Push Button NC Symbol
Definition: The Push Button NC symbol represents a momentary normally-closed (NC) push button — standardised in IEC 60617-07 and ANSI Y32.2/IEEE 315 — whose contact is closed (conducting) in the resting state and opens momentarily when the button is pressed, used as the Stop button in motor starter control circuits, with terminal designations In and Out and reference designator S per IEC 60947-5-1.
Also known as: push button normally closed, NC push button, Stop button, momentary NC switch, break push button, normally closed momentary switch.
What the Push Button NC symbol means
The Push Button NC (normally closed) symbol represents a momentary contact switch that is in the closed (conducting) state when not actuated and opens its contact when the button cap is pressed and held. The contact returns to its closed state automatically when the button is released, making it a momentary action device with NC (break) contact function.
In industrial motor starter control circuits, the NC push button is universally used as the Stop button: it is wired in series with the contactor coil control circuit so that pressing the Stop button momentarily breaks the control circuit, de-energising the contactor coil and stopping the motor. The designation S is used for push buttons per IEC standards, and Stop buttons are typically coloured Red per IEC 60204-1 and NEMA/ANSI Z535 safety colour standards.
How to identify the Push Button NC symbol
The Push Button NC symbol is drawn as two short horizontal parallel lines (representing the contact blades in the closed position) with a diagonal slash line across or through the contact gap — indicating the break action — and a downward-pointing arrow or actuator symbol above, representing the push button operating mechanism. In IEC 60617-07 notation, the NC push button shows a make-break contact symbol with a spring-return actuator. The ANSI symbol is similar: a break-contact with a manual push actuator indicator. The symbol reads as 'this contact is closed at rest and opens when pushed'.
Function in a circuit
The NC push button, when not pressed, forms a conducting path between its In and Out terminals, allowing current to flow in the control circuit. When the button is pressed, the contact mechanism forces the contact blades apart, opening the circuit and interrupting current flow for the duration the button is held. Upon release, the spring mechanism restores the contact to its closed position. In a DOL motor starter, pressing the Stop button (NC push button) opens the control circuit, de-energising the contactor coil, causing the main contacts to open and the motor to coast to stop.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-07 defines the normally-closed (break) contact symbol with a spring-return push actuator. IEC 60947-5-1 governs low-voltage control circuit devices including push buttons, specifying ratings, IP protection, and colour coding. IEC 60204-1 (Safety of machinery) requires Stop buttons to be coloured Red and mandates NC-type stop circuits for category 0 and 1 stops. |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2 / IEEE 315 represents the NC push button as a break-contact symbol with a push actuator line. NEMA ICS standards govern push button assembly ratings for North American industrial applications. ANSI/NEMA Z535 and NFPA 79 require Stop push buttons to be Red and use NC contacts in series with the motor control circuit. |
| Key difference | IEC 60617-07 and ANSI Y32.2/IEEE 315 NC push button symbols are very similar: both show a break-contact with a spring-return push actuator. IEC schematics typically show the actuator as a short arrow or 'T-bar'; ANSI schematics show a small push button actuator circle or arrow. The contact function and wiring are identical. |
Terminals / pins
| Pin | Name |
|---|---|
| in | In |
| out | Out |
Typical values
Contact configuration: 1 NC (break). Rated voltage: 230/400 V AC, 24–250 V DC. Rated current: 6–10 A (AC-15 category per IEC 60947-5-1). Actuation force: 1.5–5 N. Contact resistance: < 10 mΩ. Mechanical life: ≥ 1,000,000 operations. IP rating (front): IP65 standard for panel-door-mount push buttons. Colour: Red (Stop function per IEC 60204-1). Terminal designators: In and Out (no standardised terminal number for push buttons; identified by circuit position).
