(21)
 In the unlikely event that abnormal operation is observed in external power and automatic protection does not operate, the crew may manually trip the XPC (operate the appropriate EXT PWR switch) thus disconnecting the faulty source from the sync bus.

 (22)
 External power receptacle No. 1 also supplies 115 volts ac to the ac ground handling and ground service buses.

 (23)
 When the BCU determines the 115-volt ac external power is good, the BCU will energize the ground handling relay coil and transfer the 115 volts ac to the ac ground handling bus (Ref 24-51-00).

 (24)
 There are two external power transformer rectifier units (TRUs). The TRUs are static devices which convert the 115-volt ac 3-phase power from the ac ground handling bus to unregulated 28-volt dc power for the dc loads required during ground operation. Each TRU is rated for a continuous output load of 20 amps.

 (25)
 The external power TRUs utilize a Y-Y primary/secondary connection and a 3 phase full-wave silicon rectifier circuit.

 (26)
 The unit output voltage ripple does not exceed 5 percent rms of the dc output voltage over the full load range.

 (27)
 The TRU is designed to deliver an overload current of 30 amps (150 percent of rated load) for 5 minutes.

 (28)
 The two TRUs and their output connections to the dc ground handling buses are protected from overloads by thermal circuit breakers on the 3-phase ac input to each TRU. Cooling is provided by natural convection only.

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 ALL  ú ú 03 Page 8 ú Feb 10/91
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400 FAULT ISOLATION/MAINT MANUAL
AC EXTERNAL POWER
COMPONENT SHT 102 FIG.  QTY  ACCESS/AREA  REFERENCE 
CIRCUIT BREAKERS 1  FLT COMPT, P6 
BUS CONT UNIT 1, C10054 1  6G6  *
 BUS CONT UNIT 2, C10052  1  6G7  * 
CIRCUIT BREAKERS 1  117AL, MAIN EQUIP CTR, P180 
BCU 1, C10053 1  180B14  *
 BCU 2, C10051  1  180B15  * 
CIRCUIT BREAKERS 1  117AL, MAIN EQUIP CTR, P714,P715 
EXT PWR NO. 1, C860 1  P714  *
 EXT PWR NO. 2, C861 1  P715  * 
L SYNC BUS, C859 1  P714  *
 R SYNC BUS, C862  1  P715  *
RECEPTACLE - EXT PWR NO. 1, D51  1  1  117AL, MAIN EQUIP CTR, FLOOR  24-41-03 
RECEPTACLE - EXT PWR NO. 2, D52 RELAY - (REF 31-01-36,-39, FIG. 101) BCU 2, STBY PWR TRANSFER, R7302 BCU 1, STBY PWR TRANSFER, R7301 1  1  117AL, MAIN EQUIP CTR, FLOOR  24-41-03 
RELAYS  2  117AL, MAIN EQUIP CTR, P714,P715 
EXT PWR CONTACTOR 1, R12 1  P714  24-41-04 
EXT PWR CONTACTOR 2, R13 RELAY - (REF 31-01-42,-45, FIG. 101) GND PWR SENSING, LEFT, R68 GND PWR SENSING, RIGHT, R69 TRANSFORMER - (REF 31-01-42,-45, FIG. 101) EXT PWR 1, GND PWR CURRENT, T4113 EXT PWR 2, GND PWR CURRENT, T4114 EXT PWR 1, SYNC BUS DIFF PROTECTION CURRENT, T4022 EXT PWR 2, SYNC BUS DIFF PROTECTION CURRENT, T4023  1  P715  24-41-04 
UNIT - EXT PWR TRU A, T97  2  1  117AL, MAIN EQUIP CTR  24-41-05 
UNIT - EXT PWR TRU B, T1  2  1  117AL, MAIN EQUIP CTR  24-41-05 

* SEE WDM EQUIPMENT LIST
AC External Power - Component Index
Figure 101

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 24-41-00
 ALL  ú
 ú  01  Page 101
 ú  Dec 10/88 

BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A

747-400
FAULT ISOLATION/MAINT MANUAL

P714 PANEL 
P715 PANEL 
P180 PANEL 
SEE A MAIN EQUIP CTR  NO. 1, D51 EXTERNAL POWER RECEPTACLE  EXTERNAL POWER NO. 2, D52 RECEPTACLE FLT COMPT  FWD EXTERNAL POWER RECEPTACLE DOOR, 118A P6 PANEL 

FLOOR PANELS (REF) LOWER LOBE SEE  B  B EXTERNAL POWER RECEPTACLES (OUTSIDE VIEW) 
A  FWD 

EXTERNAL POWER 
RECEPTACLE 
NO. 1, D51  FWD 
　
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