Abstract: A plug-in sensing unit includes a detector base and a detector head constructed and arranged for mounting on the detector base, with contacts of the detector head engaging terminals of the detector base when the detector head is mounted on the detector base for extending power to a sensor housed within the detector head. A cam on the detector head moves a shorting spring to interconnect a pair of terminals on the detector base to complete a circuit path indicative of the presence of a detector head in the detector base, a further cam on the detector head cooperates with a cam on the detector base to enable the contact spring to restore to its rest position when the detector head is removed from the detector base. The shorting spring can be moved to its operating position manually during testing of the loop in which the detector base is connected, the contact spring being restored to its rest position automatically during the initial stage of mounting of a detector head on the detector base.