On an NO NC switch, normally means unactuated, not unpowered. NO is normally open and NC is normally closed, and both describe the contact position when nothing is pressing on the actuator, regardless of whether the circuit has power. COM is the common terminal that both throws share, and it only appears on a changeover part. Swiclick is a switch manufacturer whose micro switch and slide switch datasheets use these labels throughout.
Three Labels, Three Jobs
COM is the terminal your circuit enters through. On a three-terminal changeover part it is connected to one throw or the other in each stable position.
NC is the throw that COM rests on when nothing is actuating the switch. Press the actuator and that connection breaks.
NO is the throw that COM moves to when the switch is actuated. It is open at rest.
The labels describe positions, not intentions. A part is not safer because you used NC, and it is not more efficient because you used NO. Which one you wire is a design decision about what the absence of actuation should mean in your circuit. Full detail on the three-terminal arrangement sits on the SPDT switch page.
NO and NC Without a COM
Two-terminal parts also carry NO and NC labels and they mean something slightly different.
An SPST switch has no common because there is nothing to be common to. A two-terminal part is built either as normally open or as normally closed, and the two are different parts even when they share a body and a footprint.
This is where the labels become a purchasing problem rather than a wiring one. An SPST-NO and an SPST-NC part can be visually identical and differ by one character in the part number. A board built with the wrong one passes continuity test if the tester only checks the actuated state.
Write the full suffix into the BOM, and add a test step that checks the resting state.
Where the Labels Show Up
The same three labels behave differently depending on the mechanism.
Micro switches. Changeover micro switches are a common home for these labels, because their snap-action mechanism transfers a movable contact between two fixed contacts, so COM, NO and NC all physically exist on the part. SPST micro switch variants also exist and do not necessarily expose all three terminals. The micro limit switch format uses them for position detection.
Tact switches. Most are two-terminal and normally open. Normally closed tact switches exist and are used for lid and cartridge detection, but they are the exception rather than the default.
Slide switches. Three-terminal bodies have a common, but datasheets more often number the terminals than label them NO and NC, because there is no obvious unactuated state on a maintained switch that rests wherever it was left.
That last point is worth holding onto. Normally open and normally closed only carry a clear meaning on a switch that returns to a defined resting position by itself.
Reading the Contact Diagram
The abbreviation is a summary. The contact diagram is the specification, and three things live in it that the labels do not carry.
Terminal numbering. Which physical pin is COM. This varies between manufacturers and between series in the same catalogue.
Overlap behaviour. Whether COM touches both throws briefly during transition. A shorting or make-before-break part does. It matters when the two throws are separate sources.
Operating and release points. Where in the actuator travel the transfer happens, and how far back it has to come before it returns. On a snap action switch these are the numbers that decide whether the part works in a position application.
Requirements and rating rules for mechanical switches of this class sit in IEC 61058-1-1, which is where the marking and classification conventions come from.
When NC Is Not a Safety Measure
A normally closed contact is often described as fail-safe, and for personnel safety that description is not reliable.
The reasoning goes like this: wire the guard through an NC contact, so if the guard opens the circuit breaks and the machine stops. That works when the switch works. It does not cover a welded contact, a broken actuator, a failed spring or a switch that has been physically bypassed.
Devices intended for safety-related guard interlocking use certified positive-opening operation, where the contact is forced apart by a direct mechanical link rather than relying on a spring to return it. Requirements for control circuit devices used in controlling, signalling and interlocking sit in IEC 60947-5-1, and the complete safety function including circuit architecture and fault monitoring has to match the assessed risk level.
Swiclick does not currently offer certified positive-opening safety position switches. Using a standard NC snap-action part for non-safety interlocking is normal. Using one where a person’s safety depends on the contact opening is not, and no wiring arrangement fixes that.
FAQ
Normally open and normally closed. They describe the contact position when the actuator is at rest, whether or not the circuit is powered.
No. Normally refers to the unactuated mechanical state. Power has nothing to do with it.
It is the common terminal that both throws share on a changeover part. Your circuit enters through COM and is routed to NO or NC depending on the switch position.
Yes, as a build variant rather than as separate terminals. An SPST part is either normally open or normally closed, and the two are different part numbers.
Not by itself. It does not cover welded contacts, broken actuators or failed springs. Safety-related guard interlocking needs certified positive-opening operation and an appropriate circuit architecture.
Because a maintained switch has no self-defined resting state. Slide switch datasheets usually number the terminals and show the connection per position instead.
