E. The 115 volt ac power, provided by external power No. 2 or APU generator No. 2, powers the ac main deck cargo handling bus. Loads supplied by the ac main deck cargo handling bus are only powered for main deck cargo operations and maintenance.
F. External power No. 1, APU generator No. 1, or ac bus 1 can supply power to the ac ground service bus. The ac ground service bus can be powered by ac bus 1 either on the ground or in flight.
G. The captain's and first officer's transfer buses supply power to loads such as flight instruments. These buses are switched automatically to an alternate power source if the primary power source is lost.
H. Autotransformers are used to supply power to the 28 volt ac load buses.
7. DC Electrical Load Distribution (AMM 24-60-00)
_______________________________
A. The four main dc buses supply the majority of the dc loads in the airplane. The input to the TRUs are protected by TCBs as well as the individual loads supplied by the main buses.
B. The main dc bus No. 3 supplies the main and APU battery buses and is also used for standby power to supply power to the flight critical loads if the primary dc power source is lost.
(1)
If the dc main bus No. 3 fails, the main and APU hot battery buses supply the main and APU battery buses and are also used for standby power to supply power to the flight critical loads if the primary dc power source is lost.
C.
Remote controlled circuit breakers (RCCBs) are used to provide overload protection at the input to the main and APU battery buses and to the static inverter from dc bus No. 3.
D.
The dc ground handling bus (supplied by external power TRUs) supplies power to loads required for ground operations and maintenance.
8. Operation
_________
A. Functional Description (Fig. 1)
(1)
There are many ac and dc buses throughout the EPGS. Power from these buses is distributed to all the electrical loads required for flight and ground operations.
(2)
The BCUs, GCUs, and AGCUs receive inputs from the flight deck control panel, current transformers, and current sensors to monitor and control the contactors, breakers, and relays to provide distribution of both ac and dc power to the different buses.
(3)
The 115 volt ac power from the four IDGs is applied to the four main ac buses; ac bus 1, 2, 3, and 4 through the GCBs.
(4)
The output of the four IDGs is normally operated in parallel through the BTBs, sync bus, and the SSB.
(5)
For ground operations, provided external power is not available, the APU generators can power the main ac buses through the APBs, sync bus, and the BTBs. The APU generator can not be paralleled through the SSB.
(6)
External power applied to the airplane supplies the main ac buses through the XPCs, sync bus, and the BTBs. External power can not be paralleled through the SSB.
(7)
If only one APU generator or one side of the external power is available, both the right and the left sync bus halves can be powered through the SSB.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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747-400
MAINTENANCE MANUAL
RELAY XFR
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Electrical Power Simplified SchematicFigure 1
M49504
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