How do intermittent faults happen?

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electrofelon

electrofelon

Senior Member
Location
Cherry Valley NY, Seattle, WA
Occupation
Electrician
Last week had a client with a tripping breaker. Long story short, I isolated it between two receptacles, about 6 foot run, ramen in residential, studs, drywall. I reset the breaker and when I would touch the blacks together at the recep before the fault, the breaker would hold and there wasnt even the slightest visible spark on the wires, indicating no or very little current (didnt bother metering it). If I left he wires connected in several minutes it would trip again. One time I heard the fault in the wall, so it was definitely right there. Rogue nail? overdriven staple? Rodent damage? Anyway the question is what exactly is the mechanism that causes an intermittent fault? I have seen this quite a few times. There must be some sort of high resistance material that for some reason rapidly breaks down losing it dielectric capability, then the fault happens, causes a spark which forms more of the temporary primordial mystery substance. Is it carbon? How does this work?
 
hbiss said:
Why didn't you cut the wall open and find out for yourself? Wouldn't you have to do that anyway to replace the run?

-Hal
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A piece of NM cable in the wall between two boxes would only have two staples, one at each box, securing it. It's entirely possible to remove the boxes, wedge your hand thru the ~2" x 4" opening, pop out the staples, and pull out the affected cable without cutting the wall open.

As to what is causing the intermittent fault, once the cable is out of the wall he should be able to tell what caused the damage, whether it is an installation error, rodent damage, overheating, etcetera
 
hbiss said:
Why didn't you cut the wall open and find out for yourself? Wouldn't you have to do that anyway to replace the run?

-Hal
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electrofelon said:
I was dying to cut 'er open and see, but I was able to fish a new wire in.
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Because the customer is much happier if you can make it work again without having to cut a hole in the wall.

I have a house I just wired in the last year that has a ground fault between two kitchen receptacles. First time I though it was a overtightened clamp on a cable connector into the panel. after loosening it it worked - but the problem came back and I traced it to between two receptacles. My guess is the guys that installed counter top for the raised bar above these receptacles probably drove a long screw into my cable, and it is intermittently making contact between conductors. My plan when I return is to cut cables in the basement and put a couple junction boxes and a "jumper" around the bad segment and abandon it in place. Beats opening walls or cutting holes in cabinets/counters just to find a bad spot that needs replaced or abandoned anyway.
 
Why are some of these faults intermittent? Possibly changes in temperature or humidity cause enough movement to cause change in the contact status of a fault condition.
 
I like to think of copper wire that conducts electricity as a living thing. As it expands and contracts with temperature changes or if enough current is applied it will move. As the previous post indicated a nail that is in close proximity to the wire may have eventually worn through the insulation and is just barely making enough contact. It probably Burns itself away each time it makes enough contact to trip the breaker but eventually the wire ends up moving back up to it and making contact again. If the wire is above the screw gravity even plays a role in this, when you combine all these forces random events are a probable outcome.
 
kwired said:
Why are some of these faults intermittent? Possibly changes in temperature or humidity cause enough movement to cause change in the contact status of a fault condition.
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Also, nearby heavy equipment operating, vibration from trains, even slamming a heavy door.

I've seen a few badly damaged sections of NM cable that didn't fault. Worst was in an addition, when the walls were opened up, there was a 2' piece of NM that had all the jacket and individual wire insulation missing, courtesy of an industrious squirrel that made its home nearby. Somehow, the hot, neutral, and ground were not touching each other despite only millimeter separation in spots. There was no evidence of arcing in the wall, no scorching, no fire. The HO was very lucky, not sure about the squirrel.
 
JFletcher said:
Also, nearby heavy equipment operating, vibration from trains, even slamming a heavy door.

I've seen a few badly damaged sections of NM cable that didn't fault. Worst was in an addition, when the walls were opened up, there was a 2' piece of NM that had all the jacket and individual wire insulation missing, courtesy of an industrious squirrel that made its home nearby. Somehow, the hot, neutral, and ground were not touching each other despite only millimeter separation in spots. There was no evidence of arcing in the wall, no scorching, no fire. The HO was very lucky, not sure about the squirrel.
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If all they made contact with was their teeth, they probably were fine. Can't tell you how many times I have repaired direct buried conductors/cables that had insulation chewed off by rodents, yet never seen a carcass at the damage location - even on 277 volt to ground conductors.

They don't eat that insulation they gather it for use for nesting material.
 
kwired said:
If all they made contact with was their teeth, they probably were fine. Can't tell you how many times I have repaired direct buried conductors/cables that had insulation chewed off by rodents, yet never seen a carcass at the damage location - even on 277 volt to ground conductors.

They don't eat that insulation they gather it for use for nesting material.
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We once had a muskrat get into our lab at the Wastewater Plant. It chewed through the cord on the refrigerator and the DI water supply cord. It was very much alive when I caught it with a fishing net and took it outside
 
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