What type of relay 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)

The signal used to know when to switch to a backup source is voltage presence. A voltage relay monitors system voltage and trips or signals an automatic transfer when voltage falls below a preset threshold, indicating a power loss. This direct measurement of available power is exactly what’s needed to start the switchover. A current relay is focused on current magnitude for protection against overloads, not on whether power is present. A phase relay checks that all phases are present and properly synchronized, and may trip on phase loss or imbalance, but it’s not the primary detector of overall power availability for initiating a transfer. A temperature relay measures temperature, which has no direct relation to electrical power presence. So the voltage relay is the best choice for detecting power loss and initiating a switchover.