Relay protection phase loss circuit diagram

A phase loss (phase failure) relay circuit protects three-phase motors by detecting missing or incorrect phases and automatically disconnecting the load via a contactor.

Overview of Phase Loss Protection

A phase failure relay monitors the three-phase supply (L1, L2, L3) for voltage presence, phase sequence, and under/over-voltage conditions. If one phase is lost or the sequence is incorrect, the relay trips, de-energizing the contactor and cutting off power to the motor, preventing overheating, reverse rotation, or insulation damage .

Basic Components

• Phase Failure Relay (PF Relay): Detects phase loss, phase sequence, and voltage deviations.
• Contactor: Switches the motor supply on/off based on relay output.
• Overload Relay: Provides thermal protection for the motor.
• MCB (Miniature Circuit Breaker): Protects the circuit from short circuits.
• Push Buttons: Green (start) and red (stop) for manual control.

Functional Wiring Diagram

1. Connect the three-phase supply to the MCB and then to the PF relay input terminals (L1, L2, L3).
2. The PF relay output is connected to the contactor coil. When all phases are present and correct, the relay energizes the coil, closing the contactor and supplying the motor.
3. Integrate the overload relay in series with the motor to provide additional protection.
4. Include start (green) and stop (red) push buttons in the control circuit for manual operation.
5. If a phase is lost or sequence is incorrect, the PF relay de-energizes the contactor coil, opening the contacts and disconnecting the motor supply .

Protection Logic

• Normal Operation: All three phases present → PF relay energizes → Contactor closes → Motor runs.
• Phase Loss or Incorrect Sequence: PF relay de-energizes → Contactor opens → Motor stops.
• Under/Over Voltage: Relay trips if voltage exceeds preset limits, preventing motor damage.
• Time Delay: Adjustable ON/OFF delay timers prevent nuisance tripping due to transient voltage fluctuations .

Practical Notes

• Ensure the relay and contactor ratings match the motor's voltage and current specifications.
• Always isolate power before wiring.
• Test the circuit carefully after installation to verify correct operation under phase loss and phase reversal conditions . This setup ensures automatic protection against single-phasing, phase imbalance, and incorrect rotation, safeguarding three-phase motors and industrial equipment.