0.4s

0.4s

O = circuit breaker opening / RR = rapid reclosing / SR = slow reclosing / In = normal load current / If = fault current / I0 = zero current

fig. C11: automatic reclosing cycles of a circuit breaker controlling a radial HV distributor. HV/LV distribution substations - C13


C 1.3 some operational aspects of HV distribution networks (continued) underground cable networks Faults on underground cable networks are sometimes the result of careless workmanship by cable jointers or by cablelaying contractors, etc., but are more commonly due to damage from tools such as pick-axes, pneumatic drills and trench excavating machines, and so on, used by other utilities. Insulation failures sometimes occur in cableterminating boxes due to overvoltage, particularly at points in a HV system where an overhead line is connected to an underground cable. The overvoltage in such a case is generally of atmospheric origin, and electromagnetic-wave reflection effects at the joint box (where the natural impedance of the circuit changes abruptly) can result in overstressing of the cable-box insulation to the point of failure. Overvoltage protection devices, such as lightning arresters, are frequently installed at these locations. Faults occurring in cable networks are less frequent than those on overhead (O/H) line systems, but are almost invariably permanent faults, which require more time for localization and repair than those on O/H lines. Where a cable fault occurs on a ring main, supplies can be quickly restored to all consumers when the faulty section of cable has been determined. If, however, the fault occurs on a radial distributor, the delay in locating the fault and carrying out repair work can amount to several hours, and will affect all consumers downstream of the fault position. In any case, if supply continuity is essential on all, or part of, an installation, a standby source must be provided. Standby power equipment is described in Chapter F section 2.1.

centralized remote control, based on SCADA (Supervisory Control And Data Acquisition) systems and recent developments in IT (Information Technology) techniques, is becoming more and more common in countries in which the complexity of highlyinterconnected systems justifies the expenditure.


remote control of HV networks Remote control of HV circuit breakers and switchgear, and tapchangers, etc. from a central control room is possible, while similar control facilities are also available from the console of a mobile control centre.

2. consumers HV substations

C Large consumers of electricity are invariably supplied at HV. On LV systems operating at 120/208 V (3-phase 4-wires), a load of 50 kVA might be considered to be “large”, while on a 240/415 V 3-phase system a “large” consumer could have a load in excess of 100 kVA. Both systems of LV distribution are common in many parts of the world. As a matter of interest, the IEC recommends a “world” standard of 230/400 V for 3-phase 4-wire systems. This is a compromise level and will allow existing systems which operate at 220/380 V and at 240/415 V, or close to these values, to comply with the proposed standard simply by adjusting the off-circuit tapping switches of standard distribution transformers. The distance over which the load has to be transmitted is a further factor in considering an HV or LV service. Services to small but isolated rural consumers are obvious examples. The decision of a HV or LV supply will depend on local circumstances and considerations such as those mentioned above, and will generally be imposed by the power-supply authority for the district concerned. When a decision to supply power at HV has been made, there are two widely-followed methods of proceeding.

(1) The power-supplier constructs a standard substation close to the consumer’s premises, but the HV/LV transformer(s) is (are) located in transformer chamber(s) inside the premises, close to the load centre. (2) The consumer constructs and equips his own substation on his own premises, to which the power supplier makes the HV connection. In method (1) the power supplier owns the substation, the cable(s) to the transformer(s), the transformer(s) and the transformer chamber(s), to which he has unrestricted access. The transformer chamber(s) is (are) constructed by the consumer (to plans and regulations provided by the supplier) and include plinths, oil drains, fire walls and ceilings, ventilation, lighting, and earthing systems, all to be approved by the supply authority. The tariff structure will cover an agreed part of the expenditure required to provide the service. Whichever procedure is followed, the same principles apply in the conception and realization of the project. The following notes refer to procedure (2).

2.1 procedures for the establishment of a new substation the consumer must provide certain data to the power-supply organization at the earliest stage of the project.

preliminary information

the power-supply organization must give specific information to the prospective consumer.

project studies

Before any negotiations or discussions can be initiated with the supply authorities, the following basic elements must be established: c maximum anticipated power (kVA) demand Determination of this parameter is described in Chapter B, and must take into account the possibility of future additional load requirements. Factors to evaluate at this stage are: v the utilization factor (ku), v the simultaneity factor (ks), c layout plans and elevations showing location of proposed substation Plans should indicate clearly the means of access to the proposed substation, with

From the information provided by the consumer, the power-supplier must indicate: c the type of power supply proposed and define: v the kind of power-supply system: overheadline or underground-cable network, v service connection details: single-line service, ring-main installation, or parallel feeders, etc., v power (kVA) limit and fault current level.

dimensions of possible restrictions, e.g. entrances corridors and ceiling height, together with possible load (weight) bearing limits, and so on, keeping in mind that: v the power-supply personnel must have free and unrestricted access to the HV equipment in the substation at all times, v only qualified and authorized consumer’s personnel are allowed access to the substation. c degree of supply continuity required The consumer must estimate the consequences of a supply failure in terms of its duration, v loss of production, v safety of personnel and equipment.

c the nominal voltage and rated voltage (Highest voltage for equipment) Existing or future, depending on the development of the system. c metering details which define: v the cost of connection to the power network, v tariff details (consumption and standing charges).


2. consumers HV substations (continued)

C 2.1 procedures for the establishment of a new substation (continued) the power-supply organization must give official approval of the equipment to be installed in the substation, and of proposed methods of installation.

implementation

after testing and checking of the installation by an independent test authority, a certificate is granted which permits the substation to be put into service.

commissioning


Before any installation work is started, the official agreement of the power-supplier must be obtained. The request for approval must include the following information, largely based on the preliminary exchanges noted above: c location of the proposed substation, c one-line diagram of power circuits and connections, together with earthing-circuit proposals,

Commissioning tests must be successfully completed before authority is given to energize the installation from the powersupply system. The verification tests include the following: c measurement of earth-electrode resistances, c continuity of all equipotential earth-and safety bonding conductors, c inspection and testing of all HV components, c insulation checks of HV equipment, c dielectric strength test of transformer oil (and switchgear oil if appropriate), c inspection and testing of the LV installation in the substation, c checks on all interlocks (mechanical key and electrical) and on all automatic sequences, c checks on correct protective-relay operation and settings. It is also imperative to check that all equipment is provided, such that any properly executed operational manœuvre can be carried out in complete safety. On receipt of the certificate of conformity: c personnel of the power-supply authority will energize the HV equipment and check for correct operation of the metering, c the installation contractor is responsible for testing and connection of the LV installation.

c full details of electrical equipment to be installed, including performance characteristics, c layout of equipment and provision for metering components, c arrangements for power-factor improvement if eventually required, c arrangements provided for emergency standby power plant (HV or LV) if eventually required.

When finally the substation is operational: c the substation and all equipment belongs to the consumer, c the power-supply authority has operational control over all HV switchgear in the substation, e.g. the two incoming load-break switches and the transformer HV switch (or CB) in the case of a RMU, together with all associated HV earthing switches, c the power-supply personnel has unrestricted access to the HV equipment, c the consumer has independent control of the HV switch (or CB) of the transformer(s) only, c the consumer is responsible for the maintenance of all substation equipment, and must request the power-supply authority to isolate and earth the switchgear to allow maintenance work to proceed. The power supplier must issue a signed permit-to-work to the consumers maintenance personnel, together with keys of locked-off isolators, etc. at which the isolation has been carried out.

3. substation protection schemes

C The subject of protection in the electricalpower industry is vast: it covers all aspects of safety for personnel, and protection against damage or destruction of property, plant, and equipment. These different aspects of protection can be broadly classified according to the following objectives: c protection of personnel and animals against the dangers of overvoltages and electric shock, fire, explosions, and toxic gases, etc., c protection of the plant, equipment and components of a power system against the stresses of short-circuit faults, atmospheric surges (lightning) and power-system instability (loss of synchronism) etc., c protection of personnel and plant from the dangers of incorrect power-system operation, by the use of electrical and mechanical interlocking. All classes of switchgear (including, for example, tap-position selector switches on transformers, and so on...) have well-defined operating limits. This means that the order in which the different kinds of switching device can be safely closed or opened is vitally important. Interlocking keys and analogous electrical control circuits are frequently used to ensure strict compliance with correct operating sequences. It is beyond the scope of a guide to describe in full technical detail the numerous schemes of protection available to power-systems engineers, but it is hoped that the following sections will prove to be useful through a discussion of general principles. While some of the protective devices mentioned are of universal application, descriptions generally will be confined to those in common use on HV and LV systems only, as defined in Sub-clause 1.1 of this Chapter. Where some technical explanation is necessary to simplify an understanding of the text, reference is made to a related Appendix.

3.1 protection against electric shocks and overvoltages protection against electric shocks and overvoltages is closely related to the achievement of efficient (low resistance) earthing and effective application of the principles of equipotential environments.

protection against electric shocks Protective measures against electric shock are based on two common dangers: c contact with an active conductor, i.e. which is alive with respect to earth in normal circumstances. This is referred to as a “direct contact” hazard, c contact with a conductive part of an apparatus which is normally dead, but which has become alive due to insulation failure in the apparatus. This is referred to as an “indirect contact” hazard. It may be noted that a third type of shock hazard can exist in the proximity of HV or LV (or mixed) earth electrodes which are passing earth-fault currents. This hazard is due to potential gradients on the surface of the ground and is referred to as a “step-voltage” hazard; shock current enters one foot and leaves by the other foot, and is particular dangerous for four-legged animals. A variation of this danger, known as a “touch voltage” hazard can occur, for instance, where an earthed metal fence is situated in an area in which potential gradients exist. Touching the fence would cause shock current to pass through the hand and both feet (Appendix C2).

Animals with a relatively long front-to-hind legs span are particularly sensitive to stepvoltage hazards and cattle have been killed by the potential gradients caused by a low voltage (240/415 V) neutral earth electrode of insufficiently low resistance. Potential gradients on the surface of the ground can be reduced to safe values by measures such as those shown in Appendix C2. Potential-gradient problems of the kind mentioned above are not normally encountered in electrical installations of buildings, providing that equipotential conductors properly bond all exposed metal parts of equipment and all extraneous metal (i.e. not part of an electrical apparatus or the installation - for example structural steelwork, etc.) to the protective-earthing conductor.


3. substation protection schemes (continued)

C 3.1 protection against electric shocks and overvoltages (continued) Direct-contact protection The main form of protection against direct contact hazards is to contain all live parts in housings of insulating material, by placing out of reach (behind insulated barriers or at the top of poles) or by means of obstacles. Where insulated live parts are housed in a metal envelope, for example transformers, electric motors and many domestic appliances, the metal envelope is connected to the installation protective earthing system. For LV appliances this is achieved through the third pin of a 3-pin plug and socket. Total or even partial failure of insulation to the metal, can (depending on the ratio of the resistance of the leakage path through the insulation, to the resistance from the metal envelope to earth) raise the voltage of the envelope to a dangerous level.

in the case of a HV fault to a metallic enclosure, it may not be possible to limit the touch voltage to the safe value of 50 V*. The solution is to create an equipotent-situation as described in Sub-clause 1.1 "Earthing connections".

Indirect-contact protection A person touching the metal envelope of an apparatus of which the insulation is faulty, as described above, is said to be making an indirect contact. An indirect contact is characterized by the fact that a current path to earth exists (through the protective earthing (PE) conductor) in parallel with the shock current through the person concerned. c case of fault on L.V. system. Extensive tests have shown that, providing the potential of the metal envelope is not greater than 50 V* with respect to earth, or to any conductive material within reaching distance, no danger exists. c indirect-contact hazard in the case of a HV fault. If the insulation failure in an apparatus is between a HV conductor and the metal envelope, it is not generally possible to limit the rise of voltage of the envelope to 50 V or less, simply by reducing the earthing resistance to a low value. The solution in this case is to create an equipotential situation, as described in Sub-clause 1.1 “Earthing connections”. * in dry locations, 25 V in wet locations (bathrooms, etc.).


protection against overvoltages The situation mentioned immediately above, describing an indirect-contact hazard resulting from faulty HV insulation, is one of a number of ways in which an abnormal overvoltage condition can occur. Methods of eliminating danger to personnel in such a case are described in Sub-clause 1.1. Other situations which can cause overvoltages to occur on HV and LV systems include: c surges of atmospheric origin, c a short-circuit earth fault on an unearthed (or high-impedance earthed) 3-phase system, c ferro-resonance, c energization of capacitor banks, c circuit breaker opening or fuse melting to break short-circuit current. Overvoltages created by the causes listed above can be divided according to characteristies such as: c duration: permanent, temporary, transient, c frequency: industrial frequency, harmonics of industrial frequency, high frequency, unidirectional surges. Overvoltages of atmospheric origin Protection against this kind of danger must be provided when a substation is supplied directly from an overhead-line system. The most common protective device used at present is a non-linear resistor-type of lightning arrester, which is connected (one for each phase) between a phase conductor and the substation earthing system, as close to the point of entry into the substation as possible. For consumers' substations, this protection is achieved by: c lightning arresters (one per phase conductor, which are sometimes connected in series with a device for automatic tripping of a circuit breaker) (see Chapter L) and/or by c the reduction of the substation-earthing resistance to the lowest possible value to avoid (as far as possible) a breakdown of LV insulation due to the rise in potential of the earthing system when discharging the surge current. Where it is advisable to protect a substation against direct strokes, lightning-discharge electrodes (Franklin type) and shield wires should be installed and connected to the substation earthing system. It may be noted that, at the voltage levels being considered (i 35 kV), switching surges are generally less severe than lightning surges, and so devices which are suitable for satisfactory lightning protection are adequate to protect against overvoltages due to switching surges.

C Earth faults on IT-earthed systems In normal conditions the phase conductors of a 3-phase IT system are all approximately at phase volts with respect to earth. The exact values depend on the capacitance and insulation resistance of each conductor to earth. On an unfaulted system these parameters are sensibly equal in all three phases so that the vector relationship of phase voltages will be as shown below in figure C12, and the neutral point of the transformer secondary winding will be at approximately zero volts with respect to earth. A short-circuit to earth on one phase will change the values of phase conductor voltages with respect to earth; the phase-tophase voltage values and their phase displacement relationships however, will remain unchanged. This latter feature,

together with the fact that current passing through the earth-fault path will be too small to constitute a hazard, are the reasons for adopting the IT system, where supply continuity must be maintained even in “firstfault” conditions. With one phase short-circuited to earth, and the neutral point of the transformer isolated: c the neutral point will rise to phase volts above earth, c the faulty phase conductor will be at zero volts with respect to earth, c the other two phases will rise to etimes the phase voltage, with respect to earth. As indicated above, this is a 50 Hz (or 60 Hz) stable (i.e. permanent) condition, and transformers, cabling and all appliances must be suitably insulated with respect to earth, when used on IT systems.

unearthed secondary winding of power transformer 1 insulation resistance

conductor capacitance

N

2

3

fault current normally restricted to several milli-amps depending on the size of the installation

V1

V1 3 I(C+R)

I(C+R)1

I(C+R)2

√3 I(C+R)1

√3 I(C+R)2 VNE

<90°

V3

I(C+R)3

V2

normal voltages and capacitive / resistive currents

V3 = 0

V2

voltage conditions and current flowing in an earth fault on an IT system

fig. C12: earth fault on IT-earthed systems.



C 3.1 protection against electric shocks and overvoltages (continued) Ferro-resonance Ferro-resonance is a spontaneous condition which occurs due to a complex interaction between intrinsic power-system capacitances and the non-linear voltage-dependent inductances of transformers or reactors, chokes, etc. when their magnetic circuits are in a highly-saturated state (generally due to an abnormal system disturbance). The resonant condition may be at any frequency, and can be a parallel or series resonance, not corresponding exactly to the classical formulae for LC resonant circuits (which are based on assumed linearity of the LC components). Moreover, resonance may occur on one or two phases only of a 3-phase system. All types of transformer can be affected, including instrument voltage transformers; the capacitor-type VT (not normally used at the HV levels considered in this guide) is especially prone to sub-harmonic resonance (1/3 of the fundamental frequency). Electro-magnetic VTs (which are very commonly used at HV levels covered by this guide) counter the possibility of resonance by: c designing the transformer cores to operate at low levels of flux density, c incorporating damping resistors in the transformer secondary or tertiary circuits. Apart from the obvious problems presented by false signals given by instrument transformers, permanent overvoltage conditions can be established. Unless the precautions mentioned above are taken, the following situation may arise (and often did in the past, before the phenomenon was identified). The problem concerns IT-earthed systems, in which the potential of the neutral point becomes displaced (from approx. earth potential) with the result that excessive values of phase voltage with respect to earth occur on two phases, as shown in the vector diagram of figure C13. The condition is due to the saturation of two (of the three) single-phase cores of a voltage transformer, the windings of which are connected between phase and earth, as shown in figure C14, and may be provoked by a transitory* overvoltage condition such as that described above and shown in figure C12. The overvoltage saturates the two singlephase VT cores, which then present a (nonlinear, but average) inductance which is much lower than its normal value. The parallel combination of phase-to-earth capacitance and phase-to-earth inductance which, under normal conditions, behaves overall as a capacitance K (because the capacitive reactance < inductive reactance) suddenly changes character to that of an inductance. The two states, viz: before saturation and during saturation are shown in figure C14 (a) and (b) respectively. In C14 (a1) the three capacitances and the three inductances each form an independent balanced 3-phase star-connected group, i.e. there is no exchange of current between them. * for example, a bird causing a brief short-circuit to earth, and falling clear of the line, or wind-blown debris, etc.


V1E

V1N

N V3N

V2N

V3E

V2E

E

fig. C13: vector diagram of a displaced neutral due to ferro-resonance at 50 Hz.

C In C14 (b2) however, it will be seen that the two inductances, together with one capacitance, constitute an unbalanced 3-phase group, the star point of which is the earth. It is clear that an unbalanced 3-phase load on a 3-wire system will displace the “floating” neutral point of the source. A simple calculation in Appendix C3 shows how the vector diagram of figure C13 was determined. Note: it should be appreciated that the vector representation shown in figure C13 gives an approximate, but qualitatively useful, picture of the actual phenomenon, since harmonic voltages and currents are also present. However, in the 50 Hz (or 60 Hz) resonant condition, the power-frequency values are predominant. The circuit behaviour therefore, is mainly governed by these power-frequency quantities, and justifies an approximate representation by vectors. Field measurements have confirmed the validity of such a representation. For interested readers, further information on ferroresonance can be found in Cahier Technique No. 31 : "Ferroresonance" published by Merlin Gerin. 1

= = (a2)

(a1)

N 3

2 source

L

L

L

C

C

C

K

K

K

K
(a) circuits in normal operation

= (b1)

(b2) N

source

H
H

H

K

XH > XK at a resonant frequency

(b) circuits with phase 1 and phase 2 VT cores saturated

fig. C14: equivalent circuits for ferro-resonant condition.



C 3.2 electrical protection overcurrents due to overloading or to short-circuit faults (between phases and/or to earth) are detected by protective devices up-stream and down-stream of the power transformer(s). These devices cause the faulty circuit to be cut-off electrically from the power supply. The devices may be: c fuses which clear the faulty circuit directly, or together with a mechanical tripping attachment, which opens an associated threephase load-break switch, c direct-acting tripping coils which form part of a LV circuit breaker, and are operated by the fault (or overload) current passing through them, c relays which act indirectly such as: v electrical relays supplied from current and/or voltage transformers, v pressure-operated relays (pressostats), v temperature-operated relays (thermostats), v gas-detection relays (Buchholz, etc.), v oil-surge operated relays.

general The circuits and equipment in a substation must be protected so that excessive currents and/or voltages are rapidly removed from the system before causing danger, damage or destruction. All equipments normally used in power-system installations have (standardized) short-time withstand ratings for overcurrent and overvoltage conditions, and the role of protective schemes is to ensure that these withstand limits can never be exceeded. In general, this means that fault conditions must be cleared as fast as possible within the limits set by considerations of the highest attainable reliability. Overcurrents due to overloading can normally be tolerated for longer periods than those of short-circuits and some protective devices are designed to operate with increasing speed as the degree of overloading increases (i.e. an inverse-time/current characteristic). In addition to the protection against overvoltages mentioned in section 3.1, electrical protection is routinely provided against the following abnormal conditions: c overloading (i.e. excessive currents not due to faults), c transformer faults, c short-circuit faults between phases, c short-circuit faults to earth, and is commonly realized by: c a circuit breaker downstream of the transformer, c detection and trip-initiating devices which are integral parts of the transformer, c a circuit breaker or fuses (with or without an associated load-break switch) upstream of the transformer. The choice and sophistication of the protective schemes will depend on the characteristics of the substation, and are discussed later. Protective devices upstream of the transformer must be co-ordinated with downstream devices, as noted in Chapter H2, Sub-clause 4.6.

overload protection Overloading is frequently due to the coincidental demand of a number of small loads, or to an increase in the apparent power (kVA) demand of an installation, due to expansion of an enterprise, with consequent building extensions, and so on.-Load increases raise the temperature of the circuit conductors concerned, together with that of the transformer. When the temperature exceeds the normal design limits of the equipment involved, the deterioration rate (ageing) of the insulation materials is increased, and the working life of the equipment is correspondingly reduced. Overload protection devices are usually located downstream of the transformer in consumer-type substations, but are commonly provided on the upstream side in public-supply substations.


C transformer protection Overloads The protection against overloading of a transformer is provided by a time-delayed overload relay (either a thermal bi-metal strip device or an electronic device) which acts to trip the downstream-side circuit breaker. The time delay inherent in this relay ensures that the transformer will not be unnecessarily tripped for overloads of short duration. Other alternatives are: c for pole-mounted transformers “thermalimage” relays are frequently used. Such relays artificially simulate the temperature of the transformer windings with an accuracy which is sufficient to safeguard the insulation, c dry-type transformers use heat sensors embedded in the hottest part of the windings insulation for alarm and/or tripping, while c larger oil-immersed transformers frequently have thermostats with two settings, one for alarm purposes and the other for tripping. Internal faults The protection of transformers by transformer-mounted devices, against the effects of internal faults, is provided on transformers which are fitted with airbreathing conservator tanks (see figure C15), by the classical Buchholz mechanical relay. These relays can detect a slow accumulation of gases which results from the arcing of incipient faults in the winding insulation or from the ingress of air due to an oil leak. This first level of detection generally gives an alarm, but if the condition deteriorates further, a second level of detection will trip the upstream circuit breaker. An oil-surge detection feature of the Buchholz relay will trip the upstream circuit breaker "instantaneously" if a surge of oil occurs in the pipe connecting the main tank with the conservator tank. Such a surge can only occur due to the displacement of oil caused by a rapidlyformed bubble of gas, generated by an arc of short-circuit current under the oil. All transformers are fitted with some kind over-pressure relief device, which limits the maximum pressure to a value well below that at which the transformer tank will rupture. By specially designing the cooling-oil radiator elements to perform a concertina action, “totally filled” types of transformer as large as 10 MVA are now currently available. Expansion of the oil is accommodated without an excessive rise in pressure by the “bellows” effect of the radiator elements. A full description of these transformers is given in Sub-clause 4.4 (see figure C16). Evidently the Buchholz devices mentioned above cannot be applied to this design; a modern counterpart has been developed however, which measures: c the accumulation of gas, c overpressure, c overtemperature, the first two conditions trip the upstream circuit breaker, and the third condition trips the downstream circuit breaker of the transformer. The device, referred to as a "DGPT" unit (Detection of Gas, Pressure and Temperature) is mentioned further in 4.4, under "Liquid-filled transformers".

fig. C15: transformer with conservator tank.

fig. C16: total-fill transformer.



C 3.2 electrical protection (continued) protection against short-circuits Short-circuits may occur between phase conductors, between a phase conductor and earth, or in any combination of these conditions on the three phases. The (extremely unlikely) occurrence of a shortcircuit fault between the high-voltage windings and the low-voltage windings will constitute a short-circuit-to-earth fault on the HV winding if the secondary winding is earthed, which is generally the case. Unearthed star-connected LV secondary windings of IT-system transformers have an overvoltage device which will operate in these circumstances to connect the LV neutral point of the transformer directly to earth. Earth faults on the HV winding present a particular danger to personnel, due to the Transferred Potential hazard mentioned in Sub-clause 1.1: “Earthing connections”. For this reason, high-speed sensitive earthfault protection is standard on the HV side of power transformers in many public-supply distribution and consumer-type substations. The scheme is shown below in figure C17 and can be applied to transformers having delta or unearthed-star primary windings. This arrangement is called a “Restricted Earth-Fault" (REF) protection because it will detect earth faults only on the HV windings or on the circuit downstream of the CTs (current transformers) to the winding terminals. The advantages of the scheme include: c simplicity and low cost, c instantaneous operation, c high sensitivity, c virtual elimination of the dangers of Transferred Potential (because of its instantaneous operation), c no problems of coordination with downstream protection; LV earth faults appear as phase/phase faults on the HV side of the transformer, thereby being undetected by the REF relay (see fig. AC1-2(c) of Appendix C1). Again, in public-supply systems in general, there is no CB on the LV side, simply a loadbreak switch. Overcurrent protective relays (2 only) are connected in series with the REF current transformers, as shown dotted in figure C17 (see note). These overcurrent relays afford protection against overloading and short-circuit faults downstream of the CTs, but must be carefully co-ordinated with LV overcurrent protective devices. Note: where short-circuit fault levels are low, it is recommended to use 3 overcurrent relays (rather than 2) since on delta/star transformers a phase/phase short-circuit at LV gives a 2:1:1 fault-current distribution at HV (see fig. AC1-2(b) of Appendix C1). In view of the effectiveness of REF protection against the hazards of transferred potentials, and its uncomplicated application, it is strongly recommended to be included in any protection scheme which includes a HV circuit breaker.


choice of protective devices on the upstream side of the transformer in a consumer-type substation In certain national standards*, the choice is made according to two current values: c the reference current Ib, the value of which will be: v in the case of metering at low voltage: the nominal rated current of the transformer, v in the case of metering at high voltage: the sum of the nominal rated currents of transformers and other HV plant (e.g. motors, etc.), c the minimum value of HV 3-phase shortcircuit current at the installation. * There is no equivalent IEC standard.

HV

LV

1

1

2

2

3

3 N E/F relay

fig. C17: protection against earth fault on the HV winding.

C When the reference current is less than 45 A and there is only one transformer, the protection may be by fuses or by a circuit breaker. When the reference current is equal to or greater than 45 A, or when there is more than one transformer, the protection will be by a circuit breaker. The maximum IEC standard kVA ratings of transformers corresponding to a HV full-load current not exceeding 45 A are given in the table C18. primary voltage (kV) rated 3.6 7.2

12 17.5 24 36 40.5

nominal 3 3.3 4.16 5.5 6 6.6 10 11 13.8 15 20 22 33 36.5

maximum IEC standard ratings for transformers (kVA) 250

500

800 1,250 1,600 2,500 3,150

table C18: power limits of transformers with a maximum primary current not exceeding 45 A. c protection by fuses The relationships between the reference current Ib, as defined above, the rated current In of the fuse, and the short-circuit current Ic at the primary terminals of the transformer, are determined according to the national standards previously referred to, as follows: v when the substation consists of a single HV/LV transformer, the rated current In of the fuse must satisfy the following relationships: In > 1.4 Ib and In < Ic/6 Where: In = rated current of the fuse, Ib = rated primary current of the transformer, Ic = the minimum current at the primary side of the transformer when the secondarywinding terminals are short-circuited, v when the substation is supplied from an overlead line, or when the installation is sensitive to unbalanced-voltage conditions (for example three-phase motor loads), it is recommended that the failure of a fuse causes all three phases to be cut off, by automatic tripping of the HV load-break switch (i.e. a combined switch-fuse). Standard current ratings for fuses according to IEC 282-1 are listed in table C19.



C 3.2 electrical protection (continued) supply voltages (kV) rated nominal 3.6 3 3.3 7.2 4.16 5.5 6 6.6 12 10 11 17.5 13.8 15 24 20 22 36 33 40.5 36.5

nominal transformer ratings (kVA) 25 50 100 125 160 16 25 40 50 50 16 25 40 50 50 10 25 31.5 40 50 10 16 25 31.5 40 10 16 25 31.5 31.5 10 16 25 25 31.5 6.3 10 16 25 25 6.3 10 16 25 25 6.3 6.3 10 16 25 6.3 6.3 10 16 16 6.3 6.3 10 10 16 6.3 6.3 10 10 10 6.3 6.3 6.3 6.3 6.3 6.3 6.3 6.3 6.3 6.3

200 63 63 50 40 40 40 31.5 25 25 25 16 16 6.3 6.3

250 80 80 63 50 50 40 31.5 31.5 25 25 25 25 16 16

315 80 80 80 63 50 50 40 31.5 31.5 31.5 25 25 16 16

400 100 100 80 63 63 63 50 40 31.5 31.5 31.5 25 16 16

500 125 125 100 80 80 80 50 50 40 40 31.5 31.5 16 16

630 160 160 125 100 80 80 63 63 50 50 40 31.5 16 16

800 200 200 160 125 100 100 80 63 63 50 50 40 31.5 25

1,000 250 250 200 160 125 125 80 80 63 63 50 50 31.5 25

1,250 1,600 2,000 2,500

250 200 160 160 100 100 80 80 63 50 40 31.5

250 200 200 125 125 100 80 80 63 50 40

250 250 160 160 160 100 80 80 63 50

200 200 160 160 160 160 80 63

table C19: rated current (A) of HV fuses for transformer protection according to IEC 282-1. It is strongly recommended that, following the operation of a fuse (or fuses) to clear a fault or overload condition, all three fuses be replaced, since it is possible that the fuse (or fuses) that had not operated may have deteriorated, due to the passage of excessive current during the disturbance.

no disturbance occurring within the installation shall cause the operation of any protective relaying in the power-supply network.


c protection by circuit breaker When the substation is supplied through a HV circuit breaker, it will be a contractual condition that no disturbance occurring within the installation shall cause the operation of any protective relaying in the power-supply network. To ensure that this condition can be complied with, the supply authority must specify the longest times permissible for clearing the following faults on the installation: c short-circuit fault between all 3 phases, c short-circuit fault between any 2 phases, c short-circuit fault of one phase to earth, c short-circuit fault of any 2 phases to earth. The maximum level of a 3-phase short-circuit at the installation was known at the outset of the project, in order to purchase adequatelyrated equipment. To ensure correct operation of the protective devices, the minimum value of 3-phase short-circuit current must also be stated by the supply authority. When planning the protection scheme for the installation, the general principle of coordination is that the circuit breaker closest to the power source will have the longest tripping time. In the present case, this is the HV circuit breaker. This longest tripping time must not, however, exceed that given by the supply authority; a constraint which generally is satisfied only by protective relays at the HV circuit breaker, to supplement the transformer-mounted protective devices previously mentioned. As far as earth faults are concerned, there is no co-ordination problem, provided that the transformer HV winding is a delta- or unearthed-star connection, since, as already mentioned, earth faults on the LV system will then appear as phase-to-phase faults on the HV system. HV earth faults occurring in the substation can therefore be cleared instantaneously by a REF scheme. Instantaneous tripping for phase-to-phase short-circuit faults occurring on the HV side of

the transformer can also be achieved very simply by devices which are sometimes referred to as “high-set” relays. The “high-set” principle depends on the fact that, if the current is sufficiently high to operate the relay, then the short-circuit must be on the HV side of the transformer, because a short-circuit on the LV terminals or windings of the transformer will not produce sufficient current on the HV side to cause the relay to operate. The high-set relays (2 or 3 as noted in Subclause "protection against short-circuits") will each be connected in series with one of the inverse-time/overcurrent relays, shown dotted in figure C17, and for distribution-type transformers are generally set to operate at 25 times the full-load current of the transformer. By these simple means therefore, instantaneous clearance of shortcircuit faults on the HV side of a transformer can be achieved, without affecting the coordination scheme for downstream protection. At periods of the lowest levels of short-circuit fault current, however, the high-set scheme may not be sufficiently sensitive, i.e. the current may not be high enough to operate the relay (there is no similar problem with earth faults, the REF scheme being very sensitive). In extreme cases, where the difference between maximum and minimum fault levels is very large, it may be necessary to provide a differential-protection scheme for the transformer. Differential-protection schemes compare the currents entering the primary windings with those leaving the secondary windings (after correction for current-level and phase changes) and any significant difference will operate the relay, which trips the circuit breakers controlling the transformer. Such protection will provide adequate sensitivity with high-speed tripping, and will not affect co-ordination of downstream protection. It may be noted that the high-speed relays used for the REF, high-set and differential protection schemes are stabilized against false operation due to CT saturation (for example, when energizing the transformer). Overcurrent, REF, and high-set relays are commonly contained in a single relay case.

C choice of downstream protective devices The protective device (circuit breaker or fuse-switch)* downstream of the transformer must include and comply with the following requirements (IEC 364). The device must: c include an isolating switch (for the protection of persons) of which the open switch contacts are clearly visible, c have a current rating adequate for the transformer concerned, c have a breaking-current rating where appropriate*, adequate for the secondary 3-phase short-circuit current, c have the correct number of poles according to the earthing scheme of the installation, transformer rated power (kVA) transformer current Ir (A) oil-immersed transformer Isc (kA) cast-resin transformer Isc (kA)

Psc = 250 MVA Psc = 500 MVA Psc = 250 MVA Psc = 500 MVA

50 69 1.71 1.71 1.14 1.14

100 137 3.40 3.42 2.28 2.28

160 220 5.41 5.45 3.63 3.65

250 344 8.38 8.49 5.63 5.68

315 433 10.5 10.7 7.07 7.14

400 550 13.2 13.5 8.93 9.04

500 687 16.4 16.8 11.1 11.3

v 4 poles for IT scheme with neutral conductor, TT and TNS, v 3 poles for IT scheme without neutral conductor, and for TNC. By way of an example, table C20 lists the nominal currents and corresponding shortcircuit currents at the secondary terminals of IEC-standard 20 / 0.4 kV transformers. From these data it can be seen that the short-circuit impedances are in the range of 4% (for the 100 kVA transformer) to 6.9% (for the 2,000 kVA transformer). * where no LV circuit breaker or fuse-switch is installed, a non-automatic LV load-break isolating switch must be provided, and overload protection must be effected at HV.

630 866 20.4 21.0 13.9 14.1

800 1100 17.4 17.9 17.4 17.9

1000 1375 21.5 22.2 21.5 22.2

1250 1718 26.4 27.5 26.4 27.5

1600 2199 33.1 34.8 33.1 34.8

2000 2749 40.4 43.0 40.4 43.0

2500 3437 49.1 52.9 49.1 52.9

table C20: 3-phase short-circuit currents of typical distribution transformers.

discrimination (selectivity) between the protective devices on the upstream and downstream sides of the transformer The consumer-type substation with LV metering requires discriminative operation between the HV fuses and the LV circuit breaker or fuses. The calibre of the HV fuses will have been chosen according to the characteristics of the transformer. The tripping characteristics of the LV circuit breaker must be such, that for an overload or short-circuit condition downstream of its location, the breaker will trip sufficiently quickly to ensure that the HV fuses will not be adversely affected by the passage of overcurrent through them. The tripping performance curves for HV fuses and LV circuit breakers are given by graphs of time-to-operate against current passing through them. Both curves have the general inverse-time/current form (with an abrupt discontinuity in the CB curve at the current value above which “instantaneous” tripping occurs). These curves are shown typically in figure C21. c in order to achieve discrimination: v all parts of the fuse curve must be above and to the right of the CB curve, c in order to leave the fuses unaffected (i.e. undamaged): v all parts of the minimum pre-arcing fuse curve must be located to the right of the CB curve by a factor of 1.35 or more (e.g. where, at time T, the CB curve passes through a point corresponding to 100 A, the fuse curve at the same time T must pass through a point corresponding to 135 A, or more, and so on...) and, v all parts of the fuse curve must be above the CB curve by a factor of 2 or more (e.g. where, at a current level I the CB curve passes through a point corresponding to 1.5 seconds, the fuse curve at the same current level I must pass through a point corresponding to 3 seconds, or more, etc.). The factors 1.35 and 2 are based on standard maximum manufacturing tolerances for

HV fuses and LV circuit breakers.* In order to compare the two curves, the HV currents must be converted to the equivalent LV currents, or vice-versa. Figure C21 illustrates these requirements. Where an LV fuse-switch is used, similar separation of the characteristic curves of the HV and LV fuses must be respected. This question is considered in Appendix C1 (figure AC1-3). time

minimum pre-arcing time of HV fuse B/A u 1.35 at any moment in time D/C u 2 at any current value

D circuit breaker tripping characteristic

C

current

A B

fig. C21: discrimination between HV fuse operation and LV circuit breaker tripping, for transformer protection. Note: In the simple and widely used case, where a HV circuit breaker incorporates REF, high-set and inverse-time/overcurrent relays as previously described, the only electrical protection of the LV windings and the LV connections from the transformer terminals to the upstream terminals of the LV circuit breaker is that provided by the HV inversetime/overcurrent relays. * Merlin Gerin “catalogue distribution HT/MT 96” page G29.



C 3.2 electrical protection (continued) There is no compelling need for discrimination between these HV relays and the LV circuit breaker protection, since a short-circuit on the upstream or downstream side of the LV breaker would result in a total loss of supply, in either case. In general, a 3-phase short-circuit at the LV terminals of a distribution transformer will cause a current of 14-25 times the transformer full-load current to flow in the LV and HV circuits (at times of maximum shortcircuit fault levels on the system). If the fault is a short-circuit of one phase to earth, immediately upstream of the LV circuit breaker however, then this value will be reduced to approximately 8-14 times full-load

current on the HV side of the transformer and will flow in two lines only. The tripping time of the inverse-time/overcurrent relays may, in this case, be unacceptably long. The conventional solution to the problem is to make these LV connections “fault-free” by enclosing the (generously-insulated) conductors in vermin-proof metal bus-ducts, a method which, in view of the location (in an area prohibited to all except authorized personnel) is generally considered to be satisfactory. A 100 % solution would be to install overall protection from the HV circuit breaker to the LV circuit breaker, as provided by the differential protective scheme previously described.

HV earth-fault relay settings Earth-fault relays have low current-setting ranges, i.e. they are sensitive instruments, and can, in consequence, act to clear a developing short-circuit fault in its early stages, thereby minimizing the damage to insulation at the fault position, and reducing the risk of fire. Care must be taken however, to avoid increasing the sensitivity (by reducing the operating-current setting) to a point where the relay can be caused to operate when an earth fault occurs on a nearby circuit, the circuit on which the relay is installed being (perfectly) healthy (unfaulted). This false operation is due to the inherent capacitance to earth of the power-system phase conductors and connected loads, and is especially likely to occur on impedanceearthed systems (which are common at the HV levels covered by this guide). In normal circumstances, the capacitive current from each phase to earth has practically the same magnitude (Ic) and the three currents summate in the earth to give the so-called “residual” current, which, because of the balanced conditions (in this case) is theoretically zero. A short-circuit of one phase to earth, as already mentioned in “Earth faults on ITearthed systems” (Sub-clause 3.1) will cause: c the voltage of the faulted conductor, and that of all conductors of the faulty phase over a wide area surrounding the fault location, to fall to practically zero volts, c the voltage of the two healthy phase conductors over the same area to increase by (up to) etimes their original value with respect to earth. As a result of this, the residual current in all circuits affected by the voltage changes will no longer approximate to zero. On healthy circuits close to the fault position, the residual current will, in fact, amount to almost 3 Ic, as illustrated in figure C22. If a healthy circuit has a significant capacitance to earth (a long overhead line or a section of underground cable) then 3 Ic will be detected by its earth-fault relay which, if given an oversensitive setting, will trip the circuit breaker of the unfaulted circuit. A conventional minimum value of earth-faultcurrent setting intended to avoid this problem, commonly adopted in the power-distribution industry, is 6 Ic (i.e. a safety factor of 2). This phenomenon is only of interest to a HV installation-design engineer in cases where the HV circuit breaker and protective relays C28 - HV/LV distribution substations

are some distance from the transformer, especially if the supply is by underground cable, and the HV nominal voltage is high, e.g. u 20 kV. The foregoing discussion of the presence of residual capacitive components in the earthfault current of impedance-earthed systems, leads conveniently to an explanation of the principle of the Petersen coil. System earthing on overhead-line HV systems by means of a Petersen coil In the resistance-earthed system described above, it was shown that the current through the earth fault is the sum of the residual capacitive currents of the system and the current which flows through the resistor. This resistor current is in phase with the voltage of the faulted phase (the voltage vector being reversed during the fault period) as shown in the vector diagram of figure C22. The phase displacement between the resistor current and the residual capacitive current is practically 90 degrees. If now, the resistor is replaced by a reactor, the reactor current will lag the (reversed) faulted phase voltage by 90 degrees and will, therefore, be in phase opposition to the residual capacitive current. By a suitable choice of reactance value the residual capacitive current through the fault can, in principle, be exactly cancelled, i.e. there will be no fault current flowing to earth, as shown in figure C23. This is the principle of Petersen Coil operation. In practice, absolute cancellation of fault current is not possible, owing to conductorand fault- path resistances, and to the impracticality of precise coil tuning at all times. These effects prevent perfect mutual cancellation of the opposing currents, but providing the fault-path resistance is low, the fault current can be reduced to very small values. Petersen coils are provided with a number of tapping steps on the coil to cover a range of system capacitance values. These coils are associated mainly with isolated O/H line power networks in the HV voltage ranges covered by this guide.

C c damage at the fault position is limited, owing to the restricted current level, c disturbance to neighbouring systems at the instant of fault is practically non-existent.

Operational advantages Advantages of the system include: c continuity of supply in the (common) event of an earth fault. In principle, the system can operate indefinitely with an earth fault on one phase, A

earth-fault protection relays of the healthy circuits A and C will detect an apparent earth fault

eIC1A

1

eIC2A

2 3 0

0

V3

3ICA B

eIC1B

1

eIC2B

2 3 0

IF

0

V3

IF

IF

3ICB

power supply source C

IF

I’F eIC1C

1

eIC2C

2 3 0

0

V3

3ICC eIc1

power supply

eIc2

IF IF I’F

I’F eIC1 + eIC2 = 3IC

simplified diagram showing current division

IF

I’F >> 3IC IF = I’F + 3IC

V1

V1

V1

3IC IC2

√3IC2

√3IC2

VNE

IC1

I'F √3IC1

V3

IC3 V2

normal voltages and capacitive currents

V3 = 0

V2

√3IC1

VNE V2

V3 = 0 voltages during short-circuit to earth on phase 3

V1

3IC

residual current on healthy circuits during fault

V2

fault current IF is the vector sum of the neutral resistor current I’F and the residual capacitive currents of the system 3IC

fig. C22: earth-fault diagram. HV/LV distribution substations - C29


C 3.2 electrical protection (continued) eIc1

power supply

eIc2

Petersen coil

L

IF = 0 0

0

I’F

I’F = 3IC

I’F = eIC1 + eIC2 = 3IC simplified diagram showing current division when XC / XL = 3 (where XC = capacitive reactance of one phase to earth)

V1

3IC eIC2

VNE eIC1 E

V2

I’F vector diagram for condition I’F = 3 IC

V1

V1

V1

3IC IC2

V3

eIC2

VNE

IC1

IC3 V2

V3 = 0

V2

V3 = 0 voltages during short-circuit to earth on phase 3

normal voltages and capacitive currents fig. C23: earth fault diagram (with Petersen coil). C30 - HV/LV distribution substations

√3IC1

residual current on healthy circuits during fault

V2

C 3.3 protection against thermal effects The risk and consequences of a fire are particularly serious. The prefabricated equipments are conceived and manufactured in a way that avoids excessive temperature rise in normal use. Where an installation includes one or several liquid-insulated transformers, the regulations and arrangements relative to the protection and construction details must be fully respected, and are described in Subclause 4.3: “choice of HV/LV transformers”.

3.4 interlocks and conditioned manœuvres Mechanical and electrical interlocks are included on mechanisms and in the control circuits of apparatus installed in substations, as a measure of protection against an incorrect sequence of manœuvres by operating personnel. Mechanical protection is afforded by: c compartments enclosing specific parts of equipment in pre-fabricated HV cells, c key-transfer interlocking.

an interlocking scheme is intended to prevent any operational manœuvre which would expose operating personnel to danger.

key interlocking The most widely-used form of locking/ interlocking depends on the principle of keytransfer. The principle is based on the possibility of freeing or trapping one or several keys, according to whether or not the conditions of safety are satisfied. These conditions can be combined in unique and obligatory sequences, thereby guaranteeing the safety of personnel by the avoidance of an incorrect operational procedure. For example, access to a HV panel requires a certain number of operations which must be carried out in a pre-determined order. It is necessary to carry out manœuvres in the reverse order to restore the system to its former condition. Non-observance of the correct sequence of manœuvres in either case may have extremely serious consequences for the operating personnel, as well as for the equipment concerned. Note: It is important to provide for a scheme of interlocking in the basic design stage of planning a HV/LV substation. In this way, the apparatuses concerned will be equipped during manufacture in a coherent manner, with assured compatibility of keys and locking devices.

interlocks in substations equipped with metalclad switchgear In a HV/LV distribution substation which includes: c a single incoming HV panel or two incoming panels (from parallel feeders) or two incoming/outgoing ring-main panels, c a transformer switchgear-and-protection panel, which can include a load-break/ isolating switch with HV fuses and an earthing switch, or a circuit breaker and lineisolating switch together with an earthing switch,

c a transformer compartment interlocks allow manœuvres and access to different panels in the following conditions: v operation of the load-break/isolating switch if the panel door is closed and the associated earthing switch is open, v operation of the line-isolating switch of the transformer switchgear - and protection panel - if the door of the panel is closed, and - if the circuit breaker is open, and the earthing switch(es) is (are) open. v closure of an earthing switch if the associated isolating switch(es) is (are) open*, v access to the interior of each panel if the isolating switch for the panel is open and the earthing switch(es) in the panel is (are) closed, v closure of the door of each panel or compartment if the earthing switch(es) in the panel is (are) closed, v access to the HV fuses of a substation supplied by two incomers from parallel feeders if the two isolating switches are open and the two earthing switches in the panel are closed, v access to the compartment(s) occupied by the VT(s) if the HV isolating switch is open, and if the LV isolating device is open, v operation of the isolating switches in the VT panel if the door of the panel is closed. * if the earthing switch is on an incoming circuit, the associated isolating switches are those at both ends of the circuit, and these should be suitably interlocked.



C 3.4 interlocks and conditioned manœuvres (continued) practical example In a consumer-type substation with LV metering, the interlocking scheme most commonly used is HV/LV/TR (high voltage/ low voltage/transformer). The aim of the interlocking is: c to prevent access to the transformer compartment if the earthing switch has not been previously closed, c to prevent the closure of the earthing switch in a transformer switchgear-and-protection panel, if the LV circuit breaker of the transformer has not been previously locked “open” or “withdrawn”. Access to the HV or LV terminals of a transformer, protected upstream by a HV switchgear-and-protection panel, containing a HV load-break / isolating switch, HV fuses, and a HV earthing switch) must comply with the strict procedure described below, and is illustrated by the diagrams of figure C24. Note: The transformer in this example is provided with plug-on type HV terminal connectors which can only be removed by unlocking a retaining device common to all three phase connectors*. The HV load-break / isolating switch is mechanically linked with the HV earthing switch such that only one of the switches can be closed, i.e. closure of one switch automatically blocks the closure of the other.

S

S

* or may be provided with a common protective cover over the three terminals.

Procedure for the isolation and earthing of the power transformer, and removal of the HV plug-type shrouded terminal connections (or protective cover). c initial conditions: v HV load-break/isolating switch and LV circuit breaker are closed, v HV earthing switch locked in the open position by key “O”, v key “O” is trapped in the LV circuit breaker as long as that circuit breaker is closed, c step 1: v open LV CB and lock it open with key “O”, v key “O” is then released, c step 2: v open the HV switch, v check that the “voltage presence” neon indicators extinguish when the HV switch is opened, c step 3: v unlock the HV earthing switch with key “O” and close the earthing switch, v key “O” is now trapped, c step 4: v the access panel to the HV fuses can now be removed (i.e. is released by closure of the HV earthing switch). Key “S” is located in this panel, and is trapped when the HV switch is closed, v turn key “S” to lock the HV switch in the open position, v key “S” is now released, c step 5: v key “S” allows removal of the common locking device of the plug-type HV terminal connectors on the transformer or of the common protective cover over the terminals, as the case may be. In either case, exposure of one or more terminals will trap key “S” in the interlock.


HV switch and LV CB closed

O

S O

S

HV fuses accessible

S

O

S O transformer HV terminals accessible legend key absent key free key trapped panel or door

fig. C24: example of HV/LV/TR interlocking.

C The result of the foregoing procedure is that: a) the HV switch is locked in the open position by key “S”. Key “S” is trapped at the transformer terminals interlock as long as the terminals are exposed. b) the HV earthing switch is in the closed position but not locked, i.e. may be opened or closed. When carrying out maintenance work, a padlock is generally used to lock the earthing switch in the closed position, the key of the padlock being held by the engineer supervizing the work. c) the LV CB is locked open by key “O”, which is trapped by the closed HV earthing switch. The transformer is therefore safely isolated and earthed. It may be noted that the upstream terminal of the load-break switch may remain alive in the procedure described. There are three reasons for this: c the terminals in question are located in a separate inaccessible compartment in the particular switchgear under discussion, c the open contacts of the switch have an earthed screen interposed between them, c the envelope containing the switch is moulded from insulating material, filled with SF6 gas, and sealed for life. In the general case, the upstream terminals of such a switch (or circuit breaker) will be exposed within the compartment, and an earthing switch will be provided, interlocked mechanically with a line-isolating switch. Or again, it may be necessary (depending on the type of switchgear) to isolate and lock off the incoming supply cable at its remote end, before closing the local earthing switch. Any scheme of interlocking must evidently include appropriate procedures, along similar lines to those described above.


4. the consumer substation with LV metering

C 4.1 general A consumer substation with LV metering is an electrical installation connected to a publicsupply system at a nominal voltage of 1 kV - 35 kV, and includes a single HV/LV transformer generally not exceeding 1,250 kVA.

functions The substation All component parts of the substation are located in one chamber, either in an existing building, or in the form of a prefabricated housing exterior to the building. Connection to the HV network Connection at HV can be: c either by a single service cable or overhead line, or, c via two mechanically interlocked load-break switches with two service cables from duplicate supply feeders, or, c via two load-break switches of a ring-main unit. The transformer Since the use of PCB*-filled transformers is prohibited in most countries, the preferred available technologies are: c oil-immersed transformers for substations located outside premises, c dry-type, vacuum-cast-resin transformers for locations inside premises, e.g. multistoreyed buildings, buildings receiving the public, and so on... * polychlorinated biphenyl.

one-line diagrams The diagrams on the following page (figure C25) represent: c the different methods of HV service connection, which may be one of four types: v single-circuit service, v single-circuit (for later change to ring-main service), v duplicate service (interlocked mechanically), v ring-main service, c HV protective functions and HV/LV transformation, c LV metering and general isolation functions, c LV protective and distribution functions, c zones of access for interested parties.


Metering Metering at low voltage allows the use of small metering transformers at modest cost. Most tariff structures take account of transformer losses. LV installation circuits A low-voltage circuit breaker, suitable for isolation duty with visible contacts and locking off facilities, to: c supply a distribution board, c protect the transformer against overloading and the downstream circuits against shortcircuit faults.

C power supply system

service connection

HV protection and HV/LV transformation

supplier/consumer interface

LV metering and isolation

transformer LV terminals

LV distribution and protection

downstream terminals of LV isolator

protection

protection

single-line service

(permitted if IHV nominal i 45 A and one transformer )

single-line service (equipped for extension to form a ring main)

protection

duplicatesupply service

(permitted if IHV nominal i 45 A and one transformer )

protection + auto-changeover switch

ring-main service protection

automatic LV standby source

(always permitted)

authorized access limits

consumer

consumer testing authority

power-supply authority

fig. C25: consumer substation with LV metering.


4. the consumer substation with LV metering (continued)

C 4.2 choice of panels standards and specifications The SF6 switchgear and equipments described below are rated for 1 kV - 24 kV systems and conform to the following international and national standards: c international: IEC 56-1, 129, 265-1, 298, 694, c national: French: UTE, EDF, British: BS, German: VDE, American: ANSI.

type of material All kinds of switchgear arrangements are possible when using modular compartmented panels, and provisions for later extensions are easily realized. Compact substations of modular panels are particularly applicable in the following cases: c ring-main substations (a 3-function monobloc assembly), c severe climatic or heavily-polluted conditions (integral insulation), c insufficient space for “classical” switchboards. This “all - SF6” equipment is distinguished by its reduced dimensions, its integrated functions and by its operational flexibility.

operational safety of compartmented metalclad panels Description The following notes describe a “state-of-theart” load-break / isolating-switch panel (see figure C26) incorporating the most modern developments for ensuring: c operational safety, c minimum space requirements, c extendibility and flexibility, c minimum maintenance requirements. Each panel includes 4 compartments: c switchgear: the load-break switch is incorporated in an SF6-filled hermetically sealed (for life) molded epoxy-resin unit, c connections: by cable at terminals located on the molded load-break switch unit, c busbars: modular, such that any number of panels may be assembled side-by-side to form a continuous switchboard, c control and indication: a control and instrument compartment which can accommodate automatic control and relaying equipment. An additional compartment may be mounted above the existing one if required. Cable connections Cable connections are provided inside a cable-terminating compartment at the front of the unit, to which access is gained by removal of the front panel of the compartment. The units are connected electrically by means of prefabricated sections of busbars. Site erection is effected by following the assembly instructions. Operation of the switchgear is simplified by the grouping of all controls and indications on a control panel at the front of each unit. The technology of these switchgear units is essentially based on operational safety, ease of installation and low maintenance requirements.


fig. C26: compartmented SF6 HV load-break isolating switch.

C state of isolation clearly apparent The load-break/isolating switch fully satisfies the requirement of “isolation clearly apparent" as defined in IEC 129, by means of: c a position indicator accurately reflecting the open state of the contacts, c an earthed metal barrier interposed between the open contacts. Interlocks c closure of the switch is not possible unless the earth switch is open and the access panel to the cable-terminations compartment* is closed, c closure of the earthing switch is only possible if the load-break/isolating switch is open, c opening of the access panel to the cableterminations compartment* is only possible if the earthing switch is closed, c the load-break/isolating switch is blocked in the open position when the above-mentioned access panel is open. Operation of the earthing switch is then possible. * where HV fuses are used they are located in this compartment.

Apart from the functional interlocks noted above, each switchgear panel includes: c built-in padlocking facilities, c 5 predrilled sets of fixing holes for possible future interlocking locks. Manœuvres c operating handles, levers, etc. required for switching manœuvres are grouped together on a clearly illustrated panel, c all closing-operation levers are identical on all units (except those containing a circuit breaker), c operation of a closing lever requires very little effort, c opening or closing of a load-break/isolating switch can be by lever or by push-button for automatic switches, c conditions of switches (Open, Closed, Spring-charged), are clearly indicated. Choice of short-circuit withstand ratings short-circuit (MVA) (kV) 3 3.3 4.16 65 70 90 75 85 105 85 90 115 110 120 150 135 150 190 165 180 227

for nominal system voltages 5 110 125 140 180 230 275

5.5 120 135 150 200 250 300

6 130 150 165 220 275 330

6.6 145 165 185 240 300 360

10 215 250 280 365 455

11 240 275 305 400 500

13.8 300 345 385 500

ITH/1 sec.(1) Isc(2) 15 20 22 33 (kA) r.m.s. 325 435 475 715 12.5 375 500 550 825 14.4 415 555 610 915 16 545 20 25 31.5

ICL(3) (kA) peak 31.5 36.5 40 50 62.5 79

table C27: standard short-circuit MVA and current ratings at different levels of nominal voltage. (1) I /1 sec: thermal withstand current for 1 second (2) Isc: short-circuit current (3) I : peak rated closing current TH

CL



C 4.3 choice of HV switchgear panel for a transformer circuit Three types of HV switchgear panel are generally available: c load-break switch and separate HV fuses in the panel, c load-break switch/HV fuses combination, c circuit breaker. Seven parameters influence the optimum choice: c the primary current of the transformer, c the insulating medium of the transformer, c the position of the substation with respect

to the load centre, c the kVA rating of the transformer, c the distance from switchgear to the transformer, c the use of separate protection relays (as opposed to direct-acting trip coils). Note: the fuses used in the load-break/fuseswitch combination have striker-pins which ensure tripping of the 3-pole switch on the operation of one (or more) fuse(s).

4.4 choice of HV/LV transformer a transformer is characterized in part by its electrical parameters, but also by its technology and its conditions of use. the rated power of the transformer is chosen according to the maximum apparent power, as determined in B.4.6.


characteristic parameters of a transformer A transformer is characterized in part by its electrical parameters, but also by its technology and its conditions of use. Electrical characteristics c rated power (Pn): the conventional apparent-power in kVA on which other design-parameter values and the construction of the transformer are based. Manufacturing tests and guarantees are referred to this rating, c frequency: for power distribution systems of the kind discussed in this guide, the frequency will be 50 Hz or 60 Hz, c rated primary and secondary voltages: for a primary winding capable of operating at more than one voltage level, a kVA rating corresponding to each level must be given. The secondary rated voltage is its opencircuit value, c rated insulation levels: are given by overvoltage-withstand test values at power frequency, and by high-voltage impulse tests which simulate lightning discharges. At the voltage levels discussed in this guide, overvoltages caused by HV switching operations are generally less severe than those due to lightning, so that no separate tests for switching-surge withstand capability are made. IEC standards define the rated (powerfrequency) voltage and the “highest voltage for equipment” in exactly the same terms, as noted in Sub-clause 1.1 of this Chapter, c off-circuit tap-selector switch: generally allows a choice of up to ± 2.5 % and ± 5 % level about the rated voltage of the highest voltage winding. The transformer must be de-energized before this switch is operated; however, on-load tap-changers are available (e.g. ± 12.5%) where circumstances require it, c winding configurations: are indicated in diagrammatic form by standard symbols for star, delta and inter-connected-star windings; (and combinations of these for special duty, e.g. six-or twelve-phase rectifier transformers, etc.) and in an IEC-recommended alphanumeric code. This code is read from left-toright, the first letter refers to the highest voltage winding, the second letter to the next highest, and so on,

v capital letters refer to the highest voltage winding D = delta Y = star Z = interconnected-star (or zigzag) N = neutral connection brought out to a terminal v lower-case letters are used for tertiary and secondary windings d = delta y = star z = interconnected-star (or zigzag) n = neutral connection brought out to a terminal v a number from 0 to 11, corresponding to those, on a clock dial (“O” is used instead of “12”) follows any pair of letters to indicate the phase change (if any) which occurs during the transformation. If a neutral terminal is available then the number appears after the N (or n). A very common winding configuration used for distribution transformers is that of a Dyn 11 transformer, which has a delta HV winding with a star-connected secondary winding the neutral point of which is brought out to a terminal. The phase change through the transformer is +30 degrees, i.e. phase 1 secondary voltage is at “11 o’clock” when phase 1 of the primary voltage is at “12 o’clock”, as shown in figure C36. All combinations of delta, star and zigzag windings produce a phase change which (if not zero) is either 30 degrees or a multiple of 30 degrees. IEC 76-4 describes the “clock code” in detail. Characteristics related to the technology and utilization of the transformer This list is not exhaustive: c choice of technology. The insulating medium is: v liquid (mineral oil) or, v solid (epoxy resin and air), c for interior or exterior installation, c altitude (i 1,000 m is standard), c temperature (IEC 76-2), v maximum ambient air: 40 °C, v daily maximum average ambient air: 30 °C, v annual maximum average ambient air: 20 °C. For non-standard operating conditions, refer to C1.1 "Influence of the Ambient temperature and altitude on the rated current".

C description of insulation techniques There are two basic classes of distribution transformer presently available: c dry type (cast in resin), c liquid filled (oil-immersed). Dry type transformers The windings of these transformers are insulated by resin cast under vacuum (which is patented by major manufacturers). It is recommended that the transformer be chosen according to the CENELEC standards documents HD 46451, as follows: c environment class E2 (frequent condensation and/or high level of pollution), c climatic conditions class C2 (utilization, transport and stockage down to -25 °C), c fire resistance (transformers exposed to fire risk with low flammability and selfextinguishing in a given time). The following description refers to the process developed by a leading European manufacturer in this field. The encapsulation of a winding uses three components: c epoxy-resin based on biphenol A with a viscosity that ensures complete impregnation of the windings, c anhydride hardener modified to introduce a degree of resilience in the moulding, essential to avoid the development of cracks during the temperature cycles occurring in normal operation, c pulverulent additive composed of trihydrated alumina Al (OH)3 and silica which enhances its mechanical and thermal properties, as well as giving exceptional intrinsic qualities to the insulation in the presence of heat. This three-component system of encapsulation gives Class F insulation (∆θ = 100 K) with excellent fire-resisting qualities and immediate self-extinction. These transformers are therefore classified as nonflammable. The mouldings of the windings contain no halogen compounds (chlorine, bromine, etc.) or other compounds capable of producing corrosive or toxic pollutants, thereby guaranteeing a high degree of safety to personnel in emergency situations, notably in the event of a fire. It also performs exceptionally well in hostile industrial atmospheres of dust, humidity, etc. See figure C28. Liquid-filled transformers The most common insulating/cooling liquid used in transformers is mineral oil. Mineral oils are specified in IEC 296. Being flammable, safety measures are obligatory in many countries, especially for indoor substations. The DGPT unit (Detection of Gas, Pressure and Temperature) ensures the protection of oil-filled transformers. In the event of an anomaly, the DGPT causes the HV supply to the transformer to be cut off very rapidly, before the situation becomes dangerous. Mineral oil is bio-degradable and does not contain PCB (polychlorinated biphenyl), which was the reason for banning askerel, i.e. Pyralène, Pyrolio, Pyroline...

On request, mineral oil can be replaced by an alternative insulating liquid, by adapting the transformer, as required, and taking appropriate additional precautions if necessary. The insulating fluid also acts as a cooling medium; it expands as the load and/or the ambient temperature increases, so that all liquid-filled transformers must be designed to accommodate the extra volume of liquid without the pressure in the tank becoming excessive.

fig. C28: dry-type transformer.



C 4.4 choice of HV/LV transformer (continued) There are two ways in which this pressure limitation is commonly achieved: c hermetically-sealed totally-filled tank (up to 10 MVA at the present time) Developed by a leading French manufacturer in 1963, this method was adopted by the national power authority in 1972, and is now in world-wide service. Expansion of the liquid is compensated by the elastic deformation of the oil-cooling passages attached to the tank. The “total-fill” technique has many important advantages over other methods: v oxydation of the dielectric liquid (with atmospheric oxygen) is entirely precluded, v no need for an air-drying device, and so no consequent maintenance (inspection and changing of saturated dessicant), v no need for dielectric-strength test of the liquid for at least 10 years, v simplified protection against internal faults by means of a DGPT device is possible, v simplicity of installation: lighter and lower profile (than tanks with a conservator) and access to the HV and LV terminals is unobstructed, v immediate detection of (even small) oil leaks; water cannot enter the tank. c air-breathing conservator-type tank at atmospheric pressure Expansion of the insulating liquid is taken up by a change in the level of liquid in an expansion (conservator) tank, mounted above the transformer main tank, as shown is figure C30. The space above the liquid in the conservator may be filled with air which is drawn in when the level of liquid falls, and is partially expelled when the level rises. When the air is drawn in from the surrounding atmosphere it is admitted through an oil seal, before passing through a dessicating device (generally containing silica-gel crystals) before entering the conservator. In some designs of larger transformers the space above the oil is occupied by an impermeable air bag so that the insulation liquid is never in contact with the atmosphere. The air enters and exits from the deformable bag through an oil seal and dessicator, as previously described. A conservator expansion tank is obligatory for transformers rated above 10 MVA (which is presently the upper limit for “total-fill” type transformers).


fig. C29: hermetically-sealed totally-filled tank.

fig. C30: air-breathing conservator-type tank at atmosphere pressure.

C choice of technology As discussed above, the choice of transformer is between liquid-filled or drytype. For ratings up to 10 MVA, totally-filled units are available as an alternative to conservator-type transformers. A choice depends on a number of considerations, including: c safety of persons in proximity to the transformer. Local regulations and official recommendations may have to be respected, c economic considerations, taking account of the relative advantages of each technique. Regulations affecting the choice c dry-type transformer: v in some countries a dry-type transformer is obligatory in high apartment blocks, v dry-type transformers impose no constraints in other situations, c transformers with liquid insulation: v this type of transformer is generally forbidden in high apartment blocks, v for different kinds of insulation liquids, installation restrictions, or minimum protectagainst fire risk, vary according to the class of insulation used, c some countries in which the use of liquid dielectrics is highly developed, classify the several categories of liquid according to their fire performance. This latter is assessed according to two criteria: the flash-point temperature, and the minimum calorific power. The principal categories are shown in Table C31 in which a classification code is used for convenience. code O1 K1 K2 K3 L3

dielectric fluid mineral oil high-density hydrocarbons esters silicones insulating halogen liquids

flash-point (°C) < 300 > 300 > 300 > 300 -

minimum calorific power (MJ/kg) 48 34 - 37 27 - 28 12

table C31: categories of dielectric fluids. National standards exist which define the conditions for the installation of liquid-filled transformers. No equivalent IEC standard has yet been established. The national standard is aimed at ensuring the safety of persons and property and recommends, notably, the minimum measures to be taken against the risk of fire. The main precautions to observe are indicated in Table C32. c for liquid dielectrics of class L3 there are no special measures to be taken, c for dielectrics of classes 01 and K1 the measures indicated are applicable only if there are more than 25 litres of dielectric liquid in the transformer, c for dielectrics of classes K2 and K3 the measures indicated are applicable only if there are more than 50 litres of dielectric liquid in the transformer.



C 4.4 choice of HV/LV transformer (continued) class of dielectric fluid

O1 K1

K2 K3 L3

no. of litres above which measures must be taken 25

locations chamber or enclosed area reserved to qualified and authorized personnel, and separated from any other building by a distance D D>8m 4m
50

no special measures

no special measures

interposition of a fire-proof screen (1 hour rating)

fire-proof wall (2 hour rating) against adjoining building interposition of a fire-proof screen (1 hour rating)

reserved to trained personnel and isolated from work areas by fire-proof walls (2 hours rating) no openings with opening(s) measures (1 + 2) or 3 or 4

measures (1 + 2 + 5) measures or 3 (1A + 2 + 4)(c) or (4 + 5) or 3

no special measures

measures 1A or 3 or 4

no special measures

table C32: safety measures recommended in electrical installations using dielectric liquids of classes 01, K1, K2 or K3. Measure 1: arrangements such that if the dielectric escapes from the transformer, it will be completely contained (in a sump, by sills around the transformer, and by blocking of cable trenches, ducts and so on, during construction). Measure 1A: in addition to measure 1, arrange that, in the event of liquid ignition there is no possibility of the fire spreading (any combustible material must be moved to a distance of at least 4 metres from the transformer, or at least 2 metres from it if a fire-proof screen [of 1 hour rating] is interposed). Measure 2: arrange that burning liquid will extinguish rapidly and naturally (by providing a pebble bed in the containment sump). Measure 3: an automatic device (DGPT or Buchholz) for cutting off the primary power supply, and giving an alarm, if gas appears in the transformer tank. Measure 4: automatic fire-detection devices in close proximity to the transformer, for cutting off primary power supply, and giving an alarm. Measure 5: automatic closure by fire-proof panels (1/2 hour minimum rating) of all openings (ventilation louvres, etc.) in the walls and ceiling of the substation chamber. Notes: (a) a fire-proof door (rated at 2 hours) is not considered to be an opening. (b) - transformer chamber adjoining a workshop and separated from it by walls, the fire-proof characteristics of which are not rated for 2 hours, - areas situated in the middle of workshops the material being placed (or not) in a protective container. (c) it is indispensable that the equipment be enclosed in a chamber, the walls of which are solid, the only orifices being those necessary for ventilation purposes.


other chambers or locations (b)

measures 1 or 3 or 4

C the determination of optimal power Oversizing a transformer results in: c excessive investment and unecessarily high no-load losses, but, c lower on-load losses. Undersizing a transformer, causes: c a reduced efficiency when fully loaded, (the highest efficiency is attained in the range 50% - 70% full load) so that the optimum loading is not achieved, c on long-term overload, serious consequences for: v the transformer, owing to the premature ageing of the windings insulation, and in extreme cases, resulting in failure of insulation and loss of the transformer, v the installation, if overheating of the transformer causes protective relays to trip the controlling circuit breaker.

the chamber is ventilated by natural convection or forced ventilation.

Definition of optimal power In order to select an optimal power (kVA) rating for a transformer, the following factors must be taken into account: c list the power of installed power-consuming equipment as described in Chapter B, c decide the utilization (or demand) factor for each individual item of load, c determine the load cycle of the installation, noting the duration of loads and overloads, c arrange for power-factor correction, if justified, in order to: v reduce cost penalties in tariffs based, in part, on maximum kVA demand, v reduce the value of declared load (P(kVA) = P (kW)/cos ø), c select, among the range of standard transformer ratings available, taking into account all possible future extensions to the installation. It is important to ensure that cooling arrangements for the transformer are adequate.

ventilation orifices In the general case of cooling by natural air circulation (AN) the ventilation of the chamber is arranged to remove the heat (produced by losses in the transformer) by natural convection. A good system of ventilation allows cool air to enter through an orifice of sectional area S at floor level, and to leave the chamber through an orifice of sectional area S' on the opposite wall to that of the air entry and at a height H above the incoming-air orifice, as shown in figure C33. It is important to note that any restriction to the free flow of a sufficient volume of air will result in a reduction of power available from the transformer, if the rated temperature limit is not to be exceeded. Natural ventilation The formulae for calculating the sectional area of the ventilation orifices are as follows: S = 0.18 P/√H and S’ = 1.1 S Where: P = the sum of the no-load losses and the full-load losses expressed in kW S = the sectional area of the incoming-air orifice (area of louvres or grill to be deducted) expressed in mm2 S’ = the sectional area of the outgoing-air orifice (area of louvres or grill to be deducted) expressed in mm2 H = height (centre to centre) of the outgoing air orifice above the incoming-air orifice, expressed in metres. The formulae are valid for a mean ambient temperature of 20 °C and up to an altitude of 1,000 m. Forced ventilation Forced (i.e. electric-fan assisted) ventilation of the chamber is necessary for ambient temperatures exceeding 20 °C, or if the chamber is badly ventilated; frequent overloading of the transformer, and so on... The fan can be controlled by thermostat. Recommended air-flow rate, in cubic metres per second at 20 °C: c totally-filled transformer: 0.081 P, c dry-type Class F transformer: 0.05 P where P = total losses in kW.

S'

H

S

fig. C33: natural ventilation.


5. a consumer substation with HV metering

C 5.1 general A consumer substation with HV metering is an electrical installation connected to a public supply system at a nominal voltage of 1 kV 35 kV and generally includes a single HV/LV transformer which exceeds 1,250 kVA, or several smaller transformers. The rated current of the HV switchgear does not normally exceed 400 A.

functions The substation According to the complexity of the installation and the manner in which the load is divided, the substation: c might include one room containing the HV switchboard and metering panel(s), together with the transformer(s) and low-voltage main distribution board(s), c or might supply one or more transformer rooms, which include local LV distribution boards, supplied at HV from switchgear in a main substation, similar to that described above. These substations may be installed, either: c inside a building, or c outdoors in prefabricated housings. Connection to the HV network Connection at HV can be: c either by a single service cable or overhead line, or c via two mechanically interlocked load-break switches with two service cables from duplicate supply feeders, or c via two load-break switches of a ring-main unit. Metering Before the installation project begins, the agreement of the power-supply authority regarding metering arrangements must be obtained. A metering panel will be incorporated in the HV switchboard. Voltage transformers and current transformers, having the necessary metering accuracy, may be included in the main incoming circuit breaker panel or (in the case of the voltage transformer) may be installed separately in the metering panel.

one-line diagrams The diagrams shown in figure C34 represent: c the different methods of HV service connection, which may be one of four types: v single-circuit service, v single-circuit (for later change to ring-main service), v duplicate service (interlocked mechanically), v ring-main service, c general protection at HV, and HV metering functions, c protection of outgoing HV circuits, c protection of LV distribution circuits.


Transformer rooms If the installation includes a number of transformer rooms, HV supplies from the main substation may be by simple radial feeders connected directly to the transformers, or by duplicate feeders to each room, or again, by a ring-main, according to the degree of supply security desired. In the two latter cases, 3-panel ring-main units will be required at each transformer room. Local emergency generators Emergency standby generators are intended to maintain a power supply to essential loads, in the event of failure of the power supply system. Capacitors Capacitors will be installed, according to requirements: c in stepped HV banks at the main substation, or c at LV in transformer rooms. Transformers For additional supply-security reasons, transformers may be arranged for automatic changeover operation, or for parallel operation.

C power supply system

service connection

HV protection and metering

HV distribution and protection of outgoing circuits downstream terminals of HV isolator for the installation

supplier/consumer interface

LV distribution and protection

LV terminals of transformer

single-line service

protection LV

I nominal of transformer u 45 A single-line service (equipped for extension to form a ring main)

a single transformer

HV

LV

automatic LV/HV standby source

duplicatesupply service protection + automatic changeover feature

protection

ring-main service automatic LV standby source

authorized access limits

consumer testing authority power-supply authority

fig. C34: consumer substation with HV metering.


5. a consumer substation with HV metering (continued)

C 5.2 choice of panels A substation with HV metering includes, in addition to the panels described in 4.2, panels specifically designed for metering and, if required, for automatic or manual changeover from one source to another.

metering and general protection These two functions are achieved by the association of two panels: c one panel containing the VT, c the main HV circuit breaker panel, containing the CTs for measurement and protection. The general protection is usually against overcurrent (overload and short-circuit) and earth faults. Both schemes use protective relays which are sealed by the power-supply authority.

metering VT panel

main HV CB panel with metering and protection CTs

HV distribution panels

power-supply network

fig. C35: typical arrangement of switchgear panels for HV metering.

power-supply changeover schemes Some national standards recommend a supplementary protection when an installation includes an emergency automatic changeover to a local generator. The standard states that operation of the standby plant must not, in any circumstances, result in perturbations on the power-supply network. This means that, apart from protective devices intended to protect the generator: c either a scheme of interlocking must preclude any possibility of parallel operation of the generator with the power system, or c a suitable automatic de-coupling scheme agreed with the power-supply authority, which will trip the paralleling circuit breaker in the event of a short-circuit, or other anomaly, occurring on the power supply system, or on the installation. HV distribution panels for which standby supply is required

automatic changeover panel

In the second case, the tripping command to the decoupling circuit breaker must operate reliably on undervoltage and reverse-power protection. Setting of protection relays is carried out by the power-supply authority and made inaccessible to the consumer by sealing, or some equivalent means. The tripping supply and switchgear-control switch(es) must also be inaccessible to the consumer.

busbar transition panel to remainder of the HV switchboard

from standby generator P i 20,000 kVA

fig. C36: section of HV switchboard including standby supply panel. C46 - HV/LV distribution substations

C small generators operating in parallel with public supply networks The following notes indicate some basic considerations to be taken into account when parallel operation of consumer’s generators with the public power-supply networks is planned. A voltage regulator controlling an alternator is generally arranged to respond to a reduction of voltage at its terminals by automatically increasing the excitation current of the alternator, until the voltage is restored to normal. When it is intended that the alternator should operate in parallel with others, the AVR (Automatic Voltage Regulator) is switched to “parallel operation” in which the AVR control circuit is slightly modified (compounded) to ensure satisfactory sharing of kvars with the other parallel machines. When a number of alternators are operating in parallel under AVR control, an increase in the excitation current of one of them (for example, carried out manually after switching its AVR to Manual control) will have practically no effect on the voltage level. In fact, the alternator in question will simply operate at a lower power factor (more kVA, and therefore more current) than before. The power factor of all the other machines will automatically improve, such that the load power factor requirements are satisfied, as before. Consider now the case of a standby generator at a consumer’s substation, which is operating in parallel with all the generation of the public power supply system. Supposing the power system voltage is reduced for operational reasons (it is common to operate HV systems within a range of ± 5% of nominal voltage, or even more, where load-flow patterns require it). An AVR set to maintain the voltage within ± 3% (for example) will immediately attempt to raise the voltage by increasing the excitation current of the alternator. Instead of raising the voltage, the alternator will simply operate at a lower power factor than before, thereby increasing its current output, and will continue to do so, until it is eventually tripped out by its overcurrent protective relays. This is a well-known problem and is usually overcome by the provision of a “constantpower-factor” control switch on the AVR unit. By making this selection, the AVR will automatically adjust the excitation current to match whatever voltage exists on the power system, while at the same time maintaining the power factor of the alternator constant at the pre-set value (selected on the AVR control unit). In the event that the alternator becomes decoupled from the power-system, the AVR must be automatically (rapidly) switched back to “constant-voltage” control.

Note: The problem is essentially that of a “small” generator and a “large” system. The terms “small” and “large” are relative, so that, for example, even a small generator at the end of a long line could probably operate satisfactorily on constant-voltage control. To be more specific, if the system impedance viewed from the generator location is high (i.e. the short-circuit fault level is low) then constant-voltage control may be satisfactory. In highly-developed networks where the short-circuit fault levels are high, then constant-power-factor operation is generally obligatory. A technical discussion with the power-supply authority will be necessary to resolve the question, but it is recommended that facilities for both kinds of control be specified when purchasing the generator sets.


5. a consumer substation with HV metering (continued)

C 5.3 parallel operation of transformers The need for operation of two or more transformers in parallel often arises due to: c load growth, which exceeds the capactiy of an existing transformer, c lack of space (height) for one large transformer, c a measure of security (the probability of two transformers failing at the same time is very small), c the adoption of a standard size of transformer throughout an installation.

total power (kVA) The total power (kVA) available when two or more transformers of the same kVA rating are connected in parallel, is equal to the sum of the individual ratings, providing that the percentage impedances are all equal and the voltage ratios are identical. Transformers of unequal kVA ratings will share a load practically (but not exactly) in proportion to their ratings, providing that the voltage ratios are identical

and the per-centage impedances (at their own kVA rating) are identical, or very nearly so. In these cases, a total of more than 90% of the sum of the two ratings is normally available. It is recommended that transformers, the kVA ratings of which differ by more than 2: 1, should not be operated permanently in parallel.

conditions necessary for parallel operation All paralleled units must be supplied from the same network. The inevitable circulating currents exchanged between the secondary circuits of paralleled transformers will be negligibly small providing that: c secondary cabling from the transformers to the point of paralleling have approximately equal lengths and characteristics, c the transformer manufacturer is fully informed of the duty intended for the transformers, so that:

v the winding configurations (star, delta, zigzag star) of the several transformers have the same phase change between primary and secondary voltages, v the short-circuit per-centage impedances are equal, or differ by less than 10%, v voltage differences between corresponding phases must not exceed 0.4%, v all possible information on the conditions of use, expected load cycles, etc. should be given to the manufacturer with a view to optimizing load and no-load losses.

common winding arrangements

voltage vectors 1

As described in 4.4 “Electrical characteristics winding configurations” the relationships between primary, secondary, and tertiary windings depend on: c type of windings (delta, star, zigzag), c connection of the phase windings. Depending on which ends of the windings form the star point (for example), a star winding will produce voltages which are 180° displaced with respect to those produced if the opposite ends had been joined to form the star point. Similar 180° changes occur in the two possible ways of connecting phaseto-phase coils to form delta windings, while four different combinations of zigzag connections are possible, c the phase displacement of the secondary phase voltages with respect to the corresponding primary phase voltages. As previously noted, this displacement (if not zero) will always be a multiple of 30° and will depend on the two factors mentioned above, viz type of windings and connection (i.e. polarity) of the phase windings. By far the most common type of distribution transformer winding configuration is the Dyn 11 connection.

1

V12 2

N 3

2 3

1

1

N

2

2 windings correspondence

3

V12 on the primary winding produces V1N in the secondary winding and so on ...

fig. C37: phase change through a Dyn 11 transformer. C48 - HV/LV distribution substations

3

6. constitution of HV/LV distribution substations

C HV/LV substations are constructed according to the magnitude of the load and the kind of power system in question. Substations may be built in public places, such as parks, residential districts, etc. or on private premises, in which case the power supply authority must have unrestricted access. This is normally assured by locating the substation, such that one of its walls, which includes an access door, coincides with the boundary of the consumers premises and the public way.

6.1 different types of substation Substations may be classified according to metering arrangements (HV or LV) and type of supply (O/H ou U/G). The substations may be installed: c either indoors in chambers specially built for the purpose, or incorporated in an apartment block, etc., or c an outdoor installation mounted on a pole, or poles, (“H” structure or 4-pole arrangement) or in a brick-built, concrete or prefabricated housing. Prefabricated housings mounted on a concrete base provide a particularly simple, rapid and competitive choice.

6.2 indoor substations equipped with metal-enclosed switchgear conception Figure C38 shows a typical equipment layout recommended for a LV metering substation. Remark: the use of a cast-resin dry-type transformer would obviate the need for a fireprotection oil sump. HV connections to transformer (included in a panel or free-standing)

LV connections from transformer

LV switchgear

2 incoming HV panels

HV switching and protection panel

current transformers provided by power-supply authority

connection to the power-supply network by single-core or three-core cables, with or without a cable trench

transformer

oil sump

LV cable trench

fig. C38: typical arrangment of switchgear panels for LV metering.


6. constitution of HV/LV distribution substations (continued)

C 6.2 indoor substations equipped with metal-enclosed switchgear (continued) service connections and equipment interconnections At high voltage c connections to the HV system are made by, and are the responsibility of the power-supply authority, c connections between the HV switchgear and the transformers may be: v by short copper bars where the transformer is housed in a panel forming part of the HV switchboard, v by single-core unarmoured cables with synthetic insulation, v single-core unarmoured cables to 250 A (or more) plug-in type terminals at the transformer. At low voltage c connections between the LV terminals of the transformer and the LV switchgear may be: v single-core unarmoured cables, v solid copper bars (circular or rectangular section) with heat-shrinkable insulation. Metering c metering current transformers are generally installed in the protective cover of the power transformer LV terminals, the cover being sealed by the supply authority, c alternatively, the current transformers are installed in a sealed compartment within the main LV distribution cabinet, c the meters are mounted on a panel which is completely free from vibrations, c placed as close to the current transformers as possible, and c are accessible only to the power-supply authority. The dials and graduations of the meters should be at a height of approximately 1.65 m. above floor level, not lower than 0.7 m, and not higher than 1.8 m.

100

common earth busbar for the substation

800 mini safety accessories

fig. C39: plan view of typical substation with LV metering.


meters

C earthing circuits The substation must include: c an earth electrode for all exposed conductive parts of electrical equipment in the substation and exposed extraneous metal including: v protective metal screens, v reinforcing rods in the concrete base of the substation, v the common point of all current-transformer secondary windings. Note: Metal doors and ventilation louvres are not connected to earth. c an earth electrode for the LV neutral point of the transformer*, c removable links at strategic points for measuring continuity and the resistances of individual electrodes, c an earth electrode for the installation*. * in small areas, the resistance zones of earth electrodes overlap. In such cases all electrodes are interconnected to form a common earthing system for HV and LV equipments, as discussed in “Earthing connections” in Sub-clause 1.1 of this Chapter.

substation lighting Supply to the lighting circuits can be taken from a point upstream or downstream of the main incoming LV circuit breaker. In either case, appropriate overcurrent protection must be provided. A separate automatic circuit (or circuits) is (are) recommended for emergency lighting purposes. Operating switches, pushbuttons, etc. are normally located immediately adjacent to entrances. Lighting fittings are arranged such that: c switchgear operating handles and positionindication markings are adequately illuminated, c all metering dials and instruction plaques and so on, can be easily read.

materials for operation and safety The substation must be provided with: c materials for assuring safe exploitation of the equipment including: v a wooden stool and/or an insulating mat (rubber or synthetic), v a pair of insulated gloves stored in an envelope provided for the purpose, v a voltage-detecting device for use on the HV equipment, v earthing attachments (according to type of switchgear), c fire-extinguishing devices of the powder or CO2 type, c warning signs, notices and safety alarms: v on the external face of all access doors, a DANGER warning plaque and prohibition of entry notice, together with instructions for first-aid care for victims of electrical accidents, v inside the substation: a first-aid panel as noted above, v a DANGER plaque (skull and cross-bones, or a local equivalent sign) on each removable panel providing access to live parts.



C 6.3 outdoor substations pole-top public distribution substations

lightning arresters

Field of application These substations are mainly used to supply isolated rural consumers from HV overhead line distribution systems: c at voltage levels between 1 - 24 kV, c from a single transformer not exceeding 160 kVA and at a preferred LV voltage level of 230/400 V (3-phase 4-wires), c with low-voltage metering. Constitution These substations are commonly supplied by a single 3-wire line, with no local switchgear or fuses at the HV side of the transformer. Lightning arresters are provided, however, to protect the transformer and consumers as shown in figure C40. Protection of the LV circuit is generally provided by two LV circuit breakers (D1) and (D2), shown in figure C41: c circuit breaker D1 protects the transformer against overloading and the LV service connection against short-circuit faults. This circuit breaker is mounted on the pole and has inverse-time/current-relay tripping characteristics, or may be tripped by a thermal-image relay monitoring the transformer-windings temperature, c circuit breaker D2 is the main LV circuit breaker for the installation. Tripping discrimination between these two circuit breakers must be established, the settings to be made by the power-supply authority, and sealed.

LV circuit breaker D1 earthing conductor 25 mm2 copper

protective conductor cover safety earth mat

fig. C40: pole-mounted transformer substation.

installation main circuit breaker

LV circuit breaker at the transformer D1

D2 metering

remote pole-mounted load-break/ isolating switch

pole-mounted transformer lightning arresters

fig. C41: diagram showing the principles of a pole-mounted transformer substation. General arrangement of equipment As previously noted the location of the substation must allow easy access, not only for personnel but for equipment handling (raising the transformer, for example) and the manœuvring of heavy vehicles. Earthing electrodes are commonly separated as discussed in Sub-clause 1.1 of this Chapter. See figure C42.

installation

Rp

RB

RA

Rp must have a maximum value derived in the same way as that shown for Rs of case "E" in figure C7. RB < 3 ohms to limit the voltage appearing at the consumer's installation in the event of a breakdown of HV/LV insulation due to back-flashover, or other causes.

fig. C42: separated earth electrodes.


C prefabricated housings for outdoor substations For more elaborate substations requiring the use of ring-main units or a switchboard of several circuit breakers, compact weatherproof and vermin-proof housings are commonly used. These prefabricated units require the minimum civil work, being mounted on a simple concrete base, and are used for both urban and rural substations. Among the advantages offered by these units, are: c an optimization of materials and safety by: v an appropriate choice from a wide range of available housings, v conformity with all existing and foreseeable international standards, c a reduction in study and design time, and in the cost of implementation, by: v minimal co-ordination between the several disciplines of building construction and site works, v realization, independent of the main building construction, v obviating the need for a temporary “hookup” at the beginning of the site preparation work, v simplification of civil work, which consists only of the provision of a reinforced-concrete plinth, c greatly simplified equipment installation and connection.

fig. C43: cut-away view of typical HV/LV substation using a prefabricated housing.



C 6.3 outdoor substations (continued) Other kinds of outdoor substation are common in some countries, based on weatherproof equipment exposed to the elements. These comprise a fenced area in which three or more concrete plinths are installed: c for a ring-main unit, or one or more switchfuse or circuit breaker unit(s), c for one or more transformer(s), and c for one or more LV distribution pillar(s). The simplicity of this arrangement is countered by the high cost of weatherproof switchgear, cable boxes, etc. and by an adverse visual impact. This class of substation is not favoured in residential areas or in other locations where visual amenities are important, and for these reasons have been largely supplanted by prefabricated housings and indoor-type equipment in many countries.


7. example in coordination of the characteristics of an HV switch-fuse combination protecting an HV/LV transformer

C This appendix is based on Appendices A and B of IEC 420, and is intended to clarify some of the operational features of these combination units. The following example is based on a 400 kVA, 11 kV/LV transformer with a maximum fault level at its HV terminals of 16 kA. c the full-load current is 21 A, c the permissible periodic overload is 150% F.L., c the off-circuit tapping switch is selected to the - 5% tap, so that the primary current in the overload condition is: 21 x 1.5 x 1.05 = 33 A, c the magnetizing in-rush current surge is: 21 x 12 = 252 A maximum for a duration of 0.1 second (Clause 4a of IEC 787), c the ambient-air temperature at site is 45 °C, i.e. 5 °C higher than the IEC standard. The user has selected a 12 kV switch-fuse combination from a certain manufacturer to protect the transformer. The manufacturer will provide a list of fuses which are suitable for use in the combination, and will recommend those fuses necessary for this particular application. The choice of the manufacturer will be based on factory type-tests to the appropriate IEC specifications covering this class of HV switch-fuse combination. Suppose the manufacturer recommends a 12 kV, 40 A, 16 kA (at least) fuse of a given type from a certain fuse manufacturer. To justify this choice, the manufacturer will have ascertained that: 1. The fuse can withstand the 252 A of inrush current for 0.1 second, without any modification to its subsequent performance. This will be done by reference to the timecurrent characteristic of the fuse, and/or in consultation with the fuse manufacturer. 2. The normal current rating of the combination when using the recommended fuses is adequate to carry 33 A periodically in an ambient-air temperature of 45 °C, i.e. it matches the transformer overload capability. Note: the normal current rating of the combination when fitted with the recommended 40 A fuses may, in fact, be less than 40 A, particularly in the abovestandard ambient temperature conditions. Temperature-rise tests carried out by the switch-fuse manufacturer, or calculations based on such tests, may indicate a normal current rating of (for example) 35 A at 45 °C. Such a rating is evidently satisfactory for this application.

3. The pre-arcing current in the fuse is low enough in the 10-second region of the fuse time-current characteristic, to ensure satisfactory protection of the transformer (Clause 4c of IEC 787). This information will be obtained by the switch-fuse manufacturer, from the fuse characteristic curves, and/or in consultation with the fuse manufacturer. 4. The fuses alone will clear a solid 3-phase short-circuit fault at the LV terminals of the transformer, i.e. the maximum short-circuit primary currrent (based on 5% transformer reactance) is greater than the transfer current (current at which the switch operates concurrently with the fuse(s))* when the combination includes the recommended 40 A fuses. Figure AC1-1 shows that the transfer current in this case is 280 A. 5. The transfer current of the combination when fitted with the 40 A fuses concerned is less than its rated transfer current, assumed in this case to be 1,000 A. * transfer current is defined below.

The installation designers must check that the fuse discriminates with the highest rating of a LV fuse (if existing) in the event of a phaseto-phase fault on the LV system. This is generally the worst condition for discrimination, since the LV fuses pass 0.87 Isc3 while one HV fuse (only) passes Isc3, as shown in figure AC1-2 (b). To ensure discrimination in this case, the timecurrent characteristics of the HV and LV fuses should intersect at a current value which is greater than that of the maximum possible short-circuit current on the LV system, as shown in figure AC1-3. Discrimination between HV fuses and LV circuit breakers has been covered in Chapter C Sub-clause 3.2 (figure C21) and Chapter H2 Sub-clause 4.6 (figures H2-56 and H2-57).

Appendix C1 - 1

C 7.1 transfer current and take-over current transfer current

Figure AC1-1 is intended only to show the basic principles involved, and takes no account of maximum and minimum tolerances in the fuse pre-arcing curves, etc. For greater detail, reference should be made to IEC 420.

The transfer current of a combination depends on both the fuse-initiated (striker) opening time of the switch, and the timecurrent characteristic of the fuse. Near the transfer-current level, during a LV 3-phase short-circuit (at the transformer terminals), the fastest fuse to melt clears one pole and operates its striker pin. The two remaining poles are then passing a reduced current (87%), which will be interrupted by the switch or by the fuses. The transfer point is that at which the switch opens and one or both remaining fuses melt simultaneously. For this to occur, the second fuse must melt at the instant of switch opening (by striker action of the first fuse to operate). By calculation (demonstrated in Appendix B of IEC 420) it is shown that the level of 3-phase fault current* which will cause the second fuse to melt at a time (equal to the switch-opening time) after the operation of the first fuse, is that corresponding to a period of 0.045 seconds after fault initiation, as shown on the time-current fuse-characteristic curve.

430 A

10.7 kA

430 A 10.7 kA 10.7 kA

430 A (a) 3 - phase S.C. 430 A 215 A

9.3 kA 215 A (b) phase-to-phase S.C. 0

10.7 kA

248 A

* this is the transfer-current value. Its reduction to 87%, following operation of the first fuse, is taken into account when calculating the operating time of the second fuse.

248 A

take-over current

(c) 1 - phase-to-earth S.C.

The take-over current of the combination is the level of overcurrent at which the fuses take over the duty of protection from overload relays, i.e. from the take-over current level up to and beyond the transfer current level, the switch will be tripped by striker-pin action. time (secs.) 10 full-load current 50% overload current

fig. AC1-2: short-circuit currents for the transformer of the example, assuming a 242-420 V secondary winding.

not to scale minimum rated breaking current of fuse

take-over current level

1.0 overload relay characteristic

40 A fuse characteristic 0.1

transfer current level

rated transfer current of the combination 0.045 secs. fault current (amps) 0.01 10 21 33 100 252 420 1000 280 faults cleared by fuses only faults cleared overload cleared by (strikers operate but fault is cleared switch only on operation by fuses and switch through striker action before switch contacts open) of overload relay

fig. AC1-1: principles of HV/LV transformer protection by HV switch-fuse combination.

2 - Appendix C1

C 7.2 types of faults involved in the transfer region Primary-side protective devices are particularly concerned with faults in the secondary-terminals zone of the transformer, upstream of the LV protection devices. The primary short-circuit currents arising from solid short-circuits at the transformer secondary terminals are shown in figure AC1-2. The breaking of solid (i.e. non-arcing) 3-phase faults is associated with severe TRV values which the switch in the combination is not designed to interrupt. This type of fault therefore must be cleared by the fuses only, i.e. before the striker-operated switch opens its contacts. It is necessary therefore, that the transfercurrent limit (280 A in the example) shall always be lower than that of a 3-phase LV-terminals short-circuit (430 A at HV), as shown in figure AC1-1. This condition being fulfilled, then 3-phase short-circuit transfer currents correspond to faults for which LV arc impedance reduces the magnitude of both the current and TRV values, as well as improving the power factor of the fault current. For a phase-to-phase LV terminal fault, the diagram (b) of figure AC1-2 shows that the HV fault current of one phase is equal to that of a 3-phase LV short-circuit. The fuse of that phase will clear rapidly; the current in the two remaining phases will then reduce to practically zero*, and be cleared in the transfer-current region by two of the switch contacts acting in series. This feature improves the switch performance for breaking current which is (in the present case) mainly transformer-magnetizing current. For a phase-to-earth LV terminal fault, diagram (c) of figure AC1-2 shows that the HV fault current is less than the calculated value (280 A) for the transfer-current limit. As in the previous case, following the operation of one fuse (assuming that both fuses do not clear simultaneously), the fault current will reduce to a very low value. The switch contacts breaking this low-valued (but highly-inductive) current, are acting in series, with the advantage of enhanced switchgear performance noted above. time (seconds) minimum time-current characteristic of HV fuse maximum fault current at LV (referred to HV side) maximum operating time of LV fuse (referred to HV side)

430 A

HV fault current (amps)

fig. AC1-3: discrimination between HV and LV fuses. * because the LV voltages induced in the faulted phases will then be sensibly equal in magnitude, but will be in direct phase opposition around the fault-current loop.

Appendix C1 - 3

8. ground-surface potential gradients due to earth-fault currents

C When earth-fault current flows between an earthing electrode and the surrounding soil, potential gradients exist in the soil and on the ground surface. Close to the location of the buried electrode, the potential gradients, both in the soil and on the ground surface, are generally at their maximum values and are therefore (for the ground-surface gradients) the most dangerous.

0.9 0.8

ground surface 0.7 0.6 0.5

0.4

nature of the potential gradients A vertical-rod electrode is very commonly used, either singly, or in interconnected groups; a single-rod electrode is taken as an example in the following notes. The current flow and associated potential contours associated with a rod electrode are shown in figure AC2-1, and are based on the following simplifying assumptions: c perfectly homogeneous soil, c the origin of the fault current (i.e. a shortcircuit to earth) is at an infinite distance from the electrode, thereby creating a perfectly symmetrical current-flow between the electrode and the surrounding soil. Earth rod at 1.0 pu (per unit) volts with respect to remote-earth.

current-flow lines equipotential contours

fig. AC2-1: idealized current-flow pattern and associated equipotential contours of a single-rod vertically-driven earth electrode. Note: current-flow lines are identical to the lines of the electric field.

"remote-earth" and "local-earth" zero-potential references In classical theory "remote-earth" is an earthreference point at zero potential, located at an infinite distance from the electrode concerned. The following notes show that, where a current circulates between two electrodes, a local zero-potential-earth reference is available, which can be used in practical electrode-resistance tests. The resulting measured values correspond closely to those calculated by the classical theory.

When two electrodes exchange fault current, i.e. the power-source electrode and the electrode of an installation at which an earth fault occurs, there exists (at some point between the two electrodes) an equipotential vertical-plane surface of infinite area, perpendicular to the flow lines of the fault current, as shown in figure AC2-2.

typical current-flow electrode B lines

electrode A

+0.3 +0.2 +0.1

-0.1

-0.2

-0.3

+0.5 +0.4

(a) plan view of ground-surface equipotential contours

-0.4 -0.5

0 "local-earth" zero-voltage equipotential reference plane

ground-surface A + 1.0 pu

0

-1.0 pu

B

typical current-flow

+ 0.6 + 0.5 + 0.4 + 0.3 + 0.2 + 0.1

positive potential region

+ 1.0

vp

- 0.6 lines - 0.5 - 0.4 - 0.3 - 0.2 - 0.1

negative potential region vp

A

zero

(b) equipotential surfaces in the earth

(c) voltage profile not to scale

B

RI

-1

VI

VI = (V - vp) V in zone A

zero A

O

(d) measured resistance for different locations of electrode P

Vg

VI = (V + vp) in zone B B

distance from A of test-electrode P

Vg = voltage of test-instrument generator RI = resistance in ohms indicated by an electrode-resistance measuring instrument VI = voltage applied to test instrument V = voltage at electrode A with respect to local-earth reference Vp = voltage at probe P with respect to local-earth reference For an accurate measurement of resistance of electrode A, test-electrode P must be located at 0, the position of which is not known exactly (see text for fig. F52 Chapter F).

fig. AC2-2: zero-voltage local-earth reference for two electrodes. Appendix C2 - 1

C The vertical plane surface is the locus at which the strength of the positive* electric field from one electrode (A) is exactly equal to the strength of the negative field of the other electrode (B). This means that the resultant polarity at the plane is neither positive nor negative. It therefore constitutes an equipotential surface at zero volts with respect to the two electrodes. When "local-earth" is mentioned in these notes, it refers to a point on the ground at which the edge of this plane surface is located. From the foregoing description, it can be seen that the zero-voltage equipotential surface is also the boundary of the two "zones of influence" of the two electrodes, sometimes also referred to as "zones of resistance". As a matter of interest, the electric fields of the two electrodes (X) and (C) for the electrode-resistance test, shown in figure F52 (Chapter F) are essentially similar to those mentioned above. The location of local-earth in each of the diagrams of F52 (and of diagram (d) in AC2-2) is indicated by 0 (i.e. zero). * Since a.c. systems are being considered, each electrode will change its polarity at every half cycle.

potential gradients of a vertical-rod electrode It will be seen in figure AC2-1 that the high current density at the (pointed) tip of the rod results in steep potential gradients in the soil beneath the buried extremity of the electrode, as indicated by the close proximity of adjacent equipotential contours in that region. It can also be seen that the gradient at the ground surface is less severe than those below the surface, and that the maximum gradient at the surface occurs immediately adjacent to the point at which the rod emerges from the soil, as shown in figure AC2-3. Vpu

0.8 m step length ground surface

maximum gradient at the rod-soil interface 1.0 pu

0.5 0.32 pu step voltage local-earth datum (zero volts) 1

0.5 rod 0.5 position

1

metres

fig. AC2-3: voltage profile of the single-rod electrode of figure AC2-1. Note: the equipotential contours on the surface of the ground, when viewed from above, are concentric circles around the rod location (in the idealized conditions assumed).

2 - Appendix C2

Investigations have shown that, in a homogeneous soil, between 0.5 pu and 0.8 pu of the voltage at the rod is measured between the rod and a point on the ground at 1 metre from it (i.e. approximately the length of a step) for very long-rod and very short-rod electrodes, respectively. It is clear therefore, that, where earth-fault currents are high, the area close to such an electrode will be dangerous. Measures taken to reduce such dangers are described later in this appendix.

reason for potential gradients The resistance to a flow of current in a conducting medium is given in ohms by the ρl formula R = where a ρ is the resistivity coefficient of the medium, expressed in ohm-metres, l is the length in metres of the conducting path (in the direction of the current-flow lines), a is the cross-sectional area in square metres through which the current flows. So that, for a given distance (l) along, the current-flow lines R ∝ 1 a The soil in contact with the rod has an area (a) equal to that of the rod surface, while at points progressively further from the rod, the current passes through increasingly-large areas. For equal lengths of current-flow path, therefore, the resistance becomes progressively lower and lower, and the voltage drops across equally-spaced intervals become less and less, so that the voltage gradients are smaller and smaller with distance from the rod, as shown in figure AC2-3. Another way of considering the formation of potential gradients is shown in figure AC2-1, in which the equipotential contours are shown at 0.1 pu voltage intervals. Since the same current is passing through all the equipotential surfaces, the resistance of the mass of soil between any two adjacent equipotential surfaces will have the same value. This means that, as the areas of successive surfaces are larger, the lengths of the current paths between successive surfaces must increase for R to remain constant (reminder R = ρl/a). The spacing between successive surfaces must therefore be greater (i.e. l/a must be constant) and this results in progressively lower potential gradients.

C voltage gradients associated with earthing grids The purpose of an earthing grid (or mat) is to provide a close approximation to an equipotential condition at the ground surface over a large area, generally that of a switchyard or substation. In practice, potential gradients will always occur when earth-fault currents are flowing, but providing the grid meshes are appropriately dimensioned (i.e. not too large), the permissible maximum values of gradient at the highest anticipated levels of earth-fault current will not be exceeded. During an earth fault, the whole of the earthing grid and all metallic parts connected to it (together with any personal present) may be raised to several hundreds (or thousands) of volts. Potential gradients in the grid meshes will have the general form shown in figure AC2-4. This figure also shows that connecting a metallic boundary fence to the earthing grid can be dangerous, unless adequate precautions are taken.

(a)

α long rod short rod

β

The resistance of a rod electrode is approximately inversely proportional to its length.

(b) α

gradient α with original soil

β gradient β with special earth fill

reducing potential gradients due to earth faults Some of the methods commonly employed in the reduction of potential gradients include: c reduction of earth-fault-current levels: v by using resistance- or reactance-earthed sources (generators or transformers as appropriate), v where transformers or generators (according to the case) operate permanently in parallel, some of them are left unearthed, c for electrodes generally (figure AC2-5) by: (a) increasing the length and/or number of rods to reduce the electrode resistance and therefore the voltage rise at the electrode, (b) using special low-resistance "soils" in the space surrounding the electrodes, (c) reducing earthing-grid-mesh sizes, and using "grading electrodes" at the grid boundaries. The same "grading-electrode" technique is sometimes used around the base of transmission-line towers, boundary fences, etc. to reduce the severity of the gradients, and therefore that of both "touch" and "step" voltages, (d) insulating driven rods from contact with the earth over the upper section, i.e. from the surface of the ground to a depth of approximately 1 m.

special low-resistance earth fill

(c)

metal fence fence is out of reach

S1 grid voltage with respect to local-earth

S2

S3

with grading β

α zero

without grading

S1, S2 and S3 are strip grading electrodes running parallel to the fence and connected to it at frequent intervals.

