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 17-4.  Kidde continuous-loop system. Combination Fire and Overheat Warning 17-5 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, such as would be caused by leakage of hot  bleed air or combustion gas into the engine compartment. It  could also be an early warning of fire and would alert the  crew to appropriate action to reduce the engine compartment  temperature. The second-level response is at a level above  that attainable by a leaking hot gas and is the fire warning. The resistance of a sensor also varies inversely with its  length, the increments of length being resistances in parallel.  The heating of a short length of sensor out of a given length requires that the short length be heated above the temperature  alarm point, so the total resistance of the sensor decreases to  the alarm point. This characteristic permits integration of all  temperatures throughout the length of the installation rather  than sensing only the highest local temperature. The two  wires encased within the thermistic material of each inconel  tube form a variable resistance network between themselves,  between the detector wire and the inconel tube, and between  each adjacent incremental length of sensor. These variable  resistance networks are monitored by the application of  28 volts direct current (DC) to the detector wire from the  detector control unit. The analog signal produced by the sensing element loop  as its temperature changes is converted to signals suitable  for flight deck display to 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 integrity of the continuous-loop fire detection system  may be tested by actuating a test switch in the flight deck  that switches one end of the sensing element loop from its  control circui t to a test circuit built into the control unit,  which simula tes the sensing element resistance change  due to fire.  [F igure 17-5]  If the sensing element loop is  unbroken, the resistance detected by the control circuit is  that of the simulated fire, and the alarm is activated. The  test demonstrates, in addition to the continuity of the sensing  element loop, the integrity of the alarm indicator circuit and  the proper functioning of the control circuits. The thermistic  properties of the sensing element remain unchanged for  the life of the element (no irreversible changes take place  when heated); the element functions properly as long as it is  electrically connected to the control unit.  System Test Temperature Trend Indication