A320 Electrical Source Priority and the EXT PWR Indications
The overhead panel does not tell you the outcome as plainly as most pilots assume.
Three AC source types can be available to an A320 on the ground at the same time: engine generators, external power, and the APU generator. Counted individually, the engine-generator source consists of 𝗚𝗘𝗡 𝟭 and 𝗚𝗘𝗡 𝟮. The aircraft resolves them by a fixed priority hierarchy, and the overhead panel does not tell you the outcome as plainly as most pilots assume.
This page answers the questions that come up most often on the oral exam about electrical source priority and the external power indications. Every answer is anchored to the Flight Crew Operating Manual and has been verified against the manual text rather than summarized from memory.
The Short Answer
Engine generators have priority over both external power and the APU generator. External power has priority over the APU generator when the 𝗘𝗫𝗧 𝗣𝗪𝗥 pushbutton is selected ON. So the order is engine generators, then external power, then APU generator.
The 𝗔𝗩𝗔𝗜𝗟 light on the EXT PWR pushbutton means external power is plugged in and its parameters are normal. It does not mean external power is supplying the aircraft.
The blue 𝗢𝗡 light must not be used as a real-time indication that external power is feeding the aircraft. The FCOM states that the ON light stays on even when the engine generators supply the aircraft. To determine what is actually powering the buses, use the ECAM ELEC page.
Frequently Asked Questions
What are the electrical power sources on the A320 in normal configuration?
In normal configuration the A320 electrical power system is constituted of two engine generators and one APU generator. On the ground, either the APU generator or external power may supply the complete system. If normal AC power is not available, an emergency generator can provide AC power. If all AC power is not available, DC power from the batteries can be inverted into AC power by the static inverter.
For normal configuration, name the two engine generators and one APU generator. Then add the condition: on the ground, either the APU generator or external power may supply the complete system. For the wider electrical architecture, include external power, the emergency generator, the static inverter, transformer rectifiers, and the two main batteries.
Be precise about what each one is. The 𝗲𝗺𝗲𝗿𝗴𝗲𝗻𝗰𝘆 𝗴𝗲𝗻𝗲𝗿𝗮𝘁𝗼𝗿 is an emergency AC power source. The 𝘀𝘁𝗮𝘁𝗶𝗰 𝗶𝗻𝘃𝗲𝗿𝘁𝗲𝗿 is not a generator; it converts DC power from Battery 1 into AC power for part of the AC essential bus. Describing the static inverter as if it were another source equivalent to GEN 1, GEN 2, or the APU generator is a classification error an examiner will pick up.
Each engine generator is driven by an 𝗜𝗻𝘁𝗲𝗴𝗿𝗮𝘁𝗲𝗱 𝗗𝗿𝗶𝘃𝗲 𝗚𝗲𝗻𝗲𝗿𝗮𝘁𝗼𝗿 and produces 115/200 V three-phase AC at a constant 400 Hz, rated up to 90 kVA. The APU generator produces identical output ratings. The generators are 𝗻𝗲𝘃𝗲𝗿 𝗰𝗼𝗻𝗻𝗲𝗰𝘁𝗲𝗱 𝗶𝗻 𝗽𝗮𝗿𝗮𝗹𝗹𝗲𝗹.
What is the electrical source priority on the A320?
The hierarchy has three tiers.
First, 𝗲𝗻𝗴𝗶𝗻𝗲 𝗴𝗲𝗻𝗲𝗿𝗮𝘁𝗼𝗿𝘀. Any operating engine-driven generator has priority over both the APU generator and external power.
Second, 𝗲𝘅𝘁𝗲𝗿𝗻𝗮𝗹 𝗽𝗼𝘄𝗲𝗿. Between the two non-engine sources, external power has priority over the APU generator when the EXT PWR pushbutton is selected ON.
Third, the 𝗔𝗣𝗨 𝗴𝗲𝗻𝗲𝗿𝗮𝘁𝗼𝗿, which sits below both.
The practical phrasing for an oral is 𝗲𝗻𝗴𝗶𝗻𝗲 𝗴𝗲𝗻𝗲𝗿𝗮𝘁𝗼𝗿𝘀, 𝘁𝗵𝗲𝗻 𝗲𝘅𝘁𝗲𝗿𝗻𝗮𝗹 𝗽𝗼𝘄𝗲𝗿, 𝘁𝗵𝗲𝗻 𝗔𝗣𝗨 𝗴𝗲𝗻𝗲𝗿𝗮𝘁𝗼𝗿. A candidate who has memorized only the second-tier rule tends to describe external power as the top-priority ground source. It is not. The engine generators are.
What does the AVAIL light on the A320 EXT PWR pushbutton mean?
The 𝗔𝗩𝗔𝗜𝗟 light means external power is plugged in and its parameters are normal. It does not mean external power is supplying the aircraft.
