What Is a Limit Switch? Types, NO/NC Contacts and Wiring Diagram
How a limit switch works; plunger, roller, lever and spring-rod actuators, snap vs slow action contacts, NO/NC wiring, positive opening and safety limit switches.
A limit switch is a position switch that changes its contact state when a moving machine part touches its actuator. It detects end of travel, stops a motor or reports position to the control system. Contact properties are defined in IEC 60947-5-1; actuator type, contact arrangement and protection rating decide the choice.
> Related Category: Browse models by actuator and housing in Limit Switches, Safety Limit Switches and Micro and Miniature Switches.
1. Types by actuator
| Actuator | Operating direction | Use |
|---|---|---|
| Plunger | Straight push along the axis | Precise position, cover and door detection |
| Roller plunger | Axial, via cam or ramp | Sliding parts |
| Roller lever | From the side, 30–45° rotation | Conveyors, cranes, doors; the most common type |
| Adjustable roller lever | Lever length and angle adjustable | Parts passing at different heights |
| Adjustable rod | Side contact on a long rod | Wide-tolerance detection |
| Spring rod (wobble stick) | Contact from any direction | Irregular parts, uncertain direction |
| Fork lever | Maintained in two positions | Reversing in back-and-forth motion |
On roller types the ramp angle should not exceed 30° and the part must not push the actuator past its permitted overtravel, or the head breaks.
2. Contacts: NO, NC and action type
A standard limit switch has one normally open (NO, 13-14) and one normally closed (NC, 21-22) contact.
| Contact action | Behaviour | Use |
|---|---|---|
| Snap action | At a set point NC opens and NO closes together, independent of actuator speed | Slow-moving parts, general control |
| Slow action | Contacts follow the actuator; NC opens first, NO closes later | Safety circuits, sequenced switching |
Positive opening: the NC contact is forced open directly by the actuator, with no spring in between, so it separates even if welded. Any limit switch used for a safety function must have it, shown by an arrow-in-circle symbol on the housing.
3. Limit switch wiring diagram
End-of-travel stop with a contactor (NC contact):
Control line → stop button → limit switch NC (21-22) → start button with holding contact → contactor coil A1; A2 → neutral
When the part reaches the end and presses the actuator, the NC contact opens, the coil drops out and the motor stops. In reversing systems each direction has its own switch: the forward limit in series with the forward contactor coil, the reverse limit with the reverse coil, so travel away from the limit stays possible. For the holding logic see Contactor Latching Circuit.
Position signal to a PLC (NO contact):
+24 V → limit switch NO (13-14) → PLC digital input
- Use the NC contact for stopping and safety: a broken wire opens the circuit and the machine stops safely.
- The NO contact is for information and signalling only.
- Mount with the cable entry facing down and seal it with a suitable cable gland.
4. Limit switch families compared
| Type | Property | Use |
|---|---|---|
| Standard limit switch | Metal or plastic housing, IP65–IP67, exchangeable head | Machines, conveyors, doors |
| Safety limit switch | Positive-opening NC, separate tongue or hinge design | Guard and door interlocking |
| Micro switch | Small body, low operating force, fast response | Inside devices, lid detection, appliances |
| Rotary limit switch | Cam contacts set by shaft revolutions | Hoist limits, shutters, blind motors |
5. Limit switch or sensor?
| Property | Limit switch | Inductive sensor |
|---|---|---|
| Detection | Mechanical contact | Contactless, metal only |
| Output | Dry contact; AC and DC, higher current | Transistor (NPN / PNP), low current |
| Supply | None needed | 10–30 V DC |
| Life | Mechanical wear (millions of operations) | No wear |
| Switching rate | Low | High |
| Strength | Simple, drives a contactor coil directly, safety circuits | Dirty areas, fast and frequent detection |
Sensor wiring is covered in Inductive, Capacitive and Photoelectric Sensor NPN vs PNP Wiring.
6. Selection criteria
- Contact rating: typically 3 A / 240 V in AC-15; it must exceed the coil current.
- Protection: IP65 inside machines, IP67 for wash-down and outdoors.
- Housing: metal where impact is likely, plastic in corrosive areas.
- Dimension standard: EN 50041 and EN 50047 housings interchange between brands.
- Verify mechanical life and ambient temperature in the maker's data.
Frequently asked questions
What does a limit switch do?
It detects by mechanical contact that a moving part has reached a set position, then stops the motor, reverses it or signals the control system.
Is a limit switch wired NO or NC?
Use the NC contact for stopping and safety, so a broken wire also stops the machine. If only position information is needed, wire the NO contact to a PLC input.
What is the difference between a limit switch and a micro switch?
A limit switch is a rugged IP65–IP67 position switch used on machines. A micro switch is a small, low-force contact element mounted inside equipment.
What sets a safety limit switch apart from a standard one?
A positive-opening NC contact and a design that resists defeat. Even with welded contacts the actuator movement opens the circuit; guards require this type.
Why do limit switches fail?
Most often from being pushed past overtravel, too steep a ramp, water or oil entering at the cable entry, and switching current above the contact rating.