Last weekend I decided to add GFCI protection to a couple of bathrooms. The house is 1976 vintage so many current NEC rules are not followed, such as to provide separate circuits for outlets in each bathroom. In this house, the outlet in two baths are on the same branch circuit, and unfortunately, so are many other outlets.
My intended approach was to use one GFCI receptacle wired at the first outlet of the branch circuit to protect all other outlets down stream. I found the first outlet for the two baths in the bedroom next to the first bath.
After carefully making sure all power was off at the outlet and studying the wiring to make sure to make the correct connections I proceded to complete the installation. When I was sure everything was correct, I closed the circuit breaker and returned to the GFCI to run the test. The GFCI would not reset to normal. I'd push the RESET and it would immediately trip, acting as though a ground fault existed.
My first thought was a mistake with my wiring. This may be a typical reaction for an engineer
I checked the wiring and everything looked OK. So, it was time for some deeper trouble shooting.
I soon found that the neutral on the load side of the GFCI wiring was grounded. Knowing that the neutral should not be grounded except in the sevice entrance panelboard, I knew what the problem was. I spent several hours trying to located where the neutral was being grounded and never found it.
To achieve the needed ground fault protection, separate GFCI receptacles were installed in each outlet needing the protection. This approach works fine.
I've been trying to figure how the neutral down stream of the first outlet is grounded. There are many possibilities. The most likely one I think is that the neutral wires from two different branch circuits are incorrectly connected together in an outlet box (one I did not open most likely.) When these two neutrals are connected, even though one of the neutrals is disconnected in the first outlet, the neutral returns to the neutral bus in the panelboard through the connection with the other circuit's neutral.
This situation is probably not serious, do you think?