A320 Electrical System

The A320's electrical system supplies three-phase 115/200 V AC at a constant 400 Hz and 28 V DC. In normal flight two engine-driven generators each power their own side. Behind them stand the APU generator, external power, an emergency generator driven through the blue hydraulic system, two batteries and a static inverter, and a hierarchy of buses keeps the essential loads powered longest.

The general principles are covered in electrical power distribution, AC generators, CSD and IDG and aircraft batteries.

System overview and power sources

Each generator, engine or APU, can supply all the bus bars, and part of its AC output is converted to DC. The generators are never connected in parallel: bus tie contactors (BTCs) open and close to give each bus one source at a time.

A fourth transformer rectifier (TR), the TR entertainment, feeds the DC entertainment bus (DC BUS Ent) used by the in-flight entertainment system.

Generators and control units

Each engine drives its generator through an IDG, which turns the variable N2 speed of the engine into the constant speed the generator needs for 400 Hz. A generator control unit (GCU) for each generator regulates its voltage and frequency and protects the network by opening the generator line contactor (GLC) when it detects a fault. Selecting the GEN pushbutton OFF de-energises the field, opens the GLC and resets the fault circuit, without affecting the IDG. Its amber FAULT light comes on when the GCU trips the generator or the GLC opens; after a differential fault, resetting has no effect after two attempts.

The IDG FAULT light comes on if the IDG oil outlet temperature exceeds 180 °C or its oil pressure is low, the pressure part being inhibited below 14 % N2. The ECAM ELEC page pulses the oil temperature as an advisory above 147 °C, and the IDG disconnects itself automatically at 200 °C. The crew's IDG disconnect uses the guarded IDG pushbutton, held for no more than 3 s, and only with the engine running or windmilling, or the IDG will be damaged at the next start. Only maintenance can reconnect it.

Where a ground and auxiliary power control unit (GAPCU) is fitted, it regulates the APU generator through the APU electronic control box and controls both the external power contactor and the APU generator line contactor; a ground power control unit (GPCU), where fitted, controls the external power contactor. The emergency generator has its own GCU.

AC distribution and essential buses

The AC buses are AC BUS 1 and AC BUS 2, the AC essential bus (AC ESS BUS), the AC shed essential bus (AC SHED ESS BUS, also written AC ESS SHED) and the AC static inverter bus (AC STAT INV BUS). In normal flight each engine generator supplies its own AC bus through its GLC. AC BUS 1 normally supplies the AC ESS BUS, which in turn feeds the AC SHED ESS BUS; the AC STAT INV BUS is unpowered.

The AC ESS FEED pushbutton selects the essential bus source. In its normal position AC BUS 1 supplies the AC ESS BUS; pressed, showing ALTN, AC BUS 2 does. With AC ESS FEED auto switching, AC BUS 2 takes over automatically if AC BUS 1 is lost, but there is no automatic return: the crew reselect NORMAL when AC BUS 1 is back. The amber FAULT light means that the AC ESS BUS has no supply.

With the BUS TIE pushbutton at AUTO, one BTC closes when an engine generator supplies its own side and the APU generator or external power the other; both close when a single source supplies the whole network. BUS TIE OFF opens both, and the APU generator, which sits on the tie bar behind them, can then reach neither main bus.

For ground servicing, the AC and DC ground/flight buses (GND/FLT BUS) can be powered alone. The MAINT BUS switch in the forward entrance area, controlling the maintenance bus (MAINT BUS) supply, latches magnetically when external power is connected and normal, powering only the AC and DC GND/FLT buses. It trips when external power is removed, and the DC GND/FLT bus and its AC counterpart return to DC BUS 2 and AC BUS 2.

DC distribution, TRs and batteries

The DC buses are the main buses, DC BUS 1 and DC BUS 2, the DC battery bus (DC BAT BUS), the DC essential bus (DC ESS BUS), the DC shed essential bus (DC SHED ESS BUS), the DC entertainment bus and two hot battery buses. In normal flight TR 1 supplies DC BUS 1, the DC BAT BUS and the DC ESS BUS, which feeds the DC SHED ESS BUS; TR 2 supplies DC BUS 2 alone. The essential transformer rectifier (ESS TR), identical to the others, is not used normally.

Each TR opens its own contactor on overheat or minimum current. If TR 1 fails, TR 2 takes DC BUS 1, DC BUS 2 and the DC BAT BUS, and the ESS TR, fed from the AC ESS BUS, supplies the DC ESS BUS. If both main TRs fail, DC BUS 1, DC BUS 2 and the DC BAT BUS are lost, but the ESS TR keeps the DC ESS BUS. After a loss of AC BUS 1, DC BUS 2 picks up DC BUS 1 and the DC BAT BUS after 5 s, and the ESS TR supplies the DC ESS BUS.

The aircraft batteries (BAT 1 and BAT 2) are permanently connected to HOT BUS 1 and HOT BUS 2, which stay powered even with the aircraft otherwise dead. They are connected to the DC BAT BUS only when needed: for charging, for an APU start and in abnormal configurations. Each has a battery charge limiter (BCL) that monitors charging and controls the battery contactor. Charging starts when battery voltage falls below 26.5 V, at once on the ground and after 30 minutes in flight, and ends when the charging current drops below 4 A. On the ground, with the BAT pushbuttons at AUTO and no external or generator power, the battery automatic cut-off opens the contactors on low voltage to prevent a complete discharge; selecting OFF and then AUTO resets them.

