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I had a sub-panel with a loose neutral wire that caused substantial arcing between the wire and bus bar. Arcing generates a staggering amount of heat (thousands of degrees in either F or C) and in this case, it literally melted one of the wires clean through! See here. This is obviously very bad and it resulted in me replacing the entire panel and everything in it.

But electrical arcs are typically feared not because they result in having to replace panels and breakers and such but because they can (and do) cause fires. That makes complete sense for arcing in outlet boxes or inside walls or through attics and the like. The arc results in temperatures far beyond what is needed for wood to ignite and so fire is almost inevitable in some cases.

What I don't understand is how electrical arcs can cause a fire in an electrical panel.

The panel is steel and everything inside it will either be steel, copper, aluminum, or some kind of plastic that melts or chars but doesn't burn. In fact, I can't think of anything in a panel that even could catch fire.

Is it because the panel itself gets hot enough to ignite the wood framing touching it? In my case, the panel certainly was hot to touch -- but not so hot that I couldn't keep my hand on it. That is, not even remotely close to the ignition temperature of any building material I can think of.

What am I missing? How does an arc inside a steel panel cause any sort of fire?

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    So you managed to catch your arc before it started a fire - good thing! Did you just let it keep going? What if you had been on vacation for a week? Panels should contain some energy release for some time, much better than wires in the wall. They are not perfect though.
    – Jon Custer
    Nov 30, 2023 at 13:18
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    I’m voting to close this question because it's about physics and not home improvement.
    – isherwood
    Nov 30, 2023 at 13:48
  • @JonCuster - I did find the arc relatively quickly but what if I hadn't? That's the crux of this question. Say it was arcing for a week. How would that start a fire outside of that panel? Nov 30, 2023 at 21:53
  • Steel doesn’t melt until about 2500F, wood easily ignites by 700F. So, yes, the wood studs supporting the panel can ignite even though the arc is contained within the panel.
    – Jon Custer
    Nov 30, 2023 at 21:58
  • Also , depending on what insulation is on the wires, they can melt or catch fire. Rats could have built a nest there while you were away and it could catch fire. The possibilities are endless. Nov 30, 2023 at 22:51

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or some kind of plastic that melts or chars but doesn't burn.

plastic burns

just look at all the pictures of burned plastic: https://www.electrical-forensics.com/CircuitBreakers/CircuitBreakers.html

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  • That was a fascinating page, thank you. I don't see evidence that the plastic in those pictures actually burned/combusted (creating heat), though -- it looks like it melted (reacting to heat). One of those cases did describe how it could work, where the arc went through the panel and siding and ignited the wood framing inside! Nov 30, 2023 at 16:13

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