Whole house megging

[Volta]

Ok, it's a 1953 residential duplex. Lighting hit a tree in the front yard. Some of the current flowed along the metal underground water pipe to the house, through the GEC(s), and out through the system (supplemental GECs to rods, siding, one TV). Minimal known damage to utilization equipment or GFCIs.

Normally to test I'd disconnect all sensitive equipment, unscrew lamps, and test each branch individually.

For simple pass/fail results might the testing start at the load side of the meter base(s)? Specifically, is there any chance of damage to the MCBs (100 amp SqD HOMs, then normal branch circuit breakers) with a 1000v crank?

If the wiring passes that way, several hours of individual branch testing might be saved per side.

 

[kwired]

I would say do not test GFCI or AFCI breakers this way. Standard breakers would likely stand up to the challenge.

 

[ActionDave]

I would say do not test GFCI or AFCI breakers this way. Standard breakers would likely stand up to the challenge.

I would say you are correct. I just did a job yesterday where the service neutral was lost. All of the GFCI and AFCI breakers were cooked and needed replaced.

 

[Volta]

I would say do not test GFCI or AFCI breakers this way. Standard breakers would likely stand up to the challenge.

Thanks, that was my feeling. I think that I've heard of people megging a breaker per se, though I haven't done it myself. Looking through the 1951 NEC it seems that conductors in NM are going to be rated at 600v, but I couldn't really prove it to conclusively. But 1000v for a minute shouldn't compromise the insulation at all should it?

 

[kwired]

Thanks, that was my feeling. I think that I've heard of people megging a breaker per se, though I haven't done it myself. Looking through the 1951 NEC it seems that conductors in NM are going to be rated at 600v, but I couldn't really prove it to conclusively. But 1000v for a minute shouldn't compromise the insulation at all should it?

I'm definitely not an expert in this area but my understanding is it is recommended to perform insulation testing with at least twice the operating voltage. Since the conductor in question is designed to operate up to 600 volts I wouldn't think 1000 volts for a short time should have much of a degrading effect.

 

[K8MHZ]

Here is an interesting take on the subject:

Resistance in the normal current path due to a damaged connection caused by the energy of a Lightning Strike would never be detected by a megger test. Increased resistance in the normal current path caused by a Lighting Strike can create a poor electrical connection which can build up sufficient enough heat to start a fire in an outlet or junction box.

The Megger test sends a 500 volt to 1,000 volt DC current across an electrical system that is designed for 240 volts AC. The test voltage is two to four times greater than the normal operating voltage, and can be very destructive. The test current has been known to destroy lights bulbs and printed circuit boards that have been inadvertently left in the electrical system as it is being tested. So instead of testing for damage the megger test can cause damage.

From: http://cooper-electric.net/currentsafe/lightning-strikes/

 

[kwired]

Here is an interesting take on the subject:



From: http://cooper-electric.net/currentsafe/lightning-strikes/

Your quoted information is exactly what happens with any insulation testing measurement. The person performing the test needs to know what they are connecting to, disconnect items from the circuit that will be damaged by the test current, and confirm there is no low impedance continuity before even applying the high voltage test voltage. Then one must also realize such testing only verifies that there is or is not any current leakage at the test voltage but does not confirm that the insulation is in perfect condition. You could have a long section of melted insulation in a cable and still have good insulation testing between two conductors.

 

[Sierrasparky]

Take a piece of NM and slit the sheath open ,cut some insulation off one conductor. Tape it back together. As long as the missing insulation is not near the bare ground you will show good. Get it wet and you may have a different scenario.