JohdotaA limit switch is a mechanical switch that a moving machine part presses at the end position. It stops the movement before the machine runs to the end and is damaged.
Wiring
- The NC contact is wired in series with the control of the contactor for the direction that the limit restricts. When the movement reaches the limit, the contact opens and the contactor drops out. The machine can still be run in the opposite direction.
- With the NO contact the limit can start the next movement, for example in an automatic back-and-forth movement.
Direct opening contact
In safety-related applications, limit switches are used whose NC contact has direct opening action, often called positive opening. The actuator separates the contacts through a rigid mechanism without a spring, so even a contact that has welded shut is forced open. Such a switch is marked with an arrow inside a circle.
Stopping is always done with an NC contact, because then a broken wire also stops the movement.
In your region
Showing notes for
United States, Canada
On ladder diagrams a limit switch is marked LS, for example LS1. Its symbol has a small wedge under the moving blade.
Source: Siemens: Basics of Control Components, part 1 (STEP 2000 course) · Siemens: Basics of Control Components, part 4 (STEP 2000 course) · Square D (Schneider Electric): Wiring Diagram Book 0140CT9201, symbols and examples
UK & Ireland, Europe, Australia & New Zealand, Other IEC countries
No regional notes on this page yet. If something is different in your country, please send feedback.
Frequently asked questions
What is a limit switch?
A mechanical switch that a moving machine part presses at the end position. It stops the movement before the machine runs too far and is damaged.
How do you wire a limit switch to a motor?
Wire the NC contact in series with the contactor control for the direction the limit restricts. At the limit the contact opens and the contactor drops out, but the opposite direction still works. The NO contact can start the next movement.
Is a limit switch normally open or normally closed?
Stopping always uses the NC contact, because it also stops the movement if a wire breaks. In safety applications it also has direct opening action, so even a welded contact is forced open.
Sources
- Positive opening (direct opening action) contacts in limit switches (IEC 60947-5-1, Annex K)
- Siemens: Basics of Control Components, part 1 (STEP 2000 course)
- Siemens: Basics of Control Components, part 4 (STEP 2000 course)
- Square D (Schneider Electric): Wiring Diagram Book 0140CT9201, symbols and examples