A tripper unit for actuating a device (e.g. valve-closing actuator) upon contact with water. Basically, the tripper unit is comprised of a housing having a replaceable cartridge therein which, in turn, is wrapped with an adsorbent paper which prevents a plunger from moving downward. When a leak occurs, water will flow into the housing and into contact with the absorbent paper which will quickly disintegrate to allow a compression spring in said housing to move the plunger downward to thereby actuate the unit.