NO & NC Contacts Explained – Normally Open vs Normally Closed (Working & Uses)

“NO & NC Contacts- the Building Blocks in Electrical system!“

One of my college professor had spoken the above statement. He also said Electrical system is standing on these two blocks.

At that time, I didn’t take it seriously. But now, I know the importance of these very well. I myself using these building blocks to create a logic in Electrical system.

‘NO & NC Contacts‘ What are they? Where & How, These are useful in Electrical system. All of these, Is going to be explained in this post.

Push button blocks NC & NC — Push button blocks NC(left)-NO(Right)

What are NO & NC Contacts ?

NO Stands for ‘Normally Open‘
NC Stands for ‘Normally Closed‘

(Normally means ‘at initial state’, ‘a stage without operation’ or ‘a default state’.)

NO represent a switch which is open at default state or at initial state. But when this switch get pressed, the open contact of NO get closed.

After that if we release that switch, It comes back to its initial open state.

NC represent a switch which is closed or connected at default initial state.

When this switch is get pressed, the closed contacts of NC get opened. Further if we release that pressed switch; it regain its initial closed state.

How these duos are being used ?

From a basic Circuit to big complex circuits, NO & NC contacts play a very important role.

“NO –A simple BELL Switch is the Best example of NO contact in which contact closes only when it get pressed. After that switch again retain its initial NO state when switch get released.”
“NC -Refrigerator’s door light switch is type of NC Contact. When door of refrigerator get opened; the switch get unpressed & switch’s contact get closed, At this stage light get ON. On the other hand, when door get closed; switch get pressed. Switch get Opened which results light get OFF.“

Next, some of the examples been explained which states Where & How these duos are being used !

IN MOTOR STARTERS

In dol Starter

DOL stands for direct on Line. It is the most important starter for small rating Induction motors. It’s circuit is very simple & easy to understand.

Wiring Diagram of DOL Starter

In this circuit,
NC contact is used at two places. 1st NC used for stop button & 2nd NC for overload protection. Both the NC’s are connected in series connection.
Whereas NO is used in start button & in hold circuit.
Actually, Contactor’s 3 main power contacts are also type of NO contacts.

Similarly, combination of NO & NC is being used in other starters.

IN CREATING LOGIC

In Electrical, Logic is a wired or digital program which is used to obtain output as per according to presence of Inputs.

To explain the use of NO & NC contacts in creating logic, logic gates are explained next. Outputs & Inputs, both have probability of two states which is either 1(High) or 0(low). The output state entirely depend upon the logic in combination with state of the inputs. These logics gates can also implemented in Electrical system with little assumptions, Like

At Input side 1 represents switch is Pressed & 0 represents unpressed.
At Output side 1 represents output is ON & 0 represents output is OFF.

Next, example of logic gates with 2 inputs & 1 output is explained !

Explaining AND Gate

In And gate if we look at its table as shown in an image below, Output is ON only when both the switches are pressed. Rest of the cases output always remains OFF.

This logic can be obtain if both the NO contacts are connected in series connections.

Explaining OR Gate

In OR Gate, Output is OFF only when none of the input switch is pressed. Rest of the cases, output remain ON whether 1 switch is pressed or both the switches are pressed.

OR gate can be obtain if both the NO contacts are connected in parallel connections.

Explanation of AND & OR Gate — AND & OR Logic Gate

Explaining NAND Gate

NAND gate is the inverted gate of AND.

In NAND gate output is off only when both the inputs are pressed, this can be achieve by connecting both the NC contacts in parallel connections.

Explaining NOR Gate

NOR gate is the inverted gate of OR Gate. In this gate, Output is ON only when none of the input is pressed. This is possible when both the NC blocks are connected in series.

Explanation of NAND & NOR Gate — NAND & NOR logic Gate

In above examples, It is clearly seen that entirely different output can be obtained by using two input contacts. Any type of electrical circuit is possible with these two contacts.

USE IN RELAYS

Relay is the device which electromagnetically or Electronically control the circuit when a signal is given to it.

For this, A 5 pins electromagnetic changeover relay is explained next. In which 1st & 2nd pins are coil’s pins, 3rd pin is common, 4th pin is NC & 5th pin is NO.

Schematic diagram of Electromagnetic relay — 5pins Electromagnetic changeover relay when coil is not energized.

As shown above, At normal state when coil is not energized; common pin is connected to NC with the help of spring. But when coil get energized, Electromagnet pushes the common pin towards the NO pin.

