Which device is mostly used to detect a power loss and initiate a switchover to another power source?

• A. Current Relay
• B. Phase Relay
• C. Temperature Relay
• D. Voltage Relay

Answer: (D)

A voltage relay is used to detect if the incoming supply voltage is present and within an acceptable range. It continuously monitors the voltage level, and when it drops below a preset threshold for a defined time, it changes state and signals the automatic transfer switch to connect the load to the alternate power source. This directly addresses power loss conditions, enabling an immediate switchover to a backup source.

A current relay focuses on current magnitude and is mainly for overcurrent or fault protection, not detecting loss of input voltage. A phase relay checks for the presence and sequence of the three phases, which is about phase integrity rather than a general loss of power, and a temperature relay protects equipment from overheating. Thus, the voltage relay best fits the need to detect power loss and trigger a switchover.