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1. If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To st art viewing messages, select the forum that you want to visit from the selection below. + Reply to Thread Results 1 to 5 of 5 Discuss Theory behind doing an IR test at the Inspection, Testing and Certification of Electrical Installations within the ElectriciansForums; Hi Guys, Can anyone explain how an IR test works for me
please? I understand how to do it and the readings readings I should get ...
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1. 17-09-2010 #1 parm
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Theory behind doing an IR test Hi Guys, Can anyone explain how an IR test works for me please? I understand how to do it and t he readings I should get but I'd like to understand why I'm getting those readings please. Probably a pointless question but would like to be enlightened...
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3. 17-09-2010 #2 Jonesy83
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Re: Theory behind doing an IR test What is "insulation resistance testing" (IR)?
In insulation resistance testing you are trying to answer the question "Is the resistance of the insulation high enough?" You apply a voltage and very carefully measure the current. You then calculate the insulation resistance using Ohm's Law (R = V/I).
Why high voltage test?
You use a hipot test to make sure you have good isolation between the parts of a circuit. Having good isolation helps to guarantee the safety and quality of electrical circuits. Hipot tests are helpful in finding nicked or crushed insulation, stray wire strands or braided shielding, conductive or corrosive contaminants around the conductors, terminal spacing problems, and tolerance errors in IDC cables. All of these conditions might cause a device to fail. lerance errors in IDC cables. All of these conditions might cause a device to fail. Hipot Testing FAQ - Introduction to Hipot Testing o
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4. The Following User Says Thank You to Jonesy83 For This Useful Post: parm (17-09-2010)
5. 17-09-2010 #3 widdler
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Join Date Aug 2009 Location Berkshire, Surrey & West London. Posts 1,681 Thanks 167 Thanked 436 Times in 280 Posts
Re: Theory behind doing an IR test Insulation Resistance Testing - Insulation Resistance Testers and Test Equipment This test is the same as the withstanding voltage test in that it is mandatory to prevent electric shock and fire accidents from using the equipment and that it checks the functionality or performance of the insulator. The withstanding voltage test detects insulation defects by checking whether dielectric breakdown occurs. The insulation resistance test detects insulation defects by measuring the resistance. After absorbing the moisture of the equipment (sometimes this is not done), a specific DC voltage that is 5 to 10 times higher than the normal voltage is applied, and the resistance is measured from the amount of current that flows. If the insulation resistance is sufficient, the equipment meets the requirements for preventing e lectric shock and fire accidents. Why DC voltage is used to perform insulation resistance testing? The insulation resistance test measures the resistive component of the insulator. The capacitive component is ignored. The equipment is only safe if at least a given insulation resistance (a value specified by a standard) is maintained. The insulation resistance test is performed to check this resistance. If the insulation resistance test is performed using an AC voltage, we end up measuring the impedance of the capacitive component and prevents us from obtaining the required insulation resistance. This is the reason why the insulation resistance test is performed using a DC voltage. o
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Earthing limits duration of touch voltages, bonding limits the value of touch voltages
2391-10 anyone?? Reply With Quote
6. The Following User Says Thank You to widdler For This Useful Post: parm (17-09-2010)
7. 17-09-2010 #4 telectrix
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Re: Theory behind doing an IR test good answer from widdler, but, to put it simply, if you, for example, used a multimeter ( on ohms ) across phase/neutral/cpc, it would only show up a dead short due to the low voltage applied. when applying 500V DC any leakage due to t rapped Phase/Earth, neutral/Earth or moisture problem will show up . This will tell you if when upgrading an installation whether or not RCD will trip and also if you have a potential safety/shock problem. o
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9. 17-09-2010 #5 parm
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Re: Theory behind doing an IR test Thanks to you both for this comprehensive explanation, its one thing doing the test and its another knowing why your getting the re ading for the test. Thanks again, you brought back painful memories of A-Level Physics, I knew it would come in useful one day o
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+ Reply to Thread « Testing incomplete | help with testing »
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Why have an insulation testing program? A regular program of testing insulation resistance is strongly recommended to prevent electrical shocks, assure safety of personnel and to reduce or eliminate down time. It helps to detect deterioration of insulation in order to schedule repair work such as: vacuum cleaning, steam cleaning, drying and rewinding. It is also helpful when evaluating the quality of the repairs before the equipment is put back into operation. What causes insulation failure? Some of the more common causes of insulation failure include: excessive heat or cold, moisture, dirt, corrosive vapors, oil, vibration, aging and nicked wiring. What tests are used to detect insulation deterioration? There are numerous maintenance tests for assessing insulation quality. The three tests discussed here are used primarily to test motor, generator and transformer insulation. What equipment is necessary for conducting insulation resistance tests?
