Single Phasing in an Induction motor arises when one of the supply lines out of three phase open circuits. In this condition, motor continue to operate delivering load not exceeding 57.7% of the normal rating with the same temperature rise compared to three phase supply delivering full load.
Some of the problems associated with single phasing are it may cause extreme magnetic unbalance, reduction in the torque of the motor, and overheating due to negative phase sequence currents. Operation of the motor under this condition is not advisable at it damages the motor. Therefore Thermal overload relays are provided for the protection of motor against single phasing.