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ELECTRICAL SYSTEM
GENERAL ATA 24 - 00

ATA 24 ELECTRICAL POWER SYSTEM

For Training Purposes Only

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ELECTRICAL SYSTEM
GENERAL ATA 24 - 00
ELECTRICAL POWER SYSTEM
General DC Power
The electrical power system consists of both 115 Vac and 28 Vdc distribution The left and right dc buses supply power to those loads requiring dc power.
buses. Each main dc bus is divided into independent sections. When either bus is
unpowered the dc tie control unit automatically energizes the dc tie relay.
AC Power
The dc standby bus supplies power to certain essential airplane loads and
The main ac buses supply all of the essential ac loads in the airplane. Each will transfer sources in case of primary source loss.
bus is divided into independent sections.
The dc ground handling bus supplies dc power for ground handling equip-
An ac tie bus provides interconnection between the main buses under certain ment and is energized on the ground only.
conditions.
A main battery/battery charger system provides a dedicated source of power
The utility buses supply non essential loads such as passenger entertainment for operation of the standby and autoland systems.
and reading lights.
The separate APU battery/battery charger system provides power for APU
Galley power is also considered as non--essential load. The non essential starting.
load can be de--energized automatically for load shedding purposes.
The APU starter motor is energized by the APU TR unit if the right main ac Hydraulic Motor Generator System
bus is energized during APU starting. The hydraulic motor generator system provides a non--time limited alternate
The ground service bus supplies both inflight and ground loads. These source of ac and dc power after loss of all generator power.
include interior lights, battery chargers and cooling fans. An ac generator supplies captain’s flight instrument, left and right transfer
The ground handling bus supplies loads such as cargo handling equipment buses. A dc generator can power the hot battery bus.
that are used only during ground operations. This bus is only powered on the
ground.
The center buses supply both ac and dc power to the center channel
equipment of the autoland system. During category III autoland operation,
the buses are supplied from alternate sources independent of the main buses.
The flight instrument transfer buses supply power to selected captain’s and
first officer’s flight instruments and allows automatic transfer to an alternate
power source in case of normal source failure.
The ac standby bus supplies single phase power to essential flight loads and
will automatically transfer power sources in case of primary source loss.
For Training Purposes Only

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RECURRENT TRAINING BOEING 767 - 300

ELECTRICAL SYSTEM
GENERAL ATA 24 - 00
For Training Purposes Only

Figure 1 Electrical Power System

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ELECTRICAL SYSTEM
GENERAL ATA 24 - 00
ELECTRICAL SYSTEM PANELS
General Utility Bus Switches
Normal operation of the electrical power system is performed at the electrical The alternate action utility bus switches provide manual control of the power
system control panel and the auxiliary electrical system control panel. Both relays connecting utility and galley buses to the main ac buses. The ON legend
momentary and alternate action pushbutton switches are used on the electrical is seen when the switch is latched (ON). The amber OFF light illuminates when
panels. The alternate action switches are normally in the latched (ON) position. the associated utility bus relay (UBR) is open.
This allows automatic operation of the electrical system breakers and relays
under the control of the system‘s computers. Generator Drive Disconnect Switches
Switch indicator lights are powered by the master dim and test system. Pushing the momentary generator drive disconnect switches causes a
mechanical disconnection between the integrated drive generator and the
Electrical System Control Panel (P5) engine when the associated engine fuel control switch is not off. An amber
The momentary external power switch controls the position of the external DRIVE light on the switch illuminates for either high IDG oil temperature or
power contactor. A white AVAIL light indicates power is of proper quality. low IDG oil pressure.
The white ON light illuminates when the external power contactor is closed. Battery Switch
Generator Control Switches The alternate action battery switch controls connection of the battery bus to
the left dc bus or the hot battery bus. The ON legend is seen when the switch
The generator control switches provide a control signal which closes the
is latched (ON). An amber OFF light illuminates when the battery switch is
generator control relay (GCR), and, when proper power is available, enables
unlatched during normal ground and flight operations.
automatic closing of the generator circuit breaker (GCB). The flow bar and
ON legend indicate switch position. An amber OFF light illuminates when the Battery Discharge Light
associated generator circuit breaker is open.
For the main battery, a dedicated battery discharge light illuminates if the
battery is discharging.
AC Bus Tie Switches Standby Power Switch
The ac bus tie switches allow manual or automatic control of the bus tie The standby power mode of operation is controlled by the standby power
breaker (BTB). When a switch is unlatched, the associated BTB is locked selector switch. To turn the switch off, push the switch in when going from
open, isolating the associated main ac bus from the ac tie bus. In this case, the AUTO to the OFF position.
the AUTO indication is not visible and the amber ISLN light is illuminated.
An amber standby power bus OFF light illuminates when the ac or dc standby
Latching an ac bus tie switch (ON) enables automatic operation of the
bus is unpowered.
associated bus tie breaker. In this case, the AUTO light normally illuminates
and the ISLN light is OFF. If the ISLN light illuminates when the bus tie switch Auxiliary Electrical System Control Panel (P61)
For Training Purposes Only

is latched, then a fault has tripped and locked the BTB open.
The momentary generator field manual reset switch is pushed to open and
AC BUS OFF Lights close the generator field control relay when the generator control switch is
unlatched. A white FIELD OFF light illuminates when the generator control
An ac BUS OFF indicator light illuminates when the associated main ac bus is
relay (GCR) is open.
de-energized.

