Electrics - Main AC & DC Power Show Schematic only

Amendment: Description amended to include Flight Instrument Bus Power Switch.

The electrical system generates and distributes AC and DC power to other airplane systems, and is comprised of:

System operation is automatic. Electrical faults are automatically detected and isolated.

Main AC Electrical System

The AC electrical system is the main source for airplane electrical power.

filler graphic. AC Electrical System. Start APU Connect External Power Shutdown Left Engine Shutdown Right Engine AC Buses and Load Shed System Generator Drive Disconnect Generator Drive Disconnect AC Bus Tie System AC Bus Tie System Generator Control Switch Generator Control Switch Ground Service Switch Ground Service Bus Ground Handling Bus - The Ground Handling Bus can only be powered on the ground by the APU or External Power. It cannot be powered from the engine-driven generators Left AC Bus OFF Light Right AC Bus OFF Light Left Flight Instrument Transfer Relay Right Flight Instrument Transfer Relay Left AC Bus Right AC Bus Captain's Flight Instruments Transfer Bus First Officer's Flight Instruments Transfer Bus

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:

The entire airplane AC electrical load also can be supplied by External Power.

The power sources operate isolated from one another and cannot be paralleled. For most normal operations, both engine driven generators are available. To use both at the same time, the electrical system is divided into two sections, left and right. As a result, each engine driven generator powers an essentially independent electrical system.

Integrated Drive Generators

Each engine has an IDG which combines a generator driven by a Constant Speed Drive Unit (CSDU). Each IDG has automatic control and system protection functions.

Power from the generator is routed to the main AC bus through the Generator Control Breaker. When an engine starts, with the GEN CTRL switch selected ON, the Generator Control Breaker closes and the IDG automatically powers the respective Main AC Bus. The previous power source is disconnected from that bus.

Generator Switch The IDG can be electrically disconnected from the buses by pushing the respective GEN CTRL Switch to OFF. Selecting OFF opens the generator control breaker and resets fault trip circuitry. The IDG can also be electrically disconnected from its respective bus by selecting External Power prior to engine shutdown.

Generator OFF The OFF Light in the GEN CTRL switch illuminates, and the EICAS Advisory / Caution message L or R GEN OFF displays whenever the generator control breaker is open.

Generator Drive The DRIVE light illuminates and the EICAS Advisory message L or R GEN DRIVE displays when:

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 buses, and may be used in flight as a replacement to an IDG source SCHEMATIC.

Generator Switch Electrical power from the APU generator is routed through the APU generator breaker. When the APU Generator Control Switch is ON, the APU generator breaker operates automatically.

If no other power source is available when the APU generator becomes available, the APU generator automatically connects to both main AC buses through the Bus Tie Breakers. If the External source is powering both main buses, the external source continues to power both main buses.

If only one engine-driven generator is powering both the left and right systems when the APU generator becomes available, it causes the operating generators' BTB to open. This isolates the generator, but does not trip it off. The APU the powers one side of the system while the operating generator powers the other SCHEMATIC.

Selecting OFF:

  • Opens the APU Generator control breaker
  • Resets fault trip circuitry

Generator OFF The APU Generator OFF light illuminates, and the EICAS advisory message APU GEN OFF displays when:

When the APU Generator switch is ON and a fault is detected, the APU Generator cannot connect to the buses.

External Power

Ground Power Receptacle

A receptacle aft of the nose wheel well is used to connect external ground power to the airplane. External power can power the left and right main buses, and can be used instead of APU power. Both cannot be used simultaneously SCHEMATIC. External power must be manually selected.

External Power Switch. When the external power source voltage and frequency are within limits, the external power AVAIL light illuminates. When the AVAIL light is illuminated, the EXT PWR switch controls the External Power Contactor (EPC).

Pushing the EXT PWR switch ON connects external power to both main buses and removes the IDGs and the APU generator from the buses, if they were powering the buses. When external power is connected to a main bus, the EXTERNAL POWER ON light illuminates.

