I'm in two minds about this. I have installed 2 smoke sets recently, one off the lighting circuit and one off a spare way in the CU. My thoughts are that the separate MCB circuit is better...if you think the green light won't show and that would alert you, yes..but frankly in rental properties I don't think you can rely on that. Tenants will notice if the lights are off, but not if the separate circuit with the smokes is dead.. However, I do feel that on balance it is more likely that a dedicated lighting circuit may blow when a lamp breaks down...these circuits are more "volatile"...so, with, say hall lights left on overnight, a lamp failure might kill the whole circuit, whereas it is much less likely for a smoke to trip if it is on its own circuit, as there is no other draw, and the "volatility" is therefore much lower. I would think that a lighting circuit with, say, 6 lamps, would be much more prone to blow than a dedicated smokes circuit with say 3 alarms drawing next to nothing...and as I only install smokes with 10 year lithium batteries, there is no real risk of being without cover even if the mains fails.
End result, if you install 10 year Li units, with radio-interconnect, it doesn't really matter which method you use, but on either, you will get an emergency warning tone from the alarms when the battery is getting low, regardless of the mains failure. However, in contrast, if your smokes do not have the battery backup, the less volatile installation is maybe better...off a spare mcb...less prone to tripping when a lamp fails...which will happen.
