As part of my basement finish, I need to replace a breaker with a AFCI.
This circuit currently services the new bedroom (outlets already there) and an outlet outside. One of the outlets is already GFCI but I decided to use GFCI and AFCI at the breaker.
I bought a GFCI/AFCI combo and replaced the old one. When I was replacing it, I discovered the cable coming in was 14/3 style, with the red and black wires each on their own breaker (call them breaker 1 and 2). I replaced breaker 1 with the new unit. So now the red is on breaker 1 (the AF/GFCI) the black is still on breaker 2, the white wire is connected to the AF/GFCI , and the curly pigtail from the new unit is connected to the neutral bar, as the instructions state.
With both breakers 1 and 2 off, if I turn on breaker 1 (the new one), everything seems to work. When I then turn on breaker 2, breaker 1 trips. The LED for it indicates the following from the instructions: "Ground fault/grounded neutral. Current has found an alternate path to ground, or the neutral and ground are in contact downstream of the breaker."
I replaced 2 others in the box with no issues, but these don't have GFCI outlets downstream, and they don't have a 14/3 cable. I plan to replace the GFCI outlet to a normal one next, but my hunch is that these 2 circuits sharing a neutral is the cause.
Does anything jump out to anybody about this setup? How would I resolve it? Do both of those circuits need to have AFCI/GFCI breakers?