Limit Switch NC Symbol
Definition: The Limit Switch NC symbol represents a mechanically-actuated, normally-closed electrical switch that carries current in its rest (unactuated) state and opens the circuit when the actuator is physically depressed by a moving machine part, drawn in schematics per IEC 60617-07 as a switch contact symbol (two parallel lines with a diagonal break line) with a mechanical actuator indicator, designated by the reference designator S or SQ followed by a number.
Also known as: normally closed limit switch, NC limit switch, position switch NC, travel switch NC, end-of-travel switch NC, mechanical position sensor NC.
What the Limit Switch NC symbol means
The Limit Switch NC symbol denotes a switch whose contacts are held in the closed (conducting) position by its internal spring mechanism when no external mechanical force is applied. When a machine element (cam, lever, door, conveyor, or actuator) contacts the switch's plunger or roller, the contacts open, interrupting the circuit. This normally-closed behaviour is the inverse of an NO (normally-open) limit switch and is chosen for fail-safe circuit designs where circuit continuity signals a safe condition and circuit interruption signals either a fault or a position trigger.
In industrial automation and machine control schematics, limit switch NC symbols frequently appear in safety interlocks, emergency stops, guard door interlocks, and end-of-travel stop circuits. The NC contact configuration means that if the switch wiring is cut or the switch fails open, the circuit behaves as if the switch were actuated, triggering the controlled device to stop or alarm — a fundamentally safer default than a normally-open circuit that silently fails closed.
How to identify the Limit Switch NC symbol
The Limit Switch NC symbol consists of two terminal lines representing the In and Out connections with a contact symbol between them. The NC contact is drawn as two horizontal lines (or stubs) connected by a diagonal line or angled bar indicating a closed path, overlaid with a small perpendicular arrow or actuator symbol showing mechanical operation. In IEC 60617-07 the NC mechanical contact is drawn as two approaching contact symbols with a diagonal slash and an upward-pointing mechanical actuator arrow. In ANSI/IEEE 315 the NC contact is represented as two horizontal lines connected by a diagonal slash, indicating the contact is normally in the closed position.
Function in a circuit
A normally-closed limit switch functions as a binary position sensor and circuit interruptor. In the unactuated (rest) position, current flows freely from the In terminal to the Out terminal through the closed contacts. When a machine element depresses the switch's mechanical actuator (plunger, roller lever, or rotary arm), the contacts separate, opening the circuit and interrupting current flow. This contact opening is used to stop a motor, signal a PLC input, de-energise a solenoid, or trigger a safety relay. The switch returns to its closed (conducting) state automatically by internal spring return when the actuating force is removed.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-07 defines the schematic symbols for mechanically-actuated switches and contacts. The normally-closed mechanically-actuated contact (limit switch NC) is shown as an NC contact symbol with a perpendicular actuation bar and arrow. IEC 60947-5-1 governs the electrical requirements and ratings for position switches (limit switches) used in industrial control circuits. |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2-1975 / IEEE 315-1975 defines the limit switch NC symbol as a normally-closed contact (two lines connected by a diagonal slash) with a short perpendicular line indicating mechanical actuation, consistent with the general switch notation in IEEE 315. |
| Key difference | IEC 60617-07 uses a more detailed mechanical-actuator arrow notation above or beside the contact symbol to indicate the direction and type of actuation (plunger, lever, roller). ANSI/IEEE 315 uses a simpler perpendicular line or abbreviated actuator indicator. The basic NC contact representation (diagonal slash indicating closed path) is functionally the same in both standards. |
Terminals / pins
| Pin | Name |
|---|---|
| in | In |
| out | Out |
Typical values
Contact voltage rating: 24 V DC to 600 V AC (typical industrial ratings); Contact current rating: 1–10 A (depending on series); Contact resistance: < 100 mΩ (typical silver alloy contacts); Mechanical life: 1–30 million operations; Electrical life: 100,000–1,000,000 operations at rated load; Operating force: 2–30 N (plunger types); Operating frequency: up to 10 Hz (mechanical, continuous duty); IP protection: IP65–IP67 (panel-mount industrial types).
Where the Limit Switch NC symbol is used
- End-of-travel stops on CNC machines, lathes, and machining centres: the NC contact de-energises the axis drive motor when the table reaches its mechanical limit
- Safety guard door interlocks on machinery: the NC contact in a guard door limit switch ensures the machine stops if the guard is opened
- Conveyor belt positional control: NC limit switches detect jams or misalignment, stopping the belt drive when the contact opens
- Elevator and lift systems: NC limit switches at the top and bottom of the shaft act as over-travel stops on the main drive contactor circuit
- Industrial robot arm end-stops: NC contacts on each axis limit the range of motion and provide hard stops for homing sequences
- Pneumatic and hydraulic cylinder end-of-stroke detection: the NC contact signals PLC when the cylinder reaches its retracted or extended position
Example
In a conveyor-belt motor control circuit, a normally-closed limit switch (S3) is wired in series with the normally-closed emergency stop button and the coil of the main contactor K1. When the conveyor belt reaches the end of its travel and actuates the plunger on S3, the NC contact opens, dropping out contactor K1 and stopping the belt motor. The schematic shows the limit switch NC symbol (labelled S3) in the control circuit between the E-stop button and contactor coil, with the In pin connected to the control supply rail and the Out pin connected to the contactor coil.
