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747-400
MAINTENANCE MANUAL

MISCELLANEOUS SWITCHING MODULE, M7317
SEE A
FLIGHT COMPARTMENT

FIRE/OVHT TEST
A
Miscellaneous Switching Module Installation
Figure 401

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747-400 MAINTENANCE MANUAL
 ENGINE FIRE AND OVERHEAT DETECTION SYSTEM - DESCRIPTION AND OPERATION
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General (Fig. 1)
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 A.  Dual-loop fire and overheat detection systems are installed on each engine. These systems monitor engine temperatures and provide alerts for both fire and overheat conditions.
 B.  The eight system (4 engine fire and 4 overheat detections) use power from the 28-volts dc bus 1 through 4 and battery bus. The circuit breakers for each system are on P6 main power distribution panel.
 C.  Each fire detection system is composed of two loops. Each loop has three detector elements electrically connected in series, and any element is able to trigger an alarm or fault signal for the loop. Each overheat detection system is composed of two loops, with one detector element in each loop.
 D.  Each detection loop has a control card, and each engine has an automatic fire overheat logic test system card. All systems can be tested by the fire/overheat test switch (M7317) or CMC ground test.
 E.  All systems operate in AND logic. Thus, both loops of a system must sense an input in order for an alarm to be given. The detection loops are continuously monitored for faults. If a fault is detected by one of the loops, the system automatically reconfigures to single loop operation.
Engine Fire and Overheat Detectors
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 A.  Engine fire detector information follows:
 (1)  The detector is able to respond in two modes: A localized flame or heat will cause a discrete temperature rise which will cause the core material to release gas to increase the pressure. The other mode is a general increase in temperature over a large area. Both of these modes are reversible. Should the temperature decrease, the gas pressure will decrease and the system will return to normal.
 B.  The fire detectors are installed on the lower forward, upper and lower aft section of the engine. An overheat detector is installed on upper forward section of the engine.
Fire and Overheat Detection Cards
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 A.  Each detection loop has a corresponding detection control card. These cards constantly monitor and process signals produced by each loop. The cards output alarm or fault signals to the automatic fire overheat logic test system card for the appropriate system.
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A
747-400
MAINTENANCE MANUAL

FLIGHT COMPARTMENT

TEST SWITCH

ENGINE FIRE
SWITCH (RED)

Engine Fire and Overheat Detection System - Component Location
Figure 1 (Sheet 1)

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