Phase Failure Relay

For large motors thermal overload relays provide inadequate protection against overheating of rotor due to the unbalance voltages. Unbalancing of voltages may be due to single phase loading, imperfect transposition of feeders or blown fuses in power factor correction plant. Over loading caused by unbalance voltages or phase failure condition due to fuse failure, loose connections or loss of phase from supply itself is not detected by conventional bi-metallic over load relays, and most of the motor burnouts are because of phase failure (single phasing). The detection of phase failure by sensing the negative phase sequence components of voltage or current.