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• ground service and ground handling busses
• four galley busses
• four utility busses
• two standby busses
AC Main Busses
Each IDG normally powers its related AC bus through a generator control breaker
(GCB). Each AC bus is connected to the synchronous bus by a bus tie breaker
(BTB). With the Bus Tie switch in AUTO, the related BTB controls the AC bus
connection to the synchronous bus.
If power on an AC bus is unacceptable, the related BTB opens and the bus
disconnects from the synchronous bus. However, the AC bus remains powered by
its IDG. If the IDG is not able to maintain acceptable power quality, the GCB
opens and the BTB closes to provide power from the synchronous bus.
The EICAS message ELEC BUS ISLN displays if the BTB is open. Pushing the
Bus Tie switch OFF, then AUTO, resets logic circuitry and allows the BTB to
close if the fault is corrected. TheAC bus is reconnected to the synchronous bus.
The main AC busses power individual equipment items such as:
• transformer-rectifier units (TRUs)
• navigation radios and flight control computers
• flight deck lighting
• pitot and window heat
December 01, 1999
747 Operations Manual
Copyright © The Boeing Company. See title page for details.
6.20.4 D6-30151-425
Electrical -
System Description
The main AC busses also power other AC busses:
• AC bus 1 powers the ground service bus and provides back-up power for
both transfer busses
• AC bus 2 powers the First Officer’s transfer bus
• Each AC bus powers a utility and galley bus
• AC bus 3 powers the Captain’s transfer bus and the main standby bus
Ground Service Bus
The ground service bus is powered on the ground and in flight whenever AC bus
1 is powered. The ground service bus powers individual equipment items such as:
• main and APU battery chargers
• fuel pumps for APU start
• upper deck doors
• flight deck flood, navigation, and service lights
• miscellaneous service outlets and equipment
• horizontal stabilizer fuel pump for defueling
On the ground when AC bus 1 is not powered, the ground service bus can be
connected to the same source powering the ground handling bus by pushing the
Ground Service switch on the flight attendant’s panel at door 2L.
Ground Handling Bus
The ground handling bus is powered on the ground when APU generator 1 or
external power 1 is available. If both are available, external power is automatically
used. The ground handling bus powers individual equipment items such as:
• lower cargo handling equipment and compartment lights
• fueling system
• auxiliary hydraulic pump 4
Transfer Busses
Two transfer busses provide AC power to critical flight-related equipment. The
transfer busses have a common back-up power source. Transfer to the back-up
power source is automatic.
Captain’s Transfer Bus
The Captain’s transfer bus is normally powered by AC bus 3. If AC bus 3 is
unpowered, AC bus 1 powers the Captain’s transfer bus. The Captain’s transfer bus
powers the APU standby bus.
The Captain’s transfer bus powers individual equipment items such as:
• centerEIU
• leftHF
December 01, 1999
747 Operations Manual
Electrical -
System Description
Copyright © The Boeing Company. See title page for details.
D6-30151-425 6.20.5
First Officer’s Transfer Bus
The First Officer’s transfer bus is normally powered by AC bus 2. If AC bus 2 is
unpowered, AC bus 1 powers the First Officer’s transfer bus. The First Officer’s
transfer bus powers individual equipment items such as:
• FMCS autothrottle servo
• lower EICAS display
• right ADC, right EFIS control, right EIU, right FMC
• right CDU, right HF, right ND, right PFD
Utility and Galley Busses
Each main AC bus powers one utility bus and one galley bus. Each utility/galley
bus is controlled by an electrical load control unit (ELCU) that protects the
electrical system from utility and galley bus faults, and provides load management
through automatic load shedding. With the Left Utility Power switch ON, utility
busses 1 and 2 and galley busses 1 and 2 are powered according to ELCU logic.
With the Right Utility Power switch ON, utility busses 3 and 4 and galley busses
3 and 4 are powered according to ELCU logic.
Utility busses power individual equipment items such as:
• forward main fuel pumps 2 and 3
• forward override/jettison pumps 2 and 3
• center override/jettison pump
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