(d) without insulation

β

fence

voltage profiles at the ground surface

α

voltage profiles

with insulation

insulation

ground surface

grid voltage with respect to local-earth

step voltage

touch voltage

zero potential

fig. AC2-4: voltage profile and potential gradients of an earthing grid.

Note: the connection at the top of the rod, as well as the connecting lead, must also be insulated. An alternative method is to bury the rod completely with the top of the rod below ground level, and connected to an insulated connecting lead.

fig. AC2-5: voltage profiles and methods of reducing maximum potential gradients in some common earthing arrangements.

Appendix C2 - 3

C other methods of reducing the dangers of ground-surface potential gradients The easiest method (but wasteful in terms of space) is simply to fence off the area around the electrode(s) with warning notices. More commonly, measures are taken to reduce the current passing through a person's feet by providing an insulated floor covering indoors, such as plastic tiles, rubber mats, etc., while for outdoor locations, highly-resistive surfaces, such as crushed rock, thick layers of asphalt, or clean gravel or pebbles are frequently used. Gravel or pebbles provide a very effective high-resistance surface, even when wet, providing the stones are clean. Leaf mould, or mud etc. between the stones greatly reduces the insulating performance of such surfaces.

safe levels of ground-surface potential gradient There is no IEC-recommended safe value of maximum long-duration (> 10 seconds) touch voltage for HV installations at the time of writing (1994), but many authorities have adopted the 50 V AC (or 25 V AC in wet conditions) criteria of IEC 364-4-41. Standards differ in various countries, and in certain cases, long-duration touch-voltage values which are significantly higher than 50 V AC maximum are permitted, while there is an even greater difference above the recommended IEC 364 values for allowable short-duration (e.g. 0.03 to 0.5 seconds) touch voltages. In the present circumstances, therefore, it is necessary to comply with the appropriate local regulations. At the time of writing, Cenelec Technical Committee 112 is preparing a European HV installation standard. Chapter 9 (according to present proposals) of this future standard will include recommendations for safe touchvoltage/time duration levels. It is recognized that, for a given voltage level, a touch-voltage is more dangerous to human beings than a step-voltage (since, in the former case, a substantial part of the current passes through the vital organs in the thorax). If the gradients are such that the touchvoltage criterion is satisfied, then the stepvoltage condition is also considered to be satisfactory.

4 - Appendix C2

9. vector diagram of ferro-resonance at 50 Hz (or 60 Hz)

C Figure C13 of Sub-clause 3.1 Chapter C shows how the neutral of an unearthed 3-phase source can be displaced from its normal near-zero potential, as a result of the particular ferro-resonant condition described. The vector diagram can be constructed as follows. Per-unit notation is used to generalize and simplify the calculations: c 1 pu voltage is the nominal phase-toneutral system voltage, c 1 pu impedance is equal to the normal capacitive reactance of one phase-to-earth at power frequency (i.e. K in figure C14 of Chapter C), c the source impedance is assumed to be negligible. 1 L1 imaginary neutral C3

N 3

E

L2

2

calculation of IN In figure C14 of Chapter C it was stated that, in the resonant condition, XL > XC. In these calculations XC = -j 1 pu impedance, and XL = j 10 pu. 1 90° I1 = = 0.1 0° = 0.1 + j 0 10 90° 1 -30° I2 = = 0.1 -120° = -0.05 - j 0.0866 10 90° 1 210° I3 = = 1 300° = 0.5 - j 0.866 1 -90° IN = 0.55 - j 0.953 = 1.1 -60°

calculation of ZNE ZNE is the parallel combination of XL1, XL2 and Xc. XL1 in parallel with XL2 = j 5 j 5 in parallel with XC3 = j 5 x (-j 1) 5 = = -j 1.25 j5-j1 j4 ZNE = - j 1.25 pu ohms.

calculation of VNE

source

1.1 -60° x 1.25 -90° = 1.375 -150° or 1.375 pu 210° V1

complete vector diagram

not to scale

N V3 VNE

I2

Other values shown in the vector diagram are easily obtained from the above calculations. ➀, ≠ and ➂ are the power-supply terminals. I1

1 VNE = 1.375 pu

V2

I1 + I2

2.066 pu IC3 N

I1 + I2 + I3 = IN

fig. AC3-1: calculation of VNE - circuit and vector diagrams. The procedure is as follows: c compute the current IN in an imaginary neutral of negligible impedance (i.e. ZN = 0) by summating the individual phase currents, as shown in figure AC3-1. According to Thévenin's theorem IN also equals VNE where ZNE + Z VNE is the voltage between N and E when no neutral connection exists, ZNE is the impedance of the network measured between terminals N and E when no neutral connection exists, ZN is the impedance of the neutral conductor (zero in the present case). This means that VNE = IN ZNE

V 3 E = 0.375 pu

1

not to scale 1 2

3 E

2.066 pu IL1

IL2 IL1 + IL2 = 0.375 pu

fig. AC3-2: vector diagram for the resonant condition.

Appendix C3 - 1

1. low-voltage public distribution networks

D 1.1 low-voltage consumers the most-common LV supplies are within the range 120 V single phase to 240/415 V 3-phase 4-wires. Loads up to 250 kVA can be supplied at LV, but power-supply organizations generally propose a HV service at load levels for which their LV networks are marginally adequate. An international voltage standard for 3-phase 4-wire LV systems is recommended by the IEC to be 230/400 V.*

the adjoining table is extracted from the document "World Electricity Supplies", fourth edition.

table D1: survey of electricity supplies in various countries around the world. Bracketed letters relate to the circuit diagrams at the end of the table, while bracketed numbers refer to the notes which follow the diagrams.

Low-voltage consumers are, by definition, those consumers whose loads can be satisfactorily supplied from the low-voltage system in their locality. The voltage of the local LV network may be 120/208 V or 240/415 V, i.e. the lower or upper extremes of the most common 3-phase levels in general use, or at some intermediate level, as shown in table D1. An international voltage standard for 3-phase 4-wire LV systems is recommended by the IEC to be 230/400 V.* country

Loads up to 250 kVA can be supplied at LV, but power-supply organizations generally propose a HV service at load levels for which their LV networks are marginally adequate.

Australia

frequency & tolerance Hz & % 50 ± 0.1

Western Algeria

50 50 ± 1.5

250/440 (A) 127/220 (E) 220 (L) (1)

(9) 220/380 (A) 127/220 (A)

Argentina

50 ± 1.0

225 (L) (1) 220 (L) (1)

225/390 (A) 220/380 (A) 220 (L)

Brazil

60

220 (L) (1) 127 (L) (1)

220/380 (A) 127/220 (A)

Belgium

50 ± 3

220/380 (A) 127/220 (A) 220 (F)

220/380 (A) 127/220 (A) 220 (F)

Bolivia Cambodia

50 ± 1 50

Canada

60 ± 0.02

115/230 (H) 120/208 (A) 120 (L) 120/240 (K)

115/230 (H) 220/380 (A) 120/208 (A) 347/600 (A) 480 (F) 240 (F) 120/240 (K) 120/208 (A)

Chile China Colombia

50 50 60 ± 1

Costa Rica

60

220 (L) (1) 220 (L) (1) 120/240 (G) 120 (L) 120 (L) (1)

Czechoslovakia

50 ± 0.1

230/380 (A) 220 (L)

220/380 (A) (1) 220/380 (A) 120/240 (G) 120 (L) 120/240 (K) 120 (L) (1) 220/380 (A) 220 (L)

Denmark

50 ± 0.4

220/380 (A) 220 (L)

220/380 (A) 220 (L)

Egypt (AR)

50 ± 1

220/380 (A) 220 (L)

220/380 (A) 220 (L)

Finland

50 ± 0.1

220 (L) (1)

220/380 (A)

France

50 ± 1

230/400 (A) 220/380 (A) 220 (L)

230/400 (A) 220/380 (A) 220/380 (D)

127/220 (E) 127 (L) 220/380 (A) 220 (L)

380 (B)

Germany Ex-DRG

50 ± 0.3

Ex-FRG

50 ± 0.3

Greece

50 ± 1

Hong Kong (and Kowloon)

50 ± 2

domestic

commercial

industrial

240/415 (A) (E) 240 (L)

240/415 (A) 250/440 (A) 440 (N) (6)

22 kV 11 kV 6.6 kV 240/415 (A) 250/440 (A) (9) 10 kV 5.5 kV 6.6 kV 220/380 (A) 13.2 kV 6.88 kV 225/390 (A) 220/380 (A) 13.8 kV 11.2 kV 220/380 (A) 127/220 (A) 15 kV 6 kV 220/380 (A) 127/220 (A) 220 (F) 115/230 (H) (3) 220/380 (A) (3) 120/208 (A) 7.2/12.5 kV 347/600 (A) 120/208 600 (F) 480 (F) 240 (F) 220/380 (A) (3) 220/380 (A) (3) 13.2 kV 120/240 (G) 120/240 (G) (3)

220/380 (A) 220 (L)

220/380 (A) 220 (L) 127/220 (A) 127 (L) 220 (L) (1)

220/380 (A) 220 (L)

200/346 (A) 200 (L) (1)

11 kV 200/346 (A) 220/380 (A) 200 (L)

6.6 kV 220/380 (A)

22 kV 15 kV 6 kV 3 kV 220/380 (A) 30 kV 10 kV 220/380 (A) 11 kV 6.6 kV 220/380 (A) 380/660 (A) 500 (B) 220/380 (A) (D)

low-voltage tolerance % ±6

± 6 (10) + 5 and + 10

± 10 (9)

+ 5 (day) ± 10 (night)

±5 (9) ±4 - 8.3

(9) ±7 ± 10 (9) ± 10

± 10 + 10 ± 10 ± 10

20 kV 15 kV 230/400 380 (B) 220/380 (A) (D) 20 kV 10 kV 220/380 (A) 10 kV 6 kV 380/660 (A) 220/380 (A) 22 kV 20 kV 15 kV 6.6 kV 220/380 (A) 11 kV 200/346 (A) 220/380 (A) (3)

± 10

±5

±5

±6

* IEC 38 (1983).

low-voltage service connections - D1

1. low-voltage public distribution networks (continued)

D 1.1 low-voltage consumers (continued) country

Hungary

frequency & tolerance Hz & % 50 ± 2

Iceland

50 ± 0.1

220/380 (A) 220 (L)

220/380 (A) 220 (L)

India (4) Bombay

50 ± 1

250/440 (A) 230 (L)

250/440 (A) 230 (L)

11 kW 250/440 (A)

+4

New Delhi

50 ± 3

230/400 (A) 230 (L)

230/400 (A) 230 (L)

11 kV 230/400 (A)

+6

Romakrishnapuram (2)

230/400 (A) 230 (L) 230/460 (P) 127/220 (A)

+6

220 (L) (1)

220/400 (A) 230 (L) 230/460 (P) 220/380 (A) 127/220 (A) 220/380 (A)

22 kV and 11 kV (9) (9) 220/380 (A) (3)

Iran

50 ± 3 25 d.c. 50 ± 1 -2 50 ± 5

Iraq

50

220 (L) (1)

220/380 (A)

Ireland (Northern)

50 ± 0.4

230 (L) (1) 220 (L) (1)

230/400 (A) 220/380 (A)

Ireland (Republic of) Israel

50

220 (L) (1)

220/380 (A)

50 ± 0.2

230/400 (A) 230 (L)

230/400 (A) 230 (L)

Italy

50 ± 0.4

220/380 (A) 127/220 (E) 220 (L)

220/380 (A) 127/220 (E)

Japan (East) (4)

50 ± 0.2 (5)

100/200 (K) 100 (L)

100/200 (H) (K)

Japan (West) (4)

60 ± 0.1 (5)

105/210 (K) 100/200 (K) 100 (L)

105/210 (H) (K) 100/200 (K) 100 (L)

Korea (North)

60 + 0 -5

220 (L)

220/380 (A)

22 kV 6.6 kV 105/210 (H) 100/200 (H) 220/380 (A)

Korea (South) Kuwait Luxembourg

60 50 50 ± 0.5

Malaysia

50 ± 1.0

100 (L) 240 (L) (1) 220/380 (A) 127/220 (A) 120/208 (A) 240 (L) (1)

100/200 (K) 240/415 (A) 220/380 (A) 127/220 (A) 120/208 (A) 240/415 (A)

(9) 240/415 (A) (3) 20 kV 15 kV 5 kV 240/415 (A) (3)

Mexico

60 ± 0.2

127/220 (A) 220 (L) 120 (M)

127/220 (A) 220 (L) 120 (M)

Morocco

50

220/380 (A)

Netherlands

50 ± 0.4

127/220 (A) 115/200 (A) 220/300 (A) 220 (E) (L)

13.8 kV 13.2 kV 277/480 (A) 127/220 (B) 220/380 (A) (3)

New Zealand

50 ± 1.5

230/400 (A) (E) 230 (L) 240 (L)

Nigeria

50 ± 1

230 (L) (1) 220 (L) (1)

240/415 (A) (E) 230/400 (A) (E) 230 (L) 240 (L) 230/400 (A) 220/380 (A)

Norway

50 ± 0.2

230 (B)

220/380 (A) 230 (B)

Pakistan

50

230 (L) (1)

Philippines

60 ± 0.16

110/220 (K)

230/400 (A) 230 (L) 13.8 kV 4.16 kV 2.4 kV 110/220 (H)

Manila

60 ± 5

240/120 (H) (K) 240/120 (H)

240/120 (H) (K) 240/120 (H)

Peru

60

225 (B) (M)

225 (B) (M)

Indonesia

table D1: survey of electricity supplies in various countries around the world. Bracketed letters relate to the circuit diagrams at the end of the table, while bracketed numbers refer to the notes which follow the diagrams (continued). D2 - low-voltage service connections

domestic

commercial

industrial

220/380 (A) 220 (L)

220/380 (A)

20 kV 220 (L) 10 kV 220/380 (A) 220/380 (A) (3)

220/380 (A)

20 kV 11 kV 231/400 (A) 220/380 (A) 11 kV 6.6 kV 3 kV 220/380 (A) 230/400 (A) (3) 220/380 (A) 10 kV 220/380 (A) 22 kV 12.6 kV 6.3 kV 230/400 (A) 20 kV 15 kV 10 kV 220/380 (A) 220 (C) 6.6 kV 100/200 (H) 200 (G) (J)

10 kV 3 kV 220/380 (A) 11 kV 230/400 (A) 240/415 (A) 440 (N) (6) 15 kV 11 kV 230/400 (A) 220/380 (A) 20 kV 10 kV 5 kV 220/380 (A) 230 (B) 230/400 (A) (3)

low-voltage tolerance % + 5 - 10

(9)

+5 + 15

+5

+6 (9) +6

± 5 (urban) ± 10 (rural)

± 10

± 10

+ 6.8 - 13.6 (10) (9) (9) ± 5 and ± 10 +5 - 10 ±6

(9) ±6 ±5

±5

± 10

(9)

13.8 kV 4.16 kV 2.4 kV 440 V (B) 110/220 (H)

±5

20 kV 6.24 kV 3.6 kV 240/120 (H) 10 kV 6 kV 225 (B)

±5

(9)


table D1: survey of electricity supplies in various countries around the world. Bracketed letters relate to the circuit diagrams at the end of the table, while bracketed numbers refer to the notes which follow the diagrams (continued).

Poland

frequency & tolerance Hz & % 50 ± 1

domestic

commercial

industrial

220 (L) (1)

220/380 (A)

15 kV 5 kV 220/380 (A) 220 (L) 220/380 (L)

15 kV 6 kV 220/380 (A) 15 kV 5 kV 220/380 (A)


Portugal

50 ± 1

220/380 (A) 220 (L)

Rumania

50 ± 1

220 (L) (1)

Saudi Arabia

60 ± 0.5

127/220 (A)

Singapore

50 ± 0.5

230/400 (A) 230 (L)

Spain

50 ± 3

South Africa

50 ± 2.5 25 (8)

Sweden

50 ± 0.2

220/380 (A) (E) 220 (L) 127/220 (A) (E) 127 (L) 250/433 (A) (7) 230/400 (A) (7) 220/380 (A) 220 (L) 230/400 (A) (7) 220/380 (A) 220/380 (A) 220 (L)

Syria

50

220 (L) (1) 115 (L) (1)

Taiwan

60 ± 4

Tunisia

50 + 2

220/380 (A) 220 (L) 110/220 (K) 110 (L) 220/380 (A) 220 (L)

220/380 (A) 220 (L)

Turkey

50 ± 2

220 (L) (1)

220/380 (A)

United Kingdom

50 ± 1

240 (L) (1)

240/415 (A)

U.S.A. (4) Charlotte (North Carolina)

60 ± 0.06

120/240 (K) 120/208 (A)

265/460 (A) 120/240 (K) 120/208 (A)

14.4 kV 7.2 kV 2.4 kV 575 (F) 460 (F) 240 (F) 265/460 (A) 120/240 (K) 120/208 (A)

+ 5 - 2.5

Detroit (Michigan)

60 ± 0.2

120/240 (K) 120/208 (A)

480 (F) 120/240 (H) 120/208 (A)

13.2 kV 4.8 kV 4.16 kV 480 (F) 120/240 (H) 120/208 (A)

+ 4 - 6.6

Los Angeles (California)

60 ± 0.2

120/240 (K)

4.8 kV 120/240 (G)

4.8 kV 120/240 (G)

±5

Miami (Florida)

60 ± 0.3

120/240 (K) 120/208 (A)

120/240 (K) 120/240 (H) 120/208 (A)

13.2 kV 2.4 kV 480/277 (A) 120/240 (H)

±5

New York (New York)

60

120/240 (K) 120/208 (A)

120/240 (K) 120/208 (A) 240 (F)

12.47 kV 4.16 kV 277/480 (A) 480 (F)

(9)

Pittsburgh (Pennsylvania)

60 ± 0.03

120/240 (K)

265/460 (A) 120/240 (K) 120/208 (A) 460 (F) 230 (F)

13.2 kV 11.5 kV 2.4 kV 265/460 (A) 120/208 (A) 460 (F) 230 (F)

± 5 (lighting) ± 10 (power)

Portland (Oregon)

60

120/240 (K)

227/480 (A) 120/240 (K) 120/208 (A) 480 (F) 240 (F)

19.9 kV 12 kV 7.2 kV 2.4 kV 277/480 (A) 120/208 (A) 480 (F) 240 (F)

(9)

±5

20 kV 10 kV 6 kV 220/380 (A) 13.8 kV 220/380 (A) 22 kV 6.6 kV 230/400 (A) 15 kV 11 kV 220/380 (A)

±5

11 kV 6.6 kV 3.3 kV 250/433 (A) (7) 220/380 (A)

11 kV 6.6 kV 3.3 kV 500 (B)

±5

220/380 (A) 220 (L)

20 kV 10 kV 6 kV 220/380 (A) 220/380 (A) (3) 115/200 (A)

± 10

127/220 (A) 220/380 (A) 6.6 kV 230/400 (A) 220/380 (A) 127/220 (A)

220/380 (A) 220 (L) 115/200 (A) 115 (L) 220/380 (A) 110/220 (H)

22.8 kV 11.4 kV 220/380 (A) 220 (H) 15 kV 10 kV 220/380 (A) 15 kV 6.3 kV 220/380 (A) 22 kV 11 kV 6.6 kV 3.3 kV 240/415 (A)

±5 ±3 ±7

(9)

± 5 and ± 10

+ 10 ± 10 ±6




San Francisco (California)

frequency & tolerance Hz & % 60 ± 0.08

domestic

commercial

industrial

120/240 (K)

277/480 (A) 120/240 (K)

20.8 kV 12 kV 4.16 kV 277/480 (A) 120/240 (G) 12.47 kV 7.2 kV 4.8 kV 4.16 kV 480 (F) 277/480 (A) 120/208 (A) 220/380 (A) (3)

Toledo (Ohio)

60 ± 0.08

120/240 (K) 120/208 (A)

277/480 (C) 120/240 (H) 120/208 (K)

USSR (former)

50

220/380 (A) 220 (L)

Viet-Nam

50 ± 0.1

Yugoslavia

50

220/380 (A) 220 (L) 127/220 (A) 127 (L) 220 (L) (1) 120 (L) (1) 220/380 (A) 220 (L)

220/380 (A) 120/208 (A) 220/380 (A) 220 (L)

15 kV 220/380 (A) 10 kV 6.6 kV 220/380 (A)


±5

(9)

± 10 (9)

table D1: survey of electricity supplies in various countries around the world. Bracketed letters relate to the circuit diagrams at the end of the table, while bracketed numbers refer to the notes which follow the diagrams.

D4 - low-voltage service connections

D circuit diagrams*

(a) three-phase star; four-wire: earthed neutral

(b) three-phase star: three-wire

(d) three-phase star; four-wire: non-earthed neutral

(e) two-phase star; three-wire: earthed neutral

(g) three-phase delta; four-wire: earthed mid point of one phase

(k) single-phase; three-wire: earthed mid point

V

(c) three-phase star; three-wire: earthed neutral

(f) three-phase delta: three-wire

(h) three-phase open delta; four-wire: earthed mid point of one phase

(l) single-phase; two-wire: earthed end of phase

(j) three-phase open delta: earthed junction of phases

(m) single-phase; two-wire: unearthed

kV

(n) single-wire: earthed return (swer)

(p) d.c.: three-wire: unearthed

* Windings (A) (B) (C) (D) and (F) may be transformer-secondary windings or alternator stator windings.

Notes (1) The supply to each house is normally single-phase using one line and one neutral conductor of systems (A) or (G). (2) Frequencies below 50 Hz and d.c. supplies are used in limited areas only. The examples given show the diversity of possibilities existing. (3) Information on higher voltage supplies to factories is not available. (4) More than one area of country is given to illustrate the differences which exist.

(5) Frequency is 50 Hz (eastern area) and 60 Hz (western area). The dividing line from north to south passes through Shizuoka on Honshu Island. (6) Some remote areas are supplied via a Single Wire Earthed Return (SWER) system. (7) A few towns only have this supply. (8) Refers to isolated mining districts. (9) Information not available. (10) Observed values.



D 1.1 low-voltage consumers (continued) residential and commercial consumers The function of a LV "mains" distributor (underground cable or overhead line) is to provide service connections to a number of consumers along its route. The current-rating requirements of distributors are estimated from the number of consumers to be connected and an average demand per consumer. The two principal limiting parameters of a distributor are: c the maximum current which it is capable of carrying indefinitely, and c the maximum length of cable which, when carrying its maximum current, will not exceed the statutory voltage-drop limit. system 120 V 1-phase 2-wire 120/240 V 1-phase 3-wire 120/208 V 3-phase 4-wire 220/380 V 3-phase 4-wire 230/400 V 3-phase 4-wire 240/415 V 3-phase 4-wire table D2.

These constraints mean that the magnitude of loads which power-supply organizations are willing to connect to their LV distribution mains, is necessarily restricted. For the range of LV systems mentioned in the second paragraph of this sub-clause (1.1) viz: 120 V single phase to 240/415 V 3-phase, typical maximum permitted loads connected to a LV distributor might* be:

assumed max. permitted current per consumer service 60 A 60 A 60 A 120 A 120 A 120 A

kVA 7.2 14.4 22 80 83 86

* The table D2 values shown are indicative only, being (arbitrarily) based on 60 A maximum service currents for the first three systems, since smaller voltage drops are allowed at these lower voltages, for a given percentage statutory limit. The second group of systems is (again, arbitrarily) based on a maximum permitted service current of 120 A.

Practices vary considerably from one powersupply organization to another, and no "standardized" values can be given. Factors to be considered include: c the size of an existing distributor to which the new load is to be connected, c the total load already connected to the distributor, c the location along the distributor of the proposed new load, i.e. close to the substation, or near the remote end of the distributor, etc.

In short, each case must be examined individually. The load levels listed above are adequate for all normal domestic consumers, and will be sufficient for the installations of many administrative, commercial and similar buildings.

medium-size and small industrial consumers (with dedicated LV lines direct from a public-supply HV/LV substation)

For these reasons, dedicated supply lines at LV are generally applied (at 220/380 V to 240/415 V) to a load range of 80 kVA to 250 kVA. Consumers normally supplied at low voltage include: c domestic dwellings, c shops and commercial buildings, c small factories, workshops and filling stations, c restaurants, c farms, etc.

Medium and small industrial consumers can also be satisfactorily supplied at low-voltage. For loads which exceed the maximum permitted limit for a service from a distributor, a dedicated cable can usually be provided from the LV distribution fuse- (or switch-) board from which the mains distributors emanate, in the power-supply authority substation. In principle, the upper load limit which can be supplied by this means is restricted only by the available spare transformer capacity in the substation. In practice, however: c large loads (e.g. > 300 kVA) require correspondingly large cables, so that, unless the load centre is close to the substation, this method can be economically unfavourable, c many power-supply organizations prefer to supply loads exceeding 200 kVA (this figure varies with different suppliers) at high voltage.


D 1.2. LV distribution networks in cities and large towns, standardsized LV distribution cables form a network through link boxes. Some links are removed, so that each (fused) distributor leaving a substation forms a branched openended radial system, as shown in figure D3.

In European countries the standard 3-phase 4-wire distribution voltage levels are 220/380 V, 230/400 V, or 240/415 V. Many countries are currently converting their LV systems to the latest IEC standard of 230/400 V nominal (IEC 38-1983). The target date for completion is the year 2003. Medium to large-sized towns and cities have underground cable distribution systems. HV/LV distribution substations, mutually spaced at approximately 500-600 metres, are typically equipped with: c a 3-or 4-way HV switchboard, often made up of incoming and outgoing load-break switches forming part of a ring main, and one or two HV circuit breakers or combined fuse/ load-break switches for the transformer circuits, c one or two 1.000 kVA HV/LV transformers, c one or two (coupled) 6-or 8-way LV 3-phase 4-wire distribution fuse boards, or moulded-case circuit breaker boards, control and protect outgoing 4-core distribution cables, generally referred to as “distributors". The output from a transformer is connected to the LV busbars via a load-break switch, or simply through isolating links.

In densely-loaded areas, a standard size of distributor is laid to form a network, with (generally) one cable along each pavement and 4-way link boxes located in manholes at street corners, where two cables cross. Recent trends are towards weather-proof cabinets above ground level, either against a wall, or where possible, flush-mounted in the wall. Links are inserted in such a way that distributors form radial circuits from the substation with open-ended branches (see figure D3). Where a link box unites a distributor from one substation with that from a neighbouring substation, the phase links are omitted or replaced by fuses, but the neutral link remains in place. This arrangement provides a very flexible system in which a complete substation can be taken out of service, while the area normally supplied from it is fed from link boxes of the surrounding substations. Moreover, short lengths of distributor (between two link boxes) can be isolated for fault-location and repair.

4-way link box

HV/LV substation

service cable

phase links removed

fig. D3: showing one of several ways in which a LV distribution network may be arranged for radial branched-distributor operation, by removing (phase) links. low-voltage service connections - D7


D 1.2 LV distribution networks (continued) in less-densely loaded urban areas a more-economic system of tapered radial distribution is commonly used, in which conductors of reduced size are installed as the distance from a substation increases.

Where the load density requires it, the substations are more closely spaced, and transformers up to 1.500 kVA are sometimes necessary. Other forms of urban LV network, based on free-standing LV distribution pillars, placed above ground at strategic points in the network, are widely used in areas of lower load density. This scheme exploits the principle of tapered radial distributors in which the distribution cable conductor size is reduced as the number of consumers downstream diminish with distance from the substation. In this scheme a number of large-sectioned LV radial feeders from the distribution board in the substation each supply the busbars of a distribution pillar, from which smaller distributors supply consumers immediately surrounding the pillar. Distribution in market towns, villages and rural areas generally has, for many years, been based on bare copper conductors supported on wooden, concrete or steel poles, and supplied from pole-mounted or ground-mounted transformers.

improved methods using insulated twisted conductors to form a polemounted aerial cable are now standard practice in many countries.

In recent years, LV insulated conductors, twisted to form a two-core or 4-core selfsupporting cable for overhead use, have been developed, and are considered to be safer and visually more acceptable than bare copper lines. This is particularly so when the conductors are fixed to walls (e.g. under-eaves wiring) where they are hardly noticeable. As a matter of interest, similar principles have been applied at higher voltages, and selfsupporting “bundled” insulated conductors for HV overhead installations are now available for operation at 24 kV. Where more than one substation supplies a village, arrangements are made at poles on which the LV lines from different substations meet, to interconnect corresponding phases at times of emergency. The neutral conductors are permanently connected.

in Europe, each public-supply distribution substation is able to supply at LV an area corresponding to a radius of approximately 300 metres from the substation. North and Central American systems of distribution consist of a HV network from which numerous (small) HV/LV transformers each supply one or several consumers, by direct service cable (or line) from the transformer location.

In Europe, each public-supply distribution substation is able to supply at LV an area corresponding to a radius of approximately 300 metres from the substation. North and Central American practice differs fundamentally from that in Europe, in that LV networks are practically nonexistent, and 3-phase supplies to domestic premises in residential areas are rare. The distribution is effectively carried out at high voltage in a way, which again differs from standard European practices. The HV system is, in fact, a 3-phase 4-wire system from which single-phase distributors (phase and neutral conductors) supply numerous single-phase transformers, the secondary windings of which are centre-tapped to produce 120/240 V single-phase 3-wire supplies. The central conductors provide the LV neutrals, which, together with the HV neutral conductors, are solidly earthed at intervals along their lengths. Each HV/LV transformer normally supplies one or several premises directly from the transformer position by radial service cable(s) or by overhead line(s).


D Many other systems exist in these countries, but the one described appears to be the most common. Figure D4 shows the main features of the two systems. for primary voltages > 72.5 kV (see note) primary winding may be : – delta with on-load – earthed star tap changer – earthed zigzag depending on the country concerned

}

13.8 kV / 2.4-4.16 kV N 1

2

3

each HV/LV transformer shown represents many similar units tertiary delta normally (not always) used if the primary winding is not delta

2 3 N 2.4 kV / 120-240 V 1 ph - 3 wire distribution transformer

1 ph HV / 230 V service transformer to isolated consumer(s) (rural supplies)

}

HV (1)

Ph

N 1

1 N

N

resistor replaced by a Petersen coil on O/H line systems in some countries

HV (2)

N 2

2 N

1 2 3 N main 3 ph and neutral HV distributor

3 ph HV / 230/400 V 4-wire distribution transformer

N

N 1 2 3 LV distribution network (1): 132 kV for example (2): 11 kV for example

fig. D4: widely-used American and European-type systems. Note: at primary voltages greater than 72.5 kV in bulk-supply substations, it is common practice in some European countries to use an earthed-star primary winding and a delta secondary winding. The neutral point on the secondary side is then provided by a zigzag earthing reactor, the star point of which is connected to earth through a resistor. Frequently, the earthing reactor has a secondary winding to provide LV 3-phase supplies for the substation. It is then referred to as an “earthing transformer”.



D 1.3 the consumer-service connection service components and metering equipment were formerly installed inside a consumer's building. The modern tendency is to locate these items outside in a weatherproof cabinet.

In the past, an underground cable service or the wall-mounted insulated conductors from an overhead line service, invariably terminated inside the consumer's premises, where the cable-end sealing box, the supplyauthority fuses (inaccessible to the consumer) and meters were installed. A more recent trend is (as far as possible) to locate these service components in a weatherproof housing outside the building. The supply-authority/consumer interface is often at the outgoing terminals of the meter(s) or, in some cases, at the outgoing terminals of the installation main circuit breaker (depending on local practices) to which connection is made by supply-authority personnel, following a satisfactory test and inspection of the installation. A typical arrangement is shown in figure D5.

fig. D5: typical service arrangement for TT-earthed systems. A = Service cable tee-joint F = Supply authority fuses C = Metering equipment S = Isolating link DB = Installation main circuit breaker

LV consumers are normally supplied according to the TN or TT system, as described in chapters F and G. The installation main circuit breaker for a TT supply must include a residualcurrent earth-leakage protective device. For a TN service, overcurrent protection by circuit breaker or switch-fuse is required.


A MCCB which incorporates a sensitive residual-current earth-fault protective feature is mandatory at the origin of any LV installation forming part of a TT earthing system. The reason for this feature and related leakage-current tripping levels are discussed in Clause 3 of Chapter G. A further reason for this MCCB is that the consumer cannot exceed his (contractual) declared maximum load, since the overload trip setting, which is sealed by the supply authority, will cut off supply above the declared value. Closing and tripping of the MCCB is freely available to the consumer, so that if the MCCB is inadvertently tripped on overload, or due to an appliance fault, supplies can be quickly restored following correction of the anomaly.

In view of the inconvenience to both the meter reader and consumer, the location of meters is nowadays generally outside the premises, either: c in a free-standing pillar-type housing as shown in figures D6 and D7, c in a space inside a building, but with cable termination and supply authority’s fuses located in a flush-mounted weatherproof cabinet accessible from the public way, as shown in figure D8.

D

fig. D6: typical rural-type installation. In this kind of installation it is often necessary to place the main installation circuit breaker some distance from the point of utilization, e.g. saw-mills, pumping stations, etc.

fig. D7: semi-urban installations (shopping precincts, etc.). The main installation CB is located in the consumer's premises in cases where it is set to trip if the declared kVA load demand is exceeded.

fig. D8: town centre installations. The service cable terminates in a flushmounted wall cabinet which contains the isolating fuse links, accessible from the public way. This method is preferred for esthetic reasons, when the consumer can provide a suitable metering and main-switch location.



D 1.3 the consumer-service connection (continued) c for private domestic consumers, the equipment shown in the cabinet in figure D5 is installed in a weatherproof cabinet mounted vertically on a metal frame in the front garden, or flush-mounted in the boundary wall, and accessible to authorized personnel from the pavement. Figure D9 shows the general arrangement, in which removable fuse links provide the means of isolation. Experiments are now well-advanced in the field of electronic metering; reading, and recording on magnetic cards is now possible, using information technology (IT) techniques, and it is confidently predicted that, in addition to remote reading and recording, the modification of tariff structures for a given meter will be possible from a central control location, in areas where it is economically justified.

supply authority/ consumer interface overhead line LV distributor service cable isolation by fuse links

installation

meter

meter cabinet

main installation circuit breaker

fig. D9: typical LV service arrangement for domestic consumers.


D 1.4 quality of supply voltage The quality of the LV network supply voltage in its widest sense implies: c compliance with statutory limits of magnitude and frequency, c freedom from continual fluctuation within those limits, c uninterrupted power supply, except for scheduled maintenance shutdowns, or as a result of system faults or other emergencies, c preservation of a near-sinusoidal wave form. In this Sub-clause the maintenance of voltage magnitude only will be discussed, the remaining subjects are covered in Clause 2 of chapter F. In most countries, power-supply authorities have a statutory obligation to maintain the level of voltage at the service position of consumers within the limits of ± 5% (or in some cases ± 6% or more-see table D1) of the declared nominal value. Again, IEC and most national standards recommend that LV appliances be designed and tested to perform satisfactorily within the limits of ± 10% of nominal voltage. This leaves a margin, under the worst conditions (of minus 5% at the service position, for example) of 5% allowable voltage drop in the installation wiring. The voltage drops in a typical distribution system occur as follows: the voltage at the HV terminals of a HV/LV transformer is normally maintained within a ± 2% band by the action of automatic onload tapchangers of the transformers at bulk-supply substations, which feed the HV network from a higher-voltage subtransmission system.

an adequate level of voltage at the consumers supply-service terminals is essential for satisfactory operation of equipment and appliances. Practical values of current, and resulting voltage drops in a typical LV system, show the importance of maintaining a high Power Factor as a means of reducing voltage drop.

If the HV/LV transformer is in a location close to a bulk-supply substation, the ± 2% voltage band may be centred on a voltage level which is higher than the nominal HV value. For example, the voltage could be 20.5 kV ± 2% on a 20 kV system. In this case, the HV/LV distribution transformer should have its HV off-circuit tapping switch selected to the + 2.5% tap position. Conversely, at locations remote from bulksupply substations a value of 19.5 kV ± 2% is possible, in which case the off-circuit tapping switch should be selected to the - 5% position. The different levels of voltage in a system are normal, and depend on the system powerflow pattern. Moreover, these voltage differences are the reason for the term “nominal” when referring to the system voltage.

104% at no-load*, when nominal voltage is applied at HV, or is corrected by the tapping switch, as described above. This would result in a voltage band of 102% to 106% in the present case. A typical LV distribution transformer has a short-circuit reactance voltage of 5%. If it is assumed that its resistance voltage is one tenth of this value, then the voltage drop within the transformer when supplying full load at 0.8 power factor lagging, will be: V% drop = R% cos ø + X% sin ø = 0.5 x 0.8 + 5 x 0.6 = 0.4 + 3 = 3.4% The voltage band at the output terminals of the fully-loaded transformer will therefore be (102 - 3.4) = 98.6% to (106 - 3.4) = 102.6%. The maximum allowable voltage drop along a distributor is therefore 98.6 - 95 = 3.6%. This means, in practical terms, that a medium-sized 230/400 V 3-phase 4-wire distribution cable of 240 mm2 copper conductors would be able to supply a total load of 292 kVA at 0.8 PF lagging, distributed evenly over 306 metres of the distributor. Alternatively, the same load at the premises of a single consumer could be supplied at a distance of 153 metres from the transformer, for the same volt-drop, and so on... As a matter of interest, the maximum rating of the cable, based on calculations derived from IEC 287 (1982) is 290 kVA, and so the 3.6% voltage margin is not unduly restrictive, i.e. the cable can be fully loaded for distances normally required in LV distribution systems. Furthermore, 0.8 PF lagging is appropriate to industrial loads. In mixed semi-industrial areas 0.85 is a more common value, while 0.9 is generally used for calculations concerning residential areas, so that the volt-drop noted above may be considered as a “worst case” example. * Transformers designed for the 230/400 V IEC standard will have a no-load output of 420 V, i.e. 105% of the nominal voltage.

practical application With the HV/LV transformer correctly selected at its off-circuit tapping switch, an unloaded transformer output voltage will be held within a band of ± 2% of its no-load voltage output. To ensure that the transformer can maintain the necessary voltage level when fully loaded, the output voltage at no-load must be as high as possible without exceeding the upper + 5% limit (adopted for this example). In present-day practice, the winding ratios generally give an output voltage of about low-voltage service connections - D13

2. tariffs and metering

D tariffs and metering No attempt will be made in this guide to discuss particular tariffs, since there appears to be as many different tariff structures around the world as there are distribution authorities. Some tariffs are very complicated in detail but certain elements are basic to all of them and are aimed at encouraging consumers to manage their power consumption in a way which reduces the cost to the supply authority of generation, transmission and distribution. The two predominant ways in which the cost of supplying power to consumers can be reduced, are: c reduction of power losses in the generation, transmission and distribution of electrical energy. In principle the lowest losses in a power system are attained when all parts of the system operate at unity power factor, c reduction of the peak power demand, while increasing the demand at low-load periods, thereby exploiting the generating plant more fully, and minimizing plant redundancy.

reduction of losses Although the ideal condition noted in the first possibility mentioned above cannot be realized in practice, many tariff structures are based partly on kVA demand, as well as on kWh consumed. Since, for a given kW loading, the minimum value of kVA occurs at unity power factor, the consumer can minimize billing costs by taking steps to improve the power factor of the load (as discussed in Chapter E). The kVA demand generally used for tariff purposes is the maximum average kVA demand occurring during each billing period, and is based on average kVA demands, over fixed periods (generally 10, 30 or 60 minute periods) and selecting the highest of these values. The principle is described below in "principle of kVA maximum-demand metering".

reduction of peak power demand The second aim, i.e. that of reducing peak power demands, while increasing demand at low-load periods, has resulted in tariffs which offer substantial reduction in the cost of energy at: c certain hours during the 24-hour day, c certain periods of the year. The simplest example is that of a domestic consumer with a storage-type water heater (or storage-type space heater, etc.). The meter has two digital registers, one of which operates during the day and the other (switched over by a timing device) operates during the night. A contactor, operated by the same timing device, closes the circuit of the water heater, the consumption of which is then indicated on the register to which the cheaper rate applies. The heater can be switched on and off at any time during the day if required, but will then be metered at the normal rate. Large industrial consumers may have 3 or 4 rates which apply at different periods during a 24-hour interval, and a similar number for different periods of the year. In such schemes the ratio of cost per kWh during a period of peak demand for the year, and that for the lowest-load period of the year, may be as much as 10: 1. D14 - low-voltage service connections

meters It will be appreciated that high-quality instruments and devices are necessary to implement this kind of metering, when using classical electro-mechanical equipment. Recent developments in electronic metering and micro-processors, together with remote ripple-control* from a supply-authority control centre (to change peak-period timing throughout the year, etc.) are now operational, and facilitate considerably the application of the principles discussed. * Ripple control is a system of signalling in which a voicefrequency current (commonly at 175 Hz) is injected into the LV mains at appropriate substations. The signal is injected as coded impulses, and relays which are tuned to the signal frequency and which recognize the particular code will operate to initiate a required function. In this way, up to 960 discrete control signals are available.

D In most countries, certain tariffs, as noted above, are partly based on kVA demand, in addition to the kWh consumption, during the billing periods (often 3-monthly intervals). The maximum demand registered by the meter to be described, is, in fact, a maximum (i.e. the highest) average kVA demand

registered for succeeding periods during the billing interval. Figure D10 shows a typical kVA demand curve over a period of two hours divided into succeeding periods of 10 minutes. The meter measures the average value of kVA during each of these 10 minute periods.

maximum average value during the 2 hour interval average values for 10 minute periods

kVA

0

1

time

2 hrs

fig. D10: maximum average value of kVA over an interval of 2 hours.

principle of kVA maximumdemand metering A kVAh meter is similar in all essentials to a kWh meter but the current and voltage phase relationship has been modified so that it effectively measures kVAh (kilo-volt-amphours). Furthermore, instead of having a set of decade counter dials, as in the case of a conventional kWh meter, this instrument has a rotating pointer. When the pointer turns it is measuring kVAh and pushing a red indicator before it. At the end of 10 minutes the pointer will have moved part way round the dial (it is designed so that it can never complete one revolution in 10 minutes) and is then electrically reset to the zero position, to start another 10 minute period. The red indicator remains at the position reached by the measuring pointer, and that position, corresponds to the number of kVAh (kilo-volt-ampere-hours) taken by the load in 10 minutes. Instead of the dial being marked in kilo-VAhours at that point however it can be marked in units of average kVA. The following figures will clarify the matter. Supposing the point at which the red indicator reached corresponds to 5 kVAh. It is known that a varying amount of kVA of apparent power has been flowing for 10 minutes, i.e. 1/6 hour. If now, the 5 kVAh is divided by the number of hours, then the average kVA for the period is obtained. In this case the average kVA for the period will be: 1 5x = 5 x 6 = 30 kVA 1/6 Every point around the dial will be similarly marked i.e. the figure for average kVA will be 6 times greater than the kVAh value at any given point. Similar reasoning can be applied to any other reset-time interval.

At the end of the billing period, the red indicator will be at the maximum of all the average values occurring in the billing period. The red indicator will be reset to zero at the beginning of each billing period. Electro-mechanical meters of the kind described are rapidly being replaced by electronic instruments. The basic measuring principles on which these electronic meters depend however, are the same as those described above.


1. power factor improvement

E 1.1 the nature of reactive energy alternating current systems supply two forms of energy: c "active" energy measured in kilowatt hours (kWh) which is converted into mechanical work, heat, light, etc. c "reactive" energy, which again takes two forms: v "reactive" energy required by inductive circuits (transformers, motors, etc.), v "reactive" energy required by capacitive circuits (cable capacitance, power capacitors, etc).

All inductive (i.e. electromagnetic) machines and devices that operate on a.c. systems convert electrical energy from the powersystem generators into mechanical work and heat. This energy is measured by kWh meters, and is referred to as "active" or "wattful" energy. In order to perform this conversion, magnetic fields have to be established in the machines, and these fields are associated with another form of energy to be supplied from the power system, known as "reactive" or "wattless" energy. The reason for this is that inductive plant cyclically absorbs energy from the system (during the build-up of the magnetic fields) and re-injects that energy into the system (during the collapse of the magnetic fields) twice in every power-frequency cycle. The effect on generator rotors is to (tend to) slow them during one part of the cycle and to accelerate them during another part of the cycle. The pulsating torque is stricly true only for single-phase alternators. In three-phase alternators the effect is mutually cancelled in the three phases, since, at any instant, the reactive energy supplied on one (or two) phase(s) is equal to the reactive energy being returned on the other two (or one) phase(s) of a balanced system. The nett result is zero average load on the generators, i.e. the reactive current is "wattless". An exactly similar phenomenon occurs with shunt capacitive elements in a power system, such as cable capacitance or banks of power capacitors, etc. In this case, energy is stored electrostatically. The cyclic charging and discharging of capacitive plant reacts on the generators of the system in the same manner as that described above for inductive plant, but the current flow to and from capacitive plant is in exact phase opposition to that of the inductive plant. This feature is the basis on which powerfactor improvement schemes depend. It should be noted that while this "wattless" current (more accurately, the wattless component of a load current) does not draw power from the system, it does cause power losses in transmission and distribution systems by heating the conductors. In practical power systems, wattless components of load currents are invariably inductive, while the impedances of transmission and distribution systems are predominantly inductively reactive. The combination of inductive current passing through an inductive reactance produces the worst possible conditions of voltage drop (i.e. in direct phase opposition to the system voltage).

For these reasons, viz: c transmission power losses and c voltage drop. The power-supply authorities reduce the amount of wattless (inductive) current as much as possible. Wattless (capacitive) currents have the reverse effect on voltage levels and produce voltage-rises in power systems. The power (kW) associated with "active" energy is usually represented by the letter P. The reactive power (kvar) is represented by Q. Inductively-reactive power is conventionally positive (+ Q) while capacitively-reactive power is shown as a negative quantity (- Q). Sub-clause 1.3 shows the relationship between P, Q, and S. S represents kVA of "apparent" power. Figure E1 shows that the kVA of apparent power is the vector sum of the kW of active power plus the kvar of reactive power.

S (kVA)

Q (kvar)

P (kW)

fig. E1: an electric motor requires active power P and reactive power Q from the power system.

power factor improvement - E1

1. power factor improvement (continued)

E 1.2 plant and appliances requiring reactive current items of plant which require reactive energy.

All a.c. plant and appliances that include electromagnetic devices, or depend on magnetically-coupled windings, require some degree of reactive current to create magnetic flux. The most common items in this class are transformers and reactors, motors and discharge lamps (i.e. the ballasts of). The proportion of reactive power (kvar) with respect to active power (kW) when an item of plant is fully loaded varies according to the item concerned being: c 65-75% for asynchronous motors, c 5-10% for transformers. The power factor at which a synchronous motor operates may be changed, by adjustment of the excitation current. These machines can be made to operate at lagging (underexcited) or leading (overexcited) power factors. In the latter condition, a synchronous motor is sometimes referred to as a "synchronous condenser". Before capacitor technology had developed sufficiently to guarantee the high standard of reliability of modern capacitors, synchronous condensers were widely used on transmission systems to provide reactivepower compensation, for optimum transmission-line performance under changing load conditions.

fig. E2: power consuming items that also require reactive energy.

1.3 the power factor the power factor is the ratio of kW to kVA. The closer the power factor approaches its maximum possible value of 1, the greater the benefit to consumer and supplier. PF = P (kW) ≈ cos ϕ S (kVA) P = active power S = apparent power

E2 - power factor improvement

definition of power factor The power factor of a load, which may be a single power-consuming item, or a number of items (for example an entire installation), is given by the ratio of P/S i.e. kW divided by kVA at any given moment. The value of a power factor will range from 0 to 1. The power diagram of figure E3 shows that the ratio mentioned above gives the cosine value for the angular displacement between the kW vector and the kVA vector. Conventionally, this angle is given the symbol ϕ, in which case power factor = cos ϕ.

The accuracy of this equivalence depends on an absence of harmonic currents and voltages on the system. It is generally assumed that these effects are small, so that cos ϕ and power factor are considered to be exact equivalents, for all practical purposes. A power factor close to unity means that the reactive energy is small compared with the active energy, while a low value of power factor indicates the opposite condition.

E power quantities kA, kVA and kvar are double-frequency functions and cannot be represented on a simple vector diagram. A static diagram for these quantities (figure E3) can be obtained, however, to provide a visual representation, with the aid of the true vector diagram of current components and one phase-voltage (figure E4). Since, in the diagram, the power quantities have direction and magnitude, they are referred to as "vectors" for convenience.

power vector diagram Active power P (in kW) c single phase (1 phase and neutral): P = VI cos ϕ c single phase (phase to phase): P = UI cos ϕ c three phase (3 wires or 3 wires + neutral): P = e UI cos ϕ* Reactive power Q (in kvar) c single phase (1 phase and neutral): Q = VI sin ϕ c single phase (phase to phase): Q = UI sin ϕ c three phase (3 wires or 3 wires + neutral): Q = e UI sin ϕ* Apparent power S (in kVA) c single phase (1 phase and neutral): S = VI c single phase (phase to phase): S = UI c three phase (3 wires or 3 wires + neutral): S = e UI* where: V: voltage between phase and neutral U: voltage between phases 2

S=

P +Q

ϕ

P

Q

kW

S

kVA kvar

fig. E3: power diagram. P: active power Q: reactive power S: apparent power

2

* for balanced and near-balanced loads on 4-wire systems.

current and voltage vectors, and derivation of the power diagram The power "vector" diagram is a useful artifice, derived directly from the true rotating vector diagram of currents and voltage, as follows: The power-system voltages are taken as the reference quantities, and one phase only is considered on the assumption of balanced 3-phase loading. The reference phase voltage (V) is co-incident with the horizontal axis, and the current (I) of that phase will, for practically all power-system loads, lag the voltage by an angle ϕ. The component of I which is in phase with V is the wattful component of I and is equal to I cos ϕ, while VI cos ϕ equals the active power (in kV) in the circuit, if V is expressed in kV. The component of I which lags 90 degrees behind V is the wattless component of I and is equal to I sin ϕ, while VI sin ϕ equals the reactive power (in kvar) in the circuit, if V is expressed in kV. If the vector I is multiplied by V, expressed in kV, then VI equals the apparent power (in kVA) for the circuit. The above kW, kvar and kVA values per phase, when multiplied by 3, can therefore conveniently represent the relationships of kVA, kW, kvar and power factor for a total 3-phase load, as shown in figure E3.

ϕ

P = VI cos ϕ (kW)

V

S = VI (kVA) Q = VI sin ϕ (kvar)

fig. E4: current and voltage vector diagram per phase.

1.4 tan ϕ tan ϕ = Q (kvar) P (kW)

Some electricity tariffs are partly based on this factor, which shows the amount of reactive power supplied per kW. A low value of tan ϕ corresponds to a high power factor and to a favourable consumer bill.


1. power factor improvement (continued)

E 1.5 practical measurement of power factor The power factor (or cos ϕ) can be measured, either: c by a direct-reading cos ϕ meter for an instantaneous value, or c a recording var meter, which allows a record over a period of time to be obtained, of current, voltage and power factor. Readings taken over an extended period provide a useful means of estimating an average value of power factor for an installation.

1.6 practical values of power factor an example of power calculations type of circuit single-phase (phase and neutral) single-phase (phase to phase) example 5 kW of load cos ϕ = 0.5 three phase 3-wires or 3-wires + neutral example motor Pn = 51 kW cos ϕ = 0.86 ρ = 0.91 (motor efficiency)

apparent power active power S (kVA) P (kW) S = VI P = VI cos ϕ S = UI P = UI cos ϕ 10 kVA 5 kW P = e UI cos ϕ 56 kW

S = e UI 65 kVA

reactive power Q (kvar) Q = VI sin ϕ Q = UI sin ϕ 8.7 kvar Q = e UI sin ϕ 33 kvar

table E5: example in the calculation of active and reactive power.

in addition to other information Table E20 gives corresponding cosine and tangent values for given angles.

The calculations for the three-phase example above are as follows: Pn = delivered shaft power = 51 kW P = active power consumed = Pn 51 = = 56 kW ρ 0.91 S = apparent power = P P = = 65 kVA cos ϕ 0.86 so that, on referring to Table E20 or using a pocket calculator, the value of tan ϕ corresponding to a cos ϕ of 0.86 is found to be 0.59 Q = P tan ϕ = 56 x 0.59 = 33 kvar. Alternatively 2

Q=

2

S - P =

2

ϕ

P = 56 kW

Q = 33 kvar

S=

65

kVA

fig. E6: calculation power diagram.

2

65 - 56 = 33 kvar

average power factor values for the most commonly-used plant, equipment and appliances plant and appliances c common loaded at induction motor

0% 25% 50% 75% 100%

c incandescent lamps c fluorescent lamps (uncompensated) c fluorescent lamps (compensated) c discharge lamps c ovens using resistance elements c induction heating ovens (compensated) c dielectric type heating ovens c resistance-type soldering machines c fixed 1-phase arc-welding set c arc-welding motor-generating set c arc-welding transformer-rectifier set c arc furnace

cos ϕ 0.17 0.55 0.73 0.80 0.85 1.0 0.5 0.93 0.4 to 0.6 1.0 0.85 0.85 0.8 to 0.9 0.5 0.7 to 0.9 0.7 to 0.8 0.8

table E7: values of cos ϕ and tan ϕ for commonly-used plant and equipment.


tan ϕ 5.80 1.52 0.94 0.75 0.62 0 1.73 0.39 2.29 to 1.33 0 0.62 0.62 0.75 to 0.48 1.73 1.02 to 0.48 1.02 to 0.75 0.75

2. why improve the power factor?

E 2.1 reduction in the cost of electricity an improvement of the power factor of an installation presents several technical and economic advantages, notably in the reduction of electricity bills.

Good management in the consumption of reactive energy brings with it the following economic advantages. These notes are based on an actual tariff structure of a kind commonly applied in Europe, designed to encourage consumers to minimize their consumption of reactive energy. The installation of power-factor correcting capacitors on installations permits the consumer to reduce his electricity bill by maintaining the level of reactive-power consumption below a value contractually agreed with the power-supply authority. In this particular tariff, reactive energy is billed according to the tan ϕ criterion. As previously noted: Q (kvarh) tan ϕ = P (kWh) At the supply service position, the power supply distributor delivers reactive energy free, until: c the point at which it reaches 40% of the active energy (tan ϕ = 0.4) for a maximum period of 16 hours each day (from 06-00 h to 22-00 h) during the most-heavily loaded period (often in winter); c without limitation during light-load periods in winter, and in spring and summer. During the periods of limitation, reactiveenergy consumption exceeding 40% of the active energy (i.e. tan ϕ > 0.4) is billed monthly at the current rates.

Thus, the quantity of reactive energy billed in these periods will be: kvarh (to be billed) = kWh (tan ϕ - 0.4) where kWh is the active energy consumed during the periods of limitation, and kWh tan ϕ is the total reactive energy during a period of limitation, and 0.4 kWh is the amount of reactive energy delivered free during a period of limitation. tan ϕ = 0.4 corresponds to a PF of 0.93 so that, if steps are taken to ensure that during the limitation periods the PF never falls below 0.93, the consumer will have nothing to pay for the reactive power consumed. Against the financial advantages of reduced billing, the consumer must balance the cost of purchasing, installing and maintaining the power-factor-improvement capacitors and controlling switchgear, automatic control equipment (where stepped levels of compensation are required) together with the additional kWh consumed by the dielectric losses of the capacitors, etc. It may be found that it is more economic to provide partial compensation only, and that paying for some of the reactive energy consumed is less expensive than providing 100% compensation. The question of power-factor correction is a matter of optimization, except in very simple cases.

2.2 technical/economic optimization power factor improvement allows the use of smaller transformers, switchgear and cables, etc. as well as reducing power losses and voltage drop in an installation.

A high power factor allows the optimization of the components of an installation. Overating of certain equipment can be avoided, but to achieve the best results, the correction should be effected as close to the individual items of inductive plant as possible.

reduction of cable size Table E8 shows the required increase in the size of cables as the power factor is reduced from unity to 0.4. multiplying factor 1 1.25 1.67 2.5 for the cross-sectional area of the cable core(s) cos ϕ 1 0.8 0.6 0.4 table E8: multiplying factor for cable size as a function of cos ϕ.

reduction of losses (P, kW) in cables

reduction of voltage drop PF correction capacitors reduce or even cancel completely the (inductive) reactive current in upstream conductors, thereby reducing or eliminating voltage drops. Note: Overcompensation will produce a voltage rise at the capacitors.

increase in available power By improving the power factor of a load supplied from a transformer, the current through the transformer will be reduced, thereby allowing more load to be added. In practice, it may be less expensive to improve the power factor*, than to replace the transformer by a larger unit. This matter is further elaborated in clause 6. * Since other benefits accrue from a high value of PF, as previously noted.

Losses in cables are proportional to the current squared, and are measured by the kWh meter for the installation. Reduction of the total current in a conductor by 10% for example, will reduce the losses by almost 20%.


3. how to improve the power factor

E 3.1 theoretical principles improving the power factor of an installation requires a bank of capacitors which acts as a source of reactive energy. This arrangement is said to provide reactive energy compensation.

An inductive load having a low power factor requires the generators and transmission/distribution systems to pass reactive current (lagging the system voltage by 90 degrees) with associated power losses and exaggerated voltage drops, as noted in sub-clause 1.1. If a bank of shunt capacitors is added to the load, its (capacitive) reactive current will take the same path through the power system as that of the load reactive current. Since, as pointed out in sub-clause 1.1, this capacitive current Ic (which leads the system voltage by 90 degrees) is in direct phase opposition to the load reactive current (IL), the two components flowing through the same path will cancel each other, such that if the capacitor bank is sufficiently large and Ic = IL there will be no reactive current flow in the system upstream of the capacitors. This is indicated in figure E9 (a) and (b) which show the flow of the reactive components of current only. In this figure: R represents the active-power elements of the load L represents the (inductive) reactive-power elements of the load C represents the (capacitive) reactive-power elements of the power-factor correction equipment (i.e. capacitors). IL - IC

IC C

IL

IL L

R

load a) reactive current components only flow pattern

IL - IC = 0

IC C

IL = IC

IL L

R

load b) when IC = IL, all reactive power is supplied from the capacitor bank

IR

IC C

IR + IL

IL L

IR

load c) with load current added to case (b)

fig. E9: showing the essential features of power-factor correction. It will be seen from diagram (b) of figure E9, that the capacitor bank C appears to be supplying all the reactive current of the load. For this reason, capacitors are sometimes referred to as "generators of lagging vars". In diagram (c) of figure E9, the active-power current component has been added, and shows that the (fully-compensated) load appears to the power system as having a power factor of 1. In general, it is not economical to fully compensate an installation.


R

Figure E10 uses the power diagram discussed in sub-clause 1.3 (figure E3) to illustrate the principle of compensation by reducing a large reactive power Q to a smaller value Q' by means of a bank of capacitors having a reactive power Qc. In doing so, the magnitude of the apparent power S is seen to reduce to S'. P

ϕ ϕ' Q' S' Q S

Qc

fig. E10: diagram showing the principle of compensation: Qc = P (tan ϕ - tan ϕ'). Example: A motor consumes 100 kW at a PF of 0.75 (i.e. tan ϕ = 0.88). To improve the PF to 0.93 (i.e. tan ϕ = 0.4), the reactive power of the capacitor bank must be : Qc = 100 (0.88 - 0.4) = 48 kvar The selected level of compensation and the calculation of rating for the capacitor bank depend on the particular installation. The factors requiring attention are explained in a general way in clause 5, and in clauses 6 and 7 for transformers and motors. Note: Before embarking on a compensation project, a number of precautions should be observed. In particular, oversizing of motors should be avoided, as well as the operation of the motors in an unloaded condition. In this latter condition, the reactive energy consumed by a motor results in a very low power factor (≈ 0.17); this is because the kW taken by the motor (when it is unloaded) are very small.

E 3.2 by using what equipment? compensation at L.V. At low voltage, compensation is provided by: c fixed-valued capacitor, c equipment providing automatic regulation, or banks which allow continuous adjustment according to requirements, as loading of the installation changes.

compensation can be carried out by a fixed value of capacitance in favourable circumstances.

compensation is more-commonly effected by means of an automatically-controlled stepped bank of capacitors.

Note: When the installed reactive power of compensation exceeds 800 kvar, and the load is continuous and stable, it is often found to be economically advantageous to instal capacitor banks at high voltage.

fixed capacitors This arrangement employs one or more capacitor(s) to form a constant level of compensation. Control may be: c manual: by circuit breaker or load-break switch, c semi-automatic: by contactor, c direct connection to an appliance and switched with it. These capacitors are applied: c at the terminals of inductive devices (motors and transformers), c at busbars supplying numerous small motors and inductive appliance for which individual compensation would be too costly, c in cases where the level of load is reasonably constant.

fig. E11: example of fixed-valuecompensation capacitors.

automatic capacitor banks This kind of equipment provides automatic control of compensation, maintaining within close limits, a selected level of power factor. Such equipment is applied at points in an installation where the active-power and/or reactive-power variations are relatively large, for example: c at the busbars of a general power distribution board, c at the terminals of a heavily-loaded feeder cable.

fig. E12: example of automatic-compensation-regulating equipment.


3. how to improve the power factor (continued)

E 3.2 by using what equipment? (continued) automatically-regulated banks of capacitors allow an immediate adaptation of compensation to match the level of load.

the principles of, and reasons, for using automatic compensation A bank of capacitors is divided into a number of sections, each of which is controlled by a contactor. Closure of a contactor switches its section into parallel operation with other sections already in service. The size of the bank can therefore be increased or decreased in steps, by the closure and opening of the controlling contactors. A control relay monitors the power factor of the controlled circuit(s) and is arranged to close and open appropriate contactors to maintain a reasonably constant system power factor (within the tolerance imposed by the size of each step of compensation). The current transformer for the monitoring relay

must evidently be placed on one phase of the incoming cable which supplies the circuit(s) being controlled, as shown in figure E13. By closely matching compensation to that required by the load, the possibility of producing overvoltages at times of low load will be avoided, thereby preventing an overvoltage condition, and possible damage to appliances and equipment. Overvoltages due to excessive reactive compensation depend partly on the value of source impedance.

CT In / 5 A cl 1

varmetric relay

fig. E13: the principle of automatic-compensation control.

3.3 the choice between a fixed or automatically-regulated bank of capacitors commonly-applied rules Where the kvar rating of the capacitors is less than, or equal to 15% of the supplytransformer rating, a fixed value of compensation is appropriate. Above the 15% level, it is advisable to install an automatically-controlled bank of capacitors. The location of low-voltage capacitors in an installation constitutes the mode of compensation, which may be global (one location for the entire installation), partial (section-by-section), local (at each individual device), or some combination of the latter two. In principle, the ideal compensation is applied at a point of consumption and at the level required at any instant. In practice, technical and economic factors govern the choice.


4. where to install correction capacitors

E 4.1 global compensation where a load is continuous and stable, global compensation can be applied.

principle The capacitor bank is connected to the busbars of the main LV distribution board for the installation, and remains in service during the period of normal load.

advantages The global type of compensation: c reduces the tariff penalties for excessive consumption of kvars, c reduces the apparent power kVA demand, on which standing charges are usually based, c relieves the supply transformer, which is then able to accept more load if necessary.

n°1

comments c reactive current still flows in all conductors of cables leaving (i.e. downstream of) the main LV distribution board, c for the above reason, the sizing of these cables, and power losses in them, are not improved by the global mode of compensation.

M

M

M

M

fig. E14: global compensation.

4.2 compensation by sector compensation by sector is recommended when the installation is extensive, and where the load/time patterns differ from one part of the installation to another.

principle Capacitor banks are connected to busbars of each local distribution board, as shown in figure E14. A significant part of the installation benefits from this arrangement, notably the feeder cables from the main distribution board to each of the local distribution boards at which the compensation measures are applied.

n°1

advantages The compensation by sector: c reduces the tariff penalties for excessive consumption of kvars, c reduces the apparent power kva demand, on which standing charges are usually based, c relieves the supply transformer, which is then able to accept more load if necessary, c the size of the cables supplying the local distribution boards may be reduced, or will have additional capacity for possible load increases, c losses in the same cables will be reduced.

comments

n°2

M

n°2

M

M

M

fig. E15: compensation by sector. kW kvar conductors

c reactive current still flows in all cables downstream of the local distribution boards, c for the above reason, the sizing of these cables, and the power losses in them, are not improved by compensation by sector, c where large changes in loads occur, there is always a risk of overcompensation and consequent overvoltage problems.


4. where to install correction capacitors (continued)

E 4.3 individual compensation individual compensation should be considered when the power of motor is significant with respect to power of the installation.

principle Capacitors are connected directly to the terminals of inductive plant (notably motors, see further in Clause 7). Individual compensation should be considered when the power of the motor is significant with respect to the declared power requirement (kVA) of the installation. The kvar rating of the capacitor bank is in the order of 25% of the kW rating of the motor. Complementary compensation at the origin of the installation (transformer) may also be beneficial.

n°1

advantages Individual compensation: c reduces the tariff penalties for excessive consumption of kvars, c reduces the apparent power kVA demand, c reduces the size of all cables as well as the cable losses.

comments c significant reactive currents no longer exist in the installation.


n°2

n°3

M

n°2

n°3

M

n°3

M

fig. E16: individual compensation.

n°3

M

5. how to decide the optimum level of compensation

E 5.1 general method listing of reactive power demands at the design stage

technical-economic optimization for an existing installation

This listing can be made in the same way (and at the same time) as that for the power loading described in chapter B. The levels of active and reactive power loading, at each level of the installation (generally at points of distribution and sub-distribution of circuits) can then be determined.

The optimum rating of compensation capacitors for an existing installation can be determined from the following principal considerations: c electricity bills prior to the installation of capacitors, c future electricity bills anticipated following the installation of capacitors, c costs of: v purchase of capacitors and control equipment (contactors, relaying, cabinets, etc.), v installation and maintenance costs, v cost of dielectric heating losses in the capacitors, versus reduced losses in cables, transformer, etc., following the installation of capacitors. Several simplified methods applied to typical tariffs (common in Europe) are shown in sub-clauses 5.3 and 5.4.

5.2 simplified method general principle An approximate calculation is generally adequate for most practical cases, and may be based on the assumption of a power factor of 0.8 (lagging) before compensation. In order to improve the power factor to a value sufficient to avoid tariff penalties (this depends on local tariff structures, but is assumed here to be 0.93) and to reduce losses, volt-drops, etc. in the installation, reference can be made to table E17. From the table, it can be seen that, to raise the power factor of the installation from 0.8 to 0.93 will require 0.355 kvar per kW of load. The rating of a bank of capacitors at the busbars of the main distribution board of the installation would be Q (kvar) = 0.355 x P (kW). This simple approach allows a rapid determination of the compensation capacitors required, albeit in the global, partial or independent mode. Example It is required to improve the power factor of a 666 kVA installation from 0.75 to 0.928. The active power demand is 666 x 0.75 = 500 kW. In table E17, the intersection of the row cos ϕ = 0.75 (before correction) with the column cos ϕ = 0.93 (after correction) indicates a value of 0.487 kvar of compensation per kW of load. For a load of 500 kW, therefore, 500 x 0.487 = 244 kvar of capacitive compensation is required. Note: this method is valid for any voltage level, i.e. is independent of voltage.


5. how to decide the optimum level of compensation (continued)

E before kvar rating of capacitor bank to install per kW of load, to improve cos ϕ (the power factor) or tan ϕ, compensation to a given value tan ϕ 0,75 0.59 0.48 0.46 0.43 0.40 0.36 0.33 0.29 0.25 0.20 tan ϕ cos ϕ cos ϕ 0.80 0.86 0.90 0.91 0.92 0.93 0.94 0.95 0.96 0.97 0.98 2.29 0.40 1.557 1.691 1.805 1.832 1.861 1.895 1.924 1.959 1.998 2.037 2.085 2.22 0.41 1.474 1.625 1.742 1.769 1.798 1.831 1.840 1.896 1.935 1.973 2.021 2.16 0.42 1.413 1.561 1.681 1.709 1.738 1.771 1.800 1.836 1.874 1.913 1.961 2.10 0.43 1.356 1.499 1.624 1.651 1.680 1.713 1.742 1.778 1.816 1.855 1.903 2.04 0.44 1.290 1.441 1.558 1.585 1.614 1.647 1.677 1.712 1.751 1.790 1.837 1.98 0.45 1.230 1.384 1.501 1.532 1.561 1.592 1.628 1.659 1.695 1.737 1.784 1.93 0.46 1.179 1.330 1.446 1.473 1.502 1.533 1.567 1.600 1.636 1.677 1.725 1.88 0.47 1.130 1.278 1.397 1.425 1.454 1.485 1.519 1.532 1.588 1.629 1.677 1.83 0.48 1.076 1.228 1.343 1.370 1.400 1.430 1.464 1.497 1.534 1.575 1.623 1.78 0.49 1.030 1.179 1.297 1.326 1.355 1.386 1.420 1.453 1.489 1.530 1.578 1.73 0.50 0.982 1.232 1.248 1.276 1.303 1.337 1.369 1.403 1.441 1.481 1.529 1.69 0.51 0.936 1.087 1.202 1.230 1.257 1.291 1.323 1.357 1.395 1.435 1.483 1.64 0.52 0.894 1.043 1.160 1.188 1.215 1.249 1.281 1.315 1.353 1.393 1.441 1.60 0.53 0.850 1.000 1.116 1.144 1.171 1.205 1.237 1.271 1.309 1.349 1.397 1.56 0.54 0.809 0.959 1.075 1.103 1.130 1.164 1.196 1.230 1.268 1.308 1.356 1.52 0.55 0.769 0.918 1.035 1.063 1.090 1.124 1.156 1.190 1.228 1.268 1.316 1.48 0.56 0.730 0.879 0.996 1.024 1.051 1.085 1.117 1.151 1.189 1.229 1.277 1.44 0.57 0.692 0.841 0.958 0.986 1.013 1.047 1.079 1.113 1.151 1.191 1.239 1.40 0.58 0.665 0.805 0.921 0.949 0.976 1.010 1.042 1.076 1.114 1.154 1.202 1.37 0.59 0.618 0.768 0.884 0.912 0.939 0.973 1.005 1.039 1.077 1.117 1.165 1.33 0.60 0.584 0.733 0.849 0.878 0.905 0.939 0.971 1.005 1.043 1.083 1.131 1.30 0.61 0.549 0.699 0.815 0.843 0.870 0.904 0.936 0.970 1.008 1.048 1.096 1.27 0.62 0.515 0.665 0.781 0.809 0.836 0.870 0.902 0.936 0.974 1.014 1.062 1.23 0.63 0.483 0.633 0.749 0.777 0.804 0.838 0.870 0.904 0.942 0.982 1.030 1.20 0.64 0.450 0.601 0.716 0.744 0.771 0.805 0.837 0.871 0.909 0.949 0.997 1.17 0.65 0.419 0.569 0.685 0.713 0.740 0.774 0.806 0.840 0.878 0.918 0.966 1.14 0.66 0.388 0.538 0.654 0.682 0.709 0.743 0.775 0.809 0.847 0.887 0.935 1.11 0.67 0.358 0.508 0.624 0.652 0.679 0.713 0.745 0.779 0.817 0.857 0.905 1.08 0.68 0.329 0.478 0.595 0.623 0.650 0.684 0.716 0.750 0.788 0.828 0.876 1.05 0.69 0.299 0.449 0.565 0.593 0.620 0.654 0.686 0.720 0.758 0.798 0.840 1.02 0.70 0.270 0.420 0.536 0.564 0.591 0.625 0.657 0.691 0.729 0.769 0.811 0.99 0.71 0.242 0.392 0.508 0.536 0.563 0.597 0.629 0.663 0.701 0.741 0.783 0.96 0.72 0.213 0.364 0.479 0.507 0.534 0.568 0.600 0.634 0.672 0.712 0.754 0.94 0.73 0.186 0.336 0.452 0.480 0.507 0.541 0.573 0.607 0.645 0.685 0.727 0.91 0.74 0.159 0.309 0.425 0.453 0.480 0.514 0.546 0.580 0.618 0.658 0.700 0.88 0.75 0.132 0.82 0.398 0.426 0.453 0.487 0.519 0.553 0.591 0.631 0.673 0.86 0.76 0.105 0.255 0.371 0.399 0.426 0.460 0.492 0.526 0.564 0.604 0.652 0.83 0.77 0.079 0.229 0.345 0.373 0.400 0.434 0.466 0.500 0.538 0.578 0.620 0.80 0.78 0.053 0.202 0.319 0.347 0.374 0.408 0.440 0.474 0.512 0.552 0.594 0.78 0.79 0.026 0.176 0.292 0.320 0.347 0.381 0.413 0.447 0.485 0.525 0.567 0.75 0.80 0.150 0.266 0.294 0.321 0.355 0.387 0.421 0.459 0.499 0.541 0.72 0.81 0.124 0.240 0.268 0.295 0.329 0.361 0.395 0.433 0.473 0.515 0.70 0.82 0.098 0.214 0.242 0.269 0.303 0.335 0.369 0.407 0.447 0.489 0.67 0.83 0.072 0.188 0.216 0.243 0.277 0.309 0.343 0.381 0.421 0.463 0.65 0.84 0.046 0.162 0.190 0.217 0.251 0.283 0.317 0.355 0.395 0.437 0.62 0.85 0.020 0.136 0.164 0.191 0.225 0.257 0.291 0.329 0.369 0.417 0.59 0.86 0.109 0.140 0.167 0.198 0.230 0.264 0.301 0.343 0.390 0.57 0.87 0.083 0.114 0.141 0.172 0.204 0.238 0.275 0.317 0.364 0.54 0.88 0.054 0.085 0.112 0.143 0.175 0.209 0.246 0.288 0.335 0.51 0.89 0.028 0.059 0.086 0.117 0.149 0.183 0.230 0.262 0.309 0.48 0.90 0.031 0.058 0.089 0.121 0.155 0.192 0.234 0.281 table E17: kvar to be installed per kW of load, to improve the power factor of an installation.


0.14 0.99 2.146 2.082 2.022 1.964 1.899 1.846 1.786 1.758 1.684 1.639 1.590 1.544 1.502 1.458 1.417 1.377 1.338 1.300 1.263 1.226 1.192 1.157 1.123 1.091 1.058 1.007 0.996 0.966 0.937 0.907 0.878 0.850 0.821 0.794 0.767 0.740 0.713 0.687 0.661 0.634 0.608 0.582 0.556 0.530 0.504 0.478 0.450 0.424 0.395 0.369 0.341

0.0 1 2.288 2.225 2.164 2.107 2.041 1.988 1.929 1.881 1.826 1.782 1.732 1.686 1.644 1.600 1.559 1.519 1.480 1.442 1.405 1.368 1.334 1.299 1.265 1.233 1.200 1.169 1.138 1.108 1.079 1.049 1.020 0.992 0.963 0.936 0.909 0.882 0.855 0.829 0.803 0.776 0.750 0.724 0.698 0.672 0.645 0.620 0.593 0.567 0.538 0.512 0.484

E 5.3 method based on the avoidance of tariff penalties in the case of certain (common) types of tariff, an examination of several bills covering the most heavily-loaded period of the year allows determination of the kvar level of compensation required to avoid kvarh (reactive-energy) charges. the pay-back period of a bank of power-factor-improvement capacitors and associated equipment is generally about 18 months.

The following method allows calculation of the rating of a proposed capacitor bank, based on billing details, where the tariff structure corresponds with (or is similar to) the one described in sub-clause 2.1 of this chapter. The method determines the minimum compensation required to avoid these charges which are based on kvarh consumption. The procedure is as follows: c refer to the bills covering consumption for the 5 months of winter (in France these are November to March inclusive). Note: in tropical climates the summer months may constitute the period of heaviest loading and highest peaks (owing to extensive airconditioning loads) so that a consequent variation of high-tariff periods is necessary in this case. The remainder of this example will assume Winter conditions in France. c identify the line on the bills referring to "reactive-energy consumed" and "kvarh to be charged". Choose the bill which shows the highest charge for kvarh (after checking that this was not due to some exceptional situation). For example: 15,966 kvarh in January. c evaluate the total period of loaded operation of the installation for that month, for instance: 220 hours (22 days x 10 hours). The hours which must be counted are those occurring during the heaviest load and the highest peak loads occurring on the power system. These are given in the tariff documents, and are (commonly) during a 16 hour period each day, either from 06.00 H to 22.00 H or from 07.00 H to 23.00 H according to the region. Outside these periods, no charge is made for kvarh consumption.

c the necessary value of compensation kvarh billed in kvar = = Qc number of hours of operation* * in the billing period, during the hours for which reactive energy is charged for the case considered above: 15,966 kvarh Qc = = 73 kvar 220 h The rating of the installed capacitor bank is generally chosen to be slightly larger than that calculated. Certain manufacturers can provide "sliderules" especially designed to facilitate these kinds of calculation, according to particular tariffs. These devices and accompanying documentation advise on suitable equipment and control schemes, as well as drawing attention to constraints imposed by harmonic voltages on the power system. Such voltages require either overdimensioned capacitors (in terms of heat-dissipation, voltage and current ratings) and/or harmonic-suppression inductors or filters (see Appendix E3).

5.4 method based on reduction of declared maximum apparent power (kVA) for 2-part tariffs based partly on a declared value of kVA, a table (E16) allows determination of the kvar of compensation required to reduce the value of kVA declared, and to avoid exceeding it.

For consumers whose tariffs are based on a fixed charge per kVA declared, plus a charge per kWh consumed, it is evident that a reduction in declared kVA would be beneficial. The diagram of figure E18 shows that as the power factor improves, the kVA value diminishes for a given value of kW (P). The improvement of the power factor is aimed at (apart from other advantages previously mentioned) reducing the declared level and never exceeding it, thereby avoiding the payment of an excessive price per kVA during the periods of excess, and/or tripping of the the main circuit breaker. Table E17 indicates the value of kvar of compensation per kW of load, required to improve from one value of power factor to another.

Example: A supermarket has a declared load of 122 kVA at a power factor of 0.7 lagging, i.e. an active-power load of 85.4 kW. The particular contract for this consumer was based on stepped values of declared kVA (in steps of 6 kVA up to 108 kVA, and 12 kVA steps above that value, this is a common feature in many types of two-part tariff). In the case being considered, the consumer was billed on the basis of 132 kVA. Referring to table E17, it can be seen that a 60 kvar bank of capacitors will improve the power factor of the load from 0.7 to 0.95 (0.691 x 85.4 = 59 kvar in the table). The declared value of kVA will then be 85.4 = 90 kVA, i.e. an improvement of 30%. 0.95

P = 85.4 kW

ϕ ϕ' Q' cos ϕ = 0.7 cos ϕ' = 0.95 S = 122 kVA S' = 90 kVA Q = 87.1 kvar Qc = 59 kvar Q' = 28.1 kvar

S' Q S

Qc

fig. E18: reduction of declared maximum kVA by power-factor improvement. power factor improvement - E13

6. compensation at the terminals of a transformer

E 6.1 compensation to increase the available active power output the installation of a bank of capacitors can avoid the need to change a transformer in the event of a load increase.

Steps similar to those taken to reduce the declared maximum kVA, i.e. improvement of the load power factor, as discussed in subclause 5.4, will maximise the available transformer capacity, i.e. to supply more active power. Cases can arise where the replacement of a transformer by a larger unit, to cater for load growth, may be avoided by this means. Table E20 shows directly the power (kW) capability of fully-loaded transformers at different load power factors, from which the increase of active-power output, as the value of power factor increases, can be obtained. Example: Refer to figure E19. An installation is supplied from a 630 kVA transformer loaded at 450 kW (P1) with a mean power factor of 0.8 lagging. The apparent power S1 = 450 = 562 kVA 0.8 The corresponding reactive power

Total reactive power required by the installation before compensation: Q1 + Q2 = 337 + 102 = 439 kvar. So that the minimum size of capacitor bank to instal: Qkvar = 439 - 307 = 132 kvar. It should be noted that this calculation has not taken account of peak loads and their duration. The best possible improvement, i.e. correction which attains a PF of 1 would permit a power reserve for the transformer of 630 - 550 = 80 kW. The capacitor bank would then have to be rated at 439 kvar. Q

S2

Q2 Q

Q1= S12 − P12 = 337 kvar The anticipated load increase P2 = 100 kW at a power factor of 0.7 lagging. The apparent power S2 = 100 = 143 kVA 0.7 The corresponding reactive power Q2 = S22 − P22 = 102 kvar What is the minimum value of capacitive kvar to be installed, in order to avoid a change of transformer? Total power now to be supplied: P = P1 + P2 = 550 kW. The maximum reactive power capability of the 630 kVA transformer when delivering 550 kW is: Qm = S 2 - P 2

P2

S1 S Q1

Qm

P1

P

fig. E19: compensation Q allows the installation-load extension S2 to be added, without the need to replace the existing transformer, the output of which is limited to S.

Qm = 630 2 - 550 2 = 307 kvar

tan ϕ

cos ϕ

0.00 0.20 0.29 0.36 0.43 0.48 0.54 0.59 0.65 0.70 0.75 0.80 0.86 0.91 0.96 1.02

1 0.98 0.96 0.94 0.92 0.90 0.88 0.86 0.84 0.82 0.80 0.78 0.76 0.74 0.72 0.70

nominal kVA rating of transformers (in kVA) 100 160 250 315 400 100 160 250 315 400 98 157 245 309 392 96 154 240 302 384 94 150 235 296 376 92 147 230 290 368 90 144 225 284 360 88 141 220 277 352 86 138 215 271 344 84 134 210 265 336 82 131 205 258 328 80 128 200 252 320 78 125 195 246 312 76 122 190 239 304 74 118 185 233 296 72 115 180 227 288 70 112 175 220 280

500 500 490 480 470 460 450 440 430 420 410 400 390 380 370 360 350

630 630 617 605 592 580 567 554 541 529 517 504 491 479 466 454 441

800 800 784 768 752 736 720 704 688 672 656 640 624 608 592 576 560

1000 1000 980 960 940 920 900 880 860 840 820 800 780 760 740 720 700

1250 1250 1225 1200 1175 1150 1125 1100 1075 1050 1025 1000 975 950 925 900 875

table E20: active-power capability of fully-loaded transformers, when supplying loads at different values of power factor.


1600 1600 1568 1536 1504 1472 1440 1408 1376 1344 1312 1280 1248 1216 1184 1152 1120

2000 2000 1960 1920 1880 1840 1800 1760 1720 1680 1640 1600 1560 1520 1480 1440 1400

E 6.2 compensation of reactive energy absorbed by the transformer where metering is carried out at the HV side of a transformer, the reactive-energy losses in the transformer may (depending on the tariff) need to be compensated.

the reactive power absorbed by a transformer cannot be neglected, and can amount to (about) 5% of the transformer rating when supplying its full load. Compensation can be provided by a bank of capacitors. in transformers, reactive power is absorbed by both shunt (magnetizing) and series (leakage flux) reactances. Complete compensation can be provided by a bank of shunt-connected LV capacitors.

the nature of transformer inductive reactances All previous references have been to shuntconnected devices such as those used in normal loads, and PF-correcting capacitor banks etc. The reason for this is that shuntconnected plant requires (by far) the largest quantities of reactive energy in power systems; however, series-connected reactances, such as the inductive reactances of power lines and the leakage reactance of transformer windings, etc., also absorb reactive energy. Where metering is carried out at the HV side of a transformer, the reactive-energy losses in the transformer may (depending on the tariff) need to be compensated. As far as reactive-energy losses only are concerned, a transformer may be

reactive-power absorption in series-connected (leakage flux) reactance A simple illustration of this phenomenon is given by the vector diagram of figure E21. The reactive-current component through the load = I sin ϕ so that kvarL = VI sin ϕ. The reactive-current component from the source = I sin ϕ' so that kvars = EI sin ϕ' where V and E are expressed in kV. It can be seen that E > V and sin ϕ' > sin ϕ. The difference between EI sin ϕ' and VI sin ϕ gives the kvar per phase absorbed by XL. It can be shown that this kvar value is equal to I2XL (which is analogous to the I2R activepower (kW) losses due to the series resistance of power lines, etc.). From the I2XL formula it is very simple to deduce the kvar absorbed at any load value for a given transformer, as follows: If per-unit values are used (instead of percentage values) direct multiplication of I and XL can be carried out. XL

E source

V load

E V ϕ

ϕ'

I sin ϕ

I sin ϕ'

fig. E21: reactive power absorption by series inductance. Example: A 630 kVA transformer with a short-circuit reactance voltage of 4% is fully loaded. What is its reactive-power (kvar) loss? 4% = 0.04 pu Ipu = 1 loss = I2XL = 12 x 0.04 = 0.04 pu kvar where 1 pu = 630 kVA

IXL

represented by the elementary diagram of figure E22. All reactance values are referred to the secondary side of the transformer, where the shunt branch represents the magnetizing-current path. The magnetizing current remains practically constant (at about 1.8 % of full-load current) from no load to full load, in normal circumstances, i.e. with a constant primary voltage, so that a shunt capacitor of fixed value can be installed at the HV or LV side, to compensate for the reactive energy absorbed. perfect transformer primary winding

secondary winding

leakage reactance magnetizing reactance

fig. E22: transformer reactances (per phase). The 3-phase kvar losses are 630 x 0.04 = 25.2 kvar (or, quite simply, 4% of 630 kVA). At half load i.e. I = 0.5 pu the losses will be 0.52 x 0.04 = 0.01 pu = 630 x 0.01 = 6.3 kvar and so on... This example, and the vector diagram of figure E21 show that: c the power factor at the primary side of a loaded transformer is different (normally lower) than that at the secondary side (due to the absorption of vars), c that full-load kvar losses due to leakage reactance are equal to the transformer percentage reactance (4% reactance means a kvar loss equal to 4% of the kVA rating of the transformer), c that kvar losses due to leakage reactance vary according to the current (or kVA loading) squared. To determine the total kvar losses of a transformer the constant magnetizing-current circuit losses (approx. 1.8% of the transformer kVA rating) must be added to the foregoing "series" losses. Table E24 shows the no-load and full-load kvar losses for typical distribution transformers. In principle, series inductances can be compensated by fixed series capacitors (as is commonly the case for long HV transmission lines). This arrangement is operationally difficult, however, so that, at the voltage levels covered by this guide, shunt compensation is always applied. In the case of HV metering, it is sufficient to raise the power factor to a point where the transformer plus load reactive-power consumption is below the level at which a billing charge is made. This level depends on the tariff, but often corresponds to a tan ϕ value of 0.31 (cos ϕ of 0.955). As a matter of interest, the kvar losses in a transformer can be completely compensated by adjusting the capacitor bank to give the load a (slightly) leading power factor. In such a case, all of the kvar of the transformer is being supplied from the capacitor bank, while the input to the HV side of the transformer is at unity power factor, as shown in figure E23.


6. compensation at the terminals of a transformer (continued)

E 6.2 compensation of reactive energy absorbed by the transformer (continued) E (input voltage) IXL

I ϕ

V (load voltage) I0 compensation current

load current

fig. E23: overcompensation of load to completely compensate transformer reactive-power losses. In practical terms, therefore, compensation for transformer-absorbed kvar is included in the capacitors primarily intended for powerfactor correction of the load, either globally, partially, or in the individual mode. Unlike most other kvar-absorbing items, the transformer absorption (i.e. the part due to the leakage reactance) changes significantly with variations of load level, so that, if individual compensation is applied to the transformer, then an average level of loading will have to be assumed. Fortunately, this kvar consumption generally forms only a relatively small part of the total reactive power of an installation, and so mismatching of compensation at times of load change is not likely to be a problem. Table E24 indicates typical kvar loss values for the magnetizing circuit (“no-load kvar” columns), as well as for the total losses at full load, for a standard range of distribution transformers supplied at 20 kV (which include the losses due to the leakage reactance). rated power kVA 50 100 160 250 315 400 500 630 800 1 000 1 250 1 600 2 000 2 500

reactive power (kvar) to be compensated oil immersed type cast resin type no load full load no load full load 1.5 2.9 2.5 5.9 2.5 8.2 3.7 9.6 3.7 12.9 5.3 14.7 5.0 19.5 6.3 18.3 5.7 24.0 7.6 22.9 6.0 29.4 9.5 28.7 7.5 36.8 11.3 35.7 8.2 45.2 20 66.8 10.4 57.5 24.0 82.6 12.0 71.0 27.5 100.8 15.0 88.8 32.0 125.9 19.2 113.9 38.0 155.3 22.0 140.6 45.0 191.5 30.0 178.2

table E24: reactive power consumption of distribution transformers with 20 kV primary windings. Note: for a 630 kVA transformer, the range of kvar losses extends from 11.3 at no load to 35.7 kvar at full load. These values correspond closely to those given in the worked example above.


7. compensation at the terminals of an induction motor

E 7.1 connection of a capacitor bank and protection settings individual motor compensation is recommended where the motor power (kVA) is large with respect to the declared power of the installation.

general precautions Because of the small kW consumption, the power factor of a motor is very low at no-load or on light load. The reactive current of the motor remains practically constant at all loads, so that a number of unloaded motors constitute a consumption of reactive power which is generally detrimental to an installation, for reasons explained in preceding sections. Two good general rules therefore are that unloaded motors should be switched off, and motors should not be oversized (since they will then be lightly loaded).

connection The bank of capacitors should be connected directly to the terminals of the motor.

special motors It is recommended that special motors (stepping, plugging, inching, reversing motors, etc.) should not be compensated.

effect on protection settings After applying compensation to a motor, the current to the motor-capacitor combination will be lower than before, assuming the same motor-driven load conditions. This is because a significant part of the reactive component of the motor current is being supplied from the capacitor, as shown in figure E25. Where the overcurrent protection devices of the motor are located upstream of the motorcapacitor connection (and this will always be the case for terminal-connected capacitors), the overcurrent relay settings must be reduced in the ratio: cos ϕ before compensation cos ϕ after compensation For motors compensated in accordance with the kvar values indicated in Table E28 (maximum values recommended for avoidance of self-excitation of standard induction motors, as discussed in sub-clause 7.2), the above-mentioned ratio will have a value similar to that indicated for the corresponding motor speed in Table E26. before compensation

speed in R.P.M. 750 1000 1500 3000

reduction factor 0.88 0.90 0.91 0.93

table E26: reduction factor for overcurrent protection after compensation.

after compensation transformer

power made available active power C

M

motor

M

reactive power supplied by capacitor

fig. E25: before compensation, the transformer supplies all the reactive power; after compensation, the capacitor supplies a large part of the reactive power.


7. compensation at the terminals of an induction motor (continued)

E 7.2 how self-excitation of an induction motor can be avoided when a capacitor bank is connected to the terminals of an induction motor, it is important to check that the size of the bank is less than that at which self-excitation can occur.

When a motor is driving a high-inertia load, the motor will continue to rotate (unless deliberately braked) after the motor supply has been switched off. The "magnetic inertia" of the rotor circuit means that an emf will be generated in the stator windings for a short period after switching off, and would normally reduce to zero after 1 or 2 cycles, in the case of an uncompensated motor. Compensation capacitors however, constitute a 3-phase "wattless" load for this decaying emf, which causes capacitive currents to flow through the stator windings. These stator currents will produce a rotating magnetic field in the rotor which acts exactly along the same axis and in the same direction as that of the decaying magnetic field. The rotor flux consequently increases; the stator currents increase; and the voltage at the terminals of the motor increases; sometimes to dangerously-high levels. This phenomenon is known as self-excitation and is one reason why a.c. generators are not normally operated at leading power factors, i.e. there is a tendency to spontaneously (and uncontrollably) selfexcite.

Notes 1. The characteristics of a motor being driven by the inertia of the load are not rigorously identical to its no-load characteristics. This assumption, however, is sufficiently accurate for practical purposes. 2. With the motor acting as a generator, the currents circulating are largely reactive, so that the braking (retarding) effect on the motor is mainly due only to the load represented by the cooling fan in the motor. 3. The (almost 90° lagging) current taken from the supply in normal circumstances by the unloaded motor, and the (almost 90° leading) current supplied to the capacitors by the motor acting as a generator, both have the same phase relationship to the terminal voltage. It is for this reason that the two characteristics may be superimposed on the graph. In order to avoid self-excitation as described above, the kvar rating of the capacitor bank must be limited to the following maximum value: Qc i 0,9 Io Un e where Io = the no-load current of the motor and Un = phase-tophase nominal voltage of the motor in kV. Table E28 gives appropriate values of Qc corresponding to this criterion. Example A 75 kW, 3 000 Rpm, 400 V, 3-phase motor may have a capacitor bank no larger than 17 kvar according to Table E28. The table values are, in general, too small to adequately compensate the motor to the level of cos ϕ normally required. Additional compensation can, however, be applied to the system, for example an overall bank, installed for global compensation of a number of smaller appliances. High-inertia motors and/or loads In any installation where high-inertia motordriven loads exist, the circuit breakers or contactors controlling such motors should, in the event of total loss of power supply, be rapidly tripped. If this precaution is not taken, then selfexcitation to very high voltages is likely to occur, since all other banks of capacitors in the installation will effectively be in parallel with those of the high-inertia motors.

fig. E27: connection of the capacitor bank to the motor. The protection scheme for these motors should therefore include an overvoltage tripping relay, together with reverse-power checking contacts (the motor will feed power to the rest of the installation, until the stored inertial energy is dissipated). An undervoltage relay would not be suitable because the voltage is not only maintained, but will increase, immediately following the loss of power supply.


E If the capacitor bank associated with a highinertia motor is larger than that recommended in Table E28, then it should be separately controlled by a circuit breaker or contactor, which trips in unison with the main motorcontrolling circuit breaker or contactor, as shown in figure E27. Closing of the main contactor is commonly subject to the capacitor contactor being previously closed. 3-phase motors 230/400 V nominal kvar to be installed power speed of rotation (RPM) kW hp 3000 1500 1000 750 22 30 6 8 9 10 30 40 7.5 10 11 12.5 37 50 9 11 12.5 16 45 60 11 13 14 17 55 75 13 17 18 21 75 100 17 22 25 28 90 125 20 25 27 30 110 150 24 29 33 37 132 180 31 36 38 43 160 218 35 41 44 52 200 274 43 47 53 61 250 340 52 57 63 71 280 380 57 63 70 79 355 482 67 76 86 98 400 544 78 82 97 106 450 610 87 93 107 117 table E28: maximum kvar of P.F. correction applicable to motor terminals without risk of self-excitation. Note Exact sizing of capacitor unit for a particular motor is only possible when the "no-load current" or "no-load magnetising" kvar is known.


8. example of an installation before and after power-factor correction

E installation before P.F. correction

installation after P.F. correction

*→→→ kVA=kW+kvar

→→→ kVA=kW+kvar

kVA kW

kvar

630 kVA

c kvarh are billed heavily above the declared level, c apparent power kVA is significantly greater than the kW demand, c the corresponding excess current causes losses (kWh) which are billed, c the installation must be over-dimensioned. * the arrows denote vector quantities.

Characteristics of the installation 500 kW cos ϕ = 0.75 c transformer is overloaded c the power demand is P 500 S= = = 665 kVA cos ϕ 0.75 S = apparent power

c the consumption of kvarh is v eliminated, or v reduced, according to the cos ϕ required, c the tariff penalties v for reactive energy where applicable v for the entire bill in some cases are eliminated, c the fixed charge based on kVA demand is adjusted to be close to the active power kW demand.

kVA kW

Characteristics of the installation 500 kW cos ϕ = 0.928 c transformer no longer overloaded c the power-demand is 539 kVA c there is 14% spare-transformer capacity available.

630 kVA

400 V

400 V

c the current flowing into the installation downstream of the circuit breaker is P I= = 960 A eU cos ϕ

c the current flowing into the installation through the circuit breaker is 778 A.

c losses in cables are calculated as a function of the current squared: (960)2 P=I2R

c the losses in the cables are (778)2 reduced to = 65% of the former value, (960)2 thereby economizing in kWh consumed.

cos ϕ = 0.75 c reactive energy is supplied through the transformer and via the installation wiring, c the transformer, circuit breaker, and cables must be over-dimensioned.

cos ϕ = 0.928 c reactive energy is supplied by the capacitor bank. kvar

Capacitor bank rating is 250 kvar in 5 automatically-controlled steps of 50 kvar.

cos ϕ = 0.75 workshop

cos ϕ = 0.928 workshop

Note: In fact, the cos ϕ of the workshop remains at 0.75 but cos ϕ for all the installation upstream of the capacitor bank to the transformer LV terminals is 0.928. As mentioned in Sub-clause 6.2, the cos ϕ at the HV side of the transformer will be slightly lower, * due to the reactive power losses in the transformer. * moreso in the pre-corrected case.

fig. E29: technical-economic comparison of an installation before and after power-factor correction.


9. the effect of harmonics on the rating of a capacitor bank

E 9.1 problems arising from power-system harmonics Equipment which uses power electronics components (variable-speed motor controllers, thyristor-controlled rectifiers, etc.) have, in recent years, considerably increased the problems caused by harmonics in powersupply systems. Harmonics have existed from the earliest days of the industry and were (and still are) caused by the non-linear magnetizing impedances of transformers, reactors, fluorescent lamp ballasts, etc. Harmonics on symmetrical 3-phase power systems are generally* odd-numbered: 3rd, 5th, 7th, 9th..., and the magnitude decreases as the order of the harmonic increases. All of these features may be used in various ways to reduce specific harmonics to negligible values - total elimination is not possible. In this section, practical means of reducing the influence of harmonics are recommended, with particular reference to capacitor banks. Capacitors are especially sensitive to harmonic components of the supply voltage due to the fact that capacitive reactance decreases as the frequency increases. In practice, this means that a relatively small percentage of harmonic voltage can cause a significant current to flow in the capacitor circuit. The presence of harmonic components causes the (normally sinusoidal) wave form of voltage or current to be distorted; the

greater the harmonic content, the greater the degree of distortion . If the natural frequency of the capacitor bank/ power-system reactance combination is close to a particular harmonic, then partial resonance will occur, with amplified values of voltage and current at the harmonic frequency concerned. In this particular case, the elevated current will cause overheating of the capacitor, with degradation of the dielectric, which may result in its eventual failure. Several solutions to these problems are available, which aim basically at reducing the distortion of the supply-voltage wave form, between the equipment causing the distortion, and the bank of capacitors in question. This is generally accomplished by shunt connected harmonic filter and/or harmonic-suppression reactors.

countering the effects of harmonics

Harmonic distortion of the voltage wave frequently produces a "peaky" wave form, in which the peak value of the normal sinusoidal wave is increased. This possibility, together with other overvoltage conditions likely to occur when countering the effects of resonance, as described below, are taken into account by increasing the insulation level above that of "standard" capacitors. In many instances, these two counter measures are all that is necessary to achieve satisfactory operation.

* With the advent of power electronics devices, and associated non-linear components, even-numbered harmonics are now sometimes encountered.

9.2 possible solutions harmonics are taken into account mainly by oversizing capacitors and including harmonic-suppression reactors in series with them.

The presence of harmonics in the supply voltage results in abnormally high current levels through the capacitors. An allowance is made for this by designing for an r.m.s. value of current equal to 1.3 times the nominal rated current. All series elements, such as connections, fuses, switches, etc., associated with the capacitors are similarly oversized, between 1.3 to 1.5 times nominal rating.

countering the effects of resonance Capacitors are linear reactive devices, and consequently do not generate harmonics. The installation of capacitors in a power system (in which the impedances are predominantly inductive) can, however, result in total or partial resonance occurring at one of the harmonic frequencies. The harmonic order ho of the natural resonant frequency between the system inductance and the capacitor bank is given by SSC / Q where Ssc = the level of system short-circuit kVA at the point of connection of the capacitor Q = capacitor bank rating in kvar; and ho = the harmonic order of the natural frequency fo i.e. fo/50 for a 50 Hz system, or fo/60 for a 60 Hz system.

For example: SSC / Q may give a value for ho of 2.93 which shows that the natural frequency of the capacitor/system-inductance combination is close to the 3rd harmonic frequency of the system. From ho = fo/50 it can be seen that fo = 50 ho = 50 x 2.93 = 146.5 Hz The closer a natural frequency approaches one of the harmonics present on the system, the greater will be the (undesirable) effect. In the above example, strong resonant conditions with the 3rd harmonic component of a distorted wave would certainly occur.


9. the effect of harmonics on the rating of a capacitor bank (continued)

E 9.2 possible solutions

(continued)

countering the effects of resonance (continued) In such cases, steps are taken to change the natural frequency to a value which will not resonate with any of the harmonics known to be present. This is achieved by the addition of a harmonic-suppression inductor connected in series with the capacitor bank. On 50 Hz systems, these reactors are often adjusted to bring the resonant frequency of the combination, i.e. the capacitor bank + reactors to 190 Hz. The reactors are adjusted to 228 Hz for a 60 Hz system. These frequencies correspond to a value for ho of 3.8 for a 50 Hz system, i.e. approximately mid-way between the 3rd and 5th harmonics.

In this arrangement, the presence of the reactor increases the fundamental-frequency (50 Hz or 60 Hz) current by a small amount (7-8%) and therefore the voltage across the capacitor in the same proportion. This feature is taken into account, for example, by using capacitors which are designed for 440 V operation on 400 V systems.

9.3 choosing the optimum solution A choice is made from the following parameters: c Gh = the sum of the kVA ratings of all harmonic-generating devices (static converters, inverters, speed controllers, etc.) connected to the busbars from which the capacitor bank is supplied. If the ratings of some of these devices are quoted in kW only, assume an average power factor of 0.7 to obtain the kVA ratings. c Ssc = the 3-phase short-circuit level in kVA at the terminals of the capacitor bank. capacitors supplied at LV via transformer(s) c general rule valid for any size of transformer Ssc i Gh i Ssc Gh i Ssc 120 120 70 standard capacitors capacitor voltage rating increased by 10% (except 230 V units) c simplified rule if transformer(s) rating Sn i 2 MVA Gh i 0.15 Sn 0.15 Sn < Gh i 0.25 Sn standard capacitors capacitor voltage rating increased by 10% (except 230 V units)

c Sn = the sum of the kVA ratings of all transformers supplying (i.e. directly connected to) the system level of which the busbars form a part. If a number of transformers are operating in parallel, the removal from service of one or more, will significantly change the values of Ssc and Sn. From these parameters, a choice of capacitor specification which will ensure an acceptable level of operation with the system harmonic voltages and currents, can be made, by reference to the following table.

Gh > Ssc 70 capacitor voltage rating increased by 10% + harmonic-suppression reactor 0.25 Sn < Gh i 0.60 Sn capacitor voltage rating increased by 10% + harmonic suppression reactor

Gh > 0.60 Sn filters

table E30: choice of solutions for limiting harmonics associated with a LV capacitor bank.


E examples Three cases are presented, showing (respectively) situations in which standard, overdimensioned, and overdimensioned plus harmonic-suppression-equipped capacitor banks should be installed. Example 1: 500 kVA transformer having 4% short-circuit voltage. Total rating of harmonic-generating devices Gh = 50 kVA 100 Ssc = 500 x = 12,500 kVA 4 Ssc 12,500 = = 104 120 120 Ssc Gh = 50 i 120 Solution: use standard capacitors. Example 2: 1,000 kVA transformer having 6% short-circuit voltage. Total rating of harmonic-generating devices Gh = 220 kVA 100 Ssc = 1,000 x = 16,667 kVA 6 Ssc 16,667 = = 139 120 120 Ssc 16,667 = = 238 70 70 Ssc Ssc Gh = 220 is between and 120 70 Solution: use overated (440 V) capacitors.

Example 3: 630 kVA transformer having 4% short-circuit voltage. Total rating of harmonic-generating devices Gh = 250 kVA 100 Ssc = 630 x = 15,750 kVA 4 Ssc 15,750 = = 225 70 70 Scc Gh = 250 > 70 Solution: use overated (440 V) capacitors and harmonic-suppression reactors.

