# Megger Test



## jcrispy3 (Sep 2, 2011)

What type of cable are you meggering? Unless it is a paper insulated cable I don't see humidity affecting your megger test. If it is a paper insulated cable, the cable should be stored and installed in a controlled environment in order to mitigate any atmospheric contamination. If done correctly, humidity won't be an issue.


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## jcrispy3 (Sep 2, 2011)

Also, industry wouldn't create a "sliding ruler" for humidity. The cable should be able to withstand whatever environment (and the humidity associated with it) that it was designed for.


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## 76nemo (Aug 13, 2008)

There is no industry standard or sliding scale per se.

Is thee equipment under test being done on site under normal operating conditions, or off site, say a motor shop?


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## Zog (Apr 15, 2009)

Lithium said:


> Do you know if there is an industry standard sliding scale for humid conditions for megger testing?


No, the effects of humidity is mentioned in several IEEE, NEMA, and ANSi standards but there are no correction factors as there are for tempature. They all basically say record humidity at the time of test and try to do periodic tests at near the same levels. Humidity has much less of an effect than a tempature difference will.


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## RIVETER (Sep 26, 2009)

Lithium said:


> Do you know if there is an industry standard sliding scale for humid conditions for megger testing?


I would not think so. If it is humid when you test and it fails...it fails for those conditions. Those conditions will reoccur during normal ops and that is the reason for the test.


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## jcrispy3 (Sep 2, 2011)

Zog said:


> No, the effects of humidity is mentioned in several IEEE, NEMA, and ANSi standards but there are no correction factors as there are for tempature. They all basically say record humidity at the time of test and try to do periodic tests at near the same levels. Humidity has much less of an effect than a tempature difference will.


Agreed.


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## RIVETER (Sep 26, 2009)

Zog said:


> No, the effects of humidity is mentioned in several IEEE, NEMA, and ANSi standards but there are no correction factors as there are for tempature. They all basically say record humidity at the time of test and try to do periodic tests at near the same levels. Humidity has much less of an effect than a tempature difference will.


I agree with you. There are those who think on a molecular level and know that the space between the molecules of the insulation material cannot pass the size of the water molecule unless the insulation is broken down. I just had to say that.


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## McClary’s Electrical (Feb 21, 2009)

RIVETER said:


> I agree with you. There are those who think on a molecular level and know that the space between the molecules of the insulation material cannot pass the size of the water molecule unless the insulation is broken down. I just had to say that.


 

We were preparing to start a 2500 HP motor that had sat for 4 years. It failed a megger test. We called the motor winding shop that wound it and they said it was normal. It needed to be dried out. They sent us a procedure to dry the windings using a welder. It took 2 whole days of the welder shorted out, but it dried thoroughly, passed megger test, and started fine.


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## 76nemo (Aug 13, 2008)

RIVETER said:


> I agree with you. There are those who think on a molecular level and know that the space between the molecules of the insulation material cannot pass the size of the water molecule unless the insulation is broken down. I just had to say that.


 
Humidity can very well affect measurement if done outside of normal PM programming and outside of normal ambients.


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## heel600 (Oct 31, 2007)

mcclary's electrical said:


> We were preparing to start a 2500 HP motor that had sat for 4 years. It failed a megger test. We called the motor winding shop that wound it and they said it was normal. It needed to be dried out. They sent us a procedure to dry the windings using a welder. It took 2 whole days of the welder shorted out, but it dried thoroughly, passed megger test, and started fine.



Not to hijack this thread, but I would love too see that procedure.


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## RIVETER (Sep 26, 2009)

mcclary's electrical said:


> We were preparing to start a 2500 HP motor that had sat for 4 years. It failed a megger test. We called the motor winding shop that wound it and they said it was normal. It needed to be dried out. They sent us a procedure to dry the windings using a welder. It took 2 whole days of the welder shorted out, but it dried thoroughly, passed megger test, and started fine.


Yes, that is the way to do it. A low level current will generate heat to dry things out. If the insulation is damaged...different story. Just an afterthought. Probably, a person should not use a voltage setting much more...higher, than two times the expected phase voltage. IMO.


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## 76nemo (Aug 13, 2008)

RIVETER said:


> Yes, that is the way to do it. A low level current will generate heat to dry things out. If the insulation is damaged...different story. Just an afterthought. Probably, a person should not use a voltage setting much more...higher, than two times the expected phase voltage. IMO.


