crispy cable! Why wouldn't a breaker trip?

morganhammer

I lately encountered a melted cable in a new construction. And can't really figure out why. Hoping ya'll got some idears. Story goes like this.
A friend of mine had three different people work on her new house new electrical installation. After walls we're covered one of those guys was trying to decipher the others labeling system or lack thereof. While fiddling around in a switch box he head a popping sound, smelled burning and purportedly ran to the panel and flipped off the breaker, as it hadn't tripped automatically. He didn't say anything about it until somebody else noticed some crispy cable. This cable was a 14/3.
New cable was run to the same switch box. During this time the licensed electrician on the job was fired. Not for this incident but rather for being an uncooperative jerk. Thus information from him is not available. While trouble shooting the rest of the circuits an AFCI wasn't holding and it was decided to uncover the joist bay above the panel. It was found that the original 14/3 that fried had melted through the sheath and almost started a fire along a five-foot section of this run. A scary looking situation with charred joists and blackened fiberglass. This blackened section had four more cables run along side of it. The remainder of the fired 14/3 was inspected and found to have not damaged any other adjacent cables.
At this point the damaged sections of the wiring have been repaired and all runs have been accounted for, hopefully. The question remains, why didn't the breaker trip? Other then a faulty breaker I have one other hypothesis. Perhaps while this guy was testing where cables we're run with the black wire live he shorted over to the red wire that was run in the same cable on the other leg of the 240v. This guy is adamant that the red wire was not connected to anything at the panel during his testing. If he is correct perhaps he had inadvertently let it rest against one of the bus bars (no comment). In this hypothesis each hot lead is on opposing legs of the panel. Would even this scenario result in a severely burned up cable without tripping the breaker?
Sorry for the long story. Any thoughts?

McSteve

Assuming the cable in question was on an AFCI breaker, I like to think that it would have tripped, but who knows?

If a cable develops a fault due to being nicked, pinched, stapled through, whatever, that fault can have a high enough resistance not to trip the breaker. It might, for instance, only be drawing an amp or two, much less than it would take to trip the breaker. It will still generate a good amount of heat at the fault, though, which will burn up the cable.

If a wire gets broken in the cable, it could also develop a series arcing condition, where current through the circuit is actually having to jump a small gap along its path. This would also create some heat, while the circuit may seem to be operating normally. Since there'd be no more load on the circuit than whatever was plugged into it, a breaker wouldn't trip. This is one of the conditions that an AFCI breaker is designed to detect.

So which type of breaker was used on this circuit? Regular, or AFCI?

AllanJ

The breaker might be defective, not tripping when it should.

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screwy

Was the breaker sized correctly for the wire?

morganhammer

Thanks guys.
In regard to your questions, these are the assumptions I have made.
The guy that was working on the wiring at the time of the event isn't entirely dumb. So lets say the right sized 15amp breaker was used.
The panel was not yet assembled so i doubt the expensive AFCI breakers were in place. I was told it was mostly empty.
And about the conditions McSteve opines over, namely a damaged wire, either the sheathing or wire inside the sheathing. Do consider all these runs were brand new and were not powered except during the timeframe while he was testing around. So there was not that long a period of being powered on. And the part that screams out at me, was that while fiddling in the junction box, he heard a pop and smelled the smoke. It has the earmarks of a short of some sort. No? I know it could be a faulty breaker, but that seems an easy explanation that has never been the case when proposed in the past.