I just needed a reminder...
Before I started playing with wires and mapping things out, I did what I usually do when confronted by a task I'm not eager to do and stalled. I had found a few links on the web, buried in the 5th or 6th pages of results on Google that had some similar issues to what I was experiencing.
The first - - http://www.forum.digitalaquatics.com/viewtopic.php?f=4&t=8119
. Mentions breakers tripping and a host of other issues caused by line noise from metal halide lights, what I installed and seemed to cause the problem. I tried putting a line filter between the lights and the circuit, but no dice...problem persists.
The second - http://forums.finehomebuilding.com/breaktime/energy-heating-insulation/afci-tripping-load-other-circuit
is actually more relevant. It mentions the exact problem I'm having and if you read further on, you'll see advice similar to what nealtw provided.
The second thread made me curious. It describes afci circuits being tripped by loads on other circuits, and then at some point its suggested that maybe the issue is related to specific types of circuits. So before I started mapping things out, I figured I'd take my outdoor extension cord and do some tests. Basically I wanted to narrow down the different circuits within the house that would cause the trip AND see if there was any correlation between circuit types and the ability to cause a trip. So I went through the house, plugging the lights into other circuits and seeing if they could be tripped. I even would run a vacuum cleaner on neighboring circuits, as along with ceiling fan motors it seems to be the most commonly cited source of AFCI trips.
No matter what circuit I tried, I couldn't get the breaker to trip with the lights plugged in. This would seem to confirm the issue is with the original circuit. By this time it was late in the evening, and I decided I'd start mapping things out in the morning when I was fresh and had natural light to see. Rather than face my wife's wrath, I plugged the lights back into the original circuit so the cord clutter would be hidden. When I called up to her to let her know the test was done, she thought I was asking her to flip the ceiling fan upstairs on and so she did it. Nothing happened, no trip.
I was baffled. I went upstairs and did it myself. No trip. I went downstairs and turned the ceiling fan in the dining room on, no trip. I turned them both on. No trip. We turned them both on at the same time. No trip.
The lights are plugged into the same outlet, via the same cords and same spot in the surge protector. Nothing has changed, but I haven't had a trip in the last 5 weeks. I even have a single halide light that I occasionally run on a quarantine tank (don't ask, those of us with aquariums never have just 1 or 2), on the same circuit. It doesn't cause a problem either, even when turned on at close to the same time.
I delayed posting the findings because I expected this to be a brief respite, but the condition no longer exists. Then I forgot until the reminder that a new post was in this thread.