Understanding NO vs NC Contacts is key for Logic Writing in PLC Programming

Controlling processes safely and effectively in industrial automation, particularly inside PLC (Programmable Logic Controller) systems, is fundamentally based on the usage of Normally Open (NO) and Normally Closed (NC) contacts. These contact types are symbolic representations of actual input devices like push buttons, limit switches, and sensors. Designing dependable and safe control systems in ladder logic programming depends on knowledge of their behavior.

With reference to simulation examples from programming environments, this article clarifies the concepts of NO and NC contacts and analyzes their application in PLC ladder diagrams.

What are NO and NC Contacts?

An electrical relay system has two kinds of contacts: Normally Open (NO) and Normally Closed (NC). These are the only two kinds left as, at its essence, a contact can be open (off) or closed (on). Let’s examine each one’s behavior:

Definition of NO and NC Contacts

What is an NO (Normally Open) Contact?

• Under typical circumstances, a Normally Open contact is in an open (non-conducting) state.
• The contact closes, hence wrapping up the circuit, when the related field device—e.g., a push button or a proximity switch—is triggered.
• It is applied when the input is triggered by the operator or process, therefore requiring action.
• In PLC ladder logic, an NO contact is symbolized as: [ ].

Typical uses NO (Normally Open) Contact

• Start push buttons
• Activation trigger limit switches
• Pressure or level switches acting on rising conditions

What is an NC (Normally Closed) Contact?

• Under typical circumstances, a Normally Closed contact is in a closed (contact) state.
• When the device is activated, the contact opens, thereby interrupting the circuit.
• Applications where the system must run by default and only stop when an interruption happens employ NC contacts.
• In PLC ladder logic, an NC contact is symbolized as: [ / ].

Typical uses NC (Normally Closed) Contact

• Emergency stop push buttons
• Safety interlocks and door switches
• Fail-safe shutdowns

Relay System Example in Automation

The above relay system diagram shows how a control system can operate a motor utilizing a start button, an emergency stop button, and a relay interposing circuit. The relay logic and operation are broken down below:

• NO (Normally Open) Contact: Connected to the start button.
• NC (Normally Closed) Contact: Connected to the emergency stop button.

How it Works: Start Button Pressed

Pressing the start button closes the NO contact, hence activating the relay. This lets the motor run by means of the current.

How it Works: Emergency Stop Button Pressed

Pushing the emergency stop button opens the NC contact, hence eliminating the current flow and stopping the motor.

Relay Deactivated

• The relay is deactivated and the motor turned off if the start button (NO contact) is not pressed and the emergency stop button (NC contact) is pressed.

Relay Activated

• The relay becomes energized and the motor starts when the start button is pressed (NO contact closes) and the emergency stop button is not pressed (NC contact opens).

Master the Basics of Ladder Logic: Understanding Rungs and Rails: The Foundation of PLC Ladder Logic

How these Contacts Work in PLC Programming

Controlling devices like motors, lights, and other machinery via a PLC requires both NO and NC contacts in industrial automation. Ladder logic uses particular symbols to show these contacts:

• NO Contact: [ ]
• NC Contact: [/]

Here is an easy ladder logic example of their application:

Example: Ladder Logic Interpretation using NO and NC Contacts

This below section explains three programming and simulation images that represent the use of NO and NC contacts in a PLC logic setup for a pump motor control application.

Refer the below link for Converting Electrical Diagrams into PLC Programs for Industrial Automation

Step-by-Step Guide: Converting Electrical Diagrams into PLC Programs for Industrial Automation

Image 1: Programming View – Ladder Logic Design

The above depicted picture displays the programming views of a basic ladder diagram in the PLC development environment.
Get a Solid Foundation in PLC Logic: Understanding Basic Parts of Ladder Diagram (LD) in PLC Programming

Rung 0: Pump Start Logic

• Input %I0.0 represents a limit switch sensor and is configured as a Normally Open contact.
• The output %Q0.0 is labeled PUMP_START_ and is energized when %I0.0 is active (i.e., when the NO contact closes).

Avoid Costly Mistakes!: Top 6 Important Rules for PLC Ladder Diagram Programming

Rung 1: Pump Stop Logic

• Two inputs are used here: %I0.1 for the Emergency Stop and %I0.2 for the Stop Push Button.
• Both are represented as Normally Closed contacts.
• When neither %I0.1 nor %I0.2 is actuated, the contacts remain closed, and the output %Q0.1, labeled PUMP_STOP, remains energized.
• If either input is pressed (i.e., contact opens), the rung breaks and the output de-energizes.

