Electrical system

A 28 V DC system split into a left and a right channel. Two engine generators carry the load, the APU generator can share it or stand in for one or both, and two main batteries keep the essential loads going if every generator is lost.

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Power sources

Where the power comes from

DC primary architecture. Across the top: left generator, left battery, external power, APU generator, APU start, right battery and right generator. Below them the L GEN, L BATT, EXT PWR AVAIL and ON, R BATT and R GEN switches feed the left buses in red and the right buses in green, joined in the centre by the BUS TIE. Along the bottom: left AUX, MAIN and ESS buses and right ESS, MAIN and AUX buses. A ground service bus and switch sit near the top centre, and the standby instrument battery feeds the standby instrument through the STBY INST switch. — Three generators, two batteries and external power feed the left and right buses, joined by the bus tie. Tap to enlarge

Three identical, air-cooled generators: one on each engine and one on the APU. Either engine generator alone gives enough power for normal operations.
The APU generator can share the load with the engine generators at low and medium altitudes, or replace one or both of them after a failure.
Two main batteries supply every load on the ground, and the essential loads when the generators are inoperative.
The standby instrument has its own 24 V DC battery in the nose landing gear bay, switched on with STBY INST.
External DC power can be plugged in on the ground. A hydraulic motor driven generator is optional equipment.
The left and right channels are kept apart, electrically and physically. External power and the APU generator can connect to both.

Key numbers
System	28 V DC
Generator rating	12 kVA
Generator nominal output	29.5 V
Main battery voltage	24 V, nickel-cadmium
Main battery capacity	44 Ah each

Distribution

Buses, power centers and breakers

Plan view of the airplane with the electrical parts marked. In the nose, behind the cockpit: left and right circuit breaker panels and left and right secondary power centers. At the wing root, aft: left and right battery. In the aft fuselage: left and right DC power centers, the external DC control panel on the right side, the left and right generators on the engines and the APU generator in the tail cone. — Breaker panels and secondary power centers up front, the batteries aft of the left wing, DC power centers and generators at the back. Tap to enlarge

Each side has an ESS, a MAIN, an AUX and a battery bus.
The batteries connect to the essential buses, and keep the battery buses powered even with the battery switches OFF. An online generator feeds the main buses and charges its own side battery.
With two or more generators online the bus tie is open. It closes automatically when external power is selected ON, and when a generator fails with the APU generator off line.
Two DC power centers (DCPC) in the aft equipment bay switch the sources onto the buses. They couple both channels on the ground with only batteries or external power, and in flight with one generator. They also shed load automatically and isolate a bus or feeder that shorts.
Two secondary power centers on the front equipment rack take power from the DCPCs and feed the forward loads.
Six breaker panels: CB1 pilot and CB2 copilot in the cockpit, CB3 and CB4 in the left and right equipment racks, CB5 and CB6 on the left and right DC power centers.

Lost with an AUX bus
L AUX bus	Cabin AC power, pilot's foot warmers
R AUX bus	Cabin DC power, right window anti-ice, copilot's foot warmers
In flight, one engine generator and no APU generator	Both AUX buses unpowered

Flight deck

ELECTRICAL panel and synoptic

ELECTRICAL panel. Top row: L BATT with an OFF light, EXT PWR with AVAIL and ON lights, R BATT with an OFF light. STBY INST on the left. In the centre the BUS TIE switch between the L ESS and R ESS boxes, with L AUX and L MAIN on the left and R MAIN and R AUX on the right. Bottom row: L GEN with OFF, APU GEN with ON, R GEN with OFF. — The ELECTRICAL panel: battery, external power, bus tie and generator switches, with the bus boxes between them. Tap to enlarge

The panel carries annunciators for the six buses: L and R ESS, MAIN and AUX.
Battery voltage and temperature show on the ELECTRICAL synoptic and the SUMMARY page. GPU voltage shows there too, but there is no reading of the amps drawn from the GPU.
On the synoptic the EXT PWR symbol is white when available, green when ON and gone when disconnected. It appears once a GPU cable is plugged in and more than 5 V DC is sensed.
Electrical warnings come with a triple chime, cautions with a single chime.

Controls
L BATT, R BATT	Battery on or off. Selected off: OFF lights and the L (R) BATT OFF status message shows.
EXT PWR	Green AVAIL: GPU within limits. Push: ON lights and the GPU is connected. Push again to deselect it.
BUS TIE	Push: bus tie opened by hand, no automatic operation, BUS TIE MAN OPEN status message. Push again for automatic control.
L GEN, R GEN	Engine generator switches. Selected off: L (R) GEN OFF status message.
APU GEN	Controls the APU generator. Selected off: APU GEN OFF status message.
STBY INST	Switches on the standby instrument battery.

