Limit Switch Diagram
This is a free printable limit switch diagram: download the diagram as SVG or open it and print to paper or PDF.
Reference wiring diagram for mechanical limit switches showing normally open and normally closed contact configurations for position sensing applications.
A limit switch is an electromechanical device that detects physical position or motion by converting the mechanical action of a moving object (pressing, rotating, or contacting an actuator) into an electrical switching action. Limit switches are fundamental to industrial automation, machine tool control, conveyor systems, door interlock circuits, and elevator position sensing.
Contact types: A standard limit switch contains two fundamental contact configurations: - Normally Open (NO): contacts are open (no current flow) in the switch's resting, un-actuated state. When the actuator is depressed or rotated, the contacts close and allow current to flow. NO contacts are used to trigger an action when a position is reached. - Normally Closed (NC): contacts are closed (current flows) in the resting state. When actuated, they open and break the circuit. NC contacts are used for safety interlock and e-stop circuits because a wire break (open circuit) mimics the actuated state, providing fail-safe behaviour.
Many limit switches provide both NO and NC contacts in a single unit, plus a common (COM) terminal. The common connects to one side of the supply, and either NO or NC (or both in separate circuits) connect to the load or control circuit.
Actuator types include plunger/button, roller lever, adjustable roller lever, fork lever, and rotary actuator. The actuator type determines the mechanical mounting geometry and the required operating force and travel.
Wiring: In a basic NO circuit, supply feeds COM; the load connects between NO and the supply return. In an NC circuit, supply feeds COM; the load connects between NC and return — power flows until the limit is reached. For IEC-style wiring, terminals are typically labelled: COM (common), NO (normally open), NC (normally closed).
Limit switches must be rated for the load voltage, current, and duty type (resistive, inductive, or motor). Inductive loads require de-rating or the addition of a snubber circuit.
All installation must comply with applicable codes: NEC/NFPA 70, BS 7671, AS/NZS 3000, or IEC 60364. A qualified electrician must perform installation.
A limit switch is a mechanical position sensor that converts physical movement — the end-of-travel of a conveyor, press, or actuator — into an electrical signal used to stop, reverse, or interlock a machine. In both schematic and ladder diagram formats, limit switches appear as either normally open (NO) or normally closed (NC) contacts; NC limit switches are preferred for safety-critical stop functions because a broken wire or failed switch defaults to a safe (open/de-energised) state. On a ladder diagram, a limit switch contact symbol uses the standard NC diagonal line through the contact symbol, and the rung logic shows clearly how the switch interlocks with motor starter or solenoid valve coils. Build and annotate your ladder diagram limit switch rung free in the browser-based editor.
How to wire limit switch diagram
- Select the correct limit switch for the application Choose actuator type (roller lever, plunger, fork lever) based on the mechanical arrangement. Verify contact rating matches the control voltage and current. For inductive loads (contactors, relays), derate the contact current rating per the manufacturer's factor.
- Mount the switch in the correct position Mount so the actuator is struck squarely by the moving object. The object must contact within the pre-travel zone and must not exceed the over-travel limit. Misaligned mounting causes premature switch failure or unreliable actuation.
- Isolate and verify the control circuit is dead Isolate the control power supply. Verify zero voltage at the limit switch terminals with a voltmeter before connecting any wiring.
- Connect the COM terminal to the supply Connect the control supply conductor (positive rail, or switched live) to the COM terminal of the limit switch.
- Connect NO or NC as required For a position-triggered action: connect the NC terminal in series with the contactor coil (motor stop) or connect the NO terminal to trigger an action (conveyor advance). Run the output from the selected terminal to the load device.
- Add snubber or RC network for inductive loads if required Inductive loads (relay coils, contactor coils) generate back-EMF spikes on contact opening. Connect a snubber (RC network) or flyback diode (for DC circuits) across the inductive load to protect limit switch contacts from arcing.
- Test operation mechanically and electrically Manually actuate the switch and verify the contact state changes as expected using a continuity tester or voltmeter. Then test under powered conditions with the machine running slowly to confirm correct machine behaviour at the limit position.
Specifications
| Typical contact voltage rating | 250 V AC or 30 V DC (verify per specific switch) |
|---|---|
| Typical contact current rating (resistive) | 10 A AC or 5 A DC (verify per specific switch) |
| Contact configurations available | SPDT (1 NO + 1 NC) is most common; DPDT available in some models |
| Actuator types | Plunger, roller lever, adjustable roller lever, fork lever, rotary |
| Protection rating (typical industrial) | IP65 or IP67 (IEC 60529) |
| Applicable standard | IEC 60947-5-1 (low-voltage switchgear and controlgear) |
Safety warnings
- Limit switches used as safety interlock devices must be NC-wired so that a wire break, switch failure, or mounting displacement results in a safe (de-energised) state.
