Circuit with GFCI/CAFGI breaker trips on a branch that turns off

Scenario: GFCI/CAFGI circuit with a branch that is set up as: breaker then switch then light 1 then light 2. Light 1 has a socket on it already. Switch is in a box that contains switches for all the other branches on this circuit.

Situation: Breaker is on. Turn off the switch (open it). There is theoretically no power to light 1 or light 2. Squeeze the 12G wire that leads to light 2 with electrical pliers but without nicking anything.

Result: The breaker trips.

Could it be a variation on that problem?

• Which kind of breaker do you have? Some have a way of telling you the cause of the trip (ie ground, arc, or over-current fault). If yours do support diagnostics, try that, and tell us what was the cause of the trip. Jan 4, 2022 at 21:31
• GFCI/CAFGI Square D breaker. The kind with the purple button. Keep in mind that it's very mysterious that there is no current in the cable we squeeze, but it manages to trip the breaker. We squeezed the cable in preparation for stripping the wires down so we could attach the light socket. Also, we did a reading with a multimetre - no current Jan 4, 2022 at 22:09
• Another point: after the wiring for this lighting branch leaves the box that has the switch, it goes through a box that is also used by a different circuit. There is absolutely no sharing of grounds or neutrals or anything like that going on. The wiring for the lighting branch is actually two lengths of cable, joined with wire nuts inside that second box. Jan 4, 2022 at 22:34
• Are you checking to make sure the box is de-energized to work in it? Do you only have to turn off one breaker, or more than one? Google "Time Saver Diagnostics Square D" to get the 411 on getting the breaker to tell you the reason for the trip. You need a stopwatch, or the ability to actually distinguish 2 seconds vs 5 seconds. Jan 4, 2022 at 22:41

1 Answer

The only thing I can see that would cause this would be if the insulation on the neutral wire is degraded so that when squeezed, the neutral conductor contacts the (presumed bare) ground conductor.

Even though the hot is interrupted by the switch, the neutral and ground conductors are still connected to the rest of the circuit. The momentary paralleling of the neutral and ground would be detected as a ground fault by the GFCI breaker.

You can verify this by disconnecting the neutral or ground to be cable and checking that this doesn’t happen.

I’d recommend replacing the cable.

• We did the test and it flipped immediately. What a great feature that is. And I think that jives with what DoxyLover says. So we will replace 1 the wire (she said, with a sinking heart) and see if that works. I'll report bback Jan 5, 2022 at 22:57