E.  Test Switch
(1)  The fire test switch has three positions: OVHT-off-FIRE. The switch is spring-loaded to the off position. The switch permits testing of the overheat or fire detection systems by checking sensor continuity and simulating a fire or overheat condition (Fig. 2).
F.  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 (Fig. 2).
3.  Engine and APU Fire Detection Accessory Unit Components
A.  The engine and APU fire detection accessory unit (control module) components described in the following paragraphs will be only those associated with the engine fire detection system. The engine and APU fire detection accessory unit is located on electronic equipment rack E3 in the electronic compartment. For further information on the engine and APU fire detection accessory unit, refer to 26-10-0, Fire Detection.
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26-11-0 Page 6  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  Feb 15/68 

B.  Detector Control Cards
(1)  
High (fire) and low (overheat) detector control cards for each engine are located in the accessory unit. The control cards contain fire detection and short circuit discrimination circuitry necessary to continuously monitor resistance of the applicable sensing element and to trigger overheat warning, 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 discriminator circuit which can differentiate between a sensing loop resistance drop caused by a mechanical short or by heat (fire). The fire warning circuit is deactivated by the short circuit discriminator if a direct short is applied across the sensor circuits.

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 in the aural warning devices box and the master fire warning lights on lightshield module P7. The control section consists of solid s- state switching circuit mounted on cards.

(2)  
When an alarm condition occurs at either engine, the appropriate control card will supply a 28 volt dc control signal. The warning control signal will supply 28 volts dc to the fire warning lights in the fire switch handle, on the fire protection module, turn on the two transistors in the bell and light control section, and unlock a solenoid holding the fire switch handle. The transistors ground the bell and master fire warning light circuit thus causing them to actuate. When the bell cutout switch or a master fire warning light is pressed, a positive trigger signal is supplied at the gate of an SCR in the bell control circuit. The SCR then conducts and turns off the transistor in the bell control circuit. The transistor stops conducting and de-energizes the bell and extinguishes the master fire warning lights. The SCR will continue to keep the transistor turned off until the alarm condition clears. However, the fire switch handle lights cannot be de-energized as long as a fire signal exists on the selected sensing loop. If the alarm condition clears before the bell cutout switch is pressed, the control signal to the transistors is removed; the transistors stop conducting, the bell and master fire warning lights are de-energized and the fire handle lights are de-energized. Either the fire or overheat control card may supply the warning signal, depending on the position of the overheat detector switch.

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Feb 15/68  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  26-11-0 Page 7 

D.  Detector Test Switch
(1)  A pushbutton switch on the front of the accessory unit may be used to test all detector control cards for reaction to a simulated short circuit (Fig. 2). Pressing the test switch causes test relays to energize which then short the sensors and connects a ground to the detector control cards. The short discriminator circuit should detect the ground and cause the detector inoperative lights to illuminate. The lights will extinguish when the test switch is released.
E.  Detector Inoperative Lights
(1)  Four press-to-test type amber detector inoperative lights, mounted on the front panel, provide an indication of a short circuit in the engine and APU fire and overheat detection systems. A short circuit on the fire or overheat sensors will cause the steady illumination of the applicable detector inoperative light (Fig. 2).
　
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