Thermistor core Inconel tube No. 10 screw terminal Electrical conductors Sheath Airplane wiring (ref) No. 8 screw terminal Retaining nut End bracket Support tube Bushing Element loop 2 Element loop 1 Quick release clamp Thermistor Wires Figure 9-5.  Kidde continuous-loop system. than the full length of element, which requires that portion to  be heated to a higher temperature to achieve the same total  resistance change. As a result, the system responds not to a  fixed alarm temperature but to the sum of the resistances (in  parallel) that reflects a nonarithmetic “average.” The sensing  element may be routed close to nonhazardous hot spots that  may have a normal temperature well above the overall alarm temperature, without danger of causing a false alarm. This  feature permits the alarm point to be set close to the maximum  general ambient temperature, giving greater sensitivity to a  general overheat or fire without being subject to false alarms  from localized nonhazardous hot spots. indicate engine bay temperature increases from normal.  A comparison of the readings from each loop system also  provides a check on the condition of the fire detection system,  because the two loops should normally read alike. The analog signal from the thermistor sensing element permits  the control circuits to be arranged to give a two-level response  from the same sensing element loop. The first is an overheat  warning at a temperature level below the fire warning,  indicating a general engine compartment temperature  rise, which could be caused by leakage of hot bleed air or  combustion gas into the engine compartment. It could be an  early warning of fire and would alert the crew to appropriate  action to reduce the engine compartment temperature. The  second-level response would be at a level above that attainable  by the leaking hot gas and would be the fire warning. The analog signal produced by the sensing element loop as  its temperature changes can readily be converted to signals  suitable for meter or cathode ray tube (CRT) display to  The integrity of the continuous-loop fire detection system  may be tested by actuating a test switch in the flight deck,  which switches one end of the sensing element loop from  its control circuit to a test circuit, built into the control unit,  that simulates the sensing element resistance change due to  fire.  [Figure 9-6]  If the sensing element loop is unbroken,  the resistance detected “seen” by the control circuit is now  that of the simulated fire and so the alarm is signaled. This  demonstrates, in addition to the continuity of the sensing  Monitor circuit Test circuit Test switch Sensing element loop Control unit Figure 9-6.  Continuous-loop fire detection system test circuit. 9-6 Temperature Trend Indication Combination Fire and Overheat Warning System Test