What Is An Interlock

 

23 July, 2023.

In this post, we will see the concept of an interlock.

Every system when operated has some type of safety in it. Without the safety measures, there is a very high risk of operating the machine. Basically, an unsafe machine can pose threat to the nearby environment as well as personnel, because there is no control over it. So, safety measures are a must in any application. In engineering, this safety system is simply termed as an interlock. In this post, we will see the concept of an interlock.

What Is Interlock In Control System

Suppose you have a system in your factory to manufacture biscuits. There are several types of machines in the factory. There is one system in it which is used to pack the biscuits baked from oven. The packaging machine thus, has the job to pack the biscuits for final delivery. Now, if some issue occurs in the system like some mechanical part is malfunctioning or some utility becomes unavailable suddenly, then there must be some means to stop the system automatically or manually. Simply, consider that the operator has pressed emergency stop button in this case to stop the system. This input will stop the system from running then. This is called as interlock. An interlock is a means by which the system will not run unless and until it is healthy. In our example, it was healthy constantly; but when it was pressed, it became unhealthy. The operator can now run the system only after he has released the button again.

Different Types Of Interlock:

1. Mechanical Interlock

As the name implies, a mechanical interlock is a means by which some device is physically connected with some other device to get interlocked. Means, suppose device A is installed in a system. This device A is physically connected with other device B. If the device B is healthy, then only it will allow device A to run, due to it’s mechanical design. This is called mechanical interlock.

A very simple example is running your car. If the car key is not inserted, then the internal engine will not start due to it’s mechanical tie-up. So, your car key is the mechanical interlock for starting the engine.

2. Electrical Interlock

An electrical interlock is the means by which some device is electrically connected with some other device to get interlocked. Means, suppose device A is installed in a system. This device A is electrically connected with other device B. If the device B is healthy, then only it will allow device A to run, due to it’s electrical design and wiring. This is called electrical interlock.

In an automation system, suppose you have two contactors. The first contactor has an auxiliary contact (NO type) on it, which is wired to the second contactor. This means that when the contact is open, the second contactor will not start. For that, the first contactor needs to be turned on first. When turned on, it’s contact will turn on and this in turn will turn on the second contactor. So, the second contactor is electrically interlocked with the first contactor to operate itself.

3. Logical Interlock

When it comes to automation systems, it is incomplete without an interlocked logic inside it. For example, a PLC is used which logic has written in it for operating a system. Suppose, in the logic, an output needs to be turned on after 3 seconds. This program is called logical interlock. It means that an output or a process will not operate unless it’s logical interlock written in the program is healthy.

Simply, consider the same electrical interlock explained earlier. The second contactor is hardwired with first contactor to operate; but here, both the contactors will be operated through PLC outputs. The logic will be written inside to operate the second PLC output only when the first PLC output is on. This is logical interlock.

I have covered the general concept of an interlock. I have also not attempted to cover every theory of these design deeply; you can learn it easily once you get familiar with them. I have just given you an insight of these types of controls. A great system understanding is required by the engineer who is designing the project. Learn the basics and explore a new type of study in this type of automation.

Thank you guys; I hope you enjoyed reading the practices normally used for this type of study in industrial automation.