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ELECTRICAL SYSTEM
GENERAL ATA 24 - 00
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Figure 2 Electrical System Panels

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ELECTRICAL SYSTEM
GENERAL ATA 24 - 00
AC EXTERNAL POWER SYSTEM
Purpose The 115 volt ac ground handling bus is automatically powered on the ground if
The ac external power system supplies 115/200 Vac, 3 phase, 400 Hz external power is of acceptable quality. The BPCU controls the three position
ground power to the airplane ac electrical loads. ground handling relay and automatically gives external power priority over the
APU generator.
System Description
Ground service bus power is supplied to equipment which must be powered in
A ground power source can be connected at the airplane external power both air and ground modes. This bus receives power automatically from the
receptacle to supply power to the 767 ac electrical power system. Power right main ac bus if that bus is powered. If the right main bus is not powered,
is supplied to main load buses when the external power contactor (EPC) pressing the ground service switch allows supply of either external power or
and corresponding bus tie breakers (BTB‘s) close. The external power APU generator power (whichever is first available) to the ground service bus.
source also supplies power to the ground service and ground handling buses,
using the ground service select, ground service transfer, and ground handling If both external and APU generator power are available when the ground ser-
relays. vice switch is pressed, external power is used.
If a fault exists in the external power circuit, BPCU trips the EPC to protect
Component Locations the power source and loads.
An external power receptacle (P30 panel) is located on the lower fuselage,
just aft of the nose gear. All other external power system components are
located in the main equipment center. There are two control switches:
-- An EXT PWR switch is located on the P5 overhead panel.
-- A ground service switch is located on the P21 forward attendant’s panel.

Operation
The Bus Power Control Unit (BPCU) controls the EPC. The EPC controls the
flow of external power to the ac tie bus. When external power of acceptable
quality (as determined by the BPCU) is first connected, the AC CONNECTED
and PWR NOT IN USE lights on the P30 panel come on. Also, the AVAIL light
on the EXT PWR switch comes on. Pressing the momentary EXT PWR switch
closes the EPC. When the EPC closes, the ON light on the EXT PWR switch
comes on.
External power always has priority over the IDGs and APU generator when
manually selected. If both engines are started and the IDGs are ready to
For Training Purposes Only

supply electrical power, then, automatically, the EPC and BTBs are tripped
open and the GCB’s close. If both engines are operating, and external power
is supplying power to the main buses, cycling the generator control switches
(unlatch--latch) also causes the EPC and BTBs to trip open and the GCB’s to
close.

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RECURRENT TRAINING BOEING 767 - 300

ELECTRICAL SYSTEM
GENERAL ATA 24 - 00
For Training Purposes Only

Figure 3 AC External Power System

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ELECTRICAL SYSTEM
GENERAL ATA 24 - 00
BUS POWER CONTROL UNIT
Purpose Operation
The Bus Power Control Unit (BPCU) controls: The BPCU contains all of circuitry necessary for external power monitoring
and protection, load shedding on the utility and galley buses, tie bus differential
* Electrical system external power monitoring and protection, protection, and control of the external power contactor, ground handling relay,
* Load Shedding, and ground service relays. In addition, built--in test equipment (BITE) circuitry
is included to aid on--aircraft fault isolation for external power.
* Tie Bus Differential Protection,
The BPCU utilizes a microprocessor system for control, protection and BITE
* Autoland power transfer functions. The basic software timing cycle is 5 msec which gives sufficient
* BITE. time to perform the longest operating path. The approximate division of time
for flight or ground operations is protection (1 msec), control (1 msec) and
Location BITE (3 msec). The information required by the microprocessor comes from
The BPCU is located in the main equipment center, rack mounted on the peak sensing circuits that sense external power voltage, current, frequency
E2--4 shelf. and phase. Additional inputs are external power and ground service switch
inputs, external power contactor auxiliary contacts, and generator control
Physical Description/Features unit BITE information.
The BPCU is packaged in an ARINC 600 3 MCU enclosure and weighs 7.7 The switch, auxiliary contact, and voltage/current peaks are brought into the
pounds. The BPCU is held in the tray with an extractor front hold down microprocessor system through input conditioning circuits.
assembly. Forced air cooling is provided through rack. The air enters BPCU The microprocessor system in turn supplies both digital and analog outputs
through a matrix of small openings in the base and is baffled by the chassis through output conditioning circuits.
before exiting through holes in cover. A BITE display panel and controls are
A non--volatile memory in the BPCU is used to store external power system
located on the box front panel.
passive BITE and fault data. The non--volatile memory is an Electrically
Power Alterable Read Only Memory (EAROM).
The power required to operate internal BPCU circuits, external contactors and Digital data communication links between the BPCU and the three generator
relays is derived from: control units are provided within the BPCU. System status, control and BITE
-- External power, information is exchanged on these serial data links.
-- Airplane battery bus
-- The right main dc bus.
Front panel circuit breakers (CB1 and CB2) are used to isolate power to
For Training Purposes Only

