What is NO and NC contact in PLC?

Logic is developed in every control circuit and automation system based on the open or closed state of switches, sensors, or relays. As a result, understanding the idea of NO/NC is required. The words normally open (NO) and normally closed (NC) are used to describe the states of switches, sensors, or relay contacts when their coils are not energised. It is referred to as the backbone of process automation.

What is meant by open and closed?

Before we get into what generally open and normally closed mean, let’s define “open” and “closed.” As with so many other issues that we try to simplify by linking with something we know, connecting electrical current flow with water movement leads to a plethora of misunderstandings. Whereas we open a water faucet to start water flow, we close an electrical contact to start the current flow, and we open an electrical contact to stop current flow instead of closing a water faucet to stop water flow.

  • Current flow = closed
  • No current flow = open

What is meant by normally?

This is simply the state of the contact when nothing is affecting it. If it’s a relay, it’s not turned on. If it’s a switch, it’s turned off. If it’s a high limit, like a temperature alert, the present temperature is lower than the limit.

What is Normally Open contact in PLC?

A NO contact, also known as a normally open contact, is one that remains open until a specified condition is reached. Consider a limit switch as an example. A limit switch contains at least one NO contact. The limit switch’s NO contact stays open until the actuator is pressed. When the actuator is pressed, the contact closes and the current begins to flow. NO contacts in proximity switches remain open until they detect an item; similarly, in pressure switches, the contact remains open until the predetermined pressure level is met.

The graphic above depicts the states of a push-NO button’s contact during normal operation and when pressed.

What is Normally Close contact in PLC?

By function, an NC contact, also known as a usually closed contact, is the total opposite of a NO contact. It remains closed until a specific condition is reached. In this situation, let us look at how the limit switch works. When the NC contact of a limit switch is pressed in a circuit, it breaks the circuit or current flow. Similarly, unless the coil of a relay is stimulated, its contacts remain closed.

What is the difference between Normally Closed And Normally Open Inputs?

It’s a good idea to use typically open contacts as PLC inputs. However, it can be harmful for stop function. This is because open contacts can create hazardous situations when they fail.

What if one of the wires cracked?

The diagram below depicts a wire break after a typically open input actuator.

Now, when the system fails, the stop button (the typically open contact) will be useless (wire-break). One failure is the wire break, but that results in another failure:

The stop button is inoperable. Because the stop button is an essential function, this option is not recommended.

How could this be considered excellent practice?

As a stop actuator, a normally closed contact is used. This is due to the normally closed contact as an input actuator not causing unsafe situations in the event of a failure.

That is, when a failure happens (the wire break), the input will behave as if the normally closed contact has been engaged. As a result, if the wire to the stop button breaks, the same thing happens as if the stop button is engaged. The latch will snap.

The Fig below depicts a normally closed input actuator acting as a stop button.

When the input actuator is switched from typically open to normally closed, the state of the input is likewise changed. When the actuator was not turned on, the input was always 1 or ON. When the actuator isn’t actuated, the input is now 0 or OFF.

Because the input is always 1, an instruction with a result of 1 will complete the task. The result of the if closed instruction examination is 1.

The diagram below depicts It is best to keep the PLC stop function normally closed.

The stop button now works with both hardware and software. What occurs when the pause button is pressed is shown below: