Undervoltage Relay Connection Diagram and Wiring Procedure

When the voltage level of a power system goes below its minimum level then it is called Undervoltage. Undervoltage faults can happen due to so many reasons such as overload in the power system, loose connection, or a loose contact, phase failure, dead short circuit, etc. There are so many sensitive electrical machines, devices, and equipment we used for our useful work which badly affected during the Undervoltage fault. For example, an electrical motor draws a very high current during the Undervoltage fault. So, it can produce more heating further burning, or fire. So, the protection against Undervoltage fault must be taken. There are so many different types of devices for Undervoltage faults. But, in this article, we are going to see the Undervoltage Relay Connection Diagram and Wiring Procedure.

Undervoltage relay is mainly used with motor circuits to protect the electric motor from Undervoltage faults. But they can be used for other loads also. An Undervoltage relay cannot protect alone the load from Undervoltage faults, it always works with a circuit breaker. The main function of an Undervoltage relay is to sense the Undervoltage fault and send signals to the circuit breaker to disconnect the power supply. First, of all let's see the wiring diagram.

Connection Diagram

Here, you can see the connection and wiring diagram of an Undervoltage relay with a circuit breaker.

Here, you can see the Undervoltage relay is connected to an MCCB(Moulded Case Circuit Breaker). The MCCB is used to turn on/off the main power supply from source to load and it can give protection against short circuit and overload faults. Now, all three phases(R, Y, and B) and Neutral(N) are connected to the Undervoltage relay. So, it can monitor the voltage level of the three-phase system continuously. It also has common, NO, and NC contacts. So, in normal conditions when the voltage level is ok, its NO contact remains NO, and NC contact remains NC. But when the Undervoltage fault occurs, the NO contact will be NC, and the NC contact will be NO.

You can see the shunt trip coil of the MCCB is connected to with a single-phase power supply(B-N) through the common and NO contacts of the Undervoltage relay. The shunt coil of the MCCB is that which can trip the MCCB if a power supply is applied to it. So, as here the shunt coil is connected through the NO contacts, in normal condition, it does not get any power supply. But when an Undervoltage fault occurs, the relay will sense and it makes its NO contact with NC. So the shunt trip coil will get the power supply and trip the MCCB. Thus the total power supply will be turned off during the Undervoltage fault.

Wiring Procedure

1. There are different types of Undervoltage relays available in the market, so select the best one according to your requirements. Here, we have shared basic knowledge to wire any Undervoltage relay.

2. Undervoltage relays have so many terminals but here we need only the three-phase input terminals and NO, NC, and Common contact terminals. So, find out these terminals in your Undervoltage relay.

3. Connect all three phases and neutral to the input of the relay.

4. If you use an MCCB with a shunt trip coil, then make the connection as per our wiring diagram.

Read Also: 

• Overload Relay Connection Diagram and Wiring
• Voltage Monitoring Relay(VMR) Connection Diagram and Wiring
• Earth Leakage Relay Wiring and Connection Diagram
• Automatic Phase Reverse Protection Circuit using Sequence Relay
• Phase Sequence Relay Wiring Diagram and Connection

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