Megohmmeter with a timed test function
Temperature indicator
Humidity meter (not necessary if equipment temperature is above the dew point)
Test Currents in Insulation Total current in the body of the insulation is the sum of three components
Capacitance Charging Current
Absorption Current
Leakage or Conduction Current
Spot Reading Test Method For this test, the megohmmeter is connected across the insulation of the windings of the machine being tested. A test voltage is applied for a fixed period of time, usually 60 seconds and a reading is taken. The spot reading test should only be carried out when the winding temperature is above the dew point1. The operator should make a note of the winding temperature, so that it will be possible to correct the reading to a base temperature of 20¡C. Test Duration To obtain comparable results, tests must be of the same duration. Usually the reading is taken after 60 seconds. Interpretation of Results Proper interpretation of spot reading tests requires access to records of results from previous spot reading tests. For conclusive results, only use results from tests performed at the same test voltage for the same amount of time, and under similar temperature and humidity conditions. These readings are used to plot a curve of the history of insulation resistance. A curve showing a downward trend usually indicates a loss of insulation resistance due to unfavorable conditions such as: humidity, dust accumulation, etc. A very sharp
drop indicates an insulation failure. See Figure 1.
Example of the variation of insulation resistance over a period of years: At A, the effect of aging and dust accumulation is shown by decreasing values. At B, the sharp drop indicates an insulation failure. At C, the insulation resistance value after the motor has been rewound. (1) Dew point temperature is the temperature at which the moisture vapor in the air condenses as a liquid.
Time-Resistance Testing Method This method is fairly independent of temperature and often can give you conclusive information without records of past tests. It is based on the absorption effect of good insulation compared to that of moist or contaminated insulation. Simply take successive readings at specific times and note the differences in readings (see curves, Figure 2). Tests by this method are sometimes referred to as absorption tests. Good insulation shows a continual increase in resistance (see curve D) over a period of time (in the order of 5 to 10 minutes). This is caused by the absorption; good insulation shows this charge effect over a time period much longer that the time required to charge the capacitance of the insulation. If the insulation contains moisture or contaminants, the absorption effect is masked by a high leakage current which stays at a fairly constant value Ð keeping the resistance reading low (R = E/I) (see curve E). The time-resistance testing is of value because it is independent of equipment size. The increase in resistance for clean and dry insulation occurs in the same manner whether a motor is large or small.You can compare several motors and establish standards for new ones, regardless of their horsepower ratings. Figure 2 shows how a 60-second test would appear for good and bad insulation. When the insulation is in good shape, the 60-second reading is higher that the 30-second reading. A further advantage of this two reading test is that it gives you a clearer picture, even when a “spot reading” says the insulation looks ok. Time-resistance tests on large rotating electrical machinery - especially with high operating voltage - require high insulation resistance ranges and a very constant test voltage. A heavy-duty megohmmeter serves this
need. Similarly, such an instrument is better adapted for cables, bushings, transformers, and switchgear in the heavier-duty sizes. Test Methods - Time-Resistant Tests Dielectric Absorption Ratio (DAR)
The ratio of 60 seconds/30 seconds
less than 1 = failed
1.0 to 1.25 = OK
1.4 to 1.6 = excellent Note: This is not a commonly used test
Step Voltage Test Method In this test, the operator applies two or more test voltages in steps. The recommended ratio for the test voltage steps is 1 to 5. At each step, test voltage should be applied for the same length of time, usually 60 seconds. The application of increased voltage creates electrical stresses on internal insulation cracks. This can reveal aging and physical damage even in relatively dry and clean insulation which would not have been apparent at lower voltages. Test Duration A series of “steps,” each step lasting 60 seconds. Interpretation of Results Compare the readings taken at different voltage levels, looking for any excessive reduction in insulation resistance values at the higher voltage levels. Insulation that is thoroughly dry, clean, and without physical damage should provide roughly the same resistance values despite changes in test voltage levels. If resistance values decrease substantially when tested at higher voltage levels, this should serve as a warning that insulation quality may be deteriorating due to dirt, moisture, cracking, aging, etc.
