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Micro Switch NO NC COM: Contact Labels Explained

Understand micro switch no nc com terminal labels and contact states so you can interpret a switch diagram and identify what must be verified before use.

By Micro Switch Guide Editorial Team Published Updated 6 min read

Understanding micro switch no nc com labels

The phrase micro switch no nc com refers to three contact labels that describe a changeover contact arrangement: common (COM), normally open (NO), and normally closed (NC). These labels identify electrical relationships between contacts relative to a stated normal condition. They do not, by themselves, show where terminals sit on a switch body or establish how a particular device should be connected.

The key is to interpret the labels together with the exact switch’s markings or connection diagram. In the generic changeover convention, COM is the shared contact, and NO and NC describe the contact paths from COM in the normal, unactuated state. When the actuator operates, those paths typically change state in the changeover arrangement. The exact device schematic controls, so do not assume every switch has the same terminal set or behavior.

Micro switch no nc com: what the labels mean

COM means common terminal. In a generic changeover contact, it is the shared contact associated with the NO and NC paths. NO means normally open terminal, and NC means normally closed terminal. “Open” and “closed” describe whether there is electrical continuity between the relevant contacts under a stated condition. They do not describe whether a terminal is physically exposed, nor do they identify a terminal’s position on the switch housing.

The word “normal” needs a clear reference. For this explanation, the normal condition is the switch’s specified unactuated condition, before the actuator is pressed or otherwise operated. With that convention, the generic changeover relationship is:

  • In the unactuated condition, COM and NC are closed to each other, while COM and NO are open to each other.
  • In the actuated condition, the contact relationship typically changes: COM and NC open, while COM and NO close.

These statements describe the contact relationship, not the switch’s physical location, lever orientation, or mounting position. A panel may place an actuator in a position that looks engaged, but that appearance does not redefine the documented normal condition. Read the device’s schematic and identify which condition it defines as normal before interpreting NO or NC.

The terms also should not be read as a guarantee that a particular switch contains all three terminals. They describe a generic changeover arrangement. A specific device may have a different terminal arrangement, and its markings or schematic determine which contacts are present and how they relate. The labels are useful only when matched to the exact device documentation.

What happens to NO and NC when the actuator moves?

Start with the documented unactuated state. In the generic changeover arrangement, COM has continuity to NC and no continuity to NO in that state. When the actuator is operated, the contact relationship changes so COM is no longer closed to NC and is instead closed to NO. This is the basic state relationship behind the names “normally closed” and “normally open.”

The actuator’s movement is a physical action; the open or closed state is an electrical relationship between contacts. Keeping those ideas separate avoids a common misunderstanding. “Normally closed” does not mean the actuator is normally in a particular physical position on equipment. It means the contact is closed relative to the specified normal condition. Likewise, “normally open” does not mean the terminal is always open, or that it will be open whenever the switch is mounted in a certain orientation.

The generic relationship is not a substitute for the exact switch schematic. The supplied device documentation determines the normal-state reference and the contact arrangement. Some switches may not use a three-terminal changeover arrangement, and the available evidence here does not establish the internal behavior of any particular device. Treat the state description as a general explanation of changeover terminology, not as a model-specific promise.

It is also useful to distinguish a contact state from a circuit state. Saying that COM and NO are open in the unactuated condition describes the relationship between those switch contacts. It does not tell you what the rest of an equipment circuit is doing. Other components and circuit paths are outside what the three labels alone describe. For the same reason, the labels should not be used to infer a machine’s overall on, off, safe, or operating state.

How to identify COM, NO, and NC from documentation

Use the connection diagram, terminal marking, and documentation for the exact switch and equipment. First, locate the printed labels or schematic references, if present. Then read the drawing’s contact symbol in context, including any indication of the normal condition. Match that information to the terminal identifiers provided for the device. A diagram is more reliable than guessing from the order, spacing, or side of the terminals.

Do not identify COM, NO, or NC by physical position alone. The labels describe contact function, while terminal placement is a construction detail that can differ among devices. A drawing’s visual layout can also be schematic rather than a picture of the switch body. The diagram should be interpreted according to its symbols and terminal references, not treated as a universal map of where each terminal must be located.

If the switch has no clear markings, or if its documentation does not show the contact arrangement, the exact terminal map remains unresolved. Consult the documentation for that particular switch and the equipment in which it is used. A generic COM/NO/NC explanation cannot identify unmarked terminals or establish that an assumed layout applies to a specific device.

What these labels do not tell you

COM, NO, and NC identify a contact arrangement and its relationship to a stated normal condition. The labels alone do not establish a voltage or current rating, whether the switch is suitable for a particular load, or whether it is compatible as a replacement. Those details must come from the exact switch and equipment documentation, including the documented permissible load where relevant. A familiar terminal label is not evidence that two switches have the same electrical limits or intended use.

This explanation is limited to terminology and the generic changeover state relationship. It does not provide application-specific wiring instructions or a procedure for testing a switch. For an actual installation or device-specific interpretation, rely on the applicable connection diagram, terminal markings, circuit function, and permissible-load information documented for that exact switch and equipment. Where the arrangement or conditions are unclear, the general label definitions are not enough to resolve them.

In short, COM is the common contact, while NO and NC name contacts by their open or closed relationship in the specified normal condition. In the generic changeover example, COM connects to NC when unactuated and changes to NO when actuated. Use the exact device documentation rather than terminal position or a generic diagram to determine what a particular switch’s markings mean. That is the practical meaning of micro switch no nc com.

Sources and references

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