Refer to figure.
CONTINUOUS LOOP FIRE DETECTION SYSTEM
The Continuous Fire Detector is a thermal detector – it monitors temperature or heat and responds with a signal whenever the temperature exceeds a pre-determined limit. The sensing element is a thermistor device. It is a small diameter tube filled with a thermistor material in which is embedded a wire running the full length. The thermistor changes electrical resistance with temperature – as the temperature goes up, the resistance goes down; and the current flow/leakage will increase along the loop. The resistance is measured between the wire and the tube by means of an electrical connector at each end. After the temperature drops (fire is extinguished) the wire will stop conducting and the system is reset and can be used again.

In today’s transport, dual loop systems are generally employed to engine fire zones, both loops must be triggered to activate the fire warning. The system is AC supplied and has the ability to continue functioning with a single wire break. If the two loops of an engine fire detection system detect a low resistance and a low capacity, the loops are faulty.