Final answer:
If AC BUS 1 is lost, the AC ESS bus is powered by AC STAT INV or another AC BUS through bus tie connections, with an automatic switching mechanism ensuring a seamless power transfer to maintain operations of essential systems.
Step-by-step explanation:
When AC BUS 1 is lost in an aircraft, the AC ESS bus is typically powered by an alternative source of power, such as the AC STAT INV (Static Inverter) or another AC BUS like AC BUS 2 through bus tie connections. In some aircraft systems, if AC BUS 1 fails, an automatic switching mechanism redirects power from the remaining available sources to ensure that essential systems continue to operate.
This auto-transfer is usually seamless and ensures continuous power supply to critical systems. As a backup, the Emergency Power System, which may include batteries and/or a Ram Air Turbine (RAT), can also supply power to the essential bus if all other sources fail.