Key facts
- A normally-closed (NC) limit switch carries current in its rest state and opens the circuit when its mechanical actuator is depressed; the contacts return to closed when the actuating force is removed.
- The NC contact configuration provides inherent fail-safe behaviour: a broken wire, loose connection, or switch failure that causes an open circuit mimics actuation, stopping the controlled machine rather than allowing unexpected motion.
- The Limit Switch NC symbol in IEC 60617-07 schematics is an NC contact with a perpendicular mechanical actuator arrow; in ANSI/IEEE 315 schematics it is an NC contact (diagonal slash line) with a short perpendicular actuation indicator.
- IEC 60947-5-1 governs the ratings and performance requirements for limit switches (position switches) in industrial low-voltage control circuit applications.
- A limit switch NC has two pins in schematics: In and Out; it is wired in series in the circuit it controls, so opening the contact breaks the series circuit.
- Contact voltage ratings range from 24 V DC (PLC input circuits) to 600 V AC (motor contactor coil circuits), and current ratings from 1 A to 10 A depending on the switch series.
- The reference designator for a limit switch is S or SQ followed by a number (e.g. S1, SQ2) in both IEC and IEEE/ANSI schematics; the IEC standard also uses BQ for position switches in some regional conventions.
Frequently asked questions
What does the Limit Switch NC symbol look like in a schematic?
The Limit Switch NC symbol is drawn as two terminal stubs with an NC contact between them — two horizontal lines connected by a diagonal slash indicating the normally-closed (conducting) state — and a short perpendicular line or arrow above the contact indicating mechanical actuation. In IEC 60617-07 schematics a more detailed mechanical actuator arrow is shown; in ANSI/IEEE 315 schematics the indication is a simple perpendicular bar.
What does NC mean on a limit switch?
NC stands for Normally Closed: the switch contacts are in the closed (conducting) position when the switch is at rest and no mechanical force is applied to the actuator. The circuit through the switch is complete (current flows) in the normal, unactuated state. When a machine part depresses the switch actuator, the NC contacts open and interrupt the circuit.
What is the difference between a Limit Switch NC and Limit Switch NO?
A Limit Switch NC (Normally Closed) passes current at rest and opens when actuated; a Limit Switch NO (Normally Open) breaks the circuit at rest and closes when actuated. NC switches are preferred for safety-critical stop circuits because a wire break or switch failure replicates the actuated (stopped) state. NO switches are preferred for start or enable circuits where actuation should energise a load.
Why are NC limit switches used in safety circuits?
NC limit switches are used in safety interlocks and emergency-stop circuits because they provide fail-safe behaviour by default. If the switch wire is cut, the switch fails mechanically, or a connector loosens, the circuit opens — identical to the switch being actuated — which causes the controlled machine to stop. An NO switch in the same position would allow the machine to continue running if the circuit failed open, creating a dangerous undetected fault.
What standard governs limit switch symbols?
IEC 60617-07 defines the schematic symbols for mechanically-actuated switches and contacts, including the limit switch NC symbol, in international schematics. ANSI Y32.2-1975 / IEEE 315-1975 defines the equivalent NC mechanical switch symbol for North American schematics. IEC 60947-5-1 governs the electrical performance and safety ratings of limit switches (position switches) used in industrial control circuits.
What is the reference designator for a limit switch?
The reference designator for a limit switch is S (switch) followed by a sequential number, e.g. S1, S2, S3, in general schematic practice. In IEC 81346-2 (reference designations for industrial systems) the designator SQ is used specifically for position sensors and limit switches, e.g. SQ1, SQ2.
Can a limit switch NC be used for a PLC input?
Yes. A normally-closed limit switch is commonly wired to a PLC digital input. In the unactuated state the input reads HIGH (24 V DC logic 1), confirming the machine element is away from the switch. When the switch is actuated (contacts open), the input reads LOW (0 V, logic 0), signalling the PLC that the limit position has been reached. This wiring allows the PLC to detect both the position event and any wire-break fault as a change from HIGH to LOW.
Place the Limit Switch NC symbol on a wiring diagram or schematic in the free online circuit diagram maker — no download required.