(Common pin– Common is the point which carries the required signal at required voltage & connects to either NO or NC at a time.)

In above example, We are controlling only 1 function with 1 signal.

Next; 14 pins relay is explained which controls 4 functions with only 1 signal. This 14 pins relay contains only single coil with 2 pins for coil. Rest of 12 pins constitute 4 relays, Each relay is composition of 3 pins. All the 4 relays work united simultaneously.(4×3=12)

Pins Description of 14 pins Electromagnetic relay

14 pins Electromagnetic relay when coil is Energized

With the help of 14 pins relay, Multiple functions can be controlled with 1 signal input which makes easy to create logic !

USE IN DEVICES AS A RELAY

Time, Temperature are the most important parameters in industry that’s why for production improvement devices like Timers & Temperature meters are being employed. These devices continuously monitors the input signal & controls output according to set parameter through the relay fitted in them.

Temperature meter TC19 Made by MULTISPAN — Temperature meter Range(0-400°C);Type-TC19 Made by MULTISPAN

Timer MDT 1310F By MULTISPAN — Timer with 2 Relays; Type- MDT 1310F Made By MULTISPAN

USE IN PROGRAMMABLE DEVICES

Programmable devices like PLC & Programmable HMI connected with multiple inputs & outputs.

In Some cases, signal voltage of input devices is not same as the input voltage required in programmable devices that’s why relay cards with NO contacts are being used.

Similarly at output side, PLC give command through relay cards.

To isolate the programmable devices from external circuitry, relays with NO & NC contacts are very important.

Outputs connected at PLC through relay’s NO contacts| SIEMENS LOGO !

Tips for Learners

✔ Always think of default state when identifying NO/NC in diagrams.
✔ NC contacts are often used in safety circuits, so opening them can stop machinery.
✔ In schematics, NEVER assume: check the labeled symbol or datasheet.

Frequently Asked Questions (FAQs) — NO & NC Contacts

Q1. What does NO and NC mean in electrical contacts?

Answer:
NO stands for Normally Open, meaning the contact remains open (no current flow) in its default state.
NC stands for Normally Closed, meaning the contact remains closed (current flows) in its default state.

Q2. What does “normally” mean in NO and NC contacts?

Answer:
“Normally” refers to the rest condition of the device — when no force is applied, no button is pressed, and no coil is energized.

Q3. Which contact is safer — NO or NC?
Answer:
NC contacts are generally safer and commonly used in safety circuits.
If a wire breaks or power fails, the NC circuit opens and stops the system automatically.

Q4. Why are stop push buttons always NC?
Answer:
Stop push buttons use NC contacts so that any wire break, loose terminal, or power failure will immediately stop the machine.

Q5. Why are start push buttons normally NO?
Answer:
Start buttons are NO because the circuit should energize only when the button is intentionally pressed.

Q6. How can I identify NO and NC contacts physically?
Answer:
Most devices mark terminals as NO, NC, and COM.
Always verify using the device datasheet or by continuity testing with a multimeter.

Q7. How do NO and NC contacts work in a relay?
Answer:
When the relay coil is not energized, COM is connected to NC.
When the coil is energized, COM switches to NO.

Q8. Can NO and NC contacts be used to create logic circuits?
Answer:
Yes.
NO contacts in series create AND logic.
NO contacts in parallel create OR logic.
NC contacts are used for NAND and NOR logic.

Q9. Are NO and NC contacts used in PLC systems?
Answer:
Yes. PLC ladder logic symbols are directly based on NO and NC contacts.

Q10. Can one device have both NO and NC contacts?
Answer:
Yes. Many relays, contactors, and push buttons provide both NO and NC auxiliary contacts.

Q11. What happens if I use the wrong contact type?
Answer:
Using the wrong contact can cause machine malfunction, safety failure, or unexpected operation.

Q12. Where are NO and NC contacts commonly used?
Answer:
They are used in motor starters, control panels, automation systems, alarms, and safety interlocks.

Conclusion

NO & NC contacts are real building blocks in electrical system.
NO stands for normally opened While NC stands for normally closed.
Bell switch is the simplest example of NO contact which get connects when switch get pressed.
Refrigerator door’s light switch is the simplest example of NC contact which connects when switch remains unpressed.
From simplest logic to complex programming in programmable devices, NO-NC contacts play very crucial role.

Hope! You like the post if you have any doubt, suggestion or query please do comments.

Tagged Basics of Electricals

"NO & NC Contacts- The Building Blocks! "

What are NO & NC Contacts ?

How these duos are being used ?