Light switch from one circuit trips GFCI on a different circuit

[fledermaus]

Hello, I just signed up with this forum in hopes of finding help is resolving an odd problem I'm having with a GFCI. A little background - the building is a new timber framed barn, with no heat or insulation, but I use it all year round as I have a wonderful workshop in the full basement below. I know, an odd situation, and cold at times, but it works.
There is a low-voltage cable ceiling light system on a three-way switch on the main floor of the barn that trips a GFCI in the basement every time one of the switches is turned on. The lights and the GFCI are on separate circuits. It seems to only happen in winter. It first happened a few years ago, but only a couple times, and then never again. Beginning today it happens every time I turn one of the switches to this ceiling light on. If I reset the GFCI it stays on, and will only trip again if I turn off the light upstairs, and then on again. There must be some ground fault occurring, but just how that happens between separate circuits is beyond my pay grade. Any ideas?

 

[Reach4]

Make sure there is no shared neutral after the GFCI. I presume you use the load terminals on the GFCI. Kill the circuit that feeds the GFCI. If you open both load terminals, that should not make a different circuit go dead. If that happens, you know that the neutral was shared with another circuit.

 

[Sponsor]

 

[fledermaus]

Make sure there is no shared neutral after the GFCI. I presume you use the load terminals on the GFCI. Kill the circuit that feeds the GFCI. If you open both load terminals, that should not make a different circuit go dead. If that happens, you know that the neutral was shared with another circuit.

Thank you for your response. Other than sharing a common neutral bar in the main panel, I can't see how else these different circuits could be sharing a common neutral. Then again, I'm obviously not an electrician. I'll conduct that test you mention. The light circuit is not affected after the GFCI trips, as I would expect since they are separate. The GFCI has three standard receptacles downstream, nothing else. That's about all I can add. Thank you!

 

[WorthFlorida]

A possibility;
A 14/3 or 12/3 are feeding two circuits that share a common neutral. At a junction box or outlet is where the two circuit may split to two different directions. This is an example but with 220v breaker. Two single breakers can be used. OR two circuits are in one junction box and the neutral wires are connected.

 

[fledermaus]

Thank you for the description and graphic. I always love learning new things! In my case, the circuits are on single pole breakers, and no junction boxes or shared device boxes where the two should meet. I know I'll figure it out eventually....

 

[WorthFlorida]

Thank you for your response. Other than sharing a common neutral bar in the main panel, I can't see how else these different circuits could be sharing a common neutral. Then again, I'm obviously not an electrician. I'll conduct that test you mention. The light circuit is not affected after the GFCI trips, as I would expect since they are separate. The GFCI has three standard receptacles downstream, nothing else. That's about all I can add. Thank you!

The GFCI has line-in, load out. The GFCI trips because the current on the load side is not equal the hot and neutral sides. When there is a difference it trips. I did this mistake With two circuits and just plugging in a refrigerator would trip the GFCI on the other circuit.

 

[fledermaus]

The GFCI has line-in, load out. The GFCI trips because the current on the load side is not equal the hot and neutral sides. When there is a difference it trips. I did this mistake With two circuits and just plugging in a refrigerator would trip the GFCI on the other circuit.

Did you ever find out why that was happening in your case? This is the part I struggle with. I understand how a GFCI functions, but I what I don't understand is how sending current through a light switch on a separate circuit would produce a ground (or neutral) fault on a different circuit. The two circuits travel to opposite sides, and levels, of the building and as such their wiring is not in proximity to each other, except coming out of the main panel perhaps. Maybe a wiring insulation issue inside the panel?

 

[WorthFlorida]

Did you ever find out why that was happening in your case? This is the part I struggle with. I understand how a GFCI functions, but I what I don't understand is how sending current through a light switch on a separate circuit would produce a ground (or neutral) fault on a different circuit. The two circuits travel to opposite sides, and levels, of the building and as such their wiring is not in proximity to each other, except coming out of the main panel perhaps. Maybe a wiring insulation issue inside the panel?

It was because I shared the neutral with a 14/3. One circuit was feeding light switch and the other an outlet for the refrigerator. I plug in the refrigerator and the GFCI would trip. I wasn’t wearing my hat that day. To fix it I placed everything on one breaker.