Info Byte: What Do NC and NO Mean?

Occasionally it’s important to take a step back to the basics and understand why certain terminology exists. Why is this important? If we have no idea why a device bears a certain term, the use and function of that device may also be lost. After that, we are at the mercy of application sheets and external consultants to solve problems, since we really don’t understand how or why to apply the proper solution.

Today, I present to you a pair of the most fundamental device labels in all of control automation: the ‘normally open’ and ‘normally closed’ contacts.

 

What Do Open and Closed Mean in a Circuit?

Before explaining contacts, we need to discuss two terms that are even more basic than switches.

Many people equate electronics and water, trying to draw parallels between the two systems, since the flow of water is very tangible and much easier for a novice to grasp than the mysterious interactions of sub-atomic electrons.

 

Figure 1. Symbols comparing NO vs NC contacts with NO on the left and NC on the right. These are also often called 'N/O vs N/C' or 'N.O. vs. N.C.'. Image used courtesy of the author

 

Most of the time, it works pretty well. There is one case where it’s not so clear. The words ‘open’ and ‘closed’ are used for water valves all the time. But be careful when you move into the electrical world…

 

Open and Closed in Water Systems

In a water system, a closed valve will not allow any water to flow. The system is off, and the pipe runs dry.

In contrast, opening the valve allows the water to flow freely, and the faucets will all begin to run once again.

 

Open and Closed in Electrical Systems

The definitions of these terms in electrical systems are exactly reversed. In an electrical system, an ‘open’ circuit is one that has been broken and no electricity will flow through any component that is a part of the circuit branch. Similarly, a ‘closed’ circuit is one in which the current can freely flow through any portion of the closed branch.

The concept isn’t complex, but it is quite confusing for anyone who does not have a background in electricity.

 

Figure 2. The open and closed contacts of a double-throw relay can be seen through the small viewing window. Image used courtesy of the author

 

Normally Open (N.O.) Contacts

The earliest forms of electrical switching relied on metal rods or tabs that moved across a set of wires, either completing or opening the circuit. A human operator would provide the input force that moved the metal tab, usually through a pushbutton or toggle switch (even those old knife switches). As control technology evolved, people didn’t press the button, the products themselves provided the force, so we developed limit switches, and later, discrete proximity sensors.

Including a spring return inside a switch mechanism brought about this concept called ‘normal’, which means that we can choose: when energy is not applied to the switch, will it expect to return back to on, or to off?

The decision for where a switch state remains could be the result of a safety decision, where electricity will only flow as long as the operator or the product is physically engaged, as soon as it’s removed, the circuit again de-energizes. This is indeed why normally open contacts are so common.

 

What Does Normally Open Actually Mean?

All this, just to arrive at the title question. ‘Normally open’ (also called N/O, N.O., or simply NO) means that the switch will be in the open position (no electricity flows) except when the input signal is provided.

As soon as the input energy is removed, the circuit will return back to the open position, and electricity will cease to flow.

 

Normally Closed (N.C.) Contacts

After the previous discussion, this new concept is a bit more straightforward. When NO contacts only allow electricity to flow when energized, the opposite ‘normally closed’ (again also called N/C, N.C., or just NC) means that electricity will be flowing all the time, except when the input energy is provided.

 

Figure 3. A normally closed (N/C) switch contact is seen not pressed (closed) on the left, and pressed (open) on the right. Image used courtesy of the author

 

Why Would You Use NC Contacts?

As stated previously, an advantage of the normally open switch is that the circuit is only energized with live voltage when the button is pressed. So why then would you ever wish to use a switch that provides potentially dangerous voltage when operators are not present? Two reasons come to mind: safety and energy savings.

 

Safety Switches (E-Stops)

When a dangerous situation arises and you need to press an emergency button, it is better to remove voltage from a device, rather than to energize a whole new circuit. For this reason, all emergency stop buttons are N.C., so electricity will flow and run the machine until the button is pressed.

 

Energy Savings

Limit switches, in particular, are electro-mechanical devices that interact with the product on a line (float switches may be considered the same way). If they drive a high-power load, like a large relay/contactor coil when the product is present, we must weigh our options. If the product or liquid is present more than half the time, it would be reasonable to use an N.C. contact, only energizing the relay coil when product is NOT present. That way, the relay coil is off for the majority of its service life, conserving energy.

 

Do All Switches have NO/NC Contacts?

This is a very good question to ask, since nearly every schematic has switches of some kind, and they must be replaced with the right device after any failure.

No, there are a great many switches that have no spring, and they do not return to either open or closed state by default. Each morning, when you wake up, you turn on the lights in your home. These switches (2-way, 3-way, etc) will remain in place when you move your hand away. They are neither NO nor NC, and therefore, you will never ask this question when buying light switches for home electrical projects.

If the switch returns to a default position when released, then it absolutely will be rated as either normally open or normally closed. Some contact blocks even have both kinds included in a single contact; double-throw relays and some double-pole switches work in this capacity.

 

Figure 4. This button is equipped with a double-pole contact, with de-energized on the right and energized on the left. We can see that the top contact is NC, and the bottom contact is NO. Image used courtesy of the author

 

Which is the Best Switch for Me?

This isn’t a very good question in this context. There is no absolute superior switch type that should be used over the other. Instead, consult the schematics or the design requirements of your project and decide if electricity should power the load when energized, or if it should power the load only when not energized.

It’s not always an easy answer, but without understanding the different designs and purposes of each switch style, you run a much greater risk of miswiring, which can lead to equipment failure or hazards, all avoidable with a bit of understanding.

 

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