The IEEE Std 43-2000 lists the following minimum values for the polarization index for AC and DC rotating machines: Class A: 1.5 Class B: 2.0 Class C: 2.0
Absorption curve of test conducted on 350 HP Motor: Curve D indicates a good insulation with an excellent polarization index of 5. Curve E indicates a potential problem. The polarization index is only 140/95, or 1.47. (2) IEEE Std. 43-2000, “Recommended Practice for Testing Insulation Resistance of Rotating Machinery.” Available from the Institute of Electrical and Electronics Engineers, Inc., 345 E. 47th St., New York, NY 10017.
Before and after repair: Curve F shows a downward trend of insulation resistance values as the test voltage is increased. This indicates a potential problem with the insulation. Curve G shows the same equipment after it has been repaired.
Utilizing the Guard Terminal The guard terminal is useful when measuring very high resistance values.
What test voltage should I use? There are two schools of thought regarding the voltage to test insulation at. The f irst applies to new equipment or cable and can use AC or DC test voltages. When AC voltage is used, the rule of thumb is 2 x nameplate voltage + 1000. When DC voltage is used (most common on megohmmeters manufactured today) the rule of thumb is simply 2 x nameplate voltage except when higher voltages are used. See chart below for suggested values. Equipment/Cable Rating
24 to 50V 50 to 100V 100 to 240V 440 to 550V 2400V 4100V
DC Test Voltage 50 to 100VDC 100 to 250VDC 250 to 500VDC 500 to 1000VDC 1000 to 2500VDC 1000 to 5000VDC It is always advisable to contact the original equipment manufacturer to get their recommendation for the proper voltage to use when testing their equipment.
The basics of insulation resistance Sep 1, 2000 12:00 PM, DeDad, John A. 0 Comments | Related Content ShareThis10 One of the most fundamental of electrical installation and maintenance tasks is taking insulation resistance (JR) readings. This is done to verify the integrity of the insulating material, be it wire and cable insulation or motor/generator winding insulation. Any electrical insulation must have the opposite characteristic as the conductor: It should resist the flow of current, keeping it within the conductor. Using Ohm's Law
To better understand Ohm's Law (E = I x R), let's use an analogy to describe the function of resistance--it's very much like a pipe carrying water. As shown in Fig. 1, water pressure, which is provided by a pump, causes the water to flow through the pipe. There is some resistance to this water flow in the form of friction along the interior pipe wall. If the pipe springs a leak, the water pressure goes down. [Figure 1 ILLUSTRATION OMITTED] Looking at the analogy from an "electricity" point of view, voltage is the "electrical pressure" that causes current to flow along the conductor. (See Fig. 2.) There also is a resistance to flow here, but it's much less through the conductor than through the insulation. Obviously, the higher the voltage, the more current we'll have. And, the lower the conductor resistance, the more current we'll have for the same voltage. This basically is what Ohm's Law expresses. [Figure 2 ILLUSTRATION OMITTED] We all know that no insulation is perfect (having infinite resistance). As such, there's some electricity flowing along the insulation or through it to ground. This current is called leakage current. It may be only a millionth of an ampere (one micro-amp), but it's current nonetheless. And don't forget that a higher voltage will cause a higher amount of leakage current. Leakage current does not harm good insulation, but it becomes a real problem with deteriorated insulation. So, how do we determine what's "good" insulation? Based on our discussion here, it would seem that insulation with a relatively high resistance to current would qualify. We also could say "good" insulation has the capability of keeping a high resistance. That said, you would need a way to measure this resistance to make such a determination. This is the basis for IR testing. By taking measurements at regular intervals, you can do a trend analysis on the integrity of any insulation. Measuring IR To measure IR, you would use an IR tester, which is a portable instrument that's essentially a resistance meter, or ohmmeter, with a built-in, hand-cranked or line-operated DC generator that develops a high DC voltage. This voltage (usually 500V or more) causes a small current to flow through and over the insulation's surfaces. The tester provides a direct reading of IR in ohms or megohms. So, you use an IR tester and obtain measurements. What do they mean? Based on our prior theoretical discussion, a high resistance value would indicate a "good" insulation while a relatively low resistance value would point to a "poor" insulation. In the real world, however, actual resistance values can be higher or lower due to the effects of factors such as temperature, humidity, the moisture content of the insulation, even the person doing the testing. And, IR readings can be very different for the same motor tested on three different days.
