Automatic Phase Reverse Protection Circuit using Sequence Relay

In this article, we are going to make an Automatic Phase Reverse Protection Circuit using a Phase Sequence Relay. The automatic phase reverse protection system is a very important safety feature we should use in electrical systems to prevent damage or dangerous conditions that can arise when the phase sequence of a three-phase power supply is incorrect. 

In a three-phase electrical system, it must be ensured that all three phases are in the correct order (R-Y-B or L1-L2L3) that will help the electrical equipment to function properly and safely. When the phase sequence is changed or reversed (B-R-Y or L3-L2-L1), it can lead to issues such as motor damage, reduced efficiency, or equipment malfunction. Also, we know that if any two phases of a three-phase system interchanged, the motor will start rotating in opposite directions, so this also can affect our work.

That is why the Phase Reverse Protection system is required to correct the phase sequence automatically by swapping the phases back to the correct order. Automatic phase reverse protection is essential in applications where phase sequence is very important, such as industrial machinery, HVAC systems, and some types of electrical distribution systems. By detecting and responding to phase reversal events, this protection mechanism helps to prevent costly damage to equipment and ensures the safe and efficient operation of electrical systems.

Component Required

• 3-Pole MCCB - 1 PCs
• 4-Pole MCB - 1 PCs
• 3-Pole Contactor - 2 PCs
• Phase Sequence Relay - 1 PCs
• Indication Lamp - 1 PCs Red and 1 PCs Green

Circuit Diagram

Here, you can see the circuit diagram of an Automatic Phase Reverse Protection Circuit using a Phase Sequence Relay and Contactors.

Connection Procedure

1. Connect three-phase power source terminals to both MCCB and MCB.
2. Now connect the output of the MCCB to the input of both Contactors but remember that the L1 and L3 input of the second contactor should be interchanged.
3. Connect the output of both contactors in parallel with the same phase sequence.
4. Now, you can connect the load(here we have used a motor) to the output of any one contactor.
5. Connect the output of the 4-Pole MCB to the three-phase input terminals of the Sequence relay with proper phase lines such as L1-L2-L3.
6. Connect the output neutral terminal from the MCB to the negative coil terminal of both contactors and the negative terminal of both indication lamps.
7. Now, connect the common terminal of the relay to any input phase(here we have connected the B phase).
8. Connect the NC terminal of the Relay to the Positive coil terminal of the first contactor through the auxiliary NC contacts of the second contactor.
9. Connect the NO terminal of the Relay to the Positive coil terminal of the second contactor through the auxiliary NC contacts of the first contactor.
10. Now take any phase supply terminal(here we used the R phase) and connect it to the phase terminal of both indication lamps through the NO auxiliary contacts of their respective contactors which means the Green Lamp should be connected through the NO auxiliary contacts of the first contactor and the Red Lamp should be connected through the NO auxiliary contacts of the second contactor.

Operation and Working Principle

In normal conditions, when the phase sequence is correct, the first contactor will turned On as it is connected to the NC terminal of the Sequence Relay. The green lamp will glow as it is connected to the first contactor indicating the Normal Mode.

When the phase sequence is incorrect the Sequence Relay will make its NC contact NO and NO contact NC, so in this case, the Second Contactor will turned On. As the input of the second contactor is already reverse-connected, so it will provide a correct phase sequence to the load. Also, the Red indication lamp will glow as it is connected to the second contactor indicating the Phase Reversal Mode.

Thus the Phase Reverse Protection Circuit automatically corrects the phase sequence to the load when the phase sequence of the power source is reversed.

Also. I would like to remind you that both of the contactors never turned On at the same time as they are connected through the interlocking circuit.

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