## What is Contactor Interlocking?

When an electrical circuit has two contactors and only one of them needs to be turned on at a time then the interlocking system is used. The interlocking system ensures that only one contactor can be on while another one will be in off condition even if we try to manually on.

An interlocking system is very useful for motor starter and power circuits. In a three-phase star delta motor starter, two contactors are used. One provides star connection while another provides delta connection. During starting time the Star-connected contactor will on and during the running condition delta connected contactor will on. If both of them are turned on at the same time, there will be a short circuit. So the interlocking system in a star-delta starter prevents the short circuit.

The interlocking system in a changeover circuit helps to connect the load with one source of power supply and isolated it from another source of power supply.

## Contactor Interlocking Circuit Control Diagram

Here you can see the control diagram for contactor interlocking.

As per the above control diagram, you can see the coil of contactor 1 is connected through the NC points of contactor 2 and the coil of contactor 2 is connected through the NC points of contactor 1.

So when contactor 1 is in ON condition, its NC contacts become open so contactor 2 can not be turned on even we push the switch S2 to turned on contactor 2 manually.

When contactor 2 is in ON condition, its NC contacts become open so contactor 1 can not be turned on even we push the switch S1 to turned on contactor 1 manually.

## Contactor Interlocking Wiring Diagram

Here the step-by-step procedure is given below to make the wiring for interlocking of the contactor.

Step 1

Here you can see we have taken two contactors and the three-phase power connection given as below figure.

Step 2

Now connect the A2 terminal of both contactors with neutral as shown in the below figure.

Step 3

Take a connection from the 'R' Phase and connect S1, S2, S3 as shown in the below figure.

Step 4

Connect the A1 terminal of contactor 2 to the S3 switch through NC contacts of contactor 1. See the below diagram for a better understanding.

Step 5

Now, connect the A1 terminal of contactor 1 to the S2 switch through NC contacts of contactor 2. See the below diagram for a better understanding.

Step 6

Now our interlocking circuit is complete. Now we are going to make the holding circuit. Connect the NO terminals of contactor 1 across the switch S2 and connect the NO terminals of contactor 2 across the switch S3. See the below diagram.

Step 7

Now our circuit is ready. Once cross-verify your circuit with the below wiring diagram.