We just put an addition on our home to create a small office. In doing so, we added five receptacles and three light boxes to two existing circuits--one which runs in our bedroom and the other in the living room. These two circuits shared a neutral and when the electrician came to bring the "new construction" up to code, he had to put an AFCI on the circuits, since the receptacles are in living space. Given that our old box was too small for the new AFCI breaker, he had to install a second box, which houses one lonely double pole 15V AFCI breaker. All was well. We passed our inspections with no problem.
Now, two months into using the office, suddenly the AFCI has started tripping. The strange part--it does it at times when we're not putting any load on the circuits. The first couple times we noticed it, it seemed to be each evening around 7pm. Typically it was when I was in the kitchen making dinner and my husband was outside doing yard work. Neither of us were using any of the receptacles in question. We would just spontaneously lose power in the bedroom, office, and living room. Next we started noticing it around 7am. Each time we'd reset the breaker and it would be fine for another 12 hours until the next 7 rolled around.
It seemed to stop for a couple weeks, and now in the last week it has started up again--and now it's multiple times a day. 7am, 9:30am, 2pm, 7pm. Like clockwork. We don't have any appliances in the house that are on a timer. Each time it's when we're not putting load on the circuit--if anything we're in other rooms altogether.
I'm loathe to call our electrician because we did not have a good rapport with him--I had started doing the electrical work on the house before I knew the city was going to require a licensed electrician. AFCIs are admittedly beyond my area of expertise, but I can't find any evidence of arcing in the receptacles. Is there any chance that our public power company could be experiencing surges that could set off the AFCI? What am I missing?

Arc fault breakers should not be used on a shared neutral circuit. Imbalance in one circuit causes the other side to Aldo show imbalance and it trips.

SFNM,
First... where are you? If you are in NJ you are not required to add AFCI to existing circuits even when you add new receptacles or lights...
If the branch circuit is a "new" circuit that is being added, then yes, AFCI is required.
If you are not required to have it you may chose to remove it?

We're in New Mexico, where all new construction requires AFCI--so since our office is new construction, even though the circuits were pre-existing, they required us to put on AFCIs. The problem is that since they share a neutral, the AFCI had to be a double pole--otherwise they would have had to rewire both circuits entirely. We hadn't budgeted for that kind of work, considering every other bit of the reno had been done by us without a contractor. I ended up swapping the breakers back out. We'll hold onto the AFCI in case we need inspections in the future, but for now, it's too much headache to put up with. I'm still not convinced there's not surges going on from the power company setting the damn thing off since we're never in the room when it trips.

I still think it is arguable since the branch circuit was existing and all you did was to extend it
And connect additional devices whether that constitutes "a new circuit"...

I wish there was some way to communicate with the AFCI to more directly lead us to identifying why it's tripping. We may never know if there was a valid reason for the AFCI to trip the way it did...

Arc Faults will also act Like GFI's, Hot and neutral have to draw the same current or it will trip.
With 2 pole Arc Fault for a Multi circuit, thei is a slight chance of a neutral for lights get the hot from a 3way on another circuit.
ALL HOTs and NEUTRALs Have to be on their own circuit. I believe this may be the problem.
Arc Faults do have their own problems and will fail for no reason, Quality?

Strat,
Google Siemens for "Arc Fault and Neutral Sharing"... they offer two pole Arc Fault circuit breakers for this application. So they may not operate as much like GFCI's as we think...