9.4 possible effects of power-factor-correction capacitors on the power-supply system it is necessary to ensure that interaction between harmonicgenerating devices and P.F. correction capacitors, does not result in unacceptable levels of voltage and/or current wave-form distortion on the power-supply network.

Power-supply authorities generally impose a strict limit on the total-harmonic distortion (THD) permitted at the point of power supply to a consumer. The degree of distortion is measured as the ratio of the r.m.s. value of all harmonics present, with respect to the r.m.s. value of the fundamental frequency wave (50 or 60 Hz). For LV loads supplied through a transformer from a high-voltage service connection, this means that a maximum value of 4 or 5% for voltage THD is permissible at the LV terminals of the transformer.

If this value of THD is unattainable, then recourse has to be made to low-voltage L-C series filters. Such filters are shuntconnected, and are tuned to resonate at harmonic frequencies to which they present practically zero impedance. Filters connected in this way fortuitously have the added benefit of contributing to reactivepower compensation for the installation.


10. implementation of capacitor banks

E 10.1 capacitor elements technology The capacitors are dry-type units (i.e. are not impregnated by liquid dielectric) comprising metallized polypropylene self-healing film in the form of a two-film roll. They are protected by a high-quality system (overpressure disconnector used with an HPC fuse) which switches off the capacitor if an internal fault occurs. The protection scheme operates as follows: c a short-circuit through the dielectric will blow the fuse, c current levels greater than normal, but insufficient to blow the fuse sometimes occur, e.g. due to a microscopic flow in the dielectric film. Such "faults" often re-seal due to local heating caused by the leakage current, i.e. the units are said to be "self-healing"; v if the leakage current persists, the defect may develop into a short-circuit, and the fuse will blow, v gas produced by vaporizing of the metallisation at the faulty location will gradually build up a pressure within the plastic container, and will eventually operate a pressure-sensitive device to short-circuit the unit, thereby causing the fuse to blow. Capacitors are made of insulating material providing them with double insulation and avoiding the need for a ground connection.

fuse discharge resistor short-circuiting contacts overpressure device

fig. E31: cross-section of a capacitor element.

electrical characteristics standards operating rated voltage range rated frequency capacitance tolerance temperature maximum temperature range average temperature over 24 h average annual temperature minimum temperature insulation level permissible current overload permissible voltage overload current on 400 V - 50 Hz supply consumption on 230 V - 50 Hz supply


IEC 831, NF C 54-104, VDE 0560 CSA standards, UL tests 400 V 50 Hz 0 to + 5% 55 °C 45 °C 35 °C -25 °C 50 Hz 1 mn withstand voltage: 6 kV 1.2/50 µs impulse withstand voltage: 25 kV standard range H range 30% 50% 10% 20% 2 A/kvar 2.2 A/kvar 3.5 A/kvar

E 10.2 choice of protection, control devices, and connecting cables due to the possible presence of harmonic currents and to manufacturing tolerances, components must be over-sized, and based on 1.5 times rated current.

component dimensions The choice of upstream cables and protection and control devices depends on the current loading. For capacitors, the current is a function of: c the applied voltage and its harmonics, c the capacitance value. The nominal current In a capacitor of kvar rating Q, supplied from a 3-phase system having a phase/phase voltage of Un kilo-volts, is given by: Q A In = 3 Un The permitted range of applied voltage at fundamental frequency, plus harmonic components, together with manufacturing tolerances of actual capacitance (for a declared nominal value) can result in a 50% increase above the calculated value of current.

Approximately 30% of this increase is due to the voltage increases, while a further 15% is due to the range of manufacturing tolerances, so that 1.3 x 1.15 = 1.5 In. All components carrying the capacitor current therefore, must be adequate to cover this "worst-case" condition, in an ambient temperature of 50 °C maximum. In the case where higher temperatures (than 50 °C) occur in enclosures, etc. derating of the components will be necessary.

protection At the instant of closing a switch to energize a capacitor, the current is limited only by the impedances of the network upstream of the capacitor, so that high peak values of current will occur for a brief period, rapidly falling to normal operating values. This transient overcurrent however, is generally a high-frequency phenomenon, which is superimposed on the 50 Hz (or 60 Hz) current wave. The first peak of transient high-frequency or (sometimes) unidirectional* current has the greatest magnitude. The maximum value attainable, when charging an initially uncharged capacitor, will occur if the closing switch contacts touch at the instant of peak power-supply voltage. For this condition, the maximum highfrequency peak current is given by: 2C A 3 Lo Where U = system phase-to-phase voltage in Volts C = capacitance of capacitor in Farads Lo = inductance of system impedance in Henrys (system resistance is ignored). The frequency fo of the transient current surge is given by: 1 fo = Hz 2Π LoC

Ip = U

c for a single capacitor bank, the upstream cables and transformers constitute the predominant part of Lo (the system inductance), c where a bank of capacitors is automatically switched in steps, however, those units which are already in service will initially discharge into an uncharged capacitor group at the instant of switching it into service. The transient in-rush current from the previouslycharged units will then amount to an initial peak of 2C n ( )A 3 L n +1 where L = the supply cable inductance in series with each capacitor n = the number of capacitor steps already energized before closure of the switch C = capacitance of each group forming 1 step (all steps are electrically identical). The frequency f'o of the current from the energized capacitors is given by 1 fo' = Hz 2Π LC

I' P = U

The total inrush current is the sum of the two infeeds, i.e. from the system and from the previously-charged bank. Generally, the frequencies of the two infeeds will not be equal.

* In general, peak unidirectional currents are lower than the first peaks of high-frequency currents.


10. implementation of capacitor banks (continued)

E 10.2 choice of protection, control devices, and connecting cables (continued) The peak value of this transient current must not exceed 100 times the rated current of the capacitors in one step of a multi-step bank (IEC 831-1). This maximum transient current peak occurs when the last step is energized. It is sometimes necessary to install small series inductors to achieve this limitation, in which case the manufacturer of the capacitors should be consulted. In order to avoid undesirable nuisance tripping of controlling circuit breakers at the instant of energizing a capacitor bank, the instantaneous elements of overcurrent tripping relays should be given a suitably high setting. Note: The short-circuit current-breaking rating of the circuit breaker must be adequate to match the short-circuit level existing at the point of connection of the capacitor bank.

cross-sectional area of conductors The current rating of cables, as previously noted, must be based on 1.5 times the nominal current rating for the capacitor bank concerned.

voltage transients High-frequency voltage transients accompany the high-frequency current transients, the maximum voltage transient peak never (in the absence of steady-state harmonics) exceeds twice the peak value of rated voltage, when switching an uncharged capacitor into service. In the case of a capacitor being already charged at the instant of switch closure, however, the voltage transient can attain a maximum value approaching 3 times the normal rated peak value. This maximum condition occurs only if: c the existing voltage at the capacitor is equal to the peak value of rated voltage, and c the switch contacts close at the instant of peak supply voltage, and c the polarity of the power-supply voltage is opposite to that of the charged capacitor. In such a situation, the current transient will be at its maximum possible value, viz: twice that of its maximum when closing on to an initially uncharged capacitor, as previously noted.


Section H1-2 of chapter H facilitates the selection of suitable cables, or other types of conductor, as a function of their characteristics, method of installation, and ambient temperature, etc.

For any other values of voltage and polarity on the pre-charged capacitor, the transient peaks of voltage and current will be less than those mentioned above. In the particular case of peak rated voltage on the capacitor having the same polarity as that of the supply voltage, and closing the switch at the instant of supply-voltage peak, there would be no voltage or current transients. Where automatic switching of stepped banks of capacitors is considered, therefore, care must be taken to ensure that a section of capacitors about to be energized is fully discharged. The discharge delay time may be shortened, if necessary, by using discharge resistors of a lower resistance value.

11. elementary harmonic filters

E For this appendix, the more commonlyoccurring odd-numbered harmonics are shown in the diagrams. Before the advent of power-electronics, even-numbered harmonics were rarely encountered, so that the 100 Hz (on 50 Hz systems) separating one harmonic frequency from the next made the task of filters (despite manufacturing tolerances, and impedance changes with temperature, etc.) relatively straightforward, so that satisfactory results were achieved (and still are in all but exceptional cases) by the methods described below. If it is required to eliminate (almost) a harmonic voltage existing across two points A and B in a network, a series-connected LCR circuit (figure AE3-1(a)) tuned to resonate at the harmonic frequency concerned, will constitute a virtual shortcircuit to the current of that harmonic frequency, thereby reducing VAB(h) to practically zero. The same procedure can be adopted for any number harmonic frequencies known to be present, the individual filters being connected in parallel across the points A-B (figure AE3-1(b)). (a)

A C L

f=

1 2 π v LC

R B (b)

A

protected network and power source

harmonic source

An exact analysis of the combination is not simple, since each filter is affected by those in parallel with it, as well as by the powersystem source reactance shunting the filter bank (shown dotted in figure AE3-3). When all factors have been taken into account, including a degree of damping caused by the load impedance, the response of the filter bank in terms of its impedance at different frequencies is shown in figure AE3-2. IZI Ω

fh/f50 1

5

7

11

13 (harmonic order)

fig. AE3-2. It will be seen that, at each harmonic frequency for which a filter has been provided, the impedance is very low, while at intermediate frequencies, high-impedance values occur. Care should be taken to ensure that frequencies corresponding to the lowimpedance point are not close to control frequencies (such as those of ripple-control schemes used by many power companies for remote control of power-network devices). Otherwise, the control signals will be virtually short-circuited. Harmonic-producing equipment must create the harmonic e.m.f.s. and resulting currents in order to function correctly. The role of a filter bank, as described, is to allow a free flow of harmonic currents to circulate between the harmonic source and the filter bank, while practically eliminating these currents and voltages from the rest of the network.

B

fig. AE3-1.

Appendix E3 - 1

E A

Zh a

loads

Vh harmonic source 5

7

11

13

filter bank

fig. AE3-3. In figure AE3-3, it will be seen that since the filters are practically short-circuits to harmonics, most of the harmonic voltage Vh will be dropped across the internal impedance Zh of the harmonic source and that small harmonic-current components only will pass through the power-system source impedance Xs and the loads (the latter having relatively high impedance). Since at fundamental frequency the capacitive reactance of each filter is much greater than its inductive reactance, most of the power-frequency voltage will appear across the capacitors, so that a useful contribution to any power-factor correction requirement is fortuitously available.

damped harmonic filters As noted in Chapter E, Sub-clause 9.1, the magnitude of harmonic emfs diminishes as the order of the harmonic increases. The filtering requirements are not, therefore, so critical for high-order harmonics as those necessary for lower-order harmonics. For that reason, the filter for the highest harmonic of a bank, such as that shown in figure AE3-1 (b) is often damped, by connecting a resistor in parallel with the reactor. The result is a filter which is less effective (but adequate) at its tuned frequency, while at all higher frequencies, the impedance will be low (inductive/resistive), approaching the value of the resistor only (figure AE3-4) as the frequency increases (i.e. it forms a "highpass" filter). Such a high-pass filter is commonly used for the highest-order harmonic filter (the 11th or 13th for example) of a bank, as shown dotted in figure AE3-3. IZI Ω R

L C r

R

Hz fo

fig. AE3-4: damped filter circuit and characteristic impedance/frequency curve. There are several variations of damped filters and many combinations of band-pass and undamped filters in service, according to particular requirements. In fact, the successful application of power electronics devices is largely due to the development of effective filtering techniques which are, however, beyond the scope of these brief notes.

2 - Appendix E3

B

X source

12. harmonic suppression reactor for a single (power factor correction) capacitor bank

E As shown in Appendix E2, the crux of the problem for capacitor banks is that a fraction of the total component of a given harmonic current can be magnified to dangerous levels in a parallel LCR circuit if that circuit resonates at the harmonic frequency concerned. By connecting a reactor L in series with the capacitor bank, the parallel resonant condition is moved away from the harmonic frequency towards a lower frequency, as shown in figure AE4-1 (b). In fact, the circuit now resonates at two different frequencies; the lower frequency is due to the parallel Ls//LCR combination, and the upper due to the series LCR circuit. C L

(a)

LS

R

IZI Ω

parallel resonance fP

(b)

stepped banks of capacitors Power-factor correction capacitor banks are frequently made up of a number of switched sections, so that the amount of compensation can be adjusted to suit the requirements of a changing load. If all switched steps have the same kvar rating, then the series resonant frequency of each step must be the same, i.e. the first step in service must fulfill the conditions of series resonance already mentioned and shown in figure AE4-1 (b). The addition of further identical steps in parallel will not affect the two resonant frequencies fp and fs. This is because, although the capacitance has increased n times (for n steps in service), the inductance has reduced to 1/n times its original value, so that the product LC, on which the series resonant frequency depends, remains constant. By similar reasoning, it follows that mixed steps of any kvar rating may be paralleled, providing that every step is tuned to the same series resonant frequency.

series resonance fS power source impedance

f (Hz) f1

fp

fS

range of unwanted harmonic frequencies

fig. AE4-1. It is sufficient that the two resonant frequencies be lower than those of the harmonics to be protected against, to ensure complete immunity from resonance. The reason for this is that for frequencies higher than the series-resonant frequency XL > XC so that the LCR branch behaves as an inductance + resistance series circuit. This branch being in parallel with LS, the powersystem source inductance, no resonant condition is possible. Furthermore, the addition of reactor L means that changes in the power-system source reactance will have much less influence (than formerly) on the parallel-resonant frequency, since L generally has a much greater value than LS (e.g. 2 to 9 times) and the parallelresonant frequency depends on L + LS. It may be noted that, although a harmonicsuppression reactor protects the capacitor bank against the problem of resonance with the source reactance, it does not reduce the amount of harmonic current which passes through the HV/LV transformer to the source. Such currents must be eliminated by shuntconnected series filters, as described in Appendix E3.

Appendix E4 - 1

1. general

F 1.1 the principal schemes of LV distribution In a typical LV installation, distribution circuits originate at a main general-distribution board (MGDB) from which cables are installed in various kinds of cables-ways, conduits, etc. to supply local distribution and subdistribution boards. The arrangement of groups of insulated conductors and the means of fixing them and of protecting them from mechanical damage, while respecting aesthetic considerations, constitute the practical realization of an electrical installation. Circuit arrangements The creation of independent circuits to different parts of an installation allows: c the limitation of consequences, on the failure of a circuit; c simplification in locating a defective circuit; c maintenance work on, or extension of a circuit may be effected without disturbing the greater part of the installation. Division of circuits falls logically into several categories, each requiring an individual circuit or group of circuits, and, in some cases, particular kinds of cable (e.g. for fire-alarm and protection circuits). In general, the following circuit groups are required: c lighting circuits (the circuits on which the majority of insulation failures occur); c socket-outlet circuits; c heating and/or air-conditioning appliances circuits; c power circuits for motor-driven fixed plant; c power-supply circuits for auxiliary services (indication and control); c circuits for safety systems (emergency lighting, fire-protection systems and uninterruptible-power-supplies (UPS) circuits for computer systems, etc.), the installation of which is normally subject to strict national regulations and codes of practice. The most common distribution arrangements for low-voltage installations are described in the following pages.

* IEC 38 (1983).

distribution within a low-voltage installation - F1

1. general (continued)

F 1.1 the principal schemes of LV distribution (continued) radial branched distribution schemes, in which conductor sizes are progressively reduced at each point of circuit sub-division are the most commonly used systems in most countries. For socket-outlet circuits in certain countries, a ring-main circuit is standard, in which the conductor size is the same throughout the circuit. Circuit wires drawn through conduits, as well as prefabricated bus channels, are commonly used.

radial branched distribution This scheme of distribution is practically universal, and its realization generally follows arrangements similar to those illustrated below:

Maintenance or extensions to the circuit leaves the remainder of the installation in service. Conductor sizes can be tapered to suit the decreasing current levels towards the final sub-circuits.

Advantages One sub-divided circuit only will be isolated (by fuses or MCCB) in case of a fault. Location of the defect is simplified.

Disadvantages A fault occurring on one of the cables from the main distribution board will cut off supply to all circuits of related downstream distribution boards and sub-distribution boards.

Conventional wiring installation (fig. F1) in buildings intended for specific use: dwellings, hotels, agricultural activities, schools, etc.

Advantages Virtually unrestricted passage for cable ways, conduits, trays, ducts, etc.

main distribution board

distribution board "A" worhshop

power sub-distribution board

lighting & heating sub-distribution board M

M process

fig. F1: radial branched distribution by conventional wiring at 3 levels. With prefabricated bus channels at the second level of distribution (fig. F2) for industrial and tertiary sector installations.

Advantages Flexibility of installation in large nonpartitioned work-spaces, easy exploitation.

MGDB (main general distribution board) D1

D2

D3

D4

to lighting and heating distribution board

prefabricated bus channel

a second prefabricated bus channel

M

M

M

process

fig. F2: radial branched distribution using prefabricated bus channels at the second level of distribution. F2 - distribution within a low-voltage installation

F With prefabricated bus-rail and pre-wired channels at final-circuits level (fig. F3): for offices, laboratories, etc.

Advantages Aesthetically acceptable; flexible in locations where partitioning may change according to consumers requirements; easy exploitation.


A

B

C

distribution board office C

to heating control board prefabricated pre-wired columns, skirting-board channels, etc...

bus rails for luminaires

fig. F3: radial branched distribution using prefabricated pre-wired channels and lighting rails at final-circuits level.

simple (unbranched) radial distribution This scheme is used for the centralised control of an installation or process dedicated to a particular application, its control, maintenance and surveillance.

Advantages A fault (other than at the busbar level) will clear one circuit only. Disadvantages Surplus of copper due to a multiplicity of circuits. Protective-device characteristics must be at a high level (proximity of source).


M

M

M

M

fig. F4: simple radial distribution.



F 1.2 the main LV distribution board The starting point for the design of an electrical installation, and the physical location of distribution and sub-distribution boards, is the geographical division of the loads, shown on plans of the building(s) concerned. The HV/LV substation, standby-supply plant, and the main LV distribution board, should, for both technical and economic reasons, be placed as near to the electrical centre of the load area as possible.

Many other factors must be considered however, and in particular, the agreement of the power-supply authority concerning the HV/LV substation, and its related civil engineering works. In fact, very often only the main LV distribution board can be located at the load centre, the HV/LV substation being on the building line with the public way.

fig. F5: low-voltage main distribution board.

1.3 transition from IT to TN In large LV installations, two voltage levels are normally used: c one level which is generally of 380 V, 400 V or 415 V (or exceptionally 480 V) for power circuits, which are mainly motors; c and a second level of 220 V, 230 V or 240 V (or exceptionally 277 V) for lighting and socket-outlet circuits. The first set of voltages are the phase to phase voltages of 3-phase systems, and correspond respectively to the phase-toneutral voltages given in the second set. Very often, particularly in factories and in some hospitals, the system comprises the three phase wires only, and operates as an IT scheme (discussed fully in Chapter G Clause 6). In order to provide the lower voltages for lighting circuits, etc. while at the same time

retaining, for those circuits requiring them, the advantages of the IT scheme, LV/LV delta/star transformers are used, as shown in fig. F6. In this way : c a 3-phase 3-wire supply is available at the secondary side of the LV/LV transformer with phase-to-phase voltages of 220 V, 230 V or 240 V as required; c all loads are connected phase-to-phase only (see Note); c earth faults occurring on the TN system will be cleared rapidly by the TN system circuit breaker and the advantages of the IT scheme will be preserved.

residual earth fault device IT power network

PE protective earthing conductor

TN lighting, etc network

PE protective earthing conductor

Note: in this scheme of delta-connected loads, it is essential that balanced loading is maintained in all three phases.

fig. F6: use of a LV/LV transformer to provide a 3-phase 3-wire TN system from a 3-phase 3-wire IT network.

F4 - distribution within a low-voltage installation

2. essential services standby supplies

F In order to achieve the highest possible plant performance, it is necessary that the continuity and quality of the electric-power supply be assured.

2.1 continuity of electric-power supply continuity of power supply is achieved by: c appropriate division of the installation and the provision of alternative supply sources, c the provision of local emergency standby generation, c sub-division and duplication of important circuits, c the type of earthing scheme (IT for example), c discriminative protection schemes.

A high degree of power-supply continuity can be achieved by: dividing the installation; providing more than one source, e.g. a ringmain type service connection; automatic local standby generation for essential services; the sub-division of circuits; the choice of earthing system (IT, TT, TN, etc.) and the use of selective protection devices (fuses, relays).

the division of installations and the provision of more than one source Ring-main type HV supplies, and (where the installed load justifies the expense) two or more HV/LV transformers, with provision for interconnection of the LV main distribution boards, is the most common way of ensuring a high level of supply continuity from the power network. The use of several transformers allows a measure of separation of loads which would otherwise cause an unacceptable disturbance to other circuits, for example: c computer systems which are sensitive to voltage regulation (dips and peaks) and to waveform distortion (harmonics); c circuits which create harmonics, such as discharge lamps, electric converters of various kinds (thyristor-controlled rectifiers, inverters, motor-speed controllers, etc.); c circuits which create excessive voltage changes, such as large motors, arc furnaces, etc. These loads and others of similar characteristics, i. e. loads susceptible to disturbances, and loads creating them, should preferably be supplied through different HV/LV transformers. In this way, the PCC (point of common coupling) is moved from the LV busbars to the HV busbars, where the effects are considerably less between one group of loads and the next, and in some cases are completely eliminated.

A case in particular concerns 3rd harmonics and all multiples of the 3rd harmonic*. If delta/star HV/LV transformers are used, then triplen harmonic currents on the LV side of one transformer do not appear in the HVside conductors supplying it (the currents circulate internally around the delta winding) and so cannot affect neighbouring transformers. Moreover, any triplen harmonic voltages which may be present on the HV busbars (from directly-connected HV loads for example) will not be transformed down to LV by a delta/star transformer. The separation of loads through transformers in this way is sometimes referred to as "de-coupling". * Known as "triplen" harmonics. Triplen harmonics are of zero-phase-sequence on balanced 3-phase systems, which accounts for their particular behaviour in delta/star transformers.

the provision of standby emergency power supplies Examples of standby emergency power supplies include: two separate HV/LV substations, a privately-owned power plant, diesel-generator sets, uninterruptible static power supply equipment (UPS).

the sub-division of circuits Circuits are divided into groups according to their relative importance. In general, two groups, commonly referred to as "essential" and "non-essential" loads are separated and supplied from different busbars. Figure F7 shows a typical arrangement of an automatic changeover scheme to provide LV standby power to an "essential" loads distribution board.

HV LV

G standby generator ant automatic changeover contactor NORMAL-STANDBY

non-essential loads essential loads inverter

sensitive load (computer, etc.)

fig. F7: essential and non-essential loads are separated, with automatic standby supplies provided for essential loads.


2. essential services standby supplies (continued)

F 2.1 continuity of electric-power supply (continued) A sub-group of the essential loads, namely computer and information technology equipment (ITE), requires the highest degree of continuity, stable voltage level, and quality of wave form. These requirements are met by a static UPS inverter system. Supply from a HV substation

essential loads

HV from a private power plant or from a different HV substation

HV

HV

LV

LV

non-essential loads

essential loads

non-essential loads

fig. F8: an example of HV standby power supply.

choice of earthing system Where considerations of supply continuity are paramount, e.g. in continuous-process manufacturing, hospital operating theatres, etc., the IT scheme* of earthing is generally adopted. This scheme allows normal (and safe) system operation to continue in the event of an earth-fault (by far the most common type of insulation failure). A shutdown to trace the fault manually or automatically (see G 6.2) and effect repairs can then be carried out later, at any convenient time (e.g. at the end of a manufacturing process, etc.). A second earth fault (if it occurs on a different phase or on a neutral conductor) will, however, constitute a short-circuit fault, which will cause overcurrent relays to trip the circuit(s). * Chapter F Sub-clause 4-5 discusses the matter of earthing schemes in more detail.

2.2 quality of electric-power supply public and private power-supply networks are subject to diverse disturbances, the level and frequency of which must be controlled and maintained within acceptable limits. Among the most onerous are: c voltage drops, or sudden peaks and dips, c flicker, c overvoltages, c harmonic voltages and currents, particularly odd-numbered harmonics (3rd, 5th...), c high-frequency phenomena.


Power network disturbances may be of a continuous or transitory nature. The most important of these, in terms of the design and operation of a network, are: c excessive dips (occasional voltage drops, from 15 to 90% of Un, from half a cycle to 1 s) and peaks of the supply voltage at normal frequency; c flicker, i.e. repetitive voltage drops of less than 10%, e.g. due to welding machines, photocopiers, etc.; c overvoltage surges; c harmonic currents and voltages, particularly the odd harmonics (3rd, 5th...); c high-frequency phenomena.

selective discrimination by protection relays and/or fuses The prime objective in any scheme of automatic protection against insulation faults, over-loading, etc., is to trip the circuit breaker or blow the fuse(s) which control(s) the faulted circuit only, leaving all other circuit breakers and fuses unaffected. In radial branched installations, this means the nearest upstream circuit breaker or fuse(s) to the fault position. All downstream loads then being inevitably deprived of supply. The short-circuit (or overload) current will generally pass through one or more circuit breaker(s) or fuse(s) upstream of the circuit breaker (or fuses) controlling the faulted cable. By "discrimination" is meant that none of the upstream protective devices through which the fault (or overload) current flows will operate before the protective device controlling the faulted circuit has operated. In general, discrimination is achieved by increasing the operating time of relays as their location in a network becomes closer to the power source. In this way, the failure to operate of the relay closest to the fault means that the next relay upstream will operate in a slightly longer time.

closed

closed

open

fig. F9: the principle of selective discrimination.

F the undesirable effects of voltage dips are countered in various ways, depending on the apparatus in question. Some common remedies include: c automatic load shedding and reconnection, c the use of uninterruptible powersupply units, c high-torque motors, c use of lamps which do not extinguish during dips, and other solutions.

voltage depressions of short duration ("dips")

the damaging effects of overvoltages can be avoided: c for overvoltages at power-system frequency by: v assuring adequate overvoltage withstand capability for the equipment concerned, v the use of voltage limiting devices where required, in a properly co-ordinated insulation scheme. These devices are always necessary in IT earthed systems, c for transitory (generally impulsetype) overvoltages, by: v the application of lightning arresters, v correct coordination in the insulation scheme noted above.

overvoltages

Types of voltage dip According to the duration of the undervoltage condition, the origin of the dip may be due to one of the following causes: c less than 0.1 second: short-circuit faults occurring anywhere on local LV networks, and cleared by protective devices (relays, fuses, etc.). This kind of dip is the most common in "standard" systems, i.e. as opposed to networks close to heavy industry, where large disturbances are frequent; c from 0.1 to 0.5 seconds: most of the faults occurring on HV systems fall into this category; c above 0.5 seconds: on rural networks where auto-reclosing circuit breakers are common, several successive dips may be experienced before the fault is cleared. Other reasons for voltage dips exceeding 0.5 seconds include the starting of local electric motors (central station fire-alarm sirens produce cyclic dips in the neighbouring distribution network, for example); lift motors will affect local consumers, and so on... Some consequences and solutions Among the numerous undesirable consequences of voltage dips, the following may be cited: c depending on the severity of the dip and the type of loads in a given installation, there can be the risk of a heavy current surge occurring at the restoration of normal voltage, with the consequent tripping of main circuit breakers on overcurrent. A possible solution is a scheme of automatic load shedding and staged re-connection of

Types of overvoltage Overvoltages are distinguished in a general way, according to their origin: c overvoltage surges due to lightning are referred to as being of atmospheric origin. These overvoltages mainly affect overhead transmission and distribution lines, outdoor substation equipment, and switchgear and transformers, etc., connected directly to such exposed plant. The frequency of such surges occurring is related to the so-called keraunic level of the region and to the types of network concerned, i.e. underground cables or overhead lines. The keraunic level is defined as the number of days per annum on which the sound of thunder is heard at the location concerned. c operational overvoltages. Switching at high voltage can produce surges of voltage similar to those of atmospheric origin; while on LV networks, the blowing of fuses to clear fault current can also produce relatively severe surges of overvoltage. At HV distribution voltage levels, switching overvoltages are adequately suppressed by standard lightning arresters. c overvoltage on an LV installation due to faults on an HV system, for example: v a direct HV/LV fault occurring between the primary and secondary windings of a transformer, or a HV line touching a LV overhead-line distributor, etc., v due to the flow of earth-fault current from a HV fault or lightning stroke, which passes through a substation earthing system that is

apparatuses requiring high restarting currents, e.g. cold incandescent lamps and resistive heating loads; c in all computer-based applications, such as: word processing, information technology, machine-tool control, and so on, voltage dips are unacceptable, since the loss of information or destruction of a programme can occur, with catastrophic consequences. Some degree of voltage variation can be tolerated and voltage-stabilizing circuits are built-in for this purpose, but the universal solution for important installations is the use of uninterruptible power supply (UPS) units, based on trickle-charged storage cells and inverters, associated with automaticallycontrolled diesel-generator sets; c for an electric motor, the deceleration during a voltage dip (torque α V2) means that its back-e.m.f. will very likely be out-of-phase with the restored voltage. This constitutes (more or less, depending on the degree of phase difference) conditions of short-circuit, with a corresponding heavy current flow. In certain cases, excessive transient torques may occur, with a risk of damaging shafts and couplings, etc. A common remedy is to install high-inertia high peak-torque motors where the driven load allows; c some types of discharge lamp (notably mercury-vapour lamps) used for public lighting, extinguish below a certain voltage level, and require several minutes (to cool) before re-igniting. The remedy is to use other types of lamp or to mix non-extinguishing lamps, in sufficient number to maintain a safe level of illumination.

common to both the HV and LV networks. Methods of protection against the dangers of such overvoltages are described in Chapter C Sub-clause 3.1. Consequences and solutions All appliances, plant, and equipment must have a basic overvoltage withstand ability. Electric motors are particularly susceptible to winding insulation failure in the presence of high-frequency high-voltage surges, while computer installations and related electronic processing equipments are frequently provided with independent (battery based) supplies, which not only assure a high quality level of harmonic-free stable voltage as already described, but effectively isolate sensitive circuits from the kind of voltage surges in question. In industrial installations, protection against overvoltage is considered to be achieved if all components of the installation have been successfully tested for power-frequency overvoltage withstand ability, and the measures described below have been taken to protect against high-frequency high-voltage and unidirectional surge phenomena. c tests at normal power frequency. The dielectric withstand test voltage at normal power frequency for most LV materials is 2U + 1,000 volts for 1 minute (or close to this value - discussions are still underway in the IEC). In IT-earthed systems a voltage-limiting device between the supply-transformer neutral point and earth is obligatory for protection against power-frequency and possible induced-surge type overvoltages.



F 2.2 quality of electric-power supply (continued) c measures against transient impulse-type overvoltage surges. These measures depend, in addition to a basic impulse-voltage withstand capability of the insulating materials, on the application of lightning arresters at the origin of the installation, together with voltage-surge suppression devices at sensitive points in the installation (e.g. at terminals of large motors). Such schemes require careful study and are best carried out in cooperation with the relevant manufacturers. For LV installations, the transference of surge voltages through the interwinding capacitances of the HV/LV transformer reduces considerably the severity of the overvoltage on the LV side, compared to that on the HV side. Transformers with earthed screens between HV and LV windings may also be used to provide a costly but effective method of eliminating the problem. v impulse voltage withstand capability of insulating materials. nominal voltage of the installation

The basic test applies a standardized lightning voltage impulse of the form shown in figure F11, characterized by the values 1.2/50 µs. These two values (in micro seconds) indicate the time interval for the wave to attain its peak value from the (defined) instant of impulse initiation (i.e. 1.2 µs) and the time for the impulse to fall to 50% of its peak value (50 µs). These values are called "front time" and "time to half value" respectively. The peak value is designated by Uimp (imp = impulse). Other impulse test wave-forms, notably for representing switching overvoltages, are used for test purposes, but these tests are relevant only for very high system voltages, beyond the ranges used for distribution. Table F10 shows maximum values of peak overvoltage assumed to be possible at different points in a typical LV installation. Note: Materials tested to IEC standards have an impulse withstand capability of 123% of the values shown in Table F10, for altitudes of 0-2,000 metres.

levels of distribution

main distribution board 230/400 V 400/690 V

local distribution board 4 kV 6 kV

6 kV 8 kV

final circuits level 2.5 kV 4 kV

table F10: assumed levels of transient overvoltage possible at different points of a typical installation.

concerning overvoltages, IEC Publication 947 takes account of the rules governing insulation coordination and requires that LV switchgear be impulse tested according to the withstand values shown in the relevant tables.

v industrial switchgear. The levels indicated in Table F12 are abstracted from IEC Publication 947. During the several impulse-voltage tests, no break down of insulation must occur between phases between open contacts or between any phase and earth. Table F12 also includes a test for switchgear, the front face of which is insulated to class II level, but at the same time including an accessible manual-operating handle. This feature provides additional safety for operating personnel. Note: all Compact* and Masterpact* circuit breakers have the class II front face feature.

kV

9,8kV

50%

1,2

50

µs

fig. F11: standardized impulse voltage wave-form 1.2/50 µs.

* Merlin Gerin brand names.

application of impulse voltage between phases across the open circuit breaker between phases and earth

impulse-voltage values circuit breakers circuit breakers/ isolators 9.8 kV 9.8 kV 9.8 kV 12.3 kV

circuit breakers/isolators + class II front face 9.8 kV 12.3 kV

9.8 kV

14.7 kV

9.8 kV

table F12: typical levels of impulse withstand voltage of industrial circuit breakers labelled Uimp = 8 kV. v use of lightning arresters. Lightning arresters are necessary (obligatory in some countries) where an installation is supplied by a low-voltage overhead line and the keraunic level is 25 or more. Their use is


strongly recommended, regardless of the keraunic level, where equipment known to be susceptible to damage from overvoltage surges is installed.

F Arresters are commonly installed at each end of the LV line, generally on the first pole away from the HV/LV transformer position and on the pole at which the consumers service cable is connected to the line. In this arrangement the voltage will not exceed 3-4.5 kV, the wave front being chopped at this level. The withstand value of LV components is normally standardized at 6 kV for a 1.2/50 µs impulse, and equipments complying with such standards are therefore adequately protected. Assuming that the neutral conductor and the lightning arresters are connected to the same earth electrode, then the discharge current through the arresters will raise the potential of the neutral conductor, more or less, according to the resistance of the several different earth electrodes to which it is connected.

the undesirable effects of harmonic voltages and currents are minimized by: c over-sizing of components (e.g. capacitors), v increasing insulation levels v increasing current-carrying capability c isolation of a harmonic source by supplying it through a separate HV/LV transformer, c use of harmonics filters.

This potential will be transferred to the phase conductors of the installation, as described in chapter C sub-clause 3.1. If the installation earth electrode is beyond the zone of influence of the arresters electrode, then a TT-earthed system is commonly used. If not, then the more-costly TN-C-S system and equipotential "cage" earthing scheme will be necessary.

harmonic voltages and currents Sources and types of harmonics The principal sources of harmonics are: c electromagnetic machines and devices, such as: iron-cored inductances, transformers (magnetizing currents), motors and generators and so on, and results from the non-linear relationship between current and magnetic flux produced by the current in ferro-magnetic materials. This non-linearity produces odd-order harmonics (principally 3rd order) with some additional harmonics from rotating machines, which are related to winding slots in the magnetic circuits (slot ripple); c computer installations; c discharge lamps and ballasts (both lamps and ballasts are highly nonlinear); c static 3-phase converters of various kinds (inverters, speed controllers for a.c. motors, rectifiers and so on) which depend on thyristor control and current-chopping techniques. The harmonic generation is variable according to the function, but in general the 5th and 7th harmonics are prominent, while (unlike the ferromagnetic sources) the 2nd harmonic may be present; c arc furnaces create a continuous spectrum of random disturbances. If the arc is d.c., supplied through static thyristor-controlled rectifiers, then the random perturbations are of lower average amplitude and the harmonics produced by the rectifiers are relatively significant. Consequences Harmonics give rise to (among others) the following consequences: c the need to oversize certain network and installation components: v oversizing of conductors (refer to the manufacturers of the products concerned), v oversizing of neutral conductors (of a 3-phase 4-wire system) particularly for discharge- or fluorescent-lighting circuits; for example, a 33% 3rd-harmonic content in the current of each phase produces 100% 3rd-harmonic current in the neutral conductor (since 3rd harmonic currents have zerophase-sequence on 3-phase systems, and add arithmetically), v oversizing of alternators (e.g. in dieselgenerating sets). Refer to the manufacturers of single-phase static rectifiers and inverters for guidance. The value of the subtransient reactance of the alternator and the type of loads are important factors, v oversizing of transformers,

v oversizing of capacitor banks, c local overheating of magnetic circuits in motors; c possibility of resonance between network capacitances and inductances (ferroresonances) or between capacitor banks and the system source impedance (mainly inductive). For the latter case, the manufacturer of the capacitor banks should be able to advise on suitable filtering arrangements. Solutions In general, an installation cannot tolerate a significant percentage of harmonics: a value of 5% maximum is a limit often recommended. Reducing the harmonic content of a system to an acceptable level consists of: c using delta/star LV/LV transformers to isolate the 3rd (and odd multiples of the 3rd-harmonic); c installing filters. Filters are of two kinds: v shunt-connected, series-resonant: extremely effective for a particular harmonic (the 5th for example) and is used in association with others for selective filtration of harmonic voltages, to which they present a virtual short-circuit, v damped filter: less efficient, but covering a wide band of frequencies. The action of a damped filter is described in Appendix F1, v harmonic-suppression reactor, series connected to a capacitor bank. The action of a series harmonic-suppression reductor is described in Appendix F1.



F 2.2 quality of electric-power supply (continued) the undesirable effects of inductive (electric or magnetic) or commonimpedance coupling between adjacent circuits at power-system frequency (with its harmonics and superimposed high-frequency disturbances) together with highfrequency radiated electromagnetic waves, are minimized by: c the selection of appropriate materials, c specific studies.


electromagnetic compatibility (EMC) This subject concerns all cases of coupling, by common impedance and induction (electric or magnetic) at fundamental frequency and harmonic frequencies, together with unidirectional and H.F. surges, and radiated electromagnetic waves, provoked by normal (switching, etc.) and abnormal (system faults, lightning, etc.) operating conditions. The unifying feature of all inductive phenomena is that electric, magnetic, or radiated electromagnetic fields, or different combinations of any number of them, result in producing emfs in any conducting medium in their paths. Essential differences are as follows: c electric or magnetic fields at power-system frequency and its harmonics do not, for all practical purposes, leave the space immediately surrounding their point of origin, namely a charged conductor (electric field) or a current-carrying conductor (magnetic field). Moreover, the field strength in both cases varies inversely with the distance squared from the conductor*, i.e. their zone of influence rapidly diminishes with distance from the conductor. c the amount of energy leaving a conductor in the form of an electromagnetic wave depends on the acceleration of electrons. This is why, for example, at the instant of switching on a lamp, the radiation due to the initial acceleration of electrons can be heard in a radio receiver (i.e. the switching-transient current). All disturbances on power systems which cause electrons to accelerate, whether in a unidirectional or oscillatory manner, will cause a radiated wave to leave the conductor and propagate through space. The higher the frequency, the greater the acceleration of electrons, and the greater the portion of energy leaving the circuit as radiation. The field strength of a propagated wave varies inversely as the distance from the conductor, i.e. its zone of influence is much greater than that of the electric or magnetic fields noted above. c a further difference between the above cases is that a non-radiating electric field can be much stronger than its associated magnetic field, for example, in a high-voltage high-impedance (low current) circuit, and vice-versa, i.e. in a low-voltage lowimpedance (high current) circuit. Whereas in a radiated wave the energy in the electric field is exactly equal to that of the magnetic field, the circuit of origin generally being a capacitive/inductive combination where XC = XL at the frequency of natural resonance. It may be noted that in the context of the present discussion, the effects of a radiated wave have not, until recently, been very important. With the increasing use of walkie-talkies, mobile transmitters and cordless telephones, etc., however, this aspect of EMC will require closer attention than hitherto. The emfs induced by one or more of the three possible modes are generally of the order of milli- or micro-volts. However, certain modern electronic circuits have enormous amplifying power, while at the same time in

other circuits minute currents and voltages are normal and the circuit components are correspondingly fragile.

equipment It is for the foregoing reasons that electronic equipment requires special care, and full protection against interference from any propagated or direct-coupled source. Other sources, commonly causing problems, include: c "white-noise" from fluorescent and other types of discharge lamps; c radiation from ignition systems of internal combustion engines; c commercial and amateur radio transmitters, radio-directed taxis, walkie-talkies, etc.; c mains-borne interference through conductors in the installation, for example: the opening of contactor coils or circuit breaker tripping coils. The European Directive of 3rd March 1989, concerning electromagnetic compatibility, imposes a maximum level of permitted radiation from electrical installations and their component parts (the practical application of the methods to adopt is still being studied at the time of publication of this guide).

earthing arrangements and equipotential bonding-guidance for installation contractors The following notes have been abstracted from November 1993 IEC draft proposal documents. Protection against electromagnetic interference (EMI) Lightning currents in a lightning protection system (LPS) or in the vicinity of a building can cause overvoltages in electrical installations of buildings by induction. This is the case if large metal loops exist, where different electrical wiring systems for the supply of different electrical equipment, e.g. for power supply and for information technology, are installed on different routes. In practice, a very common example is the connection of earthed conductors of power supply cables and of cables for information technology systems in a wide (extensive) mesh. Furthermore, equipotential bonding systems, building constructions or pipe systems for non-electrical supplies, e.g. for water, gas, heating or air conditioning, can also present such induction loops. When non-electrical pipe-systems or metal parts of the building construction are connected with the equipotential bonding system of the building, these metal parts may contribute to a screening effect which reduces the induction and contributes to the protection against electromagnetic interference. The value of the induced voltage depends on the rate of rise (di/dt) of the lightning current, and on the size of the loop. * except in very close proximity to the conductor, where it varies inversely as the distance cubed.

F Power cables carrying large currents with a high rate of rise of current (di/dt) (e.g. the starting current of lifts or currents controlled by rectifiers) can induce overvoltages in cables of information technology systems, which may influence or damage the related electrical equipment. In or near rooms for medical use, electric or magnetic fields of electrical installations may interfere with medical electrical equipment (a new clause for Section 710 of IEC 364, concerning such situations is currently under consideration). Recommended measures for reduction in the effect of induced overvoltages depend on adequate equipotential bonding, screening, physical separation, use of filters and surge suppressors. Consideration must be given by the planner and designer of electrical installations to the following: 1. Location of potential sources of interferences relative to sensitive equipment. 2. Location of sensitive equipment relative to heavily loaded centres, busbars or equipment, e.g. lifts. 3. Provision of filters and/or surge suppressors in the circuits feeding sensitive electrical equipment. 4. Bonding of metal enclosures and screening. 5. Adequate separation of power and signal cables and crossovers at right angles. 6. Avoidance of induction loops by selection of a common route for the wiring systems. See also item 17 of this list. 7. Use of signal cables, screened and/or in twisted pairs. 8. Bonding connections should be made as short as possible. 9. Wiring systems with single core conductors should be enclosed in bonded metal enclosures. 10. Avoidance of TN-C system (see subclause F 4.2 and clause G5) in installations with sensitive equipment; see figure F13. For buildings which have, or are likely to have, significant information technology equipment installed, consideration must be given to the use of separate protective conductors (PE) and neutral conductors (N) beyond the incoming supply point, in order to minimise the possibility of over-current and EMC problems, due to the passage of neutral current through signal cables (see figures F13 and F14). 11. For TN-C-S systems, there are two possibilities depending on the arrangement for interconnection of equipment and extraneous conductive parts within the building: c avoidance of the "TN-C section" of the TN-C-S system for distribution within the building, i.e. make the separation (of the PE conductor from the PEN conductor) at the origin of the installation; c avoidance of loops between different "TN-S sections" of the TN-C-S system within the building (see figure F14).

L N PE

I equipment 1

loop equipment 2

I

fig. F13: neutral currents in a TN-S system. L PEN I1 I1

equipment 1

I6 I1 I3

loop

I6

equipment 2 I6 I5

I6

I4

I5

fig. F14: neutral currents in a TN-C system.



F 2.2 quality of electric-power supply (continued) 12. Cables and pipes (e.g. for water, gas or heating) for feeding the building should enter the building at the same place. Bonding of metal sheaths, screens and metal pipes and connections of these parts with the main equipotential bonding (MEB) of the building (see figure F15). 13. Avoidance of potential differences between different areas of equipotential bonding should be achieved by the use of metal-free fibre optic cable or other nonconducting interconnecting systems such as microwave or laser links. Note: the problem of earth differential voltages on large public telecommunication networks are the responsibility of the network operator, who may employ other methods. Provisions for electromagnetic compatibility (EMC) Signal connections In buildings which include a PEN conductor, or where there are EMC problems on signal cables due to inadequate EMC provisions in the electrical installations, the following methods may be considered to avoid or minimise the problem. 14. Use of fibre optic links for signal connections. 15. Use of Class II equipment. 16. Use of local transformers with separate windings (double wound transformers) for the supply of the information technology equipment, taking into account the requirements of IEC 364-3, sub-clause 312.2.3 and IEC 364-4, sub-clause 413.1.5, for IT* systems (local IT* systems), or of clause 413-5, for protection by electrical separation (e.g. transformers according to IEC 742). 17. Use of suitable wiring (cabling) routing in order to minimise the enclosed area of common loops formed by the supply cables and signal cables. * not to be confused with Information Technology. IT earthing systems are defined in sub-clause F 4.2.

Information Technology equipment may be subject to malfunction due to currents and voltages induced in equipment or between interconnected equipment. Further examples of basic techniques used to achieve immunity to incoming electromagnetic disturbances are: a) to provide inherent immunity in the Information Technology equipment, either electrically or by use of error correction, telephone power supply

b) to separate the Information Technology equipment from the sources of disturbance, c) to provide equipotential bonding between equipment for the relevant range of frequencies, d) to provide a low impedance earth reference plane to minimize earth potential differential voltages and provide shielding. There is a continuous range of earthing and equipotential bonding methods to achieve electromagnetic compatibility. The following methods exemplify this range. Method 1: radially connected protective conductors (see figure F16) This uses the normal protective conductors associated with the supply conductors. The protective conductor at each equipment provides a relatively high impedance path for electromagnetic disturbances (other than mains-borne transients) such that inter-unit signal cables are subject to a large proportion of the incident noise. Equipment must therefore have a high immunity to function satisfactorily. By providing a dedicated supply circuit and earthing system serving the Information Technology equipment, incident disturbances can be much reduced. In some cases, the star earthing point (e.g. the PE bar in the relevant distribution board) of the radially connected protective and functional earthing conductors for the Information Technology equipment may be earthed by a separate dedicated insulated conductor connected to the main earthing terminal. Method 2: use of a local horizontal equipotential bonding system (mesh) (see figure F17) The normal protective conductors are supplemented by equipotential bonding of the components of the Information Technology system to a local mesh (bonding mat). Depending on the frequency and the mesh spacing, this can provide a low impedance earth reference plane for signal interconnections between those system components in close proximity to the mesh. As with Method 1, additional immunity may be provided by a separate dedicated Information Technology supply circuit and earthing system, including the bonding mesh, from other supply circuits and earthing systems and extraneous conductive parts such as building metalwork. telephone power supply

earth electrode embedded in the foundation

MEB I

I V U=0

cable from the antenna

cable from the antenna

I water district heating gas waste water

V U≠0 water district heating gas waste water

I

fig. F15: introduction of armoured cables and metal pipes into buildings (examples). a) a common introduction is suitable, U = 0 b) introduction at different places is not suitable, U ≠ 0.


F Method 2 may be extended where necessary by the installation of bonding meshes on other floors. All such meshes are interconnected by (numerous) vertical bonding conductors to minimize potential differences in the meshes. Method 1 is most easily implemented, especially in existing buildings. The difficulty and cost of implementation increases through Method 2 and its possible extensions. However, these are more likely to provide an acceptable environment for unspecified future Information Technology equipment.

PE

The foregoing information concerning Methods 1 and 2 has been abstracted from a November 1993 draft proposal for a new section (548) of IEC 364 Part 5 Chapter 54. In the case of particular difficulties, it may be necessary to consult specialists. For current projects, and in the absence of more precise information, it is recommended that materials be selected which satisfy the requirements indicated in table F18.

PE

PE

signal cables

signal cables ITE

ITE

ITE

ITE - Information Technology Equipment

distribution board

main earthing terminal or earthing-bus-conductor

fig. F16: radially connected protective conductors.

PE

PE signal cables

PE signal cables

ITE

ITE

ITE

distribution board

main earthing terminal or earthing-bus-conductor

fig. F17: local horizontal bonding mesh. disturbance

reference

electrostatic discharge field strength

IEC 801-2 IEC 801-3

high speed repetitive transient "bursts" (contact bounce) transient overvoltages

IEC 801-4

current waves (lightning, switch closing)

IEC 60.2 at the origin 690 V of the installation 400 V other cases 690 V 400 V IEC 8/20 µs (in preparation)

level minimum level 3 (8 kV) level 2 (3 V/m) level 2

recommended level 4 (15 kV) level 3 (10 V/m) level 4

10 kV 7.5 kV 7.5 kV 5 kV 80 A

200 A

table F18: compatibility levels for installation materials.



F 2.2 quality of electric-power supply (continued) it is possible, within a low-voltage installation, to provide a supply of High Quality (free from disturbances) for dedicated circuits specifically intended to supply highly sensitive equipments, such as computerbased appliances, etc.


High Quality Supplies The objective is to supply sensitive equipment (information-technology devices, cash registers, micro-processors, etc.) from a source which is free from the pollution discussed above, at a reasonable cost. The diagram of figure F19 represents a scheme at the level of the main general distribution board. The High Quality supply is achieved by means of an inverter and its associated battery of storage cells and rectifier (charger), which is supplied, in normal circumstances, from one outgoing-way of the main general distribution board. Continuity of supply is assured by means of a diesel-generator set and automatic changeover switch, so that an uninterrupted power supply can be maintained indefinitely (if personnel are available to top up the fuel tank) or for several hours if the substation is unattended.

HV LV

Diesel generator

UPS

fig. F19: example of the production of High Quality power supply.

3. safety and emergency-services installations, and standby power supplies

F 3.1 safety installations the provision of safety and emergency installations is a legal obligation.

Safety and emergency-services installations are governed by statutory regulations, concerning: c establishments receiving the public; c high-rise apartment blocks; c establishments in which people are employed (offices, shops, factories, etc.). They must be provided with the means for ensuring the safe evacuation of personnel, notably: c security and safety lighting; c alarms and warning systems; c automatic fire detection; c fire-extinguishing systems; c smoke evacuation;

c air compressors for the pressure-operated fire-extinguishing system; c water pumps for re-filling the fireextinguishing system. Apart from the general rules noted above, there are certain projects for which the safety regulations are related to a particular process (petro-chemical, cement works...) or services (tunnel lighting, airport runway lighting...). Note: power supplies for security lighting are described in Chapter J, Sub-clause 4.6.

3.2 standby reserve-power supplies standby reserve-power plant is an economic necessity in numerous circumstances where loss of supply would have far-reaching consequences.

Among the many applications in which an interruption of power supply cannot be tolerated, the following may be cited: c information technology installations (protection of data concerning insurances, banking, professional practices, administrations...); c industrial processes (continuity of "feed" material for continuous processing, boiler feed-water pumps in power stations, paper production, desalination plants...); c food-processing industry (refrigeration plants, egg-hatching...); c telecommunications; c scientific research; c surgical operating theatres; c ticketing, plane reservations, cash registers...; c military.

It may be noted that where several emergency-services standby sources exist, they can also be used as reserve-power sources, on condition that any one of them is available and capable of starting and supplying all safety and emergency circuits, and that the failure of one of them does not affect the normal functioning of the others.

fig. F20: examples of reserve power supplies: central storage battery (left) and diesel-generator sets (right).


3. safety and emergency-services installations, and standby power supplies (continued)

F 3.3. choice and characteristics of reserve-power supplies Apart from perceptible (albeit very brief) cuts in power supply, imperceptible interruptions of several milli-seconds are sufficient to interfere with certain equipments. As previously noted, UPS systems are essential in these cases, and are used together with the reserve-power source, to ensure the utmost security.

principal specifications In order to satisfy the requirement of economical exploitation, the following features are imperative: c supply interruption is not tolerated: v in information technology (IT) systems, v in continuous-process operations, except for loads of high inertia which can tolerate an interruption in the order of 1 second; c period for conserving data in information technology (IT) systems: 10 minutes; c autonomy is desirable for reserve-power supplies installations; it is a function of the economics related to exploitation beyond the minimum demanded for the safety (only) of personnel.

Specifications particular to safety installations Regulations covering safety installations contain a number of conditions to be respected concerning their electric-power sources: c duration time of an interruption: according to the case, the following choices are imposed: v no break, v a break of less than 1 second, v a break of less than 15 seconds; c autonomy demanded for the reserve-power source: in general it corresponds to the time necessary to complete all safety operations for persons: for example, the time to evacuate an ERP (Establishments for Receiving the Public): 1 hour minimum. In large apartment blocks, the autonomy of the source must be 36 hours, or more.

requirement

applications applications types examples of installations

conditions allowable duration of break autonomy of source minimum and preferred solutions technique employed

programmable controllers interruptible IT equipment sequential telecommunications process

continuous process

- data banks - process control and monitoring - IT services banking insurance administration, - management systems for production processes

- indications and control of the process parameters - nuclear - chemical - biological - thermal - heavy mechanical (high inertia)

- cold-working sequence - light machining - packaging assembly chain

zero i1s i 15 s i 15 mn 10 mn 20 mn 1h

c

inverter with or without a generator to take over load of the inverter

no-break generator or start-up and take over load from an inverter

c c c (1) c (1)

c (2) c c c permanent if economical

c c

permanent generation set

(1) according to economic circumstances. (2) data-storage time limit.

table F21: table showing the choice of reserve-power supply types according to application requirements and acceptable supply-interruption times.


F 3.4. choice and characteristics of different sources The several possible solutions are characterized by their availability, i.e. immediate or delayed load pick-up time, and their autonomy, i.e. ability to supply the load for a given period without attention (refilling fuel tanks for example). It is also necessary to take account of: c constraints imposed by the installation: in particular for specialized locations, and according to the source(s) used; c complementary equipment; c operational constraints, e.g. according to manufacturers operating instructions or local statutory regulations, etc.; c routine maintenance requirements, which could impose less than ideal restrictions during the periods allotted to such work...

An overall review of the many possibilities and associated constraints often leads to an optimum solution based on an inverter scheme associated with a standby dieselgenerator set. A battery of storage cells maintains an uninterrupted supply during the start-up and load pick-up time of the standby set.

emergency and/or reserve power supply

M

battery

inverter

time required to supply load zero time (no break) c c 1 second 1 to 10 minutes (5) total time for a changeover operation zero c c related to the automatic c changeover scheme adopted for each source installation constraints Special location None. Unless (type of battery). batteries are open Special d.c. type. network. additional equipment (apart from protection and changeover devices) Charger. None. Unless Regulator, additional batteries indications and are required. meters. operational mode and constraints Special network. Automatic. System losses. Frequent checking. other parameters maintenance Periodic shut-downs None. Unless openfor checking and type batteries. maintenance work. life expectancy (3) 4 to 5 years (2). 4 to 5 years (for sealed batteries). necessary x 2 if installation is typically 2 for 1 redundance (4) permanent. and 3 for 2. reliability (4) constant checking Integrated checks. is important (numerous human errors).

cold-start diesel generator

G

load pick-up (1)

generators in permanent service c

c c c c

c

Special location (vibrations noise nuisance, access required for maintenance, fire protection). Fuel tanks.

Starter, by batteries or compressed air.

Inertial fly wheel and clutch.

Automatic synchronizing equipment.

Manual or automatic. Periodic startups

Automatic. Fixed maximum load.

Permanent operating staff.

Periodic checks, but minimal wear and very little upkeep required. 1.000 to 10.000 hrs and 5 to 10 years. batteries x 2.

Minor mechanical Periodic checks, but constraints only except on minimal wear and very clutch and coupling shaft little upkeep required. 5 to 10 years. 10.000 hrs or 1 year.

Mechanical and starter batteries.

x 2 where security is important. Mechanical particularly clutch assembly and coupling shaft.

x 2 if the installation is permanent. Mechanical and system of synchronization.

(1) A motor-generator set permanently running and equipped with a heavy flywheel. On the loss of normal supply, the pick-up of load generally requires less than 1 second. (2) Longer if the battery is of the open type. (3) Before requiring an important overhaul. (4) A study of safety requirements allows the definition of an optimal scheme. (5) According to whether the set is pre-heated or not.

table F22: table of characteristics of different sources.


3. safety and emergency-services installations, and standby power supplies (continued)

F 3.5 local generating sets the association of an inverter and local generating set is the optimum solution for ensuring a long autonomy.

In certain installations a power supply, independent of the normal public service, is needed so that a local generator (usually driven by a diesel engine) is provided, and is associated with an inverter. In this case the autonomy of the inverter, i.e. of the battery must be sufficient to cover the period of starting the diesel and coupling the generator to the load. The time required to effect a changeover from

one source to the other depends on the characteristics of the particular installation, such as: start-up sequence for the engine, possible shedding of inessential loads... The coupling is generally carried out at the LV main general distribution board, by means of an automatic changeover panel, an example of which is shown diagrammatically in fig. F23.

normal power source

diesel generator protection and distribution equipment (complementary) possible transformater *

network 1 network 2 battery charger

battery protection box

static changeover switch

inverter

manual by-pass maintenance switch protection and distribution equipment (complementary)

fig. F23: example of an inverter/generating-set changeover scheme, taken from Merlin Gerin "Guides Pratiques". In normal operation of the inverter, a.c. power passes into the rectifier section, and a very small part of the d.c. power at the output of the rectifier maintains the battery in a fullycharged condition. The remainder of the d.c. power is converted into interference-free a.c. power for the load. In the event of a changeover from normal to reserve-power generator supply, it is important (particularly if the load to be supplied from the generator is large, relative to its rating) that damaging transient torques on the generator shaft and couplings be avoided. Such torques occur for suddenlyapplied loads and are due to the oscillating transient torque of the shaft and the steady load torque adding and subtracting at the natural frequency of the shaft oscillations. In order to avoid this phenomenon, the rectifier is controlled electronically to pass a low current initially, gradually increasing the current until the load is taken entirely by the generator and the battery is receiving its * necessary in some cases, e.g. to adapt the voltages.


maintaining trickle charge. This operation lasts for 10-15 seconds. Closing down of the inverter is also carried out progressively by similar controls on the rectifier circuits. A gradual application of load also avoids the possibility of large transient currents, and fluctuations in frequency, the latter being due to inertia in the speed-regulation governor system of the prime mover. The rectifier in the conversion system creates harmonic currents which generally means that the reserve-power generator has to be derated (i.e. an oversized generator may have to be installed). This question should be discussed with the UPS equipment manufacturer. In the example shown in fig. F23, the output from the inverter is in synchronism with the input supply to the rectifier, so that, in the event of overloading or failure of the inverter, instantaneous closure of the static changeover switch will maintain supply.

4. earthing schemes

F 4.1 earthing connections in a building, the connection to an earth electrode and the interconnection (bonding) of all metal parts of the building and all exposed conductive parts of electrical equipment prevents the appearance of dangerously high voltages between any two simultaneously accessible metal parts.

definitions The following terms are commonly used in industry and in the literature. Bracketed numbers refer to fig. F24. c earth electrode (1): a conductor or group of conductors in intimate contact with, and providing an electrical connection with Earth (see F4.6); c earth: the conductive mass of the Earth, whose electric potential at any point is conventionally taken as zero; c electrically independent earth electrodes: earth electrodes located at such a distance from one another that the maximum current likely to flow through one of them does not significantly affect the potential of the other(s); c earth electrode resistance: the contact resistance of an earth electrode with the Earth; c earthing conductor (2): a protective conductor connecting the main earthing terminal (6) of an installation to an earth electrode (1) or to other means of earthing (e.g. TN systems); c exposed-conductive-part (see table F25): a conductive part of equipment which can be touched and which is not a live part, but which may become live under fault conditions,

c protective conductor (3): a conductor used for some measures of protection against electric shock and intended for connecting together any of the following parts: v exposed-conductive-parts, v extraneous-conductive-parts, v the main earthing terminal, v earth electrode(s), v the earthed point of the source or an artificial neutral; c extraneous-conductive-part (see table F25): a conductive part liable to introduce a potential, generally earth potential, and not forming part of the electrical installation (4). For example: v non-insulated floors or walls, metal framework of buildings, v metal conduits and pipework (not part of the electrical installation) for water, gas, heating, compressed-air, etc. and metal materials associated with them; c bonding conductor (5): a protective conductor providing equipotential bonding; c main earthing terminal (6): the terminal or bar provided for the connection of protective conductors, including equipotential bonding conductors, and conductors for functional earthing, if any, to the means of earthing.

connections The main equipotential bonding system The bonding is carried out by protective conductors and the aim is to ensure that, in the event of an incoming extraneous conductor (such as a gas pipe, etc.) being raised to some potential due to a fault external to the building, no difference of potential can occur between extraneousconductive-parts within the installation. The bonding must be effected as close as possible to the point(s) of entry into the building, and be connected to the main earthing terminal (6). However, connections to earth of metallic sheaths of communications cables require the authorization of the owners of the cables. Supplementary equipotential connections These connections are intended to connect all exposed-conductive-parts and all extraneous-conductive-parts simultaneously accessible, when correct conditions for protection have not been met, i.e. the original bonding conductors present an unacceptably high resistance. Connection of exposed-conductive-parts to the earth electrode(s) The connection is made by protective conductors with the object of providing a lowresistance path for fault currents flowing to earth.

3

branched protective conductors to individual consumers (3) extraneous conductive parts (4) 4

heating

3 main protective conductor

5

water 4

3

5

gas 5

6 possible TN connection

7 2 1

fig. F24: an example of a block of flats in which the main earthing terminal (6) provides the main equipotential connection. The removable link (7) allows an earth-electrode-resistance check.


4. earthing schemes (continued)

F 4.1 earthing connections (continued) the efficient bonding and connecting to earth of all accessible metal fixtures, and all exposed-conductiveparts of electrical appliances and equipment, is essential for effective protection against electric shocks.

component parts component parts to consider as exposed-conductive-parts 1. cable ways c conduits c impregnated-paper-insulated lead-covered cable, armoured or unarmoured c mineral insulated metal-sheathed cable (pyrotenax, etc.) 2. switchgear c withdrawable section 3. appliances c exposed metal parts of class 1 insulated appliances 4. non-electrical elements v metallic fittings associated with cable ways (cable trays, cable ladders, etc.) c metal objects: v close to aerial conductors or to busbars v in contact with electrical equipment.

component parts to consider as extraneous-conductive-parts 1. elements used in building construction c metal or re-inforced concrete (RC): v steel-framed structure, v re-inforcement rods, v prefabricated RC panels, c surface finishes: v floors and walls in re-inforced concrete without further surface treatment, v tiled surface, c metallic covering, v metallic wall covering, 2. building services elements other than electrical c metal pipes, conduits, trunking, etc. for gas, water and heating systems, etc., c related metal components (furnaces, tanks, reservoirs, radiators), c metallic fittings in wash rooms, bathrooms, toilets, etc., c metallized papers.

component parts not to be consider as exposed-conductive-parts 1. diverse service channels, ducts, etc. c conduits of insulating material, c mouldings in wood or other insulating material, c conductors and cables without metallic sheaths. 2. switchgear c enclosures made of insulating material. 3. appliances c all appliances having class II insulation, regardless of the type of exterior envelope.

component parts not to be consider as extraneous-conductive-parts c wooden-block floors, c rubber-covered or linoleum-covered floors, c dry plaster-block partition, c brick walls, c carpets and wall-to-wall carpeting.

table F25: list of exposed-conductive-parts and extraneous-conductive-parts.

installation and measurements of earth electrodes This subject is dealt with at the end of Subclause 4.6.


F 4.2 definition of standardized earthing schemes the different earthing schemes described, characterize the method of earthing the LV neutral point of a HV/LV transformer, and the earthing of the exposed conductive-parts of the LV installation supplied from it. The choice of these methods governs the measures necessary for protection against indirect-contact hazards. neutral

exposed-conductive-parts

Earth

Earth

* Generally the star-point of a star-connected LV winding.

neutral Earth

exposed-conductive-parts Neutral

The earthing schemes to be described characterize the method of earthing the LV neutral point of a HV/LV transformer (or of any other source) and the means of earthing exposed conductive parts of the related LV installation. The choice of earthing scheme governs the measures to be taken for the protection of persons against the hazards of indirect contact. Several different schemes of earthing can coexist in an installation if necessary.

One point* at the supply source is connected directly to earth. All exposed- and extraneous-conductive-parts are connected to a separate earth electrode at the installation. This electrode may or may not be electrically independent of the source electrode, the two zones of influence may overlap, without affecting the operation of protective devices.

PE Rn

fig. F26: TT scheme.

TN schemes The source is earthed as for the TT scheme (above). At the installation, all exposed- and extraneous-conductive-parts are connected to the neutral conductor. The several versions of TN schemes are shown below: TN-C scheme The neutral conductor is also used as a protective conductor and is referred to as a PEN (Protective Earth and Neutral) conductor. This scheme is not permitted for conductors of less than 10 mm2 and for portable equipment. The TN-C scheme requires the establishment of an efficient equipotential environment within the installation with dispersed earth electrodes spaced as regularly as possible.

the TN-S (5 wires) system is obligatory for circuits of crosssectional-area of less than 10 mm2 for copper and 16 mm2 for aluminium on mobile equipment.

L1 L2 L3 N

TT scheme (earthed neutral)

TN-S scheme The protective conductor and the neutral conductor are separate. On underground cable systems where lead-sheathed cables exist, the protective conductor is generally the lead sheath. The use of separate PE and N conductors (5 wires) is obligatory for circuits of cross-sectional area of less than 10 mm2 for copper and 16 mm2 for aluminium on mobile equipment. TN-C-S scheme The TN-C and TN-S schemes can be used in the same installation. In the scheme TN-C-S the TN-C (4 wires) scheme must never be used downstream of the TN-S (5 wires) scheme. TN-C

L1 L2 L3 PEN

Rn

fig. F27: TN-C scheme.

L1 L2 L3 N PE

Rn

fig. F28: TN-S scheme.

The point at which the PE conductor separates from the PEN (i.e. neutral) conductor, is generally at the origin of the installation.

5 x 50 mm2

TN-S

PEN

L1 L2 L3 N PE

PE 16 mm2

6 mm2

16 mm2

16 mm2

PEN bad

bad

TN-C scheme not permitted downstream of TN-S scheme

fig F29: TN-C-S scheme. distribution within a low-voltage installation - F21


F 4.2 definition of standardized earthing schemes (continued) important: in the TN-C scheme the protective conductor function of the PEN conductor takes priority. In particular, the PEN conductor must be connected directly to the earth terminal of an applicance, and the bridging connection is then made to the neutral terminal.

4 x 95 mm2

TNC

16 mm2

10 mm2

L1 L2 L3 PEN 6 mm2

6 mm2

PEN

PEN

N

correct

incorrect

correct

incorrect

S < 10 mm2 TNC prohibited

PEN connected to the neutral terminal is prohibited

fig. F30: connection of the PEN conductor in the TN-C scheme. neutral

exposed-conductive-parts

Isolated or Impedance-earthed

Earth

IT scheme (isolated neutral) No intentional connection is made between the neutral point of the supply source and earth (fig. F31). Exposed- and extraneous-conductive-parts of the installation are connected to an earth electrode. In practice all circuits have a leakage impedance to earth, since no insulation is perfect. In parallel with this (distributed) resistive leakage path there is the distributed capacitive current path, the two paths together constituting the normal leakage impedance to earth (fig. F32). HV/LV

Example: In a LV 3-phase 3-wire system, 1 km of cable will have a leakage impedance due to C1, C2, C3 and R1, R2 and R3 equivalent to a neutral earth impedance Zct of 3,000 to 4,000 ohms. HV/LV

R1 C1

C2

R2

R3

C3

fig. F32: leakage impedance in an IT scheme.

IT scheme (impedance-earthed) An impedance Zs (in the order of 1.000 to 2.000 ohms) is connected permanently between the neutral point of the transformer LV winding and earth (fig. F34). All exposed- and extraneous-conductiveparts are connected to an earth electrode. The reasons for this form of power-source earthing are to fix the potential of a small network with respect to earth (Zs is small compared to the leakage impedance) and to reduce the level of overvoltages, such as transmitted surges from the HV windings, static charges, etc. with respect to earth. It has, however, the effect of slightly increasing the first-fault current level (see G 3.4).


fig F31: IT scheme (isolated neutral).

Zct

fig. F33: equivalent impedance to leakage impedances in an IT scheme. HV/LV

Zs

fig. F34: IT scheme (impedance-earthed).

F 4.3 earthing schemes characteristics Each earthing scheme (often referred to as the type of power system or system earthing arrangement) reflects three technical choices: c earthing method; c arrangement of PE protective conductors; c arrangement of protection against indirect contact. These choices and their consequences will be described for each scheme.

The consequences are related to the following points: c electric shock; c fire; c power supply continuity; c overvoltages; c electromagnetic disturbances; c design and operation.

L1 L2 L3 N Ω Rm PE Rd

Rc

fig. F35: with a TN-S scheme, fault currents can be very high, limited only by the impedance of the live conductors (phase and PEN).

TN-C scheme Characteristics c earthing method: v the neutral point of the transformer is connected directly to earth and the neutral conductor is earthed at as many points as possible, v exposed conductive parts of equipment and extraneous conductive parts are connected to the neutral conductor; c arrangement of PE protective conductors. The PE and neutral conductors are combined in a single PEN conductor; c arrangement of protection against indirect contact. Given the high fault currents and touch voltages: v automatic disconnection is mandatory in the event of an insulation fault, v this disconnection must be provided by circuit breakers or fuses. On installations with a combined neutral and protective conductor, residual current devices cannot be used for this purpose since an insulation fault to earth also constitutes a phase-neutral short-circuit. L1 L2 L3 PEN

Rn

fig. F36: any insulation fault occurring outside a building creates a rapid rise in the potential difference outside the building. The result of a high voltage insulation fault is shown here for a TN system, all earth electrodes considered together.

Consequences c earthing method: v the neutral point of the transformer is connected directly to earth and the neutral conductor is earthed at as many points as possible; v exposed conductive parts of equipment and extraneous conductive parts are connected to the neutral conductor; c overvoltages: v under normal conditions, the neutral, exposed conductive parts and earth are at virtually the same potential; v given the localised effect of earth electrodes, the potential may vary with the distance from the electrode. Therefore, during a HV insulation fault, a current will flow through the earth electrode of the LV neutral and a power frequency voltage will appear between the exposed conductive parts of LV equipment and the distant earth; c power supply continuity, electromagnetic compatibility and fire: the current of insulation faults is not limited by any earth electrode impedance and is therefore high (several kA). During a LV insulation fault, the supply voltage drop, electromagnetic disturbances and the risk of damage (fire, motor windings and magnetic frames) is high; c overvoltages: during a LV insulation fault, the neutral point of the triangle representing the 3-phase voltage system is displaced and the voltage between phase and the exposed conductive parts of the installation exceed the phase-to-neutral voltage. In practice, a value of 1.45 Un provides a rough approximation; c protective conductors. The PE and neutral conductors are combined in a single PEN conductor.



F 4.3 earthing schemes characteristics (continued) The PEN conductor must satisfy the requirements of both its functions. In the event of a conflict, the PE function has priority. The TN-C scheme is prohibited for all circuits with cross-sectional areas less than 10 mm2 for copper conductors or 16 mm2 for aluminium conductors. It is also prohibited for flexible conductors. c fire protection The TN-C scheme is prohibited in premises where there is a high risk of fire or explosion, for example in class BE2 and BE3 premises respectively for standard NFC 15-100. The reason is that the connection of the extraneous conductive parts of the building to the PEN conductor creates a flow of current in the structures, resulting in a risk of fire and electromagnetic disturbances. During insulation faults, these circulating currents are considerably increased. These phenomena are the reason for prohibiting the use of the TN-S scheme in premises where the risk of fire is high. L1 L2 L3 N PE

Rn

fig. F37: the presence of any length of PEN conductor in a building leads to the flow of currents in the exposed conductive parts and the shielding of equipment supplied by a TN-S scheme. c electromagnetic compatibility v when a PEN conductor is installed in a building, regardless of its length, it leads to a power frequency voltage drop under normal operating conditions, creating potential differences and therefore the flow of currents in any circuit formed by the exposed conductive parts of the installation, the extraneous conductive parts of the building, coaxial cable and the shielding of computer or telecommunications systems. These voltage drops are amplified in modern installations by the proliferation of equipment generating 3rd-order harmonics. The magnitude of this harmonic is tripled in the neutral conductor instead of being cancelled out as is the case for the fundamental; TNC

v in a less apparent manner, these circulating currents correspond to an imbalance of the currents in the distribution circuit and therefore the creation of a magnetic field that can disturb cathode-ray tubes, monitors, certain medical equipment, etc. at levels as low as 0.7 A/m (i.e. 5 A passing one metre from a sensitive device). This phenomenon is amplified in the event of an insulation fault; c corrosion: corrosion has two sources, first the DC component that the PEN conductor can carry and second, the telluric currents that corrode the earth electrodes and metal structures in the case of multiple earthing; c arrangement of protection against indirect contact. Given the high fault currents and touch voltages: v automatic disconnection is mandatory in the event of an insulation fault, v this disconnection must be provided by circuit breakers or fuses. On installations with a combined neutral and protective conductor, residual current devices cannot be used for this purpose since an insulation fault to earth also constitutes a phase-neutral short-circuit; c fire: protection is not provided for certain types of faults (impedant faults) that are not instantly transformed into solid short-circuits. Only residual current devices offer this type of protection. This situation therefore presents a risk of fire; c design and operation v when using circuit breakers or fuses to protect against indirect contact, the impedance of the source, upstream circuits and downstream circuits (the ones to be protected) must be known at the design phase and subsequently remain unchanged unless the protection is also changed. This impedance must be measured after installation and then at regular intervals (depending on the type of premises concerned). The characteristics of the protection devices are determined from these elements; v when the installation can be supplied from two sources (UPS, engine generator set, etc.), the characteristics governing the opening of the circuit breaker or the blowing of the fuse must be determined for each configuration and source used;

5 x 50 mm 2

TNS

PEN

L1 L2 L3 N PE

PE 16 mm2

6 mm2

16 mm2

16 mm2

PEN incorrect

incorrect

TNC scheme not permitted downstream of TN-S scheme

fig. F38: to determine the breaking capacity of circuit breaker C, it is necessary to know the impedance of the normal source, that of the replacement source and the length of circuit C protected by circuit breaker C.


F v each circuit is designed once and for all and cannot exceed a maximum length specified in design tables as a function of the protection device used. Oversized cable cross-sections may be necessary in certain cases; v any modification to the installation requires reassessment and checking of the protection conditions. 4 x 95 mm2

TNC

16 mm2

10 mm2

L1 L2 L3 PEN 6 mm2

6 mm2

PEN

correct

incorrect

correct

PEN connected to the neutral terminal is prohibited

incorrect S < 10 mm2 TNC prohibited

fig. F39: with a TN-S scheme, fault currents can be very high, limited only by the impedance of the live conductors (phase and PE).

TN-S scheme Characteristics c earthing method; v the neutral point of the transformer (or the power supply system if the distribution uses a TN-C scheme and the installation a TN-S scheme) is earthed just once at the upstream end of the installation, v exposed conductive parts of equipment and extraneous conductive parts are connected to the protection conductors which are in turn connected to the transformer neutral; c arrangement of PE protective conductors. The PE conductors are separate from the neutral conductors and are sized for the highest fault current that can occur; c arrangement of protection against indirect contact. Given the high fault currents and touch voltages: v automatic disconnection is mandatory in the event of an insulation fault, v this disconnection must be provided by circuit breakers, fuses or residual current devices since the protection against indirect contact can be separated from the protection against phase-phase or phase-neutral shortcircuits. Consequences c earthing method: v the neutral point of the transformer (or the power supply system if the distribution uses a TN-C scheme and the installation a TN-S scheme) is earthed just once at the upstream end of the installation, v exposed conductive parts of equipment and extraneous conductive parts are connected to the protection conductors which are in turn connected to the transformer neutral; c overvoltages: under normal conditions, the neutral of the transformer, exposed conductive parts and earth electrode are at the same potential, even if transient phenomena cannot be excluded and can lead to the use of lightning arrestors on the phases, neutral and exposed conductive parts; c power supply continuity, electromagnetic compatibility and fire: the effects of HV/LV faults, HV insulation faults and LV insulation faults are similar to those already described

for the TN-C scheme. In particular, the current of insulation faults is not limited by any earth electrode impedance and is therefore high (several kA) (see points 2, 3 and 4 of the corresponding part for the TN-C scheme); c the neutral conductor cannot be earthed. This avoids creating a TN-C scheme with its inherent disadvantages, i.e. voltage drop and load currents in the protective conductor under normal operating conditions; c arrangement of PE protective conductors. The PE conductors are separate from the neutral conductors and are sized for the highest fault current that can occur; c electromagnetic compatibility: v under normal conditions, the PE conductor, as opposed to the PEN conductor, is not subject to voltage drop and all the resulting drawbacks of the TN-C scheme are therefore eliminated. The TN-S scheme is similar in this respect to the TT scheme, v in the event of an insulation fault, a high impulse voltage appears along the PE conductor, creating the same transient problems as for the TN-C scheme; c arrangement of protection against indirect contact. Given the high fault currents and touch voltages: v automatic disconnection is mandatory in the event of an insulation fault, v this disconnection must be provided by circuit breakers, fuses or residual current devices since the protection against indirect contact can be separated from the protection against phase-phase or phase-neutral shortcircuits.

fig. F40: with a TT scheme, fault currents are limited by the earth electrode resistances and the accompanying voltage drops are very small. distribution within a low-voltage installation - F25


F 4.3 earthing schemes characteristics (continued) If the protection against indirect contact is provided by overcurrent protection devices: the same characteristics apply as for the TN-C scheme; c fire: protection is not provided for impedant faults, leading to a risk of fire; c design and operation: v calculation of the impedance of the sources and that of the circuit to be protected, with checking by measurements after installation and then at regular intervals, v double determination of the disconnection conditions when the installation can be supplied from two sources (UPS, engine generator set, etc.), v the circuits have a maximum length that cannot be exceeded, v any modification to the installation requires reassessment and checking of the protection conditions.

If the protection against indirect contact is provided by residual current devices: to avoid nuisance tripping, it is often possible to use high residual operating currents in the order of 1 A or more; c fire, design and operation: v the drawbacks already discussed are eliminated and we obtain the advantages of the TT scheme, v the use of residual current devices with operating currents 500 mA helps to prevent damage of electrical origin which can occur in the event of an impedant fault or due to the high level of insulation faults.

TT scheme Characteristics c earthing method: v the neutral point of the transformer is connected directly to earth, v exposed conductive parts of equipment are connected by protective conductors to the earth electrode of the installation which is generally independent with respect to the earth electrode of the transformer neutral; c arrangement of PE protective conductors. The PE conductors are separate from the neutral conductors and are sized for the highest fault current that can occur; c arrangement of protection against indirect contact. Automatic disconnection is mandatory in the event of an insulation fault. In practice, this disconnection is carried out by residual current devices. Their operating currents must be low enough for the devices to detect the fault currents, limited by two earth electrode resistances in series. c earthing method. Consequences c the neutral point of the transformer is connected directly to earth; c exposed conductive parts of equipment are connected by protective conductors to the earth electrode of the installation which is generally independent with respect to the earth electrode of the transformer neutral; c overvoltages: although, as for the TN scheme, the potential of the exposed conductive parts and the earth electrode is the same, this may not be true for the neutral conductor which is galvanically connected to an earth electrode and the exposed conductive parts, different and in some cases relatively far away (often the case for lightning strikes in rural areas). On industrial sites or urban areas, this is not generally the case. The coupling of the two earth earth electrodes is, from an overall point of view, an acceptable compromise. The installation of lightning arrestors provides the necessary level of protection; c electromagnetic compatibility: in the event of an insulation fault, the fault current is relatively low. For instance, with an earth electrode resistance of 230V/100A z 2.3 Ω, the fault current is only 100 A. As a result, the voltage drop created by the fault, F26 - distribution within a low-voltage installation

the accompanying electromagnetic disturbances and the transient difference in potential between two devices (e.g. two interconnected PCs) connected by a shielded cable are much easier to withstand than for a TN-S scheme; c arrangement of PE protective conductors. The PE conductors are separate from the neutral conductors and are sized for the highest fault current that can occur; c electromagnetic compatibility: under normal conditions, the PE conductor is not subject to voltage drop and all the resulting drawbacks of the TN-C scheme are therefore eliminated. In the event of an insulation fault, the impulse voltage that appears along the PE conductor is low and the resulting disturbances are negligible; c design and operation: for distribution circuits, the cross-sectional area of the PE conductor can be less than for a TN-S scheme; c arrangement of protection against indirect contact: v automatic disconnection is mandatory in the event of an insulation fault. In practice, this disconnection is carried out by residual current devices. Their operating currents must be low enough for the devices to detect the fault currents, limited by two earth electrode resistances in series, v residual current devices are added in the form of relays for circuit breakers and in the form of RCCBs for fuses. They can protect a single circuit or a group of circuits and their operating currents are chosen according to the maximum value of the resistance R of the earth electrode for the exposed conductive parts, v the presence of residual current devices minimises the design and operating constraints. It is unnecessary to know the upstream source impedance and there is no limit concerning the length of the circuits (except to avoid excessive voltage drop). An installation can be modified or extended without calculations or in-situ measurements, v the use of a replacement source by the distribution utility or the operator is straight forward; c fire: the use of residual current devices with operating currents i 500 mA helps prevent fires of electrical origin;

F c electromagnetic compatibility: insulation fault currents last only a short time, less than 100 ms (or less than 400 ms on distribution circuits) and are low in magnitude. HV/LV

R1 C1

C2

R2

R3

C3

fig. F41: with an IT scheme, fault currents are limited by the earthing of the neutral and by the overvoltage limiter.

IT scheme Characteristics c earthing method The neutral point of the transformer is isolated from earth or earthed through an impedance and an overvoltage limiter. Under normal conditions, its potential is maintained close to that of the exposed conductive parts by the earth leakage capacitances of the trunking and equipment. Exposed conductive parts of equipment and extraneous conductive parts of the building are connected to the building’s earth electrode; c arrangement of PE protective conductors. The PE conductors are separate from the neutral conductors and are sized for the highest fault current that can occur; c arrangement of protection against indirect contact. The fault current in the event of a single insulation fault is low and does not represent a hazard. The occurrence of a second fault should be made highly improbable by installing an insulation monitoring device that will detect and indicate the occurrence of a first fault that can then be promptly located and eliminated. Consequences c earthing method. The neutral point of the transformer is isolated from earth or earthed through an impedance and an overvoltage limiter. Under normal conditions, its potential is held close to that of the exposed conductive parts by the earth leakage capacitances of the trunking and equipment. Exposed conductive parts of equipment and extraneous conductive parts of the building are connected to the building’s earth electrode; c overvoltages: v under normal conditions, the neutral conductor, exposed conductive parts and earth electrode are at virtually the same potential, v an overvoltage limiter should be installed to prevent a rise in potential between the live parts and the exposed conductive parts that could exceed the withstand voltage of the LV equipment in the event of a fault originating in the high voltage installation. The protection against overvoltages should be implemented according to the criteria common to all the earthing schemes;

c power supply continuity and electromagnetic compatibility: v the current of a first insulation fault is low, a result of the capacitances between the live conductors and the expoed conductive parts such as those of the load circuits and HF filters, v a first low voltage insulation fault does not produce any voltage drop on the mains or electromagnetic disturbance over a wide frequency range corresponding to the occurrence of a classical insulation fault current. c overvoltages: after a first fault, the equipment continues to be supplied with power and the phase-to-phase voltage gradually appears between the healthy phases and the exposed conductive parts. Equipment must be chosen with this constraint in mind. Notes: c standard IEC 950 (or EN 60950) defines a category of information processing equipment that can be used on IT systems; c if lightning arrestors are used, the standards stipulate that their rated voltages should be chosen according to the phase-tophase voltage. c power supply continuity and electromagnetic compatibility: second insulation fault can occur on a different phase, creating a short-circuit and the associated hazards. The user of an IT system chooses that this situation must never occur, even if the standards allow for this possibility for safety reasons; c arrangement of PE protective conductors. The PE conductors are separate from the neutral conductors and are sized for the highest fault current that can occur; c electromagnetic compatibility: under normal conditions, and even when a first insulation fault occurs, the PE conductors show no voltage drop. A high level of equipotentiality is maintained between protective conductors, functional earthing conductors, exposed conductive parts and the extraneous conductive parts of the building to which they are connected; c arrangement of protection against indirect contact. The fault current in the event of a single insulation fault is low and does not represent a hazard.



F 4.3 earthing schemes characteristics (continued) The occurrence of a second fault should be made highly improbable by installing an insulation monitoring device that will detect and indicate the occurrence of a first fault that can then be promptly located and eliminated. The protective devices are designed to operate in the event of a double fault. If circuit breakers or fuses are used, the rules are similar to those for the TN scheme. Residual current devices can alos be used. If the two faults occur downstream of the same residual current device, the device considers the fault current as a load current and may not trip. A separate residual current device is therefore required for each circuit. If two sites have the same installation using an IT scheme, and their earth electrode systems are not connected, a residual current device must always be included at the head of each installation. This measure prevents one insulation fault on phase 1 of the first site and another on phase 2 of the second site from creating a dangerous situation; c fire: the use of an insulation monitoring device and possibly residual current devices with operating currents i 500 mA prevents fires of electrical origin; c design and operation: v trained maintenance personnel must be available for prompt locating and elimination of the first insulation fault, v the installation must be designed with great care: use of the IT scheme where justified by requirements related to continuity of supply, isolation of loads with high leakage currents (certain furnaces and certain types of computer hardware), examination of the influence of leakage currents, in particular with respect to residual current devices, division of the installation, etc., v if 30 mA residual current devices are used to protect socket circuits: - the total capacitive earth leakage current downstream of such a device must not exceed 10 mA. The value is estimated using the phase-to-phase voltage for the phase and for the phase-to-neutral voltage for the neutral, - if the loads powered by such a circuit are not critical, the residual current device can trip on a first insulation fault, thereby eliminating it immediately. Otherwise the use of sockets should be avoided or other measures taken, v comment: the earth conductor, if distributed, must be protected by 4-pole devices including neutral protection or 2-pole devices. In final distribution boxes, 1-pole + neutral protection devices are permitted as long as the ratings for the phase and neutral are the same or close, and as long as a residual current device is present upstream.


F 4.4.1 choice criteria 1st criterion No earthing scheme is universal. When it is possible to choose the earthing scheme, analyse every case separately, basing the final choice on the specific constraints of the electrical installation, the needs of the user and the rules laid down by applicable legislation or by the power distribution utility. The best solution often involves several different earthing schemes for different parts of the installation.

2nd criterion These solutions must satisfy the following fundamental criteria: c protection against electric shock; c protection against fire of electric origin; c power supply continuity; c protection against overvoltages; c protection against electromagnetic disturbances.

3rd criterion: comparison of earthing schemes A comparison of the different earthing schemes leads to the following recommendations for use: The TT scheme is recommended for installations that have only limited surveillance or installations subject to extensions or modifications. The main reason is that it is the simplest scheme to implement in private or public distribution. On the other hand, given the need for two separate LV earthing electrodes, overvoltage protection must often be provided. INFLUENCE OF EARTHING ELECTRODES Private substation with TN scheme c the inside of the equipment is not exposed (U2 = 230 V); c the incoming systems are exposed to Uf. LV consumer with TT scheme c the inside of the equipment is exposed in the event of nearby lightning strikes (Rb-If). See the section dealing with lightning arrestors; c when correctly implemented, the effects of an HV insulation fault are eliminated.

This scheme is generally implemented without medium-sensitivity residual current devices. Drawbacks however include: c insulation fault currents are high and can result in: v transient disturbances, v high risk of damage, v even fire! c a detailed study is required. If medium-sensitivity residual current devices are installed, they provide this scheme with improved protection against fire and greater flexibility both in design and use. The TN-C and TN-C-S schemes are not recommended for use, given the risk of fire and electromagnetic disturbances due to: c voltage drops along the PEN conductors; c high insulation fault currents; c currents flowing in the extraneous conductive parts, shielding, exposed conductive parts; c uneliminated impedant faults. A detailed study is required. The presence of any length of PEN conductor in a building leads to the flow of currents in the exposed conductive parts and the shielding of equipment supplied by a TN-S scheme.

4th criterion In terms of overvoltage withstand and electromagnetic disturbances, IT, TT and TN-S schemes are equally satisfactory if correctly implemented.

5th criterion When making an economic comparison, all costs must be taken into account, including those related to: c design; c maintenance; c modification or extensions; c production losses.

The IT scheme is recommended if power supply continuity is imperative. The IT scheme offers the best guarantee concerning the availability of power. It however requires: c a detailed study: v organisation of withstand to overvoltages and leakage currents; c trained maintenance personnel available at all times: v to promptly eliminate any first fault, v to supervise extensions to the installation. The TN-S scheme is recommended for installations that have a high level of surveillance or installations not subject to extensions or modifications.



F 4.4.2 comparison for each criterion 1 - level of protection against electric shock All earthing schemes provide equal protection against electric shock as long as they are implemented and used in accordance with applicable standards.

2 - protection against fire of electrical origin For TT schemes and IT schemes, in the event of a single fault, the insulation fault current is respectively low or very low. The same is true for the risk of fire. For TN schemes, protection against impedant faults is insufficient unless residual current devices are included: c in this case, it is recommended to use the TN-S scheme together with residual current devices rather than the standard TN scheme. For TN type schemes, in the event of a solid fault, the insulation fault current is high and major damage can result. The TN-C scheme presents a higher risk of fire under normal operating conditions than the other schemes. It is therefore prohibited in premises presenting a high risk of fire or explosion. A load imbalance current circulates continuously in the PEN conductors and the connected parts (e.g. metal frames, exposed conductive parts, shielding, etc.).

3 - protection against overvoltages For all neutral schemes, the following steps are necessary: 1) Evaluate the disturbances to be taken into account as a function of: c site exposure: v overvoltages due to indirect effects of lightning, v nearby direct lightning strikes; c the type of supply system: v in particular HV insulation faults; c the type of premises: v choose the appropriate level of safety. This evaluation should be carried out at power frequency and then at higher frequencies up to several MHz. 2) Decide on the number and quality of equipotential zones (room, building, site) so as to organise the protection of each. In practice, for sites with a number of buildings supplied by the same source and interconnected by communications media, one of the following solutions should be used: c equipotentiality by interconnection of the buildings in one of the two following manners: v by at least one accompanying conductor with a cross-sectional area of at least 35 mm2, sized for the possible fault currents, v by a denser mesh; c complete isolation, for example by using an optical fiber communication link without a conductive sheath.


3) Implement the necessary protection (lightning arrestors, etc.) on the lines of the different incoming and outgoing electrical systems. Comment: c the use of a TN-S scheme does not eliminate the need for the above measures; c TT installations generally require lightning arrestors (rural). Furthermore, for IT schemes, protection against overvoltages due to HV faults must be provided by an overvoltage limiter.

4 - protection against electromagnetic disturbances 1) For differential mode disturbances, the earthing scheme used is of no importance. For all common mode or differential mode disturbances with frequencies greater than 1 MHz, the earthing scheme used is of no importance. 2) If correctly implemented, TT, TN-S and IT schemes can satisfy all electromagnetic compatibility criteria. Note however that for the TN-S scheme, major disturbances are produced during an insulation fault. 3) For TN-C or TN-C-S earthing schemes, a load imbalance current circulates continuously in the PEN conductor, exposed conductive parts of equipment and cable shielding. The presence of 3rd order harmonics significantly amplifies this current in modern installations. This continuous current creates voltage drops between the exposed conductive parts of sensitive equipment connected to the PEN conductor. These schemes are therefore not recommended for use.

F 4.5 choice of earthing method - implementation After consulting local regulations and relevant codes of practice, etc. the tables F40 and F41 can be used as an aid in deciding on divisions and possible galvanic isolation of appropriate sections of a proposed installation. Division of source This technique concerns the use of several transformers instead of employing one large unit. This method has already been noted as a means of de-coupling loads, which would otherwise cause unacceptable disturbance to other loads, e.g. voltage depression during the start-up period of a large motor, and so on... The quality and continuity of supply to the whole installation are thereby improved.

The cost of switchgear is reduced (shortcircuit current level is lower). The technical/ economic appraisal must be made case by case. Network islands The creation of galvanically-separated "islands" by means of LV/LV transformers allows an open choice of earthing system to be used on the secondary side, which is independent of any imposed earthing scheme in the primary LV network. In this way, the installation network may be arranged for optimum performance on different types of load.

example overvoltage device

HV/LV

PIM

LV/LV

TN system

IT system

arc furnace

fig. F42: a workshop in which supply continuity is paramount (IT) includes an arc furnace. The most suitable arrangement is an IT scheme for the workshop, and an isolating LV/LV transformer to supply the arc furnace, in a TN earthing scheme. HV/LV

overvoltage device

LV/LV

PIM

TN system

IT system

fig. F43: a factory with a load consisting mainly of welding machines requiring a TN system of earthing, and a painting workshop for which supply continuity has top priority. The latter supply is shown to be provided by an IT Island system, via a LV/LV transformer.

conclusion The optimization of the performance of the whole installation governs the choice of earthing system (see the following Sub-clause 4.6). Including: c initial investments, and c future operational expenditure that can arise from insufficient reliability, quality of materials, safety, continuity of service, etc. which is difficult to forecast. An ideal structure would comprise: v normal power supply source, v local reserve power supply source (see Clause 3 of this chapter) and the appropriate earthing schemes.



F 4.6 installation and measurements of earth electrodes A low-impedance earth electrode improves considerably the protection of the electrical installation from external electromagnetic influences, and particularly in the case of overvoltages caused by lightning. Protection of the building against direct lightning strokes, however, requires specialized studies, and is not dealt with here. The quality of an earth electrode (resistance as low as possible) depends essentially on two factors: c installation method; c nature of the earth.

installation methods

a very effective method of obtaining a low-resistance earth connection is to bury a conductor in the form of a closed loop in the soil at the bottom of the excavation for the building foundations. The resistance R of such an electrode (in homogeneous soil) is given (approximately) in ohms by: R = 2ρ L where L = the length of the buried conductor in metres ρ = soil resistivity in ohm-metres. care must be taken to avoid the occurrence of corrosion, notably where dissimilar metals are buried in close proximity.

for n rods: R = ρ ohms nL


Three common types of installation will be discussed: A conductor-type electrode forming a ring beneath the perimeter of the building which houses the installation concerned (fig. F44) This solution is strongly recommended, particularly in the case of a new building. The electrode should be buried around the perimeter of the excavation made for the foundations. It is important that the bare conductor be in intimate contact with the soil (and not placed in the gravel or aggregate hard-core, often forming a base for concrete). At least four (widely-spaced) vertically arranged conductors from the electrode should be provided for the installation connections, and where possible any reinforcing rods in concrete work should be connected to the electrode. The conductor forming the earth electrode, particularly when it is laid in an excavation for foundations, must be in the earth, at least 50 cm below the hard-core or aggregate base for the concrete foundation. Neither the electrode nor the vertical rising conductors to the ground floor, should ever be in contact with the foundation concrete. For existing buildings, the electrode conductor should be buried around the outside wall of the premises to a depth of at least 1 metre. As a general rule, all vertical connections from an electrode to aboveground level should be insulated for the nominal LV voltage (600-1,000 V). The conductors may be: c copper - bare cable or multiple-strip u 25 mm2; Earthing rods (fig. F45) Vertically driven earthing rods are often used for existing buildings, and for improving (i.e. reducing the resistance of) earth electrodes in cases where upper-strata soil-drying can only be countered by deeper penetration into the earth. The rods may be: c copper or (more commonly) copper-clad steel. The latter are generally 1 or 2 metres long and provided with screwed ends and sockets in order to reach considerable depths, if necessary (for instance, the watertable level in areas of high soil resistivity)

c stainless steel cable or multiple strip u 35 mm2; c galvanized-steel cable. Copper is the most expensive material, but is the most suitable from considerations of corrosion. The use of more than one of these materials in the same soil is deprecated, since the elementary primary cell (e.g. zinc/copper) formed in the damp earth "electrolyte" would result in problems of corrosion. In the case noted, the zinc would be sacrificial to the copper, eventually leaving an uncoated (corroding) steel conductor of high surface-to-earth contact resistance. Steel reinforcing rods in concrete, however, have approximately the same galvanic potential, in the electro-chemical series, as copper in soil, so that copper earth electrodes may be connected to steel reinforcing rods with no danger of corrosion*. Steel rods in soil, on the other hand, will corrode if connected to steel reinforcing rods in concrete. Aluminium and lead are not suitable for use as earthing electrodes. The approximate resistance R of the electrode in ohms = 2ρ L where L = length of conductor in metres ρ = resistivity of the soil in ohm-metres (see tables F47 and F48).

fig. F44: conductor buried below the level of the foundations, i.e. not in the concrete. * Practical experience has shown that corrosion is not a problem at potential differences of less than 0.3 V.

c galvanized (see note below) steel pipe u 25 mm diameter, or rod u 15 mm diameter, u 2 metres long in each case. It is often necessary to use more than one rod, in which case the spacing between them should exceed the depth to which they are driven, by a factor of 2 to 3. The total resistance (in homogeneous soil) is then equal to the resistance of one rod, divided by the number of rods in question.

F The approximate resistance R obtained in ohms = ρ n L if the distance separating the rods > 4L where: L = the length of the rod in metres ρ = the resistivity of the soil in ohm-metres (see table F47) n = the number of rods

Lu3m

rods connected in parallel

fig. F45: earthing rods.

for a vertical plate electrode: R ≈ 0.8 ρ ohms L

data concerning earth resistivities in analogous terrains provide a useful base for designing an earth-electrode system.

Vertical plates (fig. F46) Rectangular plates, each side of which u 0.5 metres, are commonly used as earth electrodes, being buried in a vertical plane such that the centre of the plate is at least 1 metre below the surface of the soil. The plates may be: c copper of 2 mm thickness; c galvanized* steel of 3 mm thickness. The resistance R in ohms is given (approximately), by: 0.8 ρ L where: ρ = resistivity of the soil in ohm-metres L = the perimeter of the plate in metres * Note: Where galvanized conducting materials are used for earth electrodes, sacrificial cathodic protection anodes may be necessary to avoid rapid corrosion of the electrodes where the soil is aggressive. Specially prepared magnesium anodes (in a porous sack filled with a suitable "soil") are available for direct connection to the electrodes. In such circumstances specialist advice is recommended.

2 mm thickness (Cu)

fig. F46: vertical plate.

influence of the nature of the soil resistivity (in Ω.-m) 1 - 30 20 - 100 10 - 150 5 - 100 50 100 - 200 30 - 40 50 - 500 200 - 300 1500 - 3000 300 - 500 100 - 300 1000 - 5000 500 - 1000 50 - 300 800 1500 - 10000 100 - 600

nature of the terrain swampy soil, bogs silt alluvium humus, leaf mold peat, turf soft clay marl and compacted clay jurassic marl sandy clay siliceous sand stoney ground grass-covered-stoney sub-soil chalky soil limestone fissured limestone schist, shale mica schist granite and sandstone decomposed granite and sand stone table F47: resistivity (Ω-m) for different kinds of terrain. nature of the terrain heavy arable land, compacted humid banks light soil arable land, gravel, roughly banked land stoney soil, bare, dry sand, fissured rocks

mean value of resistivity (in Ω.-m) 50 500 3000

table F48: mean values of resistivity (Ω-m) for an approximate estimation of an earthelectrode resistance with respect to zero-potential earth.



F 4.6 installation and measurements of earth electrodes (continued) measurements and constancy of the resistance between an earth electrode and the earth

there must always be a (or a number of) removable link(s) to isolate an earth electrode, to allow it to be tested.


The resistance of the electrode/earth interface rarely remains constant Among the principal factors affecting this resistance are the following: c the humidity of the soil: the seasonal changes in the moisture content of the soil can be significant at depths of up to 2 meters. At a depth of 1 metre the value of resistivity (ρ) can vary in the ratio of 1 to 3 between a wet Winter and a dry Summer in temperate regions; c frost: frozen earth can increase the resistivity of the soil by several orders of magnitude. This is one of the reasons, together with that noted above, for recommending the installation of deep electrodes; c ageing: the materials used for electrodes will generally deteriorate to some extent for various reasons, for example: v chemical reactions (in acidic or alkaline soils),

v galvanic: due to stray d.c. currents in the earth from traction systems, etc. or due to dissimilar metals forming primary cells; different soils acting on sections of the same conductor can also form cathodic and anodic areas with consequent loss of surface metal from the latter areas. Unfortunately, the most favourable conditions for low earth-electrode resistance (i.e. low soil resistivity) are also those in which galvanic currents can most easily flow. c oxidization: brazed and welded joints are the locations at which oxidization is most likely to occur. Thorough cleaning of a newlymade joint, and wrapping with a suitable greased-tape binding is the preventive measure commonly adopted.

Measurement of the earth-electrode resistance There must always be removable links which allow the earth electrode to be isolated from the installation, so that periodic check tests of the earthing resistance can be carried out. To make such tests, two auxiliary electrodes are required, each consisting of a vertically driven rod. c ammeter method (fig. F49) UTt1 A = RT + Rt1 = i1 Ut1t2 B = Rt1 + Rt2 = i2 Ut2T C = Rt2 + RT = i3 A + C - B = 2RT When the source voltage U is constant (adjusted to be the same value for each test) then: U 1 RT = ( i11 + i31 - i2 ) 2 In order to avoid errors due to stray earth currents (galvanic (d.c.) or leakage currents from power and communication networks and so on) the test current should be a.c., but at a different frequency to that of the power system or any of its harmonics. Instruments using hand-driven generators to make these measurements usually produce an a.c. voltage at a frequency of between 85 Hz and 135 Hz. The distances between the electrodes are not critical and may be in different directions from the electrode being tested, according to site convenience. A number of tests at differspacings and directions are generally made for cross-checking the test results. c use of a direct-reading earthingresistance ohmmeter (fig. 50) These instruments use a hand-driven or electronic-type of a.c. generator, together with two auxiliary electrodes, the spacing of which must be such that the zone of influence of the electrode being tested should not overlap that of the test electrode (C).

The test electrode (C) furthest from the electrode (X) under test, passes a current through the earth and the electrode under test, while the second test electrode (P) picks up a voltage. This voltage, measured between (X) and (P), is due to the test current, and is a measure of the contact resistance (of the electrode under test) with earth. It is clear that the distance (X) to (P) must be carefully chosen to give accurate results. If the distance (X) to (C) is increased, however, the zones of resistance of electrodes (X) and (C) become more remote, one from the other, and the curve of potential (voltage) becomes more nearly horizontal about the point (O). In practical tests, therefore, the distance (X) to (C) is increased until readings taken with electrode (P) at three different points viz: at (P) and at approximately 5 metres on either side of (P) give similar values. The distance (X) to (P) is generally about 0.68 of the distance (X) to (C). U

t1

A T t2

fig. F49: measurement of the resistance to earth of the earth electrode of an installation by means of an ammeter.

F VG G I

V X

P

C

voltage-drop due to the resistance of electrode (X)

O

VG

voltage-drop due to the resistance of electrode (C)

a) the principle of measurement is based on assumed homogeneous soil conditions where the zones of influence of electrodes C and X everlap, the location of test electrode P is difficult to determine for satisfactory results.

