Nowadays, an Automation system is the main key in any industry. In every automation system and control circuit, logic is developed with the help of PLC based on the open and closed state of switches, sensors, and relays. Normally Open (NO) and Normally Closed (NC) are two types of contacts used to define the states of switches and relay contact in control logic.
In this article, We will discuss what are NO and NC contacts, and the main difference between NO and NC.
NO and NC are the contacts of relays and contractors that define the states of switches under when their coil is de-energized. NO stands for Normally Open and similarly NC stands for Normally Closed. They are typically used in relays and contractors.
What is NO Contact?
NO stands for Normally Open. NO is the contact of a relay or contactor switch that remains open until a certain condition is satisfied. It means that any wire connected to NO Contact will be open (OFF) when the relay is De-energized.
But when the relay is energized, the internal circuit of the relay will be triggered and make the contact closed, and hence current starts flowing through this contact.
By default NO Contact does not allow the flow of current unless it is energized that’s why it is called Normally Open (NO).
What is NC Contact?
NC stands for Normally Closed. NC is the contact of a relay or contactor switch that remains closed until a certain condition is satisfied. It means that the wire connected to NC contacts will be normally closed (ON) and current will flow when the relay is De-energized.
But when the relay is energized, the internal circuit of the relay will trigger and that makes this contact open, and hence current will now stop flowing through this contact.
That’s why it is called Normally Closed, because by default NC contact allows the flow of current unless it is energized.
Difference between NO and NC
Now coming to the Difference between NO and NC. The main difference is NO contact remains normally open and becomes closed once its coil is energized. Whereas, NC contact remains normally closed and becomes open once its coil is energized.
So here we consider a reference state to be De-energized. In a De-energized state, Contacts that are open are called NO (Normally Open), and those contacts that are closed are called NC (Normally Closed).
Also Read: Why Transformer is rated in kVA, not in kW?
But, When the device (Relay and contactor) is Energized. So it has changed its state. So, now NO contact is closed or becomes NC, and NC is open or becomes NO. Hence they toggle their state.
NO contacts are used to make the circuit ON, Whereas NC contacts are used to make the circuit OFF.
NO remains open until a certain condition is satisfied, whereas NC remains closed until a certain condition is satisfied.
These contacts (NO and NC) together in series or parallel make interlocking and other sophisticated control logic for controlling many circuits. So NO and NC are used to create binary logic in PLC for any automation system.
They are very important to understand the workings of complex power circuits. Here in the below image of the overload relay, It has both NO and NC contacts. NO Contacts are used to send feedback signals, Whereas NC contacts are used to send trip commands.
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