 
NOT true. The device under test must be tested compared to previous testing in the past under the same ambients, otherwise you will see "off" readings. Not sure where you are going........


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## RIVETER (Sep 26, 2009)

76nemo said:


> NOT true. The device under test must be tested compared to previous testing in the past under the same ambients, otherwise you will see "off" readings. Not sure where you are going........


I am going in the direction of what is acceptable...or not. VOLTAGE is not aware of AMBIENTS. If there is a difference of potential and it is able to pass through that point because TODAY it is moist, that is not acceptable.


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## 76nemo (Aug 13, 2008)

RIVETER said:


> I am going in the direction of what is acceptable...or not. VOLTAGE is not aware of AMBIENTS. If there is a difference of potential and it is able to pass through that point because TODAY it is moist, that is not acceptable.


 


You somewhat have a valid point, but you are missing the bigger picture.

An IRT must be done in a somewhat controlled environment.

If an IRT can "pass" in an environment it lays in everyday, it's subjected to the tests it's been pushed through in previous PM's.

Humidity will form in deterioration. You can't take it out of it's "norm" and expect the same readings. This is why PM's are in place. This is why it is scheduled and not performed off site.


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## RIVETER (Sep 26, 2009)

76nemo said:


> You somewhat have a valid point, but you are missing the bigger picture.
> 
> An IRT must be done in a somewhat controlled environment.
> 
> ...


I can't know your situation. I do know that a History can be determined by readings after readings...over time.
If a conductor's insulation...over time, is failing, let's keep track of that failing and try to catch it before it is catastrophic. Maybe I am off base here....again.


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## 76nemo (Aug 13, 2008)

RIVETER said:


> I can't know your situation. I do know that a History can be determined by readings after readings...over time.
> If a conductor...over time, is failing, let's keep track of that failing and try to catch it before it is catastrophic. Maybe I am off base here....again.


You are not off track. Unless your company employs off site testing, you're close. Humidity may not be a factor until it is tested outside of it's normal operating environment.

I guess I am rambling. Let's get to the point. Onsite or offsite testing???


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## RIVETER (Sep 26, 2009)

76nemo said:


> You are not off track. Unless your company employs off site testing, you're close. Humidity may not be a factor until it is tested outside of it's normal operating environment.
> 
> I guess I am rambling. Let's get to the point. Onsite or offsite testing???


Rambling is not necessarily a bad thing. Getting thoughts together takes some "rambling" sometimes. It is rambling that makes a person think abstact...whatever that means. Does it make sense to send a shipment to a dry site to see if the insulating qualities of it will suit it's environment?


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## 76nemo (Aug 13, 2008)

RIVETER said:


> Rambling is not necessarily a bad thing. Getting thoughts together takes some "rambling" sometimes. It is rambling that makes a person think abstact...whatever that means. Does it make sense to send a shipment to a dry site to see if the insulating qualities of it will suit it's environment?


Yes and no. It depends on the PM and engineering in control.

It depends on the individual PM.

In a "real" world, IRT should be taken with the equipment in place and done in a manner under normal operating conditions.

That is a matter of trending and plotting.

I'm not bucking you, I'm just saying it is not "cut and dry" as many people see it.


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## Lithium (Apr 17, 2009)

Thank you all!!!

I did some research and here is what i found:


Humidity does affect the reading of the ohmmeter. The moisture will form on the insulation surface and in some cases will be absorbed by the material causing the change of the resistance. Also the carbon and the other foreign matter existing on the surface of the conductor can become conductive under the humid conditions. 

In order to avoid the moisture, the temperature of the conductor has to be above the dew point of the ambient air. In this case the test reading normally will not be affected much by the humidity. But the conductor surface has to be free of contaminates, which have the property to absorb the moisture.


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## Lone Crapshooter (Nov 8, 2008)

Slightly off track but I think when you do Doble testing there is a humidity and temperature factors. It has been a wile sense I have seen Doble tests.


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## 76nemo (Aug 13, 2008)

Lone Crapshooter said:


> Slightly off track but I think when you do Doble testing there is a humidity and temperature factors. It has been a wile sense I have seen Doble tests.


 
No one mentioned "Doble" testing.


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