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747-400 MAINTENANCE MANUAL
SEE A
P180
P180-1
SEE B
(4 LOCATIONS) SCREW 
P180 DC POWER DISTRIBUTION PANEL 
A  TRANSFER RELAY, R7687 APU BATTERY  B 

APU Battery Bus Transfer Relay Installation Figure 401
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747-400 MAINTENANCE MANUAL
 (b)  Turn the knob on the APU battery connector (DM7432A) counterclockwise until the connector is free from the APU battery receptacle.
 S 014-004
 (4)  
Open the access doors for the P180 panel.

 S 024-020

 (5)  
Remove the transfer relay.

 (a)  
Remove the three nuts that attach the relay to the P180 panel.

 (b)  
Remove the transfer relay from the panel.

 TASK 24-31-16-404-007
 3.  APU Battery Bus Transfer Relay Installation (Fig. 401)___________________________________________
 A. References
 (1)  
AMM 24-22-00/201, Manual Control

 (2)  
AMM 45-10-00/201, Central Maintenance Computer System

 (3)  
IPC 24-33-01 Fig. 1

 (4)  
WDM 24-33-11

 (5)  
SSM 24-33-01

 B.
Access

 (1)  
Location Zone 116 Area Between Nose Gear Well and Fuselage, RH

 (2)  
Access Panel
 P180 DC Power Distribution Panel

 C.
Procedure

 S 424-021
 (1)  
Install the transfer relay.

 (a)  
Put the transfer relay in position on the P180 panel.

 (b)  
Install the three nuts that attach the relay to the panel.

 (c)  
Tighten the nuts.

 (2)  
Close the access doors for the P180 panel.

 S 414-010
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 (3)  S 434-022Connect the electrical connectors to the batteries. (a) Put the main battery connector (DM7423A) into the receptacle on the main battery. (b) Put the APU battery connector (DM7432A) into the receptacle on the APU battery. (c) Turn the knobs on the connectors clockwise with your hand until they are tight.
 (4)  S 864-012Remove the DO-NOT-OPERATE tag from the BATTERY switch.

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747-400
MAINTENANCE MANUAL

 TRANSFORMER RECTIFIER - DESCRIPTION AND OPERATION
_________________________________________________
 1.  General
_______
 A.  There are four main transformer rectifier units (TRUs), each TRU powers an associated main 28 volt dc load bus. In addition, under normal operation, the battery buses are supplied from dc bus No. 3. The TRUs are normally operated in parallel through the dc isolation relays (DCIRs), 75 amp thermal circuit breakers, and a dc tie bus. The TRUs are identical and are interchangeable.
 B.  There is an APU transformer rectifier unit (TRU) installed for APU starting. The APU TRU is used to deliver dc power to the APU stater motor for APU starting when the APU battery is not the desired power source for APU starting. The APU TRU uses galley power No. 2 which is redirected to the APU TRU which will rectify the ac input power into an output dc power for APU starting.
 2.  Main Transformer Rectifier Unit (Fig. 1)
　
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