(3)  
During normal operation, if a high temperature (fire) detector control card or sensor becomes inoperative and fails to test satisfactorily, the flight may be dispatched (or continue its flight) by unlocking (pulling) the overheat detector switch and moving it to the FIRE position. With the switch in the FIRE position, the overheat indicating components are bypassed and the warning signal from the low temperature (overheat) detector control card is fed directly to the fire warning indicating components. If a fire warning indication is presented when the overheat detector switch is in the fire position, it should be assumed a fire condition exists and appropriate action taken.

(4)  
During normal operation, an amber fire or overheat detector inoperative light will illuminate if a sensor or the monitoring circuit should fault to ground. The detector inoperative lights are located on the engine and APU fire detection accessory unit. When a short circuit occurs, the sensing loop resistance drops rapidly to a low value (less than 1/2 the alarm resistance). When the sensing loop resistance is instantly reduced below 1/2 the fire trip resistance, the fire detecting circuit in the engine and APU fire detection accessory unit does not actuate because of a trim delay circuit. The lower resistance actuates the short discriminator circuit which locks out the fire detecting circuit so that the fire warning circuit cannot actuate after the expiration of the delay period. The short discriminator circuit on the control card provides a ground path for the applicable detector inoperative light.  Normally, if a fire occurs, the resistance of the sensing loop drops relatively slowly and after reaching the fire trip resistance, will not decrease rapidly enough to reach the short circuit trip resistance before expiration of the time delay period. The fire detector circuit causes a fire alarm relay on the control card to be energized. The relay isolates the short monitoring circuit to prevent a short signal when the fire lowers the sensing loop resistance below the short circuit trip value. Most short circuits are intermittent, and the system is fully capable of detecting fire during the absence of the short. In the case of a steady short, the applicable detector "fails safe," that is, it becomes inoperative but does not give a false alarm.

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NOTE:  If an overheat or fire detection system fails to test, monitor the detector inoperative light, located on the front of the engine and APU fire detection accessory unit in the electronic compartment, prior to performing any other tests.
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B.  Test Operation
(1)  
A fire detection system continuity test can be made by moving the fire test switch, on the fire protection module, to FIRE. The fire warning indicating components should come on and then go off when the switch is released. A test relay for each engine system grounds the high temperature sensor through a resistor thus simulating a fire and providing a fire signal at each high temperature detector control card. The resistor is required to raise the resistance on the associated sensing loop above the short circuit trip resistance during the fire test.

NOTE:  Should a fire warning light fail to illuminate during a fire test, a malfunction of the detector circuit is indicated. Fire protection can be maintained by unlocking the overheat detector switch and moving it to the FIRE position.

(2)  
An overheat detection system continuity test can be made by moving the fire test to OVHT. The overheat warning indicating components should come on and then go off when the switch is released. A test relay for each engine grounds the low temperature sensor through a resistor thus simulating an overheat condition and providing an overheat signal at each low temperature detector control card. The resistor is required to raise the resistance on the associated sensing loop above the short circuit trip resistance during the fire test.

(3)  
A short circuit discriminator circuit continuity test can be made by pressing the DET TEST switch on the front of the engine and APU fire detection accessory unit. The detector inoperative lights should come on and then go off when the switch is released. A test relay for each detector system grounds the associated sensors thus creating a short across each detector control card sensing element terminals. The short circuit discriminator section of each card wil1 then illuminate the associated detector inoperative light.

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