Single Phasing - its causes and effects

Single phasing is a condition in three phase motors and transformers wherein the supply to one of the phases is cut off. Single phasing causes negative phase sequence components in the voltage. Since, motors generally have low impedances for negative phase sequence voltage. The distortion in terms of negative phase sequence current will be substantial.

Negative phase sequence currents cause heating of the motor and consequently motor failure.

Single phasing is caused by the use of single-phase protection devices such as fuses and circuit breakers. Three phase loads should be protected by devices which cause the interruption of power to all three phases simultaneously when a fault occurs.

Defective contacts in three phase breakers can also cause single phasing.

Single phasing can be identified by special protective relays which can identify and isolate the connected loads. Smaller motors rely on overcurrent and negative phase sequence relays. Motor protection relays for larger motors come readily fitted with protection against single phasing.

Single phasing can sometimes cause excessive noise and vibration in motors.