My barn had a small (6-space) main lug sub panel with four 20A 120V breakers and one 20A 240V breaker. The panel itself was protected by a 50A 240V breaker in the main service panel. It was wired up presumably before 2001 since all of the NM wire going into the panel have white sheathing -- even the 12 gauge. It worked fine until just a few days ago.
On this day, one of the 120V circuits abruptly stopped working without tripping the breaker. I noticed that the panel felt very hot so I cut power to the entire panel until the next day, when I had time to investigate.
It was then that I discovered the carnage inside of the panel. All of the neutral wires had around an inch of melted plastic that was the wire insulation and several inches of blacked/burnt plastic going up the wire. One of the neutrals (the one for the circuit that stopped working) was loose, since it had melted right through -- the hole in the neutral bar is filled with previously molten copper and/or aluminum! The neutral bar itself is very discolored and the plastic support for the bar is thoroughly deformed due to its own melted state.
Notably, none of the circuit breakers ever tripped -- not even the one with the completely melted through neutral. They are all 20+ year old breakers so none are arc-detecting.
I have since ripped the entire panel off the wall and replaced it and all the breakers with a new panel and breakers, just in case. I also cut back the neutrals to leave only non-discolored copper and wire insulation. But what I want to know is why the previous panel failed the way it did, to ensure that something like that doesn't happen again.
One theory is that the melted neutral actually worked itself loose over the decades and at some recent time, was at the right distance from the neutral bar to start arcing to the bar. This would create tremendous heat -- presumably enough to heat up the neutral bar enough to transfer the heat to the other neutrals, causing their insulation to melt and burn... and hot enough to melt the copper conductor in the problem neutral wire itself.
But can a loose neutral wire arc quite that much, especially without ever tripping the breaker?
Is there something else obvious that might be the culprit?