Megger testing

[hacim23]

What is the general rule of thumb minimum reading for feeder insulation testing?
I always forget

 

[ptonsparky]

IDK about rule of thumb. I would look at G ohm or full scale as a pass. Next would be to compare each conductor. All at 650Mohm would be good, but one at 350Mohm would flag it.

Best would be to record periodic measurements and compare.

 

[xformer]

I would say 50 MOHMS or greater.

 

[ramsy]

 

[ptonsparky]

View attachment 2554699

I would not pass anything with those values. How old is that and what was it on?

 

[ramsy]

I would not pass anything with those values. How old is that and what was it on?

That obsolete nameplate is posted here in the spirit of Internet Opinion (IO), under the Authority of Social Media & Angry Tolls (ASMAT).

Such information is used at your own risk.

No longer have the tester
Perhaps 1950's erra hand held insulation tester.

 

[WasGSOHM]

View attachment 2554699

If no progress has been made on insulation materials over decades, sure, why not use these?

Lets see. . .800A/5K is 0.2 and 50A/250k is 2E-4. Maybe the ratios still hold.

 

[ramsy]

Lets see. . .800A/5K is 0.2 and 50A/250k is 2E-4. Maybe the ratios still hold.

0.2A leakage on 1000Amp services appears to fail 30mA requirements of GFPE, but 20A/1M = 0.2mA appears to hold for 6mA Class-2 GFCI requirements.

 

[Besoeker3]

This what I have seen:

Insulation resistance should be approximately one megohm for each 1,000 volts of operating voltage, with a minimum value of one megohm. For example, a motor rated at 2,400 volts should have a minimum insulation resistance of 2.4 megohms

 

[ramsy]

Insulation resistance should be approximately one megohm for each 1,000 volts of operating voltage, with a minimum value of one megohm. For example, a motor rated at 2,400 volts should have a minimum insulation resistance of 2.4 megohms

That rule of thumb must originate from a more sophisticated engineering standard, not NFPA-70 or the NEC.

 

[Besoeker3]

That rule of thumb must originate from a more sophisticated engineering standard, not NFPA-70 or the NEC.

I think it was IEC.

 

[WasGSOHM]

Back then we had only name-plate embossed-in-metal large print that didn't change. Now we've got the fine print, the exceptions, the amendments, the "call for a quote".

I guess that's progress.

But maybe we always had the fine print, etc., but you just didn't know about the other stuff 'til a palm was crossed with green.

 

[Besoeker3]

That rule of thumb must originate from a more sophisticated engineering standard, not NFPA-70 or the NEC.

It was www.transcat.com not IEC..

 

[ramsy]

It was www.transcat.com not IEC..

Can't find it. Is there a direct link?

 

[Besoeker3]

Can't find it. Is there a direct link?

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