Yesterday one of my co-workers got a call from the trouble alert center. (We call it the P.O.C.C. I can't remember what the initials stand for). There was a power outage at a remote site. The power had come back on but the generator was still running. He went out there, found the commercial power was on, and forced the generator to shut off.

But, on the way out of the site he noticed a hole in a conduit cover. Maybe it was a bullet hole -- stupid things happen at remote sites. He got out his voltmeter and stuck the probes in the hole. I am pretty sure I would not have done that, but I might have considered removing the cover, which might have been even worse. What happened was what we call an Arc Flash -- sparks shot out so energetically the air ignited. Burned the hair off the back of his hands and temporarily blinded him.

Today we went out there (three of us) along with a power company guy. After replacing the power lines involved we determined the following: It wasn't a bullet hole. The power cable -- as thick as a man's thumb, and 50's vintage -- had a worn spot in the rubber insulation. It had been touching the side of the conduit case, and the cable must have moved enough due to wind swaying the building that it wore a little hole in the insulation. This is the main power line into the site, and there was enough amperage to burn right through a steel conduit cover. That's the top of a ladder it's on, so you can see the cover is about the size of a man's hand.

I was impressed. I'd heard about arc flashes, but this is the first result of one that I've seen. This was a small one, even. Big ones can simulate an explosion. Here's a 480 volt one on YouTube:
http://www.youtube.com/watch?v=-iClXrd50Z8