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A

747-400
MAINTENANCE MANUAL

SEE  B 
GALLEY POWER 1  UTILITY POWER 1 ELCU 
ELCU  SEE  C 
SEE  C 

P514
SEE A
P614
GALLEY POWER 2ELCU
UTILITY POWER 2 ELCU SEE C SEE C
P514 AND P614 GENERATOR 1 AND 2 POWER CENTER-LEFT UTILITY POWER 3 ELCU
A SEE C
GALLEY POWER 3
ELCU
SEE C
P515
P615
UTILITY POWER 4 ELCU
GALLEY POWER 4 SEE C
ELCU
SEE C
P515 AND P615 GENERATOR 3 AND 4 POWER CENTER-RIGHT
B
ELCU
(TYPICAL) C
Utility and Galley Power Electrical Load Control Unit Location Figure 2
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747-400 MAINTENANCE MANUAL
SEE A
SEE B
P6 POWER DISTRIBUTION PANEL
A
INSTRUMENT BUS VOLTAGE SENSE UNIT
B
Instrument Bus Voltage Sense Unit LocationFigure 3
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747-400 MAINTENANCE MANUAL
EXTERNAL POWER
NO. 1

GSSR
Ground Handling/Ground Service Simplified Schematic Figure 4
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747-400
MAINTENANCE MANUAL

P6
AC Voltage Distribution Simplified SchematicFigure 5
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747-400
MAINTENANCE MANUAL

EXTERNAL POWER RECEPTACLE
P124
MAIN LOCK
CARGO
HANDLING BUS

MAIN DECK CARGO HANDLING BUS RELAY NO. 2
Main Deck Cargo Handling Simplified Schematic
Figure 6

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747-400 MAINTENANCE MANUAL
The four main generator buses supply power to three main ac distribution centers. All 115-volt ac loads, except for ground handling loads, are connected to these centers which are located in power shields P6, P414, and P415. Power shield P414 is supplied by main generator buses 1 and 2, and P415 by main generator buses 3 and
 4. Power shield P6 is powered by all four main generator buses. Power shields P414 and P415 are in the lower E/E bay and power shield P6 is located in the lower right side of the flight compartment. Each distribution center is connected directly to its associated main generator bus. The feeders to the distribution centers are protected by generator differential protection (DP). Control and protection is provided for individual sections within each distribution center through electrical load control units (ELCUs) and/or individual ac distribution thermal circuit breakers (TCBs).The main generator buses are tied together through a symmetrical, two section sync bus. The sections are connected to each other through the SSB to provide a four channel paralleled system. The function of the sync bus is to provide for parallel operation of the main engine driven generators and to provide a flexible switching arrangement for supplying the airplane loads from external power or the APU driven generators.The sync bus feeder is connected in a ring-bus configuration between BTBs 1 through 4, the APBs, the XPCs, and the SSB. This feeder arrangement is designed to transmit 180 kva through the SSB, using two paralleled number 4 AWG copper wires per phase.The main ac distribution buses supply all the essential ac loads in the airplane. Each main ac distribution bus is divided into smaller sections and each section is supplied through separate TCBs to provide independent fault isolation between sections. All the main bus sections are supplied from the associated main generator bus.Each main ac distribution bus is protected by a separate three-phase thermal circuit breaker. A short circuit fault condition on any phase of any bus section will be isolated by tripping the appropriate three-phase thermal circuit breaker, so that other unfaulted sections of the bus will remain operative.The utility/galley buses are supplied through ELCUs, and these protective devices will operate to isolate utility/galley buses from the other buses supplied from the same source if a short circuit fault condition occurs on the utility/galley load buses.
　
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