Electrical
Insulation resistance (megger) testing field guide for electrical crews
Read the insulation before it faults: match the DC test voltage, correct the reading to 20 degrees C, trend it against the baseline, and discharge the cable when you are done.
Direct answer
Insulation resistance testing, or megger testing, applies a DC test voltage to a conductor or winding and reads the leakage current back as a resistance in megohms, so degraded, wet, or damaged insulation shows up before it faults. The test voltage matches the equipment class, and the reading is corrected to 20 degrees C before it means anything.
Key takeaways
- Match the DC test voltage to roughly twice rated voltage: 500 to 1000 V DC for 600 V-class gear, 2500 V for 5 kV, 5000 V for 15 kV.
- Insulation resistance roughly halves per 10 degrees C rise, so correct every reading to a 20 degrees C reference before comparing.
- Polarization index is the 10-minute reading divided by the 1-minute reading; above about 2 is healthy, near 1 signals moisture or contamination.
- Always discharge and ground a cable or winding after a megger test; stored capacitance charge can hold a lethal voltage after the tester is off.
- Disconnect surge devices, drives, and electronics before testing, since the DC voltage damages them and skews the reading; governing standards are NETA, IEEE 43, and IEEE 400.
Insulation resistance testing, and what the megohms tell you
Insulation resistance testing puts a DC voltage across insulation and measures how much current leaks through it, reported as a resistance in megohms. Good insulation passes almost no current, so the resistance reads high. Insulation that is wet, dirty, cracked, or aged passes more current, and the resistance reads low. The instrument that does it is a megohmmeter, which most of the trade still calls a megger after the brand that made it common.
The test does one job and does it well. It grades the health of the insulation that separates a conductor from ground and from the next phase, before that insulation fails in service. A failure there is a ground fault, an arc, a tripped feeder, or a dead motor, and the point of the test is to find the weak insulation while it is still just a number on a meter.
What it does not do is tell you the insulation will survive a specific overvoltage. That is a hipot or withstand test, a different and destructive animal. The megohmmeter measures condition. It is the routine health check, run at acceptance and run again on a schedule, not the proof test that stresses insulation to its rating.
Why insulation goes bad, and why you test it
Insulation does not last forever, and it rarely fails all at once. Heat, age, moisture, contamination, vibration, and physical damage chip away at it over years. A motor that runs hot ages its winding insulation faster. A cable nicked during the pull has a weak spot that sits fine until water finds it. Switchgear in a damp vault collects film and dust that turns into a leakage path across the surface.
Every one of those is invisible until it faults. The insulation looks the same on the outside the day before it lets go. The test is how you see the degradation early, as a falling megohm reading, instead of late, as a fault that drops a feeder or kills a motor.
There are two times you run it for two reasons. At acceptance, on new gear, you confirm nothing got damaged in shipping and installation, and you set the baseline. In maintenance, you watch the trend so the weak insulation gets caught at a planned outage, not at 2 a.m. The infrared survey catches the hot connection running under load; the insulation test catches the failing dielectric. They look at different failure modes, and a real maintenance program runs both.
The megohmmeter, the tool people call a megger
A megohmmeter is a meter with a high-voltage DC source built in. It applies a steady test voltage, commonly switchable between 250, 500, 1000, 2500, and 5000 V DC, and reads the resulting current back as a resistance on a megohm or gigohm scale. The old hand-crank analog units are still around and still work; the swing of the needle told you as much as the number, because a drifting needle showed the absorption current settling. Digital testers now do the same with a bar graph and log the readings.
The better field units run the timed tests for you. They hold the voltage and capture the 30-second, 1-minute, and 10-minute readings automatically for DAR and PI, apply a temperature correction, and store the result against an asset ID. That last part matters more than the brand, because the value of insulation testing is in the trend, and a trend needs records that do not live on a scrap of paper.
Match the tester to the job. A 500 V unit covers most 600 V-class work. Medium-voltage cable and large machines need a 2500 or 5000 V instrument. A tester rated below the voltage you need to apply is the wrong tool, and one rated well above it can over-stress what you are testing.
What test voltage do you use?
Match the DC test voltage to the equipment's rated voltage. The common practice is to test at roughly twice the rated voltage, which lands most 600 V-class gear at 500 or 1000 V DC and medium-voltage cable at 2500 or 5000 V DC. The aim is to stress the insulation enough to reveal a weakness without over-stressing healthy insulation and damaging it.
