Basic Interlocking of Electrical Circuit

We have covered a few items during the posts on fault finding and I think we should consider something else you will encounter on circuits namely, Interlocking. Reality is that it means one thing needs to happen before the next one can or if one thing has happened, the next one can’t. Sound complicated? It is not that bad. Let’s have a look at something very easy to start with.


You have two motors, A & B. You need motor A to run before motor B can be started. Each motor has its own stop and start control as well as a single emergency stop which needs to stop both motors. What this tells you is that motor A is a simple DOL circuit as we looked at previously, with one small item added to it. What we do is add a normally open contact of motor A into the circuit of motor B between the start and coil as you will see in the sketch below, and an emergency stop in front of the control circuit

Now let’s follow the circuit as we had done previously with the DOL circuit:

  • Control Breaker is on
  • Power flows to Emergency Stop
  • Emergency Stop is closed so power flows through
  • Power now follows both paths to the overloads for Motor A and Motor B
  • Overloads are closed and power flows through
  • Power flows to Stop A and Stop B
  • Both Stop buttons are closed and power flows through
  • Power now flows to the two Start Buttons and stops as both are open
  • Should you push the Start Button of Motor B, power will flow through and stop at the normally open contact CA as motor A is not running
  • If you push the Start Button Motor A, power will flow through and energize the coil of CA
  • This will allow the normally open contacts of CA to close where we have used on for the retaining circuit in parallel with the Start A and the other as the interlock contact in the circuit of Motor B
  • With the contact now closed, should you push the Start Button of Motor B, the power will flow through to the coil and energize it
  • Both motors are now running
  • Should Motor A be stopped, Motor B will also stop
  • Should the Emergency Stop be pushed while both motors are running, they will both stop

That means we have the circuit operating as per the instruction and have followed the circuit to test and it all looks good. Notice how we use the fault finding technique to test our circuit as well?

Well that describes the basic interlocking circuit. Have a bit of a play with adding different options to it and see how you go. As usual then, till next time, strive for perfection and always work safe!

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