Automatic bus transfer switches transfer to the alternate feeder when voltage drops to what percentage?

Automatic bus transfer switches are designed to ensure continuous power supply by switching to an alternative feeder when the voltage on the primary feeder falls below a certain threshold. The correct answer indicates that these switches activate when the voltage drops to a range of 60 to 70 percent of the nominal voltage. This range is crucial for maintaining system reliability, as it allows for a quick response to power interruptions while avoiding unnecessary transfers during minor voltage fluctuations. Operating within this range strikes a balance between load demands and the capacity of the alternate feeder.

Options that suggest transferring at 50-60%, 70-80%, or 80-90% do not align with the standard thresholds typically utilized in automatic bus transfer systems, as they either prompt a transfer too early, risking nuisance trips, or too late, potentially leaving loads unprotected during voltage sags. The 60-70% threshold effectively ensures that only significant drops warrant a switch to an alternate power source, maintaining operational integrity and equipment safety.