Electrical arcing. What caused it?

General Questions
#1

My coworker had a large sun room installed in the back of her house a couple years ago. An electric heater (or furnace/AC unit - I forget) was also installed, and it started acting erratically earlier this year.

An electrician found an electrical cable in the crawl space that showed evidence of arcing:

Pic 1
Pic 2
Pic 3
Pic 4
Pic 5
Pic 6

I am scratching my head trying to figure out why the arcing occurred. And I’m not even sure what the electrical path was… hot wire to neutral wire? Hot wire to metal sleeve in gas line?

Did chaffing of the cable insulation occur from being in contact with the gas line? Seems unlikely given the gas line has a soft jacket. And I don’t have any reason to believe there would be much vibration.

Could the cable insulation be chemically incompatible with the jacket on the gas line? Again, seems unlikely.

#2

Looks like the Romex rubbed against the gas line until the bare conductor was exposed and contacted the metal core of the gas line.
Why? Maybe because it’s right next to the duct, and the duct caused the them to vibrate. It doesn’t take much vibration amplitude to wear away the sheath, given enough time.

#3

I’m guessing that the cold air return had a sharp edge that cut in to to the electrical line over time.

#4

I’d bet that beowulff has it. The two lines rubbed through each other’s insulation and the Romex hot line just happened to be on that side.

#5

This reminds me of the time that the Space Shuttle Columbia experienced a significant short circuit (STS-93). A wire ran next to a screw head. A tech had overtightened the screw and raised a tiny burr–probably invisible looking at it, but maybe noticeable if you ran a fingernail on it. The cable vibrated against the burr until it wore through the insulation and shorted to ground.

#6

Looks like the gas line and the romex were in contact with each other. If they were pulled tight vibration could have worn through the insulation. And the current flow may have been from the hot to the metal of the gas line. Or hot to ground or neutral.

#7

Looks to be faulty installation & at least a couple of electrical code violations here.

  • appears that the NM cable was just laying in that space, instead of being stapled down as required by code. That might have prevented the vibration friction that caused this.
  • also appears that it was in direct contact with a gas line, one that was grounded.

Proper way to do it would have had that NM cable properly stapled down, and NOT in direct contact with the gas line. Possibilities for that would be:

  • extra staples near the gas line, and the electrical cable looped in an arc st least several inches away from the gas line.
  • a ‘bridge’ built over the gas line, out of non-conductive materials (like some 2x4 wood scraps) and the electrical cable stapled in place over that bridge.
  • installing the part of the electrical cable that crosses the gas line inside a short section of electrical conduit (metal, plastic, or flexible). (Some inspectors dislike sheathed cable like this inside conduit, but in this case it’s better than the alternative.)

I would hope that the bill for locating & fixing this has been sent to the original electrician, or to his insurance company!

#8

Not sure what code violations you see, believe the code requires support every 4 1/2 ft. I cant tell from pics if that is the case.

How do we know that that the plumber didn’t drag his gas line over the NM cable and rub through? Perhaps he saw the wire as support for his gas line

Better send the bill to both just to be sure.

#9

I’m curious to know if the electrician took a look at the breaker that controls that circuit to see if it was properly sized.

I tried to read the gauge info on the cable in Pic-3 but I can’t make it out. If the cable is #14 a space heater and other usage on the circuit might cause it to heat up if the breaker is oversized.

Add that heat to pressure points where the cable is pulled tight against any object - in this case metal object(s) - and the plastic jacket would slowly deform away to the point that it might expose the conducting core. Once it does that it would arc through the jacket onto the metal.

Solution - replace the cable with a higher rated one and verify that the breaker is properly sized. Make sure that the breaker and cable are capable of serving the AC/heater and any other electric loads with may be placed on the circuit. The electrician that discovered the problem should already have covered this.

#10

As an aside, your coworker really dodged a bullet!
Attic fires are really destructive - they are hard to detect until they are already large, and extinguishing them often causes extensive water damage to the house.
The neighbor across the street had one, and it resulted in an almost complete re-build.

#11

That cable is sheathed in orange, which would indicate #10 (30Amp) wires. That should be sufficient for the indicated load.

#12

Yep, after I posted I went back and looked at pic 1 and could see the 600V rating and realized that it probably wasn’t some weird market color code. In previous scenarios I’ve actually seen cables with colors that lie.

That makes this situation even more interesting. Most here have passed this off as wear from vibration/movement but the fact that it appears at multiple touch points gives me the impression that the cable degraded from heat due to over-current. But with a 30 amp rating there would have to be some serious load on the circuit for that as well as a circuit breaker that was defective or oversized.

I’d love to get more info from the electrician on this one.

#13

I wonder if the electromagnetic pulse from a nearby lightning strike caused the loop of wire and pipe to develop enough voltage to arc through good unworn insulation?

As I remember from 20+ years ago, in the pipeline business it is a known problem that you can’t lay a wire in the trench with a pipe without having to worry about this phenomenon. I actually got to talk to lightning expert Martin Uman about this issue.