B757 Electrical
Boeing B757 - Systems Summary [Electrical]
6.20 Electrical-System Description
Introduction
The electrical system generates and distributes AC and DC power to other airplane
systems, and is comprised of: main AC power, main DC power, battery/standby
power, and the hydraulic driven generator. System operation is automatic.
Electrical faults are automatically detected and isolated.
The electrical system generates and distributes AC and DC power to other airplane
systems, and is comprised of: main AC power, main DC power, and
battery/standby power. System operation is automatic. Electrical faults are
automatically detected and isolated.
AC Electrical System
The AC electrical system is the main source for airplane electrical power.
AC Electrical System Power Sources
The entire airplane AC electrical load can be supplied by any two main airplane
AC power sources.
The main airplane AC electrical power sources are:
• left and right engine integrated drive generators (IDGs)
• APU generator.
The entire airplane AC electrical load also can be supplied by external power.
The power sources operate isolated from one another.
Integrated Drive Generators (IDGs)
Each engine has an IDG. Each IDG has automatic control and system protection
functions.
When an engine starts, with the GENERATOR CONTROL switch selected ON,
the IDG automatically powers the respective main bus. The previous power source
is disconnected from that bus.
The IDG can be electrically disconnected from the busses by pushing the GEN
CTRL switch to OFF. The IDG can also be electrically disconnected from its
respective bus by selecting external power prior to engine shutdown.
The OFF light in the GEN CTRL switch illuminates, and the EICAS Advisory
message L or R illuminates, and the EICAS Advisory message L or R GEN OFF
displays whenever the generator control breaker is open.
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Boeing B757 - Systems Summary [Electrical]
The DRIVE light illuminates and the EICAS Advisory message L or R GEN
DRIVE displays when low oil pressure or high oil temperature is detected in an
IDG. The IDG drive can be disconnected from the engine by pushing the
respective DRIVE DISC switch. The IDG cannot be reconnected by the flight
crew.
APU Generator
The APU generator is electrically identical to the IDG generators. The APU
generator can power either or both main busses, and may be used in flight as a
replacement to an IDG source.
If no other power source is available when the APU generator becomes available,
the APU generator automatically connects to both main AC busses. If the external
source is powering both main busses, the external source continues to power both
main busses.
The APU Generator OFF light illuminates, and the EICAS advisory message APU
GEN OFF displays when the APU is operating and the APU generator control
breaker is open because of a fault or the APU GEN switch is selected OFF. When
the APU GENERATOR switch is ON and a fault is detected, the APU generator
cannot connect to the busses.
External Power
External power can power the left and right main busses. When the power source
voltage and frequency are within limits, the external power AVAIL light
illuminates.
Pushing the EXT PWR switch ON connects external power to both main busses
and removes the IDGs and the APU generator from the busses, if they were
powering the busses. When external power is connected to a main bus, the
EXTERNAL POWER ON light illuminates.
AC Electrical Power Distribution
AC power is distributed through the left and right main busses and the ground
service bus.
AC Main Busses
The right IDG normally powers the right main bus and the left IDG normally
powers the left main bus. The APU normally powers both main busses when they
are not powered by any other source. External power may also be connected and
will also power both main busses.
Bus tie breakers, controlled by BUS TIE switches, isolate or parallel the right and
left main busses. When both BUS TIE switches are set to AUTO, the bus tie
system operates automatically to maintain power to both main busses.
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Boeing B757 - Systems Summary [Electrical]
The AC bus ISLN light illuminates and the EICAS advisory message L or R BUS
ISOLATED displays when the bus tie breaker is open because of a fault or the
BUS TIE switch is OFF.
The BUS OFF light illuminates and the EICAS caution message L or R AC BUS
OFF displays if an AC bus is unpowered.
The source order for powering left and right main busses is the:
• respective IDG
• APU generator
• opposite IDG.
Utility busses
Left and right utility busses, powered by their respective main AC bus, are
controlled by UTILITY BUS switches. Left and right galley busses are powered
by their respective utility busses, and have no direct controls or indicators. The
utility bus OFF lights illuminate and the EICAS advisory message L or R UTIL
BUS OFF displays when a galley and utility bus are unpowered.
Ground Service Bus
The ground service bus is normally powered by the right main bus. Alternate
sources of power for the ground service bus are:
• the APU generator
• external power.
The ground service bus powers:
• the main battery charger
• the APU battery charger
Ground Handling Bus
• miscellaneous cabin and
system loads.
