I recently installed some AFCI/GFCI circuit breakers. One of them kept tripping immediately, so I tracked down the problem to where a neutral and ground were joined. (There was just the one neutral in that box.) To confirm that this was the problem, I disconnected the neutral from the ground and ran a neutral wire to another nearby box in the same circuit and connected it to neutral there. Problem solved!
It looks like the above. The outlet (O) on the left is fed into from earlier in the circuit. It connects to the right, which connects up to a 3-way switch (S3), which connects left to the other 3-way switch, which then feeds a regular switch (S) to a light (L) (independent of the 3-ways). S3 on the right has an isolated neutral from O that is capped off. S3 on the left therefore has no neutral to contribute to the S and L part, so they just tied the neutral there to the ground, which was my problem.
To test this, I ran a neutral between the left 3-way switch and outlet, joining their neutrals. That's what worked. But when I ran the 12-2, I also joined their grounds, and that caused the AFCI/GFCI to trip immediately when I turned it on. (Actually, with a half-second delay -- it tripped immediately if there was any load on the circuit.) And if I disconnect those grounds it works again.
So there's my question: why would having an extra ground there be the difference between AFCI/GFCI tripping or not tripping?
The bottom receptacles are all on that circuit, too, but always hot (not switched).
It's all working, so this isn't urgent, but I'm baffled and would like to know what's going on!