X

P

C

O

b) showing the effect on the potential gradient when (X) and (C) are widely spaced. The location of test electrode P is not critical and can be easily determined

fig. F50: measurement of the resistance to the mass of earth of electrode (X) using an earth-electrode-testing ohmmeter. Simplified measurement (TT-system) In a TT-earthed system, a simplified measurement of the earth-electrode resistance is possible. It consists in measuring the impedance between the earthelectrode and the neutral conductor. It equals the sum of the consumer earth-electrode resistance and the distributor earth-electrode resistance. This value is always pessimistic, but the distributor earth-electrode resistance is generally less than 5 Ω. In case of doubt, use the general method.


5. distribution boards

F a distribution board is among the most important elements in an installation. Its design and construction must conform with welldefined standards.

A main general distribution board is the point at which the incoming-power supply divides into separate circuits, each of which is controlled and protected by the fuses or switchgear of the board. In general, the power supply is connected to a set of busbars via a main switch (a circuit breaker or switch-fuse). Individual circuits, which are usually grouped according to the circuit function (lighting, heating, power and so on...), are supplied from the busbars. Some of the circuits feed directly into the busbars of local distribution boards, at which a division of circuits is made, while in extensive installations,

sub-distribution boards are sometimes necessary, thereby creating three levels of distribution. Modern practice is to enclose LV distribution boards in metal housings, which afford double protection: c the protection of switchgear, indicating instruments, relays, fusegear... against mechanical shocks, vibrations and other external influences likely to interfere with operational integrity (EMI*, dust, moisture, vermin, etc.); c the protection of personnel against the possibility of electric shock. * electromagnetic interference

5.1 types of distribution board Distribution boards, or an assembly of LV switchgear, may differ according to the kind of application, and to the design principle adopted (notably in the arrangement of the busbars).

the load requirements dictate the type of distribution board to be installed.

distribution boards according to specific applications The principal types of distribution board are: c main general distribution board (fig. F53); c local general distribution board (fig. F52); c sub-distribution board (fig. F51); c process-control i.e. "functional" distribution board. For example MCC (motor-controlcentre), heating circuits control board, and so on. The local and sub-distribution boards are dispersed throughout the installation.

The process-control boards are either: c adjacent to the main general distribution board, or c in proximity to the process concerned. Distribution boards are generally referred to in written texts by the abbreviation DB.

fig. F51: typical sub-distribution board.

fig. F52: local general distribution board.

fig. F53: an example of a large industrial main general distribution board.


F a distinction is made between: c traditional DBs in which switch- and fusegear, etc. are fixed to a chassis at the inside-rear part of the housing c functional DBs for specific applications.

realization of the two types of DB Traditional DBs Switchgear and fusegear, etc. are normally located on a chassis near the back of the enclosure. Indications and control devices (meters, lamps, pushbuttons, etc.) are mounted on the front face of the board. The placement of the components within the enclosure requires very careful study, taking into account the dimensions of each item, the connections to be made to it, and the clearances necessary to ensure safe and trouble-free operation. A quick estimation of the area required can be made by multiplying the sum of the areas of the individual items by 2.5.

Functional DBs Dedicated to specific functions, recourse is made to functional modules which include switchgear and devices, together with accessories for mounting and connections. For example, drawer-type motor control units, which comprise contactor, fuses, isolating switch, control pushbuttons, indicating lamps, and so on. Design of the board is rapid, since it is sufficient to add the number of modules required, with vacant spaces for later additional units if necessary. Using these prefabricated components greatly facilitates the assembly of the board. Moreover, the components of these boards have benefited from type tests, thereby ensuring an excellent safety performance. Figure F54 is an example of an industrial functional DB.

5.2 the technologies of functional distribution boards There are three basic technologies in general use for the realization of functional DBs.

fixed functional units

(fig. F54) The board is made up of fixed functional units such as contactors and associated relays, according to the particular function. These units are not suitable for circuit isolation (from the busbars for example) so that any intervention for maintenance, modifications and so on, requires the shutdown of the entire board. The use of removable plug-in or withdrawable units however can minimize shutdown times, which are then limited to the interval required only to remove or withdraw the unit of the circuit concerned.

are mounted on a drawer-type horizontally withdrawable chassis. The function is generally complex and often concerns motor control. Isolation is effected on both the upstream and downstream sides by the complete withdrawal of the unit.

functional units which have isolating and disconnecting features (fig. F55) Each functional unit is mounted on a removable panel and provided with a means of isolation on the upstream (busbars) side and disconnecting facilities on the downstream (circuit) side. The complete unit can therefore be removed for servicing, without requiring a general shutdown.

fig. F54: board with fixed functional units.

withdrawable chassis-mounted functional units (fig. F56) The switchgear and associated accessories

fig. F55: board with isolating and disconnecting features on each functional unit. fig. F56: board with withdrawable chassismounted units. distribution within a low-voltage installation - F37

5. distribution boards (continued)

F 5.3 standards conformity with the relevant standards is essential in order to ensure an adequate degree of operational safety.

Certains types of distribution boards (in particular, functional distribution boards) in which all component parts are individually subject to IEC 947, also conform to specific recommendations of IEC 439-1.

two elements of the standard IEC 439-1 largely contribute to operational safety: c forms of separation between adjacent functional units according to user's requirements c clearly defined individual and type tests.

IEC standard 439-1 IEC 439-1 covers LV switchgear and controlgear assemblies, manufactured and type-tested as complete units. IEC 439-1 defines four "forms" of assembly, according to the degree of internal separation, by barriers or partitions, into different compartments. The separations provide: c protection against contact with live parts of adjacent functional units; c limitation of the probability of initiating arcing faults; c protection against the passage of foreign solid bodies from one unit of the assembly to an adjacent unit. The following are typical forms of separation by barriers or partitions: v Form 1: no separation; v Form 2: separation of busbars from the functional units; v Form 3: separation of busbars from the functional units and separation of all functional units, one from another, except at their output terminals; v Form 4: as for Form 3, but including separation of the outgoing terminals of all functional units, one from another.

form 1

form 2

The form of the separation (e.g. metallic or non-metallic) shall be the subject of agreement between the manufacturer and the user. Forms 2, 3 and 4 are generally used since, in each case, the busbars are enclosed, thereby allowing safer intervention on functional units or their outgoing-circuit components, than that afforded by Form 1. Forms 3 and 4 are adopted where the space available for each functional unit is limited, so that without complete segregation between adjacent units, safe intervention for maintenance, etc., is not possible, unless the whole distribution board is shut down. c finally, individual type tests, checks and functional tests carried out during manufacture ensure conformity to the standard of the entire assembly.

form 3

form 4

fig. F57: representation of different forms of LV functional distribution boards.

5.4 centralized control the integration of functional distribution boards in a system of centralized technical management must be taken into account from the earliest design stage.


The organization of data acquisition from, and instructions to equipment, in schemes of remote control, is assuming greater importance as Centralized Technical Management techniques become more general. In the interests of economy (in communication-cable costs) all data and control-command signals should be processed at the equipment (e.g. functional DBs) in question, for transmission to and reception from the central command post.

Such signal conversions (e.g. analogue-todigital; electrical-to-optical, etc.) to suit the data-transmission links must therefore be housed, and supplied with suitable pollutionfree power, at, or very near to the distribution boards or other equipment, concerned.

6. distributors

F 6.1 description and choice two types of distribution are possible: c by insulated wires and cables, c by prefabricated pre-wired cable channels. The latter are distinguished by their ease of installation, flexibility, and by the number of connecting points possible.

types Two types of distribution are possible: Distribution by insulated conductors and cables Includes the mechanical protection and fixing of conduits, etc. The method of installation will affect the maximum current permitted, as noted in IEC 439 Parts 1 and 2. Distribution by prefabricated cable channels These channels are distinguished by their ease of installation, flexibility, and by the number of connecting points possible.

examples subdistribution board

local general distribution board

main general distribution board (MGDB)

heating, etc. general utilities distribution board

fig. F58: example 1: radial distribution wiring scheme for a hotel, using conductors in conduits and cables.


6. distributors (continued)

F 6.1 description and choice (continued) prefabricated pre-wired cable channels

bus-duct transformer to MGDB

transformer

mainbusbar trunking

prefabricated distribution busbar trunking MGDB prefabricated power and light-current distribution column local general distribution board

offices

prefabricated pre-wired cable channels

fig. F59: example 2: radial distribution with prefabricated bus trunking and cable channels for an entrepot installation.

selection of method-criteria The main considerations governing the choice of one method or the other are the first cost and the likelihood of extensive and frequent modifications. In the case of a fixed installation which is unlikely to be modified, either frequently or extensively, then the insulated wires-andconduit system is the more-economic solution. Where flexibility and ease of circuit modification are important, then the prefabricated cable channelling system should be the first choice. Design information concerning the smallest allowable (i.e. the most economic) c.s.a. of wiring conductors and cables, for the case of a wires-and-conduit installation, is given in Sub-clauses 2.1, 2.2 and 2.3 of Chapter H1.


F 6.2 conduits, conductors and cables IEC 364-5-52 provides information on the selection and erection of wiring systems, based on the principles described in IEC 364-1, concerning cables and conductors, their termination and/or jointing, their associated supports or suspensions, and their enclosures or methods of protection against external influences.

selection of wiring systems and methods of installation, according to IEC 364-5-52 (1993) The selection of a wiring system may be made from the following table. conductors and cables

bare conductors insulated conductors sheathed cables (including armoured and mineral insulated) c multi-core c single-core

method of installation without clipped conduit cable fixings direct trunking (including skirting trunking, flush floor trunking)

cable cable on supducting ladder, inport cable sulators wire tray, cable brackets

-

-

-

-

-

-

+

-

-

-

+

+

+

-

+

-

+ 0

+ +

+ +

+ +

+ +

+ +

0 0

+ +

+: Permitted. -: Not permitted. 0: Not applicable, or not normally used in practice.

table F60: selection of wiring systems. Recommended erection methods are indicated in the table below. situations

building voids

method of installation without with conduit cable fixings fixings trunking (including skirting trunking, flush floor trunking) 21, 25 0 22, 73, 73, 74 74

cable cable on supducting ladder, inport cable sulators wire tray, cable brackets 23

cable channel

43

43

41, 42

31, 32

4, 23

buried in ground embedded in structure surface mounted

62, 63

0

61

-

61

12, 13, 14, 15, 16 12, 13, 14, 15, 16 0

-

-

-

-

-

-

52, 53

51

1, 2, 5

33

24

0

-

-

-

11

3

31, 32 71, 72

4

18

-

overhead

-

-

0

34

-

18

17

immersed

81

81

0

-

0

12, 13, 14, 15, 16 12, 13, 14, 15, 16 0

-

-

The number in each box indicates the reference number in table H52 (IEC 364-5-52)*. -: Not permitted. 0: Not applicable, or not normally used in practice. Note: for current-carrying capacity see IEC 364-5-523.

table F61: erection of wiring systems. * Table H52 of IEC 364-5-52 fills seven pages. Two of these pages are reproduced below by way of example.



F 6.2 conduits, conductors and cables (continued) example 1

description 2 insulated conductors in conduits embedded in thermally insulating walls

reference 3 1

multicore cables in conduits embedded in thermally insulating walls

2

insulated conductors in surface mounted conduits

3

single or multicore cables in surface mounted conduits

3A

insulated conductors in cable ducting on a wall

4

single or multicore cables in cable ducting on a wall

4A

insulated conductors in conduits embedded in masonry

5

single or multicore cables in conduits embedded in masonry

5A

room

room

sheathed and/or armoured cables or sheathed single or multicore armoured cables


c on a wall

11

c on a ceiling

11A

c on unperforated trays

12

c on perforated trays

13

c on brackets run horizontally or vertically

14

c on cleats, spaced from a wall or a ceiling

15

c on ladders

16

F example 1

description 2 sheathed single-core or multicore cables suspended from or incorporating suspension wire

reference 3 17

bare or insulated conductors on insulators

18

table F62: some examples of installation methods. Note: the illustrations are not intended to depict actual product or installation practices but are indicative of the method described.



F 6.2 conduits, conductors and cables (continued) Designation of conduits according to the most recent IEC recommendations new designation code

3

90

3

2

obligatory marking code 1st numeral: mechanical properties average mechanical constraints: very light 1 light 2 medium 3 high 4 very high 5 2nd and 3rd numerals: classification according to temperature withstand capabilities: conduit class: -5°C 05 -25°C 25 +90°C 90 complementary marking code: 1st complementary numeral: malleability of conduits: rigid (slight bends only are possible) 1 malleable ("bendable") 2 transversally flexible (will flatten when bent) 3 flexible 4 2nd complementary numeral: electrical properties of conduits: with electrical continuity 1 intended for use as complementary insulation 2 intended for use as complementary insulation but including electrical continuity 3 3rd complementary numeral: resistance of conduits to the penetration of water, including: rain water projections of water (wind-blown rain) jets of water (from hose pipe, etc.) sea spray temporary immersion long-term immersion 4th complementary numeral: resistance to the penetration of solid bodies: conduits providing protection against: solid bodies greater than 2.5 mm solid bodies greater than 1 mm dust dust-proof (total exclusion) 5th complementary numeral: resistance to corrosion: conduits with protection: light, internal and external protection medium external, and light internal protection medium external and internal protection heavy external and light internal protection heavy external and medium internal protection heavy external and internal protection 6th complementary numeral: resistance to solar radiation: conduits with protection: low degree medium degree high degree reference number indicating the exterior diameter in mm 16-20-25-32-40-50-63

8

6

1

2

25

3 4 5 6 7 8 3 4 5 6 1 2 3 4 5 6 1 2 3

table F63: designation code for conduits according to the most recent IEC publications.


F designation code for LV conductors and cables as referred to in this sub-clause a conductor comprises a single metallic core in an insulating envelope.

a cable is made up of a number of conductors, electrically separated, but mechanically solid, and generally enclosed in a protective flexible sheath.

the term cable-way refers to conductors and/or cables together with the means of support and protection, etc. for example : cable trays, ladders, ducts, trenches, and so on... are all "cable-ways".

Definitions c a conductor: as referred to in this subclause a conductor comprises a single metallic core in an insulating envelope. c a cable: a cable is made of a number of conductors, electrically separated, but mechanically solid, and generally enclosed in a protective flexible sheath. c a cable-way: the term cable-way refers to conductors and/or cables together with the means of support and protection, etc. for example: cable trays, ladders, ducts, trenches, and so on... are all "cable-ways". Designation Most countries have national standards of codification for conductors and cables. In Europe, a code has been established by CENELEC* which "harmonizes" the several codes of member countries, each of which is progressively replacing its national code by the CENELEC version. It may be noted, that at the time of writing, certain cable types (notably XLPE insulated) have not yet been included in the harmonized code. Table F64 illustrates the form and significance of the designation code. * Comité Européen de Normalisation de l'Electrotechnique.

designation code (CENELEC)

H

07

"harmonized" cable H cable derived from a harmonized cable A cable according to a national standard FRN service voltage between conductors 300 volts maximum 03 500 volts maximum 05 750 volts maximum 07 1,000 volts maximum 1 symbols for insulation materials ethylene propylene rubber (EPR) natural rubber or equivalent (Rubber) polyvinylchloride (PVC) cross-linked polyethylene (XLPE) polychloroprene (neoprene) (PCP) symbol of sheath materials ethylene propylene rubber (EPR) natural rubber or equivalent (Rubber) polyvinylchloride (PVC) cross-linked polyethylene (XLPE) polychloroprene (neoprene) (PCP) special constructions flat divisible cable flat indivisible cable core metals copper (no code) aluminium core symbols solid single core (inflexible) core of twisted strands (inflexible) flexible core, class 5 standard flexible core (fixed installation) highly-flexible core, class 6 composition of the cables number of conductors multiplication sign if no green/yellow conductor is present sign when a green/yellow conductor is present cross-sectional area of conductor

R

N

-

-

F

3

G

1,5

B R V X N B R V X N H H2

A U R F K H X X G X

table F64: designation of conductors and cables according to CENELEC code for harmonized cables. CENELEC has undertaken a project of harmonization of different national standards, with a view to facilitating exchanges between European countries.



F 6.2 conduits, conductors and cables (continued) Example of decoding: H07 RN-F 3G 1,5 : Harmonized cable - Nominal voltage 450/750 V - Rubber insulated - Neoprene (PCP) sheathed - Flexible-3 conductors: 1 green/yellow conductor - All conductors are 1.5 mm2. conductors and cables

cross-linked polyethylene (XLPE) inflexible cables

designation according to the French national standards code U 1000 R12N U 1000 R2V U 1000 RVFV U 1000 RGPFV

inflexible cables with halogen-free insulation (1) flexible elastomericinsulated cables PVC-insulated cables

PVC-insulated conductors conductors with halogen-free insulation

core insulation

fig. F65: typical 3-core unarmoured cable. designation according to CENELEC code

number of conductors

c.s.a.-voltage mm2 V

cable standards not yet harmonized

1 to 5

1.5 - 630

1 to 5 1 to 5 1 to 5 1 to 5 1 to 5 1 to 5 2 to 5 7 to 37 2 to 5 2 to 5 2 to 5 2 1 1 1 1 1 1

1.5 - 300 1.5 - 240 1.5 - 630 1.5 - 630 1.5 - 300 1.5 - 300 1.5 - 500 1.5 - 4 1.5 - 35 1.5 - 35 0.75 - 2.5 0.75 1.5 - 400 1.5 - 400 1.5 - 240 1.5 - xxx 1.5 - xxx 1.5 - xxx

FRN 1X1X2 FRN 1X1G1 FRN 1X1X2Z4X2 FRN 1X1G1Z4G1 H 07 RN-F FRN 07 RN-7 FRN 05VV-U FRN 05VV-R H 05VV-F H 05VVH2-F H 07V-U H 07V-R H 07VK FRN 0...-U FRN 0...-R FRN 0...-

table F66: commonly used conductors and cables. (1) cable of category C1 (non-fire-propagating cable).

c rule 1 The green-and-yellow striped marking is reserved exclusively for protective conductors PE or PEN. c rule 2 Where a circuit includes a neutral conductor, it must be coloured lightblue (or marked by the number 1 for multicore cables of more than 5 conductors). Where a circuit does not include a neutral conductor, the light-blue conductor may be used as a phase conductor if it is included in a cable of more than one conductor. c rule 3 Phase conductors may be identified by any colour, except: v green-and-yellow, v green, v yellow, v light-blue (see rule 2).


identification marking of LV conductors LV wiring and cable conductors are marked either by colouring, or by numbers. These markings, as recommended in IEC 446, are governed by the following three rules: c rule 1 The green-and-yellow striped marking is reserved exclusively for protective conductors PE or PEN. c rule 2 Where a circuit includes a neutral conductor, it must be coloured light-blue (or marked by the number 1 for multicore cables of more than 5 conductors). Where a circuit does not include a neutral conductor, the light-blue conductor may be used as a phase conductor if it is included in a cable of more than one conductor. c rule 3 Phase conductors may be identified by any colour, except: v green-and-yellow, v green, v yellow, v light-blue (see rule 2). Note: if a circuit requires a protective conductor, but the cable which is available for the circuit does not include a green-andyellow striped conductor, the protective conductor may be: c either a separate conductor with green-andyellow striped insulation, or c a light-blue conductor, if the circuit does not include a neutral conductor, or c a black conductor, if the circuit includes a neutral conductor. In the two last cases, the conductor used must be marked by bands or grommets of striped green-and-yellow colours at the extremities of the cable, and along any of its exposed lengths.

non-metallic protective sheath

core

7. external influences

F Every electrical installation occupies an environment which presents a more-or-less severe degree of risk c for persons, c for the materials constituting the installation. Consequently, environmental conditions influence the definition and choice of appropriate installation materials and the choice of protective measures for the safety of persons. The environmental conditions are referred to collectively as "external influences".

7.1 classification external influences shall be taken into account when choosing: c the appropriate measures to ensure the safety of persons (in particular in special locations or electrical installations) c the characteristics of electrical equipment, such as IP degree, mechanical withstand, water ingress.

Many national standards concerned with external influences include a classification scheme which is based on, or which closely resembles, that of the international standard IEC 364-3. This standard (IEC 364-3) devotes many pages to detailed explanations of each class of influence, and for such detail the reader is referred to the standard. Following the IEC codification scheme given below, however, a concise list of external influences, extracted from Appendix A of the IEC document, is presented in table F67.

Codification Each condition of external influence is designated by a code comprising a group of two capital letters and a number as follows: The first letter relates to the general category of external influence. A = environment B =utilization C = construction of buildings The second letter relates to the nature of the external influence. The number relates to the class within each external influence. For example the code AC2 signifies: A = environment AC = environment-altitude AC2 = environment-altitude > 2,000 m Note: the codification given in this chapter is not intended to be used for marking equipment.


7. external influences (continued)

F 7.1 classification (continued)

environnement

A AA AA1 AA2 AA3 AA4 AA5 AA6 AB AC AC1 AC2 AD AD1 AD2 AD3 AD4 AD5 AD6 AD7 AD8 AE AE1

ambient (°C) -60°C +5°C -40°C +5°C -25°C +5°C -5°C +40°C +5°C +40°C +5°C +60°C humidity altitude (m) ≤ 2000 > 2000 water negligible drops sprays splashes jets waves immersion submersion foreign bodies negligible

AE2 AE3 AE4 AF AF1 AF2 AF3 AF4 AG AG1 AG2 AG3 AH AH1 AH2 AH3 AJ

BA BA1 BA2 BA3 BA4 BA5

capability ordinary children handicapped instructed skilled

BB

resistance

BC BC1

contact with earth none

BD3

CA CA1 CA2

materials non-combustible combustible

CB CB1 CB2

AK AK1 AK2

small very small dust corrosion negligible atmospheric intermittent continuous impact low medium high vibration low medium high other mechanical stresses flora no hazard hazard

AL AL1 AL2 AM AM1 AM2 AM3 AM4 AM5 AM6 AN AN1 AN2 AP AP1 AP2 AP3 AP4 AQ AQ1 AQ2 AR

fauna no hazard hazard radiation negligible stray currents electromagnetic ionization electrostatics induction solar negligible significant seismic negligible low medium high lightning negligible indirect wind

BC2 BC3 BC4

low frequent continuous

BD4

(high density/ difficult exit)

BD BD1

evacuation (low density/ easy exit) (low density/ difficult exit) (high density/ easy exit)

BE BE1 BE2 BE3 BE4

materials no risk fire risk explosion risk contamination risk

CB3

structure movement flexible

utilization

B

BD2

building

C structure negligible risk fire propagation

CB4

table F67: concise list of important external influences (taken from Appendix A of IEC 364-3).


F 7.2 protection by enclosures: IP code The degree of protection provided by an enclosure is indicated in the IP code, recommended in IEC 529 (1989). Protection is afforded against the following external influences:

c penetration by solid bodies; c protection of persons against access to live parts; c protection against the ingress of dust; c protection against the ingress of liquids.

IP

2

3

C

H

Code letters (International Protection) First characteristic numeral (numerals 0 to 6, or letter X) Second characteristic numeral (numerals 0 to 8, or letter X) Additional letter (optional) (letters A, B, C, D) Supplementary letter (optional) (letters H, M, S, W) Where a characteristic numeral is not required to be specified, it shall be replaced by the letter "X" ("XX" if both numerals are omitted). Additional letters and/or supplementary letters may be omitted without replacement. fig. F68: IP Code arrangement. Note: the IP code applies to electrical equipment for voltages up to and including 72.5 kV.



F 7.2 protection by enclosures: IP code (continued) Elements of the IP Code and their meanings A brief description of the IP Code elements is given in the following chart. Element Code letters

Numerals or letters IP

Meaning for the protection of equipment

Meaning for the protection of persons

-

-

Against ingress of solid foreign objects

First characteristic numeral

Second characteristic numeral

Additional letter (optional)

Supplementary letter (optional)

0 1 2 3 4 5 6

(non-protected) u 50 mm diameter u 12.5 mm diameter u 2.5 mm diameter u 1.0 mm diameter dust-protected dust-tight

0 1 2 3 4 5 6 7 8

Against ingress of water with harmful effects (non-protected) vertically dripping dripping (15° tilted) spraying splashing jetting powerful jetting temporary immersion continuous immersion

A B C D

H M S W

-

Supplementary information specific to: High-voltage apparatus Motion during water test Stationary during water test Weather conditions

fig. F69: elements of the IP Code.


Against access to hazardous parts with (non-protected) back of hand finger tool wire wire wire

-

Against access to hazardous parts with back of hand finger tool wire

-

F Examples of the use of letters in the IP Code The following examples are to explain the use and arrangement of letters in the IP Code. IP44 - no letters, no options. IPX5 - omitting first characteristic numeral. IP2X - omitting second characteristic numeral. IP20C - using additional letter. IPXXC - omitting both characteristics numerals, using additional letter. IPX1C - omitting first characteristic numeral, using additional letter. IP3XD - omitting second characteristic numeral, using additional letter. IP23S - using supplementary letter. IP21CM - using additional letter and supplementary letter. IPX5/IPX7 - giving two different degrees of protection by an enclosure against both water jets and temporary immersion for "versatile" application. Examples of designations with the IP Code c IP Code not using optional letters: IP 3 4 Code letters 1st characteristic numeral 2nd characteristic numeral An enclosure with this designation (IP Code) (3) - protects persons, handling tools having a diameter of 2.5 mm and greater, against access to hazardous parts - protects the equipment inside the enclosure against ingress of solid foreign objects having a diameter of 2.5 mm and greater. (4) protects the equipment inside the enclosure against harmful effects due to water splashed against the enclosure from any direction. c IP Code using optional letters: IP 2 3 C S Code letters 1st characteristic numeral 2nd characteristic numeral Additional letter Supplementary letter

An enclosure with this designation (IP Code) (2) - protects persons against access to hazardous parts with fingers - protects the equipment inside the enclosure against ingress of solid foreign objects having a diameter of 12.5 mm and greater. (3) protects the equipment inside the enclosure against the harmful effects due to water sprayed against the enclosure. (C) protects persons handling tools having a diameter of 2.5 mm and greater and a length not exceeding 100 mm against access to hazardous parts (the tool may penetrate the enclosure up to its full length). (S) is tested for protection against harmful effects due to the ingress of water when all the parts of the equipment are stationary (e.g. the rotor of a rotating machine). An extensive description of the numerous possible combinations of protective requirements is beyond the scope of this guide. For additional information and full details of application and testing requirements of the IP Code, the reader is referred to IEC Publication 529 (1989). Access to the interior of a protective enclosure In the normal operating state, access doors and removable panels provided for maintenance purposes are closed, but the majority of enclosures are provided with orifices for ventilation. Adjustments through orifices by tools (screw drivers, box spanners, etc.) from the outside are also common, while limited access to some "safe" sections of an enclosure are frequently provided in the form of hand-holes under a removable plate. Such penetrations could, unless interior arrangements are carefully designed to prevent it, lead to accidental contact with live parts. Figure F70 shows IEC test probes intended to prove the adequacy of protection against such dangers, and the corresponding IP Code for each probe.



F 7.2 protection by enclosures: IP code (continued) first numeral

addit. letter

access probe

test force

1

A

sphere 50 mm diameter

50 N ± 10%

approx. 100

4

Ø50

Ø10 rigid test sphere (metal) handle guard (insulating material) 2

B

Ø45 10 N ± 10%

jointed test finger 80 Ø12

jointed test finger (metal) insulating material 3

C

stop face (Ø50x20) 3 N ± 10%

test rod 2.5 mm diameter, 100 mm long approx. 100 Ø10

100 Ø2.5 edges free from burrs

handle (insulating material) 4, 5, 6

D

Ø35 stop face (insulating material)

rigid test rod (metal) 1 N ± 10%

test wire 1.0 mm diameter, 100 mm long approx. 100 Ø10

100 Ø1 edges free from burrs

handle (insulating material)

Ø35 stop face (insulating material)

rigid test wire (metal)

fig. F70: access probes for the tests for protection of persons against access to hazardous parts.

protection against mechanical impact The selection of equipment according to an adequate IP code can ensure safety only if the enclosure is sufficiently robust to sustain anticipated mechanical stresses, notably impact forces, without distortion which will adversely affect its IP classification. Specifications for such equipment should therefore include the appropriate AG code (AG1, AG2 or AG3) according to the severity of possible impact stresses as listed in table F67. Tests for standardized impact severities are being "harmonized" November 1993 internationally, and are based on four levels of impact energy, viz: level 1 2 3 4

energy in joules 0.255 2.0 6.0 20.0

It is recommended that these values be used in specifications, pending quantification of the AG code in IEC 364-3.


protection against corrosion For similar reasons to those mentioned above (i.e. possible reduction in the degree of protection required, due to external influences), the possibility of weakening of enclosures or enlarging of orifices and so on, caused by corrosion must also be given due consideration. The severity of the corrosive environment may be indicated in the equipment specifications by the AF code (AF1, AF2, AF3 or AF4) as listed in table F67, and defined in sub-clause 321 of IEC 364-3.

1. general

G 1.1 electric shock when a current exceeding 30 mA passes through a part of a human body, the person concerned is in serious danger if the current is not interrupted in a very short time. the protection of persons against electric shock in LV installations must be provided in conformity with appropriate national standards and statutory regulations, codes of practice, official guides and circulars, etc. Relevant IEC standards include: IEC 364, IEC 479-1, IEC 755, IEC 1008, IEC 1009 and IEC 947-2 appendix B.

electric shock An electric shock is the pathophysiological effect of an electric current through the human body. Its passage affects essentially the circulatory and respiratory functions and sometimes results in serious burns. The degree of danger for the victim is a function of the magnitude of the current, the parts of the body through which the current passes, and the duration of current flow. IEC Publication 479-1 defines four zones of current-magnitude/time-duration, in each of which the pathophysiological effects are described (fig. G1). Any person coming into contact with live metal risks an electric shock.

Curve C1 (of figure G1) shows that when a current greater than 30 mA passes through a part of a human being, the person concerned is likely to be killed, unless the current is interrupted in a relatively short time. The point 500 ms/100 mA close to the curve C1 corresponds to a probability of heart fibrillation of the order of 0.14%. The protection of persons against electric shock in LV installations must be provided in conformity with appropriate national standards and statutory regulations, codes of practice, official guides and circulars, etc. Relevant IEC standards include: IEC 364, IEC 479-1, IEC 755, IEC 1008, IEC 1009 and IEC 947-2 appendix B.

duration of current flow t

ms 10000

A

B

C2 C3

C1

5000

∂ imperceptible ∑ perceptible ∏ reversible effects: muscular contraction π possibility of irreversible effects C1: no heart fibrillation C2: 5% probability of heart fibrillation C3: 50% probability of heart fibrillation

2000 1000 500 1

3

2

4

200 100 50 20 10 0,1 0,2

0,5

1

2

5

10

20

50

100 200

500 1000 2000 5000 10000 mA

current passing through the body Is

fig. G1: curve C1 (of IEC 479-1) defines the current-magnitude/time-duration limits which must not be exceeded.

1.2 direct and indirect contact standards and regulations distinguish two kinds of dangerous contact: c direct contact, c indirect contact, and corresponding protective measures.

direct contact

indirect contact

A direct contact refers to a person coming into contact with a conductor which is live in normal circumstances.

An indirect contact refers to a person coming into contact with a conductive part which is not normally alive, but has become alive accidentally (due to insulation failure or some other cause). insulation failure 1 2 3

1

2

PE conductor

3 N Id

busbars Is

Is

fig. G2: direct contact.

fig. G3: indirect contact.

Is: touch current

Id: insulation fault current

protection against electric shocks - G1

2. protection against direct contact

G two measures of protection against direct-contact hazards are often imposed, since, in practice, the first measure may not prove to be infallible.

Two complementary measures are commonly employed as protection against the dangers of direct contact: c the physical prevention of contact with live parts by barriers, insulation, inaccessibility, etc. c additional protection in the event that a direct contact occurs, despite the above measures. This protection is based on residual-current operated high-sensitivity fastacting relays, which are highly effective in the majority of direct contact cases.

2.1 measures of protection against direct contact IEC and national standards frequently distinguish between degrees of protection c complete (insulation, enclosures) c partial or particular.

measures of complete protection Protection by the insulation of live parts This protection consists of an insulation which conforms to the relevant standards. Paints, lacquers and varnishes do not provide an adequate protection. Protection by means of barriers or enclosures This measure is in widespread use, since many components and materials are installed in cabinets, pillars, control panels and distribution-board enclosures, etc. To be considered as providing effective protection against direct-contact hazards, these equipments must possess a degree of protection equal to at least IP2X or IPXXB (see Chapter F Sub-clause 7.2). Moreover, an opening in an enclosure (door, panel, drawer, etc.) must only be removable, opened or withdrawn: c by means of a key or tool provided for the purpose, or c after complete isolation of the live parts in the enclosure, or c with the automatic action of an intervening metal shutter, removable only with a key or with tools. The metal enclosure and all metal shutters must be bonded to the protective earthing conductor for the installation.

partial measures of protection Protection by means of obstacles, or by placing out of reach This practice concerns locations to which qualified, or otherwise authorized personnel only, have access.

particular measures of protection Protection by the use of extra-low voltage SELV (Safety Extra Low Voltage) schemes This measure is used only in low-power circuits, and in particular circumstances, as described in Sub-clause G3.5.

G2 - protection against electric shocks

fig. G4: inherent direct-contact protection by the insulation of a 3-phase cable with outer sheath.

fig. G5: example of direct-contact prevention by means of an earthed metal enclosure.

G 2.2 additional measure of protection against direct contact an additional measure of protection against the hazards of direct contact is provided by the use of residualcurrent operated devices, which operate at 30 mA or less, and are referred to as RCDs of high sensitivity.

All the preceding protective measures are preventive, but experience has shown that for various reasons they cannot be regarded as being infallible. Among these reasons may be cited: c lack of proper maintenance, c imprudence, carelessness, c normal (or abnormal) wear and tear of insulation; for example, flexure and abrasion of connecting leads, c accidental contact, c immersion in water, etc. - a situation in which insulation is no longer effective. In order to protect users in such circumstances, highly-sensitive fast-tripping devices, based on the detection of residual currents to earth (which may or may not be through a human being or animal) are used to disconnect the power supply automatically, and with sufficient rapidity to prevent permanent injury to, or death by electrocution, of a normally healthy human being.

fig. G6: high-sensitivity RCD.

IEC wiring regulations impose the use of RCDs on circuits supplying socket outlets, installed in particular locations considered to be potentially dangerous, or used for special purposes. Some national wiring regulations impose their use on all circuits supplying socket outlets.

These devices operate on the principle of differential current measurement, in which any difference between the current entering a circuit and that leaving it, must (on a system supplied from an earthed source) be flowing to earth, either through faulty insulation or through contact of an earthed object, such as a person, with a live conductor. Standard residual-current devices, referred to as RCDs, sufficiently sensitive for directcontact protection are rated at 30 mA of differential current. Other standard IEC ratings for high-sensitivity RCDs are 10 mA and 6 mA (generally used for individual appliance protection). This additional protection is imposed in certain countries for circuits supplying socket outlets of ratings up to 32 A, and even higher if the location is wet and/or temporary (such as work-sites for example). Chapter L section 3 itemizes various common locations in which RCDs of high sensitivity are obligatory (in some countries), but in any case, are highly recommended as an effective protection against both direct- and indirect-contact hazards.


3. protection against indirect contact

G national regulations covering LV installations impose, or strongly recommend, the provision of devices for indirect-contact protection. the measures of protection are: c automatic disconnection of supply (at the first or second fault detection, depending on the system of earthing) c particular measures according to circumstances.

Conductive material(1) used in the manufacture of an electrical appliance, but which is not part of the circuit for the appliance, is separated from live parts by the "basic insulation". Failure of the basic insulation will result in the conductive parts becoming live. Touching a normally-dead part of an electrical appliance which has become live due to the failure of its insulation, is referred to as an indirect contact. Various measures are adopted to protect against this hazard, and include: c automatic disconnection of power supply to the appliance concerned,

c special arrangements such as: v the use of class II insulation materials, or an equivalent degree of insulation, v non-conducting location(2) - out-of-reach or interposition of barriers, v equipotential locality, v electrical separation by means of isolating transformers. (1) Conductive material (usually metal) which may be touched, without dismantling the appliance, is referred to as "exposed conductive parts". (2) The definition of resistances of the walls, floor and ceiling of a non-conducting location is given in Sub-clause G3.5

3.1 measure of protection by automatic disconnection of the supply protection against indirect-contact hazards by automatic disconnection of the supply can be achieved if the exposed conductive parts of appliances are properly earthed.

principle This protective measure depends on two fundamental requirements: c the earthing of all exposed conductive parts of equipment in the installation and the constitution of an equipotential bonding network (see Sub-clause F4-1), c automatic disconnection of the section of the installation concerned, in such a way that the touch-voltage/time safety requirements are respected for any level of touch voltage Uc(3). (3) touch voltage Uc: Touch voltage Uc is the voltage existing (as the result of insulation failure) between an exposed conductive part and any conductive element within reach which is at a different (generally earth) potential. The greater the value of Uc, the greater the rapidity of supply disconnection required to provide protection (see Tables G8 and G9). The highest value of Uc that can be tolerated indefinitely without danger to human beings is called the "conventional touch-voltage limit" (UL).

installation earth electrode

Uc

fig. G7: in this illustration the dangerous touch voltage Uc is from hand to hand.

in practice the disconnecting times and the choice of protection schemes to use depend on the kind of earthing system concerned; TT, TN or IT. Precise indications are given in the corresponding paragraphs.

reminder of the theoretical disconnecting-time limits* assumed touch voltage (V) < 50 50 75 90 120 150 220 280 350 500

maximum disconnecting time for the protective device (seconds) alternating direct current current 5 5 5 5 0.60 5 0.45 5 0.34 5 0.27 1 0.17 0.40 0.12 0.30 0.08 0.20 0.04 0.10

table G8: maximum safe duration of the assumed values of touch voltage in conditions where UL = 50 V(1). (1) The resistance of the floor and the wearing of shoes are taken into account in these values. * For most locations, the maximum permitted touch voltage (UL) is 50 V. For special locations, the limit is reduced to 25 V. See G4-1 and Clause L3.


assumed touch voltage (V) 25 50 75 90 110 150 230 280

maximum disconnecting time for the protective device (seconds) alternating direct current current 5 5 0.48 5 0.30 2 0.25 0.80 0.18 0.50 0.12 0.25 0.05 0.06 0.02 0.02

table G9: maximum safe duration of the assumed values of touch voltage in conditions where UL = 25 V.

G 3.2 automatic disconnection for a TT-earthed installation automatic disconnection for a TT-earthed installation is effected by a RCD having a sensitivity of U 50 V* I∆n i L = RA RA where RA = resistance of the installation earth electrode * 25 V in some particular cases.

principle In this scheme all the exposed and extraneous conductive parts of the installation must be connected to a common earth electrode. The supply system neutral is normally earthed at a point outside the area of influence of the electrode for the installation, but need not be so. The impedance of the earth-fault loop therefore consists mainly of the two earth electrodes (i.e. the source and installation electrodes) in series, so that the magnitude of the earth-fault current is generally too small to operate overcurrent relays or fuses, and the use of a differential-current form of protection is essential. This principle of protection is also valid if one common earth electrode only is used, notably in the case of a consumer-type substation within the installation area, where space limitations may impose the adoption of a TN earthing scheme, but where all other conditions required by the TN system cannot be fulfilled.

Automatic protection for a TT-earthed installation is assured by the use of a RCD of sensitivity: I∆n i UL = 50 V RA RA where RA = the resistance of the earth electrode for the installation. I∆n = rated differential current operating level. For temporary supplies (to work-sites etc.) and agricultural and horticultural establishments, the value of UL in the abovementioned formula must be replaced by 25 V.

example The resistance of the substation neutral earth electrode Rn is 10 ohms. The resistance of the installation earth electrode RA is 20 ohms. The earth-fault current Id = 7.7 A. The touch-voltage Uc = IdRA = 154 V and therefore dangerous, but, I∆n = 50 = 2.5 A so that a standard 300 mA 20 RCD will operate in 30 ms to clear a condition in which 50 V touch voltage, or more, appears on an exposed conductive part. HV/400V 1 2 3 4

substation earth electrode

Rn : 10 Ω

installation earth electrode

Uc

RA : 20 Ω

fig. G10: automatic disconnection for a TT-earthed installation.


3. protection against indirect contact (continued)

G 3.2 automatic disconnection for a TT-earthed installation (continued) the tripping times of RCDs are generally lower than those prescribed in the majority of national standards; this feature facilitates their use and allows the adoption of an effective scheme of discriminative protection.

specified disconnection time RCD is a general term for all devices operating on the residual-current principle. RCCB* (residual current circuit breaker) as defined in IEC 1008 is a specific class of RCD. Type G (general) and type S (selective) have tripping time/current characteristics as shown in table G11. These characteristics allow a certain degree of selective tripping between the several combinations of rating and type, as shown later in Sub-clause 4.3.

x I∆n instantaneous (ms) domestic type S (ms) industrial setting I** (ms)

1 2 5 300 150 40

>5 40

500 200 150 150 150 150 150 150

* Merlin Gerin

table G11: maximum operating times of RCCBs (IEC 1008). ** Note : the use of the term "circuit breaker" does not mean that a RCCB can break short-circuit currents. For such duties RCDs known as RCBOs ("O" for overcurrent) as defined in IEC 1009 must be employed.

3.3 automatic disconnection for a TN-earthed installation the principle of the TN scheme of earthing is to ensure that earth-fault current will be sufficient to operate overcurrent protective devices (direct-acting tripping, overcurrent relays and fuses) so that Uo Uo* Ia i or 0.8 Zs Zc

principle In this scheme all exposed and extraneous conductive parts of the installation are connected directly to the earthed point of the power supply by protective conductors. As noted in Chapter F Sub-clause 4.2, the way in which this direct connection is carried out depends on whether the TN-C, TN-S, or TN-C-S method of implementing the TN principle is used. In figure G12 the method TN-C is shown, in which the neutral conductor acts as both the Protective-Earth and Neutral (PEN) conductor. In all TN arrangements, any insulation fault to earth constitutes a phase-neutral short-circuit. High fault current levels simplify protection requirements but can give rise to touch voltages exceeding 50% of the phase-toneutral voltage at the fault position during the brief disconnection time. In practice, therefore, earth electrodes are normally installed at intervals along the neutral of the supply network, while the consumer is generally required to instal an earth electrode at the service position. On large installations additional earth electrodes dispersed around the premises are often

provided, in order to reduce the touch voltage as much as possible. In high-rise apartment blocks, all extraneous conductive parts are connected to the protective conductor at each level. In order to ensure adequate protection, the earth-fault current Id = Uo or 0.8 Uo u Ia where Zs Zc Uo = nominal phase-neutral voltage. Zs = earth-fault current loop impedance, equal to the sum of the impedances of: the source, the live phase conductors to the fault position, the protective conductors from the fault position back to the source. Zc = the faulty-circuit loop impedance (see "conventional method" Sub-clause 5.2). Note: the path through earth electrodes back to the source will have (generally) much higher impedance values than those listed above, and need not be considered. Id = the fault current. Ia = a current equal to the value required to operate the protective device in the time specified.

example B

A

N

F

3 2 1 PEN

E NS160 35 mm2 D

50 m 35 mm2 C

RnA Uc

fig. G12: automatic disconnection for a TN-earthed installation. In figure G12 the touch voltage 230 Uc = = 115 V 2 and is therefore dangerous. The impedance Zs of the loop = ZAB + ZBC + ZDE + ZEN + ZNA. If ZBC and ZDE are predominant, then: ZS = 2ρ x L = 64.3 milli-ohm, so that S Id = 230/64.3 = 3,576 A (≈ 22 In based on a 160 A circuit breaker). The "instantaneous" magnetic tripping device setting of the circuit breaker is many times G6 - protection against electric shocks

less than this value, so that positive operation in the shortest possible time is assured. Note: some authorities base such calculations on the assumption that a voltage drop of 20% occurs in the part of the impedance loop BANE. This method, which is recommended, is explained in chapter G Sub-clause 5.2 "conventional method" and, in this example, will give a fault current of 230 x 0.8 x 103 = 2,816 A (≈ 18 In) 64.3

G for TN earthing, the maximum allowable disconnection time depends on the nominal voltage of the system.

specified maximum disconnection times The times specified are a function of the nominal voltage phase/earth, which, for all practical purposes on TN systems, is the phase/neutral voltage. Uo (volts) phase/neutral 127 230 400 > 400

disconnection time (seconds) UL=50 V (see note 2) 0.8 0.4 0.2 0.1

table G13: maximum disconnection times specified for TN earthing schemes (IEC 364-4-41). Note 1: a longer time interval than those specified in the table (but in any case less than 5 seconds) is allowed under certain circumstances for distribution circuits, as well as for final circuits supplying a fixed appliance, on condition that a dangerous touch voltage is not thereby caused to appear on another appliance. IEC recommends, and

if the protection is to be provided by a circuit breaker, it is sufficient to verify that the fault current will always exceed the current-setting level of the instantaneous or short-time delay tripping unit (Im): Im < Uo or 0.8* Zs Zc * according to the "conventional" method of calculation (see sub-clause 5.2).

Ia can be determined from the fuse performance curve. In any case, protection cannot be achieved if the loop impedance Zs or Zc exceeds a certain value.

protection by means of a circuit breaker The instantaneous trip unit of a circuit breaker will eliminate a short-circuit to earth fault in less than 0.1 second. In consequence, automatic disconnection within the maximum allowable time will always be assured, since all types of trip unit, magnetic or electronic, instantaneous or slightly retarded, are suitable: Ia = Im. The maximum tolerance authorized by the relevant standard, however, must always be taken into consideration. It is sufficient therefore that the fault current Uo / Zs or 0.8 Uo / Zc determined by calculation (or established on site) be greater than the instantaneous trip-setting current, or than the very short-time tripping threshold level, to be sure of tripping whithin the permitted time limit.

certain national regulations impose, the provision of equipotential bonding of all extraneous and exposed conductive parts that are simultaneously accessible, in any area where socket-outlets are installed, from which portable or mobile equipment might be supplied. The common equipotential busbar is installed in the distribution-board cabinet for the area concerned. Note 2: when the conventional voltage limit is 25 V, the specified disconnection times are: 0.35 s. for 127 V 0.2 s. for 230 V 0.05 s. for 400 V If the circuits concerned are final circuits, then these times can easily be achieved by the use of RCDs. Note 3: the use of RCDs may, as mentioned in note 2, be necessary on TN-earthed systems. Use of RCDs on TN-C-S systems means that the protective conductor and the neutral conductor must (evidently) be separated upstream of the RCD. This separation is commonly made at the service position.

2 1

Im

Uo/Zs

I

fig. G14: disconnection by circuit breaker for a TN-earthed installation.

protection by means of fuses The value of current which assures the correct operation of a fuse can be accertained from a current/time performance graph for the fuse concerned. The fault current Uo/Zs or 0.8 Uo/Zc as determined above, must largely exceed that necessary to ensure positive operation of the fuse. The condition to observe therefore is that: Ia < Uo or 0.8 Uo as indicated in figure G15. Zs Zc Example: The nominal phase-neutral voltage of the network is 230 V and the maximum disconnection time given by the graph in figure G15 is 0.4 seconds. The corresponding value of Ia can be read from the graph. Using the voltage (230 V) and the current Ia, the complete loop impedance or the circuit loop impedance can be calculated from Zs = 230/Ia or Zc = 0.8 x 230/Ia.This impedance value must never be exceeded and should preferably be substantially less to ensure satisfactory fuse operation.

1 : instantaneous trip 2 : short time-delayed trip

t

t

tc

= 0,4 s

Ia Uo/Zs

I

fig. G15: disconnection by fuses for a TN-earthed installation.



G 3.4 automatic disconnection on a second earth fault in an IT-earthed system In this type of system: c the installation is isolated from earth, or the neutral point of its power-supply source is connected to earth through a high impedance,

in an IT scheme it is intended that a first fault to earth will not cause any disconnection.

c all exposed and extraneous conductive parts are earthed via an installation earth electrode.

first fault On the occurrence of a short-circuit fault to earth, referred to as a "first fault", the fault current is very small, such that the rule Id x RA i 50 V (see G3.2) is respected and no dangerous touch voltages can occur. In practice the current Id is feeble, a condition that is neither dangerous to personnel, nor harmful to the installation. However, in this scheme: c a permanent surveillance of the condition of the insulation to earth must be provided, together with an alarm signal (audio and/or flashing lights, etc.) in the event of a first earth fault occurring, c the rapid location and repair of a first fault is imperative if the full benefits of the IT system are to be realized. Continuity of service is the great advantage afforded by the scheme. Id2

HV/400 V

fig. G16: phases to earth insulation monitoring relay (obligatory on IT-earthed installation).

Id1 3 2 1 PE

B

Zct 1500 Ω Id1

RnA = 5 Ω

Id1

Id1

Id2

Id2

Id2 ZF

A Uc Id2

fig. G17: fault-current paths for a first (earth) fault on an IT-earthed installation. Example: For a network formed from 1 km of conductors, the leakage (capacitive) impedance to earth ZF is of the order of 3,500 ohms per phase. In normal (unfaulted) operation, the capacitive current* to earth is therefore Uo = 230 = 66 mA per phase ZF 3,500 During a phase-to-earth fault, as shown in figure G17, the current passing through the electrode resistance RnA is the vector sum of the capacitive currents in the two healthy phases. The voltages of the two healthy phases have (because of the fault) increased to √3 the normal phase voltage, so that the capacitive currents increase by the same amount. These currents are displaced, one from the other by 60°, so that when added vectorially, this amounts to 3 x 66 mA = 198 mA i.e. Id2 in the present example.

the simultaneous existence of two earth faults (if not both on the same phase) is dangerous, and rapid clearance by fuses or automatic circuit breaker tripping depends on the type of earth-bonding scheme, and whether separate earthing electrodes are used or not, in the installation concerned. G8 - protection against electric shocks

The touch voltage Uc is therefore 198 x 5 x 10-3 = 0.99 V, which is evidently harmless. The current through the short-circuit is given by the vector sum of the neutral-resistor current Id1 (= 153 mA) and the capacitive current (Id2). Since the exposed conductive parts of the installation are connected directly to earth, the neutral impedance Zct plays practically no part in the production of touch voltages to earth. * Resistive leakage current to earth through the insulation is assumed to be negligibly small in the example.

second fault situation On the appearance of a second fault, on a different phase, or on a neutral conductor, a rapid disconnection becomes imperative. Fault clearance is carried out differently in each of the following cases: 1st case: concerns an installation in which all exposed conductive parts are bonded to a common PE conductor, as shown in figure G19. In this case no earth electrodes are included in the fault current path, so that a high level of

fault current is assured, and conventional overcurrent protective devices are used, i.e. circuit breakers and fuses. The first fault could occur at the end of a circuit in a remote part of the installation, while the second fault could feasibly be located at the opposite end of the installation. For this reason, it is conventional to double the loop impedance of a circuit, when calculating the anticipated fault setting level for its overcurrent protective device(s).

G 1st case: where all exposed conductive parts are connected to a common PE conductor conventional overcurrent protection schemes (such as those used in TN systems) are applicable, with fault-level calculations and tripping/fuseoperating times suitably adapted.

c where the system includes a neutral conductor in addition to the 3 phase conductors, the lowest short-circuit fault currents will occur if one of the (two) faults is from the neutral conductor to earth (all four conductors are insulated from earth in an IT scheme). In four-wire IT installations, therefore, the phase-to-neutral voltage must be used to calculate short-circuit protective levels i.e. (1) 0.8 Uo* u Ia where 2 Zc Uo = phase/neutral voltage Zc = impedance of the circuit fault-current loop (see G3.3) Ia = current level for trip setting

c if no neutral conductor is provided, then the voltage to use for the fault-current calculation is the phase-to-phase value, i.e. (2) 0.8 e Uo* u Ia 2 Zc Specified tripping/fuse-clearance times Disconnecting times for 3-wire 3-phase IT schemes differ from those adopted for 4-wire 3-phase IT schemes, and are given for both cases in table G18. * based on the "conventional method" noted in the first example of Sub-clause 3.3.

disconnection time (seconds) UL = 50 V (1) 3-phase 3-wires 3-phase 4-wires

Uo/U (volts) Uo = phase-neutral volts U = phase-phase volts 127/220 230/400 400/690 580/1000

0.8 0.4 0.2 0.1

5 0.8 0.4 0.2

table G18: maximum disconnection times specified for an IT-earthed installation (IEC 364-4-41). c in the case of a 3-phase 4-wire scheme, 1.0 second at 127/220 V; 0.5 seconds at 230/400 V and 0.2 seconds at 400/690 V.

(1) When the conventional voltage limit is 25 V, the disconnecting times become: c in the case of a 3-phase 3-wire scheme, 0.4 seconds at 127/220 V; 0.2 seconds at 230/400 V and 0.06 seconds at 400/690 V, Example HV/400 V A

Id

B 3 2 1 PE busbars

J K NS160 160 A 50 m 35 mm2 H

Zct

Rn

F

E

G

50 m 35 mm2 D

C

RA

fig. G19: circuit breaker tripping on second (earth) fault when exposed conductive parts are connected to a common protective conductor. The current levels and protective measures depend on the switchgear and fusegear concerned: c circuit breakers In the case shown in figure G19, the levels of instantaneous and short time-delay overcurrent-trip settings must be decided. The times recommended in table G18 can be readily complied with. Example: from the case shown in figure G19, determine that the short-circuit protection provided by the 160 A circuit breaker is suitable to clear a phase-to-phase shortcircuit occurring at the load ends of the circuits concerned. Reminder: In an IT system, the two circuits involved in a phase-to-phase short circuit are assumed to be of equal length, with the same sized conductors; the PE conductors being the same size as the phase conductors. In such a case, the impedance of the circuit loop when using the "conventional method" (Sub-clause 5.2 of this chapter) will be twice that calculated for one of the circuits in the TN case, shown in Sub-clause 3.3.

So that the resistance of circuit 1 loop FGHJ = 2 RHJ = 2 ρ l mΩ a where: ρ = the resistance in milli-ohm of a copper rod 1 metre long of c.s.a. 1 mm2 l = length of the circuit in metres a = c.s.a. of the conductor in mm2 = 2 x 22.5 x 50 = 64.3 mΩ 35 and the loop resistance B, C, D, E, F, G, H, J will be 2 x 64.3 = 129 mΩ The fault current will therefore be: 0.8 x ex 230 x 103 = 2.470 A 129 c fuses The current Ia for which fuse operation must be assured in a time specified according to table G18 can be found from fuse operating curves, as described in figure G15. The current indicated should be significantly lower than the fault currents calculated for the circuit concerned, c RCCBs In particular cases, RCCBs are necessary. In this case, protection against indirect contact hazards can be achieved by using one RCCB for each circuit.



G 3.4 automatic disconnection on a second earth fault in an IT-earthed system (continued) 2nd case: where exposed conductive parts of appliances are earthed individually or in separate groups, each appliance or each group must (in addition to overcurrent protection) be protected by a RCD.

2nd case: concerns exposed conductive parts which are earthed either individually (each part having its own earth electrode) or in separate groups (one electrode for each group). If all exposed conductive parts are not bonded to a common electrode system, then it is possible for the second earth fault to occur in a different group or in a separatelyearthed individual apparatus. Additional protection to that described above for case 1, is required, and consists of a RCD placed at the circuit breaker controlling each group and each individually-earthed apparatus. The reason for this requirement is that the separate-group electrodes are "bonded" through the earth so that the phase-to-phase short-circuit current will generally be limited when passing through the earth bond, by the

case 2

case 1

HV/LV

electrode contact resistances with the earth, thereby making protection by overcurrent devices unreliable. The more sensitive RCDs are therefore necessary, but the operating current of the RCDs must evidently exceed that which occurs for a first fault. For a second fault occurring within a group having a common earth-electrode system, the overcurrent protection operates, as described above for case 1. Note 1: see also Chapter H1 Sub-clause 7.2, protection of the neutral conductor. Note 2: in 3-phase 4-wire installations protection against overcurrent in the neutral conductor is sometimes more conveniently achieved by using a ring-type current transformer over the single-core neutral conductor, as shown in figure G20 (see also Table H1-65c). HV/LV

RCD

RCD

N

N RCD

PIM

group 1 earth

group earth Rn

RCD

PIM

RA

Rn

RA 1

group 2 earth RA 2

fig. G20: the application of RCDs when exposed conductive parts are earthed individually or by groups, on IT-earthed systems.

3.5 measures of protection against direct or indirect contact without circuit disconnection extra-low voltage is used where the risks are great: swimming pools, wandering-lead hand lamps, and other portable appliances for outdoor use, etc.

the use of SELV (Safety by Extra Low Voltage) Safety by extra low voltage SELV is used in situations where the operation of electrical equipment presents a serious hazard (swimming pools, amusement parks, etc.). This measure depends on supplying power at very low voltage from the secondary windings of isolating transformers especially designed according to national or to international (IEC 742) standards. The impulse withstand level of insulation between the primary and secondary windings is very high, and/or an earthed metal screen is sometimes incorporated between the windings. The secondary voltage never exceeds 50 V rms. Three conditions of exploitation must be respected in order to provide satisfactory protection against indirect contact: c no live conductor at SELV must be connected to earth, c exposed conductive parts of SELV-supplied

the use of PELV (Protection by Extra Low Voltage) This system is for general use where low voltage is required, or preferred for safety reasons, other than in the high-risk locations noted above. The conception is similar to that of the SELV system, but the secondary circuit is earthed at one point. IEC 364-4-41 defines precisely the significance of the reference PELV. Protection against direct-contact hazards is generally necessary, except when the equipment is in the zone of equipotential bonding, and the nominal voltage does not exceed 25 V rms, G10 - protection against electric shocks

equipment must not be connected to earth, to other exposed conductive parts, or to extraneous conductive parts, c all live parts of SELV circuits and of other circuits of higher voltage must be separated by a distance at least equal to that between the primary and secondary windings of a safety isolating transformer. These measures require that: c SELV circuits must use conduits exclusively provided for them, unless cables which are insulated for the highest voltage of the other circuits are used for the SELV circuits, c socket outlets for the SELV system must not have an earth-pin contact. The SELV circuit plugs and sockets must be special, so that inadvertent connection to a different voltage level is not possible. Note: In normal conditions, when the SELV voltage is less than 25 V, there is no need to provide protection against direct-contact hazards. Particular requirements are indicated in Chapter L, Clause 3: "special locations". and the equipment is used in normally dry locations only, and large-area contact with the human body is not expected. In all other cases, 6 V rms is the maximum permitted voltage, where no direct-contact protection is provided. 230 V / 24 V

fig. G21: low-voltage supplies from a safety isolating transformer, as defined in IEC 742.

G FELV system (Functional Extra Low Voltage) Where, for functional reasons, a voltage of 50 V or less is used, but not all of the requirements relating to SELV or PELV are fulfilled, appropriate measures described in IEC 364-4-41 must be taken to ensure protection against both direct and indirect contact hazards, according to the location and use of these circuits.

the separation of electric circuits is suitable for relatively short cable lengths and high levels of insulation resistance. It is preferably used for an individual appliance.

Note: Such conditions may, for example, be encountered when the circuit contains equipment (such as transformers, relays, remote-control switches, contactors) insufficiently insulated with respect to circuits at higher voltages.

the separation of electric circuits The principle of separation of circuits (generally single-phase circuits) for safety purposes is based on the following reasoning. The two conductors from the unearthed single-phase secondary winding of a separation transformer are insulated from earth. If a direct contact is made with one conductor, a very small current only will flow into the person making contact, through the earth and back to the other conductor, via the inherent capacitance of that conductor with respect to earth. Since the conductor capacitance to earth is very small, the current is generally below the level of perception. As the length of circuit cable increases, the direct contact current will progressively increase to a point where a dangerous electric shock will be experienced. Even if a short length of cable precludes any danger from capacitive current, a low value of insulation resistance with respect to earth can result in danger, since the current path is then via the person making contact, through the earth and back to the other conductor through the low conductor-to-earth insulation resistance. For these reasons, relatively short lengths of well-insulated cable are essential in separation schemes. Transformers are specially designed for this duty, with a high degree of insulation between primary and secondary windings, or with equivalent protection, such as an earthed metal screen between the windings. Construction of the transformer is to class II insulation standards. As indicated above, successful exploitation of the principle requires that:

c no conductor or exposed conductive part of the secondary circuit must be connected to earth, c the length of secondary cabling must be limited to avoid large capacitance values*, c a high insulation-resistance value must be maintained for the cabling and appliances. These conditions generally limit the application of this safety measure to an individual appliance. In the case where several appliances are supplied from a separation transformer, it is necessary to observe the following requirements: c the exposed conductive parts of all appliances must be connected together by an insulated protective conductor, but not connected to earth, c the socket outlets must be provided with an earth-pin connection. The earth-pin connection is used in this case only to ensure the interconnection (bonding) of all exposed conductive parts. In the case of a second fault, overcurrent protection must provide automatic disconnection in the same conditions as those required for an IT scheme of power system earthing. * It is recommended in IEC 364-4-41 that the product of the nominal voltage of the circuit in volts and length in metres of the wiring system should not exceed 100 000, and that the length of the wiring system should not exceed 500 m.

separation transformer 230 V / 230 V class II

fig. G22: safety supplies from a separation transformer.

class II appliances symbol These appliances are also referred to as having "double insulation" since in class II appliances a supplementary insulation is added to the basic insulation. No conductive parts of a class II appliance must be connected to a protective conductor: c most portable or semi-fixed appliances, certain lamps, and some types of transformer are designed to have double insulation. It is important to take particular care in the exploitation of class II equipment and to verify regularly and often that the class II standard is maintained (no broken outer envelope, etc.). Electronic devices, radio and television sets have safety levels equivalent to class II, but are not formally class II appliances, c supplementary insulation in an electrical installation (IEC 364-4-41: Sub-clause 413-2). Some national standards such as NF C 15-100 (France) (annex to 413.5

active part basic insulation supplementary insulation

fig. G23: principle of class II insulation level. Chapter 41) describe in more detail the necessary measures to achieve the supplementary insulation during installation work. A simple example is that of drawing a cable into a PVC conduit. Methods are also described for distribution boards. c for distribution boards and similar equipment, IEC 439-1 describes a set of requirements, for what is referred to as "total insulation", equivalent to class II, c some cables are recognized as being equivalent to class II by many national standards. protection against electric shocks - G11


G 3.5 measures of protection against direct or indirect contact without circuit disconnection (continued)

in principle, safety by placing simultaneously-accessible conductive parts out-of-reach, or by interposing obstacles, requires also a non-conducting floor, and so is not an easily applied principle

out-of-reach or interposition of obstacles. By these means, the probability of touching a live exposed conductive part, while at the same time touching an extraneous conductive part at earth potential, is extremely low. In practice, this measure can only be applied in a dry location, and is implemented according to the following conditions: c the floor and the walls of the chamber must be non-conducting, i.e. the resistance to earth at any point must be: > 50 kΩ (installation voltages i 500 V), > 100 kΩ (500 V < installation voltages i 1000 V). Resistance is measured by means of "MEGGER" type instruments (hand-operated generator or battery-operated electronic model) between an electrode placed on the floor or against the wall, and earth (i.e. the nearest protective earth-conductor). The electrode contact area and pressure

must evidently be the same for all tests. Different instrument suppliers provide electrodes specific to their own product, so that care should be taken to ensure that the electrodes used are those supplied with the instrument. There are no universally recognized standards established for these tests at the time of writing. c the placing of equipment and obstacles must be such that simultaneous contact with two exposed conductive parts or with an exposed conductive part and an extraneous conductive part by an individual person is not possible. c no exposed protective conductor must be introduced into the chamber concerned. c entrances to the chamber must be arranged so that persons entering are not at risk, e.g. a person standing on a conducting floor outside the chamber must not be able to reach through the doorway to touch an exposed conductive part, such as a lighting switch mounted in an industrial-type cast-iron conduit box, for example.

insulated obstacles

insulated walls

2.5 m

electrical apparatus



insulated floor >2m

<2m

fig. G24: protection by out-of-reach arrangements and the interposition of non-conducting obstacles.

earth-free equipotential chambers

earth-free equipotential chambers are associated with particular installations (laboratories, etc.) and give rise to a number of practical installation difficulties.

In this scheme, all exposed conductive parts, including the floor (see *Note) are bonded by suitably large conductors, such that no significant difference of potential can exist between any two points. A failure of insulation

between a live conductor and the metal envelope of an appliance will result in the whole "cage" being raised to phase-to-earth voltage, but no fault current will flow. In such conditions, a person entering the chamber would be at risk (since he/she would be stepping on to a live floor). Suitable precautions must be taken to protect personnel from this danger (e.g. nonconducting floor at entrances, etc.). Special protective devices are also necessary to detect insulation failure, in the absence of significant fault current. *Note: extraneous conductive parts entering (or leaving) the equipotential space (such as water pipes, etc.) must be encased in suitable insulating material and excluded from the equipotential network, since such parts are likely to be bonded to protective (earthed) conductors elsewhere in the installation.

M conductive floor

insulating material

fig. G25: equipotential bonding of all exposed conductive parts simultaneously accessible. G12 - protection against electric shocks

4. implementation of the TT system

G 4.1 protective measures the application to living quarters is covered in Chapter L Clause 1.

protection against indirect contact General case Protection against indirect contact is assured by RCDs, the sensitivity I∆n of which complies with the condition: 50 V (1) I∆n i RA (1) 25 V for work-site installations, agricultural establishments, etc.

The choice of sensitivity of the differential device is a function of the resistance RA of the earth electrode for the installation, and is given in table G26.

Case of distribution circuits IEC 364-4-41 and a number of national standards recognize a maximum tripping time of 1 second in installation distribution circuits (as opposed to final circuits). This allows a degree of selective discrimination to be achieved: c at level A: RCD time-delayed, e.g. "S" type, c at level B: RCD instantaneous.

I∆n

3A 1A 500 mA 300 mA 30 mA

maximum resistance of the earth electrode (50 V) (25 V) 16 Ω 8Ω 50 Ω 25 Ω 100 Ω 50 Ω 166 Ω 83 Ω 1666 Ω 833 Ω

table G26: the upper limit of resistance for an installation earthing electrode which must not be exceeded, for given sensitivity levels of RCDs at UL voltage limits of 50 V and 25 V.

A

B

fig. G27: distribution circuits. Case where the exposed conductive parts of an appliance, or group of appliances, are connected to a separate earth electrode Protection against indirect contact by a RCD at the circuit breaker controlling each group or separately-earthed individual appliance. In each case, the sensitivity must be compatible with the resistance of the earth electrode concerned.

RA 1

RA 2 a distant location

fig. G28: separate earth electrode.

high-sensitivity RCDs IEC 364-4-471 strongly recommends the use of a RCD of high sensitivity (i 30 mA) in the following cases: c socket-outlet circuits for rated currents of i 32 A at any location(1), c socket-outlet circuits in wet locations at all current ratings(1), c socket-outlet circuits in temporary installations(1), c circuits supplying laundry rooms and swimming pools(1), c supply circuits to work-sites, caravans, pleasure boats, and travelling fairs(1). This protection may be for individual circuits or for groups of circuits, c strongly recommended for circuits of socket outlets u 20 A (mandatory if they are expected to supply portable equipment for outdoor use), c in some countries, this requirement is mandatory for all socket-outlet circuits rated i 32 A.

fig. G29: circuit supplying socket-outlets.

(1) these cases are treated in delail in Chapter L Clause 3.


4. implementation of the TT system (continued)

G 4.1 protective measures (continued) in areas of high fire risk RCD protection at the circuit breaker controlling all supplies to the area at risk is necessary in some locations, and mandatory in many countries. The sensitivity of the RCD must be i 500 mA. fire-risk area

fig. G30: fire-risk location.

protection when exposed conductive parts are not connected to earth (in the case of an existing installation where the location is dry and provision of an earthing connection is not possible, or in the event that a protective earth wire becomes broken) RCDs of high sensitivity (i 30 mA) will afford both protection against indirect-contact hazards, and the additional protection against the dangers of direct-contact .

fig. G31: unearthed exposed conductive parts (A).

4.2 types of RCD RCDs are commonly incorporated in the following components: c industrial-type moulded-case differential circuit breakers conforming to IEC 947-2 and its appendix B, c domestic-type differential circuit breakers (RCCBs)* conforming to IEC 755, 1008, and 1009 (RCBOs), *see NOTE concerning RCCBs at the end of Sub-clause 3.2.

c differential switches conforming to particular national standards, c relays with separate toroidal (ring-type) current transformers, conforming to IEC 755. RCDs are mandatorily used at the origin of TT-earthed installations, where their ability to discriminate with other RCDs allows selective tripping, thereby ensuring the level of service continuity required.

DIN-rail circuit breaker with RCD module

fig. G32: industrial-type CB with RCD module.

the international standard for industrial differential circuit breakers is IEC 947-2 and its appendix B.


Adaptable differential circuit breakers, including DIN-rail mounted units, are available, to which may be associated an auxiliary module. The ensemble provides a

comprehensive range of protective functions (isolation, short-circuit, overload, and sensitive earth-fault protection).

G the international standards for domestic circuit breakers (RCBOs) is IEC 1009.

The incoming-supply circuit breaker can also have timedelayed characteristics (type S).

"Monobloc" type of earth-fault differential circuit breakers designed for the protection of socket-outlet circuits and final circuit protection.

fig. G33: domestic earth-fault differential circuit breakers. In addition to the adaptable industrial circuit breakers which comply to industrial and domestic standards, there are ranges of

"monobloc" differential circuit breakers intended for domestic and tertiary sector applications.

Differential switches (RCCBs) are used for the protection of distribution or sub-distribution boards.

RCDs with separate toroidal CTs can be used in association with circuit breakers or contactors.

fig. G34: differential switches (RCCBs).

fig. G35: RCDs with separate toroidal current transformers.

differential switches are covered by particular national standards (NF C 61-140 for France). RCDs with separate toroidal current transformers are standardized in IEC 755.

RCCBs, RCBOs and CBRs RCCBs (Residual Current Circuit Breakers) These devices are more-accurately described in the French version of IEC 1008 as "interrupteurs" which is generally translated into English by "load-break switches". "Residual-current load-break switches" would be a more accurate description of a RCCB, which, although assigned a rated making and breaking capacity, is not designed to break short-circuit currents (the unique feature of a circuit breaker) so that the term RCCB can be misleading. As noted in sub-clause 7.3 a SCPD (ShortCircuit Protective Device) must always be series-connected with a RCCB.

Note: Both RCCBs and RCBOs as standardized in IEC 1008 and 1009 respectively, provide complete isolation when opened. These units are designed for domestic and similar installations. CBRs Amendment 1 (1992) of the product standard IEC 947-Part 2: "Circuit Breakers" includes Appendix B, which covers the incorporation of residual-current protection into industrialtype LV circuit breakers. The Appendix is based on the relevant requirements of IEC 755, IEC 1008 and IEC 1009. Circuit breakers so equipped are referred to as CBRs.

RCBOs The "O" stands for "Overcurrent" which refers to the fact that, in addition to sensitive differential earth-fault protection, overcurrent protection is provided. The RCBO has a rated short-circuit breaking capability and is properly referred to as a circuit breaker. IEC 1009 is the international reference standard.

4.3 coordination of differential protective devices Discriminative-tripping coordination is achieved either by time-delay or by subdivision of circuits, which are then protected individually or by groups, or by a combination of both methods. Such discrimination avoids the tripping of any circuit breaker, other than that immediately upstream of a fault position c with equipment currently available, discrimination is possible at three or four different levels of distribution, viz: v at the main general distribution board,

v at local general distribution boards, v at sub-distribution boards, v at socket outlets for individual appliance protection c in general, at distribution boards (and subdistribution boards, if existing) and on individual-appliance protection, devices for automatic disconnection in the event of an indirect-contact hazard occurring are installed together with additional protection against direct-contact hazards.


4. implementation of the TT system (continued)

G 4.3 coordination of differential protective devices (continued) discrimination between RCDs Discrimination is achieved by exploiting the several levels of standardized sensitivity: 30 mA, 100 mA, 300 mA and 1 A and the corresponding tripping times, as shown below in figure G36. time (ms) 10000

1000 500 300 250 200 150 130 100

300 mA selective RCDs (i.e. time-delayed) industrial (settings I and II) domestic S time delayed

II

I

60 40

RCD 30 mA general domestic and industrial setting 0

current (mA)

1000

300

60

500 600

30

150

15

100

10

1 1,5

10

100

500 1000

(A)

fig. G36: discrimination between RCDs.

discrimination at 2 levels Protection: c level A: RCD time-delayed setting 1 (for industrial device) type S (for domestic device) for protection against indirect contacts, c level B: RCD instantaneous, with high sensitivity on circuits supplying socket-outlets or appliances at high risk (washing machines, etc. See also Chapter L Clause 3).

discrimination at 3 or 4 levels Protection: c level A: RCD time-delayed (setting III), c level B: RCD time-delayed (setting II), c level C: RCD time-delayed (setting I) or type S, c level D: RCD instantaneous.

A RCD 300 mA type S B

RCD 30 mA

fig. G37. A

relay with separate toroidal CT 3 A delay time 500 ms

B

RCCB 1 A delay time 250 ms

C

RCCB 300 mA delay time 50 ms or type S D

RCCB 30 mA

fig. G38: discrimination at 3 or 4 levels.


G discriminative protection at three levels main circuit breaker

HV/LV MERLIN GERIN

differential relay with separate toroidal CT setting level ≤ 50/RA time-delay setting level II

Rp

3 2 1 N PE

Rn RA

NS80H-MA

NS400

MERLIN GERIN

differential relay with separate CT discontactor N 1 2 3 PE instantaneous 300 mA T

MCB

Vigi compact NS100 setting level 300 mA

MERLIN GERIN SM20

IN

M earth leakage current monitor

T

300 mA type S timedelayed RCD

OUT

RCD MCB discontactor

distribution box

M

N Ph PE DPN Vigi 30 mA

XC40 diff. 30 mA RCD

T TEST

30

MCB + RCD

mA

remotelycontrolled actuator

fig. G39: typical 3-level installation, showing the protection of distribution circuits in a TT-earthed system. One motor is provided with specific protection.


5. implementation of the TN system

G 5.1 preliminary conditions At the design stage, the maximum permitted lengths of cable downstream of a controlling circuit breaker (or set of fuses) must be calculated, while during the installation work certain rules must be fully respected.

imposed conditions Certain conditions must be observed, as listed below and illustrated in figure G40. 1. earth electrodes should be provided at evenly-spaced points (as far as practical conditions allow) along the PE conductor. Note : This is not normally done for a single domestic installation; one earth electrode only is usually required at the service position. 2. the PE conductor must not pass through ferro-magnetic conduit, ducts, etc. or be mounted on steel work, since inductive and/ or proximity effects can increase the effective impedance of the conductor. 3. in the case of a PEN conductor (a neutral conductor which is also used as a protective conductor), connection must be made directly to the earth terminal of an appliance (see 3 in figure G40) before being looped to the neutral terminal of the appliance. 4. where the conductor i 6 mm2 for copper or 10 mm2 for aluminium, or where a cable is movable, the neutral and protective conductors should be separated (i.e. a TN-S scheme should be adopted within the installation). 5. earth faults should be cleared by overcurrent-protection devices, i.e. by fuses and circuit breakers. The foregoing list indicates the conditions to be respected in the implementation of a TN scheme for the protection against indirect contacts.

5 2

2 5

PEN 1

5 PE N 4

3

TN-C

TN-C-S

RpnA

fig. G40: implementation of the TN system of earthing. note (1) the TN scheme requires that the LV neutral of the HV/LV transformer, the exposed conductive parts of the substation and of the installation, and the extraneous conductive parts in the sub-station and installation, all be earthed to a common earthing system. (2) for a substation in which the metering is at low-voltage, a means of isolation is required at the origin of the LV installation, and the isolation must be clearly visible. (3) a PEN conductor must never be interrupted under any circumstances. Control and protective switchgear for the several TN arrangements will be: c 3-pole when the circuit includes a PEN conductor, c preferably 4-pole (3 phases + neutral) when the circuit includes a neutral with a separate PE conductor.

5.2 protection against indirect contact Three methods of calculation are commonly used: c the method of impedances, based on the trigonometric addition of the system resistances and inductive reactances. c the method of composition. c the conventional method, based on an assumed voltage drop and the use of prepared tables.


methods of determining levels of short-circuit current In TN-earthed systems, a short-circuit to earth will, in principle, always provide sufficient current to operate an overcurrent device. The source and supply mains impedances are much lower that those of the installation circuits, so that any restriction in the magnitude of earth-fault currents will be mainly caused by the installation conductors (long flexible leads to appliances greatly increase the "fault-loop" impedance, with a corresponding reduction of short-circuit current). The most recent IEC recommendations for indirect-contact protection on TN earthing schemes only relates maximum allowable tripping times to the nominal system voltage (see table G13 in Sub-clause 3.3). The reasoning behind these recommendations is that, for TN systems, the current which must pass in order to raise the potential of an exposed conductive part to 50 V or more is so high that one of two possibilities will occur: c either the fault path will blow itself clear, practically instantaneously, or c the conductor will weld itself into a solid fault and provide adequate current to operate overcurrent devices. To ensure correct operation of overcurrent devices in the latter case, a reasonably accurate assessment of short-circuit earth-

fault current levels must be determined at the design stage of a project. A rigorous analysis requires the use of phasesequence-component techniques applied to every circuit in turn. The principle is straightforward, but the amount of computation is not considered justifiable, especially since the zero-phase-sequence impedances are extremely difficult to determine with any resonable degree of accuracy in an average LV installation. Other simpler methods of adequate accuracy are preferred. Three practical methods are: c the "method of impedances", based on the summation of all the impedances (positivephase-sequence only) around the fault loop, for each circuit, c the "method of composition", which is an estimation of short-circuit current at the remote end of a loop, when the short-circuit current level at the near end of the loop is known, c the "conventional method" of calculating the minimum levels of earth-fault currents, together with the use of tables of values for obtaining rapid results. These methods are only reliable for the case in which the cables that make up the earthfault-current loop are in close proximity (to each other) and not separated by ferromagnetic materials.

G for calculations, modern practice is to use software agreed by National Authorities, and based on the method of impedances, such as ECODIAL 2 (Merlin Gerin). National Authorities generally also publish Guides, which include typical values, conductor lengths, etc.

method of impedances This method summates the positivesequence impedances of each item (cable, PE conductor, transformer, etc.) included in the earth-fault loop circuit from which the short-circuit earth-fault current is calculated, using the formula: 2

2

I =U/ (∑R) +(∑X) where (∑R)2 = (the sum of all resistances in the loop)2

and (∑X)2 = (the sum of all inductive reactances in the loop)2 and U = nominal system phase-to-neutral voltage The application of the method is not always easy, because it supposes a knowledge of all parameter values and characteristics of the elements in the loop. In many cases, a national guide can supply typical values for estimation purposes.

method of composition This method permits the determination of the short-circuit current at the end of a loop from the known value of S.C. at the sending end, by means of the approximate formula: U Isc I= where U + Zsc Isc Isc = upstream short-circuit current I = end-of-loop short-circuit current U = nominal system phase voltage Zsc = impedance of loop

Note: in this method the individual impedances are added arithmetically* as opposed to the previous "method of impedances" procedure. * This results in a calculated current value which is less than that which would actually flow. If the overcurrent settings are based on this calculated value, then operation of the relay, or fuse, is assured.

conventional method

the maximum length of any circuit of a TN-earthed installation is: 0.8 Uo Sph Lmax = ρ (1+m) Ia

This method is generally considered to be sufficiently accurate to fix the upper limit of cable lengths. Principle: The principle bases the short-circuit current calculation on the assumption that the voltage at the origin of the circuit concerned (i.e. at the point at which the circuit protective device is located) remains at 80% or more of the nominal phase to neutral voltage. The 80% value is used, together with the circuit loop impedance, to compute the short-circuit current. This coefficient takes account of all voltage drops upstream of the point considered. In LV cables, when all conductors of a 3-phase 4-wire circuit are in close proximity (which is the normal case), the inductive reactance internal to* and between conductors is negligibly small compared to the cable resistance.

This approximation is considered to be valid for cable sizes up to 120 mm2. Above that size, the resistance value R is increased as follows: core size (mm2) value of resistance S = 150 mm2 R+15% S = 185 mm2 R+20% S = 240 mm2 R+25%

Example:

The maximum length of a circuit in a TNearthed installation is given by the formula: 0.8 Uo Sph Lmax = metres, where: ρ (1+m) Ia Lmax = maximum length in metres Uo = phase volts = 230 V for a 230/400 V system ρ = resistivity at normal working temperature in ohm-mm2/metre = 22.5 10-3 for copper = 36 10-3 for aluminium Ia = trip current setting for the instantaneous operation of a circuit breaker, or Ia = the current which assures operation of the protective fuse concerned, in the specified time. m = Sph / SPE Sph = cross-sectional area of the phase conductors of the circuit concerned in mm2 SPE = cross-sectional area of the protective conductor concerned in mm2

B A

PE

Id L

SPE

Sph C

fig. G41: calculation of L max. for a TN-earthed system, using the conventional method.

* causes proximity and skin effects, i.e. an apparent increase in resistance.


5. implementation of the TN system (continued)

G 5.2 protection against indirect contact (continued) tables

the following tables* give the length of circuit which must not be exceeded, in order that persons be protected against indirect contact hazards by protective devices.

The following tables, applicable to TN systems, have been established according to the "conventional method" described above. The tables give maximum circuit lengths, beyond which the ohmic resistance of the conductors will limit the magnitude of the short-circuit current to a level below that required to trip the circuit breaker (or to blow the fuse) protecting the circuit, with sufficient rapidity to ensure safety against indirect contact.

* Based on tables given in the guide UTE C15-105.

The tables take into account: c the type of protection: circuit breakers or fuses, c operating-current settings, c cross-sectional area of phase conductors and protective conductors, c type of earthing scheme (see fig. G47), c type of circuit breaker (i.e. B, C or D). The tables may be used for 230/400 V systems. Equivalent tables for protection by Compact and Multi 9 circuit breakers (Merlin Gerin) are included in the relevant catalogues.

Correction factor m Table G42 indicates the correction factor to apply to the values given in tables G43 to G46 according to the ratio SPH/SPE, the type of circuit, and the conductor materials. circuit 3P + N or P + N

conductor material copper aluminium

m = SPH/SPE (or PEN) m=1 m=2 1 0.67 0.62 0.42

m=3 0.50 0.31

m=4 0.40 0.25

table G42: correction factor to apply to the lengths given in tables G43 to G46 for TN systems. Circuits protected by general-purpose circuit-breakers nominal crosssectional area of conductors mm2 1.5 2.5 4 6 10 16 25 35 50 70 95 120 150 185 240

instantaneous or short-time-delayed tripping current Im (amperes) 50 103 171 274 410

63 81 136 217 326

80 64 107 171 256 427

100 51 85 137 205 342

125 41 66 109 164 273 436

160 32 53 85 126 214 342

200 25 42 68 102 171 274 428

250 20 34 54 82 137 219 342 479

320 16 26 43 64 107 171 267 374

400 13 21 34 51 85 137 213 299 406

500 10 17 27 41 68 109 171 239 325 479

560 9 15 24 36 61 97 152 214 290 427

630 8 13 21 32 54 87 135 190 258 380

700 7 12 19 29 49 78 122 171 232 342 464

800 6 10 17 25 42 68 107 150 203 299 406

875 6 10 16 23 39 62 98 136 185 274 371 469

1000 1120 1250 1600 2000 2500 3200 4000 5000 6300 8000 10000 12500 5 8 8 7 5 14 12 11 8 7 5 20 18 16 13 10 8 6 5 34 30 27 21 17 14 10 8 7 5 55 49 44 34 27 21 17 13 11 8 7 5 85 76 66 53 43 34 27 21 17 13 10 8 7 120 107 96 75 80 48 37 30 24 19 15 12 9 162 145 130 101 81 65 50 40 32 26 20 16 12 239 214 191 150 120 96 75 60 48 38 30 24 19 325 290 260 203 162 130 101 81 65 51 40 32 26 410 366 328 256 205 165 128 102 82 65 51 41 33 446 398 357 279 223 178 139 111 89 71 56 44 36 471 422 329 264 211 165 132 105 84 66 53 42 410 328 263 205 164 131 104 82 66 52

table G43: maximum circuit lengths for different sizes of conductor and instantaneous-tripping-current settings for general-purpose circuit breakers. Circuits protected by Compact* or Multi 9* circuit breakers for industrial or domestic use SPH mm2 1.5 2.5 4 6 10 16 25 35 50

rated current (A) 1 2 3 4 1227 613 409 307 681 511 1090 818

6 204 341 545 818

8 10 153 123 256 204 409 327 613 491 1022 818

13 16 94 77 157 128 252 204 377 307 629 511 1006 818

20 25 61 49 102 82 164 131 245 196 409 327 654 523 1022 818

32 38 64 102 153 256 409 639 894

40 31 51 82 123 204 327 511 716

45 27 45 73 109 182 291 454 636

50 25 41 65 98 164 262 409 572 777

63 19 32 52 78 130 208 325 454 617

80 15 28 41 61 102 164 258 358 485

100 12 20 33 49 82 131 204 288 388

table G44: maximum circuit lengths for different sizes of conductor and rated currents for type B (1) circuit breakers. * Merlin Gerin products. (1) For the definition of type B circuit breaker refer to chapter H2 Sub-clause 4.2.


G SPH mm2 1.5 2.5 4 6 10 16 25 35 50

rated current (A) 1 2 3 4 6 613 307 204 153 102 1022 511 341 256 170 818 545 409 273 818 613 409 1022 681

8 77 128 204 307 511 818

10 13 61 47 102 79 164 126 245 189 409 315 654 503 1022 786

16 38 64 102 153 256 409 639 894

20 31 51 82 123 204 327 511 716

25 25 41 65 98 164 262 409 572 777

32 19 32 51 77 128 204 319 447 607

40 15 26 41 61 102 164 256 358 485

45 14 23 36 55 91 145 227 318 431

50 12 20 33 49 82 131 204 286 389

63 10 16 26 39 65 104 162 227 309

80 8 13 20 31 51 82 128 179 243

100 6 10 16 25 41 65 102 143 194

table G45: maximum circuit lengths for different conductor sizes and for rated currents of circuit breakers of type C (1). (1) For the definition of type C circuit breakers refer to chapter H2 Sub-clause 4.2.

SPH rated current (A) mm2 1

1.6 2

2.5 3

4

6

6.3 8

10

12.5 13

16

20

25

32

40

45

50

63

80

70

53

44

35

34

27

22

18

14

11

10

9

7

5

4

730 456 365 292 243 183 122 116 88

73

58

56

46

37

29

23

18

16

15

12

9

7

93

90

73

58

47

37

29

26

23

19

14

12

140 135 110 88

70

55

44

39

35

28

21

18

234 225 183 146 117 91

73

65

58

46

35

29

374 359 292 234 187 146 117 104 93

74

58

47

584 562 456 365 292 228 183 162 146 116 88

73

1.5

438 274 219 175 146 110 73

2.5 4

730 584 467 389 292 195 186 141 117

6

876 701 584 438 292 279 211 175

10

974 730 487 465 352 292

16

779 743 564 467

25

881 730

100

35

1022 818 786 639 511 409 319 258 227 204 162 123 102

50

867 692 558 432 347 308 277 220 174 139

table G46: maximum circuit lengths for different conductor sizes and for rated currents of circuit breakers of type D or MA Merlin Gerin (1). (1) For the definition of type D circuit breakers refer to chapter H2 Sub-clause 4.2. For typical use of an MA circuit breaker, refer to Chapter J figure J5-3.

Example: A 3-phase 4-wire (230/400 V) installation is TN-C earthed. A circuit is protected by a circuit breaker rated at 63 A, and consists of an aluminium cored cable with 50 mm2 phase conductors and a neutral conductor (PEN) of 25 mm2. What is the maximum length of circuit, below which protection of persons against indirectcontact hazards is assured by the instantaneous magnetic tripping relay of the circuit breaker? Table G44 gives 617 metres, to which must be applied a factor of 0.42 (table G42 for m = SPH/SPE = 2). The maximum length of circuit is therefore: 617 x 0.42 = 259 metres.

particular case where one or more exposed conductive part(s) is (are) earthed to a separate earth electrode Protection must be provided against indirect contact by a RCD at the origin of any circuit supplying an appliance or group of appliances, the exposed conductive parts of which are connected to an independent earth electrode. The sensitivity of the RCD must be adapted to the earth electrode resistance (RA2 in figure G47). Downstream of the RCD, the earthing scheme must be TN-S.

RA 1

RA 2 a distant location

fig. G47: separate earth electrode.


5. implementation of the TN system (continued)

G 5.3 high-sensitivity RCDs IEC 364-4-471 strongly recommends the use of a RCD of high sensitivity (i 30 mA) in the following cases: c socket-outlet circuits for rated currents of i 32 A at any location(1), c socket-outlet circuits in wet locations at all current ratings(1), c socket-outlet circuits in temporary installations(1), c circuits supplying laundry rooms and swimming pools(1), c supply circuits to work-sites, caravans, pleasure boats, and travelling fairs(1). This protection may be for individual circuits or for groups of circuits, c strongly recommended for circuits of socket outlets u 20 A (mandatory if they are expected to supply portable equipment for outdoor use), c in some countries, this requirement is mandatory for all socket-outlet circuits rated i 32 A.



5.4 protection in high fire-risk locations In locations where the risk of fire is high, the TN-C scheme of earthing is often prohibited, and the TN-S arrangement must be adopted. Protection by a RCD of sensitivity 500 mA at the origin of the circuit supplying the fire-risk location is mandatory in some countries. fire-risk area



G 5.5 when the fault-current-loop impedance is particularly high When the earth-fault current is restricted due to an inevitably high fault-loop impedance, so that the overcurrent protection cannot be relied upon to trip the circuit within the prescribed time, the following possibilities should be considered: Suggestion 1: install a circuit breaker which has an instantaneous magnetic tripping element with an operation level which is lower than the usual setting, for example: 2In i Irm i 4In This affords protection for persons on circuits which are abnormally long. It must be checked, however, that high transient currents such as the starting currents of motors will not cause nuisance trip-outs. Suggestion 2: install a RCD on the circuit. The device need not be highly-sensitive (HS) (several amps to a few tens of amps). Where socket-outlets are involved, the particular circuits must, in any case, be protected by HS (i 30 mA) RCDs; generally one RCD for a number of socket outlets on a common circuit.

PE or PEN 2 i Irm i 4In unusually long cable

fig. G50: a circuit breaker with low-set instantaneous magnetic trip.

phases neutral PE

TN-S

phases PEN

TN-C

fig. G51: RCD protection on TN systems with high earth-fault-loop impedance. Suggestion 3: increase the size of the PE or PEN conductors and/or the phase conductors, to reduce the loop impedance. Suggestion 4: add supplementary equipotential conductors. This will have a similar effect to that of suggestion 3, i.e. a reduction in the earthfault-loop resistance, while at the same time improving the existing touch-voltage protection measures. The effectiveness of this improvement may be checked by a resistance test between each exposed conductive part and the local main protective conductor. For TN-C installations, bonding as shown in figure G52 is not allowed, and Suggestion 3 should be adopted.

fig. G52: improved equipotential bonding.


6. implementation of the IT system

G The basic feature of the IT scheme of earthing is that, in the event of a short-circuit to earth fault, the system can continue to function without interruption. Such a fault is referred to as a "first fault". In this scheme, all exposed conductive parts of an installation are connected via PE conductors to an earth electrode at the installation, while the neutral point of the supply transformer is isolated from earth or connected to earth through a high resistance (commonly 1,000 ohms or more). This means that the current through an earth fault will be measured in milli-amps, which will not cause serious damage at the fault position, or give rise to dangerous touch voltages, or present a fire hazard. The system may therefore be allowed to function normally until it is convenient to isolate the faulty section for repair work. In practice, the scheme requires certain specific measures for its satisfactory exploitation:

c permanent monitoring of the insulation with respect to earth, which must signal (audibly or visually) the occurrence of the first fault, c a device for limiting the voltage which the neutral point of the supply transformer can attain with respect to earth, c a "first-fault" location routine by an efficient maintenance staff. Fault location is greatly facilitated by automatic devices which are currently available, c automatic high-speed tripping of appropriate circuit breakers must take place in the event of a "second fault" occurring before the first fault is repaired. The second fault (by definition) is an earth fault affecting a different phase than that of the first fault or a neutral conductor*. The second fault results in a short-circuit through the earth and/or through PE bonding conductors. * on systems where the neutral is distributed, as shown in figure G58.

6.1 preliminary conditions Preliminary conditions are summarized in table G53 and fig. G54. minimum functions required protection against overvoltages at system frequency neutral earthing resistor (for impedance earthing variation) overall earth-fault monitor with alarm for first fault condition automatic fault clearance on second fault and protection of the neutral conductor against overcurrent location of first fault

components and devices (1) voltage limiter

examples (MG) Cardew C

(2) resistor

impedance Zx

(3) permanent insulation monitor PIM with alarm feature (4) four-pole circuit breakers (if the neutral is distributed) all 4 poles + trip

Vigilohm TR22A or XM 200 Compact circuit breaker or RCD-MS

(5) with device for fault-location on live system, or by successive opening of circuits

Vigilohm system

table G53: essential functions in IT schemes. HV/LV

4

L1 L2 L3 N 4

2

1

4

3 5

fig. G54: 3-phase 3-wire IT-earthed system.


G 6.2 protection against indirect contact first-fault condition

modern monitoring systems greatly facilitate first-fault location and repair.

Low-frequency instruments can be used on a.c. systems which generate transient d.c. components under fault conditions. Certain versions can distinguish between resistive and capacitive components of the leakage current. Modern developments permit the measurement of leakage-current evolution, so that prevention of a first fault can be achieved. Examples of equipment and devices** c manual fault-location (fig. G55). The generator may be fixed (example: XM200) or portable (example: XGR permitting the checking of dead circuits) and the receiver, together with the magnetic tongtype pick-up sensor, are portable.

The earth-fault current which flows under a first-fault condition is measured in milli-amps. The touch voltage with respect to earth is the product of this current and the resistance of the installation earth electrode and PE conductor (from the faulted component to the electrode). This value of voltage is clearly harmless and could amount to several volts only in the worst case (1,000 Ω earthing resistor will pass 230 mA* and a poor installation earth-electrode of 50 ohms, would give 12.5 V, for example). An alarm is given by the permanent earthfault monitoring. Principle of earth-fault monitoring A generator of very low frequency a.c. current, or of d.c. current, (to reduce the effects of cable capacitance to negligible levels) applies a voltage between the neutral point of the supply transformer and earth. This voltage causes a small current to flow according to the insulation resistance to earth of the whole installation, plus that of any connected appliance.

* On a 230/400 V 3-phase system. ** The equipment and devices described to illustrate the principles of fault location, are manufactured by M.G.

MERLIN GERIN XM100

XM200 P12

XGR

P50 P100

FF

ON/O

XRM

fig. G55: non-automatic (manual) fault location. c fixed automatic fault location (fig. G56) The monitoring relay XM200, together with the fixed detectors XD301 (each supplied from a toroidal CT embracing the conductors of the circuit concerned) provide a system of automatic fault location on a live installation. Moreover, the level of insulation is indicated for each monitored circuit, and two levels are checked: the first level warns of unusually low insulation resistance so that preventive measures may be taken, while the second level indicates a fault condition and gives an alarm.

MERLIN GERIN XM100

toroidal CTs

XM200

1 to 12 circuits

XD301 XD301

XD301

XD312

fig. G56: fixed automatic fault location.


6. implementation of the IT system (continued)

G 6.2 protection against indirect contact (continued) c automatic monitoring, logging, and fault location. The Vigilohm System also allows access to a printer and/or a PC which provides a global review of the insulation level of an entire installation, and records the chronological evolution of the insulation level of each circuit. The central monitor XM300C, together with the localization detectors XL308 and XL316, associated with toroidal CTs from several circuits, as shown below in figure G57, provide the means for this automatic exploitation.

MERLIN GERIN XM100

XM300 C MERLIN GERIN

MERLIN GERIN

XL08

XL16

897

678 XL308

XL316

fig. G57: automatic fault location and insulation-resistance data logging. Implementation of permanent insulationmonitoring (PIM) devices c connection The PIM device is normally connected between the neutral (or articificial neutral) point of the power-supply transformer and its earth electrode, c supply Power supply to the PIM device should be taken from a highly reliable source. In practice, this is generally directly from the installation being monitored, through overcurrent protective devices of suitable short-circuit current rating, c impedance of the PIM device In order to maintain the level of earth-fault within safe limits, the current passing through a PIM device during a short-circuit to earth is normally limited to a value < 30 mA. Where the neutral point is earthed through an impedance, the total current passing through the PIM device and the impedance (in parallel with it) must be < 500 mA. This means that a touch voltage of less than 50 V will occur in the installation as long as the installation earth-electrode resistance is less than 100 ohms, and that fire risk of electrical origin is avoided, c level settings Certain national standards recommend a first setting at 20% below the insulation level of the new installation. This value allows the detection of a reduction of the insulation quality, necessitating preventive maintenance measures in a situation of incipient failure. The detection level for earth-fault alarm will be set at a much lower level. G26 - protection against electric shocks

By way of an example, the two levels might be: v new installation insulation level: 100 kΩ v leakage current without danger: 500 mA (fire risk at > 500 mA) v indication levels set by the consumer: - threshold for preventive maintenance: 0.8 x 100 = 80 kΩ - threshold for short-circuit alarm: 300 mA. Notes: v following a long period of shutdown, during which the whole, or part of, the installation remains de-energized, humidity can reduce the general level of insulation resistance. This situation, which is mainly due to leakage current over the damp surface of healthy insulation, does not constitute a fault condition, and will improve rapidly as the normal temperature rise of current-carrying conductors reduces the surface humidity. v the PIM device (XM) can measure the resistive and the capacitive current components of the leakage current to earth, separately, thereby deriving the true insulation resistance from the total permanent current leakage.

G the case of a second fault A second earth fault on an IT system (unless occurring on the same conductor as the first fault) constitutes a phase-phase or phase-toneutral fault, and whether occurring on the same circuit as the first fault, or on a different circuit, overcurrent protective devices (fuses or circuit breakers) would normally operate to effect an automatic fault clearance. The settings of overcurrent tripping relays and the ratings of fuses are the basic parameters that decide the maximum practical length of circuit that can be satisfactorily protected, as discussed in Sub-clause 5.2. Note: in normal circumstances, the fault current path is through common PE conductors, bonding all exposed conductive

three methods of calculating shortcircuit current levels are commonly employed: c method of impedances, which takes account of complex representation of impedances, c method of composition, is a conservatively approximate method, which combines impedances arithmetically, c conventional method, is a simplified method based on an assumed minimum voltage during fault, and the use of tables.

A reasonably accurate assessment of shortcircuit current levels must be carried out at the design stage of a project. A rigorous analysis is not necessary, since current magnitudes only are important for the protective devices concerned (i.e. phase angles need not be determined) so that simplified conservatively approximate methods are normally used. Three practical methods are: c the method of impedances, based on the vectorial summation of all the (positivephase-sequence) impedances around a faultcurrent loop, c the method of composition, which is an approximate estimation of short-circuit current

the software Ecodial 2 (Merlin Gerin) is based on the "method of impedances".

Method of impedances This method as described in Sub-clause 5.2, is identical for both the IT and TN systems of earthing.

parts of an installation, and so the fault loop impedance is sufficiently low to ensure an adequate level of fault current. Where circuit lengths are unavoidably long, and especially if the appliances of a circuit are earthed separately (so that the fault current passes through two earth electrodes), reliable tripping on overcurrent may not be possible. In this case, an RCD is recommended on each circuit of the installation. Where an IT system is resistance earthed, however, care must be taken to ensure that the RCD is not too sensitive, or a first fault may cause an unwanted trip-out. Tripping of residual current devices which satisfy IEC standards may occur at values of 0.5 I∆n to I∆n, where I∆n is the nominal residual-current setting level. at the remote end of a loop, when the level of short-circuit current at the near end of the loop is known. Complex impedances are combined arithmetically in this method, c the conventional method, in which the minimum value of voltage at the origin of a faulty circuit is assumed to be 80% of the nominal circuit voltage, and tables are used based on this assumption, to give direct readings of circuit lengths. These methods are reliable only for the cases in which wiring and cables which make up the fault-current loop are in close proximity (to each other) and are not separated by ferromagnetic materials.

Method of composition This method as described in Sub-clause 5.2, is identical for both the IT and TN systems of earthing.

the maximum length of an IT earthed circuit is: c for a 3-phase 3-wire scheme 0.8 Uo ex Sph L max = 2 ρ Ia (1+m) c for a 3-phase 4-wire scheme 0.8 Uo S1 L max = 2 ρ Ia (1+m)

Conventional method The principle is the same for an IT system as that described in Sub-clause 5.2 for a TN system, viz: the calculation of maximum circuit lengths which should not be exceeded downstream of a circuit breaker or fuses, to ensure protection by overcurrent devices. It is clearly impossible to check circuit lengths for every feasible combination of two concurrent faults. All cases are covered, however, if the overcurrent trip setting is based on the assumption that a first fault occurs at the remote end of the circuit concerned, while the second fault occurs at the remote end of an identical circuit, as already mentioned in Subclause 3.4. This may result, in general, in one trip-out only occurring (on the circuit with the lower trip-setting level), thereby leaving the system in a first-fault situation, but with one faulty circuit switched out of service. c for the case of a 3-phase 3-wire installation the second fault can only cause a phase/ phase short-circuit, so that the voltage to use in the formula for maximum circuit length is eUo. The maximum circuit length is given by: 0.8 e Uo Sph Lm = metres 2 ρ (1+m) Ia

For the case of a 3-phase 4-wire installation the lowest value of fault current will occur if one of the faults is on a neutral conductor. In this case, Uo is the value to use for computing the maximum cable length, and, 0.8 Uo S1 Lm = metres 2 ρ (1+m) Ia (i.e. 50% only of the length permitted for a TN scheme). Reminder: there is no length limit for earth-fault protection on a TT scheme, since protection is provided by RCDs of high sensitivity. In the preceding formulae: Lmax = longest circuit in metres Uo = phase-to-neutral voltage (230 V on a 230/400 V system) ρ = resistivity at normal operating temperature = 22.5 x 10-3 ohms-mm2/m for copper = 36 x 10-3 ohms-mm2/m for aluminium Ia = overcurrent trip-setting level in amps or Ia = current in amps required to clear the fuse in the specified time m = Sph/SPE SPE = cross-sectional area of PE conductor in mm2 S1 = S neutral if the circuit includes a neutral conductor.



G 6.2 protection against indirect contact (continued) N

N

D

B

C

PE

A

Id

PE

Id Id

Id

fig. G58: calculation of Lmax. for an IT-earthed system, showing fault-current path for a double-fault condition.

the following tables* give the length of circuit which must not be exceeded, in order that persons be protected against indirect contact hazards by protective devices. * The tables are those shown in Sub-clause 5.2 (tables G43 to G46). However, the table of correction factors (table G59) which takes into account the ratio Sph/SPE, and of the type of circuit (3-ph 3-wire; 3-ph 4-wire; 1-ph 2-wire) as well as conductor material, is specific to the IT system, and differs from that for TN.

