Had my ELECSA assessment yesterday and passed. The assessor mentioned it was preferable to put the smoke alarms on a lighting circuit. Thing is, in the building regs Document B 1.19 about power supplies(in brief) it says can be supplied either direct from the board or from a lighting circuit, but there should be a means of isolating power to the smoke alarms without isolating the lighting. What's the difference? The smoke alarms could still get left switched off :-o Although I agree you would notice if a fault made the lights go out. I'm thinking I should supply one light (such as the hall) from the RCBO for the smoke alarms and in addition to labelling up the board correctly, stick a note on the board warning that the hall light is seperate from the rest of downstairs lights.