The ground handling bus can be powered only on the ground and only from the
APU generator or from the external power source. It is provided for loads such as
cargo handling and equipment energized only during ground operations.
Autoland
During autoland, the busses isolate to allow three independent sources to power
the three autopilots:
• the left main system powers the left autopilot and the captain’s flight
instrument transfer bus
• the right main system powers the right autopilot and the first officer’s
flight instrument transfer bus
• the battery/standby system powers the center autopilot.
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Boeing B757 - Systems Summary [Electrical]
Above 200 feet, loss of a generator results in:
• both bus tie breakers closing and the operating generator powers both left
and right AC busses
• the left main system powers the center autopilot
• NO LAND 3 appears on the Autoland Status Annunciator.
Below 200 feet, loss of a generator results in:
• both bus tie breakers remaining open
• the autopilot associated with a failed generator is unpowered
• the flight instruments remain powered through the flight instrument
transfer busses
• the autoland continues using the remaining two autopilots.
When the autopilots are disengaged or an autopilot go–around is performed, the
electrical system reverts to normal, non–isolated operation.
Flight Instrument Transfer Busses
Normally, the captain’s flight instruments are powered by the left main AC Bus,
and the first officer’s flight instruments are powered by the right main AC Bus. If
the respective bus tie breakers are in AUTO, the flight instrument transfer busses
transfer to the opposite main AC bus in the event power is lost to a main AC Bus.
If power is lost to both main AC busses, the captain’s flight instruments are
powered by the hydraulic driven generator.
Electrical Load Shedding
Electrical load shedding occurs automatically to ensure power is available to
critical and essential equipment.
If the electrical loads exceed the power available, the electrical system
automatically sheds AC loads by priority until the loads are within the capacity of
the generators. The load shedding is galleys first, then utility busses. Utility busses
are followed by individual equipment items powered by the main AC busses.
When an additional power source becomes available or the load decreases, the
electrical system automatically restores power to the shed systems (in the reverse
order).
Examples of load shedding that may be observed in the flight deck or cabin during
normal operations include:
• an electric hydraulic pump prior to engine start
• center tank fuel pumps prior to engine start
• utility busses during engine start.
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Boeing B757 - Systems Summary [Electrical]
Examples of load shedding that may be observed in the flight deck or cabin during
non–normal operations include:
• utility busses after a generator or engine failure
• center tank fuel pump after an engine failure
• cabin ceiling lights after an engine failure.
On the ground, advancing the thrust levers into the takeoff range with the engines
shut down may cause inadvertent load shedding of the utility busses to occur.
Returning the thrust levers to idle, then pushing the UTILITY BUS switches OFF,
then ON will reset this inadvertent load shedding.
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Boeing B757 - Systems Summary [Electrical]
AC Electrical System Schematic (Hydraulic Driven Generator)
APU GEN EXT PWR
O
F
F
BUS TIE
AUTO
ON
ON
BUS TIE
AVAIL
AUTO
ISLN
ISLN
UTILITY BUS
L
R
ON
ON
OFF
OFF
L BUS
BUS
OFF
L
GEN
CONT
O
F
F
L
ON
R BUS
BUS
OFF
GEN DRIVE DISC
DRIVE
R
GEN
CONT
R
DRIVE
O
F
F
ON
HYDRAULIC
DRIVEN GENERATOR
TO STANDBY
AC BUS
LEFT
MAIN
AC
BUS
LEFT AC
TRANSFER BUS
RIGHT AC
TRANSFER BUS
CAPT FLT INSTR
TRANSFER BUS
F/O FLT INSTR
TRANSFER BUS
RIGHT UTILITY
BUS
LEFT UTILITY
BUS
APU
TO
BATTERY
CHARGERS
GROUND
SERVICE BUS
GEN
APU GEN
CONTROL
SWITCH
GROUND
HANDLING BUS
LEFT
BUS
TIE
LEFT
GEN
CONTROL
RIGHT
MAIN
AC
BUS
EXTERNAL
POWER
SWITCH
RIGHT
BUS
TIE
RIGHT
GEN
CONTROL
LEFT
DRIVE
PRIMARY POWER SOURCE
SECONDARY OR BACKUP POWER SOURCE
Page 6
.
