When I had a house, I tapped off of a cold water line, to the center of my room's ceiling, where I installed a single fire sprinkler head. I also ran arc fault breakers on every circuit I grew with. I never had a false trip in 6 years of using them.
I had one close call years ago, when a timer failed. Lights stayed on, light hood cooling fan shut off... That could have been a very bad day. Hoods were blisteringly hot, and the room was 102f when I caught it, and it had only been 2 hours that way. Moral of the story, lights and light cooling always on same timer, and breaker!
When I ran a sealed 4x4x8 co2 cab in my garage, I used 2 "flame defenders" mounted on the wall.
I never, ever used cheap plastic powerstrips for more than a couple hundred watts, and never loaded breakers more than 50%.
Fire concern keeps me awake at night, and it should do that to more growers, in my opinion.
Even in a legal state, I wouldn't count on insurance paying a penny, once a grow is found at a fire scene. Not to mention the obvious chance of dying, or killing your family, a fireman, etc...
Oh yeah. Smoke detectors. Lots of 'em.
Regards, Kyle