Tables The following tables have been established according to the "conventional method" described above. The tables give maximum circuit lengths, beyond which the ohmic resistance of the conductors will limit the magnitude of the short-circuit current to a level below that required to trip the circuit breaker (or to blow the fuse) protecting the circuit, with sufficient rapidity to ensure safety against indirect contact. The tables take into account: c the type of protection: circuit breakers or fuses, c operating-current settings, c cross-sectional area of phase conductors and protective conductors, c type of earthing scheme, c correction factor: table G59 indicates the correction factor to apply to the lengths given in tables G43 to G46, when considering an IT system. circuit

conductor material

3 phases

copper aluminium 3ph + N or 1ph + N copper aluminium

m = S ph/SPE (or PEN) m=1 m=2 0.86 0.57 0.54 0.36 0.50 0.33 0.31 0.21

m=3 0.43 0.27 0.25 0.16

m=4 0.34 0.21 0.20 0.12

table G59: correction factors, for IT-earthed systems, to apply to the circuit lengths given in tables G43 to G46. Example A 3-phase 3-wire 230/400 V installation is IT-earthed. One of its circuits is protected by a circuit breaker rated at 63 A, and consists of an aluminium-cored cable with 50 mm2 phase conductors. The 25 mm2 PE conductor is also aluminum. What is the maximum length of circuit, below which protection of persons against indirect-contact hazards is assured by the instantaneous magnetic tripping relay of the circuit breaker? Table G44 indicates 617 metres, to which must be applied a correction factor of 0.36 (m = 2 for aluminium cable). The maximum length is therefore 222 metres.


G 6.3 high-sensitivity RCDs IEC 364-4-471 strongly recommends the use of a RCD of high sensitivity (i 30 mA) in the following cases: c socket-outlet circuits for rated currents of i 32 A at any location(1), c socket-outlet circuits in wet locations at all current ratings(1), c socket-outlet circuits in temporary installations(1), c circuits supplying laundry rooms and swimming pools(1), c supply circuits to work-sites, caravans, pleasure boats, and travelling fairs(1). This protection may be for individual circuits or for groups of circuits, c strongly recommended for circuits of socket outlets u 20 A (mandatory if they are expected to supply portable equipment for outdoor use), c in some countries, this requirement is mandatory for all socket-outlet circuits rated i 32 A.



6.4 in areas of high fire-risk RCD protection at the circuit breaker controlling all supplies to the area at risk, is mandatory in many countries. The sensitivity of the RCD must be i 500 mA.

fire-risk area




G 6.5 when the fault-current-loop impedance is particularly high When, during the design stage of the installation, it is found that the fault-current loop impedance of a circuit will be inevitably high, so that the overcurrent protection cannot be relied upon to operate within the prescribed time, the following possibilities should be considered: Suggestion 1: instal a circuit breaker which has an instantaneous magnetic tripping element with an operation level which is lower than the usual setting, for example: 2In i Irm i 4 In. This affords protection on circuits which are abnormally long. It must be checked, however, that high transient currents such as the starting currents of motors will not cause nuisance trip-outs.

Note: this is also the case when one (of two) earth faults occurs at the end of a long flexible lead, for example.

PE or PEN 2In i Irm i 4In unusually long cable

fig. G62: a circuit breaker with low-set instantaneous magnetic trip.

Suggestion 2: instal a RCD on the circuit of low sensitivity (several amps to a few tens of amps, since it must not operate for a first fault). If the circuit is supplying socket outlets, it will, in any case, be protected by a high-sensitivity RCD (i 30 mA).

phases neutral PE

TN-S

fig. G63: RCD protection. Suggestion 3: increase the size of the PE conductors and/or the phase conductors, to reduce the loop impedance. Suggestion 4: add supplementary equipotential conductors. This will have a similar effect to that of suggestion 3, i.e. a reduction in the earthfault-loop resistance, while at the same time improving the existing touch-voltage protection measures. The effectiveness of this improvement may be checked by a resistance test between each exposed conductive part and the local main protective conductor. For TN-C installations, bonding as shown in figure G52 is not allowed, and Suggestion 3 should be adopted.


fig. G64: improved equipotential bonding.

7. residual current differential devices (RCDs)

G 7.1 description principle The essential features are shown diagrammatically in figure G65 below. A magnetic core encompasses all the currentcarrying conductors of an electric circuit and the magnetic flux generated in the core will depend at every instant on the arithmetical sum of the currents; the currents passing in one direction being considered as positive, while those passing in the opposite direction will be negative. In a normally healthy circuit (figure G65) i1 + i2 = 0 and there will be no flux in the magnetic core, and zero e.m.f. in its coil. An earth-fault current id will pass through the core to the fault, but will return to the source

via the earth, or via protective conductors in a TN-earthed system. The current balance in the conductors passing through the magnetic core therefore no longer exists, and the difference gives rise to a magnetic flux in the core. The difference current is known as the "residual" current and the principle is referred to as the "differential current" principle. The resultant alternating flux in the core induces an e.m.f. in its coil, so that a current i3 flows in the tripping-device operating coil. If the residual current exceeds the value required to operate the tripping device, then the associated circuit breaker will trip.

P N

i1

i2

i3 S N

Ø

id

fig. G65: the principle of RCD operation.

7.2 application of RCDs earth-leakage currents exist which are not due to a fault, as well as transient overvoltages, either or both of which can lead to unwanted tripping by RCDs. Certain techniques have been developed to overcome these operational problems.

permanent earth leakage currents Every LV installation has a permanent leakage current to earth, which is mainly due to imperfect insulation, and to the intrinsic capacitance between live conductors and earth. The larger the installation the lower its insulation resistance and the greater its capacitance with consequently increased leakage current. On 3-phase systems the capacitive leakage current to earth would be zero if the conductors of all three phases had equal capacitance to earth, a condition that cannot be realized in practical installations. The capacitive current to earth is sometimes increased significantly by filtering capacitors associated with electronic equipment (automation, informatics and computer-based systems, etc.). In the absence of moreprecise data, permanent leakage current in a given installation can be estimated from the following values, measured at 230 V 50 Hz, and abstracted from "Bulletin de l'UTE" April 1992. Fax terminal 0.5 to 1.0 mA IT* workstation 1 to 2 mA Printer (IT*) < 1 mA IT* terminal 1 to 2 mA Photocopier 0.5 to 1.5 mA

transient leakage currents The initial energization of the capacitances mentioned above gives rise to high-frequency transient currents of very short duration, similar to that shown in figure G66. The sudden occurrence of a first-fault on an ITearthed system also causes transient earthleakage currents at high frequency, due to the sudden rise of the two healthy phases to phase/phase voltage above earth.

100% 90%

10 µs (f = 100 kHz)

10% t ca.0.5 µs

60%

fig. G66: standardized 0.5 µs/100 kHz current transient wave.

* Information Technology.


7. residual current differential devices (RCDs) (continued)

G 7.2 application of RCDs (continued) influence of overvoltages Electrical power networks are subjected to overvoltages of various origins; atmospheric, or due to abrupt changes of system operating conditions (faults, fuse operation, switching, etc.). These sudden changes often cause large transient voltages and currents in system inductive and capacitive circuits, before a new stable state is reached. Records have established that, on LV systems, overvoltages remain generally below 6 kV, and that they can be adequately represented by the conventional 1.2/50 µs impulse wave (figure G67). These overvoltages give rise to transient currents represented by a current impulse wave of the conventional 8/20 µs form, having a peak value of several tens of amperes (figure G68). The transient currents flow to earth via the capacitances of the installation surge arresters or through an insulation failure.

U U max

0.5 U

1.2 µs

50 µs

t

fig. G67: standardized voltage-impulse wave 1.2/50 µs. I

0.9

0.5

electromagnetic compatibility The high-frequency (or unidirectional impulse) transient overvoltages and currents mentioned above, together with other electromagnetic disturbance sources (contactor coils, relays, dry contacts), electrostatic discharges, and radiated electromagnetic waves (radio, ignition systems, etc.) are part of the increasingly important field of EMC (electromagnetic compatibility). For further details, the Technical publications nos. 120 and 149, by Merlin Gerin, may be consulted. It is essential that RCDs be immune to possible malfunction from the effects of electromagnetic-surge disturbances. In practice, the levels shown in table G70 are complied with in design and manufacturing specifications*. * Merlin Gerin products.

implementation c every RCD installed must have a minimum level of immunity to unwanted tripping in conformity with the requirements of table G70. RCDs type "S" or time-delay setting levels I or II (see figure G36) cover all transient leakage currents, including those of lightning arresters (see installation layouts in Chapter L, Sub-clause 1.3) of a duration less than 40 ms, c permanent leakage currents downstream of a RCD must be studied, particularly in the case of large installations and/or where filter circuits are present, or again, in the case of an IT-earthed installation. If the capacitance values are known, the equivalent leakage

0.1 t 8 µs 20 µs

fig. G68: standardized current-impulse wave 8/20 µs.

fig. G69: standardized symbol used in some countries, to indicate proof against incorrect operation due to transients. current for the choice of the sensitivity of a RCD is: i mA* = 0.072 C at 50 Hz i mA = 0.086 C at 60 Hz where C = capacity (in n F) of one phase to earth. Since RCDs complying with IEC and many national standards may operate within the range 0.5 I∆n - I∆n for a nominal rating of I∆n, the leakage current downstream of a RCD must not exceed 0.5 I∆n. The limitation of permanent leakage current to 0.25 I∆n, by sub-division of circuits, will, in practice, eliminate the influence of all corresponding current transients. For very particular cases, such as the extension, or partial renovation of extended IT-earthed installations, the manufacturers must be consulted. 3 i mA* = 230 V x 1009π x 10 C (n F) 10 i mA* = 0.072 C (n F) at 50 Hz

disturbance overvoltage transient current

type of test 1.2/50 µs impulse 0.5 µs/100 kHz impulse 8/20 µs impulse

switching static electricity radiated waves

repetitive transient bursts IEC 801-4 electrostatic discharges IEC 801-2 radiated electromagnetic fields IEC 801-3

required withstand quantity 6 kV peak 200 A peak* 200 A peak 60 A peak for 10 mA RCDs 5 kA peak for types "S" or time-delayed models (see note) 4 kV 8 kV 3 V/m

table G70: electromagnetic compatibility withstand-level tests for RCDs. * for RCDs having I∆n < 10 mA this test is not required (IEC 1008-1). Note: Time-delayed RCDs are normally installed near the service position of installations, where current surges of external origin are the most severe. The 5 kA peak test reflects this high-performance duty requirement.


G direct current components Auxiliary d.c. supplies for control and indication of electrical and mechanical equipment are common, and certain appliances include rectifiers (diodes, triacs, thyristors). In the event of an earth fault downstream of a rectifier, the fault current can include a d.c. component. The risk depends on the level of insulation of the d.c. circuits in an appliance, and each case must be considered individually. Problems of this kind generally concern industrial applications. The IEC classifies RCDs according to their ability to function correctly in the presence of d.c. components in the residual current.

3 classes are distinguished: Class AC: operates due to a.c. current only. Class A: operates if residual current consists of uni-directional pulses. Class B: operates on pure d.c. Note: For general use Class AC RCDs are normally installed. Class A are available for specific requirements as a special variation of Class AC devices.

recommendations concerning the installation of RCDs with separate toroidal current transformers The detector of residual current is a closed magnetic circuit (usually circular) of very high magnetic permeability, on which is wound a coil of wire, the ensemble constituting a toroidal (or ring-type) current transformer. Because of its high permeability, any small deviation from perfect symmetry of the conductors encompassed by the core, and the proximity of ferrous material (steel enclosure, chassis members, etc.) can affect the balance of magnetic forces sufficiently, at times of large load currents (motor-starting current, transformer energizing current surge, etc.) to cause unwanted tripping of the RCD. Unless particular measures are taken, the ratio of operating current I∆n to maximum phase current Iph (max.) is generally less than 1/1,000. This limit can be increased substantially (i.e. the response can be desensitized) by adopting the measures shown in fig. G71, and summarized in table G72.

Centralize the cables in the ring core

Use an oversized magnetic ring core

Insert a tubular magnetic screen.

L

L = twice the diameter of the magnetic ring core

fig. G71: means of reducing the ratio I∆n/Iph (max.). measures

diameter (mm)

sensitivity diminution factor careful centralizing of cables through the ring core 3 oversizing of the ring core ø 50 > ø 100 2 ø 80 > ø 200 2 ø 120 > ø 200 6 use of a steel or soft-iron shielding sleeve ø 50 4 c of wall thickness 0.5 mm ø 80 3 c of length 2 x inside diameter of ring core ø 120 3 c completely surrounding the conductors and overlapping the circular core equally at both ends ø 200 2 These measures can be combined. By carefully centralizing the cables in a ring core of 200 mm diameter, where a 50 mm core would be large enough, and using a sleeve, the ratio 1/1,000 could become 1/30,000. table G72: means of reducing the ratio I∆n/Iph (max.).


7. residual current differential devices (RCDs) (continued)

G 7.3 choice of characteristics of a residual-current circuit breaker (RCCB - IEC 1008) rated current The rated current of a RCCB is chosen according to the maximum sustained load current it will carry, estimated in accordance with the methods described in Chapter B Sub-clause 4.3. c if the RCCB is connected in series with, and downstream of a circuit breaker, the rated current of both items will be the same, i.e. In u In1* (fig. G73 (a)), c if the RCCB is located upstream of a group of circuits, protected by circuit breakers, as shown in fig. G73 (b), then the RCCB rated current will be given by In u ku x ks (In1 + In2 + In3 + In4).

(b)

(a)

In1 In

In

In1

In2

In3

In4

fig. G73: residual current circuit breakers (RCCBs).

* Some national standards include a thermal withstand test at a current greater than In in order to ensure correct coordination of protection.

electrodynamic withstand requirements Protection against short-circuits must be provided by an upstream SCPD (Short-Circuit Protective Device) but it is considered that where the RCCB is located in the same distribution box (complying with the appropriate standards) as the downstream circuit breakers (or fuses), the short-circuit protection afforded by these (outgoing-circuit) SCPDs is an adequate alternative. Coordination between the RCCB and the SCPDs is necessary, and manufacturers generally provide tables associating RCCBs and circuit breakers or fuses (see table G74). Coordination of circuit breakers and RCCBs- max. short-circuit current in kA (r.m.s.) upstream circuit breaker type C60a C60N C60H C60L NC100H NC100L downstream 2 p 25 A 10 16 20 45 45 RCCB 40 A 10 16 20 40 45 63 A 16 20 30 5 45 80 A 5 4 p 25 A 5 8 10 25 22 40 A 5 8 10 25 22 63 A 8 10 15 5 22 Coordination of fuses and RCCBs- max. short-circuit (not applicable to aM fuses) upstream fuses gl (not applicable to aM fuses) 16 A 25 A 32 A 40 A 50 A 63 A 80 A 100 A downstream 2 p 25 A 100 100 100 RCCB 40 A 100 100 80 10 (1) 63 A 80 50 30 20 10 (1) 80 A 30 20 4 p 25 A 100 100 100 10 (1) 40 A 100 100 80 10 (1) 63 A 80 50 30 20 10 (1) 80 A 30 20 10 (1) table G74: typical manufacturers coordination table for RCCBs, circuit breakers, and fuses. (1) A 100 A fuse with several RCCBs downstream: the thermal withstand of the RCCBs is not certain.


1. general

H1 1.1 methodology and definitions component parts of an electric circuit and its protection are determined such, that all normal and abnormal operating constraints are satisfied.

methodology Following a preliminary analysis of the power requirements of the installation, as decribed in Chapter B Clause 4, a study of cabling* and its electrical protection is undertaken, starting at the origin of the installation, through the intermediate stages to the final circuits. The cabling and its protection at each level must satisfy several conditions at the same time, in order to ensure a safe and reliable installation, e.g. it must: c carry the permanent full load current, and normal short-time overcurrents, c not cause voltage drops likely to result in an inferior performance of certain loads, for example: an excessively long acceleration period when starting a motor, etc. Moreover, the protective devices (circuit breakers or fuses) must: c protect the cabling and busbars for all levels of overcurrent, up to and including short-circuit currents,

c ensure protection of persons against indirect contact hazards, particularly in TN- and IT- earthed systems, where the length of circuits may limit the magnitude of short-circuit currents, thereby delaying automatic disconnection (it may be remembered that TT- earthed installations are obligatorily protected at the origin by a RCD, generally rated at 500 mA). The cross-sectional areas of conductors are determined by the general method described in Sub-clause 1.2 of this Chapter. Apart from this method some national standards may prescribe a minimum cross-sectional area to be observed for reasons of mechanical endurance. Particular loads (as noted in Chapter J) require that the cable supplying them be oversized, and that the protection of the circuit be likewise modified. * the term "cabling" in this chapter, covers all insulated conductors, including multi-core and single-core cables and insulated wires drawn into conduits, etc.

kVA to be supplied

short-circuit MVA at the origin of the circuit

maximum load current

short-circuit current

upstream or downstream network

IB

Isc

rated current of protective device (C.B. or fuses) In

choice of C.B. or fuses

choice of protective device

conditions of installation

short-circuit current-breaking rating of C.B. or fuses I scb

cross-sectional area of conductors of the circuit

verification of thermal withstand requirements

verification of the maximum voltage drop

IT or TN scheme

verification of the maximum length of the circuit TT scheme determination of the cross-sectional area of the conductors

confirmation of the cross-sectional area of the cabling, and the choice of its electrical protection

table H1-1: logigram for the selection of cable size and protective-device rating for a given circuit.

the protection of circuits - the switchgear - H1-1


H1 1.1 methodology and definitions (continued) definitions Maximum load current: IB c at the final circuits level, this current corresponds to the rated kVA of the load. In the case of motor-starting, or other loads which take an initially-high current, particularly where frequent starting is concerned (e.g. lift motors, resistance-type spot welding, and so on) the cumulative thermal effects of the overcurrents must be taken into account. Both cables and thermaltype relays are affected; c at all upstream circuit levels this current corresponds to the kVA to be supplied, which takes account of the factors of simultaneity (diversity) and utilization, ks and ku respectively, as shown in figure H1-2. Maximum permissible current: IZ This is the maximum value of current that the cabling for the circuit can carry indefinitely, without reducing its normal life expectancy. The current depends, for a given crosssectional area of conductors, on several parameters: c constitution of the cable and cable-way (Cu or Alu conductors; PVC or EPR etc. insulation; number of active conductors); c ambient temperature; c method of installation; c influence of neighbouring circuits.


combined factors of simultaneity (or diversity) and utilization ks x ku = 0.69

IB = 290 x 0.69 = 200 A

sub-distribution board

80 A

60 A

100 A

IB = 50 A

M

normal load motor current 50 A

fig. H1-2: calculation of maximum load current IB.

overcurrents An overcurrent occurs each time the value of current exceeds the maximum load current IB for the load concerned. This current must be cut off with a rapidity that depends upon its magnitude, if permanent damage to the cabling (and appliance if the overcurrent is due to a defective load component) is to be avoided. Overcurrents of relatively short duration can however, occur in normal operation; two types of overcurrent are distinguished: Overloads These overcurrents can occur in healthy electric circuits, for example, due to a number of small short-duration loads which occasionally occur co-incidentally; motorstarting loads, and so on. If either of these conditions persists however beyond a given period (depending on protective-relay settings or fuse ratings) the circuit will be automatically cut off.

H1-2 - the protection of circuits - the switchgear

Short-circuit currents These currents result from the failure of insulation between live conductors or/and between live conductors and earth (on systems having low-impedance-earthed neutrals) in any combination, viz: c 3 phases short-circuited (and to neutral and/or earth, or not); c 2 phases short-circuited (and to neutral and/or earth, or not); c 1 phase short-circuited to neutral (and/or to earth).

H1 1.2 overcurrent protection principles A protective device is provided at the origin of the circuit concerned. c acting to cut-off the current in a time shorter than that given by the I2t characteristic of the circuit cabling; c but allowing the maximum load current IB to flow indefinitely. The characteristics of insulated conductors when carrying short-circuit currents can, for periods up to 5 seconds following short-circuit initiation, be determined approximately by the formula: Is2 x t = k2 x S2 which shows that the allowable heat generated is proportional to the cross-sectional-area of the condutor squared. Where: t: duration of short-circuit current (seconds); S: c.s.a. of insulated conductor (mm2); Is: short-circuit current (A r.m.s.); k: insulated conductor constant (values of k2 are given in table H1-54). For a given insulated conductor, the maximum permissible current varies according to the environment. For instance, for a high ambient temperature (θa1 > θa2), IZ1 is less than IZ2 (fig. H1-5). θ means "temperature". Note: Isc means 3-phase short-circuit current. IscB means rated 3-ph. short-circuit breaking current of the circuit breaker. Ir (or Irth)* means regulated "nominal" current level; e.g. a 50 A nominal circuit breaker can be regulated to have a protective range, i.e. a conventional overcurrent tripping level (see figure H1-6) similar to that of a 30 A circuit breaker.

t maximum load current

I2t cable characteristic

circuit-breaker tripping curve

temporary overload

IB Ir Iz

ISCB PdC

I

fig. H1-3: circuit protection by circuit breaker. t I2t cable characteristic

fuse curve

temporary overload

IB

Ir cIz Iz

I

fig. H1-4: circuit protection by fuses. t

1

2

* both designations are commonly used in different standards.

θa1 > θa2

5s I2t = k2S2

Iz1 < Iz2

I

fig. H1-5: I2t characteristic of an insulated conductor at two different ambient temperatures.



H1 1.3 practical values for a protection scheme The following methods are based on rules laid down in the IEC standards, and are

representative of the practices in many countries.

loads

circuit cabling

Iz x

a lo d cu n rre t IB

m ax i

1. 45

um im

m u cu m rre pe nt rm Iz iss i

ax

bl e

m IB In zone a

ISC

1.45 Iz

Iz I2

ISCB zone c

n its om re in gu al la cu te rr d en cu t rre In nt or Ir co nv en tri tio p na cu l o rre ve nt rc I2 urr en

t

zone b

g t ui atin irc ng r c i t or ak sh t bre h n p 3- urre -c ult

fa

protective device

fig. H1-6: current levels for determining circuit breaker or fuse characteristics.

IB i In i Iz I2 i 1,45 Iz ISCB u ISC

zone a zone b zone c

general rules A protective device (circuit breaker or fuse) functions correctly if: c its nominal current or its setting current In is greater than the maximum load current IB but less than the maximum permissible current IZ for the circuit, i.e. IB i In i IZ corresponding to zone "a" in figure H1-6; c its tripping current I2 "conventional" setting is less than 1.45 IZ which corresponds to zone "b" in figure H1-6.

The "conventional" setting tripping time may be 1 hour or 2 hours according to local standards and the actual value selected for I2. For fuses, I2 is the current (denoted If) which will operate the fuse in the conventional time; c its 3-phase short-circuit fault-current breaking rating is greater than the 3-phase short-circuit current existing at its point of installation. This corresponds to zone "c" in figure H1-6.

criteria for a circuit breaker: IB i In (or Ir) i Iz and, rated short-circuit breaking current ISCB u ISC the 3-ph. short-circuit current level at the point of CB installation.

applications

criteria for fuses: IB i In i IZ k3 and, the rated short-circuit current breaking capacity of the fuse ISCF u ISC the 3-ph. short-circuit current level at the point of fuse installation.

Protection by fuses The condition I2 i 1.45 IZ must also be taken into account, where I2 is the fusing (meltinglevel) current, equal to k2 x In (k2 ranges from 1.6 to 1.9) according to the particular fuse concerned. A further factor k3 has been introduced (in the national standards from which these notes have been abstracted) such that I2 i 1.45 IZ will be valid if In i IZ/k3.

For fuses type gl: In i 10 A k3 = 1.31 10 A < In i 25 A k3 = 1.21 In > 25 A k3 = 1.10 Moreover, the short-circuit current breaking capacity of the fuse ISCF must exceed the level of 3-phase short-circuit current at the point of installation of the fuse(s).

Association of different protective devices The use of protective devices which have fault-current ratings lower than the fault level existing at their point of installation are permitted by IEC and many national standards in the following conditions: c there exists upstream, another protective device which has the necessary short-circuit rating, and c the amount of energy allowed to pass through the upstream device is less than that which the downstream device and all

associated cabling and appliances can withstand without damage. In pratice this arrangement is generally exploited in: c the association of circuit breakers/fuses; c the technique known as "cascading" in which the strong current-limiting performance of certain circuit breakers effectively reduces the severity of downstream short-circuits. Possible combinations which have been tested in laboratories are indicated in certain manufacturers catalogues.


Protection by circuit breaker By virtue of its high level of precision the current I2 is always less than 1.45 In (or 1.45 Ir) so that the condition, that I2 i 1.45 IZ (as noted in the "general rules" above) will always be respected.

Particular case: if the circuit breaker itself does not protect against overloads, it is necessary to ensure that, at a time of lowest value of short-circuit current, the overcurrent device protecting the circuit will operate correctly. This particular case is examined in Sub-clause 5.1.

H1 1.4 location of protective devices a protective device is, in general, required at the origin of each circuit.

general rule A protective device is necessary at the origin of each circuit where a reduction of permissible maximum current level occurs.

P

P2

P3

50 mm2

possible alternative locations in certain circumstances The protective device may be placed part way along the circuit: c if AB is not in proximity to combustible material, and c if no socket-outlets or branch connections are taken from AB. Three cases may be useful in practice. Consider case (1) in the diagram c AB i 3 metres, and c AB has been installed to reduce to a practical minimum the risk of a short-circuit (wires in heavy steel conduit for example). Consider case (2) c the upstream device P1 protects the length AB against short-circuits in accordance with Sub-clause H1-5.1. Consider case (3) c the overload device (S) is located adjacent to the load. This arrangement is convenient for motor circuits. The device (S) constitutes the control (start/stop) and overload protection of the motor while (SC) is: either a circuit breaker (designed for motor protection) or fuses type aM, c the short-circuit protection (SC) located at the origin of the circuit conforms with the principles of Sub-clause H1-5.1.

P4

10 mm2

25 mm2

P1 A short-circuit sc protective device

<3m

B P2

B P3

case (1)

B s overload protective device3

case (2)

case (3)

circuits with no protection Either c the protective device P1 is calibrated to protect the cable S2 against overloads and short-circuits; Or c where the breaking of a circuit constitutes a risk, e.g. v excitation circuits of rotating machines, v circuits of large lifting electromagnets, v the secondary circuits of current transformers. No circuit interruption can be tolerated, and the protection of the cabling is of secondary importance.

P1: C60 calibre 15 A 2,5 mm2 S2: 1,5 mm2

table H1-7: general rules and exceptions concerning the location of protective devices.

1.5 cables in parallel Conductors of the same cross-sectional-area, the same length, and of the same material, can be connected in parallel. The maximum permissible current is the sum of the individual-core maximum currents, taking into account the mutual heating effects, method of installation, etc. Protection against overload and short-circuits is identical to that for a single-cable circuit.

The following precautions should be taken to avoid the risk of short-circuits on the paralleled cables: c additional protection against mechanical damage and against humidity, by the introduction of supplementary protection; c the cable route should be chosen so as to avoid close proximity to combustible materials.



H1 1.6 worked example of cable calculations installation scheme The installation is supplied through a 1,000 kVA transformer. The process requires a high degree of supply continuity and this is provided by the installation of a 500 kVA 400 V standby generator, and by the adoption of a 3-phase 3-wire IT-system at the main general distribution board from which the processing plant is supplied. The remainder of the installation is isolated by a 315 kVA 400/400V transformer: the isolated network is a TT-earthed 3-phase 4-wire system.

Following the one-line diagram of the system shown in figure H1-8 below, a reproduction of the results of a computer study for the circuit C1 and its circuit breaker Q1, and C2 with associated circuit breaker Q2 are presented. These studies were carried out with ECODIAL 2.2 software (a Merlin Gerin product). This is followed by the same calculations carried out by the methods described in this guide.

TR1 1000 kVA 5% 400 V 26. 44 kA C1 8m .18% 3x (3 x 240)

Q1 M16 N1 STR 38 1600 A

B1

G1 500 kVA 721 A Q2 C801N STR35SE 800 A

Q3

Q4

C2 15m .7%

Q5

Q6

C3

C4

I1

I2

3x (1 x 240) T1 315 kVA 400 V

B2 Q7 NS630N STR35SE 630 A

Q11 NS250N TMD 250 A

Q12 NS160N TMD 160 A

Q8

Q13 NS100N TMD 80 A

fig. H1-8: one-line diagram of the installation.


Q9

Q10

H1 calculations using software Ecodial 2.2 General network characteristics earthing system neutral distributed voltage (V) frequency (Hz) Transformer TR 1 number of transformers upstream fault level (MVA) rating (kVA) short-circuit impedance voltage (%) remarks nominal current (A) resistance of transformer (mΩ) reactance of transformer (mΩ) running total of impedance RT (mΩ) running total of impedance XT (mΩ) 3-phase short-circuit current (kA) short-circuit power factor Cable C 1 maximum load current (A) type of insulation conductor material ambient temperature (°C) single-core or multi-core cable installation method number of circuits in close proximity (table H1-14) other coefficient number of phases selected cross-sectional area (mm2) protective conductor neutral conductor length (m) voltage drop ∆U (%) running total of impedance RT (mΩ) running total of impedance XT (mΩ) voltage drop ∆U total (%) 3-phase short-circuit current (kA) 1-phase-to-earth fault current (A) resistance of protective conductor RPE (mΩ) touch voltage (V) Circuit breaker Q 1 voltage (V) 3-ph short-circuit current upstream of the circuit breaker (kA) maximum load current (A) ambient temperature (°C) number of poles circuit breaker type tripping unit type rated current (A) Busbars B 1 maximum load current (A) number of phases number of bars per phase width (mm) thickness (mm) length (m) remarks impedance of busbars R (mΩ) impedance of busbars X (mΩ) voltage drop ∆U(%) running total of impedance RT (mΩ) running total of impedance XT (mΩ) voltage drop ∆U total (%) 3-ph short-circuit current (kA)

IT N 400 50 input data 1 500 1000 5

input data 1374 PRC Cu 30 UNI 13

output

1374 2.13 8.55 2.18 8.9 26.44 .23 output

1 1 3 3 x 240 1 x 240

input data 400

8 .18 2.43 9.11 .18 25.7 20334 .75 15 output

25.7 1374 40 3 M 16 N1 STR 38 1600 1374 3 1 125 5 3 .1 .45 .16 2.53 9.55 .34 24.53



H1 1.6 worked example of cable calculations (continued) Circuit breaker Q 2 voltage (V) 3-ph short-circuit current upstream of the circuit breaker (kA) maximum load current (A) ambient temperature (°C) number of poles circuit breaker type tripping unit type rated current (A) 3-phase fault current (A) protection against indirect contact assured upstream circuit breaker absolute discrimination Cable C 2 maximum load current (A) type of insulation conductor material ambient temperature (°C) single-core or multi-core cable installation method number of circuits in close proximity (table H1-14) other coefficient number of phases selected cross-sectional area (mm2) protective conductor neutral conductor length (m) voltage drop ∆U (%) running total of impedance RT (mΩ) running total of impedance XT (mΩ) voltage drop ∆U total (%) 3-phase short-circuit current (kA) 1-phase-to-earth fault current (A) resistance of protective conductor RPE (mΩ) touch voltage (V)

input data 400

output

24.53 433 40 3 NS630 N STR23SE 630 13221 M16 N1 STR38 input data 433 PRC Cu 30 UNI 13

output

1 1 3 1 x 240 1 x 70 15 .33 3.93 10.75 .67 21.18 13221 5.57 73

table H1-9: calculations carried out with ECODIAL software (M.G).

the same calculations using the methods recommended in this guide Dimensioning circuit C 1 The HV/LV 1,000 kVA transformer has a rated no-load voltage of 420 V. Circuit C 1 must be suitable for a current of In = 1,000 = In = 1,374 A per phase ex 0.42 Three single-core XLPE-insulated copper cables in parallel will be used for each phase; these cables will be laid on cable trays corresponding with reference F (see tables in Clause H1 2.2). The "K" correction factors are as follows: K1=1 K 2 = 0.82 ( 3 three-phase groups in a single layer) K 3 = 1 (temperature 30 °C). If the circuit breaker is a withdrawable or unpluggable* type, which can be regulated, one might choose: Iz = 1,374 A applying H1.2.1 Iz I’z = = 1,676 A. K1xK2xK3 Each conductor will therefore carry 558 A. Table H1-17 indicates that the c.s.a. is 240 mm2. * Withdrawable, CBs are generally mounted in drawers for maintenance purposes. Plug-in type CBs are generally moulded-case units, which may be completely removed from the fixed-base sockets.


The resistances and the inductive reactances for the three conductors in parallel are, for a length of 8 metres (see H1-4.2): 22.5 x 8 R= = 0.25 mΩ per phase 240 x 3 0.12 x 8 X= = 0.32 mΩ per phase 3 (0.12 mΩ/metre was advised by the cable maker). Dimensioning circuit C 2 Circuit C 2 supplies a 315 kVA 3-phase 400/400 V isolating transformer Ib = 315 = 433 A. 0.42 x e A multi-core XLPE cable laid on a cable tray (together with two other cables) in an ambient air temperature of 30 °C is proposed. The circuit breaker is regulated to 433 A. Iz = 433 A The method of installation is characterized by the reference letter E, and the "K" correcting factors are: K1=1 K 2 = 0.82 K 3 = 1. 433 I’z = = 528 A so that 1 x 0.82 x 1 a c.s.a. of 240 mm2 is appropriate. The resistance and inductive reactance are respectively: 22.5 x 15 R= = 1.4 mΩ per phase 240 X = 0.08 x 15 = 1.2 mΩ per phase.

H1 Calculation of short-circuit currents for the selection of circuit breakers Q 1 and Q2 *all values are to a 420 voltage base e circuits R* X* Z* components parts mΩ mΩ mΩ 500 kVA at the HV source network 0.050 0.35 HV/LV transformer 2.24 8.10 cable C 1 0.25 0.32 sub-total for Q1 2.54 8.77 9.13 busbars B1 0.75 9.85 cable C 2 1.40 1.2 sub-total for Q2 3.94 10.72 11.42 table H1-10: example of short-circuit current evaluation.

Isc* kA

26.5 24.6 21.2

Sub-clause H1-4.2 shows the formula for calculating the short-circuit current Isc at a given point in the system. If the rated no-load voltage of the transformer is 420 V: 420 Isc = 2.542 + 8.772 3 = 26.5 kA at Q 1. The inductive reactance of busbars B1 is estimated to be 0.15 x 5 = 0.75 mΩ - its resistance being negligibly small. The Isc at the location of Q 2 is computed as for Q 1, and found to be 21 kA. In order to make the final choice, features such as selectivity, isolating capability, withdrawal or unplugging facility and general ease of maintenance, and so on, must be considered, with the aid of manufacturers catalogues. The protective conductor Thermal requirements. Tables H1.60 and H1.61 show that, when using the adiabatic method (IEC 724 (1984) Clause 2) the c.s.a. for the protective earth (PE) conductor for circuit C1 will be: 26500 x √0.1 u = 47.6 mm2 176 a single 240 mm2 conductor dimensioned for other reasons mentioned later is therefore largely sufficient, provided that it also satisfies the requirements for indirect-contact protection (i.e. that its impedance is sufficiently low). For the circuit C2, the c.s.a. of its PE conductor should be: 21,000 x √0.1 u = 37.7 mm2 176 In this case a 70 mm2 conductor may be adequate if the indirect-contact protection conditions are also satisfied. Protection against indirect-contact hazards Reminder: the LV neutral point of an IT-scheme transformer is isolated from earth, or is earthed through a high resistance (1-2 kΩ) so that an indirect-contact hazard can only exist if two earth faults occur concurrently, each on a different phase (or on one phase and a neutral conductor). Overcurrent protective devices must then be relied upon to cut-off the faulty circuits, except in particular circumstances i.e. where the resistance of P.E. conductors is too high, as noted in Chapter G Sub-clauses 6.3 to 6.5. RCDs are often employed in such cases.

Circuit C 1 will be of class 2 insulation i.e. double insulation and no earthed exposed conductive parts. The only indirect-contact requirement for this circuit, therefore, is at the transformer tank. The 240 mm2 P.E. conductor mentioned above, generally connects the tank of the HV/LV transformer to the earth electrode for the installation at a common earthing busbar in the main general distribution board. This means that if one (of the two) concurrent LV phase-to-earth faults should occur in the transformer, an indirect contact danger will exist at the transformer tank. In such a case, the HV overcurrent protection for the transformer is unlikely to operate, but the protection on the second faulty LV circuit must do so infallibly to ensure protection against the indirect contact danger, as described. Since a HV fault to earth at the transformer is also always possible, and very often HV lightning arresters on the transformer are connected to earth through the P.E. conductor in question, a conductor of large c.s.a. is invariably selected for this section of the installation. Dimensioning considerations for this conductor are given in Sub-clause 6.3. For circuit C2, tables G.43 and G.59, or the formula given in Sub-clause G.6.2 may be used for a 3-phase 3-wire circuit. The maximum permitted length of the circuit is given by: 0.8 x 230 x 240 x ex 103 Lmax = 2 x 22.5 x (1.25 + 240/70) x 630 x 11.5 = 76,487 = 50 metres. 1,530 The factor 1.25 in the denominator is a 25% increase in resistance for a 240 mm2 conductor, in accordance with Chapter G Sub-clause 5.2. (The value in the denominator 630 x 11.5 = Im i.e. the current level at which the instantaneous short-circuit magnetic trip of the 630 A circuit breaker operates). This value is equal to 10 In + 15 % (the highest positive manufacturing tolerance for the tripping device). For further details of magnetic tripping devices, please refer to Chapter H2 Sub-clause 4.2. The length of 15 metres is therefore fully protected by "instantaneous" overcurrent devices. Voltage drop From table H1.29 it can be seen that: for C1 (3 x 240 mm2 per phase) 0.21 V/A/km x 1,374 A x 0.008 km ∆U = 3 = 0.77 V ∆U % = 100 x 0.77 = 0.19 %; 400 for C2 ∆U = 0.21 V/A/km x 433 A x 0.015 km = 1.36 V ∆U % = 100 x 1.36 = 0.34 %; 400 At the circuit terminals of the LV/LV transformer the percentage volt-drop ∆U % = 0.53 %.


2. practical method for determining the smallest allowable cross-sectional-area of circuit conductors

H1 2.1 general installation conditions for the conductors

maximum load current IB IB rated current In of the protective device must be equal to or greater than the maximum load current IB

determination of K factors and of the appropriate letter code

In choice of maximum permissible current IZ for the circuit, corresponding to a conductor size that the protective device is capable of protecting

fuse

circuit breaker

IZ = 1.31 In if In 10 A* IZ = 1.21 In if In 10 A* and In 25 A* IZ = 1.10 In if In 25 A* I Z1

I Z = I n*

I Z2

Determination of the size (c.s.a.) of the conductors of the circuit capable of carrying IZ1 or IZ2, by use of an equivalent current I'Z, which takes into account the influences of factor K (I'Z = IZ/K), of the letter code, and of the insulating sheath of the conductors (refer to tables H1-17 or H1-24) I 'Z S1

I 'Z S2

verification of other conditions that may be required-see figure H1.1 * or slightly greater

table H1-11: logigram for the determination of minimum conductor size for a circuit. The first step is to determine the size of the phase conductors. The dimensioning of the neutral and protective conductors is explained in H1-6 and H1-7. In this clause the following cases are considered: c unburied conductors, c buried conductors. The tables in this clause permit the determination of the size of phase conductors for a circuit of given current magnitude. The procedure is as follows: c determine an appropriate code-letter

reference which takes into account: v the type of circuit (single-phase; threephase, etc.) and v the kind of installation: and then c determine the factor K of the circuit considered, which covers the following influences: v installation method, v circuit grouping, v ambient temperature.

2.2 determination of conductor size for unburied circuits the size of a phase conductor is given in tables which relate: c the code letter symbolizing the method of installation, and c the factor of influence K. These tables distinguish unburied circuits from buried circuits.

determination of the code-letter reference The letter of reference (B to F) depends on the type of conductor used and its method of installation. The possible methods of types of conductor single-core wires and multi-core cables

multi-core cables

single-core cables

installation are numerous, but the most common of them have been grouped according to four classes of similar environmental conditions, as shown below in table H1-12.

method of installation c under decorative moulding with or without a removable cover, surface or flush-mounting, or under plaster c in underfloor cavity or behind false ceiling c in a trench, moulding or wainscoting c surface-mounted in contact with wall or ceiling c on non-perforated cable trays c cable ladders, perforated trays, or on supporting brackets c surface-mounted clear of the surface (e.g. on cleats) c catenary cables

letter code

B

C E

F

table H1-12: code-letter reference, depending on type of conductor and method of installation. H1-10 - the protection of circuits - the switchgear

H1 for circuits which are not buried, factor k characteristizes the conditions of installation, and is given by: K = K1 x K2 x K3 the three component factors depending on different features of the installation.

determination of the factor K The factor k summarizes the several features which characterize the conditions of installation. It is obtained by multiplying three correction factors K1, K2 and K3. The values of these factors are given in tables H1.13 to H1.15 below.

correction factor K1 Factor K1 is a measure of the influence of the method of installation.

factor K1 is a measure of the influence of the method of installation.

code letter B

installation details - cables installed directly in thermal-insulation materials

example

K1 0.70

- conduits installed in thermalinsulation materials

0.77

- multi-core cables

0.90

- construction cavities and closed cables trenches

0.95

C

- surface mounted on ceiling

0.95

B, C, E, F

- other cases

1

table H1-13: factor K1 according to method of circuit installation (for further examples refer to IEC 364-5-52 table 52H). Correction factor K2 Factor K2 is a measure of the mutual influence of two circuits side-by-side in close proximity.

factor K2 is a measure of the mutual influence of two circuits side-by-side in close proximity.

Two circuits are considered to be in close proximity when L, the distance between two cables, is less than double the diameter of the larger of the two cables.

code letter

location of correction factor K2 cables in close number of circuits or multicore cables proximity 1 2 3 4 5 6 7 8 9 12 16 20 B,C embedded 1.00 0.80 0.70 0.65 0.60 0.57 0.54 0.52 0.50 0.45 0.41 0.38 or buried in the walls C single layer 1.00 0.85 0.79 0.75 0.73 0.72 0.72 0.71 0.70 0.70 on walls or floors, or on unperforated cables trays single layer 0.95 0.81 0.72 0.68 0.66 0.64 0.63 0.62 0.61 0.61 on ceiling E,F single layer 1.00 0.88 0.82 0.77 0.75 0.73 0.73 0.72 0.72 0.72 on horizontal perforated trays, or on vertical trays single layer 1.00 0.87 0.82 0.80 0.80 0.79 0.79 0.78 0.78 0.78 on cable ladders, brackets, etc table H1-14: correction factor K2 for a group of conductors in a single layer


2. practical method for determining the smallest allowable cross-sectional-area of circuit conductors (continued)

H1 2.2 determination of conductor size for unburied circuits (continued) When cables are installed in more than one layer a further factor, by which K2 must be multiplied, will have the following values : 2 layers : 0.80 3 layers : 0.73 4 or 5 layers : 0.70. Correction factor K3 Factor K3 is a measure of the influence of the temperature, according to the type of insulation.

factor K3 is a measure of the influence of the temperature according to the type of insulation.

ambient temperatures

insulation elastomer (rubber)

cross-linkedpolyethylene (XLPE) butyl, ethylenepropylene-rubber (EPR) 10 1.29 1.22 1.15 15 1.22 1.17 1.12 20 1.15 1.12 1.08 25 1.07 1.07 1.04 30 1.00 1.00 1.00 35 0.93 0.93 0.96 40 0.82 0.87 0.91 45 0.71 0.79 0.87 50 0.58 0.71 0.82 55 0.61 0.76 60 0.50 0.71 65 0.65 70 0.58 75 80 table H1-15: correction factor K3 for ambient temperatures other than 30 °C. Example: A 3-phase 3-core XLPE cable is laid on a perforated cable-tray in close proximity to three other circuits, consisting of: c a 3-phase 3-core cable (circuit no. 1), c three single-core cables (circuit no. 2), c six single-core cables (circuit no. 3), circuit no. 2 and no. 3 are 3-phase circuits, the latter comprising 2 cables per phase. There are, therefore, effectively 5 3-phase circuits to be considered, as shown in figure H1-16. The ambient temperature is 40 °C. The code letter indicated in table H1-12 is E. K1 given by table H1-13 = 1. K2 given by table H1-14 = 0.75. K3 given by table H1-15 = 0.91. K = K1 x K2 x K3 = 1 x 0.75 x 0.91 = 0.68.


polyvinylchloride (PVC)

1

2

θa = 40°C

3

XLPE

fig. H1-16: example in the determination of factors K1, K2 and K3.

H1 determination of the minimum cross-sectional area of a conductor The current Iz when divided by K gives a fictitious current I'z. Values of I'z are given in table H1-17 below, together with corresponding cable sizes for different types of insulation and core material (copper or aluminium). insulation and number of conductors (2 or 3) rubber butyl or XLPE or EPR or PVC code B PVC3 PVC2 PR3 PR2 B code letter C PVC3 PVC2 PR3 PR2 C letter E PVC3 PVC2 PR3 PR2 E F PVC3 PVC2 PR3 PR2 F c.s.a. 1.5 15.5 17.5 18.5 19.5 22 23 24 26 1.5 c.s.a. copper 2.5 21 24 25 27 30 31 33 36 2.5 copper 4 28 32 34 36 40 42 45 49 4 (mm2) (mm2) 6 36 41 43 48 51 54 58 63 6 10 50 57 60 63 70 75 80 86 10 16 68 76 80 85 94 100 107 115 16 25 89 96 101 112 119 127 138 149 161 25 35 110 119 126 138 147 158 169 185 200 35 50 134 144 153 168 179 192 207 225 242 50 70 171 184 196 213 229 246 268 289 310 70 95 207 223 238 258 278 298 328 352 377 95 120 239 259 276 299 322 346 382 410 437 120 150 299 319 344 371 395 441 473 504 150 185 341 364 392 424 450 506 542 575 185 240 403 430 461 500 538 599 641 679 240 300 464 497 530 576 621 693 741 783 300 400 656 754 825 940 400 500 749 868 946 1083 500 630 855 1005 1088 1254 630 c.s.a. 2.5 16.5 18.5 19.5 21 23 25 26 28 2.5 c.s.a. aluminium 4 22 25 26 28 31 33 35 38 4 alu (mm2) 6 28 32 33 36 39 43 45 49 6 (mm2) 10 39 44 46 49 54 59 62 67 10 16 53 59 61 66 73 79 84 91 16 25 70 73 78 83 90 98 101 108 121 25 35 86 90 96 103 112 122 126 135 150 35 50 104 110 117 125 136 149 154 164 184 50 70 133 140 150 160 174 192 198 211 237 70 95 161 170 183 195 211 235 241 257 289 95 120 186 197 212 226 245 273 280 300 337 120 150 227 245 261 283 316 324 346 389 150 185 259 280 298 323 363 371 397 447 185 240 305 330 352 382 430 439 470 530 240 300 351 381 406 440 497 508 543 613 300 400 526 600 663 740 400 500 610 694 770 856 500 630 711 808 899 996 630 table H1-17: case of an unburied circuit: determination of the minimum cable size (c.s.a.), derived from the code letter; conductor material; insulation material and the fictitious current I'z.



H1 2.2 determination of conductor size for unburied circuits (continued) Example The example shown in figure H1-16 for determining the value of K, will also be used to illustrate the way in which the minimum cross-sectional-area (c.s.a.) of conductors may be found, by using the table H1-17. The XLPE cable to be installed will carry 23 amps per phase. Previous examples show that: c the appropriate code letter is E, c the factor K = 0.68. 1

2

θa = 40°C

3

XLPE

fig. H1-18: example for the determination of minimum cable sizes.

Determination of the cross-sectional areas A standard value of In nearest to, but higher than 23 A is required. Two solutions are possible, one based on protection by a circuit breaker and the second on protection by fuses. c circuit breaker: In = 25 A v permissible current Iz = 25 A v fictitious current I'z = 25 = 36.8 A 0.68 v cross-sectional-area of conductors is found as follows: In the column PR3 corresponding to code letter E the value of 42 A (the nearest value greater than 36.8 A) is shown to require a copper conductor c.s.a. of 4 mm2. For an aluminium conductor the corresponding values are 43 A and 6 mm2. c fuses: In = 25 A v permissible current Iz = K3 In = 1.21 x 25 = Iz = 30.3 A v the fictitious current I'z = 30.3 = 40.6 A 0.68 v the cross-sectional-areas, of copper or aluminium conductors are (in this case) found to be the same as those noted above for a circuit-breaker-protected circuit.

2.3 determination of conductor size for buried circuits In the case of buried circuits the determination of minimum conductor sizes, necessitates the establishement of a factor K.

for buried circuits the value of factor K characteristizes the conditions of installation, and is obtained from the following factors: K4 x K5 x K6 x K7 = K each of which depends on a particular feature of installation.

determination of factor K

factor K4 measures the influence of the method of installation.

method of installation K4 placed in earthenware ducts; in 0.8 conduits, or in decorative mouldings other cases 1 table H1-19: correction factor K4 related to the method of installation.

factor K5 measures the mutual influence of circuits placed side-byside in close proximity.

Correction factor K5 Factor K5 is a measure of the mutual influence of circuits placed side-by-side in close proximity. Cables are in close proximity when the distance L separating them is less than double the diameter of the larger of the two cables concerned.

A code letter corresponding to a method of installation is not necessary.

Factor K summarizes the global influence of different conditions of installation, and is obtained by multiplying together correction factors K4, K5, K6 and K7. The values of these several factors are given in tables H1-19 to H1-22. Correction factor K4 Factor K4 is a measure of the influence of the method of installation.

location of correction factor K5 cables side-by-side number of circuits or of multicore cables in close proximity 1 2 3 4 5 6 7 8 9 12 16 20 buried 1.00 0.80 0.70 0.65 0.60 0.57 0.54 0.52 0.50 0.45 0.41 0.38 table H1-20: correction factor K5 for the grouping of several circuits in one layer. When cables are laid in several layers, multiply K5 by 0.8 for 2 layers, 0.73 for 3 layers, 0.7 for 4 layers or 5 layers.


H1 factor K6 is a measure of the influence of the earth in which the cable is buried.

Correction factor K6 This factor takes into account the nature and condition of the soil in which a cable (or cables) is (are) buried; notably its thermal conductivity. nature of soil K6 very wet soil (saturated) 1.21 wet soil 1.13 damp soil 1.05 dry soil 1.00 very dry soil (sunbaked) 0.86 table H1-21: correction factor K6 for the nature of the soil.

factor K7 is a measure of the influence of the soil temperature.

Correction factor K7 This factor takes into account the influence of soil temperature if it differs from 20 °C. soil temperature °C

insulation polyvinyl-chloride (PVC)

cross-linked polyethylene (XLPE) ethylene-propylene rubber (EPR) 10 1.10 1.07 15 1.05 1.04 20 1.00 1.00 25 0.95 0.96 30 0.89 0.93 35 0.84 0.89 40 0.77 0.85 45 0.71 0.80 50 0.63 0.76 55 0.55 0.71 60 0.45 0.65 table H1-22: correction factor K7 for soil temperatures different than 20 °C. Example A single-phase 230 V circuit is included with four other loaded circuits in a buried conduit. The soil temperature is 20 °C. The conductors are PVC insulated and supply a 5 kW lighting load. The circuit is protected by a circuit breaker. K4 from table H1-19 = 0.8. K5 from table H1- 20 = 0.6. K6 from table H1- 21 = 1.0. K7 from table H1- 22 = 1.0. K = K4 x K5 x K6 x K7 = 0.48.

θa = 20°C 5 kW 230 V

fig. H1-23: example for the determination of K4, K5, K6 and K7.



H1 2.3 determination of conductor size for buried circuits (continued) determination of the smallest c.s.a. (cross-sectional-area) of a conductor, for buried circuits Knowing Iz and K, the corresponding crosssectional-areas are given in table H1-24 below. insulation and number of loaded conductors rubber or PVC Butyl, or cross-linked polyethylene XLPE, or ethylene-propylene rubber EPR 3 conductors 2 conductors 3 conductors 2 conductors c.s.a. 1.5 26 32 31 37 copper 2.5 34 42 41 48 (mm2) 4 44 54 53 63 6 56 67 66 80 10 74 90 87 104 16 96 116 113 136 25 123 148 144 173 35 147 178 174 208 50 174 211 206 247 70 216 261 254 304 95 256 308 301 360 120 290 351 343 410 150 328 397 387 463 185 367 445 434 518 240 424 514 501 598 300 480 581 565 677 c.s.a. 10 57 68 67 80 aluminium 16 74 88 87 104 2 (mm ) 25 94 114 111 133 35 114 137 134 160 50 134 161 160 188 70 167 200 197 233 95 197 237 234 275 120 224 270 266 314 150 254 304 300 359 185 285 343 337 398 240 328 396 388 458 300 371 447 440 520 table H1-24: case of a buried circuit: minimum c.s.a. in terms of type of conductor; type of insulation; and value of fictitious current I'z (I'z = Iz ). K Example This is a continuation of the previous example, for which the factors K4, K5, K6 and K7 were determined, and the factor K was found to be 0.48. Full load current 5.000 IB = = 22 A 230 Selection of protection A circuit-breaker rated at 25 A would be appropriate. Maximum permanent current permitted Iz = 25 A (i.e. the circuit-breaker rating In) Fictitious current Iz 25 I'z = = = 52.1 A K 0.48 C.s.a. of circuit conductors In the column PVC, 2 conductors, a current of 54 A corresponds to a 4 mm2 copper conductor. In the case where the circuit conductors are in aluminium, the same fictitious current (52 A) would require the choice of 10 mm2 corresponding to a fictitious current value (for aluminium) of 68 A.


θa = 20°C 5 kW 230 V

fig. H1-25: example for determination of the minimum c.s.a. of the circuit conductors.

3. determination of voltage drop

H1 The impedance of circuit conductors is low but not negligible: when carrying load current there is a fall in voltage between the origin of the circuit and the load terminals. The correct operation of an item of load (a motor; lighting circuit; etc.) depends on the voltage at its terminals being maintained at a value close to its rated value. It is necessary therefore to dimension the circuit conductors such, that at full-load current, the load terminal voltage is

maintained within the limits required for correct performance. This section deals with methods of determining voltage drops, in order to check that: c they conform to the particular standards and regulations in force, c they can be tolerated by the load, c they satisfy the essential operational requirements.

3.1 maximum voltage-drop limit Maximum allowable voltage-drop limits vary from one country to another. Typical values for LV installations are given below in table H1-26. maximum voltage-drop between the service-connection point and the point of utilization lighting other uses (heating and power) a low-voltage service connection from a LV 3% 5% public power distribution network consumers HV/LV substation supplied from 6% 8% a public distribution HV system table H1-26: maximum voltage-drop limits. These voltage-drop limits refer to normal steady-state operating conditions and do not apply at times of motor starting; simultaneous switching (by chance) of several loads, etc. as mentioned in Chapter B Sub-clause 4.3 (factor of simultaneity, etc.). When voltage drops exceed the values shown in table H1-26 larger cables (wires) must be used to correct the condition. The value of 8%, while permitted, can lead to problems for motor loads; for example: c in general, satisfactory motor performance requires a voltage within ± 5% of its rated nominal value in steady-state operation, c starting current of a motor can be 5 to 7 times its full-load value (or even higher). If 8% voltage drop occurs at full-load current, then a drop of 40% or more will occur during start-up. In such conditions the motor will either: v stall (i.e. remain stationary due to insufficient torque to overcome the load torque) with consequent over-heating and eventual trip-out, v or accelerate very slowly, so that the heavy current loading (with possibly undesirable low-voltage effects on other equipment) will continue beyond the normal start-up period; c finally an 8% voltage drop represents a continuous (E2/R watts) of power loss, which, for continuous loads will be a significant waste of (metered) energy. For these reasons it is recommended that the maximum value of

8% in steady operating conditions should not be reached on circuits which are sensitive to under-voltage problems. Important: In a number of countries the existing 220/380 V 3-phase systems are being uprated to operate eventually at nominal 230/400 V (the recommended IEC standard). Transformer manufacturers in these countries have recently increased the no-load secondary voltage of their distribution transformers accordingly, to 237/410 V. After several years of transition in the appliances industry, distribution transformers will be manufactured with no-load ratios of 242/420 V. The rated voltage of consumer appliances will evolve in the same time-scale. From now on, therefore, voltage-drop calculations must take account of these changes. Dangerous possible consequences for motors are: c a lightly-loaded "new" transformer and an "old" motor: risk of overvoltage on the motor, c a fully-loaded "old" transformer and a "new" motor: risk of undervoltage at the motor. Similar (but inverse) problems will arise in countries which presently operate 240/415 V systems, if the IEC 230/400 V standard is adopted by them.

HV consumer

LV consumer 8%(1) 5%(1)

load

(1) between the LV supply point and the load

fig. H1-27: maximum voltage drop. the protection of circuits - the switchgear - H1-17

3. determination of voltage drop (continued)

H1 3.2 calculation of voltage drops in steady load conditions use of formulae The table below gives formulae commonly used to calculate voltage drop in a given circuit per kilometre of length. If: IB: the full load current in amps L: length of the cable in kilometres R: resistance of the cable conductor in Ω/km 22,5 Ω.mm2/km R= for copper S (c.s.a. in mm2) 2 36 Ω.mm /km R= for aluminium S (c.s.a. in mm2) Note: R is negligible above a c.s.a. of 500 mm2.

X: inductive reactance of a conductor in Ω/km Note: X is negligible for conductors of c.s.a. less than 50 mm2. In the absence of any other information, take X as being equal to 0.08 Ω/km.

ϕ: phase angle between voltage and current in the circuit considered, generally: c lighting: cos ϕ = 1 c motor power: v at start-up: cos ϕ = 0.35 v in normal service: cos ϕ = 0.8 Un: phase-to-phase voltage. Vn: phase-to-neutral voltage. For prefabricated pre-wired ducts and bustrunking, resistance and inductive reactance values are given by the manufacturer.

single phase: phase/phase

voltage drop (∆ U) in volts ∆U = 2 IB (R cos ϕ + X sin ϕ) L

single phase: phase/neutral

∆U = 2 IB (R cos ϕ + X sin ϕ) L

balanced 3-phase: 3 phases (with or without neutral)

∆U = eIB (R cos ϕ + X sin ϕ) L

circuit

in % 100 ∆U Un 100 ∆U Vn 100 ∆U Un

table H1-28: voltage-drop formulae.

simplified table Calculations may be avoided by using the table H1-29 below, which gives, with an adequate approximation, the phase-to-phase voltage drop per km of cable per ampere, in terms of: c kinds of circuit use: motor circuits with cos ϕ close to 0.8, or lighting with a cos ϕ in the neighbourhood of unity; c of the type of cable; single-phase or 3-phase. Voltage drop in a cable is then given by: K x IB x L c.s.a. in mm2

Cu 1.5 2.5 4 6 10 16 25 35 50 70 95 120 150 185 240 300

Al

10 16 25 35 50 70 120 150 185 240 300 400 500

single-phase circuit motor power normal service cos ϕ = 0.8 24 14.4 9.1 6.1 3.7 2.36 1.5 1.15 0.86 0.64 0.48 0.39 0.33 0.29 0.24 0.21

lighting start-up cos ϕ = 0.35 10.6 6.4 4.1 2.9 1.7 1.15 0.75 0.6 0.47 0.37 0.30 0.26 0.24 0.22 0.2 0.19

cos ϕ = 1 30 18 11.2 7.5 4.5 2.8 1.8 1.29 0.95 0.64 0.47 0.37 0.30 0.24 0.19 0.15

balanced three-phase circuit motor power normal service start-up cos ϕ = 0.8 cos ϕ = 0.35 20 9.4 12 5.7 8 3.6 5.3 2.5 3.2 1.5 2.05 1 1.3 0.65 1 0.52 0.75 0.41 0.56 0.32 0.42 0.26 0.34 0.23 0.29 0.21 0.25 0.19 0.21 0.17 0.18 0.16

table H1-29: phase-to-phase voltage drop ∆U for a circuit, in volts per ampere per km.


K is given by the table, IB is the full-load current in amps, L is the length of cable in km. The column motor power cos ϕ = 0.35" of table H1-29 may be used to compute the voltage drop occurring during the start-up period of a motor (see example (1) after the table H1-29).

lighting cos ϕ = 1 25 15 9.5 6.2 3.6 2.4 1.5 1.1 0.77 0.55 0.4 0.31 0.27 0.2 0.16 0.13

H1 examples: Example 1 (figure H1-30) A three-phase 35 mm2 copper cable 50 metres long supplies a 400 V motor taking: c 100 A at a cos ϕ = 0.8 on normal permanent load; c 500 A (5 In) at a cos ϕ = 0.35 during start-up. The voltage drop at the origin of the motor cable in normal circumstances (i.e. with the distribution board of figure H1-30 distributing a total of 1.000 A) is 10 V phase-to-phase. What is the volt drop at the motor terminals: c in normal service? c during start-up? Solution: c voltage drop in normal service conditions: ∆U % = 100 ∆U/Un Table H1-29 shows 1 V/A/km so that: ∆U for the cable = 1 x 100 x 0.05 = 5 V ∆U total = 10 + 5 = 15 V = i.e. 15 x 100 = 3.75 % 400 This value is less than that authorized (8%) and is satisfactory. c voltage drop during motor start-up: ∆U cable = 0.52 x 500 x 0.05 = 13 V Owing to the additional current taken by the motor when starting, the volt drop at the distribution board will exceed 10 Volts. Supposing that the infeed to the distribution board during motor starting is 900 + 500 = 1.400 A then the volt-drop at the distribution board will increase approximately pro rata, i.e. 10 x 1400 = 14 V 1000 ∆U distribution board = 14 V ∆U for the motor cable = 13 V ∆U total = 13 + 14 = 27 V i.e. 27 x 100 = 6.75 % 400 a value which is satisfactory during motor starting. Example 2 A 3-phase 4-wire copper line of 70 mm2 c.s.a. and a length of 50 m passes a current of 150 A. The line supplies, among other loads, 3 single-phase lighting circuits, each of 2.5 mm2 c.s.a. copper 20 m long, and each passing 20 A. It is assumed that the currents in the 70 mm2 line are balanced and that the three lighting circuits are all connected to it at the same point. What is the voltage drop at the end of the lighting circuits? Solution: c voltage drop in the 4-wire line: ∆U % = 100 ∆U/Un Table H1-29 shows 0.55 V/A/km. ∆U line = 0.55 x 150 x 0.05 = 4.125 V phase-to-phase which: 4.125 V = 2.38 V phase to neutral. e c voltage drop in any one of the lighting single-phase circuits: ∆U for a single-phase circuit = 18 x 20 x 0.02 = 7.2 V The total volt-drop is therefore 7.2 + 2.38 = 9.6 V 9.6 V x 100 = 4.2 % 230 V This value is satisfactory, being less than the maximum permitted voltage drop of 6%.

1000 A

400 V

50 m / 35 mm2 Cu IB = 100 A (500 A during start-up)

fig. H1-30: example 1.

50 m / 70 mm2 Cu IB = 150 A

20 m / 2.5 mm2 Cu IB = 20 A



4. short-circuit current calculations

H1 knowing the levels of 3-phase symmetrical short-circuit currents (Isc) at different points in an installation is an essential feature of its design.

A knowledge of 3-phase symmetrical shortcircuit current values (Isc) at strategic points of an installation is necessary in order to dimension switchgear (fault current rating); cables (thermal withstand rating); protective devices (discriminative trip settings) and so on... In the following notes a 3-phase short-circuit of zero impedance (the so-called bolted short-circuit) fed through a typical HV/LV distribution transformer will be examined.

Except in very unusual circumstances, this type of fault is the most severe, and is certainly the simplest to calculate. Short-circuit currents occurring in a network supplied from an alternator and also in d.c. systems are dealt with in Chapter J Sub-clauses 1.1 and 6.1. The simplified calculations and practical rules which follow give conservative results of sufficient accuracy, in the large majority of cases, for installation design purposes.

4.1 short-circuit current at the secondary terminals of a HV/LV distribution transformer the case of one transformer

transformer rating

c as a first approximation the impedance of the HV system is assumed to be negligibly small, so that: In x 100 Isc = where Usc P x 103 In = and: eU20 P = kVA rating of the transformer, U20 = phase-to-phase secondary volts on open circuit, In = nominal current in amps Isc = short-circuit fault current in amps, Usc = short-circuit impedance voltage of the transformer in %. Typical values of Usc for distribution transformers are given in the table H1-32. Example: 400 kVA transformer, 242/420 V at no load Usc = 4% In = 400 x 103 = 550 A ÷ex 420 550 x 100 Isc = = 13.75 kA 4 c in practice Isc is slightly less than that calculated by this method, as shown in the following table (H1-33) since the HV system impedance is such that its fault level at the HV terminals of the transformer rarely exceeds 500 MVA. A level of 250 MVA, or less, is more common. transformer rated power (kVA) transformer current Ir (A) oil-immersed transformer Isc (kA) cast-resin transformer Isc (kA)

Psc = 250 MVA Psc = 500 MVA Psc = 250 MVA Psc = 500 MVA

50 69 1.71 1.71 1.14 1.14

100 137 3.40 3.42 2.28 2.28

160 220 5.41 5.45 3.63 3.65

250 344 8.38 8.49 5.63 5.68

315 433 10.5 10.7 7.07 7.14

400 550 13.2 13.5 8.93 9.04

50 to 630 800 to 2500

Usc in % type of transformer oil-immersed cast-resin 4% 6% 6% 6%

table H1-32: typical values of Usc for different kVA ratings of transformers with HV windings i 20 kV.

500 687 16.4 16.8 11.1 11.3

630 866 20.4 21.0 13.9 14.1

800 1100 17.4 17.9 17.4 17.9

1000 1375 21.5 22.2 21.5 22.2

1250 1718 26.4 27.5 26.4 27.5

1600 2199 33.1 34.8 33.1 34.8

2000 2749 40.4 43.0 40.4 43.0

2500 3437 49.1 52.9 49.1 52.9

tables H1-33: Isc at the LV terminals of 3-phase HV/LV transformers supplied from a HV system with a 3-phase fault level of 500 MVA, or 250 MVA.

the case of several transformers in parallel feeding a busbar The value of fault current on an outgoing circuit immediately downstream of the busbars (figure H1-34) can be estimated as the sum of the Isc from each transformer calculated separately. It is assumed that all transformers are supplied from the same HV network, in which case the values obtained from table H1-33 when added together will give a slightly higher fault-level value than would actually occur. Other factors which have not been taken into account are the impedance of the busbars and of the circuit breakers. The conservative fault-current value obtained however, is sufficiently accurate for basic installation design purposes.


The choice of circuit breakers and incorporated protective devices against short-circuit fault currents is described in Chapter H2 Sub-clause 4.4.

Isc1

Isc2

Isc3

Isc1 + Isc2 + Isc3

fig. H1-34.

H1 4.2 3-phase short-circuit current (Isc) at any point within a LV installation In a 3-phase installation Isc at any point is given by: Isc = U20 (amps) eZT U20 = phase-to-phase voltage of the opencircuited secondary windings of the powersupply transformer(s). ZT = total impedance per phase of the installation upstream of the fault location (in ohms).

method of calculating ZT Each component of an installation (HV network, transformer, cable, circuit breaker, busbar, and so on...) is characterized by its impedance Z, comprising an element of resistance (R) and an inductive reactance (X). It may be noted that capacitive reactances are not important in short-circuit current calculations. The parameters R, X and Z are expressed in ohms, and are related by the sides of a rightangled triangle, as shown in the impedance diagram of figure H1-35. The method consists in dividing the network into convenient sections, and to calculate the R and X values for each. Where sections are connected in series in the network all the resistive elements in the section are added arithmetically; likewise for the reactances, to give RT and XT. The impedance (Z) for the combined sections concerned is then calculated from

Z

ϕ

X

R

fig. H1-35: impedance diagram. R1 R2 X1 X2 or for reactances X3 = R1 + R2 X1 + X2 Combining two or more dissimilar circuits in parallel is (fortunately) seldom required in normal radial-type installation networks and will not be demonstrated in the main text. General methods for reducing impedances to a single equivalent impedance are given, however, in Appendix H1. R3 =

ZT = RT 2 + XT 2 Any two sections of the network which are connected in parallel, can, if, predominantly both resistive (or both inductive) be combined to give a single equivalent resistance (or reactance) as follows: Let R1 and R2 be the two resistances connected in parallel, then the equivalent resistance R3 wil be given by:

determination of the impedance of the HV network c network upstream of the HV/LV transformer (table H1-36) The 3-phase short-circuit fault level in kA or in MVA* is given by the power supply authority concerned, from which an equivalent impedance can be deduced. A formula which makes this deduction and at the same time converts the impedance to an equivalent value at LV is given, as follows Uo2 Zs = where: Psc Zs = impedance of the HV voltage network, expessed in milli-ohms, Uo = phase-to-phase no-load LV voltage, expressed in volts. Psc 250 MVA 500 MVA

Uo (V) 420 420

Psc = HV 3-phase short-circuit fault level, expressed in kVA. *Short-circuit MVA: eEL Isc where: EL = phase-to-phase nominal system voltage expressed in kV (r.m.s.). Isc = 3-phase short-circuit current expressed in kA (r.m.s.).

The upstream (HV) resistance Ra is generally found to be negligible compared with the corresponding Xa, the latter then being taken as the ohmic value for Za. If more accurate calculations are necessary, Ra may be taken to be equal to 0.15 Xa. Table H1-36 gives values for Ra and Xa corresponding to the most common HV* short-circuit levels in public power-supply networks, namely, 250 MVA and 500 MVA. * up to 36 kV

Ra (mΩ) 0.106 0.053

Xa (mΩ) 0.71 0.353

table H1-36: the impedance of the HV network referred to the LV side of the HV/LV transformer.


4. short-circuit current calculations (continued)

H1 4.2. 3-phase short-circuit current (Isc) at any point within a LV installation (continued) c transformers (table H1-37) The impedance Ztr of a transformer, viewed from the LV terminals, is given by the formula: U22O Usc Ztr = x milli-ohms where: Pn 100 U2 o = open-circuit secondary phase-tophase voltage expressed in volts. Pn = rating of the transformer (in kVA). Usc = the short-circuit impedance voltage of the transformer expressed in %. The transformer windings resistance Rtr can be derived from the total losses as follows:

transformer rated power oil-immersed transformer Usc Rtr Xtr Ztr cast-resin transformer Usc Rtr Xtr Ztr

kVA % mΩ mΩ mΩ % mΩ mΩ mΩ

50 4 95.3 104.1 141.1

100 4 37.9 59.5 70.5 6 33.5 100.4 105.8

160 4 16.2 41.0 44.1 6 18.6 63.5 66.2

250 4 9.2 26.7 28.2 6 10.7 41.0 42.4

315 4 6.9 21.3 22.4 6 8.2 32.6 33.6

400 4 5.1 16.9 17.7 6 6.1 25.8 26.5

500 4 3.9 13.6 14.1 6 4.6 20.7 21.2

630 4 2.9 10.8 11.2 6 3.5 16.4 16.8

Pcu = 3 In2 Rtr so that Rtr =

Pcu x 103 3 In2

in milli-ohms where Pcu = total losses in watts. In = nominal full-load current in amps. Rtr = resistance of one phase of the transformer in milli-ohms (the LV and corresponding HV winding for one LV phase are included in this resistance value).

Xtr = Ztr 2 − Rtr 2 For an approximate calculation Rtr may be ignored since X ≈ Z in standard distributiontype transformers. 800 6 2.9 12.9 13.2 6 2.6 13.0 13.3

1000 6 2.3 10.3 10.6 6 1.9 10.4 10.6

1250 6 1.8 8.3 8.5 6 1.5 8.3 8.4

1600 6 1.4 6.5 6.6 6 1.1 6.5 6.6

2000 6 1.1 5.2 5.3 6 0.8 5.2 5.3

2500 6 0.9 4.1 4.2 6 0.6 4.2 4.2

table H1-37: resistance, reactance and impedance values for typical distribution transformers with HV windings i 20 kV. c circuit breakers In LV circuits, the impedance of circuit breakers upstream of the fault location must be taken into account. The reactance value conventionally assumed is 0.15 mΩ per CB, while the resistance is neglected. c busbars The resistance of busbars is generally negligible, so that the impedance is practically all reactive, and amounts to approximately 0.15 mΩ/metre* length for LV busbars (doubling the spacing between the bars increases the reactance by about 10% only). * for 50 Hz systems, but 0.18 mΩ/metre length at 60 Hz.

c circuit conductors The resistance of a conductor is given by the formula: ρx L Rc = where S ρ = the resistivity constant of the conductor material at the normal operating temperature being: 22.5 mΩ.mm2/m for copper, 36 mΩ.mm2/m for aluminium, S = c.s.a. of conductor in mm2. Cable reactance values can be obtained from the manufacturers. For c.s.a. of less than 50 mm2 reactance may be ignored. In the absence of other information, a value of 0.08 mΩ/metre may be used (for 50 Hz systems) or 0.096 mΩ/metre (for 60 Hz systems). For prefabricated bus-trunking and similar pre-wired ducting systems, the manufacturer should be consulted.


c motors At the instant of short-circuit, a running motor will act (for a brief period) as a generator, and feed current into the fault. In general, this fault-current contribution may be ignored. However, for more precise calculation, particularly in the case of large motors and/or numerous smaller motors, the total contribution can be estimated from the formula: Iscm = 3.5 In from each motor i.e. 3.5 m In for m similar motors operating concurrently. The motors concerned will be the 3-phase motors only; single-phase-motor contribution being insignificant. For HV circuit breakers, the contribution from motors is often reduced to very low values at the instant of contact separation, and for that reason is frequently ignored, but with lowinertia high-speed LV CBs* the value given in the above formula is recommended. Note: for circuit breaker fault-current makingduty however, no account can be taken of diminution of fault current contribution, and each running motor will initially feed current into the fault at a level approaching its own starting current, i.e. 4 or 5 In. * and fuses

c fault-arc resistance Short-circuit faults generally form an arc which has the properties of resistance. The resistance is not stable and its average value is low, but at low voltage this resistance is sufficient to reduce the fault-current to some extent. Experience has shown that a reduction of the order of 20% may be expected. This phenomenon will effectively ease the current-breaking duty of a CB, but affords no relief for its fault-current making duty.

H1 recapitulation table system elements considered supply network table H1-33

resistance R in milli-ohms Ra ≈ 0,15 Xa R can therefore be neglected in comparison with X 3 Rtr = Pcu x 10 3In2 Rtr = is often negligible compared to Xtr for transformers > 100 kVA negligible negligible for S > 200 mm2 in the formula below: R = ρ L (1) S R = ρ L (1) S see Sub-clause 4.2 "motors" (often negligible at LV)) U2o Isc = 2 2 3 RT + XT

transformer table H1-34

circuit breaker busbars

circuit conductors (2) motors

M

three-phase short-circuit current in kA

reactance X in milli-ohms U22 0 Psc

Xa ≈ Za =

Ztr 2 − Rtr 2 U2 Usc avec Ztr = 2 0 x Pn 100 XD = 0.15 mΩ/pole XB = 0.15 mΩ/m

cables : Xc = 0.08 mΩ/m

table H1-38: recapitulation table of impedances for different parts of a power-supply system. U20: phase-to-phase no-load secondary voltage of HV/LV transformer (in volts). Psc: 3-phase short-circuit power at HV terminals of the HV/LV transformers (in kVA). Pcu: 3-phase total losses of the HV/LV transformer (in watts). Pn: rating of the HV/LV transformer (in kVA). Usc: short-circuit impedance voltage of the HV/LV transfomer (in %). R (mΩ)

X (mΩ)

RT (mΩ)

RT : total resistance. XT: total reactance (1) ρ = resistivity at normal temperature of conductors in service ρ = 22.5 milli-ohms x mm2/metre for copper ρ = 36 milli-ohms x mm2/metre for aluminium. (2) If there are several conductors in parallel per phase, then divide the resistance of one conductor by the number of conductors. The reactance remains practically unchanged. XT (mΩ)

Isc = 3

420 2 2 RT + XT

HV network 0.053 0.353 Psc = 500 MVA transformer 2.35 10.34 20 kV/420 V Pn = 1000 kVA Usc = 6% Pcu = 13.3 x 103 watts single-core cables Xc = 0.08 x 5 2.523 11.1 Rc = 22.5 x 5 5 m copper 4 240 4 x 240 mm2/phase = 0.12 = 0.40 Isc1 = 21.3 kA main RD = 0 XD = 0.15 circuit breaker busbars RB = 0 XB = 1.5 2.523 12.75 Isc2 = 18.6 kA 10 m three-core cable Xc = 100 x 0.08 Rc = 22.5 x 100 100 m 95 95 mm2 copper = 23.68 =8 26.2 20.75 Isc3 = 7.24 kA three-core cable Xc = 20 x 0.08 Rc = 22.5 x 20 20 m 10 10 mm2 copper = 45 = 1.6 71.2 22.35 Isc4 = 3.24 kA final circuits table H1-39: example of short-circuit current calculations for a LV installation supplied at 400 V (nominal) from a 1,000 kVA HV/LV transformer.

4.3 Isc at the receiving end of a feeder in terms of the Isc at its sending end The following tables, derived by the "method of composition" (mentioned in Chapter G Sub-clause 5.2) give a rapid and sufficiently accurate value of short-circuit current at a point in a network, knowing: c the value of short-circuit current upstream of the point considered c the length and composition of the circuit between the point at which the short-circuit current level is known, and the point at which the level is to be determined. It is then sufficient to select a circuit breaker with an appropriate short-circuit fault rating immediately above that indicated in the tables.

If more precise values are required, it is possible to make a detailled calculation (see Sub-Clause 4.2 above) or to use a software package, such as Ecodial*. In such a case, moreover, the possibility of using the cascading technique should be considered, in which the use of a currentlimiting circuit breaker at the upstream position would allow all circuit breakers downstream of the limiter to have a shortcircuit-current rating much lower than would otherwise be necessary (see Chapter H2 Sub-clause 4.5). * a Merlin Gerin product (see Chapter B, Clause 1, Methodology).


4. short-circuit current calculations (continued)

H1 4.3 Isc at the receiving end of a feeder in terms of the Isc at its sending end (continued) copper 230 V / 400 V

c.s.a. of length of circuit (in metres) phase conductors (in mm2) 1.5 0.8 1 1.3 1.6 3 2.5 1 1.3 1.6 2.1 2.6 5 4 0.8 1.7 2.1 2.5 3.5 4 8.5 6 1.3 2.5 3 4 5 6.5 13 10 0.8 1.1 2.1 4 5.5 6.5 8.5 11 21 16 0.9 1 1.4 1.7 3.5 7 8.5 10 14 17 34 25 1 1.3 1.6 2.1 2.6 5 10 13 16 21 26 50 35 1.5 1.9 2.2 3 3.5 7.5 15 19 22 30 37 75 50 1.1 2.1 2.7 3 4 5.5 11 21 27 32 40 55 110 70 1.5 3 3.5 4.5 6 7.5 15 30 37 44 60 75 150 95 0.9 1 2 4 5 6 8 10 20 40 50 60 80 100 200 120 0.9 1 1.1 1.3 2.5 5 6.5 7.5 10 13 25 50 65 75 100 130 250 150 0.8 1 1.1 1.2 1.4 2.7 5.5 7 8 11 14 27 55 70 80 110 140 270 185 1 1.1 1.3 1.5 1.6 3 6.5 8 9.5 13 16 32 65 80 95 130 160 320 240 1.2 1.4 1.6 1.8 2 4 8 10 12 16 20 40 80 100 120 160 200 400 300 1.5 1.7 1.9 2.2 2.4 5 9.5 12 15 19 24 49 95 120 150 190 240 2 x 120 1.5 1.8 2 2.3 2.5 5.1 10 13 15 20 25 50 100 130 150 200 250 2 x 150 1.7 1.9 2.2 2.5 2.8 5.5 11 14 17 22 28 55 110 140 170 220 280 2 x 185 2 2.3 2.6 2.9 3.5 6.5 13 16 20 26 33 65 130 160 200 260 330 3 x 120 2.3 2.7 3 3.5 4 7.5 15 19 23 30 38 75 150 190 230 300 380 3 x 150 2.5 2.9 3.5 3.5 4 8 16 21 25 33 41 80 160 210 250 330 410 3 x 185 2.9 3.5 4 4.5 5 9.5 20 24 29 39 49 95 190 240 290 390 Isc upstream Isc downstream (in kA) (in Ka) 100 94 94 93 92 91 83 71 67 63 56 50 33 20 17 14 11 9 5 90 85 85 84 83 83 76 66 62 58 52 47 32 20 16 14 11 9 4.5 80 76 76 75 75 74 69 61 57 54 49 44 31 19 16 14 11 9 4.5 70 67 67 66 66 65 61 55 52 49 45 41 29 18 16 14 11 5 4.5 60 58 58 57 57 57 54 48 46 44 41 38 27 18 15 13 10 8.5 4.5 50 49 48 48 48 48 46 42 40 39 36 33 25 17 14 13 10 8.5 4.5 40 39 39 39 39 39 37 35 33 32 30 29 22 15 13 12 9.5 8 4.5 35 34 34 34 34 34 33 31 30 29 27 26 21 15 13 11 9 8 4.5 30 30 29 29 29 29 28 27 26 25 24 23 19 14 12 11 9 7.5 4.5 25 25 25 25 24 24 24 23 22 22 21 20 17 13 11 10 8.5 7 4 20 20 20 20 20 20 19 19 18 18 17 17 14 11 10 9 7.5 6.5 4 15 15 15 15 15 15 15 14 14 14 13 13 12 9.5 8.5 8 7 6 4 10 10 10 10 10 10 10 9.5 9.5 9.5 9.5 9 8.5 7 6.5 6.5 5.5 5 3.5 7 7 7 7 7 7 7 7 7 6.5 6.5 6.5 6 5.5 5 5 4.5 4 2.9 5 5 5 5 5 5 5 5 5 5 5 5 4.5 4 4 4 3.5 3.5 2.5 4 4 4 4 4 4 4 4 4 4 4 4 3.5 3.5 3.5 3 3 2.9 2.2 3 3 3 3 3 3 3 3 3 2.9 2.9 2.9 2.8 2.7 2.6 2.5 2.4 2.3 1.9 2 2 2 2 2 2 2 2 2 2 2 2 1.9 1.9 1.8 1.8 1.7 1.7 1.4 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0.9 0.9 0.9 0.8 aluminium c.s.a. of length of circuit (in metres) 230 V / phase 400 V conductors (in mm2) 2.5 0.8 1 1.3 1.6 3 4 1 1.3 1.6 2.1 2.6 5 6 0.8 1.6 2 2.4 3 4 8 10 1.3 2.6 3.5 4 5.5 6.5 13 16 0.8 1.1 2.1 4 5.5 6.5 8.5 11 21 25 0.8 1 1.3 1.7 3.5 6.5 8.5 10 13 17 33 35 0.9 1.2 1.4 1.8 2.3 4.5 9 12 14 18 23 46 50 1.3 1.7 2 2.6 3.5 6.5 13 17 20 26 33 65 70 0.9 1.8 2.3 2.8 3.5 4.5 9 18 23 28 37 46 90 95 1.3 2.5 3 4 5 6.5 13 25 32 38 50 65 130 120 0.8 1.7 3 4 4.5 6.5 8 17 32 40 47 65 80 160 150 0.9 1.7 3.5 4.5 5 7 8.5 17 34 43 50 70 85 170 185 0.9 1 2 4 5 6 8 10 20 40 50 60 80 100 240 240 0.9 1 1.1 1.3 2.5 5 6.5 7.5 10 13 25 50 65 75 100 130 250 300 0.9 1 1.2 1.4 1.5 3 6 7.5 9 12 15 30 60 75 90 120 150 300 2 x 120 0.9 1.1 1.3 1.4 1.6 3 6.5 8 9.5 13 16 32 65 80 95 130 160 320 2 x 150 1 1.2 1.4 1.5 1.7 3.5 7 9 10 14 17 35 70 85 100 140 170 2 x 185 1.2 1.4 1.6 1.8 2 4.1 8 10 12 16 20 41 80 100 120 160 200 2 x 240 1.5 1.8 2 2.3 2.5 5 10 13 15 20 25 50 100 130 150 200 250 3 x 120 1.4 1.7 1.9 2.1 2.4 4.5 9.5 12 14 19 24 48 95 120 140 190 240 3 x 150 1.5 1.8 2.1 2.3 2.6 5 10 13 15 21 26 50 100 130 150 210 260 3 x 185 1.8 2.1 2.4 2.7 3 6 12 15 18 24 30 60 120 150 180 240 300 3 x 240 2.3 2.7 3 3.5 4 7.5 15 19 23 30 38 75 150 190 230 300 380 table H1-40: Isc at a point downstream, in terms of a known upstream fault-current value and the length and c.s.a. of the intervening conductors, in a 230/400 V 3-phase system. Note: for a 3-phase system having 230 V between phases, divide the above lengths by e= 1.732


H1 Example: The network shown in figure H1-41 typifies a case for the application of table H1-40. Select the c.s.a. of the conductor in the column for copper conductors (in this example the c.s.a. is 50 mm2). Search along the row corresponding to 50 mm2 for the length of conductor equal to that of the circuit concerned (or the nearest possible on the low side). Descend vertically the column in which the length is located, and stop at a row in the middle section (of the 3 sections of the table) corresponding to the known fault-current level (or the nearest to it on the high side). In this case 30 kA is the nearest to 28 kA on the high side. The value of short-circuit current at the downstream end of the 11 metre circuit is given at the intersection of the vertical column in which the length is located, and the horizontal row corresponding to the upstream Isc (or nearest to it on the high side). This value in the example is seen to be 19 kA. The procedure for aluminium conductors is similar, but the vertical column must be ascended into the middle section of the table. In consequence, a DIN-rail-mounted circuit breaker rated at 63 A and Isc of 50 kA (such as a NC100LH unit*) can be used for the 55 A circuit in figure H1-41. A Compact* rated at 260 A with an Isc capacity of 25 kA (such as a NS160N unit*) can be used to protect the 160 A circuit.

400 V Icc = 28 kA

50 mm2, Cu 11 m Icc = ?

IB = 55 A

IB = 160 A

fig. H1-41: determination of downstream short-circuit current level Isc using table H1-40.

* Merlin Gerin product.

4.4 short-circuit current supplied by an alternator or an inverter Please refer to Chapter J.


5. particular cases of short-circuit current

H1 5.1 calculation of minimum levels of short-circuit current if a protective device in a circuit is intended only to protect against short-circuit faults, it is essential that it will operate with certainty at the lowest possible level of short-circuit current that can occur on the circuit.

In general, on LV circuits, a single protective device protects against all levels of current, from the overload threshold through the maximum rated short-circuit current-breaking capability of the device.

In certain cases, however, overload protective devices and separate short-circuit protective devices are used.

examples of such arrangements Figures H1-42 to H1-44 show some common arrangements where overload and shortcircuit protections are effected by separate devices. As shown in figures H1-42 and H1-43, the most common circuits using separate devices control and protect motors. Figure H1-44 constitutes a derogation in the basic protection rules, and is generally used on circuits of prefabricated bustrunking, lighting rails, etc.

aM fuses (no protection against overload)

load-breaking contactor with thermal overload relay

fig. H1-42: circuit protected by aM fuses. circuit breaker with instantaneous magnetic short-circuit protective relay only

load-breaking contactor with thermal overload relay

fig. H1-43: circuit protected by circuit breaker without thermal overload relay. circuit breaker D

S1

S2 < S1 load with incorporated overload protection

fig. H1-44: circuit breaker D provides protection against short-circuit faults as far as and including the load.


H1 it is necessary that the protective device instantaneous-trip setting c Im < Isc (min) for protection by a circuit breaker; or fusion current c Ia < Isc (min) for protection by fuses.

conditions to be respected

t

The protective device must therefore satisfy the two following conditions: c its fault-current breaking rating > Isc the 3-phase short-circuit current at its point of installation, c elimination of the minimum short-circuit current possible in the circuit, in a time tc compatible with the thermal constraints of the circuit conductors, where: K2 S2 tc = (tc < 5 seconds) Isc (min) Comparison of the tripping or fusing performance curve of protective devices, with the limit curves of thermal constraint for a conductor shows that this condition is satisfied if: c Isc (min) > Im (instantaneous or short timedelay circuit-breaker trip setting current level). See figure H1-45. c Isc (min) > Ia for protection by fuses. The value of the current Ia corresponds to the crossing point of the fuse curve and the cable thermal withstand curve (figures H1-46 and H1-47).

t=

k2 S2 I2

I

Im

fig. H1-45: protection by circuit breaker. t

t=

k2 S2 I2

Ia

I

fig. H1-46: protection by aM-type fuses. t

t=

Ia

k2 S2 I2

I

fig. H1-47: protection by gl-type fuses.

in practice this means that the length of circuit downstream of the protective device must not exceed a calculated maximum length: Lmax = 0.8 x U x Sph 2 x ρ x Im

practical method of calculating Lmax The limiting effect of the impedance of long circuit conductors on the value of short-circuit currents must be checked and the length of a circuit must be restricted accordingly. The method of calculating the maximum permitted length has already been demonstrated in TN- and IT- earthed schemes for single and double earth faults, respectively (see Chapter G Sub-clauses 5.2 and 6.2). Two cases are considered below: 1. calculation of Lmax for a 3-phase 3-wire circuit The minimum short-circuit current will occur when two phase wires are short circuited at the remote end of the circuit.

* or for aluminium according to conductor material ** the high value for resistivity is due to the elevated temperature of the conductor when passing short-circuit current.

P 0.8 U

i

where ρ = resistivity of copper* at the average temperature during a short-circuit, and Sph : c.s.a. of a phase conductor in mm2 L = length in metres In order that the cable will not be damaged by heat Isc u Im 2L Im 0.8 U u ρ or Sph 0.8 U Sph Lmax = 2 ρ Im with: U = 400 V ρ = 1.5 x 0.018 = 0.027 Ω.mm2/m**. Im = magnetic trip current setting for the CB Lmax = maximum circuit length in metres. 5.926 Sph Lmax = Im

L

load

f Using the "conventional method", the voltage at the point of protection P is assumed to be 80% of the nominal voltage during a shortcircuit fault, so that 0.8 U = Isc Zd, where: Zd = impedance of the fault loop Isc = short-circuit current (ph/ph) U = phase-to-phase nominal voltage. For cables i 120 mm2, reactance may be neglected, so that 2L Zd = ρ Sph the protection of circuits - the switchgear - H1-27

5. particular cases of short-circuit current (continued)

H1 5.1 calculation of minimum levels of short-circuit current (continued) 2. calculation of Lmax for a 3-phase 4-wire 230/400 V circuit The minimum Isc will occur when the shortcircuit is between a phase conductor and the neutral. A calculation similar to that of example 1 above is required, but using the following formulae (for cable i 120 mm2 (1)). c where Sn for the neutral conductor = Sph for the phase conductor Sph Lmax = 3,421 Im c If Sn for the neutral conductor < Sph, then 6,842 Sph Sph Lmax = where m = (1+m) Im Sn (1) for larger c.s.a.'s, the resistance calculated for the conductors must be increased to account for the non-uniform

current density in the conductor (due to "skin" and "proximity" effects, previously noted in Chapter G Sub-clause 5.2). Suitable values (taken from French standard NF 15-100) are as follows: 150 mm2 : R + 15% 185 mm2 : R + 20% 240 mm2 : R + 25% 300 mm2 : R + 30% where R is the value calculated from ρ 2L R= Sph For larger c.s.a.'s than those listed, reactance values must be combined with those of resistance to give an impedance. Reactance may be taken as 0.08mΩ/metre for cables (at 50 Hz). At 60Hz the constant is 0.096 mΩ/metre.

tabulated values for Lmax Table H1-49 below gives maximum circuit lengths (Lmax) in metres, for: c 3-phase 3-wire 400 V circuits (i.e. without neutral) and c 1-phase 2-wire 400 V circuits, without neutral, protected by general-purpose circuit breakers. In other cases, apply correction factors (given in table H1-53) to the lengths obtained.

The calculations are based on the foregoing methods, with Im = 1.2 Irm Irm = regulated short-circuit current trip setting. Irm is guaranteed to be within ± 20% of the regulated value; hence the worst-case factor of 1.2 (i.e. 120%). Refer to Chapter H2 Sub-clause 4.2 for details of regulation of circuit-breaker protective elements.

operating current c.s.a. (nominal cross-sectional-area) of conductors (in mm2) level Im of the instantaneous magnetic tripping element (in A) 1.5 2.5 4 6 10 16 25 35 50 70 95 120 150 185 50 148 246 394 63 117 195 313 470 80 92 154 246 370 100 74 123 197 296 493 125 59 99 158 237 395 160 46 77 123 185 308 494 200 37 62 99 148 247 395 250 30 49 79 118 197 316 494 320 23 38 62 92 154 247 386 400 18 31 49 74 123 197 308 432 500 15 25 39 59 99 158 247 345 494 560 13 22 35 53 88 141 220 308 441 630 12 19 31 47 78 125 196 274 392 700 11 18 28 42 70 113 176 247 353 494 800 9 15 25 37 61 98 154 215 308 432 875 8 14 22 34 56 90 141 197 282 395 1000 7 12 20 30 49 79 123 173 247 345 469 1120 6 11 17 26 44 70 110 154 220 308 419 1250 6 10 16 24 39 63 99 138 197 276 375 474 1600 7 12 18 31 49 77 108 154 216 293 370 532 2000 6 10 15 24 39 62 86 123 173 234 296 425 570 2500 8 12 20 31 49 69 99 138 188 237 340 438 3200 6 9 15 25 38 54 77 108 146 185 265 340 4000 7 12 20 31 43 62 86 117 148 212 273 5000 6 10 16 25 34 49 69 94 118 170 218 6300 8 12 20 27 39 55 74 94 134 175 8000 6 10 15 21 31 43 59 74 105 136 10000 8 12 17 25 35 47 59 85 109 12500 6 10 14 20 28 37 47 67 87 table H1-49: maximum circuit lengths in metres for copper conductors (for aluminium, the lengths must be multiplied by 0.62).


240

592 462 370 296 235 185 147 118

H1 Tables H1-50 to H1-52 below give maximum circuit length (Lmax) in metres for: c 3-phase 3-wire 400 V circuits (i.e. without neutral) and c 1-phase 2-wire 400 V circuits, without neutral, protected in both cases by domestic-type circuit breakers or with circuit breakers having similar tripping/current characteristics. In other cases, apply correction factors to the lengths indicated. These factors are given in table H1-53. The calculations are carried out

according to the method described above, again, with Im = 1.2 Irm as previously noted. These circuit breakers have fixed overload (thermal) tripping elements and fixed shortcircuit (magnetic) tripping elements. Categories B, C and D differ only in the levels of short-circuit-current trip setting Im. IEC 898 is the relevant international standard for these circuit breakers. See also table H2-28 for tripping ranges.

rated current cross-sectional-area (c.s.a.) of conductors (in mm2) of circuit breakers (in A) 1.5 2.5 4 6 10 16 25 35 50 6 296 494 790 10 178 296 474 711 13 137 228 385 547 912 16 111 185 296 444 741 20 89 148 237 356 593 948 25 71 119 190 284 474 759 32 56 93 148 222 370 593 926 40 44 74 119 178 296 474 741 50 36 59 95 142 237 379 593 830 63 28 47 75 113 188 301 470 658 854 80 22 37 59 89 148 237 370 519 704 100 18 30 47 71 119 190 296 415 563 125 14 24 38 57 95 152 237 331 450 table H1-50: maximum length of copper-conductored circuits in metres protected by B-type circuit breakers. rated current cross-sectional-area (c.s.a.) of conductors (in mm2) of circuit breakers (in A) 1.5 2.5 4 6 10 16 25 35 50 6 148 247 395 593 988 10 89 148 237 356 593 948 13 68 114 182 274 456 729 16 56 93 148 222 370 593 926 20 44 74 119 178 296 474 741 25 36 59 95 142 237 379 593 830 32 28 46 74 111 185 296 463 648 880 40 22 37 59 89 148 237 370 519 704 50 18 30 47 71 119 190 296 415 563 63 14 24 38 56 94 150 235 329 446 80 11 19 30 44 74 119 185 259 351 100 9 15 24 36 59 95 148 207 281 125 7 12 19 28 47 76 119 166 225 table H1-51: maximum length of copper-conductored circuits in metres protected by C-type circuit breakers. rated current cross-sectional-area (c.s.a.) of conductors (in mm2) of circuit breakers (in A) 1.5 2.5 4 6 10 16 25 35 50 6 105 176 283 423 706 1129 10 63 105 170 254 423 639 1058 13 48 81 130 195 325 521 814 1140 16 40 65 105 158 264 422 661 925 1255 20 32 52 84 126 211 337 528 740 1004 25 25 41 67 101 169 270 423 592 803 32 20 32 52 79 132 211 330 462 627 40 16 26 42 63 105 168 264 370 502 50 12 20 33 50 84 135 211 296 401 63 10 16 26 40 67 107 167 234 318 80 8 13 21 31 52 84 132 185 251 100 6 10 16 25 42 67 105 148 200 125 5 8 13 20 33 54 84 118 160 table H1-52: maximum length of copper-conductored circuits in metres protected by D-type circuit breakers (Merlin Gerin). Note: IEC 898 provides for an upper shortcircuit-current tripping range of 10-50 In for type D circuit breakers. European standards, and the above table H1-52 however, are

based on a range of 10-20 In, a range which covers the vast majority of domestic and similar installations.


5. particular cases of short-circuit current (continued)

H1 5.1 calculation of minimum levels of short-circuit current (continued) circuit details 3-phase 3-wire 400 V circuit 3-phase 4-wire 230/400 V circuit

Sph =1 S neutral or 1 ph 2-wire 400V circuit (no neutral) or 2 ph 3-wire 230/400 V circuit (i.e. with neutral)

Sph =2 S neutral 1

0.58

0.39 (1)

1-phase 2-wire 0.58 (phase and neutral) 230 V circuit table H1-53: correction factors to apply to lengths obtained from tables H1-49 to H1-52. (1) 0.77 for the c.s.a. of the neutral conductor.

examples Example 1 In a 3-phase 3-wire installation the protection is provided by a 250 A industrial-type circuit breaker, the instantaneous short-circuitcurrent trip setting of which, is set at 2,000 A (accuracy of ± 20%), i.e. in the worst case would require 2,000 x 1,2 = 2,400 A to trip. The cable c.s.a. = 120 mm2 and the conductor material is copper. In table H1-49, the row Im = 2,000 A crosses the column c.s.a. = 120 mm2 at the value for Lmax of 296 m. The circuit breaker protects the cable against short-circuit faults, therefore, provided that its length does not exceed 296 metres. Example 2 In a single-phase 230 V (phase to neutral) system, the protection is provided by a circuit breaker with an instantaneous short-circuitcurrent trip setting of 500 A (± 20%), i.e. a worst case of 600 A to be certain of tripping. The cable c.s.a. = 10 mm2 and the conductor material is copper. In table H1-49 the row Im = 500 A crosses the column c.s.a. = 10 mm2 at the value for Lmax of 99 m. Being a 230 V single-phase circuit, a correction factor from table H1-53 must be applied. This factor is seen to be 0.58. The circuit breaker will therefore protect the cable against short-circuit current, provided that its length does not exceed 99 x 0.58 = 57 metres.


H1 5.2 verification of the withstand capabilities of cables under short-circuit conditions in general verification of the thermal-withstand capability of a cable is not necessary, except in cases where cables of small c.s.a. are installed close to, or feeding directly from, the main general distribution board.

thermal constraints When the duration of short-circuit current is brief (several tenths of a second up to five seconds maximum) all of the heat produced is assumed to remain in the conductor, causing its temperature to rise. The heating process is said to be adiabatic, an assumption that simplifies the calculation and gives a pessimistic result, i.e. a higher conductor temperature than that which would actually occur, since in practice, some heat would leave the conductor and pass into the insulation. For a period of 5 seconds or less, the relationship I2t = k2 S2 characterizes the time in seconds during which a conductor of c.s.a. S (in mm2) can be allowed to carry a current I amps, before its temperature reaches a level which would damage the surrounding insulation.

The factor k2 is given in table H1-54 below, and is abstracted from the French standards NF C 15-100. The method of verification consists in checking that the thermal energy I2t per ohm of conductor material, allowed to pass by the protecting circuit-breaker (from manufacturers catalogues) is less than that permitted for the particular conductor (as given in table H1-55 below).

insulation

conductor conductor copper (Cu) aluminium (Al) PVC 13225 5776 PR 20449 8836 table H1-54: value of the constant k2. S (mm2)

PVC XLPE copper aluminium copper k 115 76 143 k2 13225 5776 20449 1.5 0.0297 0.0130 0.0460 2.5 0.0826 0.0361 0.1278 4 0.2116 0.0924 0.3272 6 0.4761 0.2079 0.7362 10 1.3225 0.5776 2.0450 16 3.3856 1.4786 5.2350 25 8.2656 3.6100 12.7806 35 16.2006 7.0756 25.0500 50 29.839 13.032 46.133 table H1-55: maximum allowable thermal stress for cables (expressed in amperes2 x seconds x 106).

aluminium 94 8836 0.0199 0.0552 0.1414 0.3181 0.8836 2.2620 5.5225 10.8241 19.936

Example: Is a copper-cored XLPE cable of 4 mm2 c.s.a. adequately protected by a C60N circuit breaker (Merlin Gerin)? The above table shows that the I2t value for the cable is 0.3272 x 106, while the maximum "let-through" value by the circuit breaker, as given in the manufacturer's catalogue, is considerably less, and amounts to 0.094 x 106 ampere2-seconds. The cable is therefore adequately protected by the circuit breaker up to its full rated breaking capability.

electrodynamic constraints For bus-trunking and other kinds of prefabricated pre-conductored channels, rails, etc. it is necessary to verify that the electrodynamic withstand performance when carrying short-circuit currents is satisfactory. The peak value of current, limited by the circuit breaker or fuse, must be less than that for which the pre-conductored system is rated. Tables of coordination ensuring adequate protection of their products are generally published by the manufacturers of such systems.


6. protective earthing conductors (PE)

H1 6.1 connection and choice connection, choice and dimensioning of PE conductors (extracted from IEC standards and the French standard NF C 15-100)

Protective (PE) conductors provide the bonding connection between all exposed and extraneous conductive parts of an installation, to create the main equipotential bonding system. These conductors conduct fault current due to insulation failure (between a phase conductor and an exposed conductive part) to the earthed neutral of the source. P.E. conductors are connected to the main earthing terminal of the installation. The main earthing terminal is connected to the earthing electrode (see Chapter F) by the earthing conductor (grounding electrode conductor in USA). PE conductors must be: c insulated and coloured yellow and green (stripes); c be protected against mechanical and chemical damage. In IT and TN-earthed schemes it is strongly recommended that PE conductors should be installed in close proximity (i.e. in the same conduits, on the same cable tray, etc.) as the live cables of the related circuit. This arrangement ensures the minimum possible inductive reactance in the earth-fault currentcarrying circuits.

connection PE conductors must: c not include any means of breaking the continuity of the circuit (such as a switch, removable links, etc.); c connect exposed conductive parts individually to the main PE conductor, i.e. in parallel, not in series, as shown in figure H1-56; c have an individual terminal on common earthing bars in distribution boards. TT scheme The PE conductor need not necessarily be installed in close proximity to the live conductors of the corresponding circuit, since high values of earth-fault current are not needed to operate the RCD-type of protection used in TT installations. IT and TN schemes The PE or PEN conductor, as previously noted, must be installed as close by as possible to the corresponding live conductors of the circuit and no ferro-magnetic material must be interposed between them. A PEN conductor must always be connected directly to the earth terminal of an appliance, with a looped connection from the earth terminal to the neutral terminal of the appliance (figure H1-57). c TN-C scheme (the neutral and PE conductor are one and the same, referred to as a PEN conductor). The protective function of a PEN conductor has priority, so that all rules governing PE conductors apply strictly to PEN conductors; c TN-C to TN-S transition The PE conductor for the installlation is connected to the PEN terminal or bar (figure H1-58) generally at the origin of the installation. Downstream of the point of separation, no PE conductor can be connected to the neutral conductor.


PE

correct PE

incorrect

fig. H1-56: a poor connection in a series arrangement will leave all downstream appliances unprotected. PEN

fig. H1-57: direct connection of the PEN conductor to the earth terminal of an appliance. PEN

PE

N

fig. H1-58: the TN-C-S scheme.

