I replaced an old two-prong receptable with a three-prong GFCI (no ground) yesterday. It won't reset. I found out that "downstream" there is a light on the next level down that is controlled by two switches, one at the top of the stairs, one at the bottom. The original receptable is fine (resets, green) as long as that light is not turned on. As soon as it is turned on, the new GFCI receptacle trips.
Now for the weird part. I open the light and it has two cables coming in, each with a black, white and pink wire. I assume (not a great idea) these are one cable each from the two switches, and that based on the color they are hot(black), neutral(white) and switched hot (pink). I would have assumed the light fixture's bulb holders would be wired with the switched hots (combined together) as hot (to the bulb holders' black wires) and the white neutrals should be combined and attached to the white of the bulb holders, but they're not. Instead the neutrals (really I should say "white", 'cause who knows what they are) from one cable is fed to the hot of the light holders, and the neutral of the other cable is attached as neutral.
Does this make sense? I just bought the house, and the light and switches worked until I installed the GFCI, and the GFCI I just installed doesn't work either.
I would have thought that the pink switched hots should be combined and attached to the hot (black of the bulb holders) Of course, if it were that simply, why was it not that way already?
Edit2: When I took the switches out of the boxes so that I could take pictures, not disconnecting anything, the upstream GFCI would not reset, even with the switches open. When I put them back again, it would.
Again, it all worked before I changed the two prong outlet upstream with the GFCI. That is the only change I made.
When I looked in the 2nd switch box a second time I noticed there were some bare wires, maybe grounds I had not seen before.
Is it possible all this is about a ground or a neutral? If so, how do I track it down?
Add pics of the "unrelated" switch, per request. See below.
Bottomline, if I can't figure this out shortly, I'm in trouble with the wife, so I bought another standard, non-GFCI receptable so shortly I'm going to install it. I don't have a ground, but the box is metal and the cable is armored, so I think the receptacle is grounded. I gather I can test by seeing (carefully!) that I get 120V AC across the hot and the box? And if I don't, it's not grounded?