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 fault can happen due to so many reasons such as overload in power system, loose connection, or a loose contact, phase failure, dead short circuit, etc. There are so many sensitive electrical machines, devices, and the 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 fault. But, in this article, we are going to see Undervoltage Relay Connection Diagram and Wiring Procedure.


Undervoltage relay is mainly used with motor circuits to protect the electric motor from Undervoltage fault. 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.

undervoltage relay connection diagram and wiring, wiring of undervoltage relay, connection of undervoltage relay


Here, you can see the Undervoltage relay is connected with 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 the three phases(R,Y,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 into 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 are available in the market, so select the best one according to your requirement. Here, we have shared a 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, Common contact terminal. 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.


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Undervoltage Relay Connection Diagram and Wiring Procedure Undervoltage Relay Connection Diagram and Wiring Procedure Reviewed by Author on 11/01/2021 Rating: 5
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