RIGHT
DRIVE
Boeing B757 - Systems Summary [Electrical]
AC Electrical System Schematic
APU GEN EXT PWR
O
F
F
BUS TIE
AUTO
ON
ON
BUS TIE
AVAIL
AUTO
ISLN
ISLN
UTILITY BUS
L
R
ON
ON
OFF
OFF
L BUS
BUS
OFF
L
GEN
CONT
O
F
F
L
ON
R BUS
BUS
OFF
GEN DRIVE DISC
DRIVE
R
GEN
CONT
R
DRIVE
O
F
F
ON
TO STANDBY
AC BUS
LEFT
MAIN
AC
BUS
CAPT FLT INSTR
TRANSFER BUS
F/O FLT INSTR
TRANSFER BUS
LEFT UTILITY
BUS
RIGHT UTILITY
BUS
TO
BATTERY
CHARGERS
APU
GROUND
SERVICE BUS
GEN
APU GEN
CONTROL
SWITCH
GROUND
HANDLING BUS
LEFT
BUS
TIE
LEFT
GEN
CONTROL
RIGHT
MAIN
AC
BUS
EXTERNAL
POWER
SWITCH
RIGHT
BUS
TIE
RIGHT
GEN
CONTROL
LEFT
DRIVE
PRIMARY POWER SOURCE
SECONDARY OR BACKUP POWER SOURCE
Page 7
RIGHT
DRIVE
Boeing B757 - Systems Summary [Electrical]
DC Electrical System
The main DC electrical system uses transformer–rectifier units (TRUs) to produce
DC power. The TRUs are powered by the main AC busses.
The TRUs operate isolated from one another. If one TRU fails, the DC bus tie
breaker closes to keep both DC busses powered. Both BUS TIE switches must be
in AUTO for the DC bus tie breaker to close.
There are no flight deck controls for the main DC electrical system.
APU GEN EXT PWR
O
F
F
BUS TIE
AUTO
ON
ON
BUS TIE
AVAIL
AUTO
ISLN
ISLN
UTILITY BUS
L
R
ON
ON
OFF
OFF
L BUS
BUS
OFF
L
GEN
CONT
O
F
F
L
ON
R BUS
BUS
OFF
GEN DRIVE DISC
DRIVE
R
DRIVE
R
GEN
CONT
O
F
F
ON
TO STANDBY
DC BUS
BATTERY BUS
LEFT
MAIN
DC
BUS
RIGHT
MAIN
DC
BUS
MAIN
BATTERY
T
DC BUS
TIE BREAKER
TRU
TRU
FROM LEFT
AC BUS
PRIMARY POWER SOURCE
SECONDARY OR BACKUP POWER SOURCE
Page 8
FROM RIGHT
AC BUS
Boeing B757 - Systems Summary [Electrical]
Battery/Standby Power System
The battery/standby power electrical system can supply DC and AC power to
selected flight instruments, communications and navigation systems, and other
critical systems, if there are main AC and DC electrical power system failures.
The Battery/Standby Power System consists of the following busses:
• the hot battery bus
• the battery bus
• the standby DC bus
• the standby AC bus
Hot Battery Bus
The hot battery bus provides power to items which must be continuously powered,
such as the clock’s time reference.
Prior to establishing electrical power, the main battery powers the hot battery bus.
After establishing electrical power, the main battery charger powers the hot
battery bus.
Battery Bus
Prior to establishing electrical power, when the battery switch is ON, the main
battery powers the battery bus.
After establishing electrical power, the left DC system powers the battery bus, and
the main battery provides a backup source of power.
The Battery DISCH light illuminates when the main battery is discharging. If
EICAS is powered, the advisory message MAIN BAT DISCH also displays.
The battery OFF light illuminates and the EICAS advisory message BATTERY
OFF displays if the battery switch is OFF after electrical power is established.
Standby DC Bus
The Standby DC Bus can be powered by several sources. Prior to establishing
electrical power, when the battery switch is ON and the standby power selector is
in AUTO, the main battery powers the standby DC bus. The Battery DISCH light
illuminates when the main battery is discharging. After establishing electrical
power, the left DC system powers the standby DC bus and the main battery
provides a backup source of power. When the standby power selector is in BAT,
the main battery powers the standby DC bus.
The standby bus OFF light illuminates and the EICAS advisory message
STANDBY BUS OFF displays if the standby DC bus is unpowered.
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Boeing B757 - Systems Summary [Electrical]
Standby AC Bus
The Standby AC bus can be powered by several sources. Prior to establishing
electrical power, when the battery switch is ON and the standby power selector is
in AUTO, the main battery powers the standby inverter which provides AC power
to the standby AC bus. After establishing electrical power, the left AC system
powers the standby AC bus and the main battery and standby inverter provide a
backup source of power. When the standby power selector is in BAT, the main
battery and standby inverter power the standby AC bus.
