In every control circuits and automation systems, logic is developed based on the open or closed state of switches, sensors or relays. Hence it is mandatory to know the concept behind NO/NC. Normally open(NO) and Normally closed (NC) are terms used to define the states of switches, sensors or relay contacts under when its coil is not excited. It is the fundamental of process automation.
What is NO contact?
A NO contact or a normally open contact is the one that remains open until a certain condition is satisfied. For example, let us consider a limit switch. A limit switch consists of at least one NO contact in it. The NO contact in the limit switch remains open until its actuator is pressed. When the actuator is pressed the contact closes and starts conducting. In the case of proximity switches, NO contacts remain open until it senses some object similarly in the case of pressure switches, the contact remains open until the preset pressure level is reached.
The above image shows the states of the NO contact of a push-button during the normal condition and when pressed.
What is NC contact?
A NC contact or normally closed contact is the exact opposite of NO contact by function. It remains closed until a certain condition is satisfied. Let us discuss the operation of the limit switch in this case. The NC contact of a limit switch, when used in a circuit breaks the circuit or current flow when its actuator is pressed. Similarly, the contacts of a relay remain closed unless its coil is excited.
|Normally Open Contacts||Normally Closed Contacts|
|Remains open until a certain condition is satisfied||Remains closed until a certain condition is satisfied.|
The above image shows the NO and NC arrangement of a limit switch (Click to enlarge the image). The contacts 1 and 2 are normally closed, meaning it remains conductive until the actuator is pressed. When the actuator is pressed, it stops the conductor. The contacts 3 and 4 are normally open contacts, meaning that it starts conducting when the actuator is pressed.