When does the Automatic Bus Transfer Switch transfer a load from a normal source to the emergency source during a normal source failure?

The Automatic Bus Transfer Switch is designed to ensure that critical loads receive power from an emergency source when the normal power source fails. The correct answer pertains to the condition under which this transfer occurs, specifically when the normal source voltage drops below the rated voltage.

This drop indicates a failure or significant anomaly in the power supply that may not be sufficient to support connected loads. The transfer switch continuously monitors the voltage of the normal power source; if it detects that the voltage has fallen below a predefined threshold — which is considered safe operational limits — it initiates the transfer process to the emergency source.

This mechanism is crucial for maintaining power supply to essential systems during an outage or failure of the regular power source, ensuring reliability and continuity for operations that depend on uninterrupted power. Thus, the system reacts specifically to a drop in voltage, fulfilling its role by switching to an alternative source in response to actual power quality issues.