I have a shop wired without GFCI outlets. I decided that was unsafe so purchased three GFCI outlets, picked the nearest outlet and wired as described. When I powered it up a huge POP occurred in the metal box. What?
The original wiring has three outlets and two pull-chain lights plus a fluorescent light. I have disconnected all of these - open wired except white wires are still connected - hmm maybe that is the problem, the failure is in the neutral somewhere. (I'll check that by disconnecting neutrals as well. I did it on several, but not on all.)
I pulled the wiring apart again and can see no evidence of damage. So I hooked up only the line side to check operation - it works. But the other side is showing only a few ohms so I disconnected all of the outlets on the line until the short went away. Then I tried to hook up the line again without the short. The GFCI refuses to work, so I tried a different one - same result. The wires should be open. Can the GFCI determine that it has a load when there is only wires?
The building was wired in 2005 probably before the new code and it has a ground rod and two sets of ground terminals, but they are connected together and the white side is connected to the main box - could that be involved somehow?
The wires ground (bare) wire and the white wires are separated throughout and passed the tester check before the change. (Can't test them now.) But I wanted to verify that the wiring had not been destroyed, so I hooked it back up as it was and it works ok now. So what gives?
Update 7/13/2018: I discovered the enemy and it is me. I had forgotten that I had added an outdoor outlet complete with GFCI a few years ago. As it happens, the GFCI was on this circuit and as I traced it back, the box for the circuit was protected by a gob of materials I had stored there.
As I mentioned in another post, I had found an additional outlet as I had continued to trace the circuit back, but with every thing disconnected, I had a short black to white on one line. I finally traced it back to a junction box and found the feed from there that was failing was one that I thought had power on it. But unfortunately, I cut a wire assuming that it was the bad (shorted) wire and was going to wire a new circuit around it when I found this junction box and then the outdoor outlet with the GFCI so now I have to fix my damage. Life lesson: don't begin to fix until you know what you are fixing. I would have been done now had I not jumped the gun and cut the wire. It had to be it, right? No, it wasn't. I cut the wire too close to the wall so that I cannot repair it so now I will have to replace the wire as I was going to do and so needlessly if I could have figured out how to find it earlier. Even now I am guessing about where the cut wire comes out - it just has to be that one right there - but what if I am wrong again? I have no way to check - the two wires are about 8 feet apart - it could have been routed anywhere. So here goes...