What really matters is the trend of the readings over a period of time. A continued lessening of IR readings through a specific interval should be interpreted as a warning of pending problems. As such, you can get a very good sense of the condition of an insulation through good record keeping and common sense. General guidelines One important note to remember: Each of these periodic tests should be made, as much as possible, the same way. In other words, you should use the same test connections at the same applied test voltage for the same length of time. If at all possible, try to do the testing at the same temperature, or correct the measurements to the same temperature. A helpful hint is to record the relative humidity near the tested equipment at the time of each test; this will help you evaluate the readings. IR test set manufacturers have helpful temperature correction and humidity information. Based on your observations of the test data, you can make some intelligent decisions. The table below provides some useful guidelines.
An ammeter is a measuring instrument used to measure the electric current in a circuit. Electric currents are measured in amperes (A), hence the name. Instruments used to measure smaller currents, in the milliampere or microampere range, are designated as milliammeters or microammeters. Early ammeters were laboratory instruments which relied on the Earth's magnetic field for operation. By the late 19th century, improved instruments were designed which could be mounted in any position and allowed accurate measurements in electric power systems.
Contents [hide]
1 History 2 Types 3 Picoammeter 4 Application 5 See also 6 References
[edit ] History The relation between electric current, magnetic fields and physical forces was first noted by Hans Christian Ørsted who, in 1820, observed a compass needle was deflected from pointing North when a current flowed in an adjacent wire. The tangent galvanometer was used to measure currents using this effect, where the restoring force returning the pointer to the zero position was provided by the Earth's magnetic field. This made these instruments usable only when aligned with the Earth's field. Sensitivity of the instrument was increased by using additional turns of wire to multiply the effect – the instruments were called "multipliers" .[1]
[edit ] Types The D'Arsonval galvanometer is a moving coil ammeter. It uses magnetic deflection, where current passing through a coil causes the coil to move in a magnetic field. The modern form of this instrument was developed by Edward Weston (NOT the American photographer!), and uses two spiral springs to provide the restoring force. By maintaining a uniform air gap between the iron core of the instrument and the poles of its permanent magnet, the instrument has good linearity and accuracy. Basic meter movements can have full-scale deflection for currents from about 25 microamperes to 10 milliamperes and have linear scales .[2] Moving iron ammeters use a piece of iron which moves when acted upon by the electromagnetic
force of a fixed coil of wire. This type of meter responds to both direct and alternating currents (as opposed to the moving coil ammeter, which works on direct current only). The iron element consists of a moving vane attached to a pointer, and a fixed vane, surrounded by a coil. As alternating or direct current flows through the coil and induces a magnetic field in both vanes,
the vanes repel each other and the moving vane deflects against the restoring force provided by fine helical springs.[2] The non-linear scale of these meters makes them unpopular. An electrodynamic movement uses an electromagnet instead of the permanent magnet of the d'Arsonval movement. This instrument can respond to both alternating and direct current .[2] In a hot-wire ammeter, a current passes through a wire which expands as it heats. Although these instruments have slow response time and low accuracy, they were sometimes used in measuring radio-frequency current .[2] Digital ammeter designs use an analog to digital converter (ADC) to measure the voltage across
the shunt resistor; the digital display is calibrated to read the current through the shunt. There is also a whole range of devices referred to as integrating ammeters.[3][4] In these ammeters the amount of current is summed over time giving as a result the product of current and time, which is proportional to the energy transferred with that current. These can be used for energy meters (watt-hour meters) or for estimating the charge of battery or capacitor.