If the external power is powering the buses and an engine is started, that engine automatically powers its respective bus SCHEMATIC. External power is also removed from the buses by pushing the External Power Switch a second time.

AC Electrical Power Distribution

AC power is distributed through the left and right main buses and the ground service bus.

AC Main buses

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 buses when they are not powered by any other source. External power may also be connected and will also power both main buses.

From the main AC bus, the power can go to the Utility bus. Each main AC bus also supplies a transformer rectifier, which in turn supplies its respective DC bus.

AC Bus Tie System

Bus Tie Isolation

Bus Tie Breakers, controlled by BUS TIE switches, isolate or parallel the right and left main buses. When both Bus Tie Switches are set to AUTO, the bus tie system operates automatically to maintain power to both buses, whilst preventing paralleling.

  • Arms automatic AC and DC bus tie circuits
  • Arms automatic Flight Instrument Transfer Bus circuits
  • Commands the AC and DC bus tie open
  • Commands the Flight Instrument Transfer Bus Tie open
  • Resets fault trip circuitry

Bus Tie Isolation Warning The AC Bus ISLN Light illuminates and the EICAS advisory message L/R BUS ISOLATED displays when:

Bus Off Warning

The status of the AC bus is indicated by the BUS OFF Light. The BUS OFF light illuminates and the EICAS caution message L/R AC BUS OFF displays if an AC bus is unpowered SCHEMATIC.

The source order for powering left and right main buses is the:

  1. respective IDG
  2. APU generator
  3. opposite IDG

Utility (and Galley) buses

Utility Bus Off Left and Right Utility buses, powered by their respective main AC bus, are controlled by UTILITY BUS switches and the Load Shed System. Left and right galley buses are powered by their respective utility buses, and have no direct controls or indicators.

Selecting the UTILITY BUS switches OFF, disconnects the Utility buses and Galleys from the main AC buses. Selecting ON will connect the buses, but only if there is no load shed signal.

Utility Bus Off The Utility Bus OFF lights illuminate and the EICAS advisory message L/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 AC Bus. Alternate sources of power for the Ground Service bus are:

Ground Service Switch

When the Right AC Bus is not powered, the Ground Service Bus can be connected, via the Ground Handling Bus, to either External or APU power (using the Ground Service Switch on the main Cabin Attendant Panel). SCHEMATIC

The ground service bus powers:

Ground Handling Bus

The ground handling bus can be powered only on the ground and only from the APU Generator or from the External Power source and not by the engine-driven generators. It is provided for loads such as cargo handling and equipment energized only during ground operations.

The Ground Handling Bus powers the:

When external or APU generator power is available, the Ground Handling Bus is powered automatically. If both APU and external power are available, external power has priority SCHEMATIC. There are no lights or switches in the cockpit directly related to the Ground Handling bus.


During Autoland, the buses isolate to allow three independent sources to power the three autopilots:

Above 200 feet, loss of a generator results in:

Below 200 feet, loss of a generator results in:

When the autopilots are disengaged or an autopilot go-around is performed, the electrical system reverts to normal, non-isolated operation.

Flight Instrument Transfer buses

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 buses transfer to the opposite main AC bus in the event power is lost to a main AC Bus.

Flight Instrument Transfer buses

If power is lost to both main AC busses, the captain’s flight instruments are powered by the Hydraulic Driven Generator.

Flight Instrument Bus Power Switch.

Flight Instrument Bus Power Switch

G-BPEC through G-BPEK If power is lost to both main AC busses, either the captain’s or the first officer’s flight instruments are powered by the Hydraulic Driven Generator depending upon the position of the Flight Instrument Bus Power switch. With the switch in the off position, the captain’s flight instruments are powered. With the switch in the ALTN position, the first officer’s flight instruments are powered.

Flight Instrument Bus Power Switches are normally only found on British Airways aircraft.

AC Transfer buses

Left and right AC transfer buses power items considered necessary for ETOPs flights, which are not powered by the battery/standby system. Transfer buses are normally powered by their associated main AC buses, but also can be powered by the Hydraulic Driven Generator when both AC buses are unpowered.