Going too high over-stresses good insulation and can puncture insulation that was marginal but still serviceable, which means the test caused the failure. Going too low under-stresses it, and a real weakness can hide. For motors and generators, IEEE 43 gives the test-voltage guidance by machine rating. For cable, IEEE 400 and the NETA tables do. When the manufacturer specifies a test voltage for a piece of equipment, that number wins over the rule of thumb, and the project spec wins over both.
| Equipment rating | Common DC test voltage |
|---|---|
| Control and signal wiring, 300 V class | 500 V DC |
| 600 V class: motors, panels, feeders below 1 kV | 500 to 1000 V DC |
| 5 kV class cable and gear | 2500 V DC |
| 15 kV class cable and gear | 5000 V DC |
| Specific equipment | Per manufacturer and the governing standard |
How the megohm reading is made
The DC test voltage drives current through the insulation, and the meter divides the voltage by that current to display a resistance: R equals V over I, in megohms. Drive 500 V through insulation that passes one microamp and you read 500 megohms. The cleaner and drier the insulation, the less it leaks, and the higher the number climbs.
The current is not one current but three, and understanding that is what makes the timed tests make sense. There is the capacitive charging current, large at first and gone in seconds as the geometry charges like a capacitor. There is the absorption current, which decays slowly over minutes as the dielectric polarizes. And there is the leakage current, the small steady current that actually flows through and across the insulation. That last one is the current that tells you the condition.
On dry, healthy insulation the reading starts low while the capacitance charges, then climbs steadily for minutes as absorption tails off, settling at a high value. On wet or contaminated insulation the leakage current is large and steady from the start, so the reading is low and flat. The shape of the curve carries as much information as the final number, which is exactly what PI and DAR put into a ratio.
The spot reading
The spot reading is the simplest version. Apply the test voltage, wait 60 seconds for the capacitance to charge and the reading to settle, and record the megohms. It is fast, it is the one most people run, and it answers a blunt question. Is the insulation above the minimum, or is it not.
The 60-second wait is the part that gets skipped. Read too early and you catch the charging current still settling, so the number is artificially low and you fail good insulation. On a small motor or a short cable the reading settles fast. On a long cable or a large winding with real capacitance it takes the full minute or longer.
The spot reading has a real limit. It is a single number with no context. A megohm value that looks fine in isolation can be a value that has fallen by half since last year, which is the actual warning. That is why the spot reading is the floor of insulation testing, not the ceiling, and why the timed ratios and the trend exist.
Why you correct the reading to 20 degrees C
Insulation resistance changes hard with temperature, and if you do not correct for it you will chase ghosts. The rule of thumb is that the resistance roughly halves for every 10 degrees C the insulation warms, and roughly doubles for every 10 degrees C it cools. A winding read at 40 degrees C can show a quarter of the megohms it would show at 20 degrees C, on exactly the same healthy insulation.
Because of that, readings are corrected to a standard reference temperature, commonly 20 degrees C, before they are compared to each other or to a minimum. Correct to 20 degrees C and a reading taken on a hot motor in August compares honestly to one taken on a cold motor in February. Skip the correction and a normal seasonal swing looks like the insulation is failing, or worse, a real decline hides because you tested warm one year and cold the next.
Record the insulation temperature with every reading, not the room temperature. The winding or the cable can sit well above ambient after running. The correction factor comes from the standard and the insulation type. The timed ratios like PI are largely self-correcting, because the temperature barely moves during the test, which is part of why they are trusted over a single corrected number.
What is a good megger reading?
A good reading is high and stable, above the minimum for the equipment, and in line with or better than its own history. The old rule of thumb is one megohm per kV of rating plus one megohm, temperature-corrected, which sets a floor of a few megohms for low-voltage gear. It is a floor, not a target. Healthy modern insulation usually reads far above it, in the hundreds or thousands of megohms.
For motors and generators, IEEE 43 sets the minimum more carefully by machine type and age. Form-wound stators built in recent decades are commonly held to a minimum near 100 megohms corrected to 40 degrees C, while older machines and random-wound windings use lower floors. The exact minimum depends on the standard, the rating, and the construction, so pull the number from IEEE 43 or the project spec rather than carrying one figure for everything.
What matters more than clearing the floor is the trend. A motor reading 200 megohms is fine on its own and alarming if it read 2000 megohms last year. The absolute minimum catches gross failures. The trend catches the slow death, and the slow death is most of what insulation testing is for.
| Reference | Minimum (rule of thumb) | Note |
|---|---|---|
| General rule of thumb | 1 megohm per kV + 1 megohm | A floor, temperature-corrected |
| Motor or generator (IEEE 43) | By machine type and age | Modern form-wound often near 100 megohm at 40 C |
| Cable (IEEE 400 / NETA) | Compare to baseline and like cables | Trend matters more than the absolute |
What is polarization index (PI)?
