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C.  The engine fire detection system can be tested by operation of the TEST switch. The TEST switch circuit checks continuity through the system by simulating fire or overheat on both engines. This energizes overheat warning lights or the alarm bell and fire warning lights. The detector test switch on engine and APU fire detection accessory unit tests the detector inoperative circuit and causes the detector inoperative lights to illuminate.
2.  Engine and APU Fire Control Module Components (Fig. 1)
A.  Engine fire detection system components of the fire control module are the loop select switch, warning lights, test switch and bell cutout switch. The fire control module is located on aft control stand electronic panel P8.
B.  Loop Select Switches
(1)  One loop select switch for each engine permits selection of dual-loop AND-logic when set to NORMAL, or single-loop when set to A or B. When the switch is set to NORMAL, signals from both elements in a detector are processed by detection accessory unit, which actuates the appropriate fire, overheat, or fault signal. When the switch is set to A or B, the detection accessory unit permits the selected loop to monitor fire or overheat, adjusts control logic to recognize absence of the other loop, and suppresses fault signals from the other loop.
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H79590 L20623 T79898
Engine Fire Detection System Component Location  5C9 
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505  Engine Fire Detection System Component Location 
Feb 20/85  Figure 1 (Sheet 2)  26-11-02 
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Engine Fire Detection System Simplified Schematic  5C9 
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C.  Warning Lights
(1)  
Red fire switch lights in the fire switch handles provide engine fire warning. A fire signal from the detection accessory unit will cause steady illumination of the associated fire switch lights. Each warning light consists of several bulbs covered by a red translucent plastic lens in the fire switch handle.

(2)  
Amber ENGINE OVERHEAT lights provide for engine overheat warning. An overheat signal from the detection accessory unit will cause the illumination of the associated ENGINE OVERHEAT light. The overheat warning lights may be tested individually by a press-to-test feature or the master test switch.

(3)  
Detector fault lights provide detector fault warning. A fault signal from the detection accessory unit will cause steady illumination of the applicable DET FAULT light.

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 overheat or fire detection systems by checking detector continuity and simulating a fire, fault or overheat 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 extinguishes the FIRE WARN lights.
3.  Engine and APU Fire Detection Accessory Unit Components (Fig. 1)
A.  Engine 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 or overheat 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 M/C fire signal to the FIRE WARN lights.

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