1.17 The power supply for a smoke alarm system should be derived from the dwelling’s mains electricity supply................If alarm comprises no stand by power supply, [ie AC ONLY] no other electrical equipment should be connected to this circuit. [ie AC only alarms, must only be wired back to main consumer unit on dedicated circuit].
1.18 A smoke alarm, or smoke alarm system, that includes a standby power supply or supplies, [ie AC with battery back up], can operate during mains failure. It can therefore be connected to a regularly-used local lighting circuit. ...............