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Old 02-12-2009, 07:27 PM   #1
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Megger, Insulation test


Hello, Can I get some opinions on the correct way to use a digital meggar. I had 4800 volts hit a messenger line from the utility and some of the romex has burned up but I still have to meg all other branch circuits. It has 500 and 1000 volt options. I know I have to disconnect any loads, light bulbs, etc. But a little confused on how to correctly meg the branch circuits and what a good reading should be. Thanks for any help.

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Old 02-12-2009, 09:03 PM   #2
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Megger, Insulation test


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Originally Posted by hastime View Post
Hello, Can I get some opinions on the correct way to use a digital meggar. I had 4800 volts hit a messenger line from the utility and some of the romex has burned up but I still have to meg all other branch circuits. It has 500 and 1000 volt options. I know I have to disconnect any loads, light bulbs, etc. But a little confused on how to correctly meg the branch circuits and what a good reading should be. Thanks for any help.
First check that the wire still carries current (hasn't melted). The 4800v line can deliver how many kA, you think?
This link may allow you to estimate how many amps for how many seconds it takes to melt copper wires.
http://home.earthlink.net/~jimlux/hv/fuses.htm

Burned up from arcing or high current? High current would make wire insulation uniformly crispy over the length that carried the overcurrent and may ruin the copper even if it hasn't melted.

If the wire insulation cracks when you bend it over a 2" radius it is ruined.
If not, I'd think 600vac insulation should be able to withstand 1000vdc and conduct much less than 1 mA (= >>1 MΩ) at that potential.

http://en.wikipedia.org/wiki/Hipot

Some keywords to search: dielectric, breakdown, withstand

Also, if you look up the instruction manual for a Biddle megger, they have a lot of tips on appraising the quality of insulation. Trends, the whole bit.

It would also show Due Diligence and Duty of Care on your part if you asked
http://www.southwire.com/
for their take (in writing) on this, since safety is important here.

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Old 02-12-2009, 10:18 PM   #3
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Megger, Insulation test


Almost all cables are meggered at twice the rated insulation voltage.

I always use the 1000 volt range when meggering branch circuit wiring.

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Old 02-13-2009, 12:46 PM   #4
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Megger, Insulation test


Mr. "Time", I'm curious if this sequence fits what you found with the wiring.
I ask because I don't usually get to whack Romex with 4800v.

The cable is exposed to high voltage,
within 1/2 cycle the weakest part of the insulation arcs over,
the cable downstream of the arc location then sees no current and almost no voltage,
the cable upstream of the arc now sees very high current until the conductor melts.

If so, the cable upstream of the arc need replacing.

If the cable insulation downstream of the arc is somehow changed in a bad way by being exposed to 4800v for a half-cycle then it needs changing. A cable insulation specialist, if there is such a thing, could best advise you on this. FWIW, the insulation might even benefit by this kind of treatment.

Last edited by Yoyizit; 02-13-2009 at 02:22 PM.
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Old 02-13-2009, 03:41 PM   #5
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Megger, Insulation test


Make sure NOTHING is plugged in or connected to the circuits you plan to meg. If you do, chances are very good you will be replacing them also. Electronics, especially would not like 1000 volts applied to them. Be very careful.
If you need help understanding/using your megger, we can help you.
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Old 02-13-2009, 07:24 PM   #6
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Megger, Insulation test


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Originally Posted by Yoyizit View Post
FWIW, the insulation might even benefit by this kind of treatment.
Care to elaborate on this?
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Old 02-13-2009, 10:39 PM   #7
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Megger, Insulation test


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Originally Posted by WFO View Post
Care to elaborate on this?
Maybe it realigns the molecules or something? Changes the grain boundaries?

It might ". . . prevent formation of voids within the insulation. [by causing localized melting and then] The insulation applied in each superimposed layer increases the impulse breakdown stress in volts per mil of insulation thickness."

It works for dilithium crystals, but only sometimes.

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