external circuits.

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ELECTRICAL SYSTEM
GENERAL ATA 24 - 00
For Training Purposes Only

Figure 4 Bus Power Control Unit

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ELECTRICAL SYSTEM
GENERAL ATA 24 - 00
AC POWER SYSTEM
Purpose
The ac power system generates 115 Vac and provides control and protection The BPCU provides tie bus fault protection by tripping the EPC and sending
for airplane electrical loads. signals to GCU’s when a fault occurs on the tie bus. Protective functions
System Description for the external power system are provided by the BPCU when unacceptable
power is supplied or when overload conditions persist.
The primary ac system is a three--phase, four wire, wye-connected system
which operates at a nominal voltage of 115/200 Vac, nominal frequency of Electrical system status, protection and control information as well as BITE
400 Hz and is rated at 90 kva.The system is divided into left and right main information is exchanged between the bus power control unit and generator
ac channels. control units. The serial data link is bi--directional and bi--polar. The bus power
control unit establishes communications with each GCU. It sends a data word
Each channel consists of a main ac bus supplied by an associated integrated to the GCU first. The GCU then responds by returning a data word to the
drive generator (IDG). BPCU. This continues until the BPCU has sent and received data from all
An APU driven auxiliary generator provides dispatch capability if one primary three GCU’s. The BPCU sends the GCU’s information on:
source IDG is inoperative and also provides a self sufficient power source for -- Differential faults,
ground operation. An external power source can be connected to the ac tie
bus through the external power receptacle and the external power contactor -- Breaker position,
(EPC). -- External power,
-- Autoland,
General Subsystem Features
-- APU generator overload,
Both main ac buses can be supplied concurrently from any one of four isolated
power sources (left and right IDG, auxiliary generator and external power) by -- BITE request.
means of the interconnecting ac tie bus when the appropriate GCB, APB, EPC Information returned from the GCU includes:
and BTBs are closed. Operation of these contactors is controlled automatically -- Generator differential protection and overload information,
by the generator control units (GCU) and bus power control unit (BPCU) to -- Breaker position and status,
maintain power on the buses from any available source. Also, the two main ac
buses can be supplied independently from two different operating sources. -- Ground mode,
-- Dead tie bus,
Control and Protection -- Underspeed,
The three generator control units provide automatic control and protection -- APU available and
functions for each channel by monitoring IDG output, BPCU status information
and control switch positions. The GCU provides protective trip commands to -- BITE code data.
For Training Purposes Only

the GCB and/or BTB to prevent load and source equipment damage if faults BITE
or failures occur in the system.
Each GCU and the BPCU has built--in test equipment (BITE) with self--check
The BPCU wilI transmit load shedding signals to de--energize non--essential and fault diagnostics capability. The BITE display and operating controls are
load on both main ac buses if an in flight generator shutdown or power source mounted on the BPCU.
overload occurs.

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ELECTRICAL SYSTEM
GENERAL ATA 24 - 00
For Training Purposes Only

Figure 5 AC Power System

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ELECTRICAL SYSTEM
GENERAL ATA 24 - 00
HYDRAULIC MOTOR GENERATOR SYSTEM
Purpose
The hydraulic motor generator system provides a non--time limited backup
source after loss of all generator electrical power.
System Description
The Hydraulic Motor Generator (HMG) is located in the left wheel well.
The generator is driven by a hydraulic motor supplied from the center
hydraulic system. An air driven hydraulic pump (ADP), supplied by right
or left engine bleed air, can pressurize the center hydraulic system.
A motor driven hydraulic shutoff valve provides on/off control for the HMG.
Automatic activation of the hydraulic motor generator system occurs with
loss of power to both main ac buses. A flow limiter regulates hydraulic flow
to the trailing edge flaps and leading edge slats during HMG operation.
Control and protection functions are performed by the generator control unit
located in the P65 panel. A momentary test switch (P61 panel) is provided
for system checkout.
The HMG system operational status and electrical parameters are monitored
by EICAS.
For Training Purposes Only