H1 types of materials Materials of the kinds mentioned below in table H1-59 can be used for PE conductors, provided that the conditions mentioned in the last column are satisfied. type of protective earthing conductor (PE) supplementary in the same conductor cable as the phases, or in the same cable run independent of the phase conductors metallic housing of bus-trunking or of other prefabricated prewired ducting (5) external sheath of extruded, mineral- insulated conductors (e.g. "pyrotenax" type systems) certain extraneous conductive elements (6) such as: c steel building structures c machine frames c water pipes (7) metallic cable ways, such as, conduits*, ducts, trunking, trays, ladders, and so on…

IT scheme

TN scheme

TT scheme

strongly recommended

strongly recommended

correct

possible (1)

possible (1) (2)

correct

possible (3)

PE possible (3) PEN (8) PE possible (3) PEN not recommended (2) (3) PE possible (4) PEN forbidden

correct

possible (3)

possible (4)

possible

possible

conditions to be respected the PE conductor must be insulated to the same level as the phases c the PE conductor may be bare or insulated (2) c the electrical continuity must be assured by protection against deterioration by mechanical, chemical and electrochemical hazards c their conductance must be adequate

possible (4)

PE possible (4) possible PEN not recommended (2) (4) forbidden for use as PE conductors, are: metal conduits*, gas pipes, hot-water pipes, cable-armouring tapes* or wires*. * forbidden in some countries only-universally allowed to be used for supplementary equipotential conductors.

table H1-59: choice of protective conductors (PE). (1) In schemes TN and IT, fault clearance is generally effected by overcurrent devices (fuses or circuit breakers) so that the impedance of the fault-current loop must be sufficiently low to assure positive protective device operation. The surest means of achieving a low loop impedance is to use a supplementary core in the same cable as the circuit conductors (or taking the same route as the circuit conductors). This stratagem minimizes the inductive reactance and therefore the impedance of the loop. (2) The PEN conductor is a neutral conductor that is also used as a protective earth conductor. This means that a current may be flowing through it at any time (in the absence of an earth fault). For this reason an insulated conductor is recommended for PEN operation. (3) The manufacturer provides the necessary values of R and X components of the impedances (phase/PE, phase/PEN) to include in the calculation of the earth-fault

loop impedance. (4) Possible, but not recomended, since the impedance of the earth-fault loop cannot be known at the design stage. Measurements on the completed installation are the only practical means of assuring adequate protection for persons. (5) It must allow the connection of other PE conductors. NB: these elements must carry an indivual green/yellow striped visual indication, 15 to 100 mm long (or the letters PE at less than 15 cm from each extremity). (6) These elements must be demountable only if other means have been provided to ensure uninterrupted continuity of protection. (7) With the agreement of the appropriate water authorities. (8) In the prefabricated pre-wired trunking and similar elements, the metallic housing may be used as a PEN conductor, in parallel with the corresponding bar, or other PE conductor in the housing.

Table H1-60 below is based on the French national standard NF C 15-100 for LV installations. This table provides two methods of determining the appropriate c.s.a. for both PE or PEN conductors, and also for the conductor to the earth electrode. The two methods are: c adiabatic (which corresponds with that described in IEC 724) This method, while being economical and assuring protection of the conductor against overheating, leads to small c.s.a.'s compared to those of the corresponding-circuit phase conductors. The result is sometimes incompatible with the necessity in IT and TN schemes to minimize the impedance of the circuit earth-fault loop, to ensure positive operation by instantaneous overcurrent tripping devices. This method is used in

practice, therefore, for TT installations, and for dimensioning an earthing conductor*. c simplified This method is based on PE conductor sizes being related to those of the correspondingcircuit phase conductors, assuming that the same conductor material is used in each case. Thus, in table H1-60 for: Sph i 16 mm2 SPE = Sph 16 < Sph i 35 mm2 SPE = 16 mm2 Sph > 35 mm2 SPE = Sph/2 c note: when, in a TT scheme, the installation earth electrode is beyond the zone of influence of the source earthing electrode, the c.s.a. of the PE conductor can be limited to 25 mm2 (for copper) or 35 mm2 (for aluminium).

6.2 conductor dimensioning

* grounding electrode conductor.


6. protective earthing conductors (PE) (continued)

H1 6.2 conductor dimensioning (continued) The neutral cannot be used as a PEN conductor unless its c.s.a. is equal to or larger than 10 mm2 (copper) or 16 mm2 (aluminium). Moreover, a PEN conductor is not allowed in a flexible cable. Since a PEN conductor functions also as a neutral conductor, its c.s.a. cannot, in any case, be less than that necessary for the neutral, as discuss in Subclause 7.1 of this Chapter. This c.s.a. cannot be less than that of the phase conductors unless: c.s.a. of phase conductors Sph (mm2) Cu simplified i 16 method 25,35 > 35 adiabatic any size method

c.s.a. of PE conductor Alu i 16 25 35 > 35

SPE = Sph (1) SPE = 16 SPE = Sph/2 SPE = I √t (1) (2) k

c the kVA rating of single-phase loads is less than 10% of the total kVA load, and c Imax likely to pass through the neutral in normal circumstances, is less than the current permitted for the cable size selected. Furthermore, protection of the neutral conductor must be assured by the protective devices provided for phase-conductor protection (described in Sub-clause 7.2 of this Chapter).

c.s.a. of PEN conductor

c.s.a. of earthing conductor between the installation earth electrode and the main earth terminal SPEN = Sph with minimum c when protected against mechanical 10/mm2 Cu, 16/mm2 Alu damage: S = I √t (2) SPEN = Sph/2 à Sph (3) with k 2 2 minimum 16/mm Cu, 25/mm Alu c without mechanical protection, but protected against corrosion by impermeable cable sheath. Minimum 16 mm2 for copper or galvanized steel. c without either of the above protections; minimum of 25 mm2 for bare copper and 50 mm2 for bare galvanized steel.

table H1-60: minimum c.s.a.'s for PE conductors and earthing conductors (to the installation earth electrode). (1) When the PE conductor is separated from the circuit phase conductors, the following minimum values must be respected: c 2.5 mm2 if the PE is mechanically protected c 4 mm2 if the PE is not mechanically protected. (2) Refer to table H1-55 for the application of this formula. (3) According to the conditions prescribed in the introduction to this table.

values of factor k to be used in the formulae (2) These values are identical in several national standards, and the temperature rise ranges, together with factor k values and the upper temperature limits for the different classes of insulation, correspond with those published in IEC 724 (1984). k values

final temperature (°C) insulated conductors not incoporated in cables or bare conductors in contact with cable jackets copper aluminium steel conductors of a multi-core-cable copper aluminium

The data presented in table H1-61 are those most commonly needed for LV installation design.

nature of insulation polyvinylchloride (PVC)

160

cross-linked-polyethylene (XLPE) ethylene-propylene-rubber (EPR) 250

initial temperature θ initial = 30 °C

initial temperature θ initial = 30 °C

143 95 52 initial temperature θ initial = 30 °C 115 76

176 116 64 initial temperature θ initial = 30 °C 143 94

table H1-61: k factor values for LV PE conductors, commonly used in national standards and complying with IEC 724.


H1 6.3 protective conductor between the HV/LV transformer and the main general distribution board (MGDB) these conductors must be dimensioned according to national practices.

All phase and neutral conductors upstream of the main incoming circuit breaker controlling and protecting the MGDB are protected by devices at the HV side of the transformer. The conductors in question, together with the PE conductor, must be dimensioned accordingly. Dimensioning of the phase and neutral conductors from the transformer is exemplified in Sub-clause 1.6 of this chapter (for circuit C1 of the system illustrated in fig. H1-8). Recommended conductor sizes for bare and insulated PE conductors from the transformer neutral point, shown in figure H1-62, are indicated below in table H1-63. The kVA rating to consider is the sum of all (if more than one) transformers connected to the MGDB. The table indicates the c.s.a. of the conductors in mm2 according to: c the nominal rating of the HV/LV transformer(s) in kVA; c the fault-current clearance time by the HV protective devices, in seconds, P (kVA) LV voltages 127/ 230/ 220 V 400 V i 63 i 100 100 160 125 200 160 250 200 315 250 400 315 500 400 630 500 800 630 1000 800 1250

PE

MGDB

main earth bar for the LV installation

fig. H1-62: PE conductor to the main earth bar in the MGDB. c the kinds of insulation and conductor materials. If the HV protection is by fuses, then use the 0.2 seconds columns. In IT schemes, if an overvoltage protection device is installed (between the transformer neutral point and earth) the conductors for connection of the device should also be dimensioned in the same way as that described above for PE conductors.

conductor material copper t(s) aluminium t(s)

conductors bare 0.2 s 0.5 s 0.2 s 0.5 s

conductors conductors PVC-insulated XLPE-insulated 0.2 s 0.5 s 0.2 s 0.5 s 0.2 s 0.5 s 0.2 s 0.5 s

c.s.a. of PE conductors SPE (mm2)

25 25 25 25 35 50 50 70 70 95 95

25 25 25 35 35 50 70 70 95 95 120

25 25 35 35 50 70 70 95 120 120 150

25 35 50 70 70 95 120 150 150 185 185

25 25 35 50 50 70 95 95 120 120 150

25 50 50 70 95 95 120 150 185 185 240

25 25 25 25 35 35 50 70 70 70 95

25 25 25 35 50 50 70 95 95 120 120

25 35 50 50 70 95 95 120 150 150 185

table H1-63: c.s.a. of PE conductor between the HV/LV transformer and the MGDB, in terms of transformer ratings and fault-clearance times used in France.

6.4 equipotential conductor The main equipotential conductor This conductor must, in general, have a c.s.a. at least equal to a half of that of the largest PE conductor, but in no case need exceed 25 mm2 (copper) or 35 mm2 (aluminium) while its minimum c.s.a. is 6 mm2 (copper) or 10 mm2 (aluminium). Supplementary equipotential conductor This conductor allows an exposed conductive part which is remote from the nearest main equipotential conductor (PE conductor) to be connected to a local protective conductor. Its c.s.a. must be at least a half of that of the protective conductor to which it is connected. If it connects two exposed conductive parts (M1 and M2 in figure H1-64) its c.s.a. must be at least equal to that of the smaller of the two PE conductors (for M1 and M2). Equipotential conductors which are not incorporated in a cable, should be protected mechanically by conduits, ducting, etc. wherever possible. Other important uses for supplementary equipotential conductors concern the reduction of the earth-fault-loop impedance, particulary for indirect-contact protection schemes in TN- or IT-earthed installations, and in special locations with increased electrical risk (IEC 364-4-41 refers).

– between two exposed conductive parts, if SPE1 i SPE2 if SLS = SPE1 SPE1

SPE2 SLS

M1

M2

– between an exposed conductive part and a metallic structure S SLS = PE 2 SPE1 SLS metal structures (conduits, girders, …) M1 (*) with a minimum of 2.5 mm2 for mechanically protected conductors ; 4 mm2 for conductors not mechanically protected copper equivalent

fig. H1-64: supplementary equipotential conductors.



H1 The c.s.a. and the protection of the neutral conductor, apart from its current-carrying requirement, depend on several factors, namely: c the type of earthing system, TT, TN, etc.; c method of protection against indirectcontact hazards according to the methods described below.

7.1 dimensioning the neutral conductor influence of the type of earthing system TT, TN-S and IT schemes c single-phase circuits or those of c.s.a. i 16 mm2 (copper) 25 mm2 (aluminium): the c.s.a. of the neutral conductor must be equal to that of the phases; c three-phase circuits of c.s.a. > 16 mm2 copper or 25 mm2 aluminium: the c.s.a. of the neutral may be chosen to be: v equal to that of the phase conductors, or v smaller, on condition that: - the current likely to flow through the neutral in normal conditions is less than the permitted value Iz. The influence of triplen* harmonics must be given particular consideration, as previously noted in Chapter F Sub-clause 2.2, or: - the single-phase power of the circuit is less than 10% of the balanced 3-phase power of the circuit, or: - the neutral conductor is protected against short-circuit, in accordance with the following Sub-clause H1-7.2. * the 3rd and multiples of the 3rd harmonic.

7.2 protection of the neutral conductor Table H1-65 summarizes the several possible cases. The table has been based on French national standards (NF C 15-100). The following points however, should be considered when referring to the table. Circuit isolation It is considered to be good practice that every circuit be provided with the means for its isolation. Circuit breaking Table H1-65 is based on circuit breakers, which in the event of a fault, will open all poles, including the neutral pole, i.e. the circuit breakers are omnipolar. This table can also be used for fuses able to emulate this omnipolar opening. The action can only be achieved with fuses in an indirect way, in which the operation of one or more fuses provokes a mechanical trip-out of all poles of an associated series-connected load-break switch. The action is commonly caused by a strikerpin which is projected by means of an explosive cartridge (triggered by the blowing fuse) against the switch tripping mechanism. Reclosing of the switch however, must be possible only when the used cartridge has been replaced by a new one. Protection against electric shocks Table H1-65 takes into account the fact that protection against indirect-contact dangers depend either on 300 mA RCDs (TT system) or on circuit breakers (TN and IT systems).


TN-C scheme The same conditions apply in theory as those mentioned above, but in practice, the neutral conductor must not be open-circuited under any circumstances since it constitutes a PE as well as a neutral conductor (see table H160 "c.s.a. of PEN conductor" column). IT scheme In general, it is not recommended to distribute the neutral conductor, i.e. a 3-phase 3-wire scheme is preferred. When a 3-phase 4-wire installation is necesssary, however, the conditions described above for TT and TN-S schemes are applicable.

H1 earthing systems TT by RCD

reminder: protection against indirect contact

TN-C provided by circuit breakers or fuses with Ia (fuses) or Im (CB) < Isc (min)

TN-S according to the method of protection chosen

IT provided by circuit breakers or fuses and one RCD at least for each group of appliances connected to an earth electrode (see figure G20)

protected circuit 1-phase P-N phase/neutral (C)

1-phase phase/phase

2P

3-phase 3-wire

3P

3-phase 4-wire

3P-N Sn = Sph*

(A)

(A)

(C)

* Sn = c.s.a. of neutral conductor Sph = c.s.a. of phase

3P-N Sn < Sph

(B)

(B)

(C)

table H1-65: table of protection schemes for neutral conductors in different earthing systems. thermal

magnetic

Symbol for overcurrent and short-circuit tripping devices. (A) authorized for TT and TN schemes if a RCD is installed at the origin of the circuit or upstream of it, and if no artificial neutral is distributed downstream of its location. (B) authorized for TT and TN schemes if the neutral conductor is protected against shortcircuits by protective arrangements made for the phases, and if the normal service current is substantially less than the maximum permissible for the neutral conductor concerned. (C) authorized for IT schemes in certain conditions, viz: if the circuit breaker controlling a number of homogeneous (i.e. similar) final circuits, of which the ratio of the extreme circuit ratings does not exceed 2, and which are protected against a second fault occurring elsewhere in the installation by a RCD of sensitivity i 15% of that of the calibration of the final circuit having the smallest c.s.a. Refer to example 2 for CB 5.



H1 7.2 protection of the neutral conductor (continued) examples

N

Example 1: (figure H1-66) 3-phase 4-wire circuit with 3 x 95 mm2 copper phase conductors and 1 x 50 mm2 copper neutral conductor. The installation is TT-earthed with RCD protection upstream. Single-phase power of load: 70 kVA (connected phase-neutral). Three-phase power of load: 140 kVA. The condition of single-phase power being < 10% of the 3-phase power delivered (Sub-clause 7.1 TT and TN-S schemes) is not satisfied in this case, since 70/140 = 50%. A reduced neutral c.s.a. may however, be used, provided that it is correctly protected. A suitable circuit breaker for this purpose would be a 4-pole unit rated at 250 A with 3 trip units (1 for each phase) set at 250 A and 1 trip unit (for the neutral) set at 125 A. The operation of any one (or more) of these tripping units will trip all four poles of the circuit breaker. Example 2: (fig. H1-67) An installation is IT-earthed with a distributed neutral (i.e. a 3-phase 4-wire system in which the neutral is not earthed). This arrangement is not recommended, particularly for small or medium-sized installations, but it does provide two levels of voltage, e.g. 230 V and 400 V. The interposition of a LV/LV transformer in a 3-phase 3-wire IT scheme (as shown in figure H1-8) is a preferred method of obtaining the two levels of voltage, and in such a case, the neutral conductor may be earthed. Circuit breaker 1, 2 and 3 As in example 1, the circuits protected by these CBs have a neutral conductor of 50% of the c.s.a. of the phase conductors. The circuit breakers will therefore be 4-pole units, with tripping devices similar to those mentioned in example 1. 400 kVA HV LV

4-pole CB 3 - 250 A trip units 1 - 125 A

50 mm2 3 x 95 mm2

3-phase power 140 kVA 1-phase power 70 kVA

3-phase loads

1-phase loads


Circuit breaker 5 This arrangement corresponds with that described in (C) of table H1-65 concerning a circuit breaker connected directly to, and controlling the busbars from which a number of similar final circuits, (protected by 2-pole, 1-phase and neutral CBs) are supplied. Overcurrent tripping is provided on all outgoing CBs, but the 4-pole incoming CB (no 5) has only the (300 mA) RCD protection (mentioned in (C) of table H1-65) the magnetic core of which embraces all 4 conductors. It may be noted that circuit breaker 12 supplies a long lighting circuit, the neutral and phase conductors of which have the same c.s.a. A 4-pole circuit breaker having 3 tripping devices is therefore suitable (1 device per phase).

2 single-core cables 120 mm2 per phase - 1 x 120 mm2 (neutral)

PE

PE

NS100N 4-pole 32 A 300 mA

NS100N 3-pole 3d 100 A 4

compact NS400N 4-pole 3d 250 A 1d 125 A

compact NS250N 4-pole 3d 250 A 1d 125 A

PIM

PE

5

PE

7

PE

3 x 120 mm2 +1 x 70 mm2

C60N 4-pole 4d 32 A diff. HS 300 mA

NS160N 4-pole 4d 125 A 8

C60N 4-pole 4d 32 A 9

NS160N 3-pole 3d 125 A

3 x 185 mm2 +1 x 95 mm2

NS80HMA

NS160N 4-pole + MT100/160 3d 160 A diff. BS 10 A

12

10 11

3 x 35 mm2 6 4 x 6 mm2 N PE

4 x 50 mm2

4 x 6 mm2

contactor LC1 D63

3 x 50 mm2

4 x 70 mm2

thermal overload relay

PE

3 x 16 mm2 DPN

DPN

4 x 2.5 mm2

DPN 2 x 1.5 mm2

DPN 2 x 1.5 mm2

2 x 2.5 mm2

DPN

fig. H1-67: example 2. H1-38 - the protection of circuits - the switchgear

3d = 3 tripping units 4d = 4 tripping units diff. HS = high-sensitivity differential tripping diff. BS = low-sensitivity differential tripping

30 kW 58 A

outdoor lighting

H1 Example 3: (figure H1-68) TN-C/TN-S installation Three-pole circuit breakers only must be used for nos. 1, 2, 3 and 7, since no switchgear of any kind must be included in the combined protective and neutral conductor (PEN) associated with them. The total single-phase power of the load is less than 10% of the 3-phase power, so that the c.s.a. of the PEN conductor from the source (i.e. circuit 1) may be half that of the phase conductors of the circuit. The protection against indirect contact for this circuit (1) is provided by CB1 if the maximum length of the circuit is less than Lmax (see Chapter G Sub-clause 5.2). For a 630 A CB, regulated to trip instantaneously at a current level of 4 In L max = 0.8 x 230 x 240 x 103 22.5 (1.25 + 2) x Ia The 1.25 factor in the denominator is a 25% increase in resistance for a 240 mm2 c.s.a. conductor*, while Ia = 630 x 4 x 1.15 where the factor 1.15 allows for the guaranteed ± 15% tolerance of the instantaneous magnetic tripping element of the circuit breaker (i.e. the worst case, requiring the shortest Lmax). Lmax = 208 metres. * Chapter G Sub-clause 5.2

Section of the installation which is TN-S connected (PE conductor and neutral conductor separated at a point upstream) Circuit breaker 4 c.s.a. neutral = c.s.a. phase, so that a tripping device for the neutral current is not necessary. A 4-pole circuit breaker with one tripping device per phase is therefore appropriate. Circuit breaker 5 c.s.a. neutral = 50% c.s.a. phase, so that a tripping device for the neutral is required. A 4-pole circuit breaker with 3 tripping devices (set at 160 A) for the phases, and 1 tripping device for the neutral (set at 80 A) is required, as noted in (B) of table H1-65. Circuit breaker 6 The protection of a circuit supplying socket outlets, as mentioned frequently in earlier Chapters, must include a RCD of high sensitivity (generally of 30 mA). Associated circuit breaker and contactor 8 This combination provides short-circuit protection (circuit breaker) and overload protection (thermal relays on contactor to suit motor characteristics). The circuit breaker has no thermal tripping devices, while the contactor has three (one for each phase). Circuit breaker 9 Controls and protects an extensive lighting circuit, and since the phase and neutral conductors have the same c.s.a. a 4-pole circuit breaker is suitable, having 3 tripping devices (1 for each phase).

t

minimum pre-arcing time curve fuse-blown curve

4In

x In



1. the basic functions of LV switchgear

H2 the role of switchgear is that of: c electrical protection; c safe isolation from live parts; c local or remote switching.

National and international standards define the manner in which electric circuits of LV installations must be realized, and the capabilities and limitations of the various switching devices which are collectively referred to as switchgear. The main functions of switchgear are: c electrical protection; c electrical isolation of sections of an installation; c local or remote switching. These functions are summarized below in table H2-1. Electrical protection at low voltage is (apart from fuses) normally incorporated in circuit electrical protection against overload currents short-circuit currents insulation failure

breakers, in the form of thermal-magnetic devices and/or residual-current-operated tripping devices (less-commonly, residualvoltage-operated devices - acceptable to, but not recommended by IEC). In addition to those functions shown in table H2-1, other functions, namely: c over-voltage protection; c under-voltage protection are provided by specific devices (lightning and various other types of voltage-surge arrester; relays associated with: contactors, remotelycontrolled circuit breakers, and with combined circuit breaker/isolators… and so on).

isolation

control

- isolation clearly indicated by an authorized fail-proof mechanical indicator - a gap or interposed insulating barrier between the open contacts, clearly visible.

- functional switching - emergency switching - emergency stopping - switching off for mechanical maintenance

table H2-1: basic functions of LV switchgear.

1.1 electrical protection electrical protection assures: c protection of circuit elements against the thermal and mechanical stresses of short-circuit currents; c protection of persons in the event of insulation failure; c protection of appliances and apparatus being supplied (e.g motors, etc.).

The aim is to avoid or to limit the destructive or dangerous consequences of excessive (short-circuit) currents, or those due to overloading and insulation failure, and to separate the defective circuit from the rest of the installation. A distinction is made between the protection of: c the elements of the installation (cables, wires, switchgear…); c persons and animals; c equipment and appliances supplied from the installation; c the protection of circuits (see chapter H1): v against overload; a condition of excessive current being drawn from a healthy (unfaulted) installation, v against short-circuit currents due to complete failure of insulation between conductors of different phases or (in TN systems) between a phase and neutral (or PE) conductor. Protection in these cases is provided either by fuses or circuit breaker, at the distribution board from which the final circuit (i.e. the

circuit to which the load is connected) originates. Certain derogations to this rule are authorized in some national standards, as noted in chapter H1 sub-clause 1.4. c the protection of persons against insulation failures (see chapter G). According to the system of earthing for the installation (TN, TT or IT) the protection will be provided by fuses or circuit breakers, residual current devices, and/or permanent monitoring of the insulation resistance of the installation to earth. c the protection of electric motors (see chapter J clause 5) against overheating, due, for example, to long term overloading; stalled rotor; single-phasing, etc. Thermal relays, specially designed to match the particular characteristics of motors are used. Such relays may, if required, also protect the motor-circuit cable against overload. Shortcircuit protection is provided either by type aM fuses or by a circuit breaker from which the thermal (overload) protective element has been removed, or otherwise made inoperative.

The aim of isolation is to separate a circuit or apparatus, or an item of plant (such as a motor, etc.) from the remainder of a system which is energized, in order that personnel may carry out work on the isolated part in perfect safety. In principle, all circuits of an LV installation shall have means to be isolated. In practice, in order to maintain an optimum continuity of service, it is preferred to provide a means of isolation at the origin of each circuit. An isolating device must fulfil the following requirements: c all poles of a circuit, including the neutral (except where the neutral is a PEN conductor) must be open (1);

c it must be provided with a means of locking open with a key (e.g. by means of a padlock) in order to avoid an unauthorized reclosure by inadvertence; c it must conform to a recognized national or international standard (e.g. IEC 947-3) concerning clearance between contacts, creepage distances, overvoltage withstand capability, etc. and also:

1.2 isolation a state of isolation clearly indicated by an approved "fail-proof" indicator, or the visible separation of contacts, are both deemed to satisfy the national standards of many countries.

(1) the concurrent opening of all live conductors, while not always obligatory, is however, strongly recommended (for reasons of greater safety and facility of operation). The neutral contact opens after the phase contacts, and closes before them (IEC 947-1).


1. the basic functions of LV switchgear (continued)

H2 1.2 isolation

(continued) v verification that the contacts of the isolating device are, in fact, open. The verification may be: - either visual, where the device is suitably designed to allow the contacts to be seen (some national standards impose this condition for an isolating device located at the origin of a LV installation supplied directly from a HV/LV transformer), - or mechanical, by means of an indicator solidly welded to the operating shaft of the device. In this case the construction of the device must be such that, in the eventuality that the contacts become welded together in the closed position, the indicator cannot possibly indicate that it is in the open position. v leakage currents. With the isolating device open, leakage currents between the open contacts of each phase must not exceed: - 0.5 mA for a new device, - 6.0 mA at the end of its useful life. v voltage-surge withstand capability, across open contacts. The isolating device, when open must withstand a 1.2/50 µs impulse, having a peak value of 5, 8 or 10 kV according to its service voltage, as shown in table H2-2. The device must satisfy these conditions for altitudes up to 2,000 metres. Consequently, if tests are carried out at sea level, the test values must be increased by 23% to take into account the effect of altitude. See standard IEC 947 and the Note immediately preceding table F-10. service (nominal) voltage (V) 230/400 400/690 1,000

Industrial LV switchgear which affords isolation when open is marked on the front face by the symbol . This symbol may be combined with those indicating other features where a device also performs other functions as shown in figure H2-4.

fig. H2-3: symbol for a disconnector* also commonly referred to as an isolator. switch-disconnector*, also referred to as a load-break isolating switch

circuit breaker suitable for circuit isolation

fig. H2-4: symbols for circuit isolation capability incorporated in other switching devices. * IEC 617-7 and 947-3.

Note. In this guide the terms "disconnector" and "isolator" have the same meaning.

impulse withstand peak voltage (kV) 5 kV 8 kV 10 kV

table H2-2: peak value of impulse voltage according to normal service voltage of test specimen.

1.3 switchgear control switchgear-control functions allow system operating personnel to modify a loaded system at any moment, according to requirements, and include: c functional control (routine switching, etc.); c emergency switching; c maintenance operations on the power system.


In broad terms "control" signifies any facility for safely modifying a load-carrying power system at all levels of an installation. The

operation of switchgear is an important part of power-system control.

functional control This control relates to all switching operations in normal service conditions for energizing or de-energizing a part of a system or installation, or an individual piece of equipment, item of plant, etc. Switchgear intended for such duty must be installed at least: c at the origin of any installation; c at the final load circuit or circuits (one switch may control several loads). Marking (of the circuits being controlled) must be clear and unambiguous. In order to provide the maximum flexibility and continuity of operation, particularly where the switching device also constitutes the protection (e.g. a circuit breaker or switch-fuse) it is preferable to include a switch at each level of distribution, i.e. on

each outgoing way of all distribution and subdistribution boards. The manœuvre may be: c either manual (by means of an operating lever on the switch) or; c electric, by push-button on the switch or at a remote location (load-shedding and reconnection, for example). These switches operate instantaneously (i.e. with no deliberate delay), and those that provide protection are invariably omni-polar*. The main circuit breaker for the entire installation, as well as any circuit breakers used for change-over (from one source to another) must be omni-polar units. * one break in each phase and (where appropriate) one break in the neutral (see table H1-65).

H2 emergency switching emergency stop An emergency switching is intended to de-energize a live circuit which is, or could become, dangerous (electric shock or fire). An emergency stop is intended to arrest a movement which has become dangerous. In the two cases: c the emergency control device or its means of operation (local or at remote location(s)) such as a large red mushroom-headed emergency-stop pushbutton must be recognizable and readily accessible, in proximity to any position at which danger could arise or be seen; c a single action must result in a complete switching-off of all live conductors (1) (2);

c a "break glass" emergency switching initiation device is authorized, but in unmanned installations the re-energizing of the circuit can only be achieved by means of a key held by an authorized person. It should be noted that in certain cases, an emergency system of braking, may require that the auxiliary supply to the braking-system circuits be maintained until final stoppage of the machinery. (1) Taking into account stalled motors. (2) In a TN schema the PEN conductor must never be opened, since it functions as a protective earthing wire as well as the system neutral conductor.

switching-off for mechanical maintenance work This operation assures the stopping of a machine and its impossibility to be inadvertently restarted while mechanical maintenance work is being carried out on the driven machinery. The shutdown is generally carried out at the functional switching device, with the use of a suitable safety lock and warning notice at the switch mechanism.


2. the switchgear and fusegear

H2 2.1 elementary switching devices disconnector (or isolator) This switch is a manually-operated, lockable, two-position device (open/closed) which provides safe isolation of a circuit when locked in the open position. Its characteristics are defined in IEC 947-3. A disconnector is not designed to make or to break current* and no rated values for these functions are given in standards. It must, however, be capable of withstanding the passage of short-circuit currents and is assigned a rated short-time withstand capability; generally for 1 second, unless otherwise agreed between user and manufacturer. This capability is normally more than adequate for longer periods of (lower-valued) operational overcurrents, such as those of motor-starting. Standardized mechanical-endurance, overvoltage, and leakage-current tests, must also be satisfied. * i.e. a LV disconnector is essentially a deadsystem switching device to be operated with no voltage on either side of it, particularly when closing, because of the possibility of an unsuspected short-circuit on the downstream side. Interlocking with an upstream switch or circuit breaker is frequently used.

load-breaking switch This control switch is generally operated manually (but is sometimes provided with electrical tripping for operator convenience) and is a non-automatic two-position device (open/closed). It is used to close and open loaded circuits under normal unfaulted circuit conditions. It does not consequently, provide any protection for the circuit it controls. IEC standard 947-3 defines: c the frequency of switch operation (600 close/open cycles per hour maximum); c mechanical and electrical endurance (generally less than that of a contactor); c current making and breaking ratings for normal and infrequent situations. IEC 947-3 also recognizes 3 categories of load-breaking switch, each of which is suitable for a different range of load power factors, as shown in table H2-7.

fig. H2-6: symbol for a load-breaking switch. When closing a switch to energize a circuit there is always the possibility that an (unsuspected) short circuit exists on the circuit. For this reason, load-break switches are assigned a fault-current making rating, i.e. successful closure against the electrodynamic forces of short-circuit current is assured. Such switches are commonly referred to as "fault-make load-break" switches. Upstream protective devices are relied upon to clear the short-circuit fault.


fig. H2-5: symbol for a disconnector (or isolator).

H2 nature of current alternating current

utilization category frequent infrequent operation operation AC-20A AC-20B AC-21A

AC-21B

AC-22A

AC-22B

AC-23A

AC-23B

typical applications

connecting and disconnecting under no-load conditions switching of resistive loads including moderate overloads switching of mixed resistive and inductive loads, including moderate overloads switching of motor loads or other highly inductive loads

table H2-7: utilization categories of LV a.c. switches according to IEC 947-3. Category AC-23 includes occasional switching of individual motors. The switching of capacitors or of tungsten filament lamps shall be subject to agreement between manufacturer and user. The utilization categories referred to in table H2-7 do not apply to an equipment normally used to start, accelerate and/or stop individual motors. The utilization categories for such an equipment are dealt with in chapter J, table J5-4. Example: A 100 A load-break switch of category AC-23 (inductive load) must be able: c to make a current of 10 In (= 1,000 A) at a power factor of 0.35 lagging; c to break a current of 8 In (= 800 A) at a power factor of 0.35 lagging; c to withstand short-circuit currents (not less than 12 In) passing through it for 1 second, where 12 In equals the r.m.s. value of the a.c. component, while the peak value (expressed in kA) is given by a factor "n" in table XVI of IEC 947- Part 1, reproduced below for reader convenience (table H2-8). test current I (A) I i 01 500 1 500 < I i 3 000 3 000 < I i 4 500 4 500 < I i 6 000 6 000 < I i 10 000 10 000 < I i 20 000 20 000 < I i 50 000 50 000 < I

power-factor (ms) 0.95 0.9 0.8 0.7 0.5 0.3 0.25 0.2

time-constant

n

5 5 5 5 5 10 15 15

1.41 1.42 1.47 1.53 1.7 2.0 2.1 2.2

table H2-8: factor "n" used for peak-to-rms value (IEC 947-part1).

bistable switch (télérupteur) This device is extensively used in the control of lighting circuits where the depression of a pushbutton (at a remote control position) will open an already-closed switch or close an open switch in a bistable sequence. Typical applications are: c two-way switching on stairways of large buildings; c stage-lighting schemes; c factory illumination, etc. Auxiliary devices are available to provide: c remote indication of its state at any instant; c time-delay functions; c maintained-contact features.

fig. H2-9: symbol for a bistable remotelyoperated switch (télérupteur).


2. the switchgear and fusegear (continued)

H2 2.1 elementary switching devices (continued) contactor The contactor is a solenoid-operated switching device which is generally held closed by (a reduced) current through the closing solenoid (although various mechanically-latched types exist for specific duties). Contactors are designed to carry out numerous close/open cycles and are commonly controlled remotely by on-off pushbuttons. The large number of repetitive operating cycles is standardized in table VIII of IEC 947-4-1 by: c the operating duration: 8 hours; uninterrupted; intermittent; temporary of 3, 10, 30, 60 and 90 minutes; c utilization category: (for definition see table J5-4) for example, a contactor of category AC3 can be used for the starting and stopping of a cage motor; c the start-stop cycles (1 to 1,200 cyles per hour); c mechanical endurance (number of off-load manœuvres); c electrical endurance (number of on-load manœuvres);

c a rated current making and breaking performance according to the category of utilization concerned. Example: A 150 A contactor of category AC3 must have a minimum current-breaking capability of 8 In (= 1,200 A) and a minimum current-making rating of 10 In (= 1,500 A) at a power factor (lagging) of 0.35.

control circuit

power circuit

fig. H2-10: symbol for a contactor.

discontactor* A contactor equipped with a thermal-type relay for protection against overloading defines a "discontactor". Discontactors are used extensively for remote push-button control of lighting circuits, etc., and may also be considered as an essential element in a motor controller, as noted in sub-clause 2.2. "combined switchgear elements". The discontactor is not the equivalent of a

two classes of LV cartridge fuse are very widely used: c for domestic and similar installations type gG c for industrial installations type gG, gM or aM.


circuit breaker, since its short-circuit currentbreaking capability is limited to 8 or 10 In. For short-circuit protection therefore, it is necessary to include either fuses or a circuit breaker in series with, and upstream of, the discontactor contacts. *This term is not defined in IEC publications but is commonly used in some countries.

fuses Fuses exist with and without "fuse-blown" mechanical indicators. Fuses break a circuit by controlled melting of the fuse element when a current exceeds a given value for a corresponding period of time; the current/time relationship being presented in the form of a performance curve for each type of fuse. Standards define two classes of fuse: c those intended for domestic installations, manufactured in the form of a cartridge for rated currents up to 100 A and designated type gG in IEC 269-3; c those for industrial use, with cartridge types designated gG (general use); and gM and aM (for motor-circuits) in IEC 269-1 and 2. The main differences between domestic and industrial fuses are the nominal voltage and current levels (which require much larger physical dimensions) and their fault-current breaking capabilities. Type gG fuse-links are often used for the protection of motor circuits, which is possible when their characteristics are capable of withstanding the motor-starting current without deterioration. A more recent development has been the adoption by the IEC of a fuse-type gM for motor protection, designed to cover starting, and short-circuit conditions. This type of fuse is more popular in some countries than in others, but at the present time the aM fuse in combination with a thermal overload relay is more-widely used.

A gM fuse-link, which has a dual rating is characterized by two current values. The first value In denotes both the rated current of the fuse-link and the rated current of the fuseholder; the second value Ich denotes the time-current characteristic of the fuse-link as defined by the gates in Tables II, III and VI of IEC 269-1. These two ratings are separated by a letter which defines the applications. For example: In M Ich denotes a fuse intended to be used for protection of motor circuits and having the characteristic G. The first value In corresponds to the maximum continuous current for the whole fuse and the second value Ich corresponds to the G characteristic of the fuse link. For further details see note at the end of sub-clause 2.1. An aM fuse-link is characterized by one current value In and time-current characteristic as shown in figure H2-14. Important: Some national standards use a gI (industrial) type fuse, similar in all main essentails to type gG fuses. Type gI fuses should never be used, however, in domestic and similar installations.

fig. H2-11: symbol for fuses.

H2 fusing zones conventional currents

gM fuses require a separate overload relay, as described in the note at the end of sub-clause 2.1.

The conditions of fusing (melting) of a fuse are defined by standards, according to their class. c class gG fuses These fuses provide protection against overloads and short-circuits. Conventional non-fusing and fusing currents are standardized, as shown in figure H2-12 and in table H2-13. v the conventional non-fusing current Inf is the value of current that the fusible element can carry for a specified time without melting. Example: a 32 A fuse carrying a current of 1.25 In (i.e. 40 A) must not melt in less than one hour (table H2-13) v the conventional fusing current If (=I2 in fig.H2-12) is the value of current which will cause melting of the fusible element before the expiration of the specified time. Example: a 32 A fuse carrying a current of 1.6 In (i.e. 52.1 A) must melt in one hour or less (table H2-13). IEC 269-1 standardized tests require that a fuse-operating characteristic lies between the two limiting curves (shown in figure H2-12) for the particular fuse under test. This means that two fuses which satisfy the test can have significantly different operating times at low levels of overloading. v the two examples given above for a 32 A fuse, together with the foregoing notes on standard test requirements, explain why class

rated current* In (A)

gG gM

In i 4 A 4 < In < 16 A 16 < In i 63 A 63 < In i 160 A 160 < In i 400 A 400 < In

these fuses have a poor performance in the low overload range. v it is therefore necessary to install a cable larger in ampacity than that normally required for a circuit, in order to avoid the consequences of possible long term overloading (60% overload for up to one hour in the worst case). By way of comparison, a circuit breaker of similar current rating: v which passes 1.05 In must not trip in less than one hour; and v when passing 1.25 In it must trip in one hour, or less (25% overload for up to one hour in the worst case). t minimum pre-arcing time curve

1h.

fuse-blown curve

Inf I2

I

fig. H2-12: zones of fusing and non-fusing for gG and gM fuses.

conventional nonfusing current Inf 1.5 In 1.5 In 1.25 In 1.25 In 1.25 In 1.25 In

conventional fusing current If I2 2.1 In 1.9 In 1.6 In 1.6 In 1.6 In 1.6 In

conventional time h 1 1 1 2 3 4

table H2-13: zones of fusing and non-fusing for LV types gG and gM class fuses (IEC 269-1 and 269-2-1). * Ich for gM fuses

class aM fuses protect against short-circuit currents only, and must always be associated with another device which protects against overload.

c class aM (motor) fuses These fuses afford protection against shortcircuit currents only and must necessarily be associated with other switchgear (such as discontactors or circuit breakers) in order to ensure overload protection < 4 In. They are not therefore autonomous. Since aM fuses are not intended to protect against low values of overload current, no levels of conventional non-fusing and fusing currents are fixed. The characteristic curves for testing these fuses are given for values of fault current exceeding approximately 4 In (see figure H2-14), and fuses tested to IEC 269 must give operating curves which fall within the shaded area. Note: the small "arrowheads" in the diagram indicate the current/time "gate" values for the different fuses to be tested (IEC 269).

t

minimum pre-arcing time curve fuse-blown curve

4In

x In

fig. H2-14: standardized zones of fusing for type aM fuses (all current ratings).



H2 2.1 elementary switching devices (continued) rated short-circuit breaking currents A characteristic of modern cartridge fuses is that, owing to the rapidity of fusion in the case of high short-circuit current levels*, a current cut-off begins before the occurrence of the first major peak, so that the fault current never reaches its prospective peak value (fig. H2-15). This limitation of current reduces significantly the thermal and dynamic stresses which would otherwise occur, thereby minimizing danger and damage at the fault position. The rated short-circuit breaking current of the fuse is therefore based on the r.m.s. value of the a.c. component of the prospective fault current. No short-circuit current-making rating is assigned to fuses. *for currents exceeding a certain level, depending on the fuse nominal current rating, as shown below in figure H2-15A.

Reminder Short-circuit currents initially contain d.c. components, the magnitude and duration of which depend on the XL/R ratio of the faultcurrent loop. Close to the source (HV/LV transformer) the relationship I peak / Irms (of a.c. component) immediately following the instant of fault, can be as high as 2.5 (standardized by IEC, and shown in figure H2-15A). At lower levels of distribution in an installation, as previously noted, XL is small compared with R and so for final circuits I peak / I rms ~ 1.41, a condition which corresponds with figure H2-15 above and with the "n" value corresponding to a power factor of 0.95 in table H2-8.

I prospective fault-current peak rms value of the a.c. component of the prospective fault current current peak limited by the fuse 0.01s Tf Ta Ttc

0.02s

Tf: fuse pre-arc fusing time Ta: arcing time Ttc: total fault-clearance time

fig. H2-15: current limitation by a fuse.

Note on gM fuse ratings. A gM type fuse is essentially a gG fuse, the fusible element of which corresponds to the current value Ich (ch = characteristic) which may be, for example, 63 A. This is the IEC testing value, so that its time/ current characteristic is identical to that of a 63 A gG fuse. This value (63 A) is selected to withstand the high starting currents of a motor, the steadystate operating current (In) of which may be in the 10-20 A range.


maximum possible current peak characteristic i.e. 2.5Ir.m.s. (IEC)

prospective fault current (kA) peak 100

(c) 50

160A 20 100A (b)

50A

10

nominal fuse ratings

(a) 5

peak current cut-off characteristic curves

2

1 1

The peak-current-limitation effect occurs only when the prospective r.m.s. a.c. component of fault current attains a certain level. For example, in the above graph the 100 A fuse will begin to cut off the peak at a prospective fault current (r.m.s.) of 2 kA (a). The same fuse for a condition of 20 kA r.m.s. prospective current will limit the peak current to 10 kA (b). Without a current-limiting fuse the peak current could attain 50 kA (c) in this particular case. As already mentioned, at lower distribution levels in an installation, R greatly predominates XL, and fault levels are generally low. This means that the level of fault current may not attain values high enough to cause peakcurrent limitation. On the other hand, the d.c. transients (in this case) have an insignificant effect on the magnitude of the current peak, as previously mentioned.

t

0.005s

2

5

10

20

50

100

a.c. component of prospective fault current (kA) r.m.s.

fig. H2-15A: limited peak current versus prospective r.m.s. values of the a.c. component of fault current for LV fuses.

H2 This means that a physically smaller fuse barrel and metallic parts can be used, since the heat dissipation required in normal service is related to the lower figures (10-20 A). A standard gM fuse, suitable for this situation would be designated 32M63 (i.e. In M Ich). The first current rating In concerns the steady-load thermal performance of the fuselink, while the second current rating (Ich) relates to its (short-time) starting-current performance. It is evident that, although suitable for shortcircuit protection, overload protection for the motor is not provided by the fuse, and so a separate thermal-type relay is always necessary when using gM fuses. The only advantage offered by gM fuses, therefore, when compared with aM fuses, are reduced physical dimensions and slightly lower cost.

2.2 combined switchgear elements Single units of switchgear do not, in general, fulfil all the requirements of the three basic functions, viz: protection, control and isolation. Where the installation of a circuit breaker is not appropriate (notably where the switching rate is high, over extended periods) combinations of units specifically designed for such a performance are employed. The most commonly-used combinations are described below:

switch and fuse combinations Two cases are distinguished: c the type in which the operation of one (or more) fuse(s) causes the switch to open. This is achieved by the use of fuses fitted with striker pins, and a system of switch tripping springs and toggle mechanisms. This type of combination is generally used for current levels exceeding 100 A, and is commonly associated with a thermal-type overcurrent relay for overload protection (for which the fuses alone may not be suitable). If the switch is classified as AC22 or AC23, and associated with a motor-overload type of thermal relay, the ensemble, i.e. switch, striker-pin fuses and overload relay, is suitable for the control and protection of a motor circuit, and: c the type in which a non-automatic switch is associated with a set of fuses in a common enclosure. In some countries, and in IEC 947-3, the terms "switch-fuse" and "fuse-switch" have specific meanings, viz: v a switch-fuse comprises a switch (generally 2 breaks per pole) on the upstream side of three fixed fuse-bases, into which the fuse carriers are inserted (figure H2-17(a)), v a fuse-switch consists of three switch blades each constituting a double-break per phase. These blades are not continuous throughout their length, but each has a gap in the centre which is bridged by the fuse cartridge. Some designs have only a single break per phase, as shown in figures H2-17(a) and (b).

fig. H2-16: symbol for an automatictripping switch-fuse, with a thermal overload relay.

fig. H2-17 (a): symbol for a non-automatic switch-fuse.

fig. H2-17 (b): symbol for a non-automatic fuse-switch.



H2 2.2 combined switchgear elements (continued) The current range for these devices is limited to 100 A maximum at 400 V 3-phase, while their principal use is in domestic and similar installations. To avoid confusion between the first group (i.e. automatic tripping) and the second group, the term "switch-fuse" should be qualified by the adjectives "automatic" or "non-automatic".

fuse - disconnector + discontactor fuse - switch-disconnector + discontactor As previously mentioned, a discontactor does not provide protection against short-circuit faults. It is necessary, therefore, to add fuses (generally of type aM) to perform this function. The combination is used mainly for motorcontrol circuits, where the disconnector or switch-disconnector allows safe operations such as: c the changing of fuse links (with the circuit isolated); c work on the circuit downstream of the discontactor (risk of remote closure of the discontactor). The fuse-disconnector must be interlocked with the discontactor such that no opening or closing manœuvre of the fuse-disconnector is possible unless the discontactor is open (figure H2-18 (a)), since the fusedisconnector has no load-switching capability. A fuse-switch-disconnector (evidently) requires no interlocking (figure H2-18 (b)). The switch must be of class AC22 or AC23 if the circuit supplies a motor.

circuit-breaker + contactor circuit-breaker + discontactor These combinations are used in remotelycontrolled distribution systems in which the rate of switching is high, or for control and protection of a circuit supplying motors. The protection of induction motors is considered in chapter J, clause J5.


fig. H2-18 (a): symbol for a fusedisconnector + discontactor.

fig. H2-18 (b): symbol for a fuse-switchdisconnector + discontactor.

3. choice of switchgear

H2 3.1 tabulated functional capabilities After having studied the basic functions of LV switchgear (clause 1, table H2-1) and the different components of switchgear (clause 2), table H2-19 summarizes the capabilities of the various components to perform the basic functions. switchgear item isolator (or disconnector) (4) switch (5) residual device (RCCB) (5) switchdisconnector contactor bistable-switch (telerupteur) fuse circuit breaker (5) circuit breaker disconnector (5) residual and overcurrent circuit breaker (RCBO) (5) point of installation (general principle)

isolation control functional

emergency switching emergency stop switching for (mechanical) mechanical maintenance

electrical protection overload short-circuit differential

c c c

c c

c (1) c (1)

c (1) (2) c (1) (2)

c c

c

c

c (1)

c (1) (2)

c

c c

c (1) c (1)

c (1) (2)

c c

c

c (1)

c (1) (2)

c

c c

c c

c

c

c (1)

c (1) (2)

c

c

c

c

c

c (1)

c (1) (2)

c

c

c

c

origin of each circuit

all points where, for operational reasons it may be necessary to stop the process

in general at the incoming circuit to every distribution board

at the supply point to each machine and/or on the machine concerned

at the supply point to each machine



origin of circuits where the earthing system is appropriate TN-S, IT, TT

c

c

c (3)

table H2-19: functions fulfilled by different items of switchgear. (1) Where cut-off of all active conductors is provided (2) It may be necessary to maintain supply to a braking system (3) If it is associated with a thermal relay (the combination is commonly referred to as a "discontactor") (4) In certain countries a disconnector with visible contacts is mandatory at the origin of a LV installation supplied directly from a HV/LV transformer (5) Certain items of switchgear are suitable for isolation duties (e.g. RCCBs according to IEC 1008) without being explicitly marked as such.

3.2 switchgear selection Software is being used more and more in the field of optimal selection of switchgear. Each circuit is considered one at a time, and a list is drawn up of the required protection functions and exploitation of the installation, among those mentioned in table H2-19 and summarized in table H2-1. A number of switchgear combinations are studied and compared with each other against relevant criteria, with the aim of achieving: c satisfactory performance; c compatibility among the individual items; from the rated current In to the fault-level rating Icu; c compatibility with upstream switchgear or taking into account its contribution; c conformity with all regulations and specifications concerning safe and reliable circuit performance.

In order to determine the number of poles for an item of switchgear, reference is made to chapter H1, clause 7, table H1-65. Multifunction switchgear, initially more costly, reduces installation costs and problems of installation or exploitation. It is often found that such switchgear provides the best solution.


4. circuit breakers

H2 the circuit breaker/disconnector fulfills all of the basic switchgear functions, while, by means of accessories, numerous other possibilities exist.

As shown in table H2-19 the circuit breaker/ disconnector is the only item of switchgear capable of simultaneously satisfying all the basic functions necessary in an electrical installation. Moreover, it can, by means of auxiliary units, provide a wide range of other functions, for example: indication (on-off - tripped on fault); undervoltage tripping; remote control… etc. These features make a circuit-breaker/ disconnector the basic unit of switchgear for any electrical installation. functions isolation control

protection

functional emergency switching switching-off for mechanical maintenance overload short-circuit insulation faulty undervoltage

remote control indication and measurement

possible conditions c c c (with the possibility of a tripping coil for remote control) c c c c (with differential-current relay) c (with undervoltage-trip coil) c added or incorporated c (generally optional with an electronic tripping device)

table H2.20: functions performed by a circuit-breaker/disconnector.

4.1 standards and descriptions industrial circuit breakers must conform with IEC 947-1 and 947-2 or other equivalent standards. Corresponding European standards are presently being developed. Domestic-type circuit breakers should conform to IEC standard 898, or an equivalent national standard.


standards For industrial LV installations the relevant IEC standards are, or are due to be: c 947-1: general rules; c 947-2: part 2: circuit breakers; c 947-3: part 3: switches, disconnectors, switch-disconnectors and fuse combination units; c 947-4: part 4: contactors and motorstarters; c 947-5: part 5: control-circuit devices and switching elements; c 947-6: part 6: multiple function switching devices; c 947-7: part 7: ancillary equipment. Corresponding European and many national standards are presently in the course of harmonization with the IEC standards, with which they will be in very close agreement. For domestic and similar LV installations, the appropriate standard is IEC 898, or an equivalent national standard.

H2 description Figure H2-21 shows schematically the principal parts of a LV circuit breaker and its four essential functions: 1 - the circuit-breaking components, comprising the fixed and moving contacts and the arc-dividing chamber. 2 - the latching mechanism which becomes unlatched by the tripping device on detection of abnormal current conditions. This mechanism is also linked to the operation handle of the breaker.

3 - a trip-mechanism actuating device: c either: a thermal-magnetic device, in which a thermally-operated bi-metal strip detects an overload condition, while an electromagnetic striker pin operates at current levels reached in short-circuit conditions, or: c an electronic relay operated from current transformers, one of which is installed on each phase. 4 - a space allocated to the several types of terminal currently used for the main powercircuit conductors.

power circuit terminals

contacts and arc-dividing chamber

fool-proof mechanical indicator

latching mechanism

trip mechanism and protective devices

fig. H2-21: principal parts of a circuit breaker.

domestic circuit breakers conforming to IEC 898 and similar national standards perform the basic functions of: c isolation c protection against overcurrent.

fig. H2-22: domestic-type circuit breaker providing overcurrent protection and circuit isolation features.

some models can be adapted to provide sensitive detection (30 mA) of earth-leakage current with CB tripping, by the addition of a modular block, as shown in figure H2-23, while other models (complying with IEC 1009) have this residual-current feature incorporated, viz. RCBOs., and, more recently, IEC 947-2 (appendix B) CBRs. fig. H2-23: domestic-type circuit breaker as above (H2-22) plus protection against electric shocks by the addition of a modular block.


4. circuit breakers (continued)

H2 4.1 standards and descriptions (continued) apart from the above-mentioned functions further features can be associated with the basic circuit breaker by means of additional modules, as shown in figure H2-24; notably remote control and indication (on-off-fault).

1

2 3 4 5

FF -OFF O O-O O-OFF O-OFF

fig. H2-24: "Multi 9" system* of LV modular switchgear components.

moulded-case type industrial circuit breakers conforming to IEC 947-2 are now available, which, by means of associated adaptable blocks provide a similar range of auxiliary functions to those described above (figure H2-25).

SDE

OF2

SD OF1

OF2 SDE SD OF1

fig. H2-25: example of a modular (Compact NS*) industrial type of circuit breaker capable of numerous auxiliary functions. * Merlin Gerin product.


H2 heavy-duty industrial circuit breakers of large current ratings, conforming to IEC 947-2, have numerous built-in communication and electronic functions (figure H2-26).

fig. H2-26: examples of heavy-duty industrial circuit breakers. The "Masterpact"* provides many automation features in its tripping module. These circuit breakers are provided with means to adjust protective-device settings over a wide range, and also with: c a 20 mA output loop; c remote indication contacts; c load indication at the CB.

4.2 fundamental characteristics of a circuit breaker the fundamental characteristics of a circuit breaker are: c its rated voltage Ue c its rated current In c its tripping-current-level adjustment ranges for overload protection (Ir** or Irth**) and for short-circuit protection (Im)** c its short-circuit current breaking rating (Icu for industrial CBs; Icn for domestic-type CBs).

rated operational voltage (Ue) This is the voltage at which the circuit breaker has been designed to operate, in normal (undisturbed) conditions. Other values of voltage are also assigned to the circuit breaker, corresponding to disturbed conditions, as noted in sub-clause 4.3.

rated current (In)

frame-size rating

This is the maximum value of current that a circuit breaker, fitted with a specified overcurrent tripping relay, can carry indefinitely at an ambient temperature stated by the manufacturer, without exceeding the specified temperature limits of the currentcarrying parts. Example: A circuit breaker rated at In = 125 A for an ambient temperature of 40 °C will be equipped with a suitably calibrated overcurrent tripping relay (set at 125 A). The same circuit breaker can be used at higher values of ambient temperature however, if suitably "derated". Thus, the circuit breaker in an ambient temperature of 50 °C could carry only 117 A indefinitely, or again, only 109 A at 60 °C, while complying with the specified temperature limit. Derating a circuit breaker is achieved therefore, by reducing the trip-current setting of its overload relay, and marking the CB accordingly. The use of an electronic-type of tripping unit, designed to withstand high temperatures, allows circuit breakers (derated as described) to operate at 60 °C (or even at 70 °C) ambient.

A circuit breaker which can be fitted with overcurrent tripping units of different currentlevel-setting ranges, is assigned a rating which corresponds with that of the highest current-level-setting tripping unit that can be fitted.

Note: In for circuit breakers (in IEC 947-2) is equal to Iu for switchgear generally, Iu being rated uninterrupted current. * Merlin Gerin products. ** Current-level setting values which refer to the current-operated thermal and "instantaneous" magnetic tripping devices for over-load and short-circuit protection.



H2 4.2 fundamental characteristics of a circuit breaker (continued) overload relay trip-current setting (Irth or Ir) Apart from small circuit breakers which are very easily replaced, industrial circuit breakers are equipped with removable, i.e. exchangeable, overcurrent-trip relays. Moreover, in order to adapt a circuit breaker to the requirements of the circuit it controls, and to avoid the need to install over-sized cables, the trip relays are generally adjustable. The trip-current setting Ir or Irth (both designations are in common use) is the current above which the circuit breaker will trip. It also represents the maximum current that the circuit breaker can carry without tripping.

That value must be greater than the maximum load current IB, but less than the maximum current permitted in the circuit Iz (see chapter H1, sub-clause 1.3). The thermal-trip relays are generally adjustable from 0.7 to 1.0 times In, but when electronic devices are used for this duty, the adjustment range is greater; typically 0.4 to 1 times In. Example (figure H2-27): a circuit breaker equipped with a 320 A overcurrent trip relay, set at 0.9, will have a trip-current setting: Ir = 320 x 0.9 = 288 A Note: for circuit breakers equipped with non-adjustable overcurrent-trip relays, Ir = In.

rated current of the tripping unit to suit the circumstances In

0.7 In

adjustment range

overload trip current setting to suit the circuit Ir

224 A

288 A

circuit-breaker frame-size rating

320 A

400 A

I

fig. H2-27: example of a 400 A circuit breaker equipped with a 320 A overload trip unit adjusted to 0.9, to give Ir = 288 A.

short-circuit relay trip-current setting (Im) Short-circuit tripping relays (instantaneous or slightly time-delayed) are intended to trip the circuit breaker rapidly on the occurrence of high values of fault current. Their tripping threshold Im is: c either fixed by standards for domestic type CBs, e.g. IEC 898, or, c indicated by the manufacturer for industrialtype CBs according to related standards, notably IEC 947-2.

domestic breakers IEC 898 modular industrial (2) circuit breakers industrial (2) circuit breakers IEC 947-2

type of protective relay thermalmagnetic

overload protection Ir = In

thermalmagnetic

Ir = In fixed

thermalmagnetic

Ir = In fixed adjustable: 0.7 In i Ir < In

electronic

long delay 0.4 In i Ir < In

For the latter circuit breakers there exists a wide variety of tripping devices which allow a user to adapt the protective performance of the circuit breaker to the particular requirements of a load.

short-circuit protection low setting standard setting type B type C 3 In i Im < 5 In 5 In i Im < 10 In low setting standard setting type B or Z type C 3.2 In < fixed < 4.8 In 7 In < fixed < 10 In fixed: Im ≈ 7 to 10 In adjustable: - low setting : 2 to 5 In - standard setting: 5 to 10 In short-delay, adjustable 1.5 Ir i Im < 10Ir instantaneous (I) fixed I ≈ 12 to 15 In

high setting circuit type D 10 In i Im < 20 In (1) high setting type D or K 10 In < fixed < 14 In

table H2.28: tripping-current ranges of overload and short-circuit protective devices for LV circuit breakers. (1) 50 In in IEC 898, which is considered to be unrealistically high by most European manufacturers (M-G = 10 to 14 In). (2) For industrial use, IEC standards do not specify values. The above values are given only as being those in common use.


H2 t (s)

t (s)

I(A)

I(A) Ir

Im

PdC

fig. H2-29: performance curve of a circuit breaker thermal-magnetic protective scheme.

Ir

Im

I

PdC

fig. H2-30 : performance curve of a circuit breaker electronic protective scheme.

Ir: overload (thermal or short-delay) relay trip-current setting. Im: short-circuit (magnetic or long-delay) relay trip-current setting. I: short-circuit instantaneous relay trip-current setting. PdC: breaking capacity.

isolating feature A circuit breaker is suitable for isolating a circuit if it fulfills all the conditions prescribed for a disconnector (at its rated voltage) in the relevant standard (see sub-clause 1.2). In such a case it is referred to as a circuit breaker-disconnector and marked on its front face with the symbol

the short-circuit current-breaking performance of a LV circuit breaker is related (approximately) to the cos ϕ of the fault-current loop. Standard values for this relationship have been established in some standards.

All Multi 9, Compact NS and Masterpact LV switchgear of Merlin Gerin manufacture is in this category.

rated short-circuit breaking capacity (Icu or Icn) The short-circuit current-breaking rating of a CB is the highest (prospective) value of current that the CB is capable of breaking without being damaged. The value of current quoted in the standards is the r.m.s. value of the a.c. component of the fault current, i.e. the d.c. transient component (which is always present in the worst possible case of short-circuit) is assumed to be zero for calculating the standardized value. This rated value (Icu) for industrial CBs and (Icn) for domestic-type CBs is normally given in kA r.m.s. Icu (rated ultimate s.c. breaking capacity) and Ics (rated service s.c. breaking capacity) are defined in IEC 947-2 together with a table relating Ics with Icu for different categories of utilization A (instantaneous tripping) and B (time-delayed tripping) as discussed in subclause 4.3. Tests for proving the rated s.c. breaking capacities of CBs are governed by standards, and include: c operating sequences, comprising a succession of manœuvres, i.e. closing and opening on short-circuit; c current and voltage phase displacement. When the current is in phase with the supply voltage (cos ϕ for the circuit = 1), interruption of the current is easier than that at any other power factor. Breaking a current at low lagging* values of cos ϕ is considerably more difficult to achieve; a zero power-factor circuit being (theoretically) the most onerous case.

In practice, all power-system short-circuit fault currents are (more-or-less) at lagging power factors, and standards are based on values commonly considered to be representative of the majority of power systems. In general, the greater the level of fault current (at a given voltage), the lower the power factor of the fault-current loop, for example, close to generators or large transformers. Table H2-31 below extracted from IEC 947-2 relates standardized values of cos ϕ to industrial circuit breakers according to their rated Icu. Icu 6 kA < Icu i 10 kA 10 kA < Icu i 20 kA 20 kA < Icu i 50 kA 50 kA i Icu

cos ϕ 0.5 0.3 0.25 0.2

table H2-31: Icu related to power factor (cos ϕ) of fault-current circuit. (IEC 947-2). c following an open - time delay - close/open sequence to test the Icu capacity of a CB, further tests are made to ensure that v the dielectric withstand capability; v the disconnection (isolation) performance and v the correct operation of the overload protection, have not been impaired by the test.



H2 4.3 other characteristics of a circuit breaker Familiarity with the following less-important characteristics of LV circuit breakers is, however, often necessary when making a final choice.

rated insulation voltage (Ui) This is the value of voltage to which the dielectric tests voltage (generally greater than 2 Ui) and creepage distances are referred. The maximum value of rated operational voltage must never exceed that of the rated insulation voltage, i.e. Ue i Ui.

rated impulse-withstand voltage (Uimp) This characteristic expresses, in kV peak (of a prescribed form and polarity) the value of voltage which the equipment is capable of withstanding without failure, under test conditions. For further details see chapter F, clause 2.

category (A or B) and rated short-time withstand current (Icw) As already briefly mentioned (sub-clause 4.2) there are two categories of LV industrial switchgear, A and B, according to IEC 947-2: c those of category A, for which there is no deliberate delay in the operation of the "instantaneous" short-circuit magnetictripping device (figure H2-32), are generally moulded-case type circuit breakers, and, c those of category B for which, in order to discriminate with other circuit breakers on a time basis, it is possible to delay the tripping of the CB, where the fault-current level is lower than that of the short-time withstand current rating (Icw) of the CB (figure H2-23). This is generally applied to large open-type circuit breakers and to certain heavy-duty moulded-case types. Icw is the maximum current that the B category CB can withstand, thermally and electrodynamically, without sustaining damage, for a period of time given by the manufacturer.

t (s)

I(A) Im

fig. H2-32: category A circuit breaker. t (s)

I(A) Im

I

Icw

fig. H2-33: category B circuit breaker.


PdC

H2 rated making capacity (Icm) Icm is the highest instantaneous value of current that the circuit breaker can establish at rated voltage in specified conditions. In a.c. systems this instantaneous peak value is related to Icu (i.e. to the rated breaking current) by the factor k, which depends on the power factor (cos ϕ) of the short-circuit current loop (as shown in table H2-34). Example: a LV circuit breaker has a rated breaking capacity Icu of 100 kA r.m.s. Its rated making capacity Icm will be 100 x 2.2 = 220 kA peak.

in a correctly designed installation, a circuit breaker is never required to operate at its maximum breaking current Icu. For this reason a new characteristic Ics has been introduced. It is expressed in IEC 947-2 as a percentage of Icu (25, 50, 75, 100%).

many designs of LV circuit breakers feature a short-circuit current limitation capability, whereby the current is reduced and prevented from reaching its (otherwise) maximum peak value (figure H2-35). The current-limitation performance of these CBs is presented in the form of graphs, typified by that shown in figure H2-36, diagram (a).

Icu 6 kA < Icu i 10 kA 10 kA < Icu i 20 kA 20 kA < Icu i 50 kA 50 kA i Icu

cos ϕ 0.5 0.3 0.25 0.2

Icm = kIcu 1.7 x Icu 2 x Icu 2.1 x Icu 2.2 x Icu

table H2.34: relation between rated breaking capacity Icu and rated making capacity Icm at different power-factor values of short-circuit current, as standardized in IEC 947-2.

rated service short-circuit breaking capacity (Ics) The rated breaking capacity (Icu) or (Icn) is the maximum fault-current a circuit breaker can successfully interrupt without being damaged. The probability of such a current occurring is extremely low, and in normal circumstances the fault-currents are considerably less than the rated breaking capacity (Icu) of the CB. On the other hand it is important that high currents (of low probability) be interrupted under good conditions, so that the CB is immediately available for reclosure, after the faulty circuit has been repaired. It is for these reasons that a new

characteristic (Ics) has been created, expressed as a percentage of Icu, viz: 25, 50, 75, 100% for industrial circuit breakers. The standard test sequence is as follows: c O - CO - CO* (at Ics); c tests carried out following this sequence are intended to verify that the CB is in a good state and available for normal service. For domestic CBs, Ics = k Icn. The factor k values are given in IEC 898 table XIV. In Europe it is the industrial practice to use a k factor of 100% so that Ics = Icu. Note: O represents an opening operation. CO represents a closing operation followed by an opening operation.

fault-current limitation The fault-current limitation capacity of a CB concerns its ability, more or less effective, in preventing the passage of the maximum prospective fault-current, permitting only a limited amount of current to flow, as shown in figure H2-35. The current-limitation performance is given by the CB manufacturer in the form of curves (figure H2-36 diagrams (a) and (b)). c diagram (a) shows the limited peak value of current plotted against the r.m.s. value of the a.c. component of the prospective fault current ("prospective" fault-current refers to the fault-current which would flow if the CB had no current-limiting capability); c limitation of the current greatly reduces the thermal stresses (proportional I2t) and this is shown by the curve of diagram (b) of figure H2-36, again, versus the r.m.s. value of the a.c. component of the prospective fault current. LV circuit breakers for domestic and similar installations are classified in certain standards (notably European Standard EN 60 898). CBs belonging to a class (of

current limiters) have standardized limiting I2t let-through characteristics defined by that class. In these cases, manufacturers do not normally provide characteristic performance curves. Icc prospectice fault-current peak

prospectice fault-current

limited current peak limited current t

tc

fig. H2-35: prospective and actual currents.

t

n rre cu s c d i ite st m ri -li cte n a no har c

limited peak current (kA)

limited peak current (A2 x s) 4,5.105

22

2.105

(a)

prospective a.c. component (r.m.s.)

(b)

prospective a.c. component (r.m.s.)

150

150 kA

fig. H2-36: performance curves of a typical LV current-limiting circuit breaker.



H2 4.3 other characteristics of a circuit breaker (continued) current limitation reduces both thermal and electrodynamic stresses on all circuit elements through which the current passes, thereby prolonging the useful life of these elements. Furthermore, the limitation feature allows "cascading" techniques to be used (see 4.5) thereby significantly reducing design and installation costs.

the advantages of current limitation The use of current-limiting CBs affords numerous advantages: c better conservation of installation networks: current-limiting CBs strongly attenuate all harmful effects associated with short-circuit currents; c reduction of thermal effects: conductors (and therefore insulation) heating is significantly reduced, so that the life of cables is correspondingly increased; c reduction of mechanical effects: forces due to electromagnetic repulsion are lower, with less risk of deformation and possible rupture, excessive burning of contacts, etc.; c reduction of electromagnetic-interference effects: less influence on measuring instruments and associated circuits, telecommunication systems, etc. These circuit breakers therefore contribute towards an improved exploitation of: c cables and wiring; c prefabricated cable-trunking systems; c switchgear, thereby reducing the ageing of the installation.

Example: On a system having a prospective shortcircuit current of 150 kA r.m.s., a circuit breaker limits the peak current to less than 10% of the calculated prospective peak value, and the thermal effects to less than 1% of those calculated. Cascading of the several levels of distribution in an installation, downstream of a limiting CB, will also result in important economies. The technique of cascading, described in sub-clause 4.5 allows, in fact, substantial savings on switchgear (lower performance permissible downstream of the limiting CB(s)) enclosures, and design studies, of up to 20% (overall). Discriminative protection schemes and cascading are compatible, in the range Compact NS*, up to the full short-circuit breaking capacity of the switchgear. * A Merlin Gerin product.

4.4 selection of a circuit breaker the choice of a range of circuit breakers is determined by: the electrical characteristics of the installation, the environment, the loads and a need for remote control, together with the type of telecommunications system envisaged.

choice of a circuit breaker The choice of a CB is made in terms of: c electrical characteristics of the installation for which the CB is destined; c its eventual environment: ambient temperature, in a kiosk or switchboard enclosure, climatic conditions, etc.; c short-circuit current breaking and making requirements; c operational specifications: discriminative tripping, requirements (or not) for remote control and indication and related auxiliary contacts, auxiliary tripping coils, connection into a local network (communication or control and indication) etc.,

c installation regulations; in particular: protection of persons; c load characteristics, such as motors, fluorescent lighting, LV/LV transformers, etc. Problems concerning specific loads are examined in chapter J. The following notes relate to the choice of a LV circuit breaker for use in distribution systems.

choice of rated current in terms of ambient temperature The rated current of a circuit breaker is defined for operation at a given ambient temperature, in general: c 30 °C for domestic-type CBs; c 40 °C for industrial-type CBs. Performance of these CBs in a different ambient temperature depends principally on the technology of their tripping units.

ambient temperature

single CB in free air

temperature of air surrounding the circuit breakers

circuit breakers installed in an enclosure

fig. H2-37: ambient temperature.


ambient temperature

H2 circuit breakers with uncompensated thermal tripping units have a tripcurrent level that depends on the surrounding temperature.

uncompensated thermalmagnetic tripping units Circuit breakers with uncompensated thermal tripping elements have a tripping-current level that depends on the surrounding temperature. If the CB is installed in an enclosure, or in a hot location (boiler room, etc.), the current required to trip the CB on overload will be sensibly reduced. When the temperature in which the CB is located exceeds its reference temperature, it will therefore be "derated". For this reason, CB manufacturers provide tables which indicate factors to apply at temperatures different to

the CB reference temperature. It may be noted from typical examples of such tables (tables H2-38) that a lower temperature than the reference value produces an up-rating of the CB. Moreover, small modular-type CBs mounted in juxtaposition, as shown typically in figure H2-24, are usually mounted in a small closed metal case. In this situation, mutual heating, when passing normal load currents, generally requires them to be derated by a factor of 0.8.

C60a. C60H: curve C. C60N: curves B and C (reference temperature: 30 °C) rating (A) 20 °C 25 °C 30 °C 35 °C 40 °C 45 °C 50 °C 55 °C 1 1.05 1.02 1.00 0.98 0.95 0.93 0.90 0.88 2 2.08 2.04 2.00 1.96 1.92 1.88 1.84 1.80 3 3.18 3.09 3.00 2.91 2.82 2.70 2.61 2.49 4 4.24 4.12 4.00 3.88 3.76 3.64 3.52 3.36 6 6.24 6.12 6.00 5.88 5.76 5.64 5.52 5.40 10 10.6 10.3 10.0 9.70 9.30 9.00 8.60 8.20 16 16.8 16.5 16.0 15.5 15.2 14.7 14.2 13.8 20 21.0 20.6 20.0 19.4 19.0 18.4 17.8 17.4 25 26.2 25.7 25.0 24.2 23.7 23.0 22.2 21.5 32 33.5 32.9 32.0 31.4 30.4 29.8 28.4 28.2 40 42.0 41.2 40.0 38.8 38.0 36.8 35.6 34.4 50 52.5 51.5 50.0 48.5 47.4 45.5 44.0 42.5 63 66.2 64.9 63.0 61.1 58.0 56.7 54.2 51.7 NS250N/H/L (reference temperature: 40 °C) rating (A) 40 °C 45 °C TM160D 160 156 TM200D 200 195 TM250D 250 244

50 °C 152 190 238

55 °C 147 185 231

60 °C 0.85 1.74 2.37 3.24 5.30 7.80 13.5 16.8 20.7 27.5 33.2 40.5 49.2 60 °C 144 180 225

tables H2-38: examples of tables for the determination of derating/uprating factors to apply to CBs with uncompensated thermal tripping units, according to temperature. Example What rating (In) should be selected for a CB c protecting a circuit, the maximum load current of which is estimated to be 34 A; c installed side-by-side with other CBs in a closed distribution box; c in an ambient temperature of 50 °C. A circuit breaker rated at 40 A would be derated to 35.6 A in ambient air at 50 °C (see

table H2-38). To allow for mutual heating in the enclosed space, however, the 0.8 factor noted above must be employed, so that, 35.6 x 0.8 = 28.5 A, which is not suitable for the 34 A load. A 50 A circuit breaker would therefore be selected, giving a (derated) current rating of 44 x 0.8 = 35.2 A.

compensated thermal-magnetic tripping units These tripping units include a bi-metal compensating strip which allows the overload trip-current setting (Ir or Irth) to be adjusted, within a specified range, irrespective of the ambient temperature. For example: c in certain countries, the TT system is standard on LV distribution systems, and domestic (and similar) installations are protected at the service position by a circuit breaker provided by the supply authority.

This CB, besides affording protection against indirect-contact hazard, will trip on overload; in this case, if the consumer exceeds the current level stated in his supply contract with the power authority. The circuit breaker (i 60 A) is compensated for a temperature range of - 5 °C to + 40 °C. c LV circuit breakers at ratings i 630 A are commonly equipped with compensated tripping units for this range (- 5 °C to + 40 °C).



H2 4.4 selection of a circuit breaker (continued) general note concerning derating of circuit breakers It is evident that a CB rated to carry a current In at its reference ambient temperature (30 °C) would overheat when carrying the same current at (say) 50 °C. Since LV CBs are provided with overcurrent protective devices which (if not compensated) will operate for lower levels of current in higher ambient temperatures, the CB is automatically derated by the overload tripping device, as shown in the tables H2-38. Where the thermal tripping units are temperature-compensated, the tripping current level may be set at any value between 0.7 to 1 x In in the ambient temperature range of - 5 °C to + 40 °C. The reference ambient temperature in this case is 40 °C (i.e. on which the rating In is based). For these compensated units, manufacturers' catalogues generally also give derated values of In for ambient temperatures above the compensated range, e.g. at + 50 °C and + 60 °C; typically, 95 A at + 50 °C and 90 A at + 60 °C, for a 100 A circuit breaker.


H2 electronic tripping units are highly stable in changing temperature levels.

electronic tripping units An important advantage with electronic tripping units is their stable performance in changing temperature conditions. However, the switchgear itself often imposes operational limits in elevated temperatures, M25N/H/L circuit breaker A

i 40 °C 2500 1 2500 1

In (A) maximum adjustment Ir In (A) maximum adjustement Ir

circuit breaker B coeff.

as mentioned in the general note above, so that manufacturers generally provide an operating chart relating the maximum values of permissible trip-current levels to the ambient temperature (figure H2-39). 45 °C 2500 1 2500 1

50 °C 2500 1 2500 1

55 °C 2450 0.98 2350 0.94

60 °C 2400 0.96 2200 0.88

In (A)

1 2500

circuit breaker A 0.96 2400 0.94 2350 circuit breaker B

0.88 2200 θ °C 20

25

30

35

40

45

50

55

60

fig. H2-39: derating of two circuit breakers having different characteristics, according to the temperature.

selection of an instantaneous, or short-time-delay, tripping threshold Principal charasteristics of magnetic or shorttime-delay tripping units. Type classification according to IEC 898. See also table H2-28. type t

tripping unit low setting type B

applications c sources producing low-short-circuit-current levels (standby generators) c long lengths of line or cable

standard setting type C

c protection of circuits: general case

high setting type D or K

c protection of circuits having high initial transient current levels (e.g. motors, transformers, resistive loads)

12 In type MA

c protection of motors in association with discontactors (contactors with overload protection)

I t

I t

I t

I

table H2-40: different tripping units, instantaneous or short-time-delayed.



H2 4.4 selection of a circuit breaker (continued) the installation of a LV circuit breaker requires that its short-circuit breaking capacity (or that of the CB together with an associated device) be equal to or exceeds the calculated prospective short-circuit current at its point of installation.

the circuit breaker at the output of the smallest transformer must have a short-circuit capacity adequate for a fault current which is higher than that through any of the other transformer LV circuit breakers (fig. H2-42).

selection of a circuit breaker according to the short-circuit breaking capacity requirements The installation of a circuit breaker in a LV installation must fulfil one of the two following conditions: c either have a rated short-circuit breaking capacity Icu (or Icn) which is equal to or exceeds the prospective short-circuit current calculated for its point of installation, or c if this is not the case, be associated with another device which is located upstream, and which has the required short-circuit breaking capacity.

The selection of main and principal circuit breakers c a single transformer Table C-13 (in chapter C) gives the shortcircuit current level on the downstream side of a commonly-used type of HV/LV distribution transformer. If the transformer is located in a consumer's substation, certain national standards require a LV circuit breaker in which the open contacts are clearly visible*. Example (figure H2-41): What type of circuit breaker is suitable for the main circuit breaker of an installation supplied through a 250 kVA HV/LV (400 V) 3-phase transformer in a consumer's substation? In transformer = 360 A Isc (3-phase) = 8.9 kA. A 400 A CB with an adjustable tripping-unit range of 250 A-400 A and a short-circuit breaking capacity (Icu) of 35 kA* would be a suitable choice for this duty.

In the second case, the characteristics of the two devices must be co-ordinated such that the energy permitted to pass through the upstream device must not exceed that which the downstream device and all associated cables, wires and other components can withstand, without being damaged in any way. This technique is profitably employed in: c associations of fuses and circuit breakers; c associations of current-limiting circuit breakers and standard circuit breakers. The technique is known as "cascading" (see sub-clause 4.5 of this chapter).

250 kVA 20 kV/400 V

Visucompact NS400N

fig. H2-41: example of a transformer in a consumer's substation.

* A type Visucompact NS400N of Merlin Gerin manufacture is recommended for the case investigated.

c several transformers in parallel (figure H2-42) v the circuit breakers CBP outgoing from the LV distribution board must each be capable of breaking the total fault current from all transformers connected to the busbars, viz: Isc1 + Isc2 + Isc3, v the circuit breakers CBM, each controlling the output of a transformer, must be capable of dealing with a maximum short-circuit current of (for example) Isc2 + Isc3 only, for a short-circuit located on the upstream side of CBM1. From these considerations, it will be seen that the circuit breaker of the smallest transformer will be subjected to the highest level of fault current in these circumstances, while the circuit breaker of the largest transformer will pass the lowest level of short-circuit current. v the ratings of CBMs must be chosen according to the kVA ratings of the associated transformers. Note: the essential conditions for the successful operation of 3-phase transformers in parallel may be summarized as follows: 1. the phase shift of the voltages, primary to secondary, must be the same in all units to be paralleled. 2. the open-circuit voltage ratios, primary to secondary, must be the same in all units. 3. the short-circuit impedance voltage (Zsc%) must be the same for all units. For example, a 750 kVA transformer with a Zsc = 6% will H2-24 - the protection of circuits - the switchgear

HV

HV

Tr1

Tr2

LV A1

HV Tr3

LV A2

CBM

B1

B2 CBP

CBM

LV A3

CBM

B3 CBP

E

fig. H2-42: transformers in parallel.

share the load correctly with a 1,000 kVA transformer having a Zsc of 6%, i.e. the transformers will be loaded automatically in proportion to their kVA ratings. For transformers having a ratio of kVA ratings exceeding 2, parallel operation is not recommended, since the resistance/ reactance ratios of each transformer will generally be different to the extent that the resulting circulating currrent may overload the smaller transformer.

H2 Table H2-43 indicates, for the most usual arrangement (2 or 3 transformers of equal kVA ratings) the maximum short-circuit currents to which main and principal CBs (CBM and CBP respectively, in figure H2-42) are subjected. The table is based on the following hypotheses: c the short-circuit 3-phase power on the HV side of the transformer is 500 MVA; c the transformers are standard 20/0.4 kV distribution-type units rated as listed; c the cables from each transformer to its LV circuit breaker comprise 5 metres of singlecore conductors; c between each incoming-circuit CBM and each outgoing-circuit CBP there is 1 metre of busbar; c the switchgear is installed in a floormounted enclosed switchboard, in an ambient-air temperature of 30 °C. Moreover, this table shows selected circuit breakers of M-G manufacture recommended for main and principal circuit breakers in each case. number and kVA ratings of 20/0.4 kV transformers 2 x 400 3 x 400 2 x 630 3 x 630 2 x 800 3 x 800 2 x 1000 3 x 1000 2 x 1250 3 x 1250 2 x 1600 3 x 1600 2 x 2000 3 x 2000

minimum S.C. breaking capacity of main CBs (Icu)* kA 14 27 22 43 24 48 27 54 31 62 36 72 39 77

main circuit breakers (CBM) total discrimination with out going-circuit breakers (CBP) M08 N1/C 801 N ST M08 N1/C 801 N ST M10N1/CM1250/C 1001 N M10H1/CM1250/C 1001 N M12N1/CM1250/C 1251 N M12H1/CM1250/C 1251 N M16N1/CM1600 M16H2/CM1600 M20N1/CM2000 M20H1/CM2000 M25N1/CM2500 M20H2/CM2500H M32H1/CM3200 M32H2/CM3200H

minimum S.C. breaking cap. of principal CBs (Icu)* kA 27 40 42 64 48 71 54 80 60 91 70 105 75 112

rated current In of principal circuit breaker (CPB) 250 A NS 250 N NS 250 H NS 250 H NS 250 H NS 250 H NS 250 L NS 250 H NS 250 L NS 250 H NS 250 L NS 250 H NS 250 L NS 250 L NS 250 L

table H2-43: maximum values of short-circuit current to be interrupted by main and principal circuit breakers (CBM and CBP respectively), for several transformers in parallel. * or Ics in countries where this alternative is practised.

Example: (figure H2-44) c circuit breaker selection for CBM duty: In for an 800 kVA transformer = 1.126 A (at 410 V, i.e. no-load voltage) Icu (minimum) = 48 kA (from table H2-43), the CBM indicated in the table is a Compact C1251 N (Icu = 50 kA) (by Merlin Gerin) or its equivalent; c circuit breaker selection for CBP duty: The s.c. breaking capacity (Icu) required for these circuit breakers is given in the table (H2-43) as 71 kA. A recommended choice for the three outgoing circuits 1, 2 and 3 would be current-limiting circuit breakers types NS 400 L, NS 100 L and NS 250 L respectively (by MG) or their equivalents. The Icu rating in each case = 150 kA. These circuit breakers provide the advantages of: v absolute discrimination with the upstream (CBM) breakers, v exploitation of the "cascading" technique, with its attendant economy for all downstream components.

3 Tr 800 kVA 20 kV/400V CBM

CBP1

400 A

CBP3

CBP2

100 A

200 A

fig H2-44: transformers in parallel.



H2 4.4 selection of a circuit breaker (continued) short-circuit fault-current levels at any point in an installation may be obtained from tables.

cascading: a particular solution to problems of CBs insufficiently rated for S.C. breaking duty.

associating fuses with CBs avoids the need for a fuse in the neutral, except in particular circumstances on some IT systems.


Choice of outgoing-circuit CBs and final-circuit CBs c use of table H1-40 From this table, the value of 3-phase shortcircuit current can be determined rapidly for any point in the installation, knowing: v the value of short-circuit current at a point upstream of that intended for the CB concerned; v the length, c.s.a., and the composition of the conductors between the two points. A circuit breaker rated for a short-circuit breaking capacity exceeding the tabulated value may then be selected. c detailed calculation of the short-circuit current level In order to calculate more precisely the shortcircuit current, notably, when the short-circuit current-breaking capacity of a CB is slightly less than that derived from the table, it is necessary to use the method indicated in chapter H1 clause 4. c two-pole circuit breakers (for phase and neutral) with one protected pole only These CBs are generally provided with an overcurrent protective device on the phase pole only, and may be used in TT, TN-S and IT schemes. In an IT scheme, however, the following conditions must be respected: v condition (c) of table H1-65 for the protection of the neutral conductor against overcurrent in the case of a double fault; v short-circuit current-breaking rating: A 2-pole phase-neutral CB must, by convention, be capable of breaking on one pole (at the phase-to-phase voltage) the current of a double fault equal to 15% of the 3-phase short-circuit current at the point of its installation, if that current is i 10 kA; or 25% of the 3-phase short-circuit current if it exceeds 10 kA; v protection against indirect contact: this protection is provided according to the rules for IT schemes, as described in chapter G sub-clause 6.2. c insufficient short-circuit currentbreaking rating In low-voltage distribution systems it sometimes happens, especially in heavy-duty networks, that the Isc calculated exceeds the Icu rating of the CBs available for installation, or system changes upstream result in lowerlevel CB ratings being exceeded. v solution 1: check whether or not appropriate CBs upstream of the CBs affected are of the current-limiting type, allowing the principle of cascading (described in sub-clause 4.5) to be applied;

v solution 2: install a range of CBs having a higher rating. This solution is economically interesting only where one or two CBs are affected; v solution 3: associate current-limiting fuses (gG or aM) with the CBs concerned, on the upstream side. This arrangement must, however, respect the following rules: - the fuse rating must be appropriate - no fuse in the neutral conductor, except in certain IT installations where a double fault produces a current in the neutral which exceeds the short-circuit breaking rating of the CB. In this case, the blowing of the neutral fuse must cause the CB to trip on all phases.

H2 4.5 coordination between circuit breakers Preliminary note on the essential function of current limiting circuit breakers Low-voltage current-limiting CBs exploit the resistance of the short-circuit current arc in the CB to limit the value of current. An improved method of achieving currentlevel limitation is to associate a separate current-limiting module (in series) with a standard CB. A contact bar (per phase) in the module bridges two (specially-designed heavy-duty) contacts, the contact pressure of which is accurately maintained by springs. Other rigidly-fixed conductors are arranged in series with, and close to the contact bar, such that when current is passed through the ensemble, the electromagnetic force tends to move the contact bar to open its contacts. This occurs at relatively low values of shortcircuit current, which then passes through the arcs formed at each contact. The resistance of the arcs is comparable with system impedances at low voltage, so that the current is correspondingly restricted.

the technique of "cascading" uses the properties of current-limiting circuit breakers to permit the installation of all downstream switchgear, cables and other circuit components of significantly lower performance than would otherwise be necessary, thereby simplifying and reducing the cost of an installation.

in general, laboratory tests are necessary to ensure that the conditions of exploitation required by national standards are met and compatible switchgear combinations must be provided by the manufacturer.

Furthermore, the higher the current, the more the repulsive force on the bar and the greater the arc resistance as its path lengthens, i.e. the current magnitude is (to some extent) self-regulating. The circuit breaker is easily able to break the resulting low value of current, particularly since the power factor of the fault-current loop is increased by the resistive impedance of the arcs. When used in a cascading scheme as described below, the tripping of the limiting CB main contacts is briefly delayed, to allow downstream high-speed circuit breakers to clear the (limited) current, i.e. the currentlimiter CB remains closed. The contact bar in the limiter module resets under the influence of its pressure springs when the flow of short-circuit current ceases. Failure of downstream CBs to trip will result in the tripping of the current-limiting CB, after its brief time delay.

cascading Definition of the cascading technique By limiting the peak value of short-circuit current passing through it, a current-limiting CB permits the use, in all circuits downstream of its location, of switchgear and circuit components having much lower short-circuit breaking capacities, and thermal and electromechanical withstand capabilities than would otherwise be the case. Reduced physical size and lower performance requirements lead to substantial economies and to the simplification of installation work. It may be noted that, while a current-limiting circuit breaker has the effect on downstream circuits of (apparently) increasing the source impedance during short-circuit conditions, it has no such effect at any other time; for example, during the starting of a large motor (where a low source impedance is highly desirable). A new range of Compact* current-limiting circuit breakers with powerful limiting performances (namely: NS 100, NS 160, NS 250 and NS 400) is particularly interesting. Conditions of exploitation Most national standards permit use of the cascading technique, on condition that the amount of energy "let through" by the limiting CB is less than that which all downstream CBs and components are able to withstand without damage. In practice this can only be verified for CBs by tests performed in a laboratory. Such tests are carried out by manufacturers who provide the information in the form of tables, so that users can confidently design a cascading scheme based on the combination of circuit breaker types recommended. By way of an example, table H2-45 indicates the possibilities of cascading circuit breaker types* C 60 and NC 100 when installed downstream of current-limiting CBs NS 250 N, H or L for a 230/400 V or 240/415 V 3-phase installation. * Merlin Gerin products



H2 4.5 coordination between circuit breakers (continued) Advantages of cascading The limitation of current benefits all downstream circuits that are controlled by the current-limiting CB concerned. The principle is not restrictive, i.e. currentlimiting CBs can be installed at any point in an installation where the downstream circuits would otherwise be inadequately rated. The result is: c simplified short-circuit current calculations; c simplification, i.e. a wider choice of downstream switchgear and appliances; c the use of lighter-duty switchgear and appliances, with consequently lower cost; c economy of space requirements, since light-duty equipment is generally less voluminous.

Short-circuit breaking capacity of the upstream (limiter) CBs kA r.m.s. 150 NS250L 100 70 NS250H 36 NS250N 25 22 Short-circuit breaking capacity of the downstream CBs (benefiting from the cascading technique) kA r.m.s. 150 NC100LH NC100LMA 100 NC100LS 70 NC100LS NC100L NC100LH NC100LMA 50 NC100L 40 C60L i 40 C60L i 40 30 C60H C60N C60N C60L C60H C60H C60L C60L (50 to 63) (50 to 63) NC100H NC100H 25 C60N NC100H 20 C60a C60a 15 C60a tables H2-45: example of cascading possibilities on a 230/400 V or 240/415 V 3-phase installation.

discrimination may be absolute or partial, and based on the principles of current levels, or time-delays, or a combination of both. A more recent development is based on the principles of logic. A (patented) system by Merlin Gerin exploits the advantages of both current-limitation and discrimination.


discriminative tripping (selectivity) Discrimination is achieved by automatic protective devices if a fault condition, occurring at any point in the installation, is cleared by the protective device located immediately upstream of the fault, while all other protective devices remain unaffected (figure H2-46). Discrimination between circuit breakers A and B is absolute if the maximum value of shortcircuit-current on circuit B does not exceed the short-circuit trip setting of circuit breaker A. For this condition, B only will trip (figure H2-47). Discrimination is partial if the maximum possible short-circuit current on circuit B exceeds the short-circuit trip-current setting of circuit breaker A. For this maximum condition, both A and B will trip (figure H2-48).

IscA A

IscB B

absolute discrimination

Icc IccB

IrB

partial discrimination B only open A and B opens IrB

Ic

IccB

fig. H2-46: absolute and partial discrimination.

Icc

H2 t

t

B

A

B

A

Isc downstream of B Ir B

Ir A

Icc B Irm A

I

Ir B

Ir A

Irm A Isc B

B only opens

1. discrimination based on current levels. This method is realized by setting successive relay tripping thresholds at stepped levels, from downstream relays (lower settings) towards the source (higher settings).

fig. H2-48: partial discrimination between CBs A and B.

Discrimination is absolute or partial, according to the particular conditions, as noted in the above examples.

t

B

In the two-level arrangement shown, upstream circuit breaker A is delayed sufficiently to ensure absolute discrimination with B (for example: Masterpact electronic).

I

A and B open

fig. H2-47: absolute discrimination between CBs A and B.

A

I

Irm A Isc B

Irm B

2. discrimination based on stepped time delays. This method is implemented by adjusting the time-delayed tripping units, such that downstream relays have the shortest operating times, with progressively longer delays towards the source.

IscA

A t B A ∆t B Isc B

3. discrimination based on a combination of methods 1 and 2. A mechanical time-delay added to a currentlevel scheme can improve the overall discrimination performance.

Discrimination is absolute if Isc B < Irm A (instantaneous). The upstream CB has two high-speed magnetic tripping thresholds: - Irm A (delayed) or a SD* electronic timer - Irm A (instantaneous) standard (Compact type SA)

t

B

chambers of the CBs. The heated-air pressure level depends on the energy level of the arc, as described in the following pages (figures H2-54 and H2-55).

A Isc B

* short-delay.

4. discrimination based on arc-energy levels (Merlin Gerin patent) In the range of short-circuit currents, this system provides absolute discrimination between two circuit breakers passing the same fault current. This is achieved by using current-limiting CBs and initiating CB tripping by pressure-sensitive detectors in the arcing

I

Irm A delayed

Irm A instantaneo us

I

t

conventional instantaneous magnetic-trip characteristic pressure operated magnetic-trip characteristic

Irm B

Irm A

Isc

table H2-49: summary of methods and components used in order to achieve discriminative tripping.



H2 4.5 coordination between circuit breakers (continued) current-level discrimination is achieved with stepped current-level settings of the instantaneous magnetic-trip elements.

Current-level discrimination Current-level discrimination is achieved with circuits breakers, preferably limiters, and stepped current-level settings of the instantaneous magnetic-trip elements. c the downstream circuit breaker is not a current-limiter. The discrimination may be absolute or partial for a short-circuit fault downstream of B, as previously noted in 1, above. Absolute discrimination in this situation is practically impossible because Isc A z Isc B, so that both circuit breakers will generally trip in unison. In this case discrimination is partial, and limited to the Irm of the upstream circuit breaker. c the downstream circuit breaker is a current limiter. Improvement in discriminative tripping can be obtained by using a current limiter in a downstream location, e.g. for circuit breaker B. For a short-circuit downstream of B, the limited level of peak current IB would operate the (suitably adjusted) magnetic trip unit of B, but would be insufficient to cause circuit breaker A to trip. Note: All LV breakers (considered here) have some inherent degree of current limitation, even those that are not classified as currentlimiters. This accounts for the curved characteristic shown for the standard circuit breaker A in figure H2-50. Careful calculation and testing is necessary, however, to ensure satisfactory performance of this arrangement. c the upstream circuit breaker is highspeed with a short-delay (SD) feature. These circuit breakers are fitted with trip units which include a non-adjustable mechanical short-time-delay feature. The delay is sufficient to ensure absolute discrimination with any downstream high-speed CB at any value of s.c. current up to Irms (figure H2-51).

Example: circuit breaker A: Compact NS250 N fitted with a trip unit which includes a SD feature. Ir = 250 A, magnetic trip set at 2,000 A circuit breaker B: Compact NS100N Ir = 100 A The Merlin Gerin distribution catalogue indicates a discrimination limit of 3,000 A (an improvement over the limit of 2,500 A obtained when using a standard tripping unit).

I peak A current limitation curve for circuit breaker (see note) B

fault upstream of B fault downstream of B

I Isc prospective (rms)

Isc

fig. H2-50: downstream limiting circuit breaker B. t A (compact S) B

only B opens

A and B open Irm A Irm S delayed instantaneous

I

fig. H2-51: use of a "selective" circuit breaker upstream.

discrimination based on time-delayed tripping uses CBs referred to as "selective" (in certain countries). Application of these CBs is relatively simple and consists in delaying the instant of tripping of the several series-connected circuit breakers in a stepped time sequence.


Time-based discrimination This technique requires: c the introduction of "timers" into the tripping mechanisms of CBs; c CBs with adequate thermal and mechanical withstand capabilities at the elevated current levels and time delays envisaged. Two circuit breakers A and B in series (i.e. passing the same current) are discriminative if the current-breaking period of downstream breaker B is less than the non-tripping time of circuit breaker A. Discrimination at several levels An example of a practical scheme with (MG) circuit breakers Masterpact (electronic protection devices). These CBs can be equipped with adjustable timers which allow 4 time-step selections, such as: c the delay corresponding to a given step is greater than the total current breaking time of the next lower step;

c the delay corresponding to the first step is greater than the total current-breaking time of a high-speed CB (type Compact for example) or of fuses (figure H2-52). t A non tripping time of A

B

current-breaking time for B

only B open Ir B

Isc B

Isc I

fig. H2-52: discrimination by time delay.

H2 discrimination schemes based on logic techniques are possible, using CBs equipped with electronic tripping units designed for the purpose (Compact, Masterpact by MG) and interconnected with pilot wires.

recently-introduced circuit breakers such as Merlin Gerin type NS, use the principle of arc-energy levels to obtain discrimination.

Discrimination logic This discrimination system requires CBs equipped with electronic tripping units, designed for this application, together with interconnecting pilot wires for data exchange between the CBs. With 2 levels A and B (figure H2-53), circuit breaker A is set to trip instantaneously, unless the relay of circuit breaker B sends a signal to confirm that the fault is downstream of B. This signal causes the tripping unit of A to be delayed, thereby ensuring back-up protection in the event that B fails to clear the fault, and so on… This system (patented by Merlin Gerin) also allows rapid localization of the fault. Limitation and discrimination by exploitation of arc energy The technique of "arc-energy discrimination" (Merlin Gerin patent) is applied on circuits having a short-circuit current level u 25 In and ensures absolute selectivity between two CBs carrying the same short-circuit current. Discrimination requires that the energy allowed to pass by the downstream CB (B) is less than that which will cause the upstream CB (A) to trip (fig. H2-54 (a)). Operation principle Both CBs are current limiters, so that the electromagnetic forces due to a short-circuit downstream of CB (B) will cause the currentlimiting arcing contacts of both CBs to open simultaneously. The fault current will be very strongly limited by the resistance of the two series arcs. The intense heat of the current arc in each CB causes a rapid expansion of the air in the confined space of the arcing chambers, thereby producing a correspondingly rapid pressure rise. Above a certain level of current, the pressure rise can be reliably detected and used to initiate instantaneous tripping. Discrimination principle If both CBs include a pressure tripping device suitably regulated, then absolute discrimination between two CBs of different current ratings can be achieved by setting CB (B) to trip at a lower pressure level than that of CB (A) (fig. H2-54). If a short-circuit occurs downstream of CB (A) but upstream of CB (B), then the arc resistance of CB (A) only will limit the current. The resulting current will therefore be significantly greater than that occurring for a short-circuit downstream of CB (B) (where the two arcs in series cause a very strong limitation, as previously mentioned). The larger current through CB (A) will produce a correspondingly greater pressure, which will be sufficient to operate its pressure-sensitive tripping unit (diagrams (b) and (c) of fig. H2-54). As can be seen from figure H2-49 (4), the larger the short-circuit current, the faster the CB will trip. Discrimination is assured with this particular switchgear if: c the ratio of rated currents of the two CBs u 2.5; c the ratio of the two trip-unit current ratings is > 1.6, as shown (typically) in figure H2-55.

A

pilot wires

B

fig. H2-53: discrimination logic.

CB (A) Compact NS

(a) CB (B) Compact NS

Isc = 50 kA I Isc (prospective) CB (A) only CB (A) and CB (B) in series

(b)

Isc (limited) t Pressure in arcing chamber CB (A) setting

(c) CB (B) setting

t

fig. H2-54: arc-energy discrimination principles.

CB (A)

CB (B)

NS250N TM260D

NS100N TM100D

fig. H2-55: ratio of rated currents of CBs and of tripping units, must comply with limits stated in the text, to ensure discrimination.

For overcurrent conditions less than those of short-circuits i 25 In, the conventional protection schemes are employed, as previously described in this chapter. the protection of circuits - the switchgear - H2-31


H2 4.6 discrimination HV/LV in a consumer's substation In general the transformer in a consumer's substation is protected by HV fuses, suitably rated to match the transformer, in accordance with the principles laid down in IEC 787 and IEC 420, by following the advice of the fuse manufacturer. The basic requirement is that a HV fuse will not operate for LV faults occurring downstream of the transformer LV circuit breaker, so that the tripping characteristic curve of the latter must be to the left of that of the HV fuse pre-arcing curve. This requirement generally fixes the maximum settings for the LV circuit breaker protection: c maximum short-circuit current-level setting of the magnetic tripping element; c maximum time-delay allowable for the short-circuit current tripping element. See also Chapter C sub-clause 3.2.7, and Appendix C1, for further details.

63 A

full-load current 1760 A 3-phase short-circuit current level 31.4 kA

1250 kVA 20 kV / 400 V

Visucompact CM 2000 set at 1800 A

fig. H2-56: example. c short-circuit level at HV terminals of transformer: 250 MVA; c transformer HL/LV: 1,250 kVA 20/0.4 kV; c HV fuses: 63 A (table C 11); c cabling, transformer - LV circuit breaker: 10 metres single-core cables; c LV circuit breaker: Visucompact CM 2000 set at 1,800 A (Ir). What is the maximum short-circuit trip current setting and its maximum time delay allowable? The curves of figure H2-57 show that discrimination is assured if the short-time delay tripping unit of the CB is set at: c a level i 6 Ir = 10.8 kA; c a time-delay setting of step O or A. A general policy for HV fuse/LV circuit breaker discrimination, adopted in some countries, which is based on standardized manufacturing tolerance limits, is mentioned in chapter C sub-clause 3.2.7, and illustrated in figure C-21. Where a transformer is controlled and protected on the high-voltage side by a circuit breaker, it is usual to install separate CT- and/ or VT- operated relays, which energize a shunt-trip coil of the circuit breaker. Discrimination can be achieved, together with high-speed tripping for faults on the transformer, by using the methods described in chapter C sub-clause 3.2.


t (ms) 1000

CM 2000 set at 1800 A

minimum pre-arcing curve for 63 A HV fuses (current referred to the secondary side of the transformer)

200 100

10 1 Ir

4 Ir

6 Ir 8 Ir

220 1

step C step B step A

50

step 0

0,01

1800 A Ir

10 kA

Isc maxi 31,4 kA

fig. H2-57: curves of HV fuses and LV circuit breaker.

I

1. protection of circuits supplied by an alternator

J a major difficulty encountered when an installation may be supplied from alternative sources (e.g. a HV/LV transformer or a LV generator) is the provision of electrical protection which operates satisfactorily on either source. The crux of the problem is the great difference in the source impedances; that of the generator being much higher than that of the transformer, resulting in a corresponding difference in the magnitudes of fault currents.

Most industrial and large commercial electrical installations include certain important loads for which a power supply must be maintained, in the event that the public electricity supply fails: c either, because safety systems are involved (emergency lighting, automatic fire-protection equipment, smoke dispersal fans, alarms and signalization, and so on...) or: c because it concerns priority circuits, such HV

as certain equipment, the stoppage of which would entail a loss of production, or the destruction of a machine tool, etc. One of the current means of maintaining a supply to the so-called “essential” loads, in the event that other sources fail, is to install a diesel-generator set connected, via a changeover switch, to an emergency-power standby switchboard, from which the essential services are fed (figure J1-1).

G

LV

standby supply change-over switch

non essential loads

essential loads

fig. J1-1: example of circuits supplied from a transformer or from an alternator.

1.1 an alternator on short-circuit the establishment of short-circuit current (fig. J1-2) Apart from the limited magnitude of fault current from a standby alternator, a further difficulty (from the electrical-protection point of view) is that during the period in which LV circuit breakers are normally intended to operate, the value of short-circuit current changes drastically. For example, on the occurrence of a shortcircuit at the three phase terminals of an alternator, the r.m.s. value of current will immediately rise to a value of 3 In to 5 In*. An interval of 10 ms to 20 ms following the instant of short-circuit is referred to as the “sub-transient” period, in which the current decreases rapidly from its initial value. The current continues to decrease during the ensuing “transient” interval which may last for 80 ms to 280 ms depending on the machine type, size, etc. The overall phenomenon is referred to as the “a.c. decrement”. The current will finally stabilize in about subtransient period

r.m.s.