[edit ] Picoammeter A picoammeter, or pico ammeter, measures very low electrical current, usually from the picoampere range at the lower end to the milliampere range at the upper end. Picoammeters are used for sensitive measurements where the current being measured is below the theoretical limits of sensitivity of other devices, such as Multimeters. Most picoammeters use a "virtual short" technique and have several different measurement ranges that must be switched between to cover multiple decades of measurement. Other modern picoammeters use log compression and a "current sink" method that eliminates range switching and associated voltage spikes.[5]
[edit ] Application The majority of ammeters are either connected in series with the circuit carrying the current to be measured (for small fractional amperes), or have their shunt resistors connected similarly in series. In either case, the current passes through the meter or (mostly) through its shunt. They must not be connected to a source of voltage; they are designed for minimal burden, which refers to the voltage drop across the ammeter, which is typically a small fraction of a volt. They are almost a short circuit. Ordinary Weston-type meter movements can measure only milliamperes at most, because the springs and practical coils can carry only limited currents. To measure larger currents, a resistor called a shunt is placed in parallel with the meter. The resistances of shunts is in the integer to fractional milliohm range. Nearly all of the current flows through the shunt, and only a small fraction flows through the meter. This allows the meter to measure large currents. Traditionally, the meter used with a shunt has a full-scale deflection (FSD) of 50 mV, so shunts are typically designed to produce a voltage drop of 50 mV when carrying their full rated current.
Zero-center ammeters are used for applications requiring current to be measured with both polarities, common in scientific and industrial equipment. Zero-center ammeters are also commonly placed in series with a battery. In this application, the charging of the battery deflects the needle to one side of the scale (commonly, the right side) and the discharging of the battery deflects the needle to the other side. A special type of zero-center ammeter for testing high currents in cars and trucks has a pivoted bar magnet that moves the pointer, and a fixed bar magnet to keep the pointer centered with no current. The magnetic field around the wire carrying current to be measured deflects the moving magnet. Since the ammeter shunt has a very low resistance, mistakenly wiring the ammeter in parallel with a voltage source will cause a short circuit, at best blowing a fuse, possibly damaging the instrument and wiring, and exposing an observer to injury. In AC circuits, a current transformer converts the magnetic field around a conductor into a small AC current, typically either 1 A or 5 A at full rated current, that can be easily read by a meter. In a similar way, accurate AC/DC non-contact ammeters have been constructed using Hall effect magnetic field sensors. A portable hand-held clamp-on ammeter is a common tool for maintenance of industrial and commercial electrical equipment, which is temporarily clipped over a wire to measure current. Some recent types have a parallel pair of magnetically-soft probes that are placed on either side of the conductor.
[edit ] See also Ammeter is a measuring instrument used to measure the electric current in a circuit. Electric
currents are measured in amperes (A). Instruments used to measure smaller currents, in the milliampere or microampere range, are designated as milliammeters or microammeters. A picoammeter, or pico ammeter measures very low electrical current, usually picoamperes. Picoammeters are used for sensitive measurements where the current being measured is below the theoretical limits of sensitivity of other devices, such as Multimeters. Various ammeter electric circuit designs are moving coil ammeter, moving iron ammeter, electrodynamic ammeter, hot-wire ammeter, and Digital ammeters.