The standby bus OFF light illuminates and the EICAS advisory message
STANDBY BUS OFF displays if the standby AC bus is unpowered.
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Boeing B757 - Systems Summary [Electrical]
Battery/Standby System Schematic
(FROM THE GROUND SERVICE BUS)
BATTERY
CHARGER
BAT
STBY POWER
AUTO
ON
HOT BATTERY
BUS
MAIN
BATTERY
BAT
OFF
D
I
S
C
H
O
F
F
STANDBY
DC BUS
STANDBY
AC BUS
BATTERY
BUS
L MAIN
DC BUS
BATTERY
CHARGER
BAT
L MAIN
AC BUS
STBY POWER
AUTO
ON
HOT BATTERY
BUS
MAIN
BATTERY
BAT
OFF
D
I
S
C
H
O
F
F
INV
STANDBY
DC BUS
STANDBY
AC BUS
BATTERY
BUS
L MAIN
DC BUS
BATTERY
CHARGER
BAT
L MAIN
AC BUS
STBY POWER
AUTO
ON
HOT BATTERY
BUS
MAIN
BATTERY
BAT
OFF
D
I
S
C
H
O
F
F
INV
STANDBY
DC BUS
STANDBY
AC BUS
BATTERY
BUS
L MAIN
DC BUS
L MAIN
AC BUS
LIGHT ILLUMINATED
FLOW LINES
NOT ON PANEL:
DC POWER FLOWING
DC POWER NOT FLOWING
AC POWER FLOWING
AC POWER NOT FLOWING
Hydraulic Driven Generator
The hydraulic driven generator (HDG) activates automatically when both the left
and right main AC busses are unpowered. The hydraulic driven generator is
powered by the left hydraulic system. Page 11
Boeing B757 - Systems Summary [Electrical]
Hydraulic Driven Generator
The hydraulic driven generator (HDG) activates automatically when both the
left and right main AC busses are unpowered. The hydraulic driven generator
is powered by the left hydraulic system.
The hydraulic driven generator provides AC power to:
• the left AC transfer bus
• the right AC transfer bus
• the standby AC bus (through the left AC transfer bus)
• the captain’s flight instrument transfer bus
The hydraulic driven generator provides DC power to:
• the hot battery bus
• the battery bus
• the standby DC bus
The amount of DC power produced by the hydraulic driven generator is less than
the DC power produced by a fully charged battery. When the hydraulic driven
generator first begins to operate, the battery DISCH light may illuminate until the
battery power decreases to the power level produced by the hydraulic driven
generator.
Battery/Standby System Schematic (Hydraulic Driven
Generator Operating)
BAT
BATTERY
CHARGER
STBY POWER
AUTO
ON
HOT BATTERY
BUS
MAIN
BATTERY
D
I
S
C
H
OFF
OFF
O
F
F
BATTERY
BUS
L MAIN
DC BUS
LEFT
TRANSFER BUS
HYDRAULIC DRIVEN
GENERATOR
RIGHT
TRANSFER BUS
FLOW LINES
NOT ON PANEL:
DC POWER FLOWING
DC POWER NOT FLOWING
BAT
CAPT FLT INSTR
TRANSFER BUS
AC POWER FLOWING
AC POWER NOT FLOWING
Page 12
STANDBY
DC BUS
STANDBY
AC BUS
Boeing B757 - Systems Summary [Electrical]
6.30 Electrical-EICAS Messages
Electrical EICAS Messages
The following EICAS messages can be displayed.
Message
Level
L AC BUS OFF
Caution
BUS
OFF
APU GEN OFF
Advisory
OFF
APU generator control
breaker is open due to a
fault with the APU running.
BATTERY OFF
Advisory
OFF
Battery switch is OFF.
L BUS ISOLATED
Advisory
ISLN
Bus tie breaker is open due
to an AC electrical system
fault.
Advisory
DRIVE
Generator drive oil pressure
is low or generator drive oil
temperature is high.
Advisory
OFF
Generator control breaker is
open.
MAIN BAT DISCH
Advisory
DISCH
Main battery is discharging.
STANDBY BUS OFF
Advisory
OFF
Standby AC or DC bus is
unpowered.
L UTIL BUS OFF
Advisory
OFF
Galley and utility bus are
unpowered.
R AC BUS OFF
Light
R BUS ISOLATED
L GEN DRIVE
R GEN DRIVE
L GEN OFF
R GEN OFF
R UTIL BUS OFF
Page 13
Aural
Beeper
Condition
AC Bus is unpowered.
