What is phase sequence relay in electrical system ?

Three phase sequence relay how it work and its type.


2 min read

Phase preventer / sequence relay
Phase preventer / sequence relay

What is a phase sequence relay?

A phase sequence relay is an electrical device used to monitor the sequence or order in which the phases of a three-phase electrical supply system are applied. In a three-phase system, it's important that the phases are connected in the correct order to ensure proper operation of electrical equipment and prevent potential damage. Phase sequence relay also work when any one phase missing or low voltage in three phase system. Phase sequence relays help in monitoring and maintaining the correct phase sequence. Here's a simple explanation of how they work, their types, and their use.

How Phase Sequence Relays Work:-

1. Sensing:-

The phase sequence relay is connected to the three-phase power supply. It detects the order in which the phases are energized.

2. Comparison:-

The relay compares the actual phase sequence with the desired or correct sequence.

3. Output:-

If the phases are in the correct sequence, the relay remains in its normal state. If there's a phase reversal, meaning the phases are in the wrong order, the relay triggers an alarm or takes some other action to alert the user.

In basic phase sequence relay there is three terminal called ( L1, L2, L3 ) are main 3 phase input, and one NO and NC terminal for signal output. When any fault condition sensed by relay it signal by changing NO, NC point and LED on relay blow red in fault condition and green in normal operation.

Types of Phase Sequence Relays:-

1. Voltage-Sensing Relays:-

These relays monitor the voltage levels in each phase and determine the sequence based on the voltage order.

2. Current-Sensing Relays:-

Current-sensing relays use current transformers to monitor the phase currents and determine the sequence based on the current order.

3. Combination Relays:-

Some phase sequence relays combine both voltage and current sensing for more reliable phase sequence detection.

Use of Phase Sequence Relays:-

Protecting Motors:-

In industrial settings, phase sequence relays are commonly used to protect three-phase motors. Reversing the phase sequence can damage motors and reduce their efficiency. Phase sequence relays prevent this by ensuring that motors only run with the correct phase order.

Protecting Equipment:-

Phase sequence relays also protect other three-phase equipment and machinery, ensuring they operate as designed.


They provide safety by preventing equipment from operating if the phase sequence is incorrect.


Phase sequence relays can trigger alarms, lights, or other indicators to alert operators if there is a phase sequence issue.


They help identify phase sequence problems, allowing maintenance personnel to rectify them before equipment damage occurs.

In simple terms, phase sequence relays act as a "traffic cop" for the phases in a three-phase electrical system, ensuring they are in the correct order. If they detect a "traffic jam" or sequence problem, they raise the alarm to prevent equipment damage and maintain safe and efficient operation.