(4)  
The four main ac buses supply all essential ac loads in the airplane. The ELCUs connect the non-essential ac loads to the main ac buses. ELCUs provide control and protection for the non-essential loads.

 (5)  
The ac GSB is powered by ac bus 1, APU generator No. 1, or external power No. 1 through the ground service select relays (GSSRs) and the ground service transfer relays (GSTRs).

 (6)  
The ac GSB provides 115 volt ac power to such loads as interior lights, passenger lavatory loads, and battery chargers.

 (7)  
TCBs provide protection for the GSB and its individual loads.

 (8)  
The ac GHB is powered by external power No. 1 or APU generator No. 1 through the ground handling relays (GHRs). The GHB supplies power to such loads as cargo handling which are powered only on the ground.

 (9)  
TCBs provide protection for the GHB and its individual loads.

 (10)
 The ac main deck cargo handling bus is powered by external power No. 2 or APU generator No. 2 through the main deck cargo handling relays. The main deck cargo handling bus supplies power to operate the drive motors for cargo handling on the main deck which can be powered only on the ground.

 (11)
 TCBs provide protection for the main deck cargo handling bus and its individual loads.

 (12)
 The captain's and first officer's flight instrument transfer buses supply loads such as primary flight displays, navigation displays, and the flight management computer. These loads are protected by TCBs. The instrument bus voltage sense units (IBVSUs) automatically transfer these buses to their alternate source of power if the primary source is lost.

 (13)
 The 28 volt ac bus and the 28 volt ac ground service bus contain single phase TCBs that provide protection for the autotransformers and their individual loads. These buses supply power to indicators, sensors, flight deck lights, and ground service lighting.

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400 FAULT ISOLATION/MAINT MANUAL
AC ELECTRICAL LOAD DISTRIBUTION
COMPONENT  FIG. 102 SHT  QTY  ACCESS/AREA  REFERENCE 
CIRCUIT BREAKERS - 1  FLT COMPT, P6 
AC 1 DIST-P6, C841  1  6A2  * 
AC 1 DIST-P7, C3138  1  6B2  * 
AC 2 DIST-P6, C847  1  6A11  * 
AC 2 DIST-P7, C3139  1  6B14  * 
AC 3 DIST-P6, C851  1  6A20  * 
AC 3 DIST-P7, C3140  1  6B20  * 
AC 4 DIST-P6, C856  1  6A29  * 
CAPT XFR BUS - ALTN, C3135  1  6B5  * 
CAPT XFR BUS - PRIM, C3134  1  6B23  * 
F/O XFR BUS - ALTN, C3137  1  6B7  * 
F/O XFR BUS - PRIM, C3136  1  6B11  * 
28V AC XFMR - MAIN, C588  1  6A24  * 
CIRCUIT BREAKERS - 1  117AL, MAIN EQUIP CTR, P414 
AC 1 DIST UTILITY A, C3119  1  414R38  * 
AC 1 DIST UTILITY B, C3171  1  414R35  * 
AC 1 GSB UTILITY, C3118  1  414R32  * 
AC 2 DIST UTILITY A, C3159  1  414N38  * 
AC 2 DIST UTILITY B, C3172  1  414N35  * 
EXT POWER TRU A, C883  1  414B24  * 
EXT POWER TRU B, C2758  1  414B21  * 
INTER PHONE SYS, C717  1  414F19  * 
28V AC GSB DIST P6, C10312  1  414G25  * 
28V AC XFMR GND SVCE BUS, C584  1  414N25  * 
CIRCUIT BREAKERS - 1  117AL, MAIN EQUIP CTR, P415 
AC 3 DIST UTILITY A, C3160  1  415R1  * 
AC 3 DIST UTILITY B, C3180  1  415P1  * 
AC 4 DIST UTILITY A, C3161  1  415N1  * 
AC 4 DIST UTILITY B, C3169  1  415N4  * 
CIRCUIT BREAKERS - 1  117AL, MAIN EQUIP CTR, P514 
AC 1 DIST - P414, C926  1  * 
AC 1 GSB, C842 1  * 
CIRCUIT BREAKER - AC 3 DIST - P415, C928  1  1  117AL, MAIN EQUIP CTR, P515  * 
CIRCUIT BREAKERS - 1  117AL, MAIN EQUIP CTR, P614 
AC 2 DIST - P85 & P183, C3113  1  * 
AC 2 DIST A - P414, C925  1  * 
　
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