This occurred on “my job site” just last month (I was brought in to install the shower). The homeowner was doing his own demo and cut a live wire. Nothing tripped at the moment. About an hour later the lights flickered and then the circuit went dead. No physical trip of the breaker (internal?). I opened the panel to have a look, and the neutral bus bar was melted.
The electrician was a little baffled on the cause, and believes it might have been under torqued wiring. He cleaned up the wires and called it good more or less. Just curious what others experience or thoughts are on the melted wires.
Loose connections would have been my guess too.
Ok, I am learning more about the importance of properly torqued wires.
The delayed “trip” of the breaker also makes me scratch my head. Is that common/ok? Should the breaker be retired?
I will take a stab at this, and I hope a sparky will enlighten us further.
A loose connection creates heat, which is far different from overloading a circuit.
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I would be curious to know exactly what wire was cut through?
Would also be curious of the rest of the pic that was cropped off.
And now we know why acr-fault protection was invented…
And why arc fault may not have detected the above:
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Something that isn’t wired properly, probably is going to work properly…
Good find. I appreciate one of Mike Holt’s moderator explanations.
That was not a parllel arcing fault. The only type of arcing fault that the AFCI can directly detect is a parallel arcing fault. That is a fault between the grounded and ungrounded conductors. A loose connection is just that…a high resistance point where I^2R heating is produced. Some have called these series arcing faults, but as series arc is not self sustaining at dwelling unit voltages…it self extinguishes the next zero crossing. The high impedance connection just produces heat and the only way that the AFCI can detect this is when the heat melts enough insulation to create either a parellel arcing fault or a gound fault (all AFCIs have 30 to 50 mA GFP) or in this case, since the AFCI is also a standard thermal magnetic breaker, if the heat at the thermal element would reach the trip point.
Don
But considering that discussion is 18 years old, and with the advances in technology over the years, is it still relevant?
Granted, it will still apply to AFCI devices manufactured prior to 2006, but how many are really out there that haven’t already been replaced? (NEC since 1999).
Sounds like he clipped the entire romex wire (all three at once) with the breaker and switch still on. He said it didn’t “flash” (his words) and still had power to the rest of the circuit, for some time. From what I gathered, it took an hour for the lights on that circuit to act “unusual” then no power to the circuit. I entered the building the next day and said “that is odd, when ever I have cut a wire mistakenly, there is usually an arc and the breaker immediately trips.”
I don’t have more photos as I was not there on an inspection, but to lay tile.
I tried to make sense of some of the AFCI response, but honestly it lost me.
I do not know. It could be three days or 18 years old and relevant or irrelevant. 
What he said about the heat was appropriate, thus my interest.
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I read that Mike Holt claims bad connections are the number one cause of electrical fires.
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Liked that one also. The whole discussion also begs the question of how we got here. Why are the sensors in each breaker, and there’s no thermal sensor on the neutral bus in a modern panel? Span tried a clean sheet design for a panel: a failure or success is yet to be determined. But thinking of the panel as a system could result in better detecting faults without as many downsides as the present AFCI breaker solution. (One of the downsides is cost, another is vampire power)
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