An alarm device 1 comprises a base (2) which is secured to a ceiling using screw fasteners (11). A mains cable (12) is trained in through the base (2) at an aperture (19) and electrical connections are made to a terminal block (14). An alarm body (3) is then electrically and mechanically connected to the base (2) by simply sliding it in the plane of the base (2) until electrical and mechanical connections are automatically made. The alarm body (3) may be separated to insert a back-up battery or for maintenance by initially releasing a safety catch using a screwdriver and pushing in the opposite direction.