swcollings

The first one that comes to mind didn't end up helping the insured but it was close. It was a factory that one morning randomly had a fire in a cable tray. They had contractual obligations to maintain production so they had already ripped out all the cable before I got there. I asked them to ship me the cable, so they did. A few days later a semi backed up to my loading dock and dropped off six tons of burnt cable.

Six. Tons.

I spent the next few days going through every last piece of that cable until I found the culprit. Years before, a single run had been installed improperly. It had extra length, instead of cutting it short, the installer had left it coiled up in the cable tray. The extra heat from that was enough to damage the insulation a little bit every time it ran, until after several years the insulation finally failed entirely.

They were going to sue the installer, until they realized the installer was their own subsidiary...

24 man hours is a long time, really.

The typical example is if you have a damaged cord. One failure mode is that the conductor breaks, but can still make intermittent contact. That intermittent contact causes an arc, which (given the right circumstances) can ignite the insulation of the wire. Alternately, you can get an arc from hot to neutral or ground through damaged insulation, same deal. The combination arc fault breaker has pretty good chances of detecting those faults and tripping, where a regular or ground-fault breaker won't.

Amusingly, I once had an arc fault breaker in my house trip spuriously, repeatedly. Every time, it was during a specific moment of a specific episode of Samurai Jack. Turned out my power strip was sitting on my subwoofer, and the signal to generate the sound of gunfire was coupling into my power lines and tricking the breaker. Moved the cord, no more problems.

Follow the manufacturer's instructions, always, 100%.

Now, as ways of abusing extension cords go, there are worse ones than daisy-chaining. Daisy-chaining is more likely to lead to voltage drop rather than overheating, for example, and voltage drop on a motor load (or switching power supply) can result in more current draw and thus fire. But you'd have to chain a whole hell of a lot of cord to achieve that. I think the more likely failure mode is just by having so much exposed cable, you dramatically increase the odds of mechanical damage.

Of course, that's just straight daisy-chaining. Branching multiple high-current loads off one multi-tap could definitely start a fire, as /u/Ziazan points out.

Once I got into this field, I put arc fault breakers everywhere in my house. I don't understand how we're not all on fire, all the time.

I saw a school burn pretty hard because of one of those, yeah. I was able to identify the exact model, but for some reason the attorney didn't want to pursue against the manufacturer or importer. I still suspect that was a mistake on his part, but I'm not an attorney.

I saw another one in a small restaurant, started in the back office. Couldn't identify the manufacturer of that one, nothing to be done.