# Standards for Megger Testing



## Wirenuting (Sep 12, 2010)

This is what I found today while testing a 15hp..
my readings were 500m on all three phases.
Better then my last 25hp last week. LoL

I will begin to plan a replacement when I hit the -50
mark as we don’t like down time and the motors are beat to death by then anyway.









How to measure insulation resistance of a motor


If the motor is not put into operation immediately upon arrival, it is important to protect it against external factors like moisture high temperature




electrical-engineering-portal.com


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## Mbit (Feb 28, 2020)

Navyguy said:


> I have looked through a bunch of posts regarding megger testing and not seeing what I am looking for. I am looking for a “standard” that says this XX MΩ reading is a “pass” and this XX MΩ reading is a “fail”. I am sure it is an IEEE standard, but I do not know for sure.
> 
> I have a couple pages from one of the forum posts from a NETA (not even sure what it is) that basically says there is no “single standard” so here is ours.
> 
> ...


First use manufacturer's recommended values.

If they are not available use the NETA MTS for existing installations or the ATS for acceptance testing on new equipment. That's how the testing companies do it.

The NETA standards are ANSI standards as well. 

Testing is basically a separate trade in the US and CA. Most customers could care less about testing except utilities, data centers and some manufacturing. I can't comment specifically for residential, I'm a noob in that area.


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## SteveBayshore (Apr 7, 2013)

If you are going to MEG test wet stuff, it will test bad. If you test it after it drys out it should be OK if it was *clean* water. If it was flood water or gray water it might test OK after drying out but corrosion problems will most likely lead to tracking eventually. We have tested outdoor motors that have been sitting without running for long periods of time and read almost shorted. We cover them with a tarp and a portable heater for a day before testing again to get an OK reading and then running before taking an accurate test.


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## ScooterMcGavin (Jan 24, 2011)

Take a look at

https://www.testequipmentdepot.com/megger/pdf/electrical-insulation-testing_guide.pdf

There's no real simple answer to your question, but it may shed some light on what you're trying to do.


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## gpop (May 14, 2018)

On a dry motor 1M per 1000v is generally the lowest you can go and still get the motor to run. So 1/2 a meg on a 480v motor. 

On a wet/damp motor i have seen hundreds meg zero and still run when you throw fire to them. I worked food grade (wet environment) where we would shut down for 4 months and a lot of the motors would meg zero. Nearly all of the motor failures were bearings related due to the environment. Run to failure was a common practice on anything smaller than 25 hp. 

We use the meg more for testing the wires, starters, disconnects, etc. Anything less than 50M was a potential problem.


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## paulengr (Oct 8, 2017)

1 Megaohms per kilovolt was the old pre-polymer insulation standard. IEEE 43 is the standard for motors. 400 for wiring.

NETA is the clown patrol. Those idiots are testing companies writing a spec for themselves. They never met a test they didn’t like…it’s tests for testing same, not because you will use the data.

IEEE 43 for motors calls for 1 Megaohms per kilovolt on rotors only. On form wound or MV coils 100 Megaohms minimum. On everything else 1000 V or less it’s 5 Megaohms. 400 calls for 100 Megaohms. The test time is always 60 seconds. The voltages are standard dependent: So 400 calls for test voltage equal to 94% of rated (not use) voltage for field (not factory) testing. 43 calls for 500 up to 1000 V rated and 1000-2500 up to around 6 kV. The standard 250/500/1000 V IR tester works for most. Breakers and other equipment generally follow their associated yes standard except that vacuum interrupters have a manufacturer specific essentially BS test. It calls for a fairly high voltage and is intended to detect failed vacuum but fails to detect anything a Megger test won’t detect. Real vacuum tests are predictive and use a magnetometer.

These are ALL temperature corrected values with a cold motor/device. With I corrected values almost everything passes in January and fails in August because it will be ten timers higher in cold weather…so compare everything at 40 C.

You can reject all tests that use terms like good/fair/poor. What do you do with a “fair” result? Does it pass or fail?

IR testing in general is a single test. I can show you tons of motors that are bad but pass. I just checked with me yesterday with 8 Megaohms (sitting in storage in humidity) that runs but has severe air gap eccentricity…rotor out of round although it’s balanced.


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## Bluenose for rent (Nov 6, 2020)

We measure bond to neutral on any new residential circuit at 500v and 1 M ohm is considered a pass according to my AHJ inspectors, although we’d typically see something between 20 and 100.


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## SWDweller (Dec 9, 2020)

Navyguy,

As you read all of the great posts you should be understanding one critical concept. What you are testing is as important as the value. Larger amounts of conductor and motor will test differently than smaller amounts. Then there are the environmental issues. 
So a hard and fast table for testing. just is not going to happen.


Paulengr bring up a great concept, I have always used pass fail. The rest is all colors of grey. I choose not to deal that concept.

I was concerned about a 3000 amp service section when the megger read phase to phase 1.5 meg ohms. I had the factory tell me anything over a meg was oky doky.


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## paulengr (Oct 8, 2017)

SWDweller said:


> Navyguy,
> 
> As you read all of the great posts you should be understanding one critical concept. What you are testing is as important as the value. Larger amounts of conductor and motor will test differently than smaller amounts. Then there are the environmental issues.
> So a hard and fast table for testing. just is not going to happen.
> ...