Ammeter Design Types
Moving Coil Ammeters uses magnetic deflection, where current passing through a coil causes the coil to move in a magnetic field, and forces the current reading pointer on the scale. By maintaining a uniform air gap between the iron core of t he instrument and the poles of its permanent magnet, the instrument has good linearity and accuracy. Moving Iron Ammeters use a piece of iron which moves when acted upon by the electromagnetic force of a fixed coil of wire. This type of meter responds to both direct and alternating currents (as opposed to the moving coil ammeter, which works on direct current only). But the non-linear scale of these meters makes t hem unpopular. Electrodynamic Movement Ammeters use an electromagnet instead of the permanent magnet of the d’Arsonval movement. This instrument can respond to both alternating and dire ct current.
Hot-wire Ammeter – In a hot-wire ammeter, a current passes through a wire which expands as it heats. Although these instruments have slow response time and low accuracy, they were sometimes used in measuring radio-frequency curre nt. Digital Ammeter designs use an analog to digital converter (ADC) to measure t he voltage across the shunt resistor, the digital display is calibrated to read the current through the shunt resistor.
Posted by Karthikeya Test & Measurement Tools Keywords: ammeter, picoammeter, microammeter, ammeter circuits, multimeter, current signal, Current Measurement, AC current, DC current, microamperes, picoamperes, Digital ammeters, moving coil ammeter, moving iron ammeter, electrodynamic ammeter, hot-wire ammeter, High voltage ohmmeters Most ohmmeters of the design shown in the previous section utilize a battery of relatively low voltage, usually nine volts or less. This is perfectly adequate for measuring resistances under several mega- ohms (MΩ), but when extremely high resistances need to be measured, a 9 volt battery is insufficient for generating enough current to actuate an electromechanical meter movement. Also, as discussed in an earlier chapter, resistance is not always a stable (linear) quantity. This is especially true of non-metals. Recall the graph of current over voltage for a small air gap (less than an inch):
While this is an extreme example of nonlinear conduction, other substances exhibit similar insulating/conducting properties when exposed to high voltages. Obviously, an ohmmeter using a low-voltage battery as a source of power cannot measure resistance at the ionization potential of a gas, or at the breakdown voltage of an insulator. If such resistance values need to be measured, nothing but a high-voltage ohmmeter will suffice. The most direct method of high-voltage resistance measurement involves simply substituting a higher voltage battery in the same basic design of ohmmeter investigated earlier:
Knowing, however, that the resistance of some materials tends to change with applied voltage, it would be advantageous to be able to adjust the voltage of this ohmmeter to obtain resistance measurements under different conditions:
Unfortunately, this would create a calibration problem for the meter. If the meter movement deflects full-scale with a certain amount of current through it, the full-scale range of the meter in ohms would change as the source voltage changed. Imagine connecting a stable resistance across the test leads of this ohmmeter while varying the source voltage: as the voltage is increased, there will be more current through the meter movement, hence a greater amount of deflection. What we really need is a meter movement that will produce a consistent, stable deflection for any stable resistance value measured, regardless of the applied voltage. Accomplishing this design goal requires a special meter movement, one that is peculiar to megohmmeters, or meggers, as these instruments are known.
The numbered, rectangular blocks in the above illustration are cross-sectional representations of wire coils. These three coils all move with the needle mechanism. There is no spring mechanism
to return the needle to a set position. When the movement is unpowered, the needle will randomly "float." The coils are electrically connected like this:
With infinite resistance between the test leads (open circuit), there will be no current through coil 1, only through coils 2 and 3. When energized, these coils try to center themselves in the gap between the two magnet poles, driving the needle fully to the right of the scale where it points to "infinity."
