Single phasing, also known as phase failure or phase loss, is a condition in a three-phase electrical system where one of the three supply phases becomes disconnected or open-circuited while the remaining two phases continue to supply power.

This condition may occur due to:

• A blown fuse in one phase
• Broken conductors or cables
• Faulty contactors
• Loose electrical connections
• Open circuit in the supply line

When single phasing occurs, a three-phase motor connected to the supply may continue operating on the remaining two phases. However, the supply becomes unbalanced, resulting in abnormal current distribution and unsafe operating conditions.

Single phasing is particularly dangerous for induction motors because it can cause overheating and serious damage if not detected quickly.

1. Motor Continues Running

Unlike some other electrical faults, the motor may not stop immediately.

• The motor continues running because the remaining two phases still produce an unbalanced rotating magnetic field.
• However, motor performance becomes poor and unstable.

2. Excessive Current in the Remaining Phases

To maintain the required load torque, the current in the remaining energized phases increases considerably.

In a delta-connected motor:

• One winding effectively loses its proper supply.
• The other two windings attempt to carry the entire load.

As a result:

• Current in the energized windings may rise to approximately 1.73 to 2 times the normal full-load current, depending on the load condition.

3. Reduction in Torque and Speed

The motor is unable to develop normal torque under single phasing conditions.

This causes:

• Reduction in torque output
• Slight drop in motor speed
• Difficulty in carrying the connected load

The motor may struggle to continue operating efficiently.

4. Severe Overheating

The most serious effect of single phasing is rapid overheating of the motor windings.

Due to excessive current:

• Copper losses ((I^2R) losses) increase sharply.
• Winding temperature rises rapidly.

This overheating can damage insulation very quickly.

5. Winding Burnout and Motor Failure

If the condition continues:

• Insulation breakdown occurs,
• Windings may short circuit,
• The motor may eventually burn out completely.

Although thermal overload relays may trip the motor, they may not act quickly enough to prevent serious damage, especially when the motor is heavily loaded.

The most effective method of protecting a motor against single phasing is by using a Phase Failure Relay (also called a Phase Loss Relay or Single Phasing Relay).

Working Principle of Phase Failure Relay

1. Continuous Monitoring

The relay continuously monitors:

• Voltage in all three phases, or
• Current in all three phases

to ensure balanced supply conditions.

2. Detection of Phase Failure

If:

• One phase is lost,
• Voltage becomes severely unbalanced, or
• Current imbalance occurs,

the relay detects the abnormal condition immediately.

3. Tripping Action

Once single phasing is detected:

• The relay operates its contacts,
• The control circuit of the motor contactor is interrupted,
• The contactor coil becomes de-energized.

4. Disconnection of the Motor

When the contactor drops out:

• The main power contacts open,
• The motor is disconnected from the supply.

This prevents overheating and protects the motor windings from damage.

Modern Motor Protection Relays

Modern electronic motor protection relays often include additional protective features such as:

• Overcurrent protection
• Under-voltage protection
• Phase sequence protection
• Earth fault protection
• Thermal protection

These provide complete protection for the motor.

Limitations of Thermal Overload Relays

Thermal overload relays provide only indirect protection against single phasing.

• They operate based on overheating,
• But overheating may already have caused insulation damage before the relay trips.

Therefore, a dedicated phase failure relay is preferred because it detects the fault condition itself and acts much faster.