Just getting around to this question now...
in late August, I came home from a week's vacation to find that power to my shop's overhead door was lost. I checked the breaker, and sure enough, it was tripped. I flipped it back, but by the time I got back upstairs and outside, power again was lost (door would not open with remote). I got my wife to flip the breaker while I stood outside with the remote, and I got the door to open, but the power went out again after a few seconds.
After searching the shop for obvious signs of circuitry damage, I thought it was just a bad breaker. Changed the breaker, same thing happened. I moved the circuit to a different breaker in the panel, and the same thing happened.
So then I start disconnecting things in the shop circuit, to see what might be causing the problem. Well, after disconnecting EVERYTHING, and leaving the wire hang out of the primary box (so nothing is connected to the circuit at all), the breaker still trips immediately.
There are no obvious breaks/cuts in the wire running through the basement to the foundation, but then it goes underground to the shop (about a 25' run). I do not know if it is in conduit, but it is heavy grey cable (direct burial, right?). I thought I could just run a new line out there, but I can't get the current one to budge...so I bet it was not run in conduit.
What could have happened here? Lightning? Something chew through the wire? Why would the problem be intermittent at first, then become worse? Remember at first I had about 20 seconds before the breaker would trip...now it trips immediately.
Luckily there is a separate circuit running to the shop that I am able to plug my overhead door opener into...but the circuit that died runs all the other outlets, so it's very inconvenient.
Any ideas? Am I screwed? The cable is buried under a concrete slab all the way to the shop...
Thanks for your help.