What is single phasing?

Single phasing is the term used to denote the condition arising in a three-phase circuit when one phase becomes open circuited.Or, Single phasing refers to operating 3 phase motor with any one of the 3 phase connection is missing. During single phasing, no current will pass through one line while the other two lines carry approximately 2.4 times the normal current value for the load.

Generally, open circuit means ?/Causes of single phasing

The open circuit in a phase, often from a

1.blown fuse,

2. faulty contact or broken wire.

It will prevent a motor from starting but running motor may continue to operate with a fault.

3.Relay contacts may be damaged

4.Contact joints are not conducting properly

How single phasing occur ?

  1. Excessive current in the remaining supply cable.
  2. Unequal distribution of current in motor winding.

Effects of single phasing

1.When one phase breaks the current in the remaining two phases increases about 2.4 times the normal current value.

2.Due to single phasing uneven torque will be produced and it may cause vibration of the motor and the motor became noisy.

3.Generator may get overloaded

4.Single phasing may reduce the speed of the motor.

5.Motor winding may melt due to overheating

6.If single phasing happened on running motor, motor will continuously run until the wingdings burns off

7.It is impossible to start a motor with single phasing.

How Single phasing can be detected ?

It Can be detected by overload device in the supply line or through the overheating.Overheating in a stalled or running motor will cause burn out of the overloaded coil.


In a lightly loaded motor to remain undetected by electromagnetic trips on the supply line which monitor only current. Improve protection is given by thermisters placed in the winding to measure thermal effects


Protecting Devices To Avoid Single Phasing

1. Overload Relay: Which will trip the motor if the current exceed normal value

2.Thermistors: It is a type of resistor whose resistance is dependent on temperature placed on windings. Any increase in current heat the winding which is detected by the thermistor and send the signal to trip the motor.

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