Source priority and bus transfer

The electrical source priority is fixed: engine generators first, then external power if its pushbutton is ON, then the APU generator. One engine generator can supply the entire network except the DC entertainment bus, which needs two generators in flight; on the ground the APU generator, if not overloaded, or external power is enough.

If an engine generator fails, the APU generator replaces it if available, otherwise the other engine generator. With GEN 2 lost and the APU generator on, for example, BTC 2 closes and the APU feeds AC BUS 2 while GEN 1 keeps AC BUS 1, BTC 1 staying open. On the ground, starting engine 1 with external power connected and the APU running transfers the network to GEN 1; the EXT PWR ON light stays on regardless.

In the cockpit, the EXT PWR pushbutton shows a green AVAIL light when ground power is plugged in with voltage and frequency within limits, and a blue ON light when its contactor is closed. On the external power panel used by the ground staff, a white EXT PWR NOT IN USE light shows that the ground power unit is not supplying the aircraft and can be removed.

Emergency generator and CSM/G

When AC BUS 1 and AC BUS 2 are both lost above 100 kt, the RAT extends automatically and pressurises the blue hydraulic system, which drives the emergency generator (EMER GEN) through a hydraulic motor. The unit is the constant speed motor/generator (CSM/G): its GCU keeps it at constant speed. It couples about 8 s after the loss; until then the batteries supply the emergency network, with the static inverter providing AC above 50 kt. Running, the EMER GEN supplies the AC ESS BUS and AC SHED ESS BUS, and through the ESS TR the DC ESS BUS and DC SHED ESS BUS.

The EMER ELEC PWR panel carries a red RAT & EMER GEN FAULT light, on when the emergency generator is not supplying power while AC BUS 1 and 2 are unpowered, and a guarded MAN ON pushbutton that extends the RAT manually; the generator then couples 3 s after the RAT supplies it. The guarded EMER GEN TEST pushbutton, held with the normal AC buses supplied, drives the generator from the blue electric pump and connects the AC ESS and DC ESS buses, but not the shed buses, to it.

Emergency electrical configuration

The emergency electrical configuration is the aircraft with both main AC buses lost. With the EMER GEN running, the essential buses keep the equipment needed to fly and navigate: the captain's flight instruments, key avionics and VHF 1, the only VHF available. The DC BAT BUS is not supplied in flight.

On batteries alone, before the generator couples, if the RAT stalls or if the EMER GEN fails, BAT 1 drives the static inverter, which feeds the AC ESS BUS above 50 kt, and the batteries supply the DC ESS BUS. The shed busbars (AC SHED ESS and DC SHED ESS) are shed to save the batteries. On the ground below 100 kt the DC BAT BUS reconnects to the batteries and an APU start becomes possible; below 50 kt the AC ESS BUS is shed.

Load shedding and commercial loads

Galley load shedding protects the remaining generator. In flight with only one generator, the main galley, in-seat power and in-flight entertainment are shed automatically, as is the DC entertainment bus; with the galley load automatic shedding option, the secondary galley goes too. On the ground the main galley is shed when only one engine generator operates. The GALY & CAB pushbutton shows FAULT when any generator is loaded above 100 % of its rating, and OFF removes the galleys, in-seat power, in-flight entertainment and floor heating panels.

The COMMERCIAL pushbutton supplies the commercial electrical loads: cabin and cargo lights, water and toilet system, drain mast ice protection, galleys, passenger entertainment and semi-automatic cargo loading. Selecting it OFF sheds them all.

ELEC panel and ECAM ELEC page

The overhead ELEC panel draws the network as a synoptic, with each pushbutton placed on the line of the source or bus it controls.

The ECAM ELEC page shows the sources, contactors and buses, with amber marking anything out of limits:

A TR legend turns amber only when both its voltage and its current are amber. SHED appears on a shed bus, GALLEY SHED when the galleys are shed, and the IDG area shows DISC or LO PR.

Electrical smoke and abnormal procedures

The electrical smoke configuration sheds the main busbars to isolate a possible source of smoke. Its distribution is that of the emergency configuration, except that the fuel pumps are connected upstream of the GEN 1 line contactor so that the engines stay fed. About 75 % of the electrical equipment is shed, and the ELEC page looks as it does with the EMER GEN running. The GEN 1 LINE pushbutton on the EMER ELEC PWR panel, selected OFF, opens the GEN 1 line contactor, and AC BUS 1 is then supplied from GEN 2 through the bus ties.

A monitored circuit breaker, green, triggers the ECAM C/B TRIPPED caution when it has been out for more than 1 minute, naming its panel; black ones are not monitored, and the wing tip brake breakers carry red caps to prevent reset. If CLR is used while the breaker stays out, further trips on that panel go undetected. No cockpit circuit breaker is touched in flight without a cross-check.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.