Final answer:
The AC ESS bus in an electrical panel is usually supplied by the main AC bus and has contingencies like the APU or emergency power system to maintain power during main AC bus failures.
Step-by-step explanation:
The AC ESS bus in an electrical panel is normally supplied by the aircraft's main AC bus through an automatic bus transfer system. This system is designed to ensure that the essential bus, or AC ESS bus, is powered continuously, providing power to critical systems even if the main bus fails. It can receive power from the main AC bus, auxiliary power unit (APU), or the emergency power system, which could include batteries or a ram air turbine (RAT), depending on the aircraft's design. In normal operation, the AC essential bus is automatically powered by the main AC bus. If the main AC bus fails, the system will automatically switch to an alternate source, such as the APU or the emergency power system, to maintain continuous power to essential systems.The main bus is the primary source of power in the system and it distributes power to various components. The generator bus, on the other hand, supplies power from a backup generator.