Limit Switch NO Symbol
Definition: The Limit Switch NO symbol represents a mechanically-actuated, normally-open electrical switch whose contacts are open (non-conducting) at rest and close to complete the circuit when a machine element depresses the actuator, drawn in schematics per IEC 60617-07 as an open-contact symbol with a mechanical actuator indicator, and designated by the reference designator S or SQ followed by a number.
Also known as: normally open limit switch, NO limit switch, position switch NO, travel switch NO, end-of-travel switch NO, mechanical position sensor NO.
What the Limit Switch NO symbol means
The Limit Switch NO symbol denotes a switch whose internal spring holds the contacts in the open (non-conducting) position when no external force is applied. The circuit through the switch is broken until a machine element — such as a cam, conveyor carriage, robot arm, or door — physically contacts the switch actuator and pushes it to the actuation point, at which moment the contacts close, completing the circuit. This normally-open behaviour is the complement of the NC (normally-closed) limit switch and is selected for applications where actuation should energise, enable, or start a controlled circuit.
In industrial schematics the Limit Switch NO symbol appears in position-confirmation circuits, counter inputs, start interlocks, and signalling circuits. The NO contact provides a clean start signal when a machine element arrives at a target position, making it the preferred choice for presence-detection circuits — for example, confirming that a part is correctly seated before a press cycle begins — where the machine should only act when the limit switch is positively actuated.
How to identify the Limit Switch NO symbol
The Limit Switch NO symbol is drawn with two terminal stubs (In and Out) and an open-contact symbol between them: two parallel horizontal lines or stubs with a gap (no diagonal slash), indicating that the circuit is open at rest, and a short perpendicular line or arrow indicating mechanical actuation. In IEC 60617-07 the NO mechanical contact is shown as two approaching contact tips (open gap) with an upward-pointing mechanical actuator arrow above the gap. In ANSI/IEEE 315 the NO contact is represented as two horizontal lines with a clear gap between them and a short perpendicular bar indicating the mechanical actuator.
Function and usage boundary
A normally-open limit switch functions as a binary position sensor and circuit completor. In the rest position the circuit between the In and Out terminals is open; no current flows. When a machine element moves to the target position and depresses the switch plunger, roller lever, or rotary arm, the contacts bridge the gap and complete the circuit, allowing current to flow. The circuit opens again immediately when the actuating force is removed (spring return) or when the machine element moves away. The contact closure event is used to energise a relay, trigger a PLC input, pulse a counter, or actuate a solenoid at a specific machine position.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-07 defines schematic symbols for mechanically-actuated switches; the normally-open mechanical contact (limit switch NO) is drawn as an open-gap contact with a mechanical actuator arrow. IEC 60947-5-1 sets the electrical ratings and performance requirements for position switches used in low-voltage industrial control circuits. |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2-1975 / IEEE 315-1975 defines the Limit Switch NO symbol as a normally-open contact (two lines with a clear gap) with a short perpendicular mechanical actuator indicator, consistent with the general switch-symbol conventions in IEEE 315. |
| Key difference | IEC 60617-07 uses a directional mechanical-actuator arrow that indicates the direction and type of actuation; ANSI/IEEE 315 uses a simpler perpendicular bar. The fundamental NO contact representation (open gap between contact tips) is the same in both standards. |
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Logical terminals
| Logical ID | Editor terminal name |
|---|---|
| in | In |
| out | Out |
Where the Limit Switch NO symbol is used
- Part-presence confirmation in stamping and injection moulding machines: the NO switch closes only when the workpiece is correctly positioned before the press stroke begins
- Conveyor belt position detection: NO limit switches detect the arrival of a pallet or carrier at a loading station, triggering a stop or divert command to the PLC
- Elevator and lift overrun protection: NO limit switches at the top and bottom of the shaft close to energise an overspeed relay if the car travels beyond its travel limit
- Robotic arm homing sequences: NO switches confirm that each axis has reached its home position before the controller enables further motion
- Pneumatic cylinder extended-position sensing: the NO contact closes when the cylinder rod reaches its full-extension end stop, signalling the PLC to advance the machine cycle
- Door and gate position confirmation: NO limit switches signal that a machine enclosure door is fully closed and latched, enabling a start interlock relay
Key facts
- A normally-open (NO) limit switch has contacts that are open at rest and close only when its mechanical actuator is physically depressed by a machine element; the contacts return to open when the force is removed.
