Abstract: Combined intrusion detection and fire detection, or hazard detections, alarm system incorporating, for example, a plurality of intrusion, or first hazard, detection loop circuits and a plurality of fire, or second hazard, detection loop circuits, all of controlled resistance, operatively associated to a regulated voltage source. Integrated circuit operational amplifiers are operatively associated, or connected, with said detection loop circuits, or protective loops, as comparators to compare the voltage drop across said protective loops with reference voltages to provide sensing of variation in the voltage drop across the respective protective loop corresponding to a short or an open circuit condition having developed in the respective loop by detecting variance of the respective protective loop voltage drop relative to said reference voltages so that open and closed circuit detector devices may be and are connected within the same protective loop and mixed therein.