How to do Interlocking in Electrical System?


If you are from electrical, then you may listen about electrical interlocking at many times. Because in the industry, whenever two contactors operate the motor then mostly said that contactor should interlock.

So now the question comes, what is interlocking, and why is it so important? Friends, in today’s post, we will understand interlocking very well with the help of circuit diagram.

We will also understand how to do interlocking wiring to the contactor. So that in the future, if you are asked any question in interview related to interlocking, then you can answer it easily, and even if you have to do wiring, you will be able to interlock contactor yourself without any other person’s help.

What is interlocking?

If we talk in starting, what is interlocking? So this means that two contactors are connected with our motor, and one contactor is ON. Now, if we turn on the second contactor, our second contactor will not be ON. Mean, only one contactor will be ON at a time. This is called the interlocking of the contactor.

Similarly, you can see the use of interlocking in ACB. If we talk about the motor starter, we use interlocking in most of the reverse forward and star-delta starters. Now finally, let’s understand how to do interlocking.

How to do interlocking?

Interlocking is done the same way in ACB and contactor. To easily understand suppose we have two contactors. Now we have to do the wiring in such a way that if one contactor is ON, we could not turn on the second contactor, and if the second contactor is ON, then we could not turn on the first contactor.

To start any electrical load we have supply terminal. If we talk about the contactor, it is operated from A1, A2 terminal. In the image below you can see how our A1, A2 terminals looks in the contactor.


If this contactor can turn on with 220 volts supply and we give supply of 220 volts on A1, A2 then the contactor will turn on. We put this 220-volt phase on A1 and neutral on A2.

Friends, for example, let’s assume that we have two contactors connected to a motor, and both these contacts are operated from 220 volts, then we connect these two contactors to the direct supply. Now we have three switches as you can see in the photo below.normal-wiring of contactor

With two switches, we turn on individually contactors and with one switch we turn off both the contacts.

As can be seen in the photo above, one phase wire goes inside both the buttons through the red button, now if we talk about normal wiring, then first we connect our phase wire from the switch to the A1 terminal of the contactor, then Now as we press first switch, our first contactor gets turned on.

Now when we have to do the interlocking, we will connect the phase wire of the first switch to the NC terminal of the second contactor we will not give supply directly to the first contactor. You can easily understand in the photo below.


After this, we connect the output of second contactor NC terminal to the A1 terminal of the first contactor. In this way the interlocking of our first contactor is completed.

Now like this we are going to do interlocking of second contactor. We will connect the phase wire that will come out from second switch to the NC terminal of the first contactor and we connect the output of the first contactor’s NC terminal to the A1 terminal of the second contactor.two contactor interlocking

As we connect like this, both our contacts have been interlocked. Now if we turn on the first contactor, then we cannot turn on the second contactor. Similarly, if our second contactor is on, then we cannot turn on the first contactor.

So friends you have understand that how to do interlocking, but if you are asked that how interlocking work? So what you have to say let’s understand quickly.

How interlocking works?

Friends, you always have to keep one thing in mind, that whenever our contactor is on, then the NC terminal of the contactor gets open. Then, no current flows through the NC terminal.

Now whenever we turn on our first switch, current is supplied to the NC of the second contactor. Now if second contactor is not ON then in that case supply from NC terminal will go directly to A1 terminal of first contactor and first contactor will be ON.


Now if the first contactor is on and we press the second switch, then the supply from the second switch will go directly to the NC terminal of the first contactor and as we mentioned above that if the contactor is on then the NC terminal will be open and the current will not flow.

So if the first contactor is on, and we press the second button, the second contactor will not ON. Because the first contactor is on, its NC terminal will remain open. And no current will flow to second contactor. Therefore, the second contactor will not operate until the first contactor is on.

So friends we hope that you got all the information about what is interlocking and how to do practical wiring to interlock contactor. If any question left then you can comment out below or you can send question to us on Instagram Electrical Dost.”

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Thank You 🙂

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Aayush Sharma is an Assistant Engineer in a Semi-Government Company and Owner of "Engineering Dost" and the Electrical Dost YouTube Channel. He Provides you Engineering inquiry and support of engineering market facts with Practical experience.


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