The Kidde i12040CA is a 120 VAC, wire-in smoke alarm with 9 V battery backup that can function as a stand alone unit or interconnect with up to 24 Kidde devices (of which 18 can be initiating). The Kidde i12040CA provides continuous protection against smoke and fire hazards, even during power outages. This smoke alarm uses ionization sensing technology. Ionization sensing alarms may detect invisible fire particles (associated with fast flaming fires) sooner than photoelectric alarms. Photoelectric sensing alarms may detect visible fire particles (associated with slow smouldering fires) sooner than ionization alarms. May detect invisible fire particles (associated with flaming fires) sooner than photoelectric alarms 120 V Interconnectable - will interconnect with up to 24 other devices (18 of which can be initiating) 9 V battery backup (included) - provides continuous protection even during power outages Hush® button - temporarily silences nuisance alarms for approximately 8 min Simultaneously tests the unit's electronics and verifies alarm operation