Normal operation

External power, APU start and generators

MFD with the SUMMARY page on top: ECS, ELECTRICAL and HYDRAULIC blocks, the electrical block giving GEN AMPS, GEN VOLTS, BATT VOLTS and BATT TEMP for L, APU and R. Below, the ELECTRICAL synoptic: L BATT and R BATT boxes with temperature and volts, the EXT PWR symbol with its voltage, the L and R ESS buses either side of the bus tie, the AUX and MAIN buses, and the left, APU and right generators with their amps and volts. — The SUMMARY page and, below it, the ELECTRICAL synoptic. Tap to enlarge

Ground power plugs in on the lower right side of the fuselage, just below the engine pylon.
The main batteries do not have to be on to connect the GPU. With EXT PWR ON the bus tie closes and the whole DC system is powered.
With EXT PWR ON no generator comes online, and any generator already online drops off. After engine start, select EXT PWR to AVAIL, disconnect the GPU and check the generators online.
R BATT goes on before L BATT, so the APU cannot start uncommanded.
APU start: START held for 3 seconds and released. The right battery is cut off from the R ESS bus and cranks the starter. The left battery keeps both essential buses powered.
If the batteries are well discharged when a generator comes online, the charging current can give a short APU GEN OVERLOAD or L (R) GEN OVERLOAD caution.

External power
GPU accepted, approximately	24 to 32 V
GPU regulated to	28 V DC
GPU current limit	1,500 A

What not to exceed

Batteries and limits

Photo of an open compartment with two boxy main batteries side by side, each with a vent cap on its front face, cables and clear vent tubes running from the cases. — The two main batteries in their compartment aft of the left wing. Tap to enlarge

The main batteries sit in the fuselage fairing aft of the left wing.
With every generator lost, the batteries power the essential equipment for a limited time only.

Limitations
Generator load, two or three generators, continuous	400 A
Generator load, one generator on the ground, continuous	400 A
Generator load, one generator in flight, continuous	500 A
Generator load, transient for 5 minutes	600 A
Lowest battery temperature for takeoff	−20 °C
APU generator, maximum altitude	30,000 ft
APU start, maximum altitude in flight	30,000 ft
APU start, maximum altitude on the ground	14,000 ft

Alert thresholds
L (R) BATT OVERHEAT, battery internal temperature above	70 °C
L (R) GEN OVERLOAD, generator load above	500 A
APU GEN OVERLOAD, APU generator load above	500 A
STBY INST BATT FAULT, one cause: battery temperature above	80 °C

When it goes wrong

Warnings, cautions and the QRH

Quick reference card, section 4 ELECTRICAL: a red bar reading L (R) BATT OVERHEAT, item 1 Affected BATT OFF, then Proceed to ELEC 04-1. Page 2. — L (R) BATT OVERHEAT on the QRC, page 2: the only electrical Immediate Action Item. Tap to enlarge

L (R) BATT OVERHEAT (warning): battery internal temperature above 70 °C. Its Immediate Action Item is on the QRC, and the procedure goes on at ELEC 04-1.
ESSENTIAL POWER ONLY (warning): main generation lost in the air. The essential buses run on the batteries only, and the MAIN and AUX buses are shed automatically in flight.
L (R) GEN FAIL (caution): with the APU generator off line the bus tie closes automatically and, in flight, the AUX buses lose power.
L (R) BATT FAIL (caution): the battery contactor failed, or the battery was taken off line automatically to protect it from overheat.
L (R) ESS BUS FAIL and L (R) MAIN BUS FAIL: the bus is not powered. L (R) AUX BUS FAIL: not powered although a source is available. L (R) ESS BUS OFF and L (R) MAIN BUS OFF: the bus is serviceable but has no power. L (R) AUX BUS OFF: no source available, as in single generator operation.
Indications that flicker, cycle or make no sense: use Unexpected Electrical Indications. Any reset may take the affected source off line and make things worse.

On batteries only
Two main batteries, generators off	One hour
Integrated standby instrument, after loss of generated power	3.5 hours
Endurance assumes L WSHLD / WINDOW on for no more than	15 min
Endurance assumes APU start attempts, no more than	3
Each extra APU start attempt costs about	2 min

The full module

The slides take you through the sources and buses, the ELECTRICAL panel and synoptic, a normal power-up and the abnormals, then a 10-question test.

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