Which AC buses are energized during an electrical emergency before the RAT deploys?

In the context of an electrical emergency in an Embraer Regional Jet, the AC STBY BUS is the correct answer because it serves as a backup power source that is designed to energize critical systems prior to the deployment of the Ram Air Turbine (RAT). The RAT is a device that generates emergency power when normal power sources are lost, but during the initial phase of an electrical emergency, the aircraft must rely on the AC STBY BUS to ensure that key avionics and essential systems remain operational.

The AC STBY BUS is specifically designed to power certain instruments and systems that are crucial for safe operation in the event of primary power loss. This is critical as it provides a temporary power supply to maintain these systems until the RAT is fully deployed and operational.

Other buses mentioned may not provide power immediately during an electrical emergency before the RAT deploys. For example, the AC ESS BUS typically comes into play after certain configurations are switched, whereas the AC PWR BUS supports essential services but doesn't serve the same immediate purpose in an emergency scenario as the AC STBY BUS. The AC ACESS BUS is not a standard term used in the context of ERJ electrical systems, making it an unlikely choice. Thus, the AC STBY BUS is specifically