Polarization index is the ratio of the 10-minute insulation reading to the 1-minute reading, taken at one steady test voltage without interruption. PI equals the megohms at 10 minutes divided by the megohms at 1 minute. It grades the insulation by how it behaves over time rather than by a single number, which is why it is the working diagnostic for motor and generator windings and other large machines.
Healthy, dry insulation keeps absorbing charge over the full ten minutes, so the reading climbs and the ratio comes out high, commonly above 2. Wet, dirty, or aged insulation leaks a large steady current that swamps the absorption, so the reading barely moves and the ratio sits near 1. A PI around 1 is the classic signature of moisture or contamination even when the 1-minute spot value looks acceptable, which is the case PI is built to catch.
IEEE 43 gives the accepted minimums, commonly near 2 for most insulation classes and lower for some older classes. There is a known caveat: on very high readings, where the 1-minute value is already in the thousands of megohms, the ratio loses meaning and a low PI does not indicate a problem. Read PI alongside the spot value, not instead of it.
Dielectric absorption ratio (DAR)
DAR is the quick version of the same idea: the ratio of the 60-second reading to the 30-second reading at one steady voltage. DAR equals the megohms at 60 seconds divided by the megohms at 30 seconds. It captures the early shape of the absorption curve in one minute instead of ten, which is why it gets used on smaller equipment and when the schedule will not allow the full PI.
The interpretation runs the same direction as PI. A DAR above roughly 1.6 commonly indicates good insulation, the 1.25 to 1.6 range is marginal, and below about 1.25 is questionable, with values near or below 1 a sign of wet or contaminated insulation. The exact thresholds vary with the standard and the equipment, so treat them as guidance and confirm against IEEE 43 or the manufacturer.
DAR is faster and PI is more telling. On a large machine where the absorption current runs for the full ten minutes, PI sees more of the curve and is the better test. On smaller windings and field troubleshooting, DAR gives a usable read in a quarter of the time.
Step-voltage and ramp testing
A step-voltage test applies the voltage in stages, holding at each step and recording the reading, instead of reading once at a single voltage. The idea is that healthy insulation gives roughly the same megohm reading regardless of the applied voltage, while weak or moisture-damaged insulation shows a clear drop in resistance as the voltage climbs. A reading that holds at 500 V but falls off at 2500 V is telling you something the single-voltage spot test would miss.
The ramp test is the continuous version, raising the voltage smoothly and watching for the point where the current jumps and the resistance dips. It gets used on cable and on large machines where finding the voltage-dependent weakness matters, and it stays below the levels of a destructive hipot.
Both are step-ups from the basic spot test for cases where you suspect a weakness that only shows under stress. They are not the everyday test. Most insulation work is a spot reading, a PI or DAR, and a trend, with step-voltage reserved for the suspect equipment that earns the extra time.
Trending against a baseline
The single most useful thing you can do with insulation readings is plot them over time. One reading is a snapshot. A series of readings, all temperature-corrected to the same reference and taken at the same test voltage, is a trend, and the trend is where the warning lives. Insulation that drops steadily year over year is failing slowly, and the slope tells you roughly when it will reach the floor.
The baseline is the acceptance reading on new or rebuilt gear. Everything after is compared to it. A reading that has fallen to half its baseline is a flag long before it ever reaches the rule-of-thumb minimum, because the equipment was never supposed to read that low. The absolute minimum catches the equipment that is already in trouble. The trend catches the equipment that is heading there.
For the trend to work, the readings have to be comparable: same test voltage, same correction temperature, same connection, same asset. That is a recordkeeping problem, and it is the one that most often breaks down, because the readings end up in different notebooks or never get the temperature logged. A field tool like Tradeos that stamps each reading with its voltage, temperature, and asset keeps the trend honest, which is the whole reason for testing on a schedule.
Testing feeder and branch cable
On a cable you test each conductor to ground and, where it matters, each conductor to the others, with the far end isolated and floating. The pattern is conductor-to-ground for each phase, then phase-to-phase, so a fault between conductors does not hide behind a good-to-ground reading. Both ends come loose from anything that would give the current another path or that the DC voltage could damage.
