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A
747-400
MAINTENANCE MANUAL
G. The system is a dual-loop with "AND" logic.
2. Thermal Switches (Fig. 1)
________________
A. Each thermal switch has a snap-acting bimetal disk sensing element which gives make and break switch action through a spring contact arm. This disk is found at the base of a drawn steel cap and is hermetically sealed. The thermal switch is calibrated at a preset temperature and designed to close on the preset temperature. It is an on-off device that breaks contact upon a temperature decrease below the set temperature. In addition, the unit is resistant to shock and vibration.
3. Fire/Overheat Test Module (Fig. 1)
_________________________
A. The FIRE/OVHT TEST switch is located on M7317 miscellaneous switching module (P5). The switch initiates tests for the various fire detection systems. The operational test for each applicable section contains the FIRE/OVHT test. By pressing the switch, the switch sends a signal to a relay that simulates a fire/overheat indicators, and appropriate EICAS messages.
B. To replace the module, refer to AMM 26-10-02/401, Miscellaneous Switching Module.
4. Operation (Fig. 2)
_________
A. Functional Description
(1)
All thermal switches are connected in parallel. The relay is continuously monitoring the ground condition on any switches. If a thermal switch reaches the preset temperature, the thermal switch closes to complete the circuit and engernizes the relay. A signal is then sent to EIU, and EICAS messages are created. A decrease in temperature causes the thermal switches to reopen.
(2)
In normal operating conditions the thermal switches are open.
(3)
The system is tested by pressing the FIRE/OVHT TEST button. This creates appropriate EICAS messages. The test consists of testing the continuity of the circuitry.
B.
The CMC can initiate the fire detection system ground test if either of the ground tests switches is enabled and the CMC test relays are closed. The CMC initiates a ground test to determine the integrity of the fire detection card file and its inputs/outputs using the LRU's built-in test equipment.
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MAINTENANCE MANUAL
MISCELLANEOUS SWITCHING MODULE, M7317
SEE A
FIRE DETECTION CARD FILE, M826
ELECTRICAL EQUIPMENT COMPARTMENT
FIRE/OVHT
FIRE/OVHT
TEST
TEST SWITCH
A
Duct Leak Detection System Component Locations
Figure 1 (Sheet 1)
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MAINTENANCE MANUAL
THERMAL SWITCHES
(19 LOCATIONS)
SEE C
WING ISOLATION
VALVES
(2 LOCATIONS)
AIR CONDITIONING AFT CARGO HEAT
PACKS
(3 LOCATIONS) APU SHUTOFF VALVE
ì
THERMAL SWITCHES
(19 LOCATIONS)
SEE C
SEE B
SEE C
SEE D
SEE D
Duct Leak Detection System Component Location
Figure 1 (Sheet 2)
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