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General

The electrical power system makes, supplies, and controls electrical power to the airplane. The system has these subsystem:

• External power
• Generator drive system
• AC generation
• ELMS
• Distribution
• DC generation
• Indication.

Abbreviations and Acronyms

• AFDC - automatic flight director computer
• AFDS - automatic flight director system
• APB - auxiliary power breaker
• AUTO - automatic
• AVAIL - available
• BAT - battery
• BPCU - bus power control unit
• BTB - bus tie breaker
• BU - backup
• CCB - converter circuit breaker
• CONV - converter
• CPT - captain
• CPU - central processing unit
• CSD - constant speed drive
• CTRL - control
• DISC - disconnect
• EDIU - engine data interface unit
• EEC - electronic engine control
• EEU - ELMS electronic unit
• ELCU - electrical load control unit
• ELMS - electrical load management system
• EMI - electro-magnetic interference
• EP - external power
• EPC - external power contactor
• ESDS - electro-static discharge sensitive
• ETOPS - extended twin operations
• FBW - fly by wire
• FCDC - flight controls dc
• FLT - flight
• FNC - fiber nickel cadmium
• F/O - first officer
• GCB - generator circuit breaker
• GCR - generator control relay
• GCU - generator control unit
• GEN - generator
• GH - ground handling
• GHR - ground handling relay
• GND - ground
• GSSR - ground service select relay
• GSTR - ground service transfer relay
• HDLG - handling
• IDG - integrated drive generator
• ILS - instrument landing system
• INST - instrument
• I/O - input/output
• ISLN - isolation
• LRM - line replaceable module
• LRU - line replaceable unit
• PEPC - primary external power contactor
• PFC - primary flight computer
• PMG - permanent magnet generator
• POR - point of regulation
• PRI - primary
• PSA - power supply assembly
• PSU - power supply unit
• PWR - power
• QAD - quick attach/detach
• RAT - ram air turbine
• RLY - relay
• SCM - signal conditioning module
• SEC - secondary
• SEPC - secondary external power contactor
• STBY - standby
• SVC - service
• TBB - transfer bus breaker
• TRU - transformer rectifier unit
• UB - utility bus
• UTIL - utility
• XFR - transfer

• Related IDG
• APU generator
• Opposite IDG.

APB Control

When the APU is running, the APU GCU closes the APB. This permits the APU to supply power to the tie bus. If the left main ac bus has no power, the left GCU closes the left BTB. This connects the tie bus to the left main ac bus.
If the right main ac bus has no power, the right GCU closes the right BTB. This connects the tie bus to the right main ac bus.
GCB Control

When the left IDG operates, the left GCU closes the left GCB. This permits the IDG to supply power to the left main ac bus.
When the right IDG operates, the right GCU closes the right GCB. This permits the IDG to supply power to the right main ac bus.
BTB Control

One IDG can supply power to both main ac buses at the same time. For example, if the right IDG fails, the right GCU opens the right GCB and closes the right BTB. The left GCU closes the left BTB. This connects the left main ac bus to the right main ac bus.
Ground Service and Ground Handling Buses

When the right main ac bus has power, it supplies power to the ground service bus. When the right main ac bus has no power, the BPCU selects primary external power or the APU generator as the power source.
Primary external power or the APU generator also supply power to the ground handling bus through the ground handling relay.
If both the APU generator and primary external power are available, primary external power has priority.
Backup Generator Power

The backup power system supplies power to one or both transfer buses during these conditions:

• Loss of all main ac power
• Single main generator operation
• Autoland
• Backup system test after engine start.

BITE

The GCUs and backup converter protect and monitor the ac power systems for faults. They perform three kinds of tests:

• Continuous
• Power-up/manually-initiated
• Fault-initiated.

Training Information Point

There is a short time delay between when the engine reaches idle and when the IDG starts to supply power. The length of the time delay depends on engine idle speed data from the engine data interface unit (EDIU).
On the ground, if the GCU gets a satisfactory engine idle speed signal from the EDIU, the IDG starts to supply power in 6 seconds. If there is not a satisfactory engine idle speed signal from the EDIU, the GCU gets a different engine speed signal from its own PMG. In this case, the IDG starts to supply power in 16 seconds.
In the air, if the GCU gets a satisfactory engine idle speed signal from the EDIU, the IDG starts to supply power in 10 seconds. If it must get an engine
speed signal from the PMG, the IDG starts to supply power in 20 seconds.




DC GENERATION - FUNCTIONAL DESCRIPTION

Normal Power

The dc generation system normally receives power from these ac power sources:

• Left transfer bus
• Right transfer bus
• Ground service bus
• Backup generator PMGs.

The standby ac bus uses power directly from the left transfer bus.
Two battery chargers change the ac power from the ground service bus to dc power. This dc power charges the main and APU batteries. It also supplies power to the hot battery bus loads.
Three PSAs change the ac power from the backup generator PMGs to dc power. Flight control components use this dc power.
Standby Power

When all ac power is lost, the main battery supplies power to the standby loads of the dc generation system until the RAT deploys (except those on the first officer's flight instrument bus). The standby loads are on these buses:

• Hot battery
• Battery
• Battery No. 2
• Captain's flight instrument
• First officer's flight instrument
• Standby ac.

Control

The ELMS controls all the dc power system relays. (The RAT GCU can also control the TRU C1 relay.) The battery switch and the standby power switch supply manual control inputs to the ELMS.
The ELMS decreases the effect of faults in the dc generation system by normally keeping dc buses isolated. But, if there is a power loss to a dc bus, the ELMS can supply power to that bus from another dc bus. It does this by connecting the two buses. The ELMS uses these relays to connect dc buses:

• DC bus tie
• Main battery
• Battery-captain's isolation
• Captain's-first officer's bus tie
• Ground power battery.

The dc subsystem card does a check of the dc system. This is called a dc/standby self-check. It does these tests:

• Makes sure all dc/standby system relays operate correctly
• Makes sure the ac standby bus has power (this does a check of the static inverter).

• The airplane is on the ground
• Left and right ac transfer buses have power
• The battery switch is in the ON position
• The standby power switch is in the BAT position (momentary).

• There are more than 72 hours since the last dc system test occurred
• The airplane is on the ground
• Left and right ac transfer buses have power.