If External Power is available and EXT PWR is ON, what happens to the APU power source?

If External Power is available and EXT PWR is ON, what happens to the APU power source?

• A. It is disconnected
• B. It remains connected
• C. It supplies APU as well
• D. It starts the APU

Answer: (A)

When external power is available and the EXT PWR switch is ON, external power takes priority and is connected to the aircraft’s electrical network. The APU generator is then automatically isolated from the electrical buses, so it does not feed power into the system. This keeps power sources from paralleling and ensures a stable, single source for the buses. The APU itself may still be running for other purposes, but its electrical power source is disconnected from the buses.

When external power is available and the EXT PWR switch is ON, external power takes priority and is connected to the aircraft’s electrical network. The APU generator is then automatically isolated from the electrical buses, so it does not feed power into the system. This keeps power sources from paralleling and ensures a stable, single source for the buses. The APU itself may still be running for other purposes, but its electrical power source is disconnected from the buses.