ELECTRICAL
ELECTRICAL SYSTEM GENERAL:
The primary electrical system is 28 VDC supplied by:

Four Engine Driven Generators supplying

One APU Starter Generator supplying

Two nickel-cadmium Batteries supplying

One lead-acid Backup Battery supplying

External Power Cart.
28 VDC / 400A
28 VDC / 400A,
24 VDC / 44 Ahr
24 VDC / 5 Ahr
The Electrical System is controlled by the Electrical Distribution Logic (EDL) and Generator
Control Units (GCU). EDL controls all Contactors (CTCR), fault protections, and load shedding.
Failures in the system are automatically detected, isolated and buses de-energized as necessary.
Automatic functions of load shedding and electrical emergency configuration can be bypassed with the SHED BUSES knob and ESSENTIAL POWER button.
The DC Electrical System is divided in to LEFT and RIGHT Networks:
LEFT -- Engine 1 supplies Generators 1 & 3

DC Bus 1

Essential Bus 1

Shed Bus 1

Battery 1
RIGHT -- Engine 2 supplies Generators 2 & 4

DC Bus

Essential BUS 2

Shed Bus 2

Battery 2
Both LEFT and RIGHT networks connect through two Bus Tie Contactors with the Central DC
Bus. The Central Bus allows the GPU and APU Generator to join the system and allows load sharing. The Central Bus is normally powered by the RIGHT Network, or Engine 2.
A dedicated MFD Electrical Page displays System Configuration, Voltages, Amperages, Battery
Temperatures, Warnings and Cautions.
ENGINE DRIVEN GENERATORS:
There are four independent engine-driven brushless DC Generators; two installed on each engine gearbox. Each Generator is controlled and protected by a dedicated Generator Control Unit
(GCU). The Generators come on-line when the engine speed reaches 56.4% N2.
GENERATOR CONTROL UNIT PROTECTIONS:
The Electrical System provides protection to the electrically supplied equipment from a Generator
Control Unit failure. In the event of a Generator failure, over voltage, over current, or bus failure:

The associated GCU de-energizes and disconnects.

LEFT and RIGHT networks may be segregated (BTC 1&2 OPEN)

Some BUSES may be de-energized.
EXTERNAL POWER:
There is a Receptacle on the LEFT side of the aircraft to connect External Power to the plane.
External Power is used primarily for APU starting. External Power has priority over all other electrical sources.
External Power application is controlled from the GPU button in the cockpit and GPU AVAIL illuminates when available. When pressed, the Ground Power Contactor CLOSES and the GPU
AVAIL light extinguishes. Cycling the button resets External Power.

External Power protections:


External Power may not operate in parallel with a generator:
External Power and Batteries may not simultaneously supply the Buses.