0.5 seconds, or more, at a value which depends mainly on the type of excitation system, viz: c manual; c automatic (see figure J1-2). Almost all modern generator sets have automatic voltage regulators, compounded to maintain the terminal voltage sensibly constant, by overcoming the synchronous impedance of the machine as reactive current demand changes. This results in an increase in the level of fault current during the transient period to give a steady fault current in the order of 2.5 In to 4 In* (figure J1-2). In the (rare) case of manual control of the excitation, the synchronous impedance of the machine will reduce the short-circuit current to a value which can be as low as 0.3 In, but is often close to In*.

transient period

alternator with automatic voltage regulator

3 In

alternator with manual excitation control

In 0.3 In instant of fault

10 to 20 ms

0.1 to 0.3 s

t

fig. J1-2: establishment of short-circuit current for a three-phase short circuit at the terminals of an alternator. * depending on the characteristics of the particular machine.

particular supply sources and loads - J1

1. protection of circuits supplied by an alternator (continued)

J 1.1 an alternator on short-circuit (continued) Figure J1-2 shows the r.m.s. values of current, on the assumption that no d.c. transient components exist. In practice, d.c. components of current are always present to some degree in at least two phases, being maximum when the short-circuit occurs at the alternator terminals. This feature would appear to complicate still further the matter of electrical protection, but, in fact, the d.c. component in each phase simply increases the r.m.s. values already mentioned, so that calculations and trippingcurrent settings for protective devices based only on the a.c. components, as indicated below, will be conservative, i.e. the actual currents will always be either equal to or higher than those calculated. The further the point of short-circuit from the generator the lower the fault current, and the more rapidly the transient d.c. components disappear. Furthermore, the a.c. decrement also becomes negligible when the network impedance to the fault position attains ohmic values which are high compared with the reactance values of the alternator (since the overall change in impedance is then relatively small).

alternator impedance data Manufacturers furnish values of the several impedances mentioned below. Resistances are negligibly small compared to the reactances. It can be seen from the constantly-changing value of r.m.s. current that the effective reactance* changes constantly from a low value (sub-transient reactance) to a high value (synchronous reactance) in a smooth progression. The values discussed below are derived from test curves and correspond with current values measured at the instant of shortcircuit. * An explanation of the significance of the fixed reactance values and how they relate to a smooth variation of current is briefly described in Appendix J1. c the sub-transient reactance x”d is expressed in % by the manufacturer (analogous to the short-circuit impedance voltage of a transformer). The ohmic value X”d is therefore calculated as follows: x”d Un2 10-5 X”d (ohms) = Pn where: x”d is in % Un is in volts (phase/phase) Pn is in kVA c the % transient reactance x’d is given in ohms by: x'd Un2 10-5 X'd (ohms) = Pn c the % zero-phase-sequence reactance x’o is given in ohms by: x'o Un2 10-5 X'o (ohms) = Pn In the absence of more precise information, the following representative values may be used: x”d = 20% ; x’d = 30 % ; x’o = 6% Pn and Un being, respectively, the rated 3-phase power (kVA) and the rated phase/phase voltage of the alternator (volts).

J2 - particular supply sources and loads

The sub-transient reactance is used when calculating the short-circuit current-breaking rating for LV circuit breakers which have opening times of 20 ms or less, and also for the electrodynamic stresses to be withstood by CBs and other components (such as busbars, cleated single-core cables, etc.). The transient reactance is used when considering the breaking capacity of LV circuit breakers with an opening time that exceeds 20 ms, and also for the thermal withstand capabilities of switchgear and other system components. Remark: from the instant at which the shortcircuit is established, the alternator reactance will rapidly increase. This means that the currents calculated from the defined fixed values x"d and x'd (for breaking capacity) will always exceed those that will actually occur at the instant of circuit breaker contact separation, i.e. there is an inherent safety factor incorporated in the current-level calculation. These calculations for the circuit breaker short-circuit breaking capacity are based on the symmetrical a.c. components of current only, i.e. no account is taken of the d.c. unidirectional components. For the circuit breaker short-circuit making capacity, the d.c. components are crucial, as discussed in Chapter C, Sub-clause 1.1 (figure C-5).

J short-circuit current magnitude at the terminals of an alternator c the transient 3-phase short-circuit current at the terminals of an alternator is given by: Ig Isc = 100* where: x’d Ig: rated full-load current of the alternator x’d = transient reactance per phase of the alternator in %; c when these values are compared with those for a short-circuit at the LV terminals of 630 kVA 20 kV/400 V Usc = 4%

a transformer of equal kVA rating, the current from the alternator will be found to be of the order of 5 or 6 times less than that from the transformer. The difference will be even greater where (as is generally the case) the alternator rating is lower than that of the transformer. * for CBs with opening time exceeding 20 ms.

250 kVA 400 V X'd = 30%

A

non essential loads

essential loads

fig. J1-3: example of an essential services switchboard supplied (in an emergency) from a standby alternator. Example (figure J1 - 3) What is the value of 3-phase short-circuit current at point A according to the origin of supply? Circuit impedances are negligible compared with those of the sources. c transformer supply 3-phase Isc = 21.5 kA (see table C20 in Chapter C)

c alternator supply 3-phase Isc = Ig x 100 = Pn x 100 x'd x'd eUn where: Pn is expressed in kVA Un is expressed in volts x’d is expressed in % Isc is expressed in kA 3-phase Isc = 250 x 100 = 1.2 kA ex 400 x 30



J 1.2 protection of essential services circuits supplied in emergencies from an alternator the difficulty is due to the small margin between the rated current and the short-circuit current of the alternator.

The characteristics (s.c. breaking capacity and range of adjustable magnetic tripping unit) of the CBs protecting the circuits of essential loads must be defined as described below: Choice of s.c. breaking capacity This parameter must always be calculated for the condition of supply from the transformer, or other “normal” source. Adjustment of magnetic tripping units In practice, the only circuit breakers concerned are those protecting the essential services circuits at the main general distribution board. The protection of circuits from local distribution or sub-distribution boards is always calibrated at a much lower level than those at the main general distribution board, so that, except in unusual cases, adequate fault currents are available from an alternator to ensure satisfactory protective-gear operation at these lower levels. Two difficulties have to be overcome: c the first is the need for discrimination of circuit protection with the protection scheme for the alternator. For the basic protection requirements of an alternator, viz: overload protection, the curve shown in figure J1-4 is representative (see Note 1). c the second concerns protection of persons against electric shock from indirect contact, when the protection depends on the operation of overcurrent relays (for example, in IT* or TN systems). The operation of these relays must be assured, whether the supply is from the alternator or from the transformer (see Note 2). Instantaneous or short-time delay magneticrelay trip settings of the circuit breakers concerned must therefore be set to operate at minimum fault levels occurring at the extremity of the circuits they protect, when being supplied from the alternator.

Note 1. Sensitive high-speed protection of an alternator against internal faults (i.e. upstream of its CB) is always possible by using a pilot-wire and current-transformers differential scheme of protection, with the advantage that discrimination with circuit protection schemes is absolute. The problem of discriminative overload protection (as noted above) remains, however. A widely-used solution to this problem is provided by a voltage-controlled overcurrent relay, which depends on the following principle: short-circuit currents cause much lower system voltages than overload currents. An inverse-time/current overload relay is used having two operating curves, one of which corresponds to that of fig. J1-4, and is effective when system voltage levels are normal. If the system voltage falls below a pre-set value, the relay is automatically switched to operate much faster and at lower current levels than those shown in fig. J1-4. Modern low-setting magnetic tripping units, however, often provide a simpler solution as noted in 1.3 below. Note 2. Where the level of earth-fault current is not sufficient, in IT* and TN systems, to trip CBs on overcurrent, the protection against indirect-contact hazards can be provided by an appropriate use of RCDs, as indicated in Chapter G Sub-clause 6.5 Suggestion 2 (for IT circuits) and Sub-clause 5.5. Suggestion 2 (for TN circuits). time (s)

1000

100 12 10 7 3 2 1

1.1 1.2 1.5

2

3

4

5 I/IG overload

fig. J1-4: overload protection of an alternator. * Two concurrent earth faults on different phases or on one phase and on a neutral conductor, are necessary on IT systems, to create an indirect-contact hazard.


J 1.3 choice of tripping units the calculation of the minimum fault current (in IT or TN schemes) is complex. Software packages for this purpose are available.

calculation of the fault-current loop impedance (Zs) for IT and TN systems The determination of the minimum level of short-circuit current, from the calculation of the fault-loop impedance Zs (by the sum of impedances method) is difficult, mainly because of the uncertainly, in a practical installation, of the accuracy of the zerophase-sequence impedances. When conductor routes are known in sufficient detail, impedances can then be determined by the use of software, currently available commercially. Approximate methods for 3-phase and 1-phase short circuits are presented in Sub-clause 1.4.

types of suitable tripping units The choice of low-setting magnetic tripping units will generally be necessary, such as Compact NS* with STR (magnetic-trip short time delay is adjustable from 1.5 to 10 Ir) or circuit breakers Multi 9* curve B (tripping between 3 and 5 In). In practice, these CBs (or their equivalents) will always be necessary when the current rating of the CB is greater than one third of the alternator current rating and will, in most cases, obviate the need for voltage-controlled overload relays. Switchgear manufacturers often furnish tables showing recommended combinations of circuit breakers for commonly-used standby-generator schemes. * Merlin Gerin products.

characteristics of protection for essential-services circuits type of circuit fault-breaking rating (FBR)

tripping unit adjustment

dieselgenerator protection cabinet power-source changeover switch

main circuits

FBR > Isc with supply from transformer

Im or short-delay trip setting level < the minimum fault current at the far end of the circuit when supplied from the alternator (see Note 2 in Sub-clause 1.2)

suband final circuits

FBR > Isc with supply from transformer

check the protection of persons against indirect-contact hazards, particularly on IT and TN systems (see Note 2 in Sub-clause 1.2)

B

Isc: 3-ph short-circuit current Im: magnetic-tripping-relay current setting loads

fig. J1-5: the protection of essential services circuits.



J 1.4 methods of approximate calculation An installation on (normal) 630 kVA transformer supply (figure J1-6) includes an essential-services distribution board which can also be supplied from a standby 400 kVA diesel-alternator set.

What circuit breakers should be installed on the out-going ways from the essentialservices board: c if the installation is TN-earthed? c if the installation is IT-earthed?

transformer 630 kVA 20 kV/400 V

alternator 400 kVA 400 V

alternator and diesel protection equipment cabinet

PE essential circuits main distribution board

non essential circuits

NS250N STR22SE 250 A

NS160N TM400D

IB = 220 A 100 m 120 mm2

IB = 92 A 70 m 35 mm2

PE : 70 mm2

PE : 35 mm2

sub-distribution board

fig. J1-6: example.

calculation of the minimum level of 3-phase short-circuit current Table J1-7 shows the procedure for an alternator together with one or several circuits. item of plant alternator circuit total

R mΩ Ra 22.5 L S R

X mΩ X'd 0.08 x L X

Z mΩ

R2 + X 2

table J1-7: procedure for the calculation of 3-phase short-circuit current. S = c.s.a. in mm2 L = length in metres For the calculation of cable impedance, refer to Chapter H1, Sub-clause 4.2.


Isc kA

1.05xVn R2 + X 2

J Consider the 220 A circuit in figure J1-6 c alternator Ra = 0 2 2 X’d = Un x 0.30 = 400 x 0.30 = 120 mΩ Pn 400 c circuit Rc = 22.5 x 100 = 18.75 mΩ 120 Xc = 0.08 x 100 = 8 mΩ c application of the method of impedances as indicated in table J1-7; R = Ra + Rc = 0 + 18.75 = 18.75 mΩ X = X’d + Xc = 120 + 8 = 128 mΩ total impedance per phase: Z = R2 + X 2 = (18.75)2 + (128)2 = 129.4 mΩ

Isc = 1.05 Vn = 1.05 x 230 = 1.87 kA (r.m.s.) Z 0.129 Note: In practice there will always be some measure of d.c. transient current in at least two phases, so that the above value will normally be exceeded during the period required to trip the CB.

calculation of the minimum level of 1-phase to earth short-circuit fault current Table J1-8 shows the procedure for an alternator together with one or several circuits. item of plant alternator circuit total

R mΩ Ra 22.5 L (1 + m) Sph R

X mΩ 2 X'd + Xo 3 0.08 x L x 2 X

Z mΩ

R2 + X 2

Isc kA

1.05xVn R2 + X 2

table J1-8: procedure for the calculation of 1-phase to neutral short-circuit current. For the calculation of cable impedance, refer to Chapter H1, Sub-clause 4.2. Consider the 220 A circuit in figure J1-6 c alternator Ra = 0 2 Xa = (2 x 120 + 400 x 0.06) x 1 = 88 mΩ 400 3 c circuit Rc = 22.5 x 100 x (1 + 120 / 70) = 50.89 mΩ 120 Xc = 0.08 x 100 x 2 = 16 mΩ c application of the method of impedances, as for the previous example: R = Ra + Rc = 0 + 50.89 = 50.89 mΩ X = Xa + Xc = 88 + 16 = 104 mΩ The total impedance: Z = R2 + X 2 = 50.892 + 1042 = 115.8 mΩ

and Isc1 (phase/neutral) = 1.05 x 230 = 2.09 kA. 115.8



J 1.4 methods of approximate calculation (continued) maximum permissible setting of instantaneous or short-time delay tripping units c TN scheme Of the two fault conditions considered (3-phase and 1-phase/neutral) the 3-phase fault was found to give the lower short-circuit current. The setting of the protective relay must therefore be selected to a current level below that calculated. For the 220 A outgoing circuit the trip unit would be rated at 250 A and adjusted (in principle) to Isc/250, i.e. 1,870/250 = 7.4 In. Owing to a ± 20 % manufacturing tolerance however, the maximum permissible setting would be 7.4 = 6.2 In 1.2 A tripping unit type TM250D* set at 6 In on a NS250N* circuit breaker (breaking capacity = 36 kA i.e. > 21.5 kA) would be appropriate; c IT scheme In this case the protection must operate for a second earth fault occurring before the first earth fault is cleared. This condition (only) produces indirect-contact hazards on an IT system. If the neutral conductor is not distributed, then the minimum short-circuit current for the system will be the phase-to-phase value (i.e. concurrent earth faults on two different phases) which is equal to 0.866 Isc (Isc = the 3-phase s.c. current). If the neutral is distributed, the minimum s.c. current occurs when a phase-to-earth fault and a neutral-to-earth fault occur concurrently, and a protective relay setting equal to 0.5 Isc (phase to neutral) i.e. half the value of a phase-to-neutral short-circuit current, is conventionally used to ensure positive relay operation, v for the case of a non-distributed neutral, the minimum s.c. current = 0.5 x 0.866 x 1.87 = 0.81 kA The tripping unit rated at 250 A will be set at 810 x 1 = 2.7 In 250 1.2 (the factor 1.2 accounting for the ± 20 % manufacturing tolerance for tripping units). A TM250D or a STR22SE tripping unit set at 2.5 In would be appropriate, v when the neutral is distributed, the minimum s.c. current relay setting = 0.5 x 2.08 = 1.04 kA The 250 A tripping unit will be set at 1.040 x 1 250 1.2 = 3.5 In (the 1.2 factor covering manufacturing tolerance, as before) A STR22SE tripping unit, set at 3.0 In would be satisfactory. Note: The foregoing method is based on a simplified application of the following formulae: ➀ Isc (3-phase) = V ph Z1 ➁ Isc (phase/phase) = eVph Z1+Z2 ➂ Isc (phase/earth) = 3 Vph Z1+Z2+Z0


Where Z1 = positive phase-sequence impedance Z2 = negative phase-sequence impedance Z0 = zero phase-sequence impedance Simplifications: c Z1 is assumed to be equal to Z2 so that formula ➁ becomes eVph = 0.866 Vph or 0.866 Isc (3-phase) 2 Z1 Z1 c In table J1-8 the calculated cable reactance assumes that X1 = X2 = X0 for the cable, so that in formula ③ the total reactance = (X1 + X2 + X0) 1/3 = (3 X1) 1/3 = X1 * Merlin Gerin product.

J 1.5 the protection of standby and mobile a.c. generating sets Practical guides in certain national standards classify generator sets according to three categories, viz: c permanent installations (as discussed in Sub-clauses 1.1 to 1.4); c mobile sets (figure J1-9); c portable power packs (figure J1-10).

mobile sets These are used mainly to provide temporary supplies (on construction sites for example) where protection of persons against electric shock must be ensured by the use of RCDs with an operating threshold not exceeding 30 mA.

non-metallic conduit prividing supplementary insulation

PE

C32N 30 mA

T Vigicompact NS100 TM63G 30 mA

PE load circuits

fig. J1-9: mobile generating set.

portable power packs The use of hand-carried power packs by the general public is becoming more and more popular. When the pack and associated appliances are not of Class II (i.e. double insulation), 30 mA RCDs are required by most national standards. C60N 30 mA T

fig. J1-10: portable power pack with RCD protection.


2. inverters and UPS (Uninterruptible Power Supply units)

J 2.1 what is an inverter? An inverter produces an a.c. supply of high quality (i.e. an undistorted sine-wave, free from interference) from a d.c. source; its function is the inverse of that of a rectifier (figure J2-1). Its main purpose (when associated with a rectifier which provides its input) is to afford a high-quality power supply to equipment for which the interference and disturbances of a normal power-supply system cannot be tolerated (e.g. to computer systems). Power systems are subjected to many kinds of perturbation which adversely affect the quality of supply: atmospheric phenomena (lightning, freezing), accidental faults (shortcircuits), industrial parasites, the switching of large electric motors (lifts, fluorescent lighting) are among the many causes of poor quality of supplies. Apart from occasional loss of supply, the disturbances take the form of more-or-less severe voltage dips, high- and low-frequency parasites, continuous “noise” from

fluorescent-lamp circuits and (normally undetectable, but totally unacceptable to sensitive electronic systems) of miniinterruptions of several milli-seconds. By the addition of a storage battery at the input terminals of the inverter (and therefore across the output terminals of the associated rectifier), an elementary UPS system is formed. In normal circumstances, the rectifier supplies the load through the inverter, while, at the same time, a trickle charge from the rectifier maintains the battery fully charged. A loss of a.c. power supply from the distribution network would simply result in the battery automatically maintaining the output from the inverter with no discernable interruption. load

d.c. source

sinusoidal a.c. output

inverter

fig. J2-1: inverter function.

2.2 types of UPS system there are two main types of UPS system: c off-line, c on-line.


Several types of UPS system exist according to the degree of protection against powernetwork “pollution” required, and whether supply autonomy (automatic standby-supply on the loss of normal power supply) is specified, or not. The two most commonlyused types are described below. An off-line type of UPS system (figure J2-2) is connected in parallel with a supply direct from the public distribution network, as shown in figure J2-2, and is autonomous, within the capacity of its battery, on loss of the a.c. power supply. In normal operation the filter improves the quality of the current while the voltage is maintained sensibly constant at its declared value by appropriate and automatic regulation within the filter unit. When the tolerance limits are exceeded, including a total loss of supply, a contactor, which carries the normal load, changes over rapidly to the UPS unit (in less than 10 ms) the power then being supplied from the battery. On the return of normal power supply, the contactor changes back to its original condition; the battery then recharges to its full capacity. These units are normally of low rating (i 3 kVA) but are capable of passing large

transient currents such as those for motorstarting and switching on of (cold) resistive loads. The most common use for such units is the supply to multi-workstation ITE (information technology equipment) installations, such as cash registers.

An on-line type of UPS system (figure J2-3) is connected directly between the public a.c. supply network and the load, and has an autonomous capability, the period of which depends on the battery capacity and load magnitude. The total load passes through the system, which affords a supply of electrical energy within strict tolerance limits, regardless of the state of the a.c. power supply network. On loss of the latter, the battery automatically, and without interruption, maintains the pollution-free a.c. supply to the load. This system is equally suitable for small loads (i 3 kVA) or large loads (up to several MVA).

a.c. power supply network

a. c. power supply network F sensitive load inverter

rectifier/ charger

filter

battery

fig. J2-2: off-line UPS system.

sensitive load inverter

rectifier charger battery

fig. J2-3: on-line UPS system.

J Other apparatus, not assuring a no-break performance, but which protect sensitive loads from certain disturbances commonly occurring on power distribution network, include the following: c the filter-plug which is simply an a.c. plug for connecting or interconnecting loads, which has built-in HF (high-frequency) filters, in order to reduce HF parasitic interference to acceptable levels. Its principal use is on micro-informatic stand-alone PCs rated at 250 to 1,000 VA, for general office purposes; c the network (or mains) -supply conditioner is a complete system for providing an uncontaminated a.c. power supply, but without autonomy, i.e. no provision against loss of supply from the a.c. distribution network. Its principal functions are to: v filter out HF parasites, v maintain a sensibly-constant voltage level, v isolate (galvanically) the load from the a.c. power network. It is equally applicable to office or industrial systems which do not require a no-break standby supply, up to ratings of 5,000 VA; c the slim-line UPS has integral protection with autonomy for each micro-informatic stand-alone PC and its peripherals, and is installed immediately under the microprocessor. Two outputs, each with back-up from the UPS unit, supply the central processor and screen. Two further outputs, which are filtered, supply other less-sensitive units (e.g. the printer). The slim-line UPS belongs to the class of off-line UPS schemes. types of UPS units, conditioners and filters diagrams of principle

filter plug

mains-supply conditioner

slim-line UPS

off-line UPS

on-line UPS F

F

disturbances considered type of network corrective disturbance measures HF parasites c variations of voltage regulation autonomy 10 to 30 mn (according to battery capacity) rated power i 250 VA c 300 - 1,000 VA 1,000 - 2,500 VA > 2,500 VA applications minimal protection

c c

c c

c c

c c

c

c

c

c c c c

c c

c c c

c c c c

all sensitive loads

microinformatic stand-alone PC

micro-informatic terminals

highly disturbed a.c. power systems and/or heavy loads

table J2-4: examples of different possibilities and applications of inverters, in decontamination of supplies and in UPS schemes.

2.3 standards The international standard presently covering semi-conductor converters is IEC 146-4.


2. inverters and UPS (Uninterruptible Power Supply units) (continued)

J 2.4 choice of a UPS system The choice of a UPS system is determined mainly by the following parameters: c rated power, based on: v maximum value of actual estimated kVA demand, v transitory current peaks (motor starting, energization of resistive loads, transformers...). Note: in order to obtain satisfactory discrimination of protective devices for all

types of load, it may be necessary to adjust the power rating of the UPS system. c voltage levels upstream (input) and downstream (output) of the UPS unit; c duration of autonomy required (i.e. supply from the battery); c frequencies upstream (input) and downstream (output) of the UPS unit; c level of availability required.

(9)

(5)

(8)

UPS

distribution board (4)

mains 2

(6)

C/S mains 1

(2)

(1) (3)

(7)

fig. J2-5: classical arrangement of a UPS on-line installation, using an inverter. UPS 1. inverter 2. rectifier/charger 3. batteries (usual periods of autonomy 10 - 15 - 30 mn - several hours) 4. static contactor (see “availability” below) 5. isolating transformer, if galvanic isolation from upstream circuits is necessary. 6. outgoing ways 7. transformer for specific downstream-circuits voltage 8. changeover switch 9. transformer to match the upstream voltage to that of the consumer. Note: At first sight, the circuit arrangement in figure J2-5 closely resembles that of the off-line UPS system (of figure J2-2). In fact, however, it is an on-line system, in which the load is normally passing through circuit 1. The static contactor is open in this situation, but closes automatically if the UPS system becomes overloaded, or fails for any reason. In such a case, the load will then be supplied from the (reserve) circuit 2. This action is the converse of that of the off-line scheme.

the power rating of a UPS unit must take account of the peak motorstarting currents, of the possibility of future extensions to the installation, and of the overload capability of the inverter and other UPS-unit components.


Conditions will automatically return to normal if the overload, etc. is corrected. In this arrangement, the voltage output of the inverter is always maintained in synchronism with the voltage of the powersupply network (i.e. within close tolerance limits of magnitude and phase difference) thereby minimizing the disturbance in the event of “instantaneous” changeover from circuit 1 to circuit 2 operation.

power (VA) The rated power of the UPS unit must be sufficient to satisfy the steady load demand as well as loads of a transitory nature. The demand will be the sum of the apparent (VA) loads of individual items, for example, the CPU (central processing unit) and will amount to Pa, generally corrected by a factor (1.2 to 2) to allow for future extensions. However, in order to avoid oversizing of the installation, account should be taken of the overload capacity of the UPS components. For example, inverters manufactured by Merlin Gerin can safely withstand the following overloaded condition: c 1.5 In for 1 minute; c 1.25 In for 10 minutes.

Instantaneous variations of load: these variations occur at times of energizing and de-energizing of one or more items of load. For an instantaneous change of load up to 100 % of the nominal rating of the UPS unit, the output voltage generally remains between + 10 % and - 8 % of its rated value.

J Example of a power calculation Choice of a UPS unit suitable for the loads shown in figure J2-6. load circuits no.: 1 : 80 kVA 2 : 10 kVA 3 : 20 kVA 4 : 20 kVA 5 : 30 kVA

fig. J2-6: example. Assumed operating constraints: circuit no. 4 will take a transitory current equal to 4 In for a period of 200 ms when initially energized. This operation will be carried out at least once a day. The peak kVA demand, therefore, represents a supplement (over the steady-state 20 kVA demand) of 3 x 20 kVA = 60 kVA. The remaining circuits require no such transitory peak currents. In all cases the kVA values cited have taken the load power factors into account. Possible future extensions to the installation are estimated to amount to 20% of the existing load. The maximum steady-state power demand presently considered is therefore: P = 80 + 10 + 20 + 20 + 30 = 160 kVA. With allowance for extensions (of 20%) = 160 x 1.2 = 192 kVA. With an additional 200 ms peak of (3 x 20) kVA the total amounts to 192 + 60 = 252 kVA. The total of 252 kVA however, includes the 60 kVA peak current which is easily absorbed by the 1.5 In overload capability of (a M.G) UPS system, so that the rating of a suitable UPS unit would be 252 x 1/1.5 = 168 kVA for the nearest standard rating available above the calculated value, e.g. 200 kVA. For the choice of suitable protective devices, see Sub-clause 2.9.

C/S 200 kVA

fig. J2-7: solution to the example.

availability A UPS system is generally provided with an alternative (unconditioned) emergency source, a situation which affords a relatively high level of availability. By way of example, a UPS alone has a MTBF (mean time between failures) of 50,000 hours. In the usual case, where the supply is doubled as noted above (mains 1 and mains 2 in figure J2-5) the MTBF obtained is in the range 70,000 to 200,000 hours, depending on the availability of the second source. Switching from one source to the other is achieved automatically by a static (solid state) contactor. Configurations having a higher redundancy, e.g. three UPS units each rated at P/2 to supply a load of P (figure J2-8) are also sometimes installed. The calculation of their level of availability can be carried out by specialists, and the manufacturers are able to quote availability levels, relative to their own products and recommended layouts.

C/S P/2

P/2 P P/2

fig. J2-8: 3 UPS P/2 units providing a high level of availability of a power rated P.



J 2.5 UPS systems and their environment UPS system components include the means to communicate with other equipments.

UPS units can communicate with other equipments, notably with IT (information technology) systems, passing data concerning the state of the UPS components (static contactor open or closed, and so on...) and receiving orders controlling its function, in order to: c optimize the protection scheme: the UPS, for example, transmits data (such as: condition normal, supply being maintained by the battery, alarm for period of autonomy almost reached) to the computer it is supplying. The computer deduces the appropriate corrective action, and indicates accordingly; c permit remote control: the UPS transmits data concerning the state of UPS components, together with measured quantities, to the console of an operator, who is then able to carry out operational manœuvres through remote-control channels; c supervise (manage) the installation: the consumer (i.e. the “user”) has a centralized management technique facility which allows him to acquire data from the UPS unit(s) which are then stored and analysed, with anomalies indicated, and the state of the UPS is presented on a mimic board or displayed on a screen, and finally to exercise remote control of UPS functions (figures J2-9 to J2-11).

This evolution towards a general compatibility between diverse systems and related hardware requires the incorporation of new functions in the UPS systems. These functions can be designed to ensure mechanical and electrical compatibility with other equipments: standard versions are now provided with dry contacts and current loops. Interconnection facilities according to the standards RS 232, RS 422 or RS 485 can be incorporated on request. In fact, certain advanced modules include modern cards with integral protocole (JBus for example). Furthermore, they can make use of specialized software for automatic checking and fault diagnosis (e.g. Soft-Monitor on PC) which may be integrated into other systems of overall supervision (figure J2-10).

fig. J2-9: UPS units can communicate with centralized system management terminals.

fig. J2-10: software (e.g. Soft-Monitor) allows remote checking and automatic fault diagnosis of the UPS system.

fig. J2-11: UPS units are readily integrated into centralized management systems.


J 2.6 putting into service and technology of UPS systems UPS unit for individual stand-alone PCs.

location of UPS units

fig. J2-12: a UPS (slim-line) is easily accommodated under the computer of a stand-alone PC.

UPS system installed in a computer room.

fig. J2-13: for large computer installations, the UPS cabinets are generally located in the computer room.

UPS cabinets installed in an electrical services room.

fig. J2-14: large UPS systems are frequently located in an electrical services room.



J 2.6 putting into service and technology of UPS systems (continued) types of battery Two types of battery are associated with UPS systems. Maintenance-free sealed units These batteries are used for systems rated at 250 kVA or less, and provide an autonomy of up to 30 minutes. For certain installations, the natural ventilation of its location is considered to be adequate, provided that the particular conditions of charging and regulation, together with the characteristics of the battery, respect the necessary constraints. These constraints are defined in the national standards of some countries (e.g. NF C 15-100, Sub-clause 554 for France). To date, there is no IEC equivalent recommendation, so that consultation with the battery manufacturer may be advisable.

Non-sealed batteries These batteries are generally lead-acid units and are used for all large installations. The batteries must be installed in dedicated battery rooms, which usually require forcedair ventilation. For certain applications, open-type (i.e. nonsealed) cadmium-nickel batteries are preferred.

battery location For any closed location housing batteries, most national standards impose a system of ventilation, forced or natural, which relates the renewal rate of air to the size and charging rate of the battery (or batteries). A recommended air-change rate in cubic metres per hour can be calculated from the formula 0.05 NI where: N = number of cells in the battery I = maximum charging-current capability of the battery charger (in amperes).

fig. J2-15: a typical battery room.


In the case of forced ventilation, the battery charger must be automatically switched off if the fan(s) of the system fail, or if the air-flow is stopped or reduced, for any other reason. For UPS systems of large rating, the batteries are generally located in specially designed battery rooms, complying with the relevant local standards and regulations.

J 2.7 earthing schemes general In the general case the UPS system is fed from two circuits (as shown typically in figure J2-16) each of which is protected separately; they are referred to as mains 1 and mains 2. Mains 1 is a 3-phase 3-wire circuit connected to the UPS rectifier/charger input terminals, while mains 2 is a 3-phase 4-wire circuit connected to the upstream terminals of the static contactor. The downstream distribution board is supplied at 230/400 V. Where other values of voltage are required, adaptor transformers may be employed.

galvanic separation of the upstream and downstream circuits of the UPS system The measures taken to provide protection against electric shock depend on the earthing scheme, and therefore on the existence, or not, of galvanic separation of the downstream circuits from the upstream circuits. Manufacturers should be ready to provide all the necessary information. c if there is no separation, then the earthing scheme is evidently identical on both sides of the UPS system;

c if there is complete separation between the upstream and downstream sides of the UPS system, the earthing schemes upstream and downstream may be different (or identical). The Technical Notes CT 129 of Merlin Gerin explain this subject in more detail.

TT/TT scheme The neutral of the inverter cannot be permanently connected to earth, as described above, but only temporarily, i.e. when D2 is open in figure J2-16. D2 is a 4-pole circuit breaker which breaks the neutral conductor when it is open. The neutral conductor is earthed at the HV/LV transformer, and so, when D2 opens, contactor C closes automatically to reconnect the neutral busbar of the LV distribution board to earth. General protection A RCD is installed at each outgoing way of the MGDB feeding the UPS system (D1 and D2 in figure J2-16) and discrimination between these RCDs and those on the outgoing ways of the DB downstream of the UPS system, is arranged to ensure the maximum possible continuity of supply. The sensitivity of the RCDs is selected according to the value of earthing resistance (electrode plus earth-wires). Note: Certain versions of RCD are designed to avoid malfunctioning under abnormal conditions (d.c. components of current...) that are sometimes generated by UPS systems. It is recommended that the manufacturers of the UPS system be consulted concerning this aspect of their product.

Protection of the d.c. circuits of the UPS system c battery protection: most national standards and codes of practice, covering battery installations are based on stringent regulations, which, if properly observed, reduces the probability of a short-circuit fault or an accidental indirect contact occurring, sufficiently, to consider that the circuit from the battery terminals to the controlling circuit breaker adequately assures the safety of persons. Such will be the case, if: v the battery and all d.c. circuits are in the same cabinet as the other components of the UPS system, i.e. an equipotential location is created, v in the case of a battery location remote from the UPS system, class II insulation standards are respected; c for the remainder of the installation: in particular, the section from the downstream side of the battery circuit breaker and the junction of the rectifier output with the inverter input, where an insulation fault on the d.c. circuits presents a risk, an insulation monitoring scheme is strongly recommended. A suitable system of permanent surveillance injects a low-frequency test current (a XM 200* monitor, as mentioned in Merlin Gerin Technical Notes CT 129, for example). Protection of pollution-free output circuits Circuits supplying socket-outlets will be protected by RCDs of 30 mA (or less) sensitivity (for example, differential circuit breakers Multi 9 curve B 30 mA)*. Other outgoing ways should be protected by RCDs of suitable sensitivity (in general 300 mA) which must discriminate with the protection afforded by D1 and 2 (figure J2-16). * Merlin Gerin product.



J 2.7 earthing schemes (continued)

D1

D2

mains 2

UPS

LV distribution board

C/S

RCD sockets 30mA outlet circuits RCD 30mA

mains 1

RCD

RCD

C

fig. J2-16: TT/TT scheme.

TN-C / TN-S scheme c the automatic cut-off of supply by indirectcontact hazard protection is achieved in this scheme by overcurrent relays. The calculation of the impedance loop Zs however, is not possible in this case. The basic rule to be observed, is that the shortcircuit current from the inverter (which is the maximum it can pass before its internal protection operates) exceeds that of the tripping threshold of downstream overcurrent protection.

Circuit breakers with magnetic trip units of low-setting ranges are suitable for both TN-C and TN-S schemes. For TN-S installations (only) RCDs of medium sensitivity may also be used; c the d.c. section of the UPS system is protected as previously described for the TT scheme; c protection for the pollution-free output circuits will be by 30 mA RCDs for circuits supplying socket outlets, and by circuit breakers of low short-circuit tripping settings, previously mentioned.

non essential circuits LV distribution board

UPS mains 2 C/S

RCD 30mA socket outlet circuits RCD 30mA

mains 1

C

fig. J2-17: TN-C/TN-S scheme. J18 - particular supply sources and loads

J IT/IT scheme c insulation monitoring The CIC1 continuous insulation check relay at the origin of the installation (between the isolated neutral point of the HV/LV transformer and earth) is automatically replaced by the CIC2 at the output of the inverter, when mains 2 is out of service; c the choice of CIC v on the d.c. section of the system, CIC3 uses a very low frequency a.c. current injection relay, type Vigilohm XM 200*, v on the a.c. sections, the CIC1, and CIC2 relays are d.c. current-injection relays of type TR 22A*. In fact, a fault on the d.c. part of the system will be detected by CIC1 and CIC2 but these relays will not operate because the impedance measurement made by them is not correct.

CIC1

mains 1

CIC current-injection relays operating at very low frequency (type XM 200* for example) allow correct measurement of the impedance; c reminder of IT system constraints The design and operation of an IT system requires careful study and exploitation. The advantages of IT operation can only be realized if an in-depth study is completed by clear and concise operating instructions. In particular, the capacitances present in the network (cables and filters on appliances) must be taken into account, and all items of load must be insulated to withstand phase-tophase voltage. * a Merlin Gerin product.

UPS

LV distribution board

C/S

mains 2

CIC2

fig. J2-18: IT/IT scheme.

complete galvanic separation of the circuits upstream of the UPS system from those downstream Galvanic separation of the upstream and downstream circuits of the UPS system is sometimes required, and is effected by installing a 2-winding transformer upstream of the static contactor. In this case, the earthing schemes upstream and downstream of the separation can be different, so that the type of earthing required for the downstream circuits can be created at the output transformer of the inverter.

Protection of the d.c. circuits of the UPS system The d.c. circuits of the UPS system are protected as already described, and the insulation monitoring relays, if required, are selected as indicated in Sub-clause 2.7 for the IT/IT scheme.



J 2.8 choice of main-supply and circuit cables, and cables for the battery connection self-contained UPS units of small power ratings are supplied for direct connection, by plugging into their input and output sockets.

ready-to-use UPS units The UPS units for low-power applications such as individual PCs and micro-informatic installations are marketed as complete units in a metal enclosure, as shown typically in figure J2-19. All internal wiring is factory-installed and adapted to the characteristics of the components.

fig. J2-19: ready-to-use UPS unit.

in other cases, wiring and cables, for interconnection of the several elements of the UPS system, must be installed by the consumer’s contractor.

UPS systems requiring interconnection of constituent elements. For larger UPS installations the battery is generally located at some distance from the inverter, and in the case of an off-line arrangement the static contactor and filters (if installed) require interconnection. The cable sizes selected depend on the current level at each interconnection, as indicated in figure J2-20, and described below. static contactor

Iu

CS mains 2

I1

Iu load

rectifier/ charger

inverter

mains 1 Ib

battery capacity C10

fig. J2-20: currents to be considered for cable selection. Calculation of the currents I1 and Iu c current Iu is the maximum estimated utilization current of the load; c current I1 input to the rectifier/charger of the UPS system depends on: v the capacity of the battery (C10) and its charging rate, v the characteristics of the charger, v the output from the inverter; c the current Ib is the current in the battery cable. These current magnitudes are obtained from the manufacturers of the UPS equipment. Choice of cables In this application the basis of cable selection is the maximum voltage drop allowable for satisfactory performance of the load. Preferable values are for this application: c 3% for a.c. circuits; c 1% for d.c. circuits.


Each of these parameters imposes a minimum c.s.a. of conductor. Calculation of the c.s.a. of conductors may be carried out as shown in Chapter H1 Clause 2. Merlin Gerin recommends cable sizes to be used with Maxipac and EPS 2000 systems (tables J2-22 to J2-24) in normal conditions, for cable lengths of less than 100 m (voltage drop < 3 %). Table J2-21 shows the voltage drop for d.c. circuit lengths of less than 100 m of copper cable. That for a.c. cables can be calculated as described in Chapter H1 Clause 3.

J The voltage-drop values in % given in table J2-21 correspond to a nominal d.c. voltage of 324 V. For other voltage levels multiply the table values by a factor equal to the actual battery voltage divided by 324. c.s.a. In (A)

mm2 100 125 160 200 250 320 400 500 600 800 1000 1250

25 5.1

35 3.6 4.5

50 2.6 3.2 4.0

70 1.9 2.3 2.9 3.6

95 1.3 1.6 2.2 2.7 3.3

120 1 1.3 1.6 2.2 2.7 3.4

150 0.8 1 1.2 1.6 2.2 2.7 3.4

185 0.7 0.8 1.1 1.3 1.7 2.1 2.8 3.4 4.3

240 0.5 0.6 0.8 1 1.3 1.6 2.1 2.6 3.3 4.2 5.3

300 0.4 0.5 0.7 0.8 1 1.3 1.6 2.1 2.7 3.4 4.2 5.3

table J2-21: voltage drop in % of 324 V d.c. for a copper-cored cable. nominal current (A) rated power circuit 1 with battery (1) I1

3.5 kVA 5 kVA 7.5 kVA 10 kVA 15 kVA 20 kVA

Iu

c.s.a. (mm2) of copper-cored cables of length < 100 m circuit 1 circuit 2 or load 3-phase 1-phase 1-phase 400 V 230 V 230 V

16 23 34 45.5 68 91

6 10 10 10 10

circuit 2 or load

3-phase 400 V 1-phase 230 V I1 battery I1 battery I1 battery I1 battery floating on charge floating on charge 18 20 8.5 10.5 26 28 15 19 20 24 30 38 40 48

16 16

10 10 16 16 16 16

table J2-22: currents and c.s.a. of copper-cored cables feeding the rectifier, and supplying the load for UPS system Maxipac (cable lengths < 100 m). nominal current (A) rated power circuit 1 with battery 3-phase 400 V I1 floating recharging for standby period of: 10 mn 15 mn 30 mn 10 kVA 19 23 25 25 15 kVA 29 36 37 39 20 kVA 37 49 50 52 30 kVA 58 73 76 78 40 kVA 75 97 100 104 60 kVA 116 146 151 157 80 kVA 151 194 201 209

circuit 2 or load 400 V Iu

battery

15.2 22.8 30.4 45 60.8 91.2 121.6

27 40.5 54 81 108 162 216

Ib

c.s.a. (mm2) of copper-cored cables of length < 100 m circuit 1 circuit 2 battery 3-phase or 400 V load 3-phase 400 V 10 10 16 25 35 50 70

10 10 10 16 25 35 50

10 10 16 25 35 70 95

table J2-23: currents and c.s.a. of copper-cored cables feeding the rectifier, and supplying the load for UPS system EPS 2000 (cable lengths < 100 m). Battery cable data are also included.



J 2.8 choice of main-supply and circuit cables, and cables for the battery connection (continued) nominal current (A) rated power circuit 1 with battery 3-phase 400 V - I1 floating recharging for standby period of: 10 mn 15-30 mn 40 kVA 70 86 87.6 60 kVA 100 123 127 80 kVA 133 158 164 100 kVA 164 198 200 120 kVA 197 240 244 160 kVA 261 317 322 200 kVA 325 395 402 250 kVA 405 493 500 300 kVA 485 590 599 400 kVA 646 793 806 500 kVA 814 990 1005 600 kVA 967 1180 1200 800 kVA 1290 1648 1548

circuit 2 or load 3-phase 400 V Iu 60.5 91 121 151 182 243 304 360 456 608 760 912 1215

battery Ib

109 160 212 255 317 422 527 658 790 1050 1300 1561 2082

table J2-24: input, output and battery currents for UPS system EPS 5000 (Merlin Gerin). For a given power rating of a UPS system, these tables indicate the value of input current I1 to the rectifier/charger when the battery is on trickle charge (i.e. “floating”) as well as the load current Iu, together with the c.s.a. of corresponding input and output cables. The value of I1 when the battery is recharging (following a period in which the load has been temporarily supplied entirely from the battery) has no influence on the sizing of the cable, due to the short duration of the recharging cycle. The recharging current has to be taken into account however, to correctly determine the upstream protection requirements of circuit 1. Example: For a Maxipac UPS system rated at 7.5 kVA 3-phase 400 V, I1 = 15 A with the battery floating and Iu = 34 A (see table J2-22). The c.s.a. of the corresponding cables are: 10 mm2 for the (3-phase) input cable to the rectifier/charger, 16 mm2 for the (1-phase) output cable to the load.

fig. J2-25: examples of interconnections.


J 2.9 choice of protection schemes in the choice of protection schemes, it is necessary to take account of the characteristics particular to UPS systems.

In the choice of protection schemes, it is necessary to take account of the characteristics particular to UPS systems: the short-circuit current from a UPS system is always very limited, sometimes less than

twice its rated current. Manufacturers carry out tests to ensure a satisfactory coordination between the characteristics of the UPS system and the protection afforded by associated CBs.

choice of circuit breaker ratings The current ratings (In) of CBs D1, D2, D3 and Ddc (figure J2-26) must be chosen such, that: In u I1 for D1 (I1 including the battery recharging current) In u Iu for D2 In u Ide for Ddc The current rating (In) for each outgoing CB D3 depends on the current rating of the particular circuit. The currents I1 and Iu for UPS systems of Merlin Gerin manufacture, are given in tables J2-22 to J2-24. The currents Ib are given in the Merlin Gerin low-voltage distribution catalogue.

fault-current breaking capacity of the circuit breakers Circuit breakers D1 and D2 These CBs must have a fault-current breaking rating equal to or exceeding the value calculated for its location in the network. The calculation is made conventionally, as previously indicated in Chapter H1, Sub-clauses 4.1 and 4.2, for example. Circuit breaker Ddc The short-circuit current breaking level for this CB is always low. In fact, the maximum shortcircuit current from a battery is always less than 20 times its ampere-hour capacity (battery capacities are indicated in the Merlin Gerin low-voltage distribution catalogue). Circuit breakers D3 The very low level of short-circuit current available from the UPS system, gives rise to particularities concerning the organization of discriminative tripping on the one hand, and protection against indirect-contact hazards in TN systems, on the other.

c case 1: circuit configuration in which the static contactor is closed, but without any particular requirement concerning autonomy: the short-circuit current is supplied from the power network, so that the choice of CBs to ensure correct discrimination is determined by classical methods, previously covered in Chapter H2, Sub-clause 4.5; c case 2: circuit configuration without the static contactor or with delayed transfer to it, so that discrimination must be achieved by instantaneous or short time-delay overcurrent protection, operated by the limited shortcircuit current available from the UPS unit, before its internal overcurrent protection operates. For Merlin Gerin UPS units EPS 5000 or 2000* and Merlin Gerin circuit breakers, the following conditions must be complied with: In of a type B circuit breaker i In of UPS unit 2 * In the case of a Maxipac In of a type B circuit breaker i In of UPS unit 3

example 20 kV / 400 V CS power 630 kVA system Isc 22.1 kA network

D2

I1

EPS 5000 of 200 kVA

D1

Ddc

Ib autonomy 10 mn

D3

fig. J2-26: example. Selection of circuit breakers D1 and D2 Table J2-24 shows the values of normal-load currents through D1 and D2 respectively, viz: 395 A for I1 and 304 A for Iu. The short-circuit current-breaking rating of D1 and D2 at their points of installation must be, for such transformers u 22 kA.

Circuit breakers type NS400N* (400 A at 40 °C - 36 kA) would be satisfactory; regulated for overload protection (by thermal tripping device) at Irth u 395A for D1 and u 304 A for D2. * Merlin Gerin product.



J 2.10 complementary equipments transformers A two-winding transformer included on the upstream side of the static contactor of circuit 2 (see figure J2-5) allows: c a change of voltage level when the power network voltage is different to that of the load; c a different arrangement for the neutral on the load-side winding, from that of the power network. Moreover, such a transformer: c reduces the short-circuit current level on the secondary, (i.e. load) side compared with that on the power network side,

c prevents third harmonic currents (and multiples of them) which may be present on the secondary side from passing into the power-system network, providing that the primary winding is connected in delta.

anti-harmonic filter The UPS system includes a battery charger which is controlled by commutated thyristors or transistors. The resulting regularlychopped current cycles “generate” harmonic components in the power-supply network. These indesirable components are filtered at the input of the rectifier and for most cases this reduces the harmonic current level sufficiently for all practical purposes. In certain specific cases however, notably in very large installations, an additional filter circuit may be necessary.

For example, when: c the power rating of the UPS system is large relative to the HV/LV transformer supplying it; c the LV busbars supply loads which are particularly sensitive to harmonics; c a diesel (or gas-turbine, etc.) driven alternator is provided as a standby power supply. In such cases, the manufacturers of the UPS system should be consulted.

communications equipment Communication with equipment associated with informatic systems (see Sub-clause 2.5) may entail the need for suitable facilities within the UPS systems. Such facilities may be incorporated in an original design, or added to existing systems on request.

fig. J2-27: a UPS installation with incorporated communication systems.


3. protection of LV/LV transformers

J These transformers are generally in the range of several hundreds of VA to some hundreds of kVA and are frequently used for: c changing the (LV) voltage level for: v auxiliary supplies to control and indication circuits, v lighting circuits (230 V created when the primary system is 400 V 3-phase 3-wires), c changing the method of earthing for certain loads having a relatively high capacitive current to earth (informatic equipment) or resistive leakage current (electric ovens, industrial-heating processes, mass-cooking installations, etc.). LV/LV transformers are generally supplied

with protective systems incorporated, and the manufacturers must be consulted for details. Overcurrent protection must, in any case, be provided on the primary side. The exploitation of these transformers requires a knowledge of their particular function, together with a number of points described below. Note: In the particular cases of LV/LV safety isolating transformers at extra-low voltage, an earthed metal screen between the primary and secondary windings is frequently required, according to circumstances, as recommended in European Standard EN 60742, and as discussed in detail in Sub-clause 3.5 of Chapter G.

3.1 transformer-energizing in-rush current At the moment of energizing a transformer, high values of transient current (which includes a significant d.c. component) occur, and must be taken into account when considering protection schemes. The magnitude of the current peak depends on: c the value of voltage at the instant of energization, c the magnitude and polarity of magnetic flux (if any) existing in the core of the transformer, c characteristics of the load on the transformer. In distribution-type transformers, the first current peak can attain a value equal to 10 to 15 times the full-load r.m.s. current, but for small transformers (< 50 kVA) may reach values of 20 to 25 times the nominal full-load current. This transient current decreases rapidly, with a time constant θ (see figure J3-1) of the order of several milli-seconds to several tens of milli-seconds.

I Î first 10 to 25 In

In t

θ

fig. J3-1: transformer-energizing in-rush current.

3.2 protection for the supply circuit of a LV/LV transformer The protective device on the supply circuit for a LV/LV transformer must avoid the possibility of incorrect operation due to the magnetizing in-rush current surge, noted above in 3.1. It is necessary to use therefore: c selective (i.e. slightly time-delayed) circuit breakers of the type Compact NS STR* (figure J3-2) or c circuit breakers having a very high magnetic-trip setting, of the types Compact NS or Multi 9* curve D (figure J3-3).

t

50 to 70 ms

r.m.s. value of the first peak

* Merlin Gerin.

instantaneous I trip

fig. J3-2: tripping characteristic of a Compact NS STR circuit breaker. t

In r.m.s. value of the first peak

10In 20In

I

fig. J3-3: tripping characteristic of a circuit breaker according to standardized type D curve (for Merlin Gerin 10 to 14 In).


3. protection of LV/LV transformers (continued)

J 3.2 protection for the supply circuit of a LV/LV transformer (continued) Example (figure J3-4) A 400 V 3-phase circuit is supplying a 125 kVA 400/230 V transformer (In = 180 A) for which the first in-rush current peak can reach 17 In, i.e. 17 x 180 A = 3,067 A. A Compact NS250 circuit breaker with Ir setting of 200 A would therefore be a suitable protective device. A particular case: overload protection installed at the secondary side of the transformer An advantage of overload protection located on the secondary side, is that the short-circuit protection on the primary side can be set at a high value, or alternatively a circuit breaker type MA* may be used. The primary-side short-circuit protection setting must, however, be sufficiently sensitive to ensure its operation in the event of a short-circuit occurring on the secondary side of the transformer (upstream of secondary protective devices).

NS250N tripping unit STR22SE (Ir = 200)

3 x 70 mm2 400/230 V 125 kVA

fig. J3-4: example. Note: The primary protection is sometimes provided by fuses, type a M. This practice has two disadvantages: c the fuses must be largely oversized (at least 4 times the nominal full-load rated current of the transformer); c in order to provide isolating facilities on the primary side, either a load-break switch or a contactor must be associated with the fuses.

* Motor-control circuit breaker, the short-circuit protective relay of which is immune to high transient-current peaks, as shown in figure J5-3.

3.3 typical electrical characteristics of LV/LV 50 Hz transformers 3-phase kVA rating no-load losses (W) full-load losses (W) s.c. voltage (%) 1-phase kVA rating no-load losses (W) full-load losses (W) s.c. voltage (%)

5 100 250 4.5

6.3 110 320 4.5

8 130 390 4.5

10 150 500 5.5

12.5 160 600 5.5

16 170 840 5.5

20 270 800 5.5

8 105 400 5

10 115 530 5

12.5 120 635 5

16 140 730 4.5

20 150 865 4.5

25 31.5 175 200 1065 1200 4 4

25 310 1180 5.5

31.5 350 1240 5

40 350 1530 5

50 410 1650 4.5

63 460 2150 5

80 520 2540 5

100 570 3700 5.5

40 215 1400 5

50 265 1900 5

63 305 2000 4.5

80 450 2450 5.5

100 450 3950 5

125 160 525 635 3950 4335 5

125 680 3700 4.5

160 680 5900 5.5

200 790 5900 5

250 950 6500 5

315 1160 7400 4.5

400 1240 9300 6

500 1485 9400 6

630 1855 11400 5.5

800 2160 11400 5.5

table J3-5: typical electrical characteristics of LV/LV 50 Hz transformers.

3.4 protection of transformers with characteristics as tabled in J3-5 above, using Merlin Gerin circuit breakers 3-phase transformers (400 V primary) P (kVA) In (A) Usc %

circuit breakers type

5 10 16 20 25 31.5 40 50 63

7 14 23 28 35 44 56 70 89

4.5 5.5 5.5 5.5 5.5 5 5 4.5 5

80 100 125

113 141 176

5 5.5 4.5

160

225

5.5

250 315 400 500

352 444 563 704

5 4.5 6 6

630

887

5.5

C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K NC100 D NC100 D NC100 D NC100 D NS100H/L NS160H/L NS160H/L NS250N/H/L NS250N/H/L NS400N/H/L NS250N/H/L NS400N/H/L C801N/H/L C801N/H/L C801N/H/L C801NH/L C1001N/H/L C1001N/H/L C1251N/H

trip-unit current rating (A)/type no. 20 32 63 63 80 80 80 100 MA100 STR22SE STR22SE STR22SE STR22SE STR23SE STR22SE STR23SE STR35SE STR35SE STR35SE STR35SE STR35SE STR35SE STR35SE

table J3-6: protection of 3-phase LV/LV transformers with 400 V primary windings. J26 - particular supply sources and loads

J 3-phase transformers (230 V primary) P (kVA) In (A) Usc %


5 10 16 20 25 31.5 40 50 63

12 24 39 49 61 77 97 122 153

4.5 5.5 5.5 5.5 5.5 5 5 4.5 5

80

195

5

100 125 160 250

244 305 390 609

5.5 4.5 5.5 5

315

767

4.5

400

974

6

C60 / NC100 D or K C60 / NC100 D or K NC100 D NC100 D NS100H/L NS100H/L NS100H/L NS100H/L NS250N/H/L NS400N/H/L NS250N/H/L NS400N/H/L NS630N/H/L C801N/H/L C801N/H/L C801N/H/L C1001N/H/L C1001N/H/L C1251N/H C1251N/H

trip-unit current rating (A)/type no. 40 63 80 100 STR22SE STR22SE STR22SE STR22SE STR22SE STR23SE STR22SE STR23SE STR23SE STR35SE STR35SE STR35SE STR35SE STR35SE STR35SE STR35SE

table J3-7: protection of 3-phase LV/LV transformers with 230 V primary windings. 1-phase transformers (400 V primary) P (kVA) In (A) Usc %


0.1 0.16 0.25 0.4 0.63 1 1.6 2 2.5 4 5 6.3 8 10 12.5 16 20 25 31.5 40 50 63

0.24 0.39 0.61 0.98 1.54 2.44 3.9 4.88 6.1 9.8 12.2 15.4 19.5 24 30 39 49 61 77 98 122 154

13 10.5 9.5 7.5 7 5.2 4 2.9 3 2.1 1.9 1.6 5 5 5 4.5 4.5 4.5 4 4 4 5

80

195

4.5

100 125 160

244 305 390

5.5 5 5

C60 D or K C60 D or K C60 D or K C60 D or K C60 D or K C60 D or K C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K NC100 D NC100 D NS160H/L NS160H/L NS160H/L NS160H/L NS250N/H/L NS400N/H/L NS250N/H/L NS400 NS630 C801N/H/L C801N/H/L

trip-unit current rating (A)/type no. 1 1 1 2 3 6 10 10 16 20 32 40 50 63 63 80 100 STR22SE STR22SE STR22SE STR22SE STR22SE STR23SE STR22SE STR23SE STR23SE STR35SE STR35SE

table J3-8: protection of 1-phase LV/LV transformers with 400 V primary windings.


3. protection of LV/LV transformers (continued)

J 3.4 protection of transformers with characteristics as tabled in J3-5 above, using Merlin Gerin circuit breakers (continued) 1-phase transformers (230 V primary) P (kVA) In (A) Usc %


0.1 0.16 0.25 0.4 0.63 1 1.6 2 2.5 4 5 6.3 8 10 12.5 16 20 25

0.4 0.7 1.1 1.7 2.7 4.2 6.8 8.4 10.5 16.9 21.1 27 34 42 53 68 84 105

13 10.5 9.5 7.5 7 5.2 4 2.9 3 2.1 1.9 1.6 5 5 5 4.5 4.5 4.5

31.5 40

133 169

4 4

50

211

5

63 80 100 125 160

266 338 422 528 675

5 4.5 5.5 5 5

C60 D or K C60 D or K C60 D or K C60 D or K C60 D or K C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K C60 / NC100 D or K NC100 D NC100 D NC100 D NS160H/L NS160H/L NS250N/H/L NS250N/H/L NS250N/H/L NS250N/H/L NS400N/H/L NS250N/H/L NS400N/H/L NS630N/H/L C801N/H/L C801N/H/L C801N/H/L C801N/H/L C1001N/H/L

trip-unit current rating (A)/type no. 1 2 3 4 6 10 16 16 20 40 50 63 80 100 100 STR22SE STR22SE STR22SE STR22SE STR22SE STR22SE STR23SE STR22SE STR23SE STR23SE STR35SE STR35SE STR35SE STR35SE STR35SE

table J3-9: protection of 1-phase LV/LV transformers with 230 V primary windings.


4. lighting circuits

J the presence of adequate lighting contributes to the satety of persons.

The planning and realization of a lighting installation requires a sound understanding of the materials installed, together with familiarity with the rules for safety against fire hazards in establishments receiving the public.

emergency lighting is intended to facilitate the evacuation of persons in case of fire or other panic-causing situations, when normal lighting systems may have failed.

definitions Normal lighting refers to the installation designed for everyday use. Emergency lighting must ensure easy evacuation of persons from the premises concerned, in the event that the normal lighting system fails. Furthermore, emergency lighting must be adequate to allow any particular safety manœuvres provided in the premises to be carried out.

In fact, the provision of adequate illumination in the event of fire or other catastrophic circumstances is of great importance in reducing the likelihood of panic, and in permitting the necessary safety manœuvres to be carried out.

Standby lighting is intended to substitute normal lighting, where the latter fails. Standby lighting permits everyday activities to continue more or less normally, depending on the original design specification, and on the extent of the normal lighting failure. Failure of the standby lighting system must automatically switch on the emergency lighting system.

4.1 service continuity continuity of normal lighting service must be sufficient, independent of other supplementary systems.

normal lighting

in emergency lighting circuits, absolute discrimination between protective devices on the different circuits must be provided.

emergency lighting

Regulations governing the minimum requirements for ERP (Establishments Receiving the Public) in most European countries, are as follows: c installations which illuminate areas accessible to the public must be controlled and protected independently from installations providing illumination to other areas; c loss of supply on a final lighting circuit (i.e. fuse blown or CB tripped) must not result in total loss of illumination in an area which is capable of accommodating more than 50 persons; c protection by RCDs (residual current differential devices) must be divided amongst several devices (i.e. more than one device must be used).

These schemes include illuminated emergency exit signs and direction indications, as well as general lighting. c emergency exit indications In areas accommodating more than 50 persons, luminous directional indications to the nearest emergency exits must be provided; c general emergency lighting General lighting is obligatory when an area can accommodate 100 persons or more (50 persons or more in areas below ground level). A fault on a lighting distribution circuit must not affect any other circuit: v the discrimination of overcurrent-protection relays and of RCDs must be absolute, so that only the faulty circuit will be cut off, v the installation must be an IT scheme, or must be entirely class II, i.e. doubly-insulated. Sub-clause 4.7 describes different kinds of suitable power supplies.


4. lighting circuits (continued)

J 4.2 lamps and accessories (luminaires) fluorescent tubes For normal operation a fluorescent tube requires a ballast and a starter (device for initiating the luminous discharge). c the ballast is an iron-cored inductor, permanently connected in series with the tube; its function is threefold, viz: v to limit the preheating current during the (brief) starting period, v to provide a pulse of high voltage at the end of the starting period to strike the initial arc, v to stabilize the current through the luminous column (hence the term “ballast”).

c the starter is a switch, which, by breaking the (electrode-preheating) current passing through the ballast, causes a high-voltage transient pulse to appear across the tube. This causes an arc (in the form of a gaseous discharge) to be established through the tube. The discharge is then self-sustaining at normal voltage. The ballast, capacitor and the tube, engender disturbances during the periods of starting, steady operation and extinction. These disturbances are analysed in table J4-1 below.

The presence of the ballast means that the power-factor (cos ø) of the circuit is low (of the order 0.6) with the corresponding consumption of reactive energy, which is generally metered. For this reason each fluorescent lamp is normally provided with its own power-factor-correction capacitor. single-phase fluorescent lamp with its individual p.f. correction capacitor 1 2 3

single-phase twin-tube fluorescent lamp with each tube having its own starter and series ballast. One of the tubes has a capacitor connected in series with its ballast. The two sets of equipment are connected in parallel. The arrangement is known internationally as a “duo”-circuit luminaire. The capacitor displaces the phase of the current through its tube, to nullify the flicker effect, as well as correcting the overall p.f. A

ballast

switching-on disturbances c high current peak to charge capacitor; order of magnitude 10 In for 1 sec. A number of lamps on one circuit can result in peaks of 300-400 A for 0.5 ms. This can cause a CB to trip, or the welding of contacts in a contactor. In practice, limit each circuit to 8 tubes per contactor; c moderate overload at the beginning of steady operating condition (1.1-1.5 In for 1 sec) according to type of starter. c no high current peak as noted above; c same order of moderate overload at beginning of steady operating condition as for the single tube noted above. This arrangement is recommended for difficult cases.

switching-off disturbances no particular problems

c can generate a current peak at start; c can cause leakage to earth of HF current (at 30 kHz) via the phase conductor capacitances to earth.

no particular problems

1 2 3

presence of 5th and 7th harmonics at very low level

no particular problems

presence of 3 rd harmonic currents in the neutral, which can reach 70 to 80% of the nominal phase current. In this case, therefore, the c.s.a. of the neutral conductor must equal that of the phase conductors.

B starter

Advantages: Energy savings of the order of 25%. Rapid one-shot start. No flicker or stroboscopic effects.

table J4-1: analysis of disturbances in fluorescent-lighting circuits.


c star-connected lamps (3-ph 4-wire 400/230 V system) 1 2 3 N

starter

fluorescent lamp with HF ballast

steady-operating disturbances circulation of harmonic currents (sinusoidal currents at frequencies equal to whole-number multiples of 50 (or 60) Hz: c delta-connected lamps (see Appendix J2) (3-ph 3-wire 230 V system)

J 4.3 the circuit and its protection dimensions and protection of the conductors The maximum currents in the circuits can be estimated using the methods discussed in Chapter B. Accordingly, account must be taken of: c the nominal power rating of the lamp and the ballast; c the power factor. The temperature within the distribution panel also influences the choice of the protective device (see Chapter H2 Sub-clause 4.4). In general tables are available from manufacturers to assist in making a choice.

Note: for circuits in which large peak currents occur (at times of switching on) and their magnitude is such that CB tripping is a possibility, the cable size is chosen after the protective CB (with an instantaneous trip setting sufficient to remain closed during the current peaks) has been selected. See the Note following table J4-2.

factor of simultaneity ks (diversity) A particular feature of large (e.g. factory) lighting circuits is that the whole load is “on” or “off”, i.e. there is no diversity. Furthermore, even among a number of lighting circuits from a given distribution panel, the factor ks is generally near unity.

Consequently, the interior of distribution panels supplying lighting schemes are frequently at an elevated temperature, an important consideration to be taken into account when selecting protective devices.

4.4 determination of the rated current of the circuit breaker The rated current of a circuit breaker is generally chosen according to the rating of the circuit conductors it is protecting (in the particular circumstances in the Note of 4.3 above, however, the reverse procedure was found to be necessary). The circuit conductor ratings are defined by the maximum steady load current of the circuit. power (kW) 1 1.5 2 2.5 3 3.5 4 4.5 5 6 7 8 9 10

230 V 1-phase current rating In (A) 6 10 10 16 16 20 20 25 25 32 32 40 50 50

The following tables allow direct selection of circuit breaker ratings for certain particular cases.



table J4-2: protective circuit breaker ratings for incandescent lamps and resistive-type heating circuits (see Note below). Note: at room temperature the filament resistance of a 100 W 230 V incandescent lamp is approximately 34 ohms. Some milli-seconds after switching on, the filament resistance rises to 2302/100 = 529 ohms. The initial current peak at the instant of switch closure is therefore practically 15 times its normal operating current. A similar (but generally less severe) transient current peak occurs when energizing any resistivetype heating appliance.



J 4.4 determination of the rated current of the circuit breaker (continued) The following table (J4-3) is valid for 230 V and 400 V installations, with or without individual power-factor correcting capacitors. mercury vapour fluorescent lamps P i 700 W 6A P i 1000 W 10 A P i 2000 W 16 A metal-halogen mercury-vapour lamps P 275 W 6A P 1000 W 10 A P 2000 W 16 A high-pressure sodium discharge lamps P 400 W 6A P 1000 W 10 A table J4-3: maximum limit of rated current per outgoing lighting circuit, for high-pressure discharge lamps. single-phase distribution 230 V three-phase distribution + N : 400 V phase/phase types de tube number of luminaires per phase luminaires rating (W) single-phase 18 7 14 21 42 70 112 140 175 with capacitor 36 3 7 10 21 35 56 70 87 58 2 4 6 13 21 34 43 54 duo circuit 2x18= 36 3 7 10 21 35 56 70 87 with 2x36= 72 1 3 5 10 17 28 35 43 capacitor 2x58= 116 1 2 3 6 10 17 21 27 current rating of 1-,2-,3 -or 4- pole CBs 1 2 3 6 10 16 20 25

225 112 69 112 56 34

281 140 87 140 70 43

351 175 109 175 87 54

443 221 137 221 110 68

562 281 174 281 140 87

703 351 218 351 175 109

32

40

50

63

80

100

Calculation for tubes with p.f. capacitor; connected in star number of tubes per phase = 0.8 C x 0.86 V Pu x 1.25 where: C = current rating of C B, V = phase/neutral voltage, 0.86 = cos ø of circuit, 0.8 = derating factor for high temperature in CB housing, 1.25 = factor for watts consumed by ballast, Pu = nominal power rating of tube (W). three-phase 3 wire system(230 V) phase/phase types de tube number of luminaires per phase luminaires rating (W) single-phase 18 4 8 12 24 40 64 81 101 with capacitor 36 2 4 6 12 20 32 40 50 58 1 2 3 7 12 20 25 31 duo circuit 2x18= 36 2 4 6 12 20 32 40 50 with 2x36= 72 1 2 3 6 10 16 20 25 capacitor 2x58= 116 0 1 1 3 6 10 12 15 current rating of 2- or 3- pole CBs 1 2 3 6 10 16 20 25

127 64 40 64 32 20

162 81 50 81 40 25

203 101 63 101 50 31

255 127 79 127 63 39

324 162 100 162 81 50

406 203 126 203 101 63

32

40

50

63

80

100

Calculation for tubes with p.f. capacitor; connected in delta number of tubes per phase = 0.8 C x 0.86 U Pu x 1.25 x e where: U = phase/phase voltage tables J4-4: current ratings of circuit breakers related to the number of fluorescent luminaires to be protected.


J 4.5 choice of control-switching devices The advent of switching devices which combine the functions of remote control and protection, of which the remotely-controllable residual-current circuit breaker is the prototype, simplifies lighting-control circuits considerably, thereby enlarging the scope and diversity of control schemes. remote-control mode

point-to-point remote control centralized remote control point-to-point and centralized remote control control signals over communications bus control signals over timemultiplexing channels

Certain switching devices include control circuitry for operation at ELV (extra-lowvoltage, i.e. < 50 V or < 25 V according to requirements); these control circuits being insulated for 4,000 V with respect to the power circuits. The situation at the time of writing is summarized below in table J4-5.

function of corresponding switchgear and controlled equipment remote control

remote control + overcurrent protection

bistable switch

circuit breaker controlled by hard-wire system

contactor “pilot” bistable switch remote controlled switch remotely controlled switch

remotely controlled circuit breaker over communications bus remotely controlled static contactor/ circuit breaker combination

remote control + overcurrent protection + insulation monitoring and protection residual current circuit breaker controlled by hard-wire system

local control devices

centralized control devices

push-button

stairway time-switch with automatic switch-off automatic photo-electric lighting-control switches movement detectors; central clock relaying

switch push button

residual current circuit breaker controlled over communications bus

according to type

table J4-5: types of remote control.



J 4.6 protection of ELV lighting circuits A LV/ELV transformer is often located in an inaccessible position, so that protection installed on the secondary side would be equally difficult to reach. For this reason the protection is commonly provided on the primary circuit. The protective device is therefore chosen: c to provide switching control (Multi 9 type C CB, or type aM fuses); c to ensure protection against short-circuits. It must therefore be verified that: v in the case of a CB, the minimum value of short-circuit current exceeds by a suitable margin the short-circuit magnetic relay setting Im of the CB concerned, v in the case of fuses it is also necessary to ensure that the I2t energy let-through of the fuse(s) at minimum short-circuit current is well below the level of the thermal withstand capacity of the circuit conductors, c if necessary, overload protection must be provided. If the number of lamps on the circuit has been correctly chosen, however, overload protection is not necessary. Example: The s.c. current Isc2 at the secondary terminals of a single-phase LV/ELV transformer is equal to Us where Zs = Us2 x Usc % Zs Pn 100 Pn x 100 so that Isc2 = = 400 x 100 Us x Usc% 12 x 6 = 555 A which gives Isc1 = 29 A in the primary circuit. Circuit breaker type C trips if the primary current u Im1 = 10 In = 20 A, which corresponds to a secondary current of 20 x 230 = 383 A 12 The maximum resistance of the ELV (i.e. secondary) circuit* may be deduced from these two secondary s.c. currents, viz: 555 A and 383 A as follows: Rc = U2 - V2 = 12 - 12 = 0.0313 - 0.0216 Im2 Isc2 383 555 = 9.7 mΩ * from the transformer terminals to the ELV distribution board.

Note: The true value of Rc permitted is, in principle, greater than 9.7 milli-ohms, because the source impedance (i.e. U2/555, will be mainly reactive, not resistive, as (implied) in the example. However, for simplicity, and to automatically provide a safety margin under all circumstances, an arithmetic subtraction, as shown, is recommended. The maximum length of the 12 V circuit based on 9.7 mΩ will therefore be: Rc (mΩ) x S (mm2) in metres = 9.7 x 6 2 x 22.5 (µΩ.mm) 2 x 22.5 for a 6 mm2 copper cable = 1.3 m It is then necessary to check that this length is sufficient to reach the 12 V distribution board, where the outgoing ways are protected with other devices. If the length is insufficient, then an increase in the c.s.a. of the conductors, proportional to the increased length required, will satisfy the constraint for maximum Rc; for example, a conductor of 10 mm2 would allow 1.3 x 10/6 = 2.2 m of circuit length in the above case.


2A

LV ELV

230/12 V 400 VA Usc = 6%

secondary circuit

fig. J4-6: example.

J 4.7 supply sources for emergency lighting Supply sources for emergency-lighting systems must be capable of maintaining the supply to all lamps in the most unfavourable circumstances likely to occur, and for a period judged necessary to ensure the total evacuation of the premises concerned, with (in any case) a minimum of one hour.

compatibility between emergency lighting sources and other parts of the installation Emergency-lighting sources must supply exclusively the circuits installed only for operation in emergency situations. Standby lighting systems operate to maintain illumination, on failure of normal lighting circuits (generally in non-emergency circumstances). However, failure of standby lighting must automatically bring the emergency lighting system into operation.

Central sources for emergency supplies may also be used to provide standby supplies, provided that the following conditions are simultaneously fulfilled: c where there are several sources, the failure of one source must leave sufficient capacity in service to maintain supply to all safety systems, with automatic load shedding of non-essential loads (if necessary); c the failure of one source, or one equipment concerned with safety, must leave all other sources and safety equipments unaffected; c any safety equipment must be arranged to receive supply from any source.

classification of emergencylighting schemes Many countries have statutory regulations concerning safety in buildings and areas intended for public gatherings. Classification of such locations leads to the determination of suitable types of solutions, authorized for use in emergency-lighting schemes in the different areas. The following four classifications are typical. Type A The lamps are supplied permanently and totally during the presence of the public by a single central source (battery of storage cells, or a heat-engine-driven generator). These circuits must be independent of any other circuits (1). Type B The lamps are permanently supplied during the presence of the public, either: c by a battery to which the lamps are permanently connected, and which is on permanent trickle charge from a normal lighting source, or, c by a heat-engine-driven generator, the characteristics of which also assure supplies to essential loads within one second (since the set is already running and supplying the emergency lighting) in the event of failure of the normal power supply, or, c by autonomous units which are normally supplied and permanently alight from the normal lighting supply, and which remain alight (for at least one hour), on the loss of normal supply, by virtue of a self-contained battery. The battery is trickle-charged in normal circumstances. These units have fluorescent lamps for general emergency lighting, and fluorescent or incandescent lamps for exit and directionindicating signs. The circuits for all emergency lamps must be independent of any other circuits (1).

Type C The lamps may, or may not, be supplied in normal conditions and, if supplied, may be fed from the normal lighting system, or from the emergency-lighting supply. c the emergency-lighting batteries must be maintained on charge from the normal source by automatically regulated systems, that ensure a minimum of capacity equal to the full emergency-lighting load for one hour; c the heat-engine-driven generator sets must be capable of automatically picking-up the full emergency lighting load from a standby (stationary) condition, in less than 15 seconds, following the failure of normal supply. The engine start-up power is provided by a battery which is capable of six starting attempts, or by a system of compressed air. Minimum reserves of energy in the two systems of start-up must be maintained automatically. c failures in the central emergency supply source must be detected at a sufficient number of points and adequately signalled to supervisory/maintenance personnel; c autonomous units may be of the permanently-lit type or non-permanently-lit type. The circuits for all emergency lamps must be independent of any other circuits (2). Type D This type of emergency lighting comprises hand-carried battery-powered (primary or secondary cells) at the disposal of service personnel or the public. (1) Circuits for types A and B, in the case of a central emergency power source, must also be fire-resistant. Conduit boxes, junction sleeves and so on must satisfy national standard heat tests, or the circuits must be installed in protective cable chases, trunking, etc. capable of assuring satisfactory performance for at least one hour in the event of fire. (2) Cable circuits of type C are not required to comply with the conditions of (1).


5. asynchronous motors

J the asynchronous (i.e. induction) motor is robust and reliable, and very widely used. 95% of motors installed around the world are asynchronous. The protection of these motors is consequently a matter of great importance in numerous applications.

The consequences of an incorrectly protected motor can include the following: c for persons: v asphyxiation due to the blockage of motor ventilation, v electrocution due to insulation failure in the motor, v accident due to sticking (contact welding) of the controlling contactor; c for the driven machine and the process: v shaft couplings and axles, etc. damaged due to a stalled rotor, v loss of production, v manufacturing time delayed; c for the motor: v motor windings burnt out due to stalled rotor, v cost of dismantling and reinstating or replacement of motor, v cost of repairs to the motor.

specific features of motor performance influence the powersupply circuits required for satisfactory operation.

A motor power-supply circuit presents certain constraints not normally encountered in other (common) distribution circuits, owing to the particular characteristics, specific to motors, such as: c heavy start-up current (see figure J5-1) which is highly reactive, and can therefore be the cause of an important voltage drop; c number and frequency of start-up operations are generally high; c the heavy start-up current means that motor overload protective devices must have operating characteristics which avoid tripping during the starting period.

It is, therefore, the safety of persons and goods, and reliability and availability levels which must influence the choice of protective equipment. In economic terms, it is the overall cost of failure which must be considered; a penalty which is increasingly severe as the size of the motor, and difficulties of access to it increase. Loss of production is a further, and evidently important factor.

t

I" = 8 to 12 In Id = 5 to 8 In In = nominal motor current

td 1 to 10s

20 to 30 ms In

Id

I"

fig. J5-1: direct-on-line starting-current characteristics of an induction motor.

5.1 protective and control functions required functions to be provided generally include: c basic protective devices, c electronic control equipment, c preventive or limitative protection equipment.


Functions generally provided are: c basic protection, including: v isolating facility, v manual local and/or remote control, v protection against short-circuits, v protection against overload; c electronic controls consisting of: v progressive “soft-start” motor starter, or, v speed controller; c preventive or limitative protection by means of: v temperature sensors, v multi-function relays, v permanent insulation-resistance monitor or RCD (residual-current differential device). Table J5-2 below, shows diverse motor-circuit configurations commonly used in LV distribution boards.

I

J basic protection

fuse-disconnector + discontactor (using thermal relay)

circuit breaker* motor circuit + discontactor breaker* + contactor (using thermal relay)

contactor circuit breaker* ACPA

standards disconnection (or isolation)

manual control

remote control

short-circuit protection * circuit breaker includes disconnector capability

overload protection

c large power range c allow all types of starting schemes c a well-proven method c suitable for systems having high fault levels

refer also to Chapter H2, Sub-clause 2-2 electronic controls

preventive or limitative protection devices

c large power range c method is simple c avoids need to stock and compact for fuse cartridges low-power motors c disconnection is visible in certain cases c identification of the reason for tripping i.e. short circuit or overload

progressive “soft-start” starter device c limitation v current peaks I v voltage drops U v mechanical constraints during start-up period c thermal protection is incorporated

c low installation costs c no maintenance c high degree of safety and reliability c suitable for systems having high fault levels c long electrical life

speed controller c from 2 to 130 % of nominal speed c thermal protection is incorporated c possibility of communication facilities

thermal sensors Protection against abnormal heating of the motor by thermistance-type sensors in the motor windings, connected to associated relays. multi-function relays Direct and indirect thermal protection against: c the starting period excessively long, or stalled-rotor condition c imbalance, absence or inversion of phase voltages c earth fault or excessive earth-leakage current c motor running on no-load; motor blocked during start-up c pre-alarm overheating indication permanent insulation-to-earth monitor and RCD (residual-current differential relay) Protection against earth-leakage current and short-circuits to earth. Signalled indication of need for motor maintenance or replacement.

table J5-2: commonly-used types of LV motor-supply circuits.


5. asynchronous motors (continued)

J 5.2 standards The international standards covering materials discussed in this Sub-clause are: IEC 947-2, 947-3, 947-4-1, and 947-6-2. These standards are being adopted (often without any changes) by a number of countries, as national standards.

5.3 basic protection schemes: circuit breaker / contactor / thermal relay functions to be implemented are: c control (start/stop), c isolation (safety during maintenance), c protection against short-circuits, c specific protection as noted in Sub-clause 5.1 Where several different devices are used to provide protection, coordination between them is necessary.

among the many possible methods of protecting a motor, the association of a circuit breaker incorporating an instantaneous magnetic trip for shortcircuit protection and a contactor with a thermal overload relay* provides many advantages.

The control and protection of a motor can be provided by one, two or three devices, which share the required functions of: c control (start/stop); c disconnection (isolation) for safety of personnel during maintenance work; c short-circuit protection; c protection specific to the particular motor (but at least thermal relay overcurrent protection). When these functions are performed by several devices, co-ordination between them is essential. In the case of an electrical fault of any kind, none of the devices involved must be damaged, except items for which minor damage is normal in the particular circumstances, e.g. replaceable arcing contacts in certain contactors, after a given number of service operations, and so on... The kind of co-ordination required depends on the necessary degree of service continuity and on safety levels, etc. t

range 1.05 - 1.20 In

circuit breaker magnetic relay

characteristics of thermal relay

end of start-up period

contactor thermal relay

cable thermal-withstand limit

ts 1 to 10 s

limit of thermalrelay constraint

câble motor (nominal current In)

short-circuit tripping characteristic of the circuit breaker (type MA)

20 to 30 ms In

Is

I"

Imagn. circuit breaker only

l CB plus contactor (see Note) short-circuit-current breaking capacities

fig. J5-3: tripping characteristics of a circuit breaker (type MA)** and thermal-relay / contactor (1) combination. Advantages c interlocking; This combination of devices facilitates c diverse remote indications; installation work, as well as operation and c better protection for the starter for shortmaintenance, by: circuit currents up to about 30 In (see c the reduction of the maintenance work load: figure J5-3). the CB avoids the need to replace blown In the majority of cases short-circuit faults fuses and the necessity of maintaining a occur at the motor, so that the current is stock (of different sizes); limited by the cable and the wiring of the c better continuity performance: a motor starter (e.g. the direct-acting trip coil of the circuit can be re-energized immediately CB). following the elimination of a fault; c possibility of adding RCD: c additional complementary devices v an RCD of 500 mA sensitivity practically sometimes required on a motor circuit are eliminates fire risk due to leakage current, easily accommodated; v protection against destruction of the motor c tripping of all three phases is assured (short-circuiting of laminations) by the early (thereby avoiding the possibility of “singledetection of earth-fault currents (300 mA to phasing”); 30 A); c full-load current switching possibility (by c etc. CB) in the event of contactor failure, e.g. contact welding; * The association of an overload relay and a contactor is referred to as a “discontactor” in some countries. ** Merlin Gerin.


J Note: When short-circuit currents are very high, the contacts of some contactors may be momentarily forced open by electro-magnetic repulsion, so that two sets of contacts (i.e. those of the CB and those of the contactor) are acting in series. The combination effectively increases the s.c. current-breaking capacity above that of the CB alone. Conclusion The association circuit breaker / contactor / thermal relay(1) for the control and protection of motor circuits is eminently appropriate where: c the maintenance service for an installation is reduced, which is generally the case in tertiary and small-and medium-sized industrial enterprises; c the job specification calls for complementary functions; c there is an operational requirement for a load-breaking facility in the event of contact welding of the contactor. (1) a contactor in association with a thermal relay is commonly referred to as a discontactor.

standardization of the association of circuit breakers/ discontactors Categories of contactor The standard IEC 947-4 gives utilization categories which considerably facilitate the choice of a suitable contactor for a given service duty. The utilization categories advise on: c a range of functions for which the contactor may be adapted; c its current breaking and making capabilities; c standard test values for expected life duration on load, according to its utilization. The following table gives some typical examples of the utilization categories covered. utilization category AC-1 AC-2 AC-3 AC-4

application characteristics Non-inductive (or slightly inductive) loads: cos ø u 0.95 (heating, distribution) Starting and switching off of slip-ring motors Cage motors: starting, and switching off motors during running Cage motors: starting, plugging, inching

table J5-4: utilization categories for contactors (IEC 947-4). Types of co-ordination For each association of devices, a type of coordination is given, according to the state of the constituant parts following a circuit breaker trip out on fault, or the opening of a contactor on overload. IEC 947-4-1 defines two types of coordination, type 1 and type 2, which set maximum allowable limits of deterioration of switchgear, which must never present a danger to personnel. c type 1: deterioration of the contactor and/or of its relay is acceptable under 2 conditions: v no risk for the operator, v all elements other than the contactor and its relay must remain undamaged; c type 2: burning, and the risk of welding of the contacts of the contactor are the only risks allowed.

Which type to choose? The type of co-ordination to adopt depends on the parameters of exploitation, and must be chosen to satisfy (optimally) the needs of the user and the cost of installation. c type 1: v qualified maintenance service, v volume and cost of switchgear reduced, v continuity of service not demanded, or provided by replacement of motor-starter drawer; c type 2: v continuity of service imperative, v no maintenance service, v specifications stipulating this type of coordination.



J 5.3 basic protection schemes: circuit breaker / contactor / thermal relay (continued) key points in the successful association of a circuit breaker and a discontactor t Compact NS type MA

2

1 CB magnetic-trip performance curve 2 thermal-relay characteristic 3 thermal-withstand limit of the thermal relay 1 3

Isc ext.

I

fig. J5-5: the thermal-withstand limit of the thermal relay must be to the right of the CB magnetic-trip characteristic. Standards define precisely all the elements which must be taken into account to realize a correct co-ordination of type 2: c absolute compatibility between the thermal relay of the discontactor and the magnetic trip of the circuit breaker. In figure J5-5 the thermal relay is protected if its limit boundary for thermal withstand is placed to the right of the CB magnetic trip characteristic curve. In the case of a motor-control circuit breaker incorporating both magnetic and thermal devices, co-ordination is provided in the design;

it is not possible to predict the s.c. current-breaking capability of a CB + contactor combination. Laboratory tests and calculations by manufacturers are necessary to determine which type of CB to associate with which contactor, and to establish the s.c. breaking capacity of the combination. Tables are published by Merlin Gerin giving this information in their “LV Distribution” catalogue.

short-circuit current-breaking capacity of a combination circuit breaker + contactor In the studies, the s.c. current-breaking capacity which must be compared to the prospective short-circuit current is: c either, that of the CB + contactor combination, if these devices are physically close together (e.g. in the same drawer or compartment of a MCC*). A short-circuit downstream of the combination will be limited to some extent by the impedances of the contactor (see previous Note) and the thermal relay. The combination can therefore be used on a circuit for which the prospective short-circuit current level exceeds the rated s.c. current-breaking capacity of the circuit breaker. This feature very often presents a significant economic advantage; c or, that of the CB only, for the case where the contactor is separated from the CB (so that a short-circuit is possible on the intervening circuit). For such a case, IEC 947-4-1 requires the rating of the circuit breaker to be equal to or greater than the prospective short-circuit current at its point of installation. * Motor Control Centre.


c the short-circuit current breaking rating of the contactor must be greater than the regulated threshold of the CB magnetic trip relay, since it (the contactor) must be capable of breaking a current which has a value equal to, or slightly less than, the setting of the magnetic relay (as seen from figure J5-5); c a reliable performance of the contactor and its thermal relay when passing short-circuit current, i.e. no excessive deterioration of either device and no welding of contactor contacts.

M

fig. J5-6: circuit breaker and contactor mounted in juxtaposition.

M

fig. J5-7: circuit breaker and contactor separately mounted, with intervening circuit conductors.

J choice of instantaneous magnetic-trip relay for the circuit breaker The operating threshold must never be less than 12 In for this relay, in order to avoid possible tripping due to the first current peak during start-up. This current peak can vary from 8 In to 11 or 12 In.

5.4 preventive or limitative protection preventive or limitative protection devices detect signs of impending failure, so that action can be taken (automatically or by operator intervention) to avoid or limit the otherwise inevitable consequences.

The main protection devices of this type for motor are: c thermal sensors in the motor (windings, bearings, cooling-air ducts, etc.); c multifunction protections; c insulation-failure detection devices on running, or stationary motor.

thermal sensors Thermal sensors are used to detect abnormal temperature rise in the motor by direct measurement. The thermal sensors are generally embedded in the stator windings (for LV motors), the signal being processed by an associated control device acting to trip the circuit breaker (figure J5-8).

fig. J5-8: overheating protection by thermal sensors.

multi-function motor-protection relay The multi-function relay, associated with a number of sensors and indication modules, provides protection for motors, such as: c thermal overload; c rotor stalled, or starting-up period too long; c overheating; c phase current imbalance, loss of one phase, inverse rotation; c earth fault (by RCD); c running on no-load, blocked rotor on start-up. The advantages of this relay are essentially: c a comprehensive protection, providing a reliable, high-performance and permanent monitoring/control function; c efficient surveillance of all motor-operating schedules; c alarm and control indications; c possibility of communication via communication buses.

fig. J5-9: multi-function protection, typified by the Telemecanique relay, type LT8 above.



J 5.4 preventive or limitative protection (continued) preventive protection of stationary motors This protection concerns the monitoring of the level of insulation resistance of a stationary motor, thereby avoiding the undesirable consequences of insulation failure during operation, such as: c for motors used on emergency systems for example: failure to start or to perform correctly; c in manufacturing: loss of production. This type of protection is indispensable for essential-services and emergency-systems motors, especially when installed in humid and/or dusty locations. Such protection avoids the destruction of a motor by short-circuit to earth during start-up (one of the most frequently-occurring incidents) by giving a warning in advance that maintenance work is necessary to restore the motor to a satisfactory operational condition. Examples of application (figure J5-10) Fire-protection system “sprinkler” pumps. Irrigation pumps for seasonal operation, etc. Example: a vigilohm SM 20 (Merlin Gerin) relay monitors the insulation of a motor, and signals audibly and visually any abnormal reduction of the insulation resistance level. Furthermore, this relay can prevent any attempt to start the motor, if necessary.

SM20

MERLIN GERIN SM20

IN

OUT

fig. J5-10: preventive protection of stationary motors.

limitative protection Residual current differential protective devices (RCDs) can be very sensitive and detect low values of leakage current which occur when the insulation to earth of an installation deteriorates (by physical damage, contamination, excessive humidity, and so on). Some versions of RCDs, specially designed for such applications, provide the following possibilities: c to avoid the destruction of a motor (by perforation and short-circuiting of the laminations of the stator) caused by an eventual arcing fault to earth. This protection can detect incipient fault conditions by operating at leakage currents in the range of 300 mA to 30 A, according to the size of the motor (approx. sensitivity: 5 % In). Instantaneous tripping by the RCD will greatly limit the extent of damage at the fault location; c to reduce considerably the risk of fire due to earth-leakage currents (sensitivity i 500 mA). A typical RCD for such duties is type RH328A relay (Merlin Gerin) which provides: c 32 sensitivities (0.03 to 250 A); c possibility of discriminative tripping or to take account of particular operational requirements, by virtue of 8 possible timedelays (instantaneous to 1 s.); c automatic operation if the circuit from the current transformer to the relay is broken; c protected against false operation; c insulation of d.c. circuit components: class A.


RH328A

MERLIN GERIN

fig. J5-11: example using relay RH328A.

J voltage drop at the terminals of a motor during starting must never exceed 10% of rated voltage Un.

The importance of limiting voltage drop at the motor during start-up In order that a motor starts and accelerates to its normal speed in the appropriate time, the torque of the motor must exceed the load torque by at least 70%. However, the starting current is much greater than the full-load current of the motor; moreover, it is largely inductive. These two factors are both very unfavourable to the maintenance of voltage at the motor. Failure to provide sufficient voltage will reduce the motor torque significantly (motor torque is proportional to U2) and will result either in an excessively long starting time, or, for extreme cases, in failure to start.

Example: c with 400 V maintained at the terminals of a motor, its torque would be 2.1 times that of the load torque; c for a voltage drop of 10% during start-up, the motor torque would be 2.1 x 0.92 = 1.7 times the load torque, and the motor would accelerate to its rated speed normally; c for a voltage drop of 15% during start-up, the motor torque would be 2.1 x 0.852 = 1.5 times the load torque, so that the motorstarting time would be longer than normal. In general, a maximum allowable voltage drop of 10% Un is recommended during the start-up of a motor.

5.5 maximum rating of motors installed for consumers supplied at LV The disturbances caused on LV distribution networks during the start-up of large DOL (direct-on-line) a.c. motors can occasion considerable nuisance to neighbouring consumers, so that most power-supply authorities have strict rules intended to limit such disturbances to tolerable levels. The amount of disturbance created by a given motor depends on the “strength” of the network, i.e. on the short-circuit fault level at the point concerned. The higher the fault level, the “stronger” the system and the lower the disturbance (principally volt-drop) experienced by neighbouring consumers. For distribution networks in many countries, typical values of maximum allowable starting type of motor single- location or three-phase single phase three phase

dwellings others dwellings others

currents for DOL motors are shown below in table J5-12. Corresponding maximum power ratings of the same motors are shown in table J5-13. Since, even in areas supplied by one power authority only, “weak” areas of the network exist as well as “strong” areas, it is always advisable to secure the agreement of the power supplier before acquiring the motors for a new project. Other (but generally more costly) alternative starting arrangements exist, which reduce the large starting currents of DOL motors to acceptable levels; for example, star-delta starters, slip-ring motors, “soft start” electronic devices, etc. maximum starting current (A) overheadundergroundline network cable network 45 45 100 200 60 60 125 250

table J5-12: maximum permitted values of starting current for direct-on-line LV motors (230/400 V). type of motor singleor three-phase location

single-phase 230 V (kW)

dwellings others overhead line network underground cable network

three-phase 400 V

1.4 3

direct-on-line starting at full load (kW) 5.5 11

other methods of starting (kW) 11 22

5.5

22

45

table J5-13: maximum permitted power ratings for LV direct-on-line-starting motors.

5.6 reactive-energy compensation (power-factor correction) The effect of power factor correction on the amount of current supplied to a motor is indicated in table B4 in Chapter B Sub-clause 3-1, and the method of correction in Chapter E Clause 7.


6. protection of direct-current installations

J differences between a.c. and d.c. installations Although the basic design principles in each case are similar, there are differences in: c the calculations for short-circuit currents, and; c the choice of protective equipment, since the techniques employed for the interruption of direct current differ in practice from those used for alternating current.

6.1 short-circuit currents in order to calculate the maximum short-circuit current from a battery of storage cells, when the internal resistance of the battery is unknown, the following approximate formula may be used: Isc = kC where C = the rated ampere-hour capacity of the battery, and k is a coefficent close to 10 (and in any case is less than 20).

battery of storage cells (or accumulators) For a short-circuit at its output terminals, a battery will pass a current according to Ohm’s law equal to Isc = Vb/Ri where: Vb = open circuit voltage of the fullycharged battery Ri = the internal resistance of the battery (this value is normally obtained from the manufacturer of the battery, as a function of its ampere-hour capacity) When Ri is not known, an approximate formula may be used, namely: Isc = kC where C is the ampere-hour rating of the battery and k is a coefficient close to 10, and in any case is always less than 20.

Example: What is the short-circuit current level at the terminals of a battery with the following characteristics: c 500 Ah capacity; c fully-charged open-circuit voltage 240 V (110 cells at 2.2 V/cell); c discharge rate 300 A; c autonomy 1/2 hour; c internal resistance is 0.5 milli-ohm/cell so that Ri = 110 x 0.5 = 55 mΩ for the battery, 3 and Isc = 240 x 10 = 4.4 kA 55 The short-circuit currents are seen to be (relatively) low.

Isc

fig. J6-1: battery of storage cells.

direct-current generator If Vg is the open-circuit voltage of the generator and Ri its internal resistance, then: Isc = Vg / Ri. In the absence of precise data, and for a d.c. system of voltage Un, Vg may be taken as 1.1 Un.

Example: A d.c. generator rated at 200 kW, 230 V, and having an internal resistance of 0.032 ohm, will give a terminal short-circuit current of 230 x 1.1 = 7.9 kA 0.032 G =

Icc

fig. J6-2: direct-current generator.

Isc at any point in an installation V Ri + Rl Where Ri is as previously defined, V is either Vb or Vg as previously defined, Rl is the sum of the resistances of the faultcurrent loop conductors. Where motors are included in the system, they will each (initially) contribute a current of approximately 6 In (i.e. six times the nominal full-load current of the motor) so that: V Isc = + 6 (In mot) Ri + Rl where In mot is the sum of the full-load currents of all running motors at the instant of short-circuit.

+

In this case Isc =


-

fig. J6-3: short-circuit at any point of an installation.

J 6.2 characteristics of faults due to insulation failure, and of protective switchgear Devices for circuit interruption are sensitive to the level of d.c. voltage at their terminals when breaking short-circuit currents. The table below provides the means for determining these voltages, which depend on the source voltage and on the method of earthing the source.

Note: In the following text the word “pole” has two meanings, viz: 1. Referring to a d.c. source, for example: the positive pole or the negative pole of a battery or generator. 2. Referring to a switch or circuit breaker, for example: a pole of a circuit breaker makes or breaks the current in one conductor. A pole of a circuit breaker may be made up of modules, each of which contains a contact. The pole may therefore consist of one module or (particularly in d.c. circuits) several seriesconnected modules.

Voltage stresses across opening contacts are reduced by the technique of connecting a number of contacts in series per pole, as mentioned in the table below, and in the following text. types of network

earthing schemes and various fault conditions

system earthing one pole earthed at the source i

i

a +

+

U –

R B

A

fault A

fault B

fault C

–

case 1 pole (a) must break maximum Isc at U volts poles (a) and (b) must break the maximum Isc at U volts there is no short-circuit current in this case

i

a

a

+

U/2 + U/2

R B b

b C

analysis of each fault

unearthed system source is not earthed

source with mid-point earthing

U

R

–

A

B b

C

case 2 pole (a) must break maximum Isc* at U/2 volts poles (a) and (b) must break the maximum Isc at U volts as for fault A but concerning pole (b)*

A

C

case 3 there is no short-circuit in this case poles (a) and (b) must break the maximum Isc at U volts as for fault A

* U/2 divided by Ri/2 = Isc (max.)

the most unfavourable case case of a circuit breaker

fault A all the contacts participating in current interruption are series connected in the positiveconductor (or the negative conductor if the positive pole of the source is earthed). Provide an additional pole for inserting in the earthed polarity conductor, to permit circuit isolation (figure J6-6).

A=B=C see Note below the table provide in the CB pole for each conductor the number of contacts necessary to break Isc (max.) at the voltage U/2.

fault B (or faults A and C simultaneously) provide the number of contacts necessary for breaking the current indicated in the CB pole of each conductor.

table J6-4: characteristics of protective switchgear according to type of d.c. system earthing. Note: each pole is equally stressed for faults at A, B or C, since maximum Isc must be broken with U/2 across the CB pole(s) in each case.

6.3 choice of protective device for each type of possible insulation failure, the protective devices against short-circuits must be adequately rated for the voltage levels noted in table J6-4 above.

The choice of protective device depends on: c the voltage appearing across the currentbreaking element. In the case of circuit breakers, this voltage dictates the number of circuit-breaking contacts that must be connected in series for each pole of a circuit breaker, to attain the levels indicated in table J6-4; c the rated current required; c the short-circuit current level at its point of installation (in order to specify its s.c. currentbreaking capacity);

c the time constant of the fault current (L/R in milli-seconds) at the point of installation of the CB. Table J6-5 below gives characteristics (current ratings, s.c. current-breaking capacity, and the number of series-connected contacts per pole required for a given system voltage) for circuit breakers made by Merlin Gerin.


6. protection of direct-current installations (continued)

J 6.3 choice of protective device (continued) type

ratings (A)

sc current-breaking capacity kA for L/R i 0.015 seconds (the number of series-connected contacts per pole is shown in brackets) 24/48 V 125 V 250 V C32HDC 1 to 40 20 (1p) 10 (1p) 20 (2p) 10 (2p) C60a 10 to 40 10 (1p) 10 (2p) 20 (3p) 25 (4p) C60N 6 to 63 15 (1p) 20 (2p) 30 (3p) 40 (4p) C60H 1 to 63 20 (1p) 25 (2p) 40 (3p) 50 (4p) C60L 1 to 63 25 (1p) 30 (2p) 50 (3p) 60 (4p) NC100H 50 to 100 20 (1p) 30 (2p) 40 (3p) 20 (4p) NC100LH 10 to 63 50 (1p) 50 (1p) 50 (1 p) NS100N 16 to 100 50 (1p) 50 (1p) 50 (1p) NC100H 16 to 100 85 (1p) 85 (1p) 85 (1p) NS100L 16 to 100 100 (1p) 100 (1p) 100 (1p) NS160N 40 to 160 50 (1p) 50 (1p) 50 (1p) NS160H 40 to 160 85 (1p) 85 (1p) 85 (1p) NS160L 40 to 160 100 (1p) 100 (1p) 100 (1p) NS250N 40 to 250 50 (1p) 50 (1p) 50 (1p) NS250H 40 to 250 85 (1p) 85 (1p) 85 (1p) NS250L 40 to 250 100 (1p) 100 (1p) 100 (1p) NS400H MP1/MP2-400 85 (1p) 85 (1p) 85 (1p) NS630H MP1/MP2/MP3-630 85 (1p) 85 (1p) 85 (1p) C1251N-DC P21/P41-1250 50 (1p) 50 (1p) 50 (2p) M10-DC 1000 100 (3p) 100 (3p) 100 (3p) M20-DC 2000 100 (3p) 100 (3p) 100 (3p) M40-DC 4000 100 (3p) 100 (3p) 100 (3p) M60-DC 6000 100 (4p) 100 (4p) 100 (4p) M80-DC 8000 100 (4p) 100 (4p) 100 (4p)

thermal overload protection 500 V

50 (3p) 50 (2p) 85 (2p) 100 (2p) 50 (2p) 85 (2p) 100 (2p) 50 (2p) 85 (2p) 100 (2p) 85 (2p) 85 (2p) 50 (3p) 100 (3p) 100 (3p) 100 (3p)

750 V

25 (3p) 50 (4p) 50 (4p50 (4p)

1000 V

50 (4p) 50 (4p) 50 (4p)

special DC ditto AC ditto AC ditto AC ditto AC ditto AC ditto AC ditto AC ditto AC ditto AC ditto AC ditto AC ditto AC ditto AC ditto AC ditto AC no thermal relay; provide an external relay (if necessary)

coefficient for uprating the instantaneous magnetic tripping units* special DC 1.38 1.38 1.38 1.38 1.42 1.42 1.42 1.42

tripping units MP1/MP2/MP3 special for direct current

table J6-5: choice of d.c. circuit breakers manufactured by Merlin Gerin. * These tripping units may be used on a.c. or d.c. circuit breakers, but the operating levels marked on each unit correspond to r.m.s. a.c. values. When used on a d.c. circuit breaker the setting must be changed according to the co-efficient in table J6-5. For example, if it is required that the d.c. circuit breaker should trip at 800 A or more the coefficient given in table J6-5 is 1.42, then the setting required will be 800 x 1.42 = 1,136 A.

6.4 examples Example 1 Choice of protection for an 80 A outgoing d.c. circuit of a 125 V system, of which the negative pole is earthed. The Isc = 15 kA. + 125 V = -

NC100 H 3-pole 80 A load

fig. J6-6: example.

Example 2 Choice of protection for a 100 A outgoing d.c. circuit of a 250 V system, of which the midpoint is earthed. Isc = 15 kA. + 250 V = -

NC100 H 4-pole 100 A load

fig. J6-7: example.


Table J6-4 shows that the full system voltage will appear across the contacts of the positive pole. Table J6-5 indicates that circuit breaker NC100H (30 kA 2 contacts/pole 125 V) is an appropriate choice. Preferred practice is to (also) include a contact in the negative conductor of the outgoing circuit, to provide isolation (for maintenance work on the load circuit for example), as shown in figure J6-6. Note: three contacts in series, which open in unison, effectively triple the speed of contact separation. This technique is often necessary for successfully breaking d.c. current. Table J6-4 shows that each pole will be subject to a recovery voltage of U/2, i.e. 125 V for all types of s.c. fault. Table J6-5 indicates that circuit breaker NC100H (30 kA 2 contacts/pole 125 V) is suitable for cases A and C, i.e. 2 contacts in the positive and 2 contacts in the negative pole of the CB. It will be seen in the 250 V column that 4 contacts will break 20 kA at that voltage (case B of table J6-4).

