TM 1-1500-204-23-1 (b) Fenwal    system.        The    Fenwal system,   as   shown   in   figure   9-34,   uses   a   single   wire surrounded by a continuous string of ceramic beads in an Inconel   tube.      The   beads   in   the   Fenwal   detector   are wetted    with    a    eutectic    salt    which    possesses    the characteristic of suddenly lowering its electrical resistance as the sensing element reaches its alarm temperature. c. Maintenance of Fire Detection Systems.    Inspect fire detection systems for the following: • Cracked or broken sections caused by crushing or   squeezing   between   inspection   plates,   cow panels, or engine components. • Abrasion   caused   by   rubbing   of   element   on cowling, accessories, or structural members • Pieces  of  safety  wire  or  other  metal  particles which may short the spot detector terminals. • Condition   of   rubber   grommets   in   mounting clamps, which may be softened from exposure to oils, or hardened from excessive heat • Dents and kinks In sensing element sections Figure 9-34.  Fenwal Sensing Element NOTE Limits     on     the     element     diameter, acceptable dents or kinks, and degree    of    smoothness    of    tubing contour are specified by manufacturers.      No   attempt   should be made to straighten any acceptable dent  or  kink,  since  stresses  may  be set    up    that    could    cause    tubing failure. • Loose  nuts  or  broken  safety  wire  at  the  end  of the sensing elements NOTE Loose  nuts  should  be  re-torqued  to the value specified in the manufacturer's   instructions.      Some types of sensing element connections require the use of copper crush gaskets. These gaskets should be replaced any time a connection is separated • Broken or frayed flexible leads, if used. NOTE The   flexible   leads   is   made   up   of many fine metal strands woven into a protective  covering  surrounding  the inner   insulated   wire.      Continuous bending     of     the     cable     or     rough treatment can break these fine wires, especially those near the connectors Broken   strands   can   also   protrude into  the  insulated  gasket  and  short the center electrode. • Proper sensing element routing and clamping. NOTE Long     unsupported     sections     may permit excessive vibration which can cause breakage. The distance between  clamps  on  straight  runs  is usually  about  8  to  10  inches,  and  is specified  by  each  manufacturer.    At end    connectors,    the    first    support clamp is usually located about 4 to 6 inches     from     the     end     connector fittings.  In most cases, a straight run of   1   inches   is   maintained   from   all connectors  before  a  bend  is  started, and   an   optimum   bend   radius   of   3 inches is normally adhered to 9-35