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ELECTRICAL SYSTEM
GENERAL ATA 24 - 00
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Figure 6 Hydraulic Motor Generator System

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ELECTRICAL SYSTEM
GENERAL ATA 24 - 00
HMG - POWER DISTRIBUTION
General Operation Flight/Cabin Intph L ILS
AC bus transfer relays connect the left and right ac transfer buses and the RDMI Auto Pressurization
captain’s flight instrument transfer bus to the hydraulic motor generator ac
output. Two 115/28 volt ac single auto transformers supply 28 volt ac loads VOR OVHD PNL & Aisle Stand Lts
from the left and right ac transfer buses. C ILS Night & Lav Lights
A dc contactor connects the hydraulic motor generator dc output to the hot C IRU FWD/AFT Ceiling Lts
battery bus. Standby Engine Ind Thrust Reversers
The following loads can be powered by the hydraulic motor generator: Wing Anti--Ice Eng EEC Discretes
Fire Warning Engine Start Ign Bleed Valves L IRU
Stall Warning APU Start Rudder Trim Packs Control
Master Warning Hyd Pump Control Capt and Aux Pitot Heat ADF
Standby Instr RAT Control Probe Heat Ind
Air Data Computer Equipment Cooling
Clocks Man. Pressurization
Standby Power Control Outflow Valve Pos
Spoilers Capt & Aux Pitot Heat
Yaw Damper AOA Probe Heat
Flap/Slat Eng TT2 Probe Heat
Cabin/F.O. Instr Gages Cargo Heat Control
Altn Gear Extend HF Comm
Gear Indication Flap Pos Ind.
Antiskid Capt IAS/MACH
Capt Panel Flood Lights Capt Altimeter
Aisle Stand Flood Lights L Radio Altimeter
Fuel Shutoff EFIS Sym Gen
For Training Purposes Only

Fuel Crossfeed Capt EADI

Defueling Valves Capt EHSI
Fuel Quantity FMCS
DC Fuel Pump Capt VSI
VHF Comm L DME

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ELECTRICAL SYSTEM
GENERAL ATA 24 - 00
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Figure 7 HMG Power Distribution

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ELECTRICAL SYSTEM
GENERAL ATA 24 - 00
DC POWER SYSTEM
Purpose General Subsystem Features
The dc power system is provided on the airplane to supply loads requiring TRU
dc power. The TRU’s are normally operated isolated to supply their respective load
buses. In addition, under normal system operation, the battery bus, dc standby
System Description bus and the center dc bus are supplied from the left 28 volt dc bus.
The main dc system is a 2--wire system using the airframe structure as the DC Tie Bus
ground return circuit. Two main distribution buses, located in the P6 panel, A dc tie bus and automatic operating dc tie control unit allows a single TRU to
supply essential and non--essential loads. Transformer rectifier units (TRU), supply both main dc buses.
energized by the main ac buses, separately power each main dc bus.
Battery Charger
The battery/battery charger systems consist of a 24 volt vented nickel --
cadmium battery and a separate dedicated battery charger to recharge and Battery charging is constant current and temperature compensated to prevent
maintain the battery at its full state of charge. thermal runaway. A temperature sensing device and a thermal switch, both
integral to the battery, provide temperature sensing and overtemperature
A standby power system is provided to supply 28 volt dc and single phase protection signals for charger control and deactivation. The second battery/
115 Vac power to essential instrument, communication and navigation battery charger system is dedicated to supply dc power to start the auxiliary
equipment in the event of complete loss of primary ac power. power unit.
The main and APU batteries are automatically tied together using a remote APU TRU
control circuit breaker (RCCB) (Aircraft with extended stby power).
The APU start transformer rectifier unit can operate to supply dc power to the
If the HMG is not available, the batteries and static inverter supply power APU starter motor when the 115 volt ac right bus is powered.
to the standby buses. In this case, up to 90 minutes of 28 volt dc and single
phase 115 volt ac power is available to essential instrument, communication, Static Inverter
and navigation equipment. The standby system also serves as an independent The static inverter converts dc power to single phase ac power for the ac
source for the autoland system center channel. standby bus. Two switches located on the P5 panel control the standby
system.
General Component Locations The BATTERY switch controls inverter backup dc input power and the
The TRU’s, main battery/battery charger system and static inverter are located STANDBY POWER rotary Switch controls auto/manual standby modes.
in the main equipment center. A second, separate battery/battery charger System Status Indication
system and the APU transformer rectifier unit are located in the aft equipment
center. The dc system status information is displayed on EICAS and by control panel
caution lights.
For Training Purposes Only

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ELECTRICAL SYSTEM
GENERAL ATA 24 - 00
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Figure 8 DC Generation
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ELECTRICAL SYSTEM
GENERAL ATA 24 - 00
NOTES:
For Training Purposes Only

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