Why need overload relay when MCB give overload protection?


MCB VS Overload Relay


Hey, ever you thought why we need an overload when we already have the MCB as an overload protective device. You will see the uses of MCB in our domestic electrical installation to industrial electrical wiring whereas the overload relay is used in motor control circuits, starter circuits, protection circuits, etc. First of all, know what is Relay and what is an MCB. Let's talk about it. MCB means Miniature circuit breaker, basically, it is an electrical protective device that breaks a circuit during the short circuit fault or overload fault. On the other hand, the relay is a switching device that routes the power or signal. Most of the relays have two contacts one is normally open(NO) and another is Normally Closed(NC). In normal conditions, those contacts remain the same but when the relay is actuated, the NC contact becomes NO and the NO contacts become NC. 

In the case of an overload relay, it has also had two contacts. When a normal current flows through the relay its contact remains in their old position but when an overcurrent flows due to the overload fault, the NC contacts becomes NO and the NO contacts become NC.

Now, you may understand the MCB gives protection to a circuit itself but the overload relay does not protect or do not take any external action itself it just changes its contacts. So we can use an external device or circuit which can operate with the overload relay.

For example, a single MCB can be used for the protection of overload fault. Whereas an overload relay and a contactor both are required to provide overload protection.

MCB and Overload relay both have their own advantages, disadvantages, and specific applications. For static load, MCB provides a better performance whereas, for the dynamic load, overload relay provides better performance. Overload relay can be used for the integrated control circuits.

MCB provides both short circuit fault protection and overload fault protection whereas the overload relay is better for the protection of overload protection but it is not suitable for short circuit fault. It can give protection against the short fault but is not much sensitive than the MCB.

Basically, the MCB has two arrangements - the bimetallic strips for thermal overload protection and a magnetic coil for the short circuit protection.

When the overcurrent flows through the MCB the bimetallic strips become hit and they separated from each other, thus the circuit is disconnected. So during the overload fault, MCB takes some time to disconnect the circuit. When the short circuit fault occurs a very high current flows through the magnetic coil of the MCB and the coil forces the plunger to pul down and turns off the MCB.

The important advantages of an overload relay are, we can set the current flow or tripping current in the overload relay. For example, when we set 10A current on an overload relay, it will trip the circuit once the current flow through the overload relay exceeds 10A. Also, the overload relay has a RESET button. Until we press the RESET Button after tripping it will not allow the current to flow through it.


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Why need overload relay when MCB give overload protection? Why need overload relay when MCB give overload protection? Reviewed by Author on 3/07/2022 Rating: 5
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