Electrical Load Shedding

Electrical load shedding occurs automatically to ensure power is available to critical and essential equipment, and there are three conditions which result in load shedding:

Overload Load Shedding

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.

AMM This prevents the combination of 1) a passive IDG failure, and 2) a high load, from depressing system voltage and tripping the remaining IDG off line.

On the ground prior to engine start with a Center or Left electric hydraulic pump selected ON, overload protection operates in two phases. When the APU or external power exceeds its load limit, the Galley buses trip off. There is no cockpit indication of this trip off. The Utility buses trip off if the overload continues, illuminating the Utility Bus OFF lights.

If only the Galley buses trip off, they reset automatically as soon as the respective bus tie breaker opens or when all electric hydraulic pumps are selected OFF. For example, during engine start, as the generator comes on line and its bus tie opens, the galleys connected to it automatically reset. A manual reset is required if the Utility buses also trip off. Pushing the Utility Bus Switches OFF then ON again resets the Utility buses and the Galley buses.

If a separate generator is supplying each main AC bus, only the Utility and Galley buses associated with the overloaded generator are shed

The load shedding [order] is:

  1. Galley buses.
  2. Utility buses.
  3. Individual equipment items powered by the main AC buses.

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).

Engine Start Load Shedding

AMM This load shed circuit was designed to prevent a source overload (and subsequent shedding), when essential loads are turned on just prior to engine start. (Examples of essential loads are hydraulic pumps, and fuel pumps.)

Engine start load shedding occurs when the APU is used for both electrical and pneumatic power during engine start. This load shed trips the Utility and Galley buses, illuminating both Utility Bus OFF lights. By unloading the APU electrically, ample APU pneumatic power is assured for engine start.

When the first Engine Start Selector is moved to the GND position, both Utility and Galley buses trip off. The Utility Bus OFF lights illuminate. They reset automatically when either the generator breakers close or the engine start selector trips from GND to AUTO.

When the second Engine Start Selector is moved to GND, only the Utility and Galley buses connected to that engine trip Off.

Examples of load shedding that may be observed in the flight deck or cabin during normal operations include:

Generator Loss Load Shedding

AMM The Generator Loss load shed is needed to assure adequate generator voltage for hydraulic pump starting. This is needed for braking in the event of a refused takeoff due to engine failure.

During takeoff or inflight, the loss of one generator causes both Utility buses and all Galley buses to trip off. As a result, both Utility Bus OFF Lights illuminate. These buses cannot be reset manually in flight. However, on the ground with one or both Thrust Levers retarded, they can be rest by pushing the Utility Bus Switches OFF then ON again. In flight, they can only be reset by re-establishing a second generator. When a second generator comes on line, the Utility and Galley buses reset automatically.

Additional loads are automatically shed if an engine is shutdown inflight. This additional load shedding partially offsets the hydraulic system requirement for additional electrical power to operate the electric hydraulic pump.

Examples of load shedding that may be observed in the flight deck or cabin during non-normal operations include:

Arming of this load shed protection requires that the Air/Ground Logic be in the flight mode, or that the Thrust Levers both be moved into the Takeoff range.

On the ground, advancing the thrust levers into the takeoff range with the engines shut down may cause inadvertent load shedding of the utility buses to occur. Returning the thrust levers to idle, then pushing the UTILITY BUS switches OFF, then ON will reset this inadvertent load shedding.

Main DC Electrical System

The main DC electrical system uses two Transformer-Rectifier Units (TRUs) to produce DC power. The TRUs are powered by their respective main AC buses. On some airplanes, backup DC power is provided by a Hydraulic Driven Generator.

DC Bus Tie Breaker

During normal operations, the Left and Right TRUs (and buses) operate isolated from one another. However, if one TRU fails, the DC Bus Tie Breaker closes automatically to keep both DC buses powered. Both AC Bus Tie Switches must be in AUTO for the DC Bus Tie Breaker to close. If either switch is OFF, the DC bus tie breaker cannot close.

There are no flight deck controls for the main DC electrical system.

Battery & Standby Power