(2)
If a fire or overheat condition occurs in the APU area, the sensor is subjected to a temperature rise. The temperature rise will cause the resistance value of the sensor to decrease. At a predetermined resistance value corresponding to the alarm temperature, the APU detector control card will trigger a fire warning signal. The signal will activate the circuits which cause the fire bell and warning horn to sound, unlock the APU fire switch, the fire warning lights to illuminate and the APU to shut down. The fire warning horn will sound only if the airplane is on the ground. When the fire warning lights are illuminated, a fire is assumed and should be extinguished. Refer to 26-22-0, APU fire Extinguishing System. When the fire is extinguished, the temperature and sensor resistance value should return to normal. The fire signal will be removed and
The fire warning indicating components will turn off. The alarm bell and remote horn may be silenced at any time when the bell cutout switch, APU remote horn cutout switch or either master fire warning light lens caps are momentarily pressed.
(3)
During normal operation, an APU detector inoperative light will illuminate if the sensing element or the monitoring circuit should ground fault (short circuit). When a short circuit occurs, the sensing loop resistance drops rapidly to a low value. When the sensing loop resistance is instantly reduced below 1/2 the fire trip resistance, the fire warning circuit does not actuate because of a built-in time delay. The lower resistance actuates the short discriminator circuit, which nullifies the fire warning signal so that the fire warning circuit cannot actuate after the expiration of the delay period. The short discriminator circuit in the control card provides a ground path for the APU inoperative light on the fire protection system module. When a fire occurs, the resistance of the sensing loop drops relatively slowly and after reaching the fire trip resistance, will not decrease rapidly enough to reach the short circuit trip resistance before expiration of the time delay period. The fire detector circuit causes the fire alarm relay on the control card to be energized. The relay isolates the short monitoring circuit and prevents a short signal when the fire lowers the sensing loop resistance below the short circuit trip value. It should be noted that most short circuits are intermittent, and the system is capable of detecting fire during the absence of the short. In the case of a steady short, the detector "fails safe"; that is, it becomes inoperative but does not give a false alarm.
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NOTE: If the fire detection system fails to test, monitor the APU detector inoperative light prior to performing any other tests.
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B. Test Operation
(1)
A fire detection system continuity test can be made by moving the fire test switch, on the fire protection module, to FIRE. The fire warning indicating components should come on and then go off when the switch is released. A test relay for the APU system grounds the APU sensor through a resistor thus simulating a fire and providing a signal at the APU detector control card. The resistor is required to raise the resistance of the sensing loop above the short circuit trip resistance during the fire test.
NOTE: The APU will not shut down when the fire test switch is actuated.
(2)
A short circuit discriminator circuit continuity test can be made by moving the fire test switch to INOP/OVHT. The APU detector inoperative light should come on and then go off when the switch is released. A test relay for the detector system grounds the sensor thus creating a short across each detector control card sensing element terminals. The short circuit discriminator section of each card will then illuminate the detector inoperative light.
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APU FIRE DETECTION SYSTEM - TROUBLE SHOOTING
EFFECTIVITY
CA B2501 thru B2505
1. General
A. Trouble with the fire detection system will usually be indicated by absence of fire warning lights or alarm bell when the test switch is operated on the fire protection module, or by a false alarm. If a short circuit has occurred on a sensing line, the APU detector inoperative light will probably be illuminated. If an open circuit has occurred, the APU warning lights will not be illuminated when the system is tested. Lights on the fire protection module, and master fire warning lights should be tested before assuming the detection system is faulty by operating the master lights test circuits (Ref Chapter 33, Master Caution and Warning Lights).
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