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 E. The system has no self-test capability.Thermal Switches (Fig. 1) (sheet 2 & 3)
________________
 A.  Twenty thermal switches are installed along the APU bleed air duct. Fifteen switches are installed in the aft cargo compartment along the left wall. Five switches are installed in the area aft of the bulk cargo compartment along the APU bleed duct.
 B.  On all airplanes, seven more thermal switches are installed along the APU bleed air duct between the APU check valve and the APU isolation valve. The thermal switches monitor air temperature for an overheat condition caused by an APU duct leak or break.
 C.  Each thermal switch consists of a snap-acting bimetal disk which provides make and break switch action through a spring contact arm. This disk is installed at the base of a drawn steep cap and is hermetically sealed. The thermal switch is calibrated at a set temperature and designed to close on the set temperature.
Aft APU Electrical Control and Circuit Breaker Panel (P83) (Fig. 1)
__________________________________________________________
 A.  The aft APU electrical control and circuit breaker panel P83 is installed aft of the crew rest area in the E8 rack. This panel contains the APU DUCT OVHT indicator light L6, the APU DUCT OVHT RESET switch S220, the DUCT OVHT relay R686, and the APU DUCT OVHT circuit breaker.
Operation
_________
 A.  At high temperature conditions along the APU air duct, a thermal switch will close and send a ground to the DUCT OVHT relay. This relay latches a signal to the aft APU electrical control module to shut down the APU, and the APU DUCT OVHT light comes on. The system can be set to normal operation mode using the APU DUCT OVHT RESET switch after the duct overheat problem has been corrected.
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MAINTENANCE MANUAL

AFT APU EQUIPMENT CONTROL AND CIRCUIT BREAKER PANEL P83
SEE B
SEE A
AFT EQUIPMENT CENTER E8
A
DUCT OVERHEAT RELAY, R686
P83 APU DUCT OVERHEAT
R686
INDICATION, L6
APU DUCT OVERHEAT RESET SWITCH, S220
APU DUCT OVERHEAT IND LIGHT
RESET SW
5
7
7
7
5
5
APU EQUIPMENT CONTROL AND CIRCUIT BREAKER PANEL, P83 B Apu Duct Overheat Detection System Component Location Figure 1 (Sheet 1)
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THERMAL SWITCH (TYPICAL)

C

Apu Duct Overheat Detection System Component Location
Figure 1 (Sheet 2)

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WING ISOLATION 
VALVES 
(2 PLACES) 
AIR CONDITIONING  AFT CARGO HEAT 
PACKS 
(3 PLACES)  APU SHUTOFF VALVE 

SEE D
KEEL
BEAM PACK BAYA/C A/CPACK NO. 1 PACK NO. 3
LEFT ISOLATION VALVE
A/C PACK NO. 2
STA 1248
APU CHECK VALVE
RIGHT ISOLATION VALVE
WHEEL WELL
APU ISOLATION VALVE
THERMAL SWITCH
SEE C
STA 1480
-APU DUCT
APU  DTHERMAL SWITCH
APU Duct Overheat Detection System Component Location Figure 1 (Sheet 3)
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