Overvoltage

Inverse polarity.
APU STARTER GENERATOR:
The Starter Generator is driven by the APU. It can supply 28 VDC / 400A power during ground or in-flight operations as an alternative source for the failure of any Engine Generator.
The APU is started using Battery 2, or Battery 2 assisted by the Main Generators, or by External
Power. Battery 1 is isolated during APU Start to provide a stable start avoiding system voltage transients.
During APU start, the APU Starting Contactor (ASC) is CLOSED allowing the APU Starter
Generator to operate as a starter energized by the CENTRAL DC BUS. After the APU is operating the ASC will normally OPEN. If the ASC remains CLOSED an EICAS Message will be displayed.
After the APU reaches 95% RPM plus 7 seconds, the Starter Generator is available to supply the aircraft with energy. The APU Line Contactor (ALC) is normally CLOSED to connect the Starter
Generator to the load buses. An EICAS Message will be received if the ALC remains OPEN after
RPM is above 95% and the ALC has failed or the APU GEN button is not pressed. Cycling the
APU GEN button will reset the APU Generator. The APU Generator normally supplies DC Bus 2 when operating.
MAIN BATTERIES:
Two 24 VDC / 44 AH Ni-Cad Main Batteries are provided to supply essential loads during electrical emergency and ground operations. Battery 1 connects in parallel with the LEFT
Network (Gen 1 & 3), but is isolated during APU start. Battery 2 connects in parallel with the
RIGHT Network (Gen 2 & 4). With the BATT knobs in the AUTO position, the Battery Contactors
(BC 1 & 2) operate according to EDL. With the BATT knobs in the OFF position, BC 1 & 2 are
OPEN. The Main Batteries are ventilated and their temperatures monitored
ELECTRICAL EMERGENCY:
Electrical emergency is defined as a loss of all 5 generators fail in-flight. EDL should automatically de-energize the DC buses and the Shed buses, and connect the Main Batteries to the Essential buses. The ESSENTIAL POWER button forces this “electrical emergency configuration”.
The Main Batteries should provide 40 minutes of power, 45 minutes if Pitot 3 is de-selected. Only the Essential Buses will be powered. Standby Instruments (or ISIS), RMU’s and EICAS are the primary flight instruments.
AC ELECTRICAL SYSTEM:
The AC System is supplied by one 250 VAC / 400 Hz single phase Static Inverter delivering 115
VAC. The primary function is to supply the Avionics System with AC power. It is controlled by the
AC POWER button. It is normal to leave the AC Power Button pressed all the time, even after
Engines shutdown. Cycling the AC POWER button will reset the Static Inverter.
The Static Inverter is supplied by DC BUS 1. In an electrical emergency configuration, where the
DC BUS 1 is de-energized, you will NOT have AC power. Normally, if the 115 VAC BUS is deenergized EICAS will display a message.

BACKUP BATTERY:
The Backup Battery is 24 VDC / 5Ahr lead-acid provides a stable power source for the GCU's to permit protective functions to operate if there is a short circuit when system voltage may drop near zero volts.
The Backup Battery is charged when, the BACKUP Button is pressed, the BATT 1 knob is in the
AUTO position, and a generator is connected. If the BACKUP button is released, an EICAS
Message is displayed.
SHED BUSES:
The SHED Buses provide for in flight electrical load shedding when 2 or less generators are available.
On the ground, the SHED Buses can be energized with:

External Power


At least one generator and the SHED BUSES knob in the OVRD position
3 or more generators
GROUND SERVICE BUS:
The Ground Service Bus (GSB) provides electrical power through the GPU without energizing any other electrical distribution BUSES. This powers some internal lights for aircraft servicing and maintenance. The BUS is energized by connecting the GPU Connector to the aircraft
Receptacle, but will only be powered if the Battery Contactors (BC 1 and BC 2) and GPC are
OPEN.
This is what it will power:

Cockpit Dome Lights

Courtesy Lights

Stair Lights

Galley Lights

Passenger Cabin Lights

Lavatory Lights

Baggage Compartment Lights

Service Compartment Lights
BATT 1 (2) OVTEMP
Associated battery temperature is above 70C.
ELEC ESS XFR FAIL
Automatic transfer to electrical emergency condition failed.
GEN 1 (2,3,4) OVLD
Associated generator current is above 400A
GEN 1 (2,3,4) OFF BUS
Associated generator is disconnected from electrical network after engine stabilization due to generator channel failure or button released.
APU GEN OVLD
APU generator current is greater than 400A.

APU GEN OFF BUS
APU generator is disconnected from electrical network, due to ALC open, with APU RPM above
95% due to generator channel failure or button released.
APU CNTOR CLSD
APU starting contactor (ASC) or Line Contactor (ALC) is inadvertently closed.
DC BUS 1 (2)
Associated DC Bus is deenergized. If DC Bus 1 is deenergized the inverter becomes inoperative.
ESS BUS 1 (2) OFF
Associated Essential Bus is deenergized.
SHED BUS 1 (2) OFF
Associated Shed Bus is deenergized.
BATT 1 (2) OFF BUS
Associated battery is disconnected from electrical network.
BKUP BATT OFF BUS
Backup battery is disconnected from electrical network.
ELEC EMERG ABNORM
Improper transfer to electrical emergency condition occurred.
115VAC BUS OFF
115VAC Bus is deenergized.
GEN 1 (2,3,4) BRG FAIL
Associated generator bearing has failed.