This is a common oral exam trap because the word "available" reads as "in use." The AVAIL light is telling you a source is ready to be selected, not that it has been selected or that it is carrying the load. Pressing the pushbutton is what closes the external power line contactor.
Does the blue ON light mean external power is supplying the A320?
No. The blue 𝗢𝗡 light on the EXT PWR pushbutton must not be used as a real-time indication that external power is feeding the aircraft.
With no higher-priority source supplying the network, pressing the EXT PWR pushbutton closes the external power line contactor and external power supplies the aircraft. But the FCOM states that the 𝗢𝗡 𝗹𝗶𝗴𝗵𝘁 𝘀𝘁𝗮𝘆𝘀 𝗼𝗻 even when the engine generators supply the aircraft. So after engine start, when the engine generators have taken priority and external power is no longer supplying the aircraft network, that blue light can still be glowing on the overhead panel.
The light was never designed to report network state. To determine which buses are actually powered and which electrical source indications are normal, use the 𝗘𝗖𝗔𝗠 𝗘𝗟𝗘𝗖 page.
Is the APU generator connected when external power is selected ON?
No. If the APU is running with its generator available while external power is selected ON and supplying the network, the APU generator is 𝗮𝘃𝗮𝗶𝗹𝗮𝗯𝗹𝗲 𝗯𝘂𝘁 𝗻𝗼𝘁 𝗰𝗼𝗻𝗻𝗲𝗰𝘁𝗲𝗱 to the aircraft network.
The reason is the contactor logic. The APU generator line contactor closes if the APU generator parameters are normal 𝗮𝗻𝗱 the external power line contactor is open. With external power selected and powering the aircraft, the external power line contactor is closed, so the APU generator line contactor stays open.
Describing the APU generator as "online" during an external-power-active configuration is a precision error that an examiner who reads the manual carefully will pursue. The 𝗔𝗣𝗨 𝗶𝘀 𝗿𝘂𝗻𝗻𝗶𝗻𝗴, the 𝗔𝗣𝗨 𝗴𝗲𝗻𝗲𝗿𝗮𝘁𝗼𝗿 𝗽𝗮𝗿𝗮𝗺𝗲𝘁𝗲𝗿𝘀 𝗮𝗿𝗲 𝗻𝗼𝗿𝗺𝗮𝗹, and the APU generator is ready to connect when the external power line contactor opens. It is not connected while the external power line contactor is closed.
What happens electrically when you start the engines with external power connected?
The aircraft transfers by priority logic 𝘄𝗶𝘁𝗵𝗼𝘂𝘁 𝗰𝗿𝗲𝘄 𝗮𝗰𝘁𝗶𝗼𝗻.
Before engine start, with external power selected ON and supplying the aircraft, both bus tie contactors are closed so that the single source can feed 𝗔𝗖 𝗕𝗨𝗦 𝟭 and 𝗔𝗖 𝗕𝗨𝗦 𝟮 through the tie bus. The APU generator, if running, is available but not connected.
When the first engine generator comes online, its generator line contactor closes and it supplies its associated AC BUS. The engine generator has priority over external power on that side. The bus tie contactor on that side opens, ending the single-source configuration. The aircraft is now in a 𝘀𝗽𝗹𝗶𝘁-𝘀𝗼𝘂𝗿𝗰𝗲 configuration, with one engine generator on one side and external power on the other, and only one bus tie contactor remains closed.
When the second engine generator comes online, its line contactor closes, the split-source case ends, and both bus tie contactors open. The external power line contactor is open. The APU generator line contactor is still open. The blue ON light on the EXT PWR pushbutton stays on.
How do you confirm what is actually supplying the A320 buses?
Use the 𝗘𝗖𝗔𝗠 𝗘𝗟𝗘𝗖 page to determine which buses are actually powered and which electrical source indications are normal.
The source pushbuttons on the overhead panel report the state of their own selection and contactor logic, and each control has its own indication behavior published separately in the FCOM. Do not generalize the EXT PWR ON-light behavior to every electrical source pushbutton. The FCOM gives separate indication logic for the BAT, GEN, APU GEN, and EXT PWR controls.
When the question is what is powered and which source indications are normal, use the ECAM ELEC page. It displays 𝗯𝘂𝘀 𝗯𝗮𝗿 𝘀𝘁𝗮𝘁𝘂𝘀 together with the GEN, APU GEN, and EXT PWR electrical indications.
Where This Sits in the Series
This page covers Spoke 1 of the A320 Electrical System Foundation Series, which walks the normal architecture and normal operations before the failure cases. Spoke 2 covers single source loss and the automatic reconfiguration that follows. Spoke 3 covers emergency electrical configuration, the RAT, and battery-only operation.
The full Spoke 1 walkthrough, including the bus structure, the BUS TIE reconfiguration cases, and the three-case battery connection rule, is in the companion article.
