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• recirculating fans
Galley busses power the galleys located throughout the cabin.
The EICAS message ELEC UTIL BUS displays and the utility OFF light
illuminates if one or more utility or galley bus become unpowered due to a fault.
Cycling the Utility Power switch OFF, then ON restores power to the affected bus
if the fault is corrected.
A guarded ON Galley Emergency Power Off switch is located at each galley. If a
Galley Emergency Power Off switch is selected OFF, the related ECAS message
ELEC UTIL BUS displays and utility OFF light illuminates. Cycling the flight
deck Utility Power switch OFF, then ON will not reset the indications. The flight
deck Utility Power switch should remain ON after cycling, which permits the
remaining utility and galley busses to be powered and controlled by their related
ELCU.
December 01, 1999
747 Operations Manual
Copyright © The Boeing Company. See title page for details.
6.20.6 D6-30151-425
Electrical -
System Description
Electrical bus Isolation during Autoland
During automatic precision approach below 1,500 feet radio altitude,AC and DC
busses 1, 2, and 3 are automatically isolated. This provides an independent power
source for each of the three autopilots. AC bus 4 continues to power the
synchronous bus, and will power any other AC bus if its generator fails. During
autoland, the AC bus isolation lights 1, 2, and 3 do not illuminate, the EICAS
message ELEC BUS ISLN does not display, and the message “ELECTRICAL
SYNOPTIC INHIBITED FOR AUTOLAND” appears on the electrical synoptic.
Autoland is disengaged and the electrical system returned to normal configuration
under the following conditions:
• TO/GA mode active
• autopilot disengaged
• loss of autoland redundancy
AC Standby Power System
The AC standby power system provides electrical power to critical flight deck
equipment.
Major components of the system include:
• main and APU standby busses
• main and APU batteries
• main and APU standby inverters
• Standby Power selector
Main Standby Bus
The main standby bus is normally powered by AC bus 3. With the Battery switch
ON, Standby Power selector in AUTO, and AC bus 3 not powered, the main
standby bus is powered by the main standby inverter. The main standby inverter is
powered by the main battery charger through the main hot battery bus. The main
battery charger is normally powered byAC bus 1 through the ground service bus.
With the Battery switch ON, Standby Power selector in AUTO, and bothAC bus
1 and AC bus 3 not powered, the main standby bus is powered by the main standby
inverter. The main standby inverter is powered by the main battery through the
main hot battery bus.With the main battery charger unpowered, the main battery
can provide power to the main standby bus for a minimum of 30 minutes.
The main standby bus powers individual equipment items such as:
• left EIU, left FMS-CDU, left ILS, left VOR
• various flight control components
• standby ignition for all engines
• primary EICAS display, RMI, standby instrument lights
• left ADC, left EFIS control panel
December 01, 1999
747 Operations Manual
Electrical -
System Description
Copyright © The Boeing Company. See title page for details.
D6-30151-425 6.20.7
APU Standby Bus
The APU standby bus is normally powered by the Captain’s transfer bus.With the
Battery switch ON, Standby Power selector in AUTO, and the Captain’s transfer
bus not powered (loss of AC busses 1 and 3), the APU standby bus is powered by
the APU standby inverter. The APU standby inverter is powered by the APU
battery through the APU hot battery bus. With the APU battery charger
unpowered, the APU battery can provide power to the APU standby bus for a
minimum of 30 minutes.
The APU standby bus powers these equipment items:
• leftFMC
• leftND
• leftPFD
Standby Power Selector - BAT Position
With the Battery switch ON and Standby Power selector in BAT, main and APU
battery chargers are disabled. Each AC standby bus is powered by its related
battery and inverter. Each battery can provide power for a minimum of 30 minutes.
Note: The Standby Power selector must be in AUTO for flight. The BAT position
is for ground maintenance use only.
December 01, 1999
747 Operations Manual
Copyright © The Boeing Company. See title page for details.
6.20.8 D6-30151-425
Electrical -
System Description
AC Standby Power System Schematic
INVERTER
STANDBY
APU MAIN
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