D.  Test Switch
(1)  The TEST switch has three positions: FAULT-off-OVHT/FIRE. The switch is spring-loaded to the off position. The switch permits testing of the fire detection system by checking detector continuity and simulating a fire or fault condition.
E.  Bell Cutout Switch
(1)  The BELL CUTOUT switch is provided for alarm bell control. Pressing the switch provides bell cutout power to the fire detection accessory unit, which silences the bell and remote horn and extinguishes the FIRE WARN lights.
3.  Engine and APU Fire Detection Accessory Unit Components (Fig. 1)
A.  APU fire detection system components of the engine and APU fire detection accessory unit are the control cards, test switch, and fault lights. The engine and APU fire detection accessory unit is on electronic equipment rack E3 in the electronic compartment.
B.  Control Cards 
(1)  
The control cards continuously monitor resistance of the detectors. When a resistance change of a detector is sensed, fire warnings are actuated. When the resistance returns to the reset value, the cards cease the output of warning signals. If the resistance drop is instantaneous, the cards generate a fault signal and suppress alarm signals. The fault signal ceases when the fault is removed. 

(2)  
The control cards contain test circuitry which permit verification of loop continuity, capability of detection circuitry, and functioning of short discriminator circuitry.

(3)  
The control cards provide loop selection, which electrically removes any sensor loop from the system. 

(4)  
The control cards contain bell-cutout circuitry, which deactivates fire signal to alarm bell and remote horn and M/C fire signal to the FIRE WARN lights.

C.  Test Switch
(1)  The pushbutton test switch tests the short discriminator circuitry of the control cards. Actuation of the test switch will cause the fault lights on the control module and accessory unit to illuminate.
D.  Fault Lights
(1)  
The fault lights illuminate due to fault signal from the control cards or during fault testing. The lights extinguish when the fault is removed or the test switch is released. 

(2)  
The cockpit fault light will not illuminate when an alarm condition is detected by one loop while the other loop is in a fault condition.

4.  APU Remote Control Panel Components
A.  APU fire detection system components of the APU remote control panel are horn cutout switch and the fire light. The APU remote control panel P28 is located on the aft bulkhead, right side, in the main wheel well.
B.  APU Horn Cutout Switch. 
(1)  The HORN CUTOUT switch can be used to extinguish the master fire warning lights and silence the fire horn or bell from the ground.
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Feb 20/85  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  26-15-02 Page 5 

C.  Fire Warning Light
(1)  A fire in the APU will cause the fire warning light to flash. The warning light is controlled through the aural warning and master fire warning lights control section. When a bell or horn cutout switch is pressed, the light remains illuminated until the fire warning signal is gone.
5.  Alarm Bell
A.  The alarm bell warns of fire with a steady ringing. The bell is controlled by the control card in the engine and APU fire detection accessory unit. The alarm bell is in the aural warning devices box.
6.  Remote Horn
A.  The remote horn is in the right main wheel well adjacent to the APU control panel. Audio tones for the horn are generated by a circuit which is turned on and off by the control card in the accessory unit. The horn operates only when the airplane is on the ground.
7.  Master Fire Warning and Master Caution Lights (Fig. 1)
A.  The master fire warning lights are on lightshield module P7.
B.  The master fire warning lights are controlled by control card in engine and APU fire detection accessory unit. Either light may be pressed for bell cutout and extinguishing of the master fire warning lights.
8.  Detectors (Fig. 1)
A.  Detectors provide overheat and fire detection at the APU area. One detector is on the APU shroud, one is on the tailpipe, and one is on the exhaust. They are attached by quick-release clamps. Each detector consists of two sensing elements mounted in a perforated metal tube. Each sensing element runs the length of the tube and doubles back. Each detector is composed of two wires imbedded in thermister material which is encased in a heavy wall inconel tube for high strength at elevated temperatures. The electrical connectors at each end are ceramic insulated. The inconel tubes are shrouded in a perforated stainless steel tube and supported by insulated bushings at intervals. The shroud protects the sensor from breakage due to vibration, abrasion against airplane structure, and damage from maintenance activity.
　
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