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(1)  A red fire warning light in the APU fire switch handle provides APU area fire warning. The APU fire warning light is connected to the APU detector control card. A fire or overheat condition in the APU shroud will cause steady illumination of the warning light (Fig. 2). The fire warning light consists of several bulbs covered by a red translucent plastic lens cap in the fire switch handle.
D.  Test Switch
(1)  The fire test switch has three positions: INOP/OVHT-off-FIRE; The switch is spring-loaded to the off position. The switch permits testing of the APU fire detection system by checking sensor continuity and simulating a fire condition when positioned to FIRE. When in the INOP/OVHT positions the APU sensor is grounded and the inoperative light will illuminate (Fig. 2).
E.  Bell Cutout Switch
(1)  A pushbutton switch is provided for alarm bell control. Pressing the switch provides 28 volts dc to the aural warning and master fire warning light control section in the fire detection accessory unit. The control circuits then silence the bell and extinguish the master fire warning lights on P7.
3.  Engine and APU Fire Detection Accessory Unit
A.  The engine and APU fire detection accessory unit (control module) components described in the following paragraphs will be only those associated with the APU fire detection system. The engine and APU fire detection accessory unit is located on electronic equipment rack E3. For further information on the engine and APU fire detection accessory unit, refer to 26-10-0, Fire Detection.
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B.  APU Detector Control Card
(1)  
The control card contains the fire detection and short circuit discrimination circuitry necessary to continuously monitor the resistance of the sensing element and/or wiring and to trigger a fire warning or short circuit indication. The control card resistance trip level is fixed during assembly of the card and does not require or permit adjustment. The sensor is connected in series with the control card to form a closed circuit sensing loop. A zener diode regulator in each control card protects the circuit from input voltage transients.

(2)  
Each control card consists of two resistance monitoring circuits, a fire warning circuit set to trip at a resistance corresponding to a desired alarm temperature and a short circuit detector circuit which can differentiate between a sensing loop resistance drop caused by a mechanical short or by heat (fire).

C.  Aural Warning and Master Fire Warning Lights Control Section
(1)  
The aural warning and master fire warning lights control section provides control of the alarm bell on the aural warning devices box, the remote horn mounted in the main wheel well, the fire warning light on the remote APU control panel, the master fire warning lights on light shield module P7 and APU shutdown (Fig. 2).

(2)  
When an alarm condition occurs in the APU, the APU control card will cause a warning signal. The warning signal will supply 28 volt dc to the fire warning lights in the APU fire switch handle on the fire protection module, unlock a solenoid holding the fire switch handle, and energizes a relay in the control section. The energized relay will supply 28 volt dc to the following: to the APU automatic shutdown circuits, turn on the bell control transistor to the remote horn and light flasher unit which controls the flasher relay supplying a pulsating output for the remote horn and light and supplies a ground for the master fire warning lights. The transistor supplies a ground to the alarm bell circuit, thus causing the bell to ring. The flasher relay will pull in and relax, thus causing a pulsating output of the remote horn and light. When either the bell cutout switch, APU remote horn cutout switch, or a master fire warning light is pressed, a positive trigger signal is supplied at the gate of two silicon controlled rectifiers (SCR) in the control section. One SCR will conduct, removing the bias from the transistor in the bell circuit. The transistor will stop conducting and de-energize the bell and the master fire warning lights. The other SCR will conduct, providing a steady ground circuit to the flashing

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relay. The relay will remain energized, disconnect the horn circuit and cause steady remote fire light illumination on the remote APU control panel. The SCR's will continue to conduct until the alarm condition clears. If the alarm condition clears before the bell cutout switch is pressed, the control relay will relax, remove power from the control section and return the control section to its normal state. However, the fire handle lights cannot be de-energized as long as a fire signal exists on the selected sensing loop.
　
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