I have been sent out to a few of these and they can be quite the puzzle, one I stumbled upon some kind of inductive loop happening on the 14/3 that interconnected the alarms.
The HO was running a plug in electric space heater in the kids bedroom at night that happened to be on the same 15 amp circuit, when that bugger cycled on/off it would induce a few volts on that red wire, as the white/black were also supplying power to the bedroom.
In that case I isolated the 14/3 to its own circuit, not an easy task in a lived 'in' house.
I agree with this:
Take one apart, they are crude crap electronics, they have no input voltage filtering either so its very possible that small spikes in power or even harmonics can damage them too. I have designed several voltage filtering blocks in the past, and I can tell you, even low power stuff, the size of the filtering block was about half the footprint of existing fire alarm ckt board.
I remember when I first got in the trades in the early 90's they had small versions of those 24V commercial fire alarm panels you could put in a residence. They were pretty common, I remember pulling and fishing red 18/4 fire alarm cable for thoses as a 'grunt'. I have not seen one since I moved out west.
If I can find a system like that I'd switch us to it.