Any current through coil 1 (through a measured resistance connected between the test leads) tends to drive the needle to the left of scale, back to zero. The internal resistor values of the meter movement are calibrated so that when the test leads are shorted together, the needle deflects exactly to the 0 Ω position. Because any variations in battery voltage will affect the torque generated by both sets of coils (coils 2 and 3, which drive the needle to the right, and coil 1, which drives the needle to the left), those variations will have no effect of the calibration of the movement. In other words, the accuracy of this ohmmeter movement is unaffected by battery voltage: a given amount of measured resistance will produce a certain needle deflection, no matter how much or little battery voltage is present. The only effect that a variation in voltage will have on meter indication is the degree to which the measured resistance changes with applied voltage. So, if we were to use a megger to measure the resistance of a gas-discharge lamp, it would read very high resistance (needle to the far right of the scale) for low voltages and low resistance (needle moves to the left of the scale) for high voltages. This is precisely what we expect from a good high-voltage ohmmeter: to provide accurate indication of subject resistance under different circumstances. For maximum safety, most meggers are equipped with hand-crank generators for producing the high DC voltage (up to 1000 volts). If the operator of the meter receives a shock from the high voltage, the condition will be self-correcting, as he or she will naturally stop cranking the generator! Sometimes a "slip clutch" is used to stabilize generator speed under different cranking
conditions, so as to provide a fairly stable voltage whether it is cranked fast or slow. Multiple voltage output levels from the generator are available by the setting of a selector switch. A simple hand-crank megger is shown in this photograph:
Some meggers are battery-powered to provide greater precision in output voltage. For safety reasons these meggers are activated by a momentary-contact pushbutton switch, so the switch cannot be left in the "on" position and pose a significant shock hazard to the meter operator. Real meggers are equipped with three connection terminals, labeled Line, Earth, and Guard . The schematic is quite similar to the simplified version shown earlier:
Resistance is measured between the Line and Earth terminals, where current will travel through coil 1. The "Guard" terminal is provided for special testing situations where one resistance must be isolated from another. Take for instance this scenario where the insulation resistance is to be tested in a two-wire cable:
To measure insulation resistance from a conductor to the outside of the cable, we need to connect the "Line" lead of the megger to one of the conductors and connect the "Earth" lead of the megger to a wire wrapped around the sheath of the cable:
In this configuration the megger should read the resistance between one conductor and the outside sheath. Or will it? If we draw a schematic diagram showing all insulation resistances as resistor symbols, what we have looks like this:
Rather than just measure the resistance of the second conductor to the sheath (R c2-s), what we'll actually measure is that resistance in parallel with the series combination of conductor-toconductor resistance (R c1-c2) and the first conductor to the sheath (R c1-s). If we don't care about this fact, we can proceed with the test as configured. If we desire to measure only the resistance between the second conductor and the sheath (R c2-s), then we need to use the megger's "Guard" terminal:
Now the circuit schematic looks like this:
Connecting the "Guard" terminal to the first conductor places the two conductors at almost equal potential. With little or no voltage between them, the insulation resistance is nearly infinite, and thus there will be no current between the two conductors. Consequently, the megger's resistance indication will be based exclusively on the current through the second conductor's insulation, through the cable sheath, and to the wire wrapped around, not the current leaking through the first conductor's insulation. Meggers are field instruments: that is, they are designed to be portable and operated by a technician on the job site with as much ease as a regular ohmmeter. They are very useful for checking high-resistance "short" failures between wires caused by wet or degraded insulation. Because they utilize such high voltages, they are not as affected by stray voltages (voltages less than 1 volt produced by electrochemical reactions between conductors, or "induced" by neighboring magnetic fields) as ordinary ohmmeters. For a more thorough test of wire insulation, another high-voltage ohmmeter commonly called a hi-pot tester is used. These specialized instruments produce voltages in excess of 1 kV, and may be used for testing the insulating effectiveness of oil, ceramic insulators, and even the integrity of other high-voltage instruments. Because they are capable of producing such high voltages, they must be operated with the utmost care, and only by trained personnel. It should be noted that hi-pot testers and even meggers (in certain conditions) are capable of damaging wire insulation if incorrectly used. Once an insulating material has been subjected to breakdown by the application of an excessive voltage, its ability to electrically insulate will be compromised. Again, these instruments are to be used only by trained personnel.
