Single Phasing is the operation of a polyphase induction motor when one of the supply phase lines is dead. Single phasing is also referred to as open phasing or phase failure and it’s a peculiar case of unbalanced supply. 

Cause of single Phasing on 3 phase Induction motors

  • Can be caused by blowing off of a fuse somewhere in the system through an earth fault. 

  • Loose contact or burning out-of-control equipment.

  • Cut wires


Effects of single phasing on polyphase induction motor

  • The stationary motor will not start with one line broken. 

  • Even if it starts it will start slowly.

  • It also causes stalling.

  • The other phase draws more current causing overheating of rotor winding.


Protections against single phasing are:

  1. Differential thermal relay.

  2. Negative sequence relay in combination with thermistor O/L relay.