- The Limit Switch NO symbol in IEC 60617-07 schematics is an open-gap contact with a mechanical actuator arrow; in ANSI/IEEE 315 schematics it is an open-gap contact with a short perpendicular actuator bar.
- NO limit switches are preferred for presence-confirmation and start-enable circuits because the circuit only completes when the machine element is positively detected at the target position.
- IEC 60947-5-1 governs the electrical performance and safety ratings of position switches (limit switches) in industrial low-voltage control systems.
- A Limit Switch NO has two pins in schematics: In and Out; it is wired in series in the energising circuit of the load it controls.
- The reference designator for a limit switch is S or SQ followed by a number (e.g. S4, SQ3); IEC 81346-2 uses SQ specifically for position sensors.
- Unlike NC limit switches, NO limit switches do not provide fail-safe circuit behaviour: a wire break or switch failure mimics the unactuated (open) state, which may allow the machine to continue running if the NO switch was intended as a stop interlock — for safety-critical stop functions, NC contacts are preferred.
Diagrams that use this symbol
Frequently asked questions
What does the Limit Switch NO symbol look like in a schematic?
The Limit Switch NO symbol is drawn as two terminal stubs with a gap between the contact tips — indicating the contacts are normally open — and a perpendicular line or arrow above indicating mechanical actuation. In IEC 60617-07 a directional mechanical actuator arrow is shown above the open-gap contact; in ANSI/IEEE 315 a short perpendicular bar represents the actuator. There is no diagonal slash or connecting line between the contact tips because the switch is open at rest.
What does NO mean on a limit switch?
NO stands for Normally Open: the switch contacts are in the open (non-conducting) position when no mechanical force is applied. The circuit through the switch is broken in the normal, unactuated state. When a machine element depresses the switch actuator, the NO contacts close and complete the circuit, allowing current to flow.
What is the difference between Limit Switch NO and Limit Switch NC?
A Limit Switch NO (Normally Open) has open contacts at rest and closes when actuated; a Limit Switch NC (Normally Closed) has closed contacts at rest and opens when actuated. NO switches are preferred for start and enable circuits where actuation must complete the circuit. NC switches are preferred for safety-critical stop circuits because a wire break or switch failure opens the circuit, replicating the actuated (stop) state.
When should I use a NO limit switch instead of an NC limit switch?
Use a NO limit switch when you need a circuit to be completed (energised) only when a machine element reaches the target position, such as confirming a part is loaded, a door is closed, or a cylinder is extended. Use an NC limit switch when you need the circuit to remain complete during normal operation and be interrupted when the machine reaches a limit or a safety condition is triggered, providing fail-safe behaviour.
What standard defines the Limit Switch NO symbol?
IEC 60617-07 defines the mechanically-actuated normally-open contact symbol in international schematics. ANSI Y32.2-1975 / IEEE 315-1975 defines the equivalent symbol for North American schematics. IEC 60947-5-1 specifies the electrical performance, ratings, and testing requirements for limit switches (position switches) in industrial control applications.
What is the reference designator for a Limit Switch NO?
The reference designator for a limit switch is S followed by a sequential number (e.g. S1, S4) in general schematic conventions, or SQ (position sensor) followed by a number (e.g. SQ3, SQ4) per IEC 81346-2 reference designation standards for industrial control systems.
Can a Limit Switch NO and NC be in the same physical switch?
Yes. Most industrial limit switches have a single actuator mechanism that simultaneously operates both an NO contact block and an NC contact block, often in the same housing. When the actuator is depressed, the NO contacts close at the same moment the NC contacts open. Using both contacts in a circuit — with the NC in the safety interlock and the NO in the confirmation circuit — provides both fail-safe protection and positive position feedback from a single switch.
Related symbols
- Contactor NO Contact symbol
- Limit Switch NC symbol
- Overload Contact NO symbol
- Push Button NO symbol
- Relay NO Contact symbol
- Relay SPDT symbol
Physical breadboard, PCB, and panel mapping
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