Cable has real capacitance, and the longer the run the more it holds. That changes the test two ways. The reading takes longer to settle because the capacitance has to charge, so give it the full minute or more. And the cable stores a serious charge after the test, more on a long run, which is the safety issue covered below. A long medium-voltage feeder can hold a charge that will hurt you well after the tester is off.
On data center and critical-power work, cable insulation testing is part of the acceptance package and the periodic program, because a feeder fault in those rooms is measured in lost uptime, not just a tripped breaker. The grounding electrode system those rooms also depend on is a separate test with its own fall-of-potential and soil-resistivity methods, covered in the grounding guide.
Testing motor and generator windings
On a motor you test the winding to ground, reading from the motor leads to the frame with the machine de-energized and isolated. The winding-to-ground insulation is what fails when a motor floods, sits in a damp space, or ages out, and it is what the test grades. On a low-voltage motor a 500 or 1000 V spot reading plus a PI or DAR covers it. On a large or medium-voltage machine, IEEE 43 drives the test voltage, the minimum, and the PI acceptance.
Moisture is the usual culprit on a motor that reads low. A machine that sat idle in a humid space pulls moisture into the winding, and the reading climbs back up after the winding is dried out with space heaters or a controlled bake. A reading that will not recover after drying is degradation, not just damp, and that is a different decision and usually a rewind.
The rookie mistake on motors is worth stating plainly. People read the winding to ground, get a good number, and walk away without discharging it. A large winding holds a charge after a DC test the same way a cable does. Ground the leads before you handle them.
Transformers, switchgear, and bus
On a dry-type transformer you test winding to winding and each winding to ground. On switchgear and bus you test phase to ground and phase to phase with the bus isolated. The principle is identical to cable and motor work; only the connections change. NETA gives the acceptance and maintenance procedures and the comparison values for this gear.
The thing that bites people on switchgear and panels is everything that is still connected. Voltage transformers, control power transformers, surge arresters, meters, and electronics sit on the bus and will either give the test current a false path or be damaged by the DC voltage. They get disconnected or isolated before the test, which is most of the labor and the part that gets rushed.
Liquid-filled power transformers are a more involved test with their own procedures and oil considerations, and on medium and high voltage the program moves toward power factor and other diagnostics beyond a simple megohm reading. For the dry-type and low-voltage gear most crews touch, the insulation test is the routine check, run at acceptance and on the maintenance interval.
The test voltage is dangerous: de-energize and verify dead
This test applies hundreds or thousands of volts of DC on purpose, and the circuit you are testing has to be dead first. De-energize, lock out and tag out, and verify the circuit is dead with a meter you proved on a known source, the same discipline as any other work inside the gear. The megohmmeter is a source. Treat the leads as live whenever the test is running.
Never apply the test voltage to a circuit that could be energized from another source, and never let anyone touch the conductor or the far end while the test is on. On a cable, the far end is out of sight, so it gets a barrier, a tag, or a person, because someone walking up to a live far end is the accident waiting to happen.
Two hazards are specific to this test, beyond the obvious shock from the applied voltage: the stored charge after the test, and the damage the DC does to connected electronics. Both get their own treatment next, because both are routinely underestimated.
Do you have to discharge a cable after a megger test?
Yes. You discharge a cable or winding after a megger test, every time, no exceptions. The DC test voltage charges the capacitance of the cable or winding, and that charge stays after the tester is switched off. On a long cable it can hold a lethal charge for a long time. The tester output may be limited, but the stored energy in the capacitance is fully capable of killing you.
Most modern testers discharge the item automatically when you stop the test, and many show the voltage bleeding down on the display. Do not trust that alone on a cable of any size. Leave the tester connected so it can take the charge down first, then apply a grounding stick or a discharge stick to the conductor and leave the ground on while you work. A common practice is to ground the conductor for at least as long as the test ran, and longer on a large cable, because the absorption charge bleeds off slowly.
This is the step that kills people who knew better. The reading was fine, the test was done, the conductor felt like a finished job, and the stored charge was still sitting there. Discharge it, ground it, then touch it.
Disconnect the electronics and surge devices first
The DC test voltage that grades insulation will damage a lot of what is connected to the circuit. Surge protective devices and arresters conduct at the test voltage and either get damaged or clamp the reading, so you end up testing the SPD instead of the cable. Drives, electronics, meters, power supplies, and anything with semiconductors can be destroyed by the DC. Disconnect or isolate all of it before the test.
This is why both ends of a cable come loose, and why the winding gets tested at the motor leads with the starter and drive disconnected. Megger a circuit with the surge devices still on it and the best case is a low reading that sends you chasing a fault that is not there. The worst case is a bag of dead SPDs and a fried drive.
