Abstract: A heat detecting conductor embodied in a flat tape wherein conductive strips in an open circuit are separated, at acceptable temperatures, by thermoplastic insulating material which is adapted to melt at predetermined elevated temperatures and to permit the conductive strips to come into contact with each other, thereby closing the electric circuit and actuating an audible and/or visual alarm. The heat detecting conductor may be associated with a cable carrying one or more service lines or with a cable connected to one or more heat detecting devices, as a back-up therefor, or may be used alone.