What is a Phase Sequence Relay? | How does it work?


A phase sequence relay is a protective relay. It guards a 3-phase device against any potential damage due to sequence change. They are deployed anywhere with a phase-sequence change that can damage the device or circuit. They work like a conventional electric relay. Once a phase fails or if a sequence changes, the phase sequence relay cuts off the working. The high-end models pack more complex circuitry that can control the connected device as required. Also, it can even change the sequence of the relay automatically if any phase sequence shift is introduced.

How do they work?

In a 3-phase system, all the connected devices have to be in the same phase sequence to be functioning. In a 3-phase motor, it works only if the supply, starter, and the motor are connected in the same sequence.

A phase sequence relay has 3 terminals to connect the 3-phase supply cables. Besides, they have 3(NC, Common, NO) or more than 3 terminals based on the complexity of the control circuit. The required 3-phase supply is applied from the source to the phase sequence relay. In the case of a 3-phase motor, the supply to the starter is connected via the NO terminal of the phase sequence relay. The supply to the relay is directly taken from the R phase and connected to the Common terminal in the relay.

If a phase-shift or failure occurs, the relay switches the supply from NO terminal to the third control point, NC. The same applies even when a phase is failed as well. Hence, the starter will not get a supply to start the motor.

Modern Phase Sequence Sensors

The conventional phase sequence sensors are deployed for a simple switch on/off of the motor, or such devices. Even though the application remains the same, the capability of these sensors has improved significantly. Modern sensors can change the wrong-phase supplies automatically when they are connected falsely. And, phase sequence relays work on a high current environment. Modern sensors are rated even up to 3 X 12A currents, with integrated protection from high-temperatures. They have a great response time when compared with the older models.


Given the current and voltage ratings of phase sequence sensors, they are deployed in various industry-grade systems. They are most common in motor-related systems to avoid any potential phase reversal at the motor. Phase sequence sensors are also helpful in protecting the device from phase failure or imbalance as well. Air conditioning devices, elevator machines, pumps, fans, are all various applications of this sensor.


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