Boeing B757 - Systems Summary [Electrical]
6.10 Electrical-Controls and Indicators
Electrical Panel
8
1
APU GEN EXT PWR
2
O
F
F
BUS TIE
3
4
AUTO
ON
9
ON
BUS TIE
AVAIL
AUTO
ISLN
ISLN
UTILITY BUS
L
R
ON
ON
OFF
OFF
L BUS
BUS
OFF
5
L
GEN
CONT
6
7
O
F
F
L
ON
10
BUS
OFF
12
GEN DRIVE DISC
DRIVE
11
R BUS
R
R
GEN
CONT
DRIVE
13
O
F
F
ON
14
OVERHEAD PANEL
1
APU Generator (APU GEN) Control Switch
ON (bar in view) –
• arms APU generator control breaker to close automatically.
OFF (bar not visible) –
• opens APU generator control breaker
• resets fault trip circuitry.
2
APU Generator OFF Light
Illuminated (amber) –
• the APU generator control breaker is open because of a fault with APU
running
• the APU generator control switch is selected OFF.
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Boeing B757 - Systems Summary [Electrical]
3
BUS TIE Switches
AUTO –
• arms automatic AC bus tie circuits
• arms automatic DC bus tie circuits
• arms automatic flight instrument transfer bus circuits.
OFF (AUTO not visible) –
• commands the AC bus tie open
• commands the DC bus tie open
• commands the flight instrument bus tie open
• resets fault trip circuitry.
4
AC Bus Isolation (ISLN) Lights
Illuminated (amber) –
• a fault has occurred, automatically opening the AC bus tie breaker
• the BUS TIE switch is OFF.
5
AC BUS OFF Lights
Illuminated (amber) – the AC bus is unpowered.
6
Generator Control (GEN CONT) Switches
ON (bar in view) – arms the generator control breaker to close automatically when
generator power is available.
OFF (bar not visible)
• opens generator control breaker
• resets fault trip circuitry.
7
Generator OFF Lights
Illuminated (amber) – the generator control breaker is open.
8
External Power (EXT PWR) Switch
Push – if AVAIL light is illuminated, closes external power contactor
Subsequent push – opens external power contactor.
9
External Power ON Light
Illuminated (white) – external power is powering the bus(es).
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Boeing B757 - Systems Summary [Electrical]
10
External Power Available (AVAIL) Light
Illuminated (white) – external power is plugged in and power quality is
acceptable.
11
UTILITY BUS Switches
ON (bar in view) – if no load shed signal is present, connects utility and galley
busses to main AC bus.
OFF (bar not visible) –
• disconnects utility and galley busses from main AC bus
• resets overload load shed circuitry.
12
Utility Bus OFF Lights
Illuminated (amber) – the utility and galley busses are unpowered.
13
Generator Drive Disconnect (GEN DRIVE DISC) Switches
Push –
• disconnects generator drive from the engine
• requires maintenance action on the ground to reconnect the generator
drive.
14
Generator DRIVE Lights
Illuminated (amber) –
• the generator drive oil temperature is high
• the generator drive oil pressure is low.
Battery/Standby Control Panel
4
1
BAT
2
3
ON
D
I
S
C
H
STBY POWER
AUTO
OFF
BAT
OFF
O
F
F
OVERHEAD PANEL
Page 16
5
Boeing B757 - Systems Summary [Electrical]
1
Battery (BAT) Switch
ON –
• Unpowered airplane on the ground:
• a few annunciator lights illuminate
• allows the APU to be started
• Powered airplane inflight or on the ground when AC power is removed or
lost:
• the standby and battery busses are powered.
OFF (ON not visible) – turns battery power off.
2
Battery OFF Light
Illuminated (amber) – the battery switch is off.
3
Battery Discharge (DISCH) Light
Illuminated (amber) – the main battery is discharging.
4
Standby (STBY) POWER Selector –
OFF – the standby busses are unpowered.
AUTO – the standby busses transfer to battery power if normal AC power is lost.
BAT – the standby busses are powered from the main battery.
5
Standby Power Bus OFF Light
Illuminated (amber) – standby AC or DC bus not powered.
Page 17
Boeing B757 - Systems Summary [Electrical]
Hydraulic Generator Test Switch
1
HYD GEN
ACCESSORY PANEL
1
Hydraulic Generator Test Switch
Spring-loaded to center.
HYD GEN – initiates hydraulic driven generator system test.
Page 18