Environmental issues…

Low Megger readings are caused by three things:
Moisture
Contamination
Insulation breakdown/failures

Sometimes you can tell what is causing the issue. If the readings bounce all over the place it’s moisture. If you get “stair stepping” it’s insulation cracked and brittle. If it goes up then suddenly stops rising with no polarization it’s leaking most likely from contamination. But often all you get is a low reading but you can’t tell why because other than that it’s a good test. Better testing equipment gives you a chart so this is easier to see but watching closely with hand tools often tells you what’s going on, if it is possible. I just tested one on Thursday. 8 Megs on a motor that was rewound 2 years ago, never in service, 800 HP. PI was low too. No indication of problems and it passes but should have been higher. The humidity was also around 99% at the tune because it’s an ice plant…water everywhere evaporating Give it a day in service and it will dry out. Capacitance readings, resistive and inductive unbalance all normal and it didn’t let the smoke out and online tests all looked great when we ran it. At least with motors you can’t just go by one test. Had one a week ago where Megger test was excellent but rotor air gap eccentricity was horrendous (rotor issue).


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## mburtis (Sep 1, 2018)

From the maintenance viewpoint my understanding is that a single measurement doesn't really tell you that much without getting deeper into other tests. However If you have the same measurement taken at intervals it begins give some info. For instance maybe you have a motor that technically measures low and shouldn't run, but it does run and has had the same measurement for years, probably not worth worrying about. However if you have a motor that has a high reading for several years then all of a sudden drops significantly or begins to steadily decrease, further investigation is needed.


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## gpop (May 14, 2018)

risk verses reward always seems to come into play when meg testing.
In navyguys situation anything less then perfect should be reported as its a home and insurance will be reading the report and deciding what they are willing to pay for.

When it comes to motors if you plan to run a motor till failure then a meg test is a waste of time (unless troubleshooting). If its critical equipment then set a limit and plan accordingly. 
On the 4160 stuff (mostly 1500hp) we had a rule that any motor down for more then 8 hrs had to be meg tested before starting. If it failed then we would bag the motor and hook it to a dryer. Just the cost savings on fuses made this exercise worth while. 
Main lift station pumps we class as critical so everyone of them has a auto megger, thermal protection and seal leak detection installed. Minor lift stations have a high level buzzer.


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## paulengr (Oct 8, 2017)

mburtis said:


> From the maintenance viewpoint my understanding is that a single measurement doesn't really tell you that much without getting deeper into other tests. However If you have the same measurement taken at intervals it begins give some info. For instance maybe you have a motor that technically measures low and shouldn't run, but it does run and has had the same measurement for years, probably not worth worrying about. However if you have a motor that has a high reading for several years then all of a sudden drops significantly or begins to steadily decrease, further investigation is needed.


It only works with temperature correction and trying to keep environmental factors at bay.

A chemical plant customer was at around 3.2 GOhms 18 months ago. 2500 HP 4000 V. Now it’s 1.6 but they also did some other work where the motor sat unheated for over a week, and the last reading was in December vs July. So is it really dropping or just environment? We will see in another 12-18 months.

But on top of that if you are watching other tests you will get a much clearer picture. IR is just one test. Some tests are more definitive than others


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## CAUSA (Apr 3, 2013)

I like to use high pot testing on motors and transformers. Keeps things honest. But a expensive toy and having to get calibration certified every year bites the budget a bit, but nessesary to have that trust in your test equipment.

lots of good stuff in this thread? For testing.😉


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## paulengr (Oct 8, 2017)

CAUSA said:


> I like to use high pot testing on motors and transformers. Keeps things honest. But a expensive toy and having to get calibration certified every year bites the budget a bit, but nessesary to have that trust in your test equipment.
> 
> lots of good stuff in this thread? For testing.[emoji6]


DC hipot testing has been shown to cause electrical treeing in equipment that has been in service. The damage that it causes does not cause the hipot test to fail but the equipment fails up to 6 months later. The standard for hipot testing, IEEE 400, calls for not using it any more. It has been shown that it does not show anything that lower voltage tests don’t show so it doesn’t do anything extra. It has been completely eliminated from all standards for testing motors.



StackPath



So what you are doing is running a test that tells your customers nothing that a properly done IR/PI test already tests for but provide nothing else of value at best while running a test that can and does often cause premature equipment failures. Our motor shop and most larger reputable shops will not warranty any motor when a customer calls for this test, period.

Google partial discharge and especially the EPRI studies that show why hipot is so destructive. At one time a few “progressive” utilities routinely hipotted their equipment. However their failure rates were way above what other utilities that did not hipot experienced. This is the “assurance” that you are giving your customers.

There is a silver lining here. PD testing IS s type of high voltage testing that can predictively detect issues before they occur without causing damage. It works in motors, transformers, switchgear, and cables. It can be done online in many cases without even opening doors.


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## CAUSA (Apr 3, 2013)

Paulenger,
Thank you for the heads up.

I did understand the factors that the installation behaves similar to a capacitor when charging with DC voltage. I did not factor that it would cause the treeing breakdown following the imperfections of the bonding to the structure.

once again thanks.

Now all I have to do, Is convince the CSA, inspections division of this when we commissioning out of county products. For CSA approval.

I will keep that toy to develop a good layer of Dust in the tool crib.


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