What is NO and NC in Electrical System and Devices?


In electrical and electronics engineering the terms 'NO' and 'NC' are very important. Especially, when we talk about the automation system and control circuit, these terms are very important. So, you must have a clear knowledge of them. Because these indicate the different states of the devices, and circuits are to be controlled. For example, In an automation system, the 'NO' and 'NC' states of the sensors, relays, contactors, and switches are controlled using logic circuits or programming devices. This is very fundamental to an automation system.


What is 'NO'?


The full form of the term 'NO' is Normally Open that means in a normal condition something is open. Basically, it describes the state of contact between two ends or conductors. Until a certain condition is satisfied the contact remains open. As we know that it explains the contact between two conductors or terminals so in normal conditions it does not conduct current flow. So when a certain condition is satisfied such as coil energized, or pressing buttons the contact becomes Closed and it becomes able to conduct current.

NO or Normally Open Contact



NO Switch


There are some push button switches that have open contact in normal conditions. When the button is pressed its internal contact got closed again when we release the button it becomes open. The torchlight push button switch is an example of a NO switch. Also, you can see the use of the NO push button switch in the control circuits of the motor starter. Basically, these switches are used to control the flow of current manually.


NO contact in Contactor


A contactor is an electrically controlled device that is also used to control the flow of current. Basically, it has a magnetic coil and some NO and NC contacts. The NO contact remains open and is not able to conduct current until the coil of the contactor is energized. Once the coil is energized by an external auxiliary power supply, the NO contact becomes closed and is able to conduct current.


NO contact in Relay


Relay is also an electrically controlled device that switches or routs the electrical current flow. A simple relay has three main terminals Common, NO(Normally Open), and NC(Normally Closed). It also has a magnetic coil. Until the coil is not energized, there is no contact between the 'Common' and 'NO' terminal. But when the coil is energized the 'Common' and 'NO' terminals become connected together. 



What is 'NC'?


The full of term 'NC' is Normally Closed which means the contact remains closed in normal conditions. Once a certain condition is satisfied the contact becomes open. So we can say in the normal condition it is able to conduct current but in the operating condition, it is not able to conduct current. NC is just opposite to the NO. Not as much NO but NC contact is also used in so many systems, and devices.

NC or Normally Closed Contact


NC Switch


In normal conditions, when the button on the switch is not pressed the contacts remain closed but when the button on the switch is pressed the contact becomes open. NC push button switch is used in motor control circuits, starter circuits, pilot circuits, etc.


NC Contact in Contactor


When the coil of the contactor is not energized the NC contacts of the contactor remain closed but when the coil is energized the NC contacts become open.



NC Contact in Relay


In normal conditions, when the coil of the relay is not energized the 'Common' and 'NC' terminal remain closed but when the coil is energized the contact between the 'Common' and 'NC' terminal becomes open.


Difference between NO and NC


1. NO means Normally Open whereas NC means Normally Closed.

2. In normal conditions, NO contact remains open and not able to conduct current whereas NC contacts remain closed and able to conduct current.

3. At a certain operating condition NO contact becomes closed and able to conduct current whereas NC contact becomes open and not able to conduct current.


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What is NO and NC in Electrical System and Devices? What is NO and NC in Electrical System and Devices? Reviewed by Author on 9/13/2022 Rating: 5

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