The discipline is simple to state and easy to skip under schedule pressure. Know everything that is on the circuit, get it off, test, then put it back. The five minutes it takes to disconnect a surge device is cheaper than the device, and far cheaper than the wrong conclusion.
Moisture and contamination, the usual cause of a low reading
When a reading comes back low, moisture or surface contamination is the first suspect, not failed insulation. Water is a conductor, and a film of moisture across a winding, a termination, or a dirty insulator surface gives the leakage current an easy path that drops the megohms hard. A motor that flooded, a cable end that sat open in the weather, switchgear in a damp vault: all read low for a reason that drying or cleaning can fix.
The tell is that a low reading from moisture often recovers. Dry the winding with space heaters or a controlled bake, clean and dry the terminations and insulator surfaces, and re-test. If the reading climbs back to where it should be, the insulation was wet, not bad. If it stays low after a real dry-out, the degradation is in the insulation itself.
Surface contamination is its own case. Dust, salt, oil, and carbon tracking across the surface of an insulator create a leakage path that has nothing to do with the bulk insulation. Clean it and re-test before you condemn the equipment.
When to run the test
Run it at four points. At acceptance, on new or rebuilt equipment, to confirm nothing was damaged in shipping or installation and to set the baseline; this is the NETA acceptance testing the spec usually calls for. On a maintenance interval, to trend the readings and catch slow degradation; NETA maintenance testing gives the intervals and procedures. In troubleshooting, when equipment faults or behaves like it has a ground problem. And after any event that could have hurt the insulation: a flood, a fire, a fault, a motor that ran underwater.
The acceptance test is the one that pays off years later, because without a baseline there is no trend, and without a trend a single maintenance reading is just a number against a generic minimum. Set the baseline at acceptance and the maintenance readings have something to mean.
Critical facilities run the maintenance test on a tighter interval because the cost of a failure is higher. A data center or a hospital does not wait for a motor or a feeder to fail; the insulation program and the infrared thermography survey both run on a schedule built around the outage windows the facility can actually take.
Field checklist
Want this checklist to run itself on every job — with photo proof and a signed record crews can hand the customer? That's FieldOS.
What to document
A reading nobody can find is a reading nobody can trend, and the trend is the whole value of insulation testing. Capture enough that the next technician can reproduce the test and compare it honestly to what came before.
| Field to record | Why it matters |
|---|---|
| Asset ID and what was tested | The trend follows the asset, not the day |
| Test voltage applied | Readings only compare at the same voltage |
| Insulation temperature | Required for the 20 degrees C correction |
| Spot value and corrected value | The corrected number is the comparable one |
| PI and DAR where run | Grades the insulation independent of temperature |
| Comparison to baseline or last reading | The trend is where the warning shows |
| Connected items disconnected | Proves the SPDs and electronics were off |
| Discharge confirmed | Records that the safety step was done |
Common mistakes
- Not discharging the cable or winding after the test and grabbing a conductor holding a lethal stored charge.
- Using the wrong test voltage, over-stressing good insulation or under-stressing a real weakness.
- Reading the raw megohms without correcting to 20 degrees C, so a temperature swing looks like a failure.
- Meggering a circuit with surge devices or electronics still connected, damaging them and skewing the reading.
- Reading the spot value in isolation and ignoring the trend against the baseline.
- Testing a circuit that was not fully isolated, so the current finds another path and the reading lies.
- Calling a low reading bad insulation when it is moisture or surface dirt that drying or cleaning would fix.
- Skipping the 60-second settle and recording the charging current as the result.
Standards and references
NETA sets the acceptance and maintenance framework most of this work runs under: the ANSI/NETA acceptance testing specification for new gear and the maintenance testing specification for the periodic program, with the procedures and comparison values for cable, motors, transformers, and switchgear. When a spec calls for testing to NETA, those documents define the test and the criteria.
For rotating machines, IEEE 43, the recommended practice for testing insulation resistance of rotating machinery, is the controlling reference for the test voltage, the minimum values, and the polarization index acceptance. For cable, IEEE 400 and its companion guides cover field testing of shielded power cable, including the DC methods and their cautions. The megohmmeter manufacturer's literature, from Megger and others, is genuinely useful on technique and on the discharge and capacitance limits of the instrument.
