TM 1-1500-204-23-1 • Rate-of-temperature rise detectors • Radiation sensing detectors • Smoke detectors • Overheat detectors • Carbon monoxide detectors • Combustible mixture detectors • Fiber-optic detectors • Flame detectors • Observation by crewmembers b. Types    of    Fire    Detection    Systems.            A    fire detection  system  should  signal  the  presence  of  a  fire. Units of the system are installed in locations where there are greater possibilities of a fire Three detector systems in common use are the thermal switch system, thermocouple  system,  and  the  continuous-loop  detector system. (1) Thermal switch system.  The thermal switch system    uses    a    bimetallic    thermostat    switch    or    spot detector   similar   to   that   shown   in   figure   9-32.      Each detector unit consists of a bimetallic thermoswitch. These thermal  switches  are  heat-sensitive  units  that  complete electrical    circuits    at    a    certain    temperature.        If    the temperature rises above a set value in any one section of the  circuit,  the  thermal  switch  will  close,  completing  the light circuit to indicate the presence of a fire or overheat condition Figure 9-32.  Thermal Switch (2) Thermocouple system. The thermocouple is constructed of two dissimilar metals such as chrome and constantan.  The point where these metals are  joined  and  will  be  exposed  to  the  heat  of  a  fire  is called  a  hot  junction.    There  is  also  a  reference  junction enclosed   in   a   dead   air   space   between   two   insulation blocks.  A metal cage surrounds the thermocouple to give mechanical protection without hindering the free movement of air to the hot junction.  If there is a fire, the hot   junction   will   heat   more   rapidly   than   the   reference junction.    This  will  complete  a  circuit  from  the  aircraft power  system  to  the  coil  of  the  slave  relay  which  closes and completes the circuit to the fire-warning light (3) Continuous-loop system  .    A  continuous  loop detector    or    sensing    system    permits    more    complete coverage of a fire hazard area than any type of spot-type temperature   detectors.      Continuous-loop   systems   are versions of the thermal switch system.  They are overheat systems,    heat-sensitive    units    that    complete    electrical circuits at a certain temperature.  There is no rate-of-heat- rise  sensitivity  in  a  continuous-loop  system.    Two  widely used  types  of  continuous-loop  system  are  the  Kidde  and the Fenwal systems. (a)    Kidde  system.    The  Kidde  continuous-loop system, as shown in figure 9-33, has two wires Imbedded In a special ceramic core within an  Inconel tube.  One of the two wires In the Kidde sensing system is welded to the case  at  each  end  and  acts  as  an  internal  ground  The second  wire  is  a  hot  lead  (above  ground  potential)  that provides a current signal when the ceramic core material changes its resistance with a change in temperature. Figure 9-33.  Kidde Sensing Element 9-34