J 6.5 protection of persons

+

The rules for protection are the same as those already covered for a.c. systems. However, the conventional voltage limits and the automatic disconnection times for safety of persons are different (see tables G8 and G9 of Chapter G, Sub-clause 3.1): c all exposed conductive parts are interconnected and earthed; c automatic tripping is achieved in the timeperiod specified. RCDs are not applicable to d.c. circuits, so that in practice: c the principles of the TN scheme are used for cases 1 and 2 of Sub-clause 6.2. It is then sufficient to check that, in the case of a shortcircuit, the current magnitudes will be sufficient to trip the instantaneous magnetic relays. The checking methods are identical to those recommended for an a.c. network. c principles of the IT scheme for case 3 in Sub-clause 6.2, v the insulation level of the installation must be under permanent surveillance and any failure must be immediately indicated and alarmed: this can be achieved by the installation of a suitable monitoring relay as shown in Chapter G, Sub-clause 3.4, v the presence of two concurrent faults to earth (one on each polarity) constitutes a short-circuit, which will be cleared by overcurrent protection. As for the a.c. systems, it is sufficient to verify that the current magnitude exceeds that necessary to operate the magnetic (or short-time delay) circuit breaker tripping units.

U fixed

-

+

U variable ou fixed

-

TR5A

XM200

fig. J6-8: insulation (to earth) monitors for an IT direct-current installation.


7. short-circuit characteristics of an alternator

J The characteristics of a 3-phase alternator under short-circuit conditions are obtained from oscillogram traces recorded during tests, in which a short-circuit is applied instantaneously to all three terminals of a machine at no load, excited (at a fixed level) to produce nominal rated voltage. The resulting currents in all three phases will normally* include a d.c. component, which reduces exponentially to zero after (commonly) some tens of cycles. The curve shown below in figure AJ1-1 is the current trace, from which the d.c. component has been eliminated, of a recording made during the testing of a 3-phase 230 V 50 kVA machine. The definitions of alternator reactance values are based on such "symmetrical" curves. c b i 0

a t

fig. AJ1-1: short-circuit current of one phase of a 3-phase alternator with the d.c. component eliminated. * unless, by chance, the voltage of a phase happens to be maximum at the instant of short-circuit. In that case, there will be no d.c. transient in the phase concerned.

The reduction of current magnitude from its initial value occurs in the following way. At the instant of short-circuit, the only impedance limiting the magnitude of current is principally** the inherent leakage reactance of the armature (i.e. stator) windings, generally of the order of 10%-15%. The large stator currents are (practically) entirely inductive, so that the synchronously rotating m.m.f. produced by them acts in direct opposition to that of the excitation current in the rotor winding. The result is that the rotor flux starts to reduce, thereby reducing the e.m.f. generated in the stator windings, and consequently reducing the magnitude of the fault current. The effect is cumulative, and the reduced fault current, in turn, now reduces the rotor flux at a slower rate, and so on, i.e. the flux follows the exponential law of natural decay, its reduction rate at any instant depending on the magnitude of the quantity causing the phenomenon. Eventually, a stable state is reached, in which the (greatly reduced) rotor flux produces just enough voltage to maintain the stator current at the level of equilibrium between the three quantities, viz. current, flux and voltage. The reduction of fault current therefore is caused by a diminution of the generated e.m.f. due to armature reaction, and not, in fact, by an increase in impedance of the machine (that is why the term "effective reactance" was used in Chapter J Sub-clause 1.1). ** the sub-transient reactance, which is defined later, is very nearly equal to the leakage reactance.

As shown in figure AJ1-1, the current reduction requires a certain time, and the reason for this is that, as the rotor flux begins to diminish, the change of flux induces a current in the closed rotor circuit in the direction which, in effect, increases the excitation current, i.e. opposes the establishment of a reduced level of magnetic flux. The gradual predominance of the stator m.m.f. depends on the overall effect of rotor and stator time constants, the result of which is the principal factor in the "a.c. current decrement" shown in figure AJ1-1. If, during a short-circuit, there were no eddy currents induced in the unlaminated face of round-rotor alternators, or in damper windings (see note 1) of salient-pole alternators, the envelope of the a.c. current decrement would be similar to that of curve b in figure AJ1-1, i.e. the so-called transient-current envelope. The presence of either of the two features, mentioned above however, gives rise to the sub-transient component of current (curve C). The effect is analogous to that of the closed circuit of the rotor-excitation winding described above (i.e. the induced currents oppose the change), but having a very much shorter time constant. The overall a.c. current decrement is therefore composed of the sum of two exponentially-decaying quantities, viz. the sub-transient and the transient components, as shown in figure AJ1-2. Note 1: Damper windings are made up of heavy gauge copper bars embedded in the pole faces of salient-pole rotors, to form a squirrel-cage "winding" similar to that of an induction motor. Their purpose is to help to maintain synchronous stability of the alternator. With the rotor turning at the same speed as that of the m.m.f. due to the stator currents, no currents will be induced in the damper windings; if a difference in the speed of rotation occurs, due to loss of synchronism, then currents induced in the damper windings will be in a direction that produces a torque which acts to slow (an overspeeding rotor) or to accelerate (an underspeeding rotor). A similar, but much smaller effect occurs due to eddy currents in the surface of solid unlaminated rotors of turbo-alternators.

For advanced analytical studies of alternators, two component axes "direct" and "quadrature" are defined, and subtransient and transient reactances, etc. are derived for each component system. In the simple studies needed for 3-phase symmetrical fault levels and for circuitbreaker performance based on such faults, the direct-axis component system only is required; this accounts for the suffix "d" of reactance values, shown in figure AJ1-2. Suffix "q" is used for quadrature quantities. i" Vo/x"d

ia

enveloppe of the current, ia

i'

Vo/x'd

i t

Vo/xd substransient period

transient period

steady state

x''d = the sub-transient reactance Vo/i'' x'd = the transient reactance Vo/i' xd = the synchronous reactance Vo/i Vo = peak rated voltage of the alternator

fig. AJ1-2: a.c. component of armature current versus time, in a short-circuited alternator (no d.c. transient is shown).

Appendix J1 - 1

J The reactances are generally defined as r.m.s. voltages divided by r.m.s. currents. In the current trace of figure AJ1-2, however, it is simpler to use the projected peak values of current, so that Vo must be the rated peak voltage of the machine. Note 2: in the definition of "i" some authors use the actual voltage measured during the test, instead of Vo. Moreover, xd is generally denoted by Xs and is referred to as "synchronous reactance".

asymmetrical currents As previously noted, in general, all 3-phases of short-circuit current will include a d.c. component. These components give rise to additional electro-dynamic and thermal stresses in the machine itself, and in circuitbreakers protecting a faulted circuit. The worst condition is that of a phase in which the d.c. component is the maximum possible, i.e. the d.c. transient value at zero time (the instant of fault) is equal to the peak value of current given by Vo/xd'', as defined in figure AJ1-2. A typical test trace of this condition is shown in figure AJ1-3. stator phase current d.c. component

time

instant of short circuit The current envelope of an asymmetrical transient has the same dimensions about the d.c. transient curve, as the symmetrical envelope has about the current zero axis.

fig. AJ1-3: a fully-offset asymmetrical transient fault-current trace. The consequence of asymmetrical transient fault currents and the standardized relationship between the symmetrical and asymmetrical quantities for circuit breaker performance ratings are given in Sub-clause 1.1 of Chapter C, and are illustrated in figure C5.

2 - Appendix J1

1. domestic and similar premises

L Electrical installations for inhabited premises need a high standard of safety and reliability.

1.1 general related standards Most countries have national regulations andor standards governing the rules to be strictly observed in the design and realization of electrical installations for domestic and similar premises. The relevant international standard is the IEC publication 364.

the power distribution authority connects the LV neutral point of its HV/LV distribution transformer to earth. All LV installations must be protected therefore by RCDs (for TT and IT schemes of earthing) or by shortcircuit protective devices for TN schemes. All exposed conductive parts must be bonded together and connected to earth, either directly to an electrode at the premises (TT or IT schemes) or by means of the neutral conductor (TN schemes)*.

the power network The vast majority of power-distribution authorities connect the low-voltage neutral point of their HV/LV distribution transformers to earth. The protection of persons against electric shock therefore depends, in such cases, on the principles discussed in Chapter F, Clause 4, and Chapter G, all Clauses. The measures required depend on whether the TT, TN or IT scheme of earthing is adopted, as treated in detail in Chapter G. RCDs are essential for TT- and IT-earthed installations, but high-speed overcurrent devices (MCBs or fuses) are commonly used to clear earth faults on TN-earthed schemes. However, in particular instances (e.g. circuits feeding socket-outlets), RCDs are strongly recommended on TN installations, as being the only sure means of protection against shock when very long flexible leads of small c.s.a. are supplied from a socket. See also Clause 3 concerning special installations. * for TN-C and TN-S schemes refer to Chapter G, Sub-clause 5.1.

domestic and similar premises and special locations - L1

1. domestic and similar premises (continued)

L 1.2 distribution-board components

control and distribution board

enclosure

incoming-supply circuit breaker service connection lightning arrester lightning protection

overcurrent protection and isolation

cartridge fuses

MCB phase + neutral

protection against direct and indirect contact, and protection against fire

differential MCB

differential load switch

remote control bistable switch

THP

CDSc

energy management

IHPc fig. L1-1: presentation of realizable functions on a consumer unit.

L2 - domestic and similar premises and special locations

Isis

L the quality of electrical equipment used in inhabited premises is commonly ensured by a mark of conformity situated on the front of each item.

Distribution boards (generally one only in domestic premises) usually include the meter(s) and in some cases (notably where the supply authorities impose a TT-earthing system and/or tariff conditions which limit the maximum permitted current consumption) an incoming-supply differential circuit breaker, which includes an overcurrent trip. This circuit breaker is freely accessible to the consumer. On installations which are TN-earthed, the supply authorities usually protect the installation simply by means of sealed fuse cut-outs immediately upstream of the meter(s). See figure L1-2. The consumer has no access to these fuses.

meter

fuses or circuit breaker depending on earthing system

tableau de répartition

fig. L1-2: components of a control and distribution board.

the incoming-supply circuit breaker The consumer is permitted to operate this CB if necessary (e.g. to reclose it if the current consumption had exceeded the authorized limit; to open it in case of emergency or for isolation purposes...). The differential trip generally has a 500 mA setting to provide indirect-contact protection (and a measure of fire protection) for the whole installation. Current ratings for these circuit breakers are commonly: c 15 - 90 A two-poles, c 10 - 60 A four-poles.

fig. L1-3: incoming-supply circuit breaker.

the control and distribution board (consumer unit) This board comprises: c a control panel for mounting (where appropriate) the incoming-supply circuit breaker and other control auxiliaries, as required, c a distribution panel for housing 1, 2 or 3 rows (of 24 multi 9 units) or similar MCBs or fuse units, etc., c installation accessories for fixing conductors, and rails for mounting MCBs, fuse bases, etc. neutral busbar and earthing bar, and so on..., c service-cable ducts or conduits, surface mounted or in cable chases embedded in the wall. Note: to facilitate future modifications to the installation, it is recommended to keep all relevant documents (photos, diagrams, characteristics, etc.) in a suitable location close to the distribution board. The board should be installed at a height such that the operating handles, indicating dials (of meters) etc. are between 1 metre and 1.80 metres from the floor (1.30 metres in situations where handicapped persons or persons of advanced age are concerned).

fig. L1-4: control and distribution board.

lightning arresters Where the keraunic level of a locality exceeds 25, and the supply is taken from an overhead line, the installation of a lightning arrester at the service position of a LV installation is prescribed in many national standards and is strongly recommended for installations which include sensitive (e.g. electronic) equipment. These devices must automatically disconnect themselves from the installation in case of failure or be protected by a RCD of

appropriate sensitivity, according to the resistance of the earthing electrode for the installation. In the case of domestic installations the use of a 500 mA differential incoming-supply circuit breaker type S (i.e. slightly time-delayed) will provide effective earth-leakage protection, while, at the same time, will not trip unnecessarily each time a lightning arrester discharges the current (of an overvoltage-surge) to earth.



L 1.2 distribution-board components (continued) if, in TT schemes, the resistance of the installation earth electrode exceeds 100 ohms, then one or more 30 mA RCDs must be installed to take over the function of the differential device in the incoming-supply circuit breaker.

resistance value of the earth electrode (of a TT scheme) In the case where the resistance-to-earth is very high and exceeds the value Rt = 50 V = 100 ohms 500 mA one or several RCDs of appropriate sensitivity, i.e. 30 mA, should be used in place of the differential device of the incoming-supply circuit breaker.

1.3. protection of persons where public power-supply systems and consumers' installations form a TT-earthed scheme, the governing standards impose the use of RCDs to ensure the protection of persons.

On TT-earthed systems the protection of persons is ensured by the following measures: c protection against indirect-contact hazards by RCDs of medium sensitivity (300 or 500 mA) at the origin of the installation (incorporated in the incoming-supply circuit breaker, or on the incoming feed to the distribution board). This measure is associated with a consumer-installed earth electrode to which must be connected the protective-earth (PE) conductors from the exposed conductive parts of all class I insulated appliances and equipment, as well as those from the earthing pins of all socket outlets.

c where the CB at the origin of an installation has no RCD protection (see figure L1-7) the protection of persons shall be ensured by class II level of insulation on all circuits upstream of the first RCDs. In the case where the distribution board is metallic, care shall be taken that all live parts are double insulated (supplementary clearances or insulation, use of covers, etc.) and wiring reliably fixed, c obligatory protection by sensitive (30 mA) RCDs of socket-outlet circuits, and circuits feeding bathrooms, laundry rooms, and so on (for details of this latter obligation, refer to the table in Clause 3 of this Chapter).

incoming-supply circuit breaker with instantaneous differential relay In this case: c an insulation fault to earth could result in the shutdown of the entire installation, c where a lightning arrester is installed, its operation (i.e. discharging a voltage surge to earth) could appear to an RCD as an earth fault, with a consequent shutdown of the installation. Recommendation of suitable Merlin Gerin components c incoming-supply circuit breaker with 500 mA differential, and c RCD of type DDR-HS 30 mA (for example, differential circuit breaker 1P + N type Déclic Vigi) on the circuits supplying socket-outlets, c RCD of type DDR-HS 30 mA (for example, differential load switch type ID’clic) on circuits to bathrooms, shower rooms, laundry rooms, etc.) (lighting, heating, socket-outlets).


DB 500 mA

30 mA

diverse circuits

socket-outlets circuit

30 mA

bathroom and/or shower room

fig. L1-5: installation with incomingsupply circuit breaker having instantaneous differential protection.

L incoming-supply circuit breaker type S with retarded differential relay This type of CB affords protection against insulation faults to earth, but by virtue of a short time delay, provides a measure of discrimination with downstream instantaneous RCDs. Tripping of the incoming-supply CB and its consequences (on deep freezers, for example) is thereby made less probable in the event of lightning, or other causes of voltage surges. The discharge of voltage-surge current to earth, through the lightning arrester, will leave the type S circuit breaker unaffected. Recommendation of suitable Merlin Gerin components c incoming-supply circuit breaker with 500 mA differential, type S, and c RCD of type DDR-HS 30 mA (for example, differential circuit breaker 1P + N type DéclicVigi) on the circuits supplying socket-outlets, c RCD of type DDR-HS 30 mA (for example, differential load switch, type ID’clic) on circuits to bathrooms, shower rooms, etc. (lighting, heating, socket-outlets), c RCD of type DDR-HS 30 mA (for example, differential circuit breaker 1P + N, type DéclicVigi) on circuits supplying washing machines and dish-washing machines.

DB type S 500 mA

30 mA

30 mA

diverse high-risk location socketcircuits (laundry room) outlets circuit

30 mA bathroom and/or shower room

fig. L1-6: installation with incomingsupply circuit breaker having short time delay differential protection, type S.

incoming-supply circuit breaker without differential protection In this case the protection of persons must be ensured by: c class II level of insulation up to the downstream terminals of the RCDs, c all outgoing circuits from the distribution board must be protected by 30 mA or 300 mA RCDs according to the type of circuit concerned as discussed in Chapter G, Clause 4, c where a voltage-surge arrester is installed upstream of the distribution board (to protect sensitive electronic equipment such as microprocessors, video-cassette recorders, TV sets, electronic cash registers, etc.) it is imperative that the device automatically disconnects itself from the installation following a rare (but always possible) failure. Some devices employ replaceable fusing elements; the recommended method however as shown in figure L1-7, is to use a RCD. Recommendation of suitable Merlin Gerin components Figure L1-7 refers. 1. Incoming-supply circuit breaker without differential protection. 2. Automatic disconnection device (if a lightning arrester is installed). 3. RCD of type DDR-HS 30 mA (for example, differential circuit breaker 1P + N type Déclic Vigi) on each circuit supplying one or more socket-outlets. 4. RCD of type DDR-HS 30 mA (for example, differential load switch type ID’clic) on circuits to bathrooms and shower rooms (lighting, heating and socket-outlets) or a 30 mA differential circuit breaker per circuit. 5. RCD of type DDR-HS 300 mA (for example, differential load switch) on all the other circuits.

6. Safety and tripping discrimination are improved by the protection of circuits by means of 30 mA RCDs (for example, differential circuit breaker type Déclic-Vigi 1P + N) on a circuit supplying an apparatus which involves large quantities of water.

1

2

class II insulation

5

6

300 mA

diverse circuits

30 mA

3

30 mA

4

30 mA

high-risk circuit socket- bathroom (dish-washing outlets and/or machine) circuit shower room

fig. L1-7: installation with incomingsupply circuit breaker having no differential protection.



L 1.4 circuits the distribution and division of circuits provides comfort, and facilitates rapid location of faults.

subdivision National standards commonly recommend the sub-division of circuits according to the number of utilization categories in the installation concerned (see figure L1-8): c at least 1 circuit for lighting. Each circuit supplying a maximum of 8 lighting points, c at least 1 circuit for socket-outlets rated 10/16 A. Each circuit supplying a maximum of 8 sockets. The sockets may be single or double units (a double unit is made up of two 10/16 A sockets mounted on a common base in an embedded box, identical to that of a single unit), c 1 circuit for each appliance such as a water heater, washing machine, dish-washing machine, cooker, refrigerator, etc. Recommended numbers of 10/16 A (or similar) socket-outlets and fixed lighting points, according to the use for which the various rooms of a dwelling are intended, are indicated in the following table. room function living room bedroom, lounge, bureau, dining room kitchen bathroom, shower room entrance hall, box room WC, storage space laundry room

socketoutlets

lighting heating washing cooking machine apparatus

fig. L1-8: circuit division according to utilization.

minimum number of fixed lighting points 1 1

minimum number of 10/16 A socket-outlet 5 3

2 2 1 1 -

4 (1) 1 or 2 1 1

table L1-9: recommended minimum number of lighting and power points in domestic premises. (1) of which 2 above the working surface and 1 for a specialized circuit: in addition an independent socket-outlet of 16 A or 20 A for a cooker and a junction box or socket-outlet for a 32 A specialized circuit.


L the inclusion of a protective conductor in all circuits is required by IEC and most national standards.

protective conductors IEC and most national standards require that each circuit includes a protective conductor. This practice is strongly recommended where class I insulated appliances and equipment are installed, which is the general case. The protective conductors must connect the earthing-pin contact in each socket-outlet, and the earthing terminals in class I equipment, to the main earthing terminal at the origin of the installation. Furthermore, 10/16 A (or similarly sized) socket-outlets must be provided with shuttered contact orifices.^

cross-sectional-area (c.s.a.) of conductors The c.s.a. of conductors and the rated current of the associated protective device depend on the current magnitude of the circuit, the ambient temperature, the kind of installation, and the influence of neighbouring circuits (refer to Chapter H1). Moreover, the conductors for the phase wires, the neutral and the protective conductors of a given circuit must all be of equal c.s.a. (assuming the same material for the conductors concerned, i.e. all copper or all aluminium). Table L1-11 indicates the c.s.a. required for commonly-used appliances. Protective devices 1 phase + N in 2 x 9 mm spaces comply with requirements for isolation, and for marking of circuit current rating and conductor sizes.

fig. L1-10: circuit breaker 1 phase + N 2 x 9 mm spaces Déclic 32.

type of circuit single-phase 230 V 1 ph + N or 1 ph + N + E fixed lighting

c.s.a. of the conductors 1.5 mm2 (2.5 mm2)

maximum power

protective device

2300 W

circuit breaker fuse

16 A 10 A

10/16 A socket-outlets

2.5 mm2 (4 mm2)

4600 W


25 A 20 A

2.5 mm2 (4 mm2)

4600 W


25 A 20 A

dish-washing machine

2.5 mm2 (4 mm2)

4600 W


25 A 20 A

clothes-washing machine

2.5 mm2 (4 mm2)

4600W


25 A 20 A

cooker or hot plates (1)

6 mm2 (10 mm2)

7300 W


40 A 32 A

electric space heater

1.5 mm2 (2.5 mm2)

2300 W


16 A 10 A

individual-load circuits water heater

table L1-11: c.s.a. of conductors and current rating of the protective devices in domestic installations (the c.s.a. of aluminium conductors are shown in brackets). (1) in a 230/400 V 3-phase circuit, the c.s.a. is 4 mm2 for copper or 6 mm2 for aluminium, and protection is provided by a 32 A circuit breaker or by 25 A fuses.



L 1.5 protection against overvoltages and lightning the relevance of protective devices c disturbances Three types of disturbance often occur on electrical-power networks: v lightning and atmospheric electrical phenomena in general, with its direct and indirect consequences. The direct effects, which are fairly infrequent, concern its impact on overhead transmission and distribution lines. The indirect effects are more common and occur at lower energy levels. Such indirect phenomena are characterized by a powerful induction effect on the lines and/or by an increase of local earth potential, v operational overvoltages are transient, and are caused by abrupt changes in the circuit, such as the opening/closing of circuit breakers, load-break switches, contactors, etc., v overvoltages at normal system frequency can occur in many ways, and will do so, for example, if a neutral connection is broken on a 3-phase system, if the load is unbalanced, c the kind of installation to be protected. It is necessary to know in some detail the characteristics of the items to be protected, in order to select the most appropriate form of protection. The choice of protective device(s) depends on two factors: v the sensitivity: the ability of the equipment concerned to withstand an overvoltage condition i.e. its magnitude and duration, v the cost: which comprises the purchase price and the operational costs (possible losses, maintenance, etc.).

choice of a lightning arrester It depends on: c the level of the disturbance, c the cost, as noted above, c the connection to a LV electrical-power network, a telephone system, a buildingcontrol system bus, or to any other network, c the kind of installation-earthing scheme (see chapter F) (figure L1-12).


L installation rules Three principal rules must be respected: c it is imperative that the three lengths of cable used for the installation of the arrester (see figure L1-13) each be less than 50 cm, i.e.: v the live conductors connected to the isolating switch, v from the isolating switch to the lightning arrester, v from the lightning arrester to the main distribution board (MDB) earth bar (not to be confused with the main protective-earth (PE) conductor or the main earth terminal for the installation). The MDB earth bar must evidently be located in the same cabinet as the lightning arrester, c it is necessary to use an isolating switch of a type recommended by the manufacturer of the lightning arrester c in the interests of a good continuity of supply it is recommended that the circuit breaker be of the time-delayed or selective type. The guide "Lightning protection" treats these rules quantitively and includes information which allows the user to dimension an arrester according to his needs.


2. bathrooms and showers

L Bathrooms and shower rooms are areas of high risk, because of the very low resistance of the human body when wet or immersed in water. Precautions to be taken are therefore correspondingly rigorous, and the regulations are more severe than those for most other locations. The relevant IEC standards are 364-7-701, 479 and 669-1. Precautions to observe are based on three aspects: c the definition of zones, numbered 0, 1, 2 and 3 in which the placement (or exclusion) of any electrical device is strictly limited or forbidden, and, where permitted, the electrical and mechanical protection is prescribed, c the establishment of an equipotential bond between all exposed and extraneous metal parts in the zones concerned, c the strict adherence to the requirements prescribed for each particular zone, as tabled in Clause L3.

2.1 classification of zones Sub-clause 701.32 of IEC 364-7-701 defines the zones 0, 1, 2 and 3, as shown in the following diagrams: plan views

zone 1 * zone 2

zone 1 * zone 2

zone 3

zone 0 0.60 m

zone 0 2.40 m

2.40 m 0.60 m

vertical cross-section zone 1

zone 2

zone 3

2.25 m zone 1

zone 0

zone 1

0.60 m

2.40 m

* zone 1 is above the bath as shown in the vertical cross-section.

fig. L2-1: zones 0, 1, 2, 3 in proximity to a bath-tub.


zone 3

L zone 0 zone 1

zone 2

zone 3

0.60 m

2.40 m

zone 0 zone 1

zone 2

zone 3 2.40 m

0.60 m

zone 1

zone 2

zone 3

2.25 m zone 1 zone 0 0.60 m

2.40 m

fig. L2-2: zones 0, 1, 2, 3 in proximity of a shower with basin. fixed shower head (1)

fixed shower head (1)

0.60 m zone 1 0.60 m

0.60 m zone 1 0.60 m

zone 2

zone 2 2.40 m

2.40 m zone 3

zone 1

zone 3

zone 2

zone 3

2.25 m 0.60 m

permanent wall

fig. L2-3: zones 0, 1, 2, 3 in proximity of a shower without basin. (1) When the shower head is at the end of a flexible tube, the vertical central axis of a zone passes through the fixed end of the flexible tube.

0.60 m prefabricated shower cabinet 0.60 m

fig. L2-4: no switch or socket-outlet is permitted within 60 cm of the door opening of a shower cabinet.


2. bathrooms and showers (continued)

L 2.1 classification of zones (continued) classes of external influences

classes of external influences zone 3

AD 3 BB 2 BC 3

AD 3 BB 2 BC 3

dressing cubicles (zone 2)

AD 3 BB 3 BC 3 AD 7 BB 3 BC 3

AD 3 WC BB 2 BC 3 shower cabinets (zone 1)

fig. L2-5: individual showers with dressing cubicles. classes of external influences h<1.10 m AD 5 1.10 m
classes of external influences h<1.10 m AD 5 1.10 m
dressing cubicles zone 2

AD 3

AD 7

zone 1

BB 3

WC

BC 3

BB 2 BC 3

fig. L2-6: individual showers with separate individual dressing cubicles. classes of external influences

classes of external influences

AD 3

AD 3 dressing room zone 3

BB 2 BC 3

BB 2 BC 3

h<1.10 m AD 5 1.10 m
AD 7 zone 2

zone 1

BB 3 BC 3

BB 3 BC 3

fig. L2-7: communal showers and a common dressing room.

2.2 equipotential bonding to the earth electrode

metallic pipes h i 2m

water-drainage piping

gas

radiator lighting

metal door-frame

metal bath equipotential conductors for a bathroom

fig. L2-8: supplementary equipotential bonding in a bathroom.

2.3 requirements prescribed for each zone The table of Clause 3 describes the application of the principles mentioned in the L12 - domestic and similar premises and special locations

foregoing text and in other similar or related cases.

3. recommendations applicable to special installations and locations

L The following table summarizes the main requirements prescribed in many national and international standards. locations

protection principles

IP level

c TT or TN-S schemes 20 1 c differential protection v 500 mA if the earth electrode resistance is i 100 ohms instantaneous or short time delay (type S) v 30 mA if the earth electrode resistance is u 500 ohms c lighning arrester at the origin of the installation if: v supply is from overhead line with bare conductors, and if: v the keraunic level > 25 c a protective earth (PE) conductor on all circuits bathrooms supplementary equipotential or shower rooms bonding in (section 701) zones 0, 1, 2 and 3 zone 0 SELV 12 V only 27 1 zone 1 SELV 12 V 24 1 (1)

wiring and cables

socketoutlets switch operating protection handles and by 30 mA similar devices RCDs on distribution panels, to be mounted between 1 metre and 1.80 metre above the floor level

X class II limited to strict minimum

X X (2)

domestic dwellings and other habitations

zone 2

SELV 12 V or 30 mA RCD

zone 3

swimming baths (section 702) zone 0 zone 1 zone 2

saunas (section 703) work sites (section 704) agricultural and horticultural establishments (section 705) restricted conductive working spaces (metal tank interiors, etc) (section 706)

23 1 (1)

class II

supplementary equipotential bonding in zones 0, 1 and 2 SELV 12 V

28 1

X

SELV 12 V interior exterior

25 1 22 1 (1) 24 1 (1)

X class II

31 1

class II

35 7

mechanically protected

35 5

31 1

switchgear

X (2)

21 1 (1)

c conventional voltage limit UL reduced to 25 V c TT or TN-S scheme c conventional voltage limit UL reduced to 25 V c protection against fire risks by 500 mA RCDs

Note: Section numbers in brackets refer to sections of IEC 364-7. installation materials

X X

special special and water heater one socketclass II insulation outlet only with and protection by a a separation 30 mA RCD transformer

protection c by 30 mA RCD c by electrical separation, or c by SELV 50 V (Chapter G, Sub-clause 3.5) special (under-water lighting) X X special (under-water lighting) X X X protection c by 30 mA RCD c by electrical separation, or c by SELV 50 V (Chapter G, Sub-clause 3.5) X X adapted to temperature protection by 30 mA RCDs protection by 30 mA RCDs protection of: c portable tools: v by SELV or v by electrical separation c hand-held lamps v by SELV c fixed equipment v by SELV v by electrical separation v by 30 mA RCDs v by special supplementary equipotential bonding

(1) if subjected to water jets: IP x 5 (2) except a switch for SELV


3. recommendations applicable to special installations and locations (continued)

L locations

protection principles

IP level

wiring and cables

fountains (section 702)

protection by 30 mA RCDs and equipotential bonding of all exposed and extraneous conductive parts TN-S scheme recommended (3) TT scheme if leakage current is limited. Protective conductor 10 mm2 minimum in aluminium. Smaller sizes (in copper) must be doubled

caravan parks (section 708)

34 5

flexible underground cables

marinas and yacht basins medical centres

36 5

in conduits or buried

data processing (section 707)

fairs and exhibitions balneotherapy (cure-centre baths) motor-fuel filling stations motor vehicles

IT (medical) scheme equipotential bonding protection by 30 mA RCDs. TT or TN-S scheme individual: see section 701 (volumes 0 and 1) collective: see section 702 (volumes 0 and 1) explosion risks in security zones

protection by RCDs or by electrical separation

(3) in an IT scheme provide a LV/LV transformer to create a TN-S scheme.


switchgear

socketoutlets

protection of circuits by 30 mA RCDs (one per 6 socket-outlets) ditto protection by 30 mA RCDs

21 7

limited to the necessary minimum

installation materials

G

electrical installation practice according to IEC international standards

D 1. rules and regulations 2. electromagnetic disturbances

EMC1

2.1 disturbances by conduction 2.2. radiation

EMC2 EMC8

3. cabling of equipment and systems 3.1 earthing 3.2 masses (non-conducting metal parts) 3.3 attenuating effects 3.4 installation and cabling rules 3.5 EMC components and solutions

EMC9 EMC11 EMC15 EMC16 EMC17

4. local network problems

EMC20

low-voltage service connections Appendix- D581 EMC

1. rules and regulations

EMC Legislation on EMC throughout the world is broadly divided into two philosophies. In "liberal" countries, any parasitic interference with radio reception is illegal, but no emission-level limit for the source of interference is imposed. However, in cases of litigation the methods of measuring, and the limits of emission level laid down by the CISPR, serve as reference. For example, Japan is a "liberal country" in which the VCCI standards (that correspond technically to the international publications of CISPR) assumes that the attitude of civic responsibility prevailing is adequate at the present time. For countries more rigidly "regulated", emission levels exceeding a standardized limit are illegal. For example, in the U.S.A., Automatic Data Processing (ADP) systems are protected by obligatory emission-level standards defined by the Federal law FCC part 15. Checking procedures differ, depending on whether the level for class A (procedure for "compliance") or class B is concerned. In the case for class B (domestic environment), certification is then required. European regulations are effectively placed between the two foregoing attitudes. The parasitic-emission level and an excessivelysensitive reception device are both illegal. Compliance with the EMC standards, although only constituting a presumption of conformity to the essential requirements, is, nevertheless, the preferred means of checking. Moreover, the European regulations are applicable to all apparatuses, systems and commercialised installations, without exception.

Appendix EMC - 1

2. electromagnetic disturbances

EMC Problems of EMC (ElectroMagnetic Compatibility) often arise when equipment, which is highly sensitive to extraneous electrical disturbances (commonly referred to as "interference" or "parasites") is located in an environment subject to electromagnetic disturbances. Since sources of electromagnetic disturbance are numerous and inevitable, and desensitizing an equipment (generally electronic) to counter the effect of disturbances is difficult to achieve, consideration of the physical layout of the sensitive equipments and related cabling, relative to the sources of disturbance, becomes necessary. This is the principal means of ensuring a satisfactory degree of immunity for the large majority of sensitive electronic devices. There are two recognized modes of electromagnetic interference: c conducted disturbances propagated along cables, wires, etc. c radiated disturbances by stationary induction (magnetic or electrostatic fields*) and/or by electromagnetic (radio) waves

travelling in space. The magnitudes of electromagnetic disturbances are expressed by four parameters: two for the conduction mode and two for the radiation mode. For the conduction mode, measurements are made in the traditional quantities, viz: volts (U) and amps (I). For the radiated electromagnetic waves the electric and magnetic field strengths are measured in volts per metre (E) and in amperes per metre (H), respectively. The frequency is one of the principal features that characterise an electromagnetic wave. In EMC studies the solutions adapted differ according to whether the disturbance is at low frequency (LF) or at high frequency (HF). * Stationary (but changing in magnitude) magnetic and electric induction fields generally, are only significant in close proximity to the sources and are easily countered by placing sensitive equipment at a suitable distance from them. A notable exception is the case where some tens of thousands of amps, i.e. short-circuit fault currents, flow in a power cable.

2.1 disturbances by conduction 2.1.1 Disturbances by conduction: modes of propagation Electrical energy, whether useful signals or power, or unwanted parasites, propagate along a 2-wire circuit in one of two modes only, viz: in differential mode or in common mode. Differential mode The differential mode is the normal way of conducting current through a 2-wire circuit. This mode is sometimes referred to as series mode, normal mode, or symmetrical mode. In the differential mode the current flowing in one conductor is in exact phase opposition to that in the other conductor, i.e. flowing in the opposite direction at every instant. The voltage is measured between the two conductors.

Common mode The common mode is parasitic. It is sometimes also referred to as parallel mode, longitudinal mode or asymmetrical mode. Common-mode currents pass through all the conductors of a cable in the same direction. The return path for such currents is via the earth, earth-bonding connections and the protective earthing conductors, cable sheaths, etc. Since the earth is no longer used as a conducting medium for telefax, useful signals are no longer transmitted in the common mode through the associated cables. A potential difference in common mode is measured between the mass (local zero voltage reference terminal) and the mean value of potential of all the conductors of the cable circuit being tested. It may be present in the absence of any current flow. I 2

I U

equipment equipment

I

fig. EMC-1: signal or disturbance in the differential mode. Differential mode disturbances are the most severe at low frequencies. By low frequencies (LF) it is understood in EMC studies to concern all frequencies lower than 9 kHz. This convention means that a very large number of electrical disturbances are considered as being LF phenomena. In electrical power networks disturbances in the differential mode are numerous. One may cite, for example, interruptions of supply of short or long duration, voltage fluctuations and dips, phase instability, lamp flicker, variations of frequency: harmonics and voltage spikes. The effect of an electromagnetic disturbance depends largely on its duration. Permanent (maintained) disturbances principally affect analogue-type circuits, while transient and impulsive disturbances interfere especially with digital circuits. 2 - Appendix EMC

I 2

U

fig. EMC-2: disturbance in common mode.

EMC Electromagnetic disturbances couple readily with cables in the common mode, particularly at high frequencies (HF), since they act as radio antennae. Several kinds of coupling between neighbouring circuits can occur. The problems of common mode recur frequently in EMC cases. A conducting environment is always good for EMC, due to it equipotential quality. Only disturbances in the differential mode can be filtered locally, cable by cable. As indicated by its name, the common mode is common to all the cables of a given equipment. Common mode problems at HF are particularly critical in an insulated environment, or where the mass (the zerovoltage refernce for all electronic circuits) is "floating" with respect to earth (i.e. insulated from the earth). A common mode voltage is always detrimental. If it cannot be reduced, it is important, at least, to prevent it from developing into a differential mode disturbance.

This is the role of insulated and/or symmetrical connections. A galvanic insulation is only effective at low frequencies. A symmetrical connection, also referred to as "balanced", can remain effective up to high frequencies. The dissymmetry of a differential connection originates mainly from its end circuits. An imbalance at an end circuit can be caused by an electrical and/or geometric dissymmetry. In any case, a connection by simple coaxial cable to transmit signals at low frequencies is not recommended. Correcting measures which may produce harmful secondary effects must be combined with other precautions in order that the system effectively counters the entire range of disturbances (LF and HF, of large and small amplitudes). The combination of the different corrective measures (galvanic separation, symmetrical connections and overvoltage protection) is referred to as coordinated protection.

2.1.2 LF disturbances by conduction LF disturbances include all types of parasitic interference of which the range of significant frequencies is lower than 9 kHz. The frenquency of 9 kHz is a conventional upper limit, below which electrical phenomena may be analysed in simple terms, using customary equivalent linear circuit techniques, based on resistances, inductances (self- and mutual-) and capacitances. By definition, a LF disturbance exists for a relatively "long" time (at least a hundred microseconds). The energy level of a conducted LF disturbance can be considerable and is very easily measured. The impedance of a cable at very low frequencies is practically equivalent to its resistance only. At several kiloherz most cables of small cross-sectional-area (c.s.a.) and even at 50 Hz (for cables of large c.s.a.) the lineal inductance of a conductor is of the order of 1 µH/m, and its impedance increases linearly with frequency. For example, large 1core cables at 50 Hz, installed in trefoil, have a lineal impedance of approximately 0.3 Ω per km. This feature is important when considering harmonic frequencies in a network. Selecting a c.s.a. larger than 35 mm2 for a protective conductor would effectively reduce the heating of the conductor when carrying fault current (since its resistance would be lower) but would have a negligible effect on the equipotential distribution: the inductance of a cable being (as noted above) practically independent of its c.s.a.

Interruptions (long or transitory) An interruption is a total disappearance of the power-supply system voltage. In the case of a fault occurring on the HV network of a power-supply system, a consumer will normally experience a "voltage dip" sometimes followed by a brief interruption. This interruption will occur only if the HV system is an overhead-line (O/H line) system, and the consumer is being supplied from the section of line on which the fault has occurred. So-called "fleeting faults" on overhead lines are very common, and consist of flashovers (of insulators) to earthed metal by momentary overvoltage due to lightning or a short-circuit through a large bird, or again, directly to earth through, for example, a wet tree branch, etc. In more than 80% of these incidents such a fault will disappear during the brief period of automatic interruption, and normal supply will be restored. The automatic sequence for the elimination of fleeting faults on O/H lines is included in the protection scheme for the line. The interruptions in these schemes are normally limited to less than 0.5 seconds. An underground cable supply network reduces the number of interruptions to about 10% only of those of O/H line systems, but underground cable faults are not selfclearing, so that lengthy shutdowns are necessary to locate the fault and to effect repairs.

Appendix EMC - 3

2. electromagnetic disturbances (continued)

EMC 2.1 disturbances by conduction (continued) Flicker Flicker describes a condition of small but frequently recurring voltage dips caused by loads which require relatively heavy current for brief and regularly repeated periods. The impedance of a LV network is made up mainly of cable impedance and the impedance of the HV/LV transformer supplying the network. The greater the kVA rating of the transformer, the lower its effective impedance. In public power-supply systems the flicker problem is more common on rural systems, particularly at the end of long lines. It is a problem on a line which is supplying arc furnaces, arc-welding machines and, generally, where heavy loads are frequently switched. Flicker creates an objectionable annoyance for persons working under incandescent lighting. The effect is purely physiological, no dysfunctioning of electronic equipment will occur due to flicker. Flicker is objectionable only where heavy overloads and frequent switching are combined, or where the impedance of the system is high. Standardized parameter limits and a flicker meter are described in IEC publications 1000-3-3 and 1000-4-15. ∆U/Un in % 3 2

1

drop of voltage, caused principally by switching loads which, at the instant of energization, require a greater current than the normal rated value, e.g. small-motor starting currents, the switching on of large resistive heating devices and incandescent lamps, etc. Such dips are transitory only, but are often more severe than those classed as flicker, generally exceeding 10%. The duration of a "dip" lasts from 10 ms to approximately 1 s. Voltage reductions which exceed 10% and 1 s, due, for example, to starting large motors, or, as previously described, due to system faults, are simply referred to as "voltage drop" and the extent of the drop and its duration are specified. Voltage fluctuations have little effect on electronic circuits generally. Sensitive, precision electronic-control devices, electronic calculators of early design, and electronic (HF) fluorescent lighting tubes, however, may be adversely affected. A well-designed electronic device can tolerate, without difficulty, voltage fluctuations up to + 8%. A voltage dip at a point on a HV system is generally due to a short-circuit fault elsewhere on the same network. The closer the fault to the point in question, the more severe the dip. The severity of the dip is defined by two parameters: the magnitude of the drop as a percentage of the system nominal voltage, and its duration in milliseconds. Voltage dips are generally due to wind-blown debris (tree branches, etc.), electric storms, or faults on the lines (broken insulators) or occur on the installation of a neighbouring consumer. Ueff.

0,5

400 V 360 V depth (% de Un)

0,3

.5 .7 1

10

100

1000

fig. EMC-3: number of variations per minute. For industrial installations subject to flicker, a modification to the installation is sometimes necessary. Among the possible corrective measures available, the most effective include: separate cables for heavy loads preferably with each large load supplied through an individual HV/LV transformer, division of the load, increase the time lags in automatic control systems, reduction in the work-cycle rate, time-wise staggering and spreading of operations which require impulsive power demands, together with the installation of a static reactive-power compensator. Technically, a reduction in the source impedance is an excellent solution. In final general-distribution circuits at LV, the 3-phase symmetrical short-circuit current is usually within the 500-5,000 A range. In industry, the short-circuit current at LV may exceed 10 kA on a circuit of large c.s.a. close to the source substation. This value never, however, exceeds 100 kA. Fluctuations and voltage dips A fluctuation of voltage is a rapid change of supply voltage not exceeding + 10% (the generally accepted limits at distribution level) during normal operation. A "dip" is a sudden 4 - Appendix EMC

10

most influenced beetween the three phase to phase voltages

clearing time = 0,3 s duration = 0,4 s

time

fig. EMC-4: caracteristics of a voltage dip. Faults on VHV (very high voltage) transmission lines are rare and are usually due to lightning, or to exceptionally severe cold weather. The consequence of voltage dips (when followed by an interruption) is a complete loss of supply to electronic (and power) devices. Relays will drop out and motors controlled by electronic speed-variation and regenerativebraking devices will be deprived of brake control. Even if there is no supply interruption, a large and long (up to 1 second) voltage dip may cause similar malfunctions. Means of countering these problems at the least cost requires individual analysis of each case. To overcome the voltage-dip problem, many low-power electronic equipments have individual power packs with an autonomy of several hundred milliseconds for 100% loss of supply voltage. For heavy-duty power supplies, the period of autonomy amounts only to approximately 20 milliseconds, the limiting factor being the size of the energy-storing capacitors required. Rotating machines (motor/generators) have

EMC sufficient autonomy to cope with voltage dips. Finally, uninterruptible power supply units can suppress voltage dips, and maintain the power supply during a period of complete interruption. Unbalance The amplitude of an ac voltage is expressed by its rms (root-mean-square) value. The voltage between a phase conductor and the neutral is referred to as the phase voltage, while that measured between any two phases is called the line voltage. The line voltage equals etimes the phase voltage, on a normally balanced 3-phase system (e= 1.732). A 3-phase system may be defined simply by the amplitude of 3 voltages, either line or phase values. In order to define a sinusoidal system which is in an unbalanced state, however, the values of current and voltage of each phase are then, in the general case, the sum of 3 rotating vector components. The three components of each phase are known as: c the positive phase-sequence component c the negative phase-sequence component c the zero phase-sequence component. A balanced 3-phase system is composed of positive phase-sequence components only. An unsymmetrical system is said to be unbalanced; its negative- and zero-phase sequence components are generally both present, together with the positive phasesequence component. A common cause of imbalance is that of different levels of loading on the 3 phases. Unbalanced loading results in unbalanced voltages being applied to 3-phase motors. Increased losses occur in the rotors of the motors, and in cases of excessive imbalance, motors can be destroyed by overheating. Single-phase (line-to-line) loads are not normally adversely affected by imbalance. Small degrees of imbalance (0.5-1%) are inevitable on LV 3-phase 3-wire networks, and up to 2 or 3% can be tolerated for several minutes by all loads. When an imbalance of voltage is considered to be excessive (> 2% for example), it is advisable to correct the balance of phase loading. Where it is not possible to improve the balance, the situation may be eased by increasing the fault level at the circuit concerned by changing the supply transformer. An average HV/LV distribution transformer (> 100 kVA) has a short-circuit voltage of 5-6%. Special transformers are available with interlaced windings which limit the leakage reactance to give a short-circuit voltage of approximately 2%. A low short-circuit voltage effectively means a low source impedance (with higher faultcurrent level) a situation which improves the voltage balance, and (incidentally) improves the form of the voltage wave (if it happens to be distorted) by reducing the harmonic content of the wave. A modern method of improving a condition of imbalance, though presently rather costly, is to install a static compensator. It consists of a system which stores energy in an inductor or capacitor, and restores this energy to the system at the appropriate instants. An active filter constitutes one of the preferred solutions for limiting disturbances generated by arc furnaces during the start-up phase.

Frequency variations The European network performs, in practice, as an infinite system as far as frequency stability is concerned, in that load changes do not sensibly affect the frequency. On smaller private systems, and especially on single generators, where the rotational inertia is small and the regulating system of the prime mover is generally rudimentary, the frequency will vary (within reasonable limits) each time the load changes abruptly. Diesel engine prime movers are less stable, in terms of frequency, than turbines. Frequency variations do not unduly disturb electronic equipments. Converters based on current chopping principles are insensitive to frequency changes. All modern devices and components should be capable of correct performance during frequency changes of + 4% throughout a 10 minute period. Only very large systems with transformers operating at the limit of saturation may, when the system voltage is at its maximum, be subjected to overheating by a long-term lowfrequency condition. AC motors (locked to the frequency) will experience speed variations corresponding to those of the frequency. On the other hand, the inertia of motors tends to smooth out other sudden disturbances occurring on a network. Harmonics Any non-linear load (fluorescent lamp, Graetz bridge, arc furnace, etc.) takes a nonsinusoidal current from the network. Such a current is composed of a sinusoidal component at the frequency of the system and is known as the fundamental component, together with other sinusoidal components which are whole-number multiples of the fundamental frequency. These latter are referred to as harmonic components. Conventionally, harmonics up to the rank of 40 only are considered in power systems, i.e. 2 kHz for 50 Hz systems and 2.4 kHz for 60 Hz systems. Supplies to electronic circuits, power regulators based on Graetz bridge, and fluorescent lighting equipment, are rich in harmonics. Distortion of a voltage waveform is onerous for associated equipments; it is expressed as a percentage. It is proportional to the harmonic content of the current and to the impedance of the source. The effect of distortion is to increase the heating losses in motors. In an ADP environment, a distortion of 5% may be considered to be normal. All electronic components can tolerate a global factor of distortion including possible interharmonics of at least 8%. An inter-harmonic current has a frequency which is not a wholenumber multiple of the fundamental (i.e. system) frequency. A distinction is made between "true" inter-harmonics generated at discrete frequencies, and those forming part of a continuous spectrum. Even-numbered harmonics are generated only by asymmetrical rectifiers and load currents which contain a dc component. A dc component can readily saturate a powersupply transformer. Most non-linear loads (satured transformers, fluorescent tubes, power supply circuits which use currentchopping techniques, etc.) only generate odd-numbered harmonics.

Appendix EMC - 5


EMC 2.1 disturbances by conduction (continued) Balanced 3-phase loads supplied from a 3-phase 3-wire system (i.e no neutral wire) do not generate 3rd harmonic currents or multiples of 3rd harmonic currents. A Graetz bridge or a hexaphase thyristor-controlled regulator behave as current generators, which are practically independent of the voltage distortion. Third harmonic currents and multiples of them (known as "triplen" currents) when generated in the phases of a 3-phase 4-wire system, present a particular problem in that, being of zero-phase sequence (i.e. in phase with each other), they add arithmetically and complete their circuit through the neutral conductor. The current in the neutral conductor due to this cause is at 150 Hz and can exceed, in unfavourable circumstances, the current flowing in the phase wires. This aspect should be borne in mind when considering supplies to ADP loads and loads made up of fluorescent lighting tubes. If the neutral is not distributed, i.e. the system is 3-phase 3-wires, then 3rd harmonic currents cannot flow. A transformer with a deltaconnected primary winding, however, allows the circulation of 3rd harmonic currents, a feature that practically negates the distortion of the LV voltage wave, which would otherwise contain a large 3rd harmonic component. Reduction of the source impedance is not always effective in reducing distortion : power-factor correcting capacitors can cause a problem, if, together with the source impedance (which is predominantly inductive) the combination should form a resonant (or partially resonant) circuit at one of the harmonic or inter-harmonic frequencies. The parallel combination of inductive reactance L and capacitive reactance C can present a high impedance at its resonant frequency, particularly at low-load periods. The distortion can therefore become excessive due to the amplification effect of the resonance. A harmonic filter is a series LC circuit connected in parallel with the source and acts as a short-circuit to currents at its resonant frequency. The capacitor and inductor must be capable of carrying the maximum value of the harmonic current, while the capacitor must withstand the elevated harmonic voltage plus the system normal-frequency voltage. The inductor is adjusted to cause the combination to resonate at the exact frequency of the harmonic in question. It must not saturate or overheat. The rating of a harmonic filter varies according to the size of the installation from some kvar to several Mvar. A problem with harmonic filters is that their resonant frequencies change from one point on the network to another due to cable inductance. The longer the cabling, the lower the frequency at which the filter resonates. It is advisable moreover, to check that the currents in a number of paralleled filters are satisfactorily shared. A modern means of limiting distortion is by the use of active filters. These filters are inverters based on PWM (Pulse Width Modulation) techniques, together with reactive-energy storage.

6 - Appendix EMC

The role of an active filter, triggered by an error function, is to inject into the network a harmonic current exactly equal, but in phase opposition, to that produced downstream. The principle of operation is comparable to that of a static compensator, but at a considerably higher chopping rate. An active filter can be called upon to compensate not only first (i.e. lower) harmonics, but also reactive power (var) and flicker ; it is simply a question of dimensioning. Overvoltages Overvoltages that affect industrial-supply power networks in the differential mode (between phases) can occur for numerous reasons. The energizing of a bank of capacitors can generate an overvoltage transient associated with an energy level of several hundred Joules. The inductance of the system and the capacitance of the bank behave as a series LC circuit which oscillates transiently at its natural frequency, typically below 1 kHz. The value of the first peak (the sum of the transient and system peaks) can reach almost twice the value of the power system peak voltage, i.e. the transient can have a peak value almost equal to the system peak. A second cause of overvoltage may occur on the blowing of a wire-type fuse. A sudden release of the magnetic energy stored in the system inductances can be as much as a thousand Joules, which, when converted into electrostatic form in the system capacitances, can raise the voltage sufficiently to damage associated equipment. The effect is considerably less with cartridge fuses or with a circuit breaker. Any manœuvre on a power transmission system (opening or closing a circuit breaker, isolating switch, etc.) results in an operational disturbance. The energization of a transmission line is characterized by a wave front of voltage which (at approximately the speed of light) propagates and reflects along the line to produce a voltage-doubling phenomenon. The frequency of this highly-damped shortduration oscillation varies between 10 kHz and 1 MHz. The phenomenon is related to that of energizing a capacitor, but the frequency in this case is much greater (depending on the length of the line) while the energy level is lower. The risk of damage to equipment from this phenomenon is much less than that from overvoltages of longer duration, but the risks of malfunctions are greater. Overvoltages have little effect on electrotechnical equipments, but can disturb, weaken and even destroy electronic equipments. A LF overvoltage in common mode has no effect on supply circuits that are galvanically isolated, provided that the isolating dielectric can withstand the voltage stress without rupturing. At HF, principally by its common-mode component, an overvoltage can disturb (i.e. interfere with) sensitive electronic systems. The solution is simple: filter each equipment with respect to its own mass (metallic structure of the equipment).

EMC The installation engineer has practically only one way to protect the installation against overvoltages, and that is to install overvoltage limiting devices on the supply-circuit conductors. Overvoltages occurring on public LV distribution networks are lower in energy than those occurring in heavy-current industrial networks: the energy on public networks rarely exceeding 100 Joules. The only really dangerous case is that of a lightning stroke on a line close to the installation.

New overvoltage surge arresters based on the use of varistors of high energy-dissipation ratings allow an effective protection of all LV systems and equipments downstream of the point of arrester installation. Failure of the zinc-oxide varistor will cause a thermal fusing element (which is connected in series with it) to blow, and open the circuit, thereby avoiding a short-circuit to earth via a faulty or damaged arrester. The cable from this device must be connected by the shortest possible route to the mass of the distribution board, i.e. the common earthing bar, and not to the earthing electrode, which is generally too far away (see Sub-clause L 1.4).

2.1.3. HF disturbances by induction At HF i.e. conventionally above 1 MHz, interference phenomena become considerably more complicated. Power conductors become efficient antennae, electromagnetic fields, even when weak, produce considerable interference, all cables are affected, and some may resonate, etc. HF phenomena are severe, frequent, difficult to analyse and are cause to reconsider the established practices in electronics cable installation. The inductance of cables becomes more of a problem at HF than at low frequencies. The lineal inductance of any conducting structure following a sensibly straight route is approximately 1 µH/m. Furthermore, an interconnection of a length exceeding a thirtieth (1/30) of a wavelength becomes practically incapable of ensuring equipotentiality between the two interconnected masses. Beyond λ/30, a conductor becomes an effective radiating antenna but, if radiating, it fails to perform correctly as an equipotential conductor. The wavelength λ corresponding to a frequency of 1 MHz is 300 metres. The distance between any equipment and the main earth bar being generally greater than 10 metres, one may deduce that the nature and the quality of the earthing is of no consequence at frequencies exceeding 1 MHz. A simple dictum: a large conductor is good, but a short conductor is better. HF disturbances by conduction in common mode through cables are, in fact, considered to be the principal problem for EMC specialists. The reduction of common mode disturbances at HF through cables can be achieved by one or more of the following three stratagems: 1 - attenuation effects: close interconnection (networking) by equipotential conductors of "masses" and/or screened cables 2 - filters between conductors and mechanical mass of each equipment 3 - ferrites on "problem" cables. An electronic circuit, e.g. a card supporting chips, etc., should never be allowed to "float" relative to its conducting envelope, a condition which is to be avoided at all costs in the presence of HF interference. The natural (so-called "stray") capacitances of the card components, less than a pico-farad, can be sufficient to cause interference with an electronic circuit. To limit rapidly varying voltages between an electronic circuit and its environment, the connecting of the filter 0 V (reference voltage) terminal to an enveloping metallic housing, connected to earth or not, is an excellent preventive measure.

HF spikes The range of frequencies which presents the greatest difficulties, both in radiation and in protection against the radiated energy, is the VHF band from 30 to 300 MHz, also referred to as the "metric" band. Almost all electric arcs, sparks, electrostatic discharges and starting contacts (such as dry contacts, starting contact for striking an arc in electroluminescent tubes, operation of circuit breakers and other switching devices on HV systems) generate impulses (spikes) which are conducted in common mode and radiated. The radiation spectrum covers the range of the above-mentioned VHF band. The amplitude of the HF current spikes can attain a peak of several tens of amperes. Digital circuits are particularly vulnerable to such spikes. Respecting the standard for immunity IEC 1000-4-4 is a highlyrecommended means of achieving satisfactory protection and EMC of an installation. Maintained HF disturbances Frequency converters, electronic speed controllers, Graetz bridges and electric-motor commutating brushes also generate common mode HF disturbances. The peak value of these disturbances can reach and even exceed 1 ampere. One solution is to install an efficient filter at the supply-source and/or at the disturbed equipment. Another solution is the use of power cables that include a screen, which is earthed at both ends. For sources of heavy interference, it is recommended to form a network of equipotential interconnections of all masses in the neighbourhood of the offending source, in particular all metallic cable ways, ducts, trays, etc.

Appendix EMC - 7


EMC 2.2 radiation

8 - Appendix EMC

Electrical energy propagation is not only confined to conductors. It can also propagate in space without material support. Such propagation is referred to as electromagnetic fields or waves, or Hertzian waves. Such a wave is made up of an electric component E in volts per metre, and a magnetic component H in amperes per metre. These radiated fields, when encountering a

conductor (which acts as a receiving antenna), give rise to minute emfs and currents in the conducting material, i.e. in the form of disturbance by conduction. For cable circuits, these disturbances are in common mode. It is therefore possible to protect against these radiated fields by means of a Faraday cage arrangement or by (very often) low-pass filters.

2.2.1 LF magnetic fields At low frequencies, only the magnetic field may cause problems. Whether it is impulsive (short-circuit, lightning, electronic flash...) or maintained, the field H is generally produced in close proximity to the affected equipment. Measurement of the field strength requires an oscilloscope and a loop probe only. The magnetic field at low frequency does not propagate, but remains in the close proximity of its origin (a transformer or an induction motor, for example) and its field strength initially decreases rapidly with distance from the source according to 1/D3. At greater distances, the rate of decrease is slower and approaches 1/D2. This latter value is often used when considering the field surrounding busbars or an overhead line. The magnetic-field strength of a rectilinear current with a return path at infinity (such as that due to lightning) decreases according to 1/D. Severe sources of magnetic fields are the zero-phase sequence currents in supply cables of a TN-C scheme. Loops formed between the phase conductors and currents diverted from the neutral conductor (through equipotential bonds) are sometimes very

large; such currents can amount to several amperes. The undesirability of the TN-C scheme (in buildings) for this reason is shown in fig. F14 Chapter F, of the main text. During a short-circuit fault, the disturbance is evidently greater to a degree that depends on the fault-current magnitude. The most common consequence of a LF magnetic field is a distortion of the image of a cathode ray tube (jumps and wave-like movements of the image, and even changes of colour). A magnetically unscreened CRT (cathode-ray tube), an electron miscroscope, a mass spectrometer or a magnetic reading head, barely tolerates 1A/m at LF. Moreover, the "stray" loops formed through equipotential connections to masses are associated (naturally) with corresponding voltages. Magnetic screening from/of a magnetic field is very difficult at frequencies less than 10 kHz. The easiest solution is simply to place the sensitive equipment out of range of the offending field. Screening the sensitive equipment with a thick magnetic shield can reduce the field strength by a factor of the order 10.

2.2.2 HF electromagnetic fields At high frequencies, the E and H fields unite to form indivisible electromagnetic waves in space. At more than a sixth of a wavelength from a point source, the ratio E/H tends to a value of 120 π = 377 ohms. It is sufficient, therefore, to give the value of one component in order to deduce the field strength. Numerous industrial, scientific or medical apparatuses use radio frequencies, most often in the 1 MHz to 3 GHz range. Radio transmitters have power-radiation capabilities ranging from several milli-watts for radiocontrol devices, to several mega-watts peak for radar systems. Walky-talkies, which can be used for transmission very close to electronic equipment, are sources of disturbance, particularly for low-power analog circuits. An effective way to reduce radio-transmitter field strength, as "seen" by sensitive electronic equipments, is to use antennae as remote from the equipments as possible, and located at the greatest height attainable. Since this principle cannot be applied to portable transmitters, their use must be restricted to areas sufficiently remote from sensitive equipments to ensure trouble-free operation of the latter.

Electronic equipments are rarely affected by a field-strength of less than 1 volt per metre. Field strengths exceeding 10 volts per metre, however, very often cannot be tolerated. The range of frequencies giving the most severe effects is, again, in the VHF band. At HF a common-mode current in a cable always produces a radiated wave. The reciprocal case is also true, i.e. the arrival of a HF wave will produce a common-mode current in a cable. The methods of protection against HF fields are the same as those adopted against disturbances by conduction at the same frequencies. The antenna effect of cables carrying HF current by coupling in common mode constitutes the principal problem in EMC.

3. cabling of equipment and systems

EMC In order to cable an electronic system correctly or to correct an unsatisfactory installation, it is often sufficient to apply some simple elementary rules. From experience, the most important factor is a clear understanding of the phenomena and the recognition of their limits. The strict observance of traditional rules for correct installation and cabling has become necessary. This is the price to pay for the

achievement of EMC in modern electronic systems. Many practices which are satisfactory at LF have proved to be poor or even catastrophic at HF. Certain cabling options can be chosen with confidence. The interconnection of all the non-functional earths of a single site is one example. Factors which are always favourable should become standard practice.

The expressions "earth", "earth electrode", "earth plate", "earth rod" all refer to a conductor which is buried, and in intimate contact with the soil. The word «mass» refers to metal parts of equipment (electrical or not for example, water pipes) which, under normal conditions, are not intended to carry current. Bonding conductors, which are used to interconnect masses are also referred to as "mass". Although all masses in normal LF installation practice are connected to earth, the two words, "earth" and the above-noted equivalents should not be confused with "mass". "Mass" is commonly called "ground" in some countries.

These currents, and short-circuit-to-earth fault currents, flow mainly through the protective earthing PE conductors (coloured with yellow and green stripes) and finally back to the source substation, via the earth (TT system) or via the earth path and (mainly) through the neutral conductor in parallel (TN system). Since, in the TN case, practically all of the fault (and leakage) currents return to the source via the neutral conductor, the resistance of the installation earth electrode is not of primary importance (unless lightning arresters are to be connected to it). For the protection of electronic equipments, it is strongly recommended that common-mode currents entering the building from external cables be diverted to earth at the point of entry. A simple galvanic isolation is often insufficient: the overvoltage withstand capability of a galvanic isolation transformer is typically less that 10 kV. This value is insufficient on days of intense electrical storms. The installation of non-linear voltage-limiting devices then becomes a necessity. It is important that all incoming metallic pipes, ducts, trunking, etc., be connected to earth at their entry into the building. This policy can avoid the circulation of currents (from outside the building) in the conductors bonding the masses. The installation of overvoltage-protection devices must be carried out with the minimum possible common impedance between the external circuit and the circuit to be protected. The length of conductor in series with the voltage limiter must, consequently, be the shortest possible. The residual voltage "seen" by the protected equipment is then independent of the impedance of the earth. Even with a "bad" earth, it is possible to protect an equipment effectively against external overvoltages: it is necessary and sufficient to connect the voltage limiter to the mass of the equipment using the shortest practical length of cable.

3.1 earthing

3.1.1 the role of earthing The basic role of an earth electrode is to maintain all masses in an installation at a voltage close to zero, whether the power source is earthed or not. This is achieved in a properly designed installation, regardless of whether a faulty condition (which would otherwise raise the voltage of the installation masses) occurs in the installation circuits or on the power-supply network, or other sources external to the installation. The role of earthing, therefore, is that of the protection of persons against the dangers of electrocution. The severity of an electric shock is a function of the current which passes through the body, and equally important is the path of the current flow through the body. Established IEC rules of protection against electric shock set safe limits of voltage (referred to as conventional voltage limits) above which masses are considered to be unacceptably dangerous. For normal 50 Hz or 60 Hz power systems, these values are 50 Vrms for dry locations and 25 Vrms for wet locations, for example bathrooms and laundries (see Chapter L of the main text for more details). It is recognized that a low contact resistance of an earth electrode with the mass of the earth cannot always be obtained. Furthermore, its value is rarely constant, depending largely on soil humidity (and so prone to seasonal changes). An essential factor in maintaining safety to personnel in the event of a high earthing resistance, is that of the equipotential concept. If, for example, all masses are at a common (even normally dangerous) potential, and the earth below the building is at a similar potential, a person can touch any or several masses at the same time without danger. This is why electrical appliances with long leads (hedge trimmers, lawn mowers, etc.), which allow the user to leave the equipotential environment of the house, must be of Class II insulation level (i.e. doubly insulated). So-called normal leakage currents (no insulation is perfect) also include the minute capacitive currents of the wiring to earth.

electric-power line voltage limiter earthing conductor

protected electronic equipment

yes protected electronic equipment

no fig. EMC-5: a voltage limiter must be connected to the mass and not to earth. Appendix EMC - 9

3. cabling of equipment and systems (continued)

EMC 3.1 earthing (continued) When a voltage limiter is correctly connected to the mass, the impedance of the earth electrode is immaterial. A direct lightning stroke on the supply line close to the installation requires the dissipation of (typically) 10 kA to 100 kA of stroke current, most of which passes to earth through lightning arresters on the line external to the installation. Overvoltages within an installation where external arresters are provided rarely exceed 6 kV due to atmospheric causes. direct-stroke current lightning protection conductors internal equipments must be maintained at equipotential

fig. EMC-6: the entire equipotential "cage" will be at a high absolute potential during the brief flow of stroke current. For transmission and distribution lines at high voltage, the fault current for one phase to earth returns to the source through the earth and (where provided) through the shielding conductors above the phase conductors of the lines. The provision of an equipotential condition on the surface of the ground at the base of transmission towers and, more importantly, at substations (the source of the fault current) is a primary concern of design engineers. The principle of equipotential bonding is identical to that required for a low-voltage installation in a building. A functional earth means an earth electrode which is designed to pass load current through the earth, i.e. the earth path acts as one of the circuit conductors. There are several installations around the world that utilise this method. In some countries, d.c. is used in this (economic) way to operate a service of facsimile transmission. It may be of interest to note that when telephone cables (which use paper insulation on the conductors) have a high degree of leakage current and consequently doubtful symmetry, a low earth resistance allows the quality of the transmitted signals to be preserved. Although the magnitude of telephone signals is low (millivolts to less than 1 volt), the quality of modern cables overcomes the constraints of a "good" earth. To summarise, the protection of persons does not depend directly on a low value of earth resistance*; it is rather the establishment of a condition of equipotential between masses that is of prime importance. Thus, an aircraft in an electric storm presents no danger to the passengers, who are in a metallic envelope which is (within a few volts) perfectly equipotential. For persons or animals, the danger is not the magnitude of the absolute potential, it is the difference of potential between metal parts which can be touched simultaneously that is dangerous.

10 - Appendix EMC

An electronic equipment is not affected by the value of earth resistance. At worst, there is a risk of exposure to overvoltages from an external cable, if its protection is insufficient, or is badly cabled. So that the role of the masses is essential, and more important than that of the earthing. The only requirement for a satisfactory performance of the electronic equipments is a high degree of equipotentiality. It is evident that two earths are always less equipotential than one. Any separate earth, even if said to be «interference free» is always detrimental to equipotentiality, and so to the safety of persons and to the satisfactory functioning of interconnected equipments. Two non-functional earthelectrode systems on a common site should always be interconnected. In practice, care must be taken that there is no touch-voltage existing when working on an electronic equipment interconnection between two buildings (video, access control, local network, information technology (ADP) devices...) if the earthing systems of the two buildings are not solidly interconnected. It is, as already noted, not possible to be sure of the equipotentiality of two separate earths. * This statement is not true in certain circumstances, notably in rural areas, where a small transformer supplies an isolated community, for example. The neutral earth electrode must necessarily have the lowest possible resistance in such cases. If not, potential gradients on the surface of the ground can be dangerously high in the vicinity of the electrode during an earth fault. Animals are frequently killed due to this cause.

EMC 3.2 masses The majority of malfunctions in electromagnetic devices, sometimes wrongly imputed to software problems or human error, are found to be due to an insufficient level of equipotentiality between interconnected units (probes, cards, actuators). There are two differences between a buried conductor and a mass conductor. A buried conductor will dissipate its common mode currents, but it is always too far from the equipments to be effective at HF. A mass conductor above ground level presents two essential virtues to the good performance of electronic systems, viz: it is physically close to the circuits, and it is accessible. The equipotentiality of equipments and their masses is a functional objective. As long as interference signals circulate in the masses and not in the electronic circuits, they are harmless. On the other hand, if the masses are not all equipotential and are connected to earth in star*, for example, HF interference currents will circulate through any available path, i.e. via signal cables. Some circuits will therefore be subjected to interference and even be destroyed. Networking the mass conductors to form a closely-connected low-impedance bonding system is the only economical way to ensure a satisfactory level of equipotentiality to install all sensitive equipments in a "Faraday cage" arrangement (a room enclosed in a mesh of conductors) would be technically ideal, but is generally not economically justified. By definition, a "mass" is any conducting material which can be touched by any part of a human body, that is not normally alive, but which may become alive as the result of a fault. Two masses which are accessible and within human reach must present a potential difference, under any conceivable fault condition, that does not exceed the IECrecommended conventional voltage (UL)

safety limit (of 50 Vrms in dry locations and 25 Vrms in wet conditions for ac systems). These values are the maximum allowed that can exist indefinitely in specified conditions of external influences. A dangerous touch voltage can arise during a fault if the resistance of the equipotential conductors is not sufficiently low. In some cases, it is necessary to install supplementary equipotential conductors in parallel with existing conductors to satisfy the UL criterion. It should be noted that access to two (or more) masses is illegal, even if they are associated with different installations, if they are connected to different earthing systems which are not interconnected. The respect of safety rules is obligatory but not sufficient, in themselves, to ensure satisfactory EMC of an installation. In fact, the risk of electrocution only exists by a voltage of a high value and relatively long duration appearing between adjacent masses. An electronic equipment is sensitive to an extensive range of frequencies, or to very brief impulses. An electrostatic discharge, for example, is generally of no consequence for its source, but it could be catastrophic for an electronic device. Normal earths, commonly connected in «star» (i.e. radial) configuration for example, guarantee the safety of persons when the relevant standards are respected, but not the satisfactory operation of an installation which includes sensitive electronic components. It is certain that more and more electronic equipments are, or will be, connected to other apparatuses and devices for the exchange of information. The best way of ensuring a successful and durable installation performance is to establish a high degree of equipotentiality throughout the entire installation.

3.2.1. loops of mass and between masses A loop of mass is the area included between a working cable (metering cable, control cable, power-supply cable, local-network system cable) and the mass conductor (generally the nearest PE conductor). There are, therefore, as many loops of mass as there are cables. This is inevitable, whether the conductors are galvanically isolated or not. A galvanic isolation reduces the circulation of LF currents without, however, reducing the area of the loop. A loop can oscillate strongly at HF, so that large-area loops constitute the major problem in EMC.

If a current circulates around a mass loop, such a current in common mode may either superimpose «noise» (interference) on useful signals (in differential mode, by conversion from common mode to differential mode) or disturb the electronic circuits at each extremity. The risk is equally present for radiation from a loop as for reception of interference by the loop. The output stages of electronic circuits are as sensitive to interference as the input stages, and are more difficult to filter. The areas enclosed by mass conductors must not be confused with those referred to above as "mass loops". It is preferable to allow parasitic currents to propagate in the masses rather than in the signal cables. These loops between mass conductors are called "loops between masses".

equipment 1

equipment 2 signal cable mass loop

* i.e. by one of a number of conductors radially connected to the main earth bar, the ensemble resembling a "star".

equipment 1

equipment 2 signal cable

nearest mass conductor

mass conductor

fig. EMC-7: there is an inevitable mass loop per cable.

loop between masse mass conductor

fig. EMC-8. Appendix EMC - 11


EMC 3.2 masses (continued) If two neighbouring masses are not connected together, the differential potential difference between them may be significant. A direct connection from one to the other will always improve the equipotential condition. At least, the masses of all equipments which exchange data between them should be interconnected by mass conductors. An even more certain way of improving the state of equipotentiality is to interconnect all equipment masses, whether they exchange data or not. "Mass loops", also called "ground loops", should never be confused with «loops between masses». A mass loop is never favourable, and its area must be reduced to the minimum attainable, to reduce as far as possible the interference effects of disturbing fields. On the other hand, it is always good practice to increase the number and reduce the areas of loops between masses. 3.2.2 unity of the mass network The mass must be unique to be equipotential. There are three methods of connecting masses which preserve this unity. 1 - Earthing connections in "star": each equipment has its own earthing cable, which terminates with all other individual earthing cables on a unique earthing bar.

The more this policy is developed, the more effective is the resulting state of equipotentiality, both at LF and at HF. Connecting the masses to a mesh of interconnecting mass cables is always beneficial, regardless of the nature of the equipments concerned. equipment 1

equipment 2 greater immunity against radiation fields by reduction of area of the mass loop

greater immunity from conducted interference by multiplication and reduction in area, of loops between masses.

fig. EMC-9. Earthing in "star" can create a common impedance between two interconnected equipments. It is sometimes also assumed that the "star" system of earth connections suppresses the mass loops. Between two interconnected equipments, it is evidently not the case; the area enclosed by the mass loop can, in fact, be considerable. An electromagnetic field due, for example to a lightning discharge, will induce voltage in the mass loop greater than that occurring in any other method of earthing.

earth cables inevitably long

equipment 1

authorized method, but costly and not good for EMC, particularly for interconnected equipments

electromagnetic wave

strong d.p.d. signal cable

equipment 2

large area

earth cables radiating from the main earth bar, figuratively similar to a star

fig. EMC-10. The justification of such a philosophy is oversimple: when an equipment develops a leakage current to mass, the remaining equipments are assumed to remain at earth potential. But "earth" potential has no real meaning in practical electronics, all potentials are relative one to another, the concept of absolute zero potential (i.e. "remote earth") is abstract. It is often assumed that the "star" configuration of earthing overcomes the problem of common impedance. It is, in fact, quite the opposite! Earthing in the "star" configuration increases the common impedance (that is, forms a point of common coupling) between interconnected equipments. high value of differential potential equipment 1 difference equipment 2 (d.p.d.) disturbance on cable I mc

signal cable

PE Z

high impedance if the conductor is long I

fig. EMC-11. 12 - Appendix EMC

PE

fig. EMC-12. This long-established "star" earthing method is now only possible for an equipment which is, and will remain, isolated from any other. The method can be suitable only for electronic analog systems (as opposed to digital systems) with floating sensors, and the electronic circuits completely isolated from any other. Such cases are becoming increasingly rare. With the generalisation of information-data transmissions over great distances, local networks, shared peripherals and, in general, the exchange of signals between equipments, the "star" method of earthing must be abandoned. Moreover, even if the earth connection of each equipment by an individual conductor is not detrimental, it still remains a costly method that requires large amounts of copper and many hours of installation work. The only reasonable application for the "star" arrangement of earthing (in fact, connection to mass) is the connecting cable between an equipment and power-supply socket-outlet, or the nearest distribution board. Thus, in an ADP environment, it is reasonable to use the green-and-yellow PE conductor of the powersupply circuits to connect each equipment to

EMC the general distribution board, located in the room. From the common earth bar of the board, a single protection cable is taken to the main earth electrode for the installation. This conductor can be common to other devices, and may, with advantage, be connected to neighbouring masses. PE conductor for other installations main distribution board

3 - The shortest connection to the nearest mass. This third method of connection to the nearest mass is better than those previously described. It is based on the mesh connection of masses. The areas of mass loops are reduced to the strict minimum and the degree of equipotentiality of the masses is excellent.

sensitive equipments

earth bar main earth terminal local mass earth-electrode system

fig. EMC-13: a good earthing-system arrangement. Even if a strong source of interference is installed in the same environment as sensitive equipment, a separate earthing system for the sensitive equipment is detrimental and not recommended. At the most, it is desirable to supply the two incompatible systems on separate cables from the power-supply network. In any case, mesh-connecting the masses is favourable. Such a mesh of PE conductors has the merit of avoiding involuntary loops which can become catastrophic if not successfully countered. "Star"-system earthing can be accepted only for low-frequency installations that are, and will remain, independent and isolated from any other installation. 2 - Connection to the nearest PE conductor: a unique protection (PE) conductor, associated with several equipments.

a single PE conductor

fig. EMC-14. By using this cabling scheme, the mass loops have a small area and the common impedance between interconnected equipments is less than that with a "star"connected earthing scheme. This economic method is also recommended for safety reasons. It is easy to prove that the touch voltage between two masses connected to the same PE conductor remains less than the conventional (UL) value. The risk of using the same PE conductor for earthing two systems, one "noisy" and the other sensitive, is not negligible. Although the low impedance of the PE conductors and the good level of immunity to LF interference in common mode together limit the risks, the HF currents generated by strongly polluting sources (power converters in particular) cannot be dissipated efficiently by a single PE conductor. In such cases, it is necessary to install additional (supplementary) PE conductors in the form of a meshed network.

structures of adjacent masses (mass grid, conducting false floor, cable trays, ducts, troughs, etc)

fig. EMC-15. Note: concerning the safety of persons, this type of local connection is not generally a substitute for PE conductors. It is appropriate, therefore, to superimpose the methods 2 (or even 1) for the safety of persons, and n° 3 for EMC. Mesh connecting the masses is even more important as the area covered by the installation increases, with long interconnecting cables, or when the equipments are divided between several storeys. The mesh connecting of masses does not reduce the favourable policy of supplying sensitive equipments by different supply cables than those used to supply strongly polluting loads. However, the use of different supply cables does not signify "star" connected masses. The lengths of protective conductors (PE) mean that, at HF, their impedance is generally too high to effectively improve the equipotential situation. For example, a PE conductor 100 metres long is incapable of passing significant currents at frequencies exceeding 100 kHz. The PE conductors and earthing conductors alone are not sufficient to ensure the EMC of an installation. Additional conductors and short inter-mass connections are also necessary. PE cables, even long, and short-circuited at one end by a mass grid, function correctly at LF for the protection of persons. No interconnection between masses and no protective conductor should ever be removed, even if they appear to have become redundant following a careful meshing (close interconnection of equipments and other masses to form a "mesh") of all adjacent masses. A PE conductor is not to be considered as an earthing conductor but as a "bonding" conductor, or "earth-bonding" conductor, the principal function of which is to ensure that UL (i.e. the maximum allowable touch voltage) is never exceeded. Furthermore, there should never be more than one earthing system per installation (i.e. per site) similarly for the mass system which must be unique, and connected to the unique earthing system. If this policy is not adopted, then EMC problems will certainly be experienced via the inevitable links between adjacent installations (entry controls, video, alarms, safety measures, etc.). Appendix EMC - 13


EMC 3.2 masses (continued) In practice, any conductor can be included usefully in the equipotential network of the masses: metallic tubes, pipes, drains, cabletrays and ladders, structural members (girders, beams, reinforcing rods, etc.). Such a mesh of interconnected metal improves considerably the EMC of systems, as well as favouring the measures for safety of persons. The nature of the conductors has little importance in equipotentiality. A steel conductor at HF has approximately the same inductance as a copper conductor of equivalent cross-sectional-area and length. These connections to any and all masses in buildings are authorized and desirable. It is simpler, and ensures the best results, to interconnect all masses of every kind routinely, rather than to limit the interconnections to the masses of the equipments and devices, of the electrical and electronic systems only. In this way a "mesh of masses" or a "mass grid" (both terms are used) is formed. It is rarely useful to install an electrical conductor; it is sufficient to simply interconnect, at as many points as possible, all metallic pipes, tubes, ducts, structural girders, beams, reinforcing rods, etc. It is recommended to connect every large rack, frame, or structure, to the mass grid at intervals of approximately 1 metre. In conclusion, an effective equipotential condition of all masses favours the satisfactory performance of any electronic system, especially for rapid or highly-polluting digital systems. Whether it be for the improvement in immunity from electromagnetic interference, or for the reduction of radiation from the equipments of the installation, the mesh connection of masses affords a simple, relatively inexpensive and efficient solution for frequencies up to several tens of megahertz. If the public power-supply system benefits from 3-phase star-connected operation, it is because the supplies are (and must remain) galvanically isolated each one from the others. It should be understood that factors which are favourable for phase conductors, are not necessarily so for the masses. A HF current cannot flow easily through a phase conductor: it is only possible at low frequencies. To divide the inevitable HF common-mode currents through the multiple paths of numerous conductors of a mass grid is a guarantee of protection for the signal cables. Experience shows that when a system functions correctly in the absence of HF interference, regardless of the cabling philosophy of its masses, the routine meshing of masses has no adverse effect on the correct operation of the system; on the contrary, it often decisively improves the system performance.

14 - Appendix EMC

Except for the installation of a (very costly) "Faraday cage" type of enclosure, the mesh connecting of masses is the only practical way to guarantee a sufficient level of equipotentiality to counter effectively all types of electromagnetic interference. The notion of equipotentiality is more and more localised as the frequency increases. The equipotential condition is only obtained at HF by the free circulation of the commonmode currents in all directions, i.e. by dispersion. Thus, in an industrial environment, it is recommended to connect routinely all conducting structures to neighbouring conducting parts of the building by the shortest possible "bonding" conductors and, where appropriate (e.g. in multi-storey buildings) in three dimensions. This is the most economic way to improve the equipotentiality of an installation at all frequencies, despite some inevitable currents in the masses. Only desk-top equipments in an office environment and not connected to a network have no need be connected to a mass grid. They must, on the other hand, be very carefully screened.

EMC 3.3 attenuation effects The attenuating effect of a conducting structure (mass) is defined by the amplitude of the common-mode interference appearing on a cable installed at a location remote from any masses, with respect to the amplitude of the interference on the same cable due to the same disturbance, but with the cable installed in close proximity (i.e. clamped firmly) to mass, throughout its length.

victim cable 10 V I mass conductor

victim cable I

2V

fig. EMC-16: example of attenuating effect (in this case equal to 5). The attenuating effect is one of the key factors in EMC, being effective and not too costly. In order to exchange signals in good conditions, i.e. in limiting the interference picked up by the signal cables, it is important to reduce common-mode coupling. Any metallic structure, close to, i.e. in contact with, and longitudinally parallel to a signal cable, from one end to the other, can provide two favourable effects: 1 - A more effective meshing of the masses. For d.c. currents, the mesh does not act as an attenuator; its role is to reduce the resistance between masses, not to provide a shielding effect. The galvanic effect of the mesh is independent of the proximity of the signal cables with the mass. 2 - An attenuation (shielding) effect. The effect of proximity adds to that mentioned in above, if the word "impedance" replaces "resistance". It is achieved by connecting equipments, which are interconnected to the mass of conducting structures which are close to the signal cables. The benefit is an efficient shielding which is practically costfree. The attenuation effect being directly attributed to mutual induction, there is no attenuation of d.c. interference, as noted in 1. It should be borne in mind that any cable is potentially an excellent wide-band antenna, especially in the metric range. In order to reduce its radiation ability, a simple, efficient and inexpensive method consists in placing the cable as close as possible to a mass structure throughout its length, i.e. close to a mass cable, metal ducting, structural girder, etc. The attenuation effect produced by a mass conductor close to a signal cable is simply explained, as follows. On the occurrence of an electromagnetic-wave disturbance, a current is induced in the mass conductor. This current generates, according to Lenz’s law, a magnetic field which acts in the opposite sense to the field that produced the current. A signal cable close to the mass conductor will therefore be affected by the difference only of the original field and the reactive field of the mass-conductor current. The resulting field affecting the signal cable is known as the residual field and is evidently of

lower strength than that of the original field. A cable in close proximity to conductive mass from end to end is therefore less exposed to the most severe type of interference, viz: that of common mode. Attenuation effects can be made more effective by arranging the mass, where possible, to envelop the conductors to be protected. In this way, a woven metallic screen, incorporated in signal cables and connected to mass, protects the envelopped conductors above a frequency of 1 MHz with an attenuation factor of at least 300. It is difficult and expensive to shield all the interconnections in a installation, but it is often easy to select cable routes which provide good attenuation. To benefit from the attenuation effect, it is sufficient to fix cables on conductive mass throughout the entire cable length. Such masses must be carefully bonded together electrically and to all nearby structural framework. The quality (i.e. the low impedance) of interconnecting bonds is of primary importance. The most efficient is a direct contact of sheet metal on sheet metal. dB attenuation effect of a perforated steel sheet metal, type "dalle marine" direct contact, sheet metal on sheet metal 40

(same connection at 2 extremities) 20

metal flexible-connection tresses

10

frequency (MHz)

cable 0,1

0,3

1

3

10

30

100

fig. EMC-17. Electrical continuity from one end to the other, and the correct connection to mass at extremities, guarantee an effective attenuation factor. It is recommended to connect cable ways to conductive building structures at intervals along the cable route. The attenuation factor is not reduced by these additional contacts between masses, but the mass mesh is improved. In a single cable tray, in order to limit "cross-talk", power cables or, for example, cables of speed controllers should not be placed beside smallsignal cables. The ideal, in an industrial environment, would be to install three separate cable trays, i.e. one for measurements and similar functions, one for control and indication circuits, and one for power cables. A copper conductor provides an attenuation factor of the order 5 if it is installed throughout the entire length close to the protected signal cable. It is therefore an advantage to associate signal cables with interconnecting earthing cables in a common cable way (for instance, between two buildings). This is still true even if the earths are interconnected elsewhere. It is always possible to add a mass cable adjacent to a particularly sensitive signal cable if necessary. The mass cable is then referred to as a "cable of accompaniment". A buried cable which is passing an a.c. current in common mode creates a magnetic field in the surrounding soil. This (concentric) field gives rise to (Foucault) currents in the earth and the magnetic energy is dissipated in the form of heat. The common-mode Appendix EMC - 15


EMC 3.3 attenuation effects (continued) currents are damped by this effect, which is not exactly the same as that of the attenuation described above, but is rather analogous to the action of a transformer with a resistive load. This damping action is

particularly effective where the interference is due to repetitive trains of transient damped oscillations (i.e. "bursts"). The Foucault currents in the soil increase the degree of damping.

3.4. installation and cabling rules To resolve the majority of EMC problems, it is sufficient to respect (rigorously) a few elementary cabling rules. The first requirement is to decide to which group each cable belongs. The following classes of cable groups cover most practical installations. Group n° 1 - Measuring circuits (low-level analog signals) and supplies to analog probes. This group is sensitive. Group n° 2 - Digital circuits. This group is also sensitive (especially to impulses and bursts). It can also interfere with the circuits of Group n° 1. Group n° 3 - Control and indication circuits, including «all-or-nothing» (AON) relays. This group will interfere with Groups n° 1 and n° 2. Group n° 4 - Power-supply cables. These are power cables from the public distribution network, or from a private generating source (emergency power supply for example). Currents at this level are switched and chopped (by various power-electronics equipments, rectifiers, inverters, and so on...). In normal operation these functions generate HF current and voltage components, in and on the supply conductors. Such currents and voltages constitute a highly-polluted environment for Groups n° 1, n° 2 and n° 3. It is recommended that the cables and wires of each Group have a distinctive and different colour to the other Groups. Rule n° 1 - The "go" and "return" conductors of any circuit must always be placed as closely together as possible. This general rule applies also to powersupply cables. Do not supply in "star" (i.e. radially) two circuits that are not isolated, which exchange signals. It is necessary, even for the signals of AON relays with one common conductor, to "accompany" the active conductors with at least one common conductor per cable or per multi-core cable. For analog or digital signals, the use of two-core cables (or paired conductors) is the basic minimum precaution.

printed circuit

no

_U+

printed circuit

yes

_U+

fig. EMC-18. Rule n° 2 - All internal-circuit interconnecting conductors, cables, etc. should be fixed in close contact with equipotential structures constituting the electrical mass. This measure ensures the benefit of interference attenuation previously described, at practically no cost. Ensure that unused wires, or cables or free cores are not 16 - Appendix EMC

allowed to move unduly in an equipment. Rule n° 3 - It is recommended to use screened cables for noisy and for sensitive circuits. Screening is an effective protection against HF noise, provided that it is connected to mass at least at each end of the cable. It is quite possible to juxtapose two cables of different groups, provided that at least one (but preferably both) cable(s) is (are) screened and connected by a flexible woven metallic tress to mass at each extremity. Screened cables properly installed are immune to "cross-talk". Rule n° 4 - Only conductors of the same Group can be routed together in a cable, or in the same bundle. For flat ribbon-type multi-core cables, the conductors carrying analog signals should separated from those carrying digital data by at least two conductors mass-connected to the reference voltage of each card. For digital conductors, connecting one wire out of two, of a flat ribbon-type cable, to the zero voltage at each end, reduces the HF cross-talk between lines by a factor of 5-10. Moreover, it is detrimental to use one multi-core connecting cable link for different Groups. In practice, spacing the cables by approximately 30 cm is generally sufficient, even in an isolated environment, to reduce the cross-talk to an acceptable level. Crossing two cables from different Groups provides the lowest possible mutual coupling if the two cables cross at 90°. This practice should therefore be carried out routinely. Rule n° 5 - Any free (i.e. unused) conductors of Groups n° 2 or n° 4 should always be connected to the mass of the chassis at both ends. By this means, the attenuation effect can often reach a factor exceeding 2. These connections to mass must be easily removed to free any cores which may be needed at a later date. For Group n° 1 (at very low voltage and frequency) such a connection could be a disadvantage and is not recommended. Noise at the industrial frequency could cause unacceptable interference. Rule n° 6 - The cables of Group n° 4 need not be screened if they are filtered. It is generally necessary to filter power-supply cables at the point of entry of an equipment. On the other hand, it is difficult to filter power cables supplying speed-change controllers, especially when the peak current is high. It then becomes necessary to screen the cables by flexible metallic tresses or by a continuous metal tube connected to the mass at each end. The opposite case is also true: a cable which is well screened does not need filtering. In a common plinth, a screened signal cable has practically no problems of interference from neighbouring power-supply cables.

EMC Rule n° 7 - Noisy equipments should be supplied by separate power cables. This rule will minimize the supply-system noise in differential mode. The rule should not be confused with the practice of radial connections of the masses previously discussed. The neutral conductor should not be connected to the mass, except at a single point. This is the fundamental difference between a neutral conductor and a PE (Protective Earth) conductor*.

that the masses (chassis) of the equipments are all maintained at the same potential: radial network for power supplies, meshconnected network for masses. The connection of the main earth bar at the main distribution board of an installation (see below) to the mass grid should preferably have an inductance of less than 1 microHenry (the lower the better). A single conductor of 50 cm or two parallel conductors (not too close) of 1 m each, etc.

* The TN-C scheme for power installations uses one conductor only for both PE and neutral functions. For obvious reasons, the TN-C scheme is never used where EMC is important.

∆ transformer

∆

∆


L1 L2 L3 N

∆ PE conductor transformer

main earth bar

mass grid (equipotential mesh) noisy sensitive plant equipment to be avoided !

earth electrode

fig. EMC-20. better !

excellent !

fig. EMC-19. Since the equipments receive their power supplies individually and are isolated one from the other, supplying different equipments by separate power lines is a prudent precaution. In any case, it is advantageous

Power supply and connections to masses of an electrical equipment For supplies to an installation, it is an advantage to locate the transformer as closely as possible to the items of load, while taking account of the effect of static magnetic-induction fields.

3.5 EMC components and solutions The EMC of systems requires the use of specific components. The following is an analysis of the conditions of use and the performance of such specific components, viz: electromagnetic screens, filters, overvoltage limiters, and screened cables. 3.5.1 electromagnetic screens An electromagnetic screen serves to isolate two regions of space: one requiring to be shielded from sources of electromagnetic radiations originating in the other. Electromagnetic shielding is always composed of a conducting envelope, which is generally metallic. At low frequencies, the fields E and H are rarely coupled. Shielding against the field E is always effective, even a conductive ink is sufficient. A field H however is very difficult to shield against at LF. It requires the use of materials of very high magnetic permeability µ (soft iron, mumetal) and/or low-resistant metals (copper or aluminium). In any case, magnetic screens have to be thick to be effective. For d.c. systems, protection can be afforded only by using magnetic materials. The screen must be placed as close to the item to be protected as possible, and the thickness of the screen must be augmented, the greater its volume. It is for shielding against HF disturbances that screens are most widely used. All microcomputers, and from now on all video games, are fitted with EMC shielding. The role of the screen is to limit the radiation from the digital circuits to the antennae of the radio receivers in the neighbourhood. A screen being a passive linear bilateral

device, its action is perfectly reciprocal: its efficiency at a given frequency is identical whether protecting its inner space from external radiations or vice-versa. The primary action of an EM screen is that of a mirror by reflecting EM energy back towards its source. It is then, the reflection phenomenon. The part of the EM energy not reflected (no reflector is perfect) is propagated in the screening material, where it is dissipated as heat : this phenomenon is called absorption. If one, or other (or both) of these two features is (are) good, then the screen is fulfilling its purpose. A HF screen must be an efficient conductor (low resistance) but must, above all, present negligible leakage. «Leakage» used in this context refers to the penetration through the screen of radiated EM energy. A leak can be thought of as a small crack in the screen. The higher the frequency of the radiation, the shorter the wave-length, and the smaller the size of cracks which can be tolerated. Contrary to a widely-held belief, the nature of the materials used for shielding is of little importance at HF. The only feature which must be of the highest quality concerns the low resistance and electrical contacts: great care must be taken to avoid oxydation or other types of corrosion. For this reason, contacts are normally either tin- or nickelAppendix EMC - 17


EMC 3.5 EMC components and solutions (continued)

18 - Appendix EMC

plated. An electromagnetic shield need not necessarily be earthed to be effective. For a magnetic field simply its presence is sufficient. For electric fields, it is enough that the screen acts as potential reference for the input and output circuits. It may be concluded that a shield prevents the fields from penetrating the protected space, but also, and especially, prevents parasitic currents from entering. Thus, shields and filters are not rivals, but are complementary, one with the other. If a screen is of excellent quality, with no leakage, it is possible to install input and

output connectors at any convenient point. If, on the contrary, a screen performs badly, with excessive leakage (display, keyboard, printed-circuit board or disk reader) then, it is an advantage to group all the input and output connectors on a common chassis, remote from the leakage, the role of this chassis being that of a reference-potential point. It may be noted that all modern microcomputers have their cables grouped at the rear face, remote from the disk units which are mounted on the front face.

3.5.2 EMC filters An EMC filter is a protection against interference by conduction and is generally made up of a combination of capacitors and inductors. Its role is to allow the passage of energy or signals within the band of useful frequencies and to reject parasitic frequencies. Filters in the power-supply circuits are all lowpass filters which allow power-frequency currents to flow, but which suppress currents of higher frequencies. For an interconnecting coaxial cable, a high-pass filter is antiparasitic: it allows the HF signal to pass, but rejects any LF interference current. The cable can then be connected to mass at both ends without any difficulty. A filter at the input to a radio receiver is a band-pass filter which rejects signals outside the band of frequencies required (as well as any interferences). Finally, a harmonic filter is a notch filter, which is tuned to act as a shortcircuit (generally phase/phase) at a harmonic frequency, usually two or more filters for the first several odd-numbered harmonics above the fundamental frequency, since these invariably have the greatest magnitude. An EMC filter being a linear circuit as long as the inductors remain unsaturated) and passive, is also bilateral. It is equally effective at a given frequency from the interior to the exterior, as in the opposite sense. A filter functions firstly by reflection, i.e. by sending the energy back towards the source, due to a mismatch of the filter/line impedances, then by absorption, i.e. loss of energy in the form of heat, as it passes through the filter. Since inductors are low-loss components at LF, the L-C filters function principally by reflection. The effectiveness of a filter also depends on the upstream and downstream impedances. If these impedances vary, the efficiency of the filter referred to as "insertion losses", will vary. Remark: if a filter can mismatch a line, there is the possibility that it may also match a line. This is a phenemenon which may be observed on LF power-supply line filters: a resonance (even partial) of the filter results in a deterioration, at LF, of the level transmitted, compared to that when no filter is present. It should be verified that the resonant frequency of the filter is not likely to be a problem (it should be below that of the current-chopping frequency, for example). Filters in power-supply circuits usually use inductors in common mode, also called "current-compensation coils" or "compensated inductors".

Such filters evidently present different degrees of effectiveness in common mode than in differential mode. If the inductor of a filter is saturated by the current flowing through it, the effectiveness of the filter is greatly impaired. In order to respect the EMC standards, a filter is practically obligatory in power-supply circuits. Where no filter has been installed, it is often necessary to select one having an efficacity in the order of 30 dB in common mode at 100 MHz. A power-supply filter must, in order to perform efficiently at HF, be installed according to three rules: 1 - Screw the filter sheet-metal to sheet-metal in order to limit its impedance to the mass. 2 - Arrange the supply cable to enter the filter at the opposite face to that of the output circuit, in order to limit common-mode upstream/downstream coupling. 3 - Fix the cables firmly (i.e. clamped) against the sheet-metal of the unit to limit radiation from the upstream conductors affecting the downstream circuit. Preferred practice is to install all filters of an equipment on the same metal base which serves as the potential reference. The notion of equipotentiality at HF is local: each equipment has to provide, by means of its conducting envelope, its own potential reference to the input and output filters and to shielded connection cables. The signal filters are often R-C combinations. A simple resistor of the order of 1 kΩ in series with a sensitive line can suffice to reinforce its immunity. Small inductors in common mode can also be used, with 2, 4, or more, wires wound "two wires in hand". It is interesting to note that these components reduce common-mode interference, without affecting the useful signals transmitted in differential mode.

EMC 3.5.3 overvoltage protection The role of an overvoltage limiter, sometimes referred to as "surge diverter" or "lightning arrester" (depending on its intended location) is to reduce the risk of destruction to components or entire equipments by interferences which may occur at excessive voltage levels. An overvoltage limiter is generally a nonlinear unilateral device: it limits the peak value of voltage at a level which is much lower than that of the incoming surge. This reduced level being, in principle, lower than the rated impulse withstand capability of all downstream plant and equipment. The limiting of the voltage peak, however, does not reduce the HF radiation field strength. Conversely, a low-pass filter does not limit the voltage peak, the duration of which, at half-peak level, considerably exceeds the response time of the filter. Thus, an efficient filter which suppresses frequencies above 10 kHz would have a risetime of about 35 µs. This filter cannot limit an overvoltage due to lightning, the tail-time of which to half-peak is standardized at 50 µs. The first voltage limiters used in telephone systems were gas-filled discharge devices. A gas-filled glass envelope contains two electrodes separated one from the other by a calibrated space. An overvoltage ionizes the gas which allows a discharge to occur between the electrodes, thereby reducing the potential and allowing the gas to de-ionize. Such a component is robust and has only a small parasitic effect. Its occasional failure, often by short-circuiting of the electrodes (i.e. following an overvoltage discharge, the ionized gas sometimes provokes a short-circuit at normal working voltage) means that its reliability cannot be guaranteed. In order to protect a

public service supply line, the low arc-voltage, several tens of volts, requires the installation of a varistance in series, to extinguish the arc when the surge has been dissipated. Analogous components exist at high voltage ("horned spark-gaps" for example). At low voltage, "silicon spark-gaps" such as "Trisil" of Thomson (a triac controlled by a Zener diode in the trigger circuit) are well adapted to the protection of telecommunication lines and circuits. The highly non-linear metal-oxide varistance components are well adapted to the protection of supply-circuits. A disk of zincoxide becomes conductive when the voltage applied to its two faces exceeds a "knee-point" value. That voltage, proportional to the thickness of the disk, varies from some tens of volts to several kilovolts. The energy that a component can tolerate depends on the volume of the disk: from tens of Joules to some tens of thousand Joules. The main drawback of varistances is their degradation during periods of conduction. Zener diodes of very low dynamic resistance have a precise knee-point voltage and a short response time. Their low energy handling capacity, of a fraction of a Joule to several Joules, limits the use of these components to the protection of signal circuits. Failure of such a diode always occurs as a short-circuit, a condition that guarantees "fail-safe" protection for the circuits. In all cases, an overvoltage device in common mode should be connected directly at the mass of the item to be protected, and not, as is still often the case, by a long cable connected radially from a distant earth bar. The response time of an overvoltage limiting device depends on the length of its connections.

Appendix EMC - 19

4. local network problems

EMC Local networks present at least one particular problem: the numerous equipments are spread out, relatively distant one from another, and are installed for user convenience rather than for EMC coordination considerations, are often supplied from different lines, and interconnected by conventional wiring practices.

∆I/∆t = 100 kA/µs

signal cable

∆H ∆t


aera ≈ 300 m2

400 m

insulation stressed by 15 kV


I signal cable control unit

peripheral

field H

supply cable

supply cable

fig. EMC-21: interconnection of equipment forms loops with earthed conductors. Such an installation invariably creates a number of mass loops of very large area. The interconnection of equipments creates large mass loops. One of the most serious dangers for local networks is the magnetic field in the areas of the mass loops, created by the current from a lightning stroke. It may be noted that a surge induced in the interior of a building, on average once a year, gives rise to an overvoltage which can attain, or exceed, 100 volts per square metre of loop area. The meshing of masses should be carried out in the three dimensions (laterally and vertically), especially in multistorey buildings with network equipments on several floors. Two adjacent floors must be meshed together by all conducting metal work which passes through the intervening floor. The multiplication of these conductors affords the following advantages: 1 - Improvement of the "vertical equipotentiality" of the building by effectively reducing the value of loop inductances, and connecting them in parallel. 2 - Improvement of the "horizontal equipotentiality" of the building and the symmetrical flow of surge current directly to earth. 3 - Reduction of induction from the magnetic field in the interior of the building. At a point midway between two parallel conductors passing equal currents in the same direction, the magnetic field strength H is zero. Experience has shown that if the masses are ineffectively meshed, and interconnecting cables are without attenuation effects, some printed-circuit boards can be expected to be destroyed by the induction of even a distant lightning stroke. On the other hand, if the masses are reasonably well interconnected, with conducting cable trays screwed firmly to the equipment metal frames, a lightning stroke (even direct) produces minor interference, and causes no destruction of electronic circuits. In a badly-meshed environment, only equipments completely disconnected or well shielded are out of danger from lightning.

20 - Appendix EMC

fig. EMC-22: lightning produces interference more often by induction than by a direct-stroke current. The current induced in a mass loop by the magnetic field of a lightning stroke has the same form as that of the stroke current; it can exceed 100 A in the case of a large loop. The best solution for limiting the risks of destruction is to bring together, for example in a common tray, the signal cables and the power-supply cables. A shielded cable, with its screen properly connected to mass at each end is free from cross-talk interference. The presence of a mass cable in intimate (e.g. clamped) contact with a cable (signal or power-supply) reduces typically by a factor of 3 or 4 the interference caused by lightning, provided that the mass cable is connected to mass (at least) at each end. A bolted metal cable duct throughout the entire route of a cable has an attenuation factor of the order 30. The woven metal screen of a shielded cable, with short, direct connections to mass at both ends, reduces the induced voltage by a factor of about 100. Local networks processing large amounts of data require that the characteristic impedance of interconnecting signal cables be matched to the input/output impedances of the interconnected equipments, in order to avoid losses by reflections due to mismatched impedances. If one of the two matching units of a long line is disconnected, transmission becomes impossible. A frequent problem on local networks, apart from software parameters, is the loss of availability caused by electromagnetic interference. The software "filters" errors, but the useful output is sometimes severely reduced. The user only notices these problems in the rare cases of permanent interference. The simple observance of the serveral EMC installation rules cited in the foregoing text is sufficient to resolve these problems.

list of "cahiers techniques"

EMC N° CT 114 141 144 145 147 148 149 150 152 154 155 156 158 159 160 161 162 163 166 167 172 173 179

Residual current devices Les perturbations électriques en BT Introduction to dependability design Etude thermique des tableaux électriques BT Initiation aux réseaux de communication numérique High availability electrical power distribution EMC: Electromagnetic Compatibility Development of LV circuit breakers to standard IEC 947-2 Harmonics in industrial networks LV circuit breaker breaking techniques MV public distribution networks throughout the world Sûreté de fonctionnement et tableaux électriques BT Calculation of short-circuit currents Inverters and harmonics (case studies of non-linear loads) Harmonics upstream of rectifiers in UPS Automatic transfering of power supplies in HV and LV networks Les efforts électrodynamiques sur les jeux de barres en BT LV breaking by current limitation Enclosures and degrees of protection Energy-based discrimination for low-voltage protective devices Earthing systems in LV Earthing systems wolrdwide and evolutions Surtensions et parafoudres en BT, Coordination de l'isolement en BT

English X X

French X X X X X

Spanish X X X

X X X

X X X

X

X X X

X X X

X X X

X X

X X X

X X X

X X

X X X

X

X X X

X X X

X X

X X

X X X

X X

Appendix EMC - 21

IEC Electrical-Installation Guide

Recommend Documents