I wonder if anyone can help me
I want to fit a mains smoke alarm with battery back up in my house
and wonder if it can be feed from the lighting circuit, and if so can i take another smoke alarm looped from the first one.
Yes & yes,
find perm. live & neutral at nearest light, use twin/earth to first alarm, loop between alarms using 3core /earth.
this supplies power to second alarm & third wire is trigger so whichever alarm goes off they both(all) sound, together.