Odd electrical problem

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.

Thing we need to know:

1. Is the breaker a GFI circuit?
2. Take off the white & black both ends and ohm the wires-- white to ground, black to ground and white to black.

1. not sure what you mean here. The first outlet in the circuit is a GFI outlet, but the breaker trips without this even being hooked up. The breaker itself is a regular 20A style...nothing special about it.

2. I did this a few weeks ago (all wires diconnected from power):

Test 1. Touch VOM leads together (obvious first test for continuity) = "0" reading. That's good.

Test 2. Touch one lead to black, and one to white = "1.86" reading.

Test 3. Touch one lead to black, and one to ground = "1.35" reading.

I didn't test white to ground.

Thanks

Any work happen around the house? someone hang a picture? new siding? Could be a nail or screw piercing the cable in the wall. Could have happened several years ago and finally got bad enough to short. Could be a cable clamp too tight where the cable enters a box.

That's the thing! Nothing happened...we were on vacation and came home to find this.

The cable run goes from the box (in the basement), along the floor joists, throug a hole drilled in the poured concrete foundation. From there, it travels under a large concrete pad to the shop. It comes up through the pad in the shop, through the bottom wall sill, and up the wall to the first outlet, which is a GFI. So I can see the entire run that isn't underground...no damage. Plus, even with everything on the circuit disconnected, and the wires hanging out of that first box, the breaker trips. It's gotta be underground...right?

Ugh...

Sounds like it's in the conduit....from shop to house...you got mice. They have been known to eat anything.

If it's in conduit, pull it and inspect it. Pull a rope in to pull the cable back. If it is not in conduit then suspect that a rock may have pierced the cable. It just happens over time if the rock was pushing against cable.