Interpretation of Ladder Diagram:

• This ladder design ensures that the pump starts when the limit switch is triggered and stops safely if either the emergency stop or stop push button is pressed.

How Reliable is Your PLC?: Understanding PLC Redundancy: Cold, Warm & Hot Redundancy

Image 2: Simulation View – NO Contact Active (Start Condition)

The above shown picture demonstrates a simulation of the ladder logic under the following conditions:

• %I0.0 (LIMIT_SWITCH_SENSOR) is active. As a Normally Open contact, it is now closed, allowing the signal to pass.
• The output coil %Q0.0 (PUMP_START_) is energized.
• %I0.1 (Emergency Stop) and %I0.2 (Stop PB) are not pressed, meaning their Normally Closed contacts remain closed.
• As a result, %Q0.1 (PUMP_STOP) is also energized.

System Behavior:

• The pump is running, as both the start logic and stop logic conditions are met.
• This reflects a normal operating condition where the process continues without interruption.

Not All Racks Are the Same: Understanding PLC Racks and Chassis: Types, Differences, and Purposes

Image 3: Simulation View – NC Contact Activated (Stop Condition)

The above shown picture shows the impact of turning on the stop or emergency devices:

• %I0.0 remains active (limit switch engaged), so the NO contact stays closed.
• However, either %I0.1 (Emergency Stop) or %I0.2 (Stop PB) has been pressed.
• Pressing these inputs causes their NC contacts to open.
• The rung associated with %Q0.1 (PUMP_STOP) is now broken, resulting in a de-energized output.

Logic Made Simple: Logic Gates in PLC Programming: A Guide with Truth Tables and Ladder Logic Diagrams

System Behavior

• The STOP condition overrides the START command.
• The deactivation of the stop output simulates a halt in the pump or process, depending on how %Q0.1 is logically interpreted (e.g., controlling a stop relay or safety interlock).

Application Scenarios NO and NC Contacts

Device Type	Contact Type	PLC Symbol	Functionality Description
Start Push Button	NO	[ ]	Closes when pressed to initiate the process
Emergency Stop PB	NC	[ / ]	Opens when pressed to stop the process immediately (fail-safe)
Limit Switch	NO	[ ]	Closes when target is reached, triggering an operation
Safety Door Switch	NC	[ / ]	Opens when door is opened, interrupting operation for safety
Pressure Switch	NO or NC	Varies	Depending on design, used to start/stop operations based on pressure

Key Design Principles in PLC Programming

Use NO Contacts for Events and Actions

• Devices that activate on a particular event or condition change.
• Best for typical operations where activity is intentional.

Use NC Contacts for Safety and Fail-Safe Logic

• Devices that, should wiring breaks or is disconnected, must default to a safe condition.
• Guarantees that during hardware failures vital processes cease.

Note: Best practice in safety system design is to use NC contacts for emergency stop buttons. This setup guarantees that the system will go to a safe state and stop running automatically if a wire is severed or the button is broken.

Hands-On Learning!: PLC Programming Example: Using Timers to Control Motors with a Single Switch

Understand Input Type and Device Logic

• Field wiring has to match PLC programming logic.
• Misunderstanding NO vs NC behavior could lead to unintentional actions.

Use Descriptive Tags

• In the ladder logic, always mark inputs and outputs explicitly.
• For instance, EM_STOP, LIMIT_SWITCH_SENSOR, PUMP_START_, etc.

In PLC programming, knowing the distinction between NO and NC contacts is absolutely vital for both functionality and safety. While NC contacts are favored for stopping processes or starting safety logic, NO contacts are usually utilized for starting activities when conditions are satisfied. Correct execution guarantees consistent system performance, improved diagnostics, and safer operation in industrial settings.

Unlock the Secrets of PLC Datasheets: How to Read the PLC Datasheet: Allen-Bradley 1762 Model Step-by-Step Guide

Control engineers, electricians, and automation technicians working on design, programming, or debugging PLC-based systems find this subject to be basic.

Test Your knowledge on PLC (Programmable Logic Controller) 

Refer the below link to test your knowledge on PLC (Programmable Logic Controller) 

PLC (Programmable Logic Controller) Knowledge Quiz