insulation and meggers

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Does a megger ever say the insulation is bad, yet the insulation holds up for months or years at 120v because 120v is much less stress than the megger voltage?
If so, does this lead to confusion in troubleshooting GFIs?
Thanks.
 
bad

bad

From Biddle, bad is less than 1 meg for each 1 kV with a minimum of 1 meg.
I've seen other posts that say realistic values are much higher.

http://www.biddlemegger.com/biddle/A Stitch In Time.pdf

I guess another possibility is that the megger causes the insulation to fail; an unintentional destructive test. Maybe that only happens if the insulation was already CTD (circling the drain).

Can you ever go wrong using a megger to troubleshoot nuisance tripping in a GFI?
At 4 mA minimum leakage and 120v, (~30k) you could almost use an ohmmeter.
 
If they made a megger with a big light that said either "bad" or "good", that would cause some confusion. There are rule of thumb to go by. One of them is to not meg 120V circuits at 1KV, as you seem to be suggesting. I have always suggested twice the normal operating voltage for most building wire and cables, unless there's some special reason to do differently (like modular and mobile home testing). The short answer, however is, YES. Yes, there can be "bad" readings when Meggering cables and the cable will continue in service for a spell. The level of bad-ness is a personal thing, unless you're testing to some specific NETA spec. The level of bad-ness today, with today's temperature and humidity, will be totally different next week with a different temperature and humidity as well.

I don't know if any of the above makes any sense, but that's a stream of consciousness on the matter for the moment.
 
langjahr@comcast.net said:
Can you ever go wrong using a megger to troubleshoot nuisance tripping in a GFI?
At 4 mA minimum leakage and 120v, (~30k) you could almost use an ohmmeter.
Click to expand...
Not in my opinion. Here's why...

GFCI's trip intermittently most normally. The fact that they stay reset for a good while means that the fault current level is much lower than 4ma for a spell, then it spikes once in a while. If a 4ma fault existed all the time, you could certainly find it with a good ohm meter. They only seem to trip intermittently, which is where the megger pulls it weight in the troubleshooting process.
 
Moving on.....

Moving on.....

After joining this forum, it amazed me the level of some techs/electricians that have never used a megger. There are no solid answers for meggering in any given scenario as Marc pointed out with temperature and humidity.

You can't take a megger on WIRING and say inconclusively that the wiring is good or not. Equipment, motors, trannys, etc.,...MAYBE, but alot of other things must be ruled out. I have been asked alot about meggering. I couple that question with soldering. There is NOT any proper training to teach someone good soldering skills. I have been asked to show different people, but all I can give you is fundamentals, and work side by side with you from there. Like soldering for instance, it is one of those things you must do over and over again until you have it right. In other words, you must develope a "knack" for it. There are so many different angles and scenario's, that you can't say,..."This IS how it's done everytime."

And for anyone who thinks they can solve EVERY problem with a megger REAL quick,..... back up a minute.

Ever tried to solder???? I could sit down with you for 12 hours and you might do O.K., but you won't master it for some time to come. That's the best way I can describe the art of using a megohmeter.

That's my initial reply.
 
Can you ever go wrong using a megger to troubleshoot nuisance tripping in a GFI?
Click to expand...

YES with GFP went to a job to investigate, why the GFP and associated controls were not allow the switch to be closed. The contractor had meggered the switchboard with the control power on, appears he fried the phase failure relay.

BUT, if you know what you are dong IMO no.
 
Thanks for your thoughts. I cut-and-pasted this back-and-forth into a file for further study. Otherwise, when I need it I won't remember it.

I take it NETA is The InterNational Electrical Testing Association?
 
langjahr@comcast.net said:
Thanks for your thoughts. I cut-and-pasted this back-and-forth into a file for further study. Otherwise, when I need it I won't remember it.

I take it NETA is The InterNational Electrical Testing Association?
Click to expand...

If you'd like more studies, than I'd be happy to post some.
 
more studies

more studies

Due to the hazardous output voltage, in some states it is forbidden to diddle with the Biddle.


Sure, I can always use more info. . .
 
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