What device can be used to detect a condition of reversed phase and prevent a motor from operating?

What device can be used to detect a condition of reversed phase and prevent a motor from operating?

• A. Overcurrent relay
• B. Ground fault relay
• C. Differential relay
• D. Reverse phase relay

Answer: (D)

Detecting and guarding against the wrong order of the three-phase supply is essential because motors can spin in the opposite direction or suffer mechanical stress if the phase sequence is reversed. A reverse phase relay monitors the order of the three-phase voltages (or currents) and will trip if the sequence reverses or a phase is missing, stopping the motor to prevent damage or unintended operation. The other devices protect against different faults: an overcurrent relay fires on excessive current but doesn’t sense phase order; a ground-fault relay looks for leakage to ground; a differential relay compares currents to detect internal faults. None of these monitor phase sequence, so they won’t reliably prevent operation when phases are reversed.

Detecting and guarding against the wrong order of the three-phase supply is essential because motors can spin in the opposite direction or suffer mechanical stress if the phase sequence is reversed. A reverse phase relay monitors the order of the three-phase voltages (or currents) and will trip if the sequence reverses or a phase is missing, stopping the motor to prevent damage or unintended operation.

The other devices protect against different faults: an overcurrent relay fires on excessive current but doesn’t sense phase order; a ground-fault relay looks for leakage to ground; a differential relay compares currents to detect internal faults. None of these monitor phase sequence, so they won’t reliably prevent operation when phases are reversed.