- Never use a limit switch as the primary means of stopping a machine in an emergency stop (e-stop) circuit without a proper e-stop relay or safety relay module that meets the required safety integrity level (SIL or PLd/PLe).
- Verify the switch contact rating exceeds the actual load current and voltage, including inductive de-rating if the load is a coil or motor.
- All industrial control wiring must comply with NEC/NFPA 70, BS 7671, AS/NZS 3000, IEC 60364, or applicable machinery safety standards (IEC 60204-1).
- Isolate and lock out all energy sources before mounting, adjusting, or replacing limit switches on machinery.
Tools needed
- Digital multimeter with continuity function
- Insulated screwdrivers (flat and Phillips)
- Wire strippers and ferrule crimper
- Lockout/tagout kit
- Appropriate mounting hardware and drill bits
- Torque screwdriver (for terminal tightening to specification)
Common mistakes
- Using NO contacts for safety interlock circuits where NC is required for fail-safe behaviour — a wire break on a NO safety circuit leaves the machine unprotected.
- Exceeding the switch's over-travel limit by not adjusting mounting position correctly, causing mechanical damage to the switch.
- Wiring the limit switch directly into motor power conductors without a contactor, exceeding contact ratings.
- Not adding a snubber or flyback diode on inductive loads, causing premature contact erosion from arcing.
- Mounting the switch without verifying the actuator geometry — diagonal strike angles cause the actuator to deflect sideways and miss the switch actuation point.
Troubleshooting
- Machine does not stop when limit switch is actuated
- Cause: NO contact used instead of NC in the coil circuit; switch not being mechanically actuated due to misalignment Fix: With power off, manually actuate the switch and use a continuity tester to verify NC contacts open. Verify mounting alignment so the moving object strikes the actuator correctly within the pre-travel zone.
- Limit switch actuated continuously (machine stopped from start)
- Cause: Switch mechanically stuck in actuated state; NC contacts always open; or switch wired to NC when NO was intended Fix: Manually inspect the actuator for mechanical binding. Test NC continuity with the actuator manually released. Verify wiring connects to the intended terminal (NO or NC) per the circuit design.
- Switch contacts are burning or pitting rapidly
- Cause: Inductive load arcing without snubber; contact current exceeding rated capacity Fix: Add an appropriate snubber or flyback diode across the inductive load. Verify actual load current does not exceed the switch's contact rating (derated for inductive loads). Consider using the limit switch to drive a relay or contactor instead.
Frequently asked questions
What is the difference between a normally open (NO) and normally closed (NC) limit switch contact?
A NO contact is open at rest and closes when actuated. An NC contact is closed at rest and opens when actuated. NC contacts are preferred for safety interlock and emergency stop circuits because a wire break gives the same result as actuation, providing inherent fail-safe behaviour.
What does COM mean on a limit switch terminal?
COM (common) is the shared terminal connected internally to both the NO and NC contact mechanisms. The supply or control voltage is connected to COM, and the NO or NC terminals connect to the load or control circuit.
How do I wire a limit switch to stop a motor when it reaches a position?
Wire the NC contact in series with the motor contactor coil circuit. When the limit switch is actuated (position reached), the NC contact opens, breaking the coil circuit, which drops out the contactor and stops the motor. This is a fundamental machine control safety circuit.
Can I use a limit switch directly to switch a motor?
Only if the motor current and voltage are within the switch's rated contact capacity. Most limit switches are rated for small control currents. For larger motors, the limit switch should switch the coil of a contactor or relay, not the motor power directly.
What is a pre-travel and over-travel in a limit switch specification?
Pre-travel is the actuator movement required to change the switch state from its rest position. Over-travel is additional movement permitted beyond the switch point without damaging the mechanism. Exceeding over-travel limits damages the switch. Both are specified in degrees or millimetres.
How is a limit switch represented in a ladder diagram?
In a ladder diagram, a limit switch is drawn as a contact symbol on a rung — a normally open (NO) limit switch appears as two vertical lines (like a standard NO contact) and a normally closed (NC) limit switch has a diagonal line through the contact. The limit switch contact is placed in series or parallel with other contacts to control an output coil such as a motor starter (M1) or solenoid. NC limit switches are standard for travel-limit stop circuits because if the switch or wiring fails the coil de-energises, stopping the machine safely — this is the fail-safe principle.
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