Pull the actual minimums, PI thresholds, and test voltages from the current edition of the standard that governs the equipment, and let the manufacturer's instructions and the project specification override the rule of thumb. The numbers in this guide are field guidance. The standard and the equipment rating control the call, and two things are not up for debate on any job: correct every reading for temperature, and discharge the item after the test.
Units, terms, and conversions
Insulation testing has its own vocabulary, and the same idea shows up under a few names across a tester, a standard, and a spec.
The reading is in megohms, often written as a value with the megohm symbol, and large or new gear reads in gigohms, where 1 gigohm equals 1000 megohms. The test itself goes by insulation resistance testing, IR testing, or megger testing after the instrument. The timed ratios are the polarization index (PI) and the dielectric absorption ratio (DAR). Test voltage is always DC, stated in volts. Temperature correction is to a reference, commonly 20 degrees C, though some standards reference 40 degrees C, so confirm which one a given minimum uses before you compare to it.
- Megohm
- Unit of insulation resistance, one million ohms; 1 gigohm equals 1000 megohms
- IR / megger test
- Insulation resistance test, named after the megohmmeter that performs it
- Polarization index (PI)
- Ratio of the 10-minute to the 1-minute reading; above about 2 is commonly good
- Dielectric absorption ratio (DAR)
- Ratio of the 60-second to the 30-second reading, the quick version of PI
- Leakage current
- The small steady current through and across insulation that sets the megohm reading
- Absorption current
- The slowly decaying polarization current that makes a healthy reading climb over time
FAQ
What is insulation resistance testing used for?
Insulation resistance testing grades the health of the insulation separating a conductor from ground and from other phases, before it faults. It is run at acceptance to set a baseline, on a maintenance interval to trend the readings, and in troubleshooting after a flood, fault, or fire that could have damaged the insulation.
What is a good megger reading?
A good megger reading is high, stable, above the equipment minimum, and in line with or better than the asset's own history. The rule-of-thumb floor is one megohm per kV plus one, but healthy insulation usually reads in the hundreds or thousands of megohms. The trend against the baseline matters more than the absolute number.
What is polarization index?
Polarization index is the ratio of the 10-minute insulation reading to the 1-minute reading at one steady test voltage. A PI above about 2 commonly indicates dry, healthy insulation, while a PI near 1 points to moisture or contamination. IEEE 43 sets the accepted minimums for motors and generators.
Do you have to discharge a cable after a megger test?
Yes, always. The DC test voltage charges the cable's capacitance, and that charge stays after the tester is off, holding a potentially lethal voltage on a long cable. Let the tester discharge it, then ground the conductor with a discharge stick and leave the ground on while you work.
What test voltage do I use for a 480 V motor?
For a 480 V motor, a 500 or 1000 V DC test voltage is the common choice, roughly twice the rated voltage. Larger and medium-voltage machines use higher voltages per IEEE 43. The manufacturer's specified test voltage, where given, overrides the rule of thumb, so check it before testing.
Why is my insulation resistance reading low?
A low reading usually means moisture or surface contamination before it means failed insulation. A flooded motor, a damp vault, or a dirty insulator surface gives the leakage current an easy path. Dry or clean the equipment and re-test. If the reading recovers, it was wet; if it stays low, the insulation is degraded.
What is the difference between PI and DAR?
PI is the ratio of the 10-minute to 1-minute reading; DAR is the 60-second to 30-second ratio. DAR gives a usable result in one minute instead of ten, so it suits smaller equipment and field troubleshooting. PI sees more of the absorption curve and is the better test on large machines and windings.
Why do I correct the megger reading to 20 degrees C?
Insulation resistance roughly halves for every 10 degrees C the insulation warms, so an uncorrected reading swings with temperature. Correcting to a 20 degrees C reference lets a hot summer reading compare honestly to a cold winter one. Without the correction, a normal seasonal swing reads like the insulation is failing.
Can I megger a circuit with surge protectors connected?
No. Surge protective devices and arresters conduct at the DC test voltage, so they skew the reading and can be damaged. Drives, meters, and electronics can be destroyed outright. Disconnect or isolate all of it and float both ends of a cable before applying the test voltage, then reconnect after.
What does NETA require for insulation resistance testing?
NETA gives an acceptance testing specification for new gear and a maintenance testing specification for the periodic program, with procedures and comparison values for cable, motors, transformers, and switchgear. The specific test voltages, intervals, and minimums come from the current NETA edition and IEEE 43 for machines; verify against the edition the spec adopts.
People also ask
Codes cited in this guide
This guide is written and reviewed against the published standards below. Always confirm the current adopted edition with the authority having jurisdiction.