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A 737-300/400/500MAINTENANCE MANUAL
8. APU Ground Control Panel
________________________
A. APU ground control panel P28 contains APU fire warning light and the horn cutout switch.
B. The horn cutout switch can be used to extinguish the master fire warning lights and cut off the fire warning horn and bell.
C. A fire in the APU will cause pulsating illumination of the fire warning light. The warning light is controlled through the accessory control card. When a bell or horn cutout switch is pressed, the light illuminates continuously until the fire warning signal is gone.
D. P28 is on the right side of the main wheel well.
9. Operation (Fig. 2)
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A. Normal Operation
(1)
Normal fire detection is automatic when power is applied to the airplane and the fire detection and aural warning circuit breakers are closed.
(2)
During normal operation, the APU control card monitors the resistance of the detector circutis.
(a)
An APU detector inoperative light will illuminate if the sensing element of the monitoring circuit should ground fault (short circuit). When a short circuit occurs, the sensing loop resistance drops instantly below the fire trip resistance, which actuates the short discriminator circuit, disarming the fire warning circuit. The short discriminator circuit in the control card provides a ground path for the APU inoperative light on the fire protection system module.
(b)
When a fire occurs, the resistance of the sensing loop drops relatively slowly and, after reaching the fire trip resistance, activates a time delay, disarming the short discriminator circuit and energizing the fire warning circuit. 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.
NOTE: If the fire detection system fails to test, monitor the
____ APU detector inoperative light prior to performing any other test.
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A 737-300/400/500MAINTENANCE MANUAL
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A
737-300/400/500MAINTENANCE MANUAL
(c) 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 valve of the sensor to decrease. At a predetermined resistance valve corresponding to the alarm temperature, the APU detector control card will trigger a fire warning signal. The signal will activate the indicator circuits.
B. Test Operation
(1) A fire detection system continuity test can be made by moving the TEST switch to OVHT/FIRE. The fire warning indication components should come on and then go off when the switch is released. When the switch is activated, a fire is simulated causing an alarm condition.
NOTE: The APU will not shut down when the fire test switch is
____
actuated.
(2) A test of the fault discrimination circuit can be made by moving the TEST switch to FAULT/INOP. The APU detector inoperative light should come on and then go off when the switch is released.
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