Heat-sensing element Bracket Clamp hinge Grommet Clamp screw Figure 17-28.  Inspection of fire detector loop clamp.  Fire Extinguisher System Maintenance Container Pressure Check Fire Detection System Troubleshooting Discharge Cartridges 17-20 Figure 17-29.  Fire extinguisher container pressure-temperature  chart. 5.  Failure to obtain an alarm signal when the test switch  is actuated may be caused by a defective test switch or  control unit, the lack of electrical power, inoperative  4.  Moisture in the detection system seldom causes a  false fire alarm. If,  however, moisture does cause an  alarm, the warning persists until the contamination is  removed, or boils away, and the resistance of the loop  returns to its normal value. 3.  Kinks and sharp bends  in the sensing element can  cause an internal wire to short intermittently to the  outer tubing. The fault can be located by checking the  sensing element with an ohm meter while tapping the  element in the suspected areas to produce the short. 2.  Fire alarms and warning lights can occur when no  engine fire or overheat  condition exists. Such false  alarms can be most easily located by disconnecting  the engine sensing loop connections from the control  unit. If the false alarm ceases when the engine sensing  loop is disconnected, the fault is in the disconnected  sensing loop, which should be examined for areas  that have been bent into  contact with hot parts of the  engine. If no bent element can be found, the shorted  section can be located  by isolating the connecting  elements consecutively around the entire loop. 1.  Intermittent alarms are most often caused by an  intermittent short in the  detector system wiring. Such  shorts may be caused by a  loose wire that occasionally  touches a nearby terminal, a frayed wire brushing  against a structure or a sensing element rubbing  against a structural member long enough to wear  through the insulation.  Intermittent faults often can  be located by moving wires to recreate the short. The following troubleshooting procedures represent the  most common difficulties encountered in engine fire  detection systems: The service life of fire extinguisher discharge cartridges is  calculated from the manufacturer’s date stamp, which is  usually placed on the face of the cartridge. The cartridge  service life recommended by the manufacturer is usually in  terms of years. Cartridges are available with a service life  of 5 years or more. To determine the unexpired service life  of a discharge cartridge, it is usually necessary to remove  Fire extinguisher containers are checked periodically to  determine that the pressure is between the prescribed  minimum and maximum limits. Changes of pressure with  ambient temperatures must also fall within prescribed  limits. The graph shown in  Figure 17-29  is typical of the  pressure-temperature curve graphs that provide maximum  and minimum gauge readings. If the pressure does not fall  within the graph limits, the extinguisher container is replaced. Regular maintenance of fire extinguisher systems typically  includes such items as the inspection and servicing of fire  extinguisher bottles (containers), removal and reinstallation  of cartridge and discharge valves, testing of discharge tubing  for leakage, and electrical wiring continuity tests. The  following paragraphs contain details of some of the most  typical maintenance procedures. indicator light, or an opening in the sensing element  or connecting wiring. When the test switch fails to  provide an alarm, the continuity of a two-wire sensing  loop can be determined by opening the loop and  measuring the resistance. In a single-wire, continuous- loop system, the center conductor should be grounded. Temperature (°F) Pressure (psig) −40 −30 −20 −10  0  10  20  30  40  50  60  70  80 900 800 700 600 500 400 300 200 100 0 Min. gauge reading Max. gauge reading