Following pushback, what is the configuration of the electrical system when engine 1 is running and the APU is operating?

When engine 1 is running and the APU is operational following pushback, the correct configuration of the electrical system involves the APU generator and engine generator interacting with the various AC buses. In this scenario, engine 1 generates power, supplying AC bus 1, while the APU generator simultaneously provides power to AC bus 2.

The system is designed to allow a smooth transition of power from the APU to the engines. When the APU is running, it serves as a backup power source until the engines are available to take on the electrical load. Once the APU generator and engine generator are both online, they manage power distribution effectively between the buses, ensuring continuity of service and redundancy.

This configuration enhances operational safety by allowing the aircraft to remain powered even if one generator fails, as it utilizes two sources of electrical power. Therefore, it's clear why the answer indicates that GEN 1 powers AC bus 1, while the APU generator powers AC bus 2.