Where the Push Button NC symbol is used
- DOL motor starter Stop button: NC push button in series with contactor coil circuit, de-energises coil and stops motor when pressed
- Emergency stop (E-stop) mushroom head: large-head NC push button (latching) used as safety stop, though latching E-stops require specific safety relay circuits
- Conveyor belt Stop station: NC push button at each pull-cord station along the conveyor allows any operator to stop the belt
- Machine tool stop control: NC push button on machine control panel provides operator stop function
- Alarm reset: NC push button silences or resets an active alarm by momentarily opening the alarm latch circuit
- PLC input wiring: NC push button wired to PLC digital input to provide software-readable stop command
Example
In a standard IEC DOL motor starter control circuit wiring diagram, the Red Stop push button (NC, terminals In-Out) is drawn in series between the L1 phase supply and the Green Start push button (NO): pressing Stop opens the control circuit, de-energising the KM1 contactor coil (A1-A2) and disconnecting the 3-phase motor. The holding (self-seal) contact of the contactor is in parallel with the Start button, so releasing the Stop button does not restart the motor — the motor must be restarted by pressing the Start button again.
Key facts
- The Push Button NC is a momentary normally-closed (break) contact switch: it is closed (conducting) at rest and opens only while the button is pressed, returning to closed on release.
- IEC 60947-5-1 governs push button control devices; IEC 60204-1 requires Stop push buttons to be Red and use normally-closed contacts wired in series with the motor control circuit.
- In a DOL motor starter, the NC Stop push button is wired in series with the contactor coil circuit: pressing it opens the control circuit, de-energises the coil, and stops the motor.
- The reference designator for push buttons in IEC schematics is S (e.g. S1 for Stop, S2 for Start); in ANSI/IEEE schematics the designator is PB (push button, e.g. PB1).
- IEC 60617-07 defines the break-contact symbol with spring-return actuator used for the NC push button; ANSI Y32.2/IEEE 315 uses a visually similar break-contact with push actuator indicator.
- Stop push buttons (NC type) must be Red per IEC 60204-1 and ANSI/NEMA Z535; Start push buttons (NO type) must be Green — this colour coding is mandatory in industrial machine safety applications.
- Mechanical life of industrial push buttons is typically ≥ 1,000,000 operations; electrical life (rated current) is typically ≥ 100,000 operations per IEC 60947-5-1.
Diagrams that use this symbol
- contactor wiring diagram
- forward reverse motor control diagram
- 3 phase contactor wiring diagram
- contactor diagram
- single phase contactor wiring diagram
- motor control diagram
- 3 phase forward reverse motor control circuit diagram
- ac contactor wiring diagram
Frequently asked questions
What does the push button NC symbol look like?
The Push Button NC symbol shows two short parallel lines in the closed position (indicating the contact is conducting at rest) with a diagonal line or slash mark indicating break action, and a small arrow or actuator symbol above representing the push button. In IEC 60617-07 notation the symbol shows a break-contact with a spring-return push actuator. The ANSI/IEEE symbol is visually very similar.
What does NC mean on a push button?
NC stands for Normally Closed — meaning the push button contact is in the closed (conducting) state when the button is not being pressed. The contact opens (circuit breaks) only while the button is actively pressed and held down. When released, the spring returns the contact to its normally closed position. NC push buttons are used as Stop buttons in motor control circuits.
Why is the Stop button a NC (normally closed) push button?
The Stop button uses an NC (normally closed) push button because if the push button wiring breaks or a connection is lost, the open circuit de-energises the contactor coil and stops the motor — the safest fail-safe condition. This is a safety design principle per IEC 60204-1: a broken Stop button wire creates a fail-safe stop condition rather than an unsafe loss of stop control.
What is the difference between a NC and NO push button?
An NC (normally closed) push button is closed at rest and opens when pressed — used as a Stop button. An NO (normally open) push button is open at rest and closes when pressed — used as a Start button. In a motor starter control circuit, the NC Stop button is in series with the contactor coil, and the NO Start button is in parallel with the holding contact across the coil.
What standard defines the push button NC symbol?
IEC 60617-07 defines the normally-closed break-contact symbol with spring-return actuator used for the NC push button in IEC schematics. IEC 60947-5-1 specifies the construction, ratings, and testing of push button devices. ANSI Y32.2/IEEE 315 defines the equivalent break-contact symbol for North American schematic practice.
What colour should a NC Stop push button be?
IEC 60204-1 (Safety of Machinery) and ANSI/NEMA Z535 both require Stop push buttons to be coloured Red. This is a mandatory safety colour coding convention in industrial machine control panels worldwide: Red = Stop/Off/Emergency Stop; Green = Start/On; Yellow = Caution/Override. Using the wrong colour is a non-conformance with both IEC and ANSI safety standards.
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