The normally closed (NC) push button symbol represents a switch that passes current in its resting state and breaks the circuit when actuated. In IEC 60617 schematics, the NC push button symbol is drawn as a continuous line broken by a diagonal slash that touches a solid dot. In North American NEMA/JIC drawings, it appears as two parallel vertical lines crossed by a diagonal slash. Understanding these variations and their corresponding terminal pinouts is critical for wiring control circuits and safety interlocks correctly.

The Complete NC Push Button Symbol Reference Table

Before wiring any control panel, identify the exact switch type on your schematic. The table below maps the graphical symbols to their physical terminal markings and operational behavior. Use this as your bench reference when reading prints or ordering replacement contact blocks.

Switch Type IEC 60617 Graphic NEMA / JIC Graphic Terminal Markings Practical Meaning & Application
Standard NC Push Button Line with diagonal slash touching a solid dot Two parallel vertical lines crossed by a diagonal slash 11, 12 Passes current at rest. Used for stop buttons, interlocks, and permissive circuits. Breaks circuit when pressed.
NC with Mechanical Latch (E-Stop) Standard NC symbol with a red mushroom-head indicator or mechanical lock hash Standard NC slash with a mechanical detent/latch symbol adjacent 11, 12 (often dual 21, 22) Breaks circuit and physically locks in the open state. Requires a twist or pull to reset. Mandatory for emergency stops.
NC Timed-Closed (Delay on Release) Standard NC symbol with a small left-pointing arrow or clock hash Standard NC slash with a delayed-action arrow 11, 12 (with timer module) Breaks immediately when pressed, but delays re-closing for a set duration after release. Used for motor braking or cooldowns.
NC Push-Pull Switch Standard NC symbol with a bidirectional arrow through the actuator line Standard NC slash with bidirectional actuator arrow 11, 12 Breaks circuit when pushed OR pulled. Commonly used for dual-action machine start/stop or specific safety interlocks.

Regional Standards: IEC vs. NEMA vs. Old UK

Reading a schematic printed in 1985 in Manchester requires a different mental model than reading a 2024 schematic from a German OEM. The graphical representation of the NC push button symbol changes based on the governing standard.

  • IEC 60617 (International / Europe / Modern Global): The dominant global standard. The symbol relies on a single line representing the conductor, with a diagonal slash indicating the movable contact resting on a solid dot (the fixed contact). When actuated, the slash moves away from the dot. Terminal numbering strictly follows the 11-12 convention for NC contacts.
  • NEMA ICS / JIC (North America): Prevalent in the US and Canada. The symbol uses two parallel vertical lines to represent the fixed contacts, with a diagonal slash representing the movable bridge. While the graphic differs, modern NEMA panels have largely adopted the IEC 11-12 terminal numbering for consistency, though older JIC panels may use 1-2 or NC1-NC2.
  • Old UK (BS 3939 - Deprecated): You will still encounter this in legacy British industrial sites. The BS 3939 NC symbol often featured a cross-hatched or boxed contact representation. If you are retrofitting a panel built before the late 1990s in the UK, verify the physical terminal block numbering, as it may not align with modern IEC 60617 conventions.

Which applies to you? If your equipment was manufactured after 2005, it almost certainly uses IEC 60617, regardless of your region. For older North American machinery, default to NEMA/JIC interpretation but always verify with a multimeter. For deeper symbol references, consult the Electronics Tutorials switch guide.

Rows and Symbols People Get Wrong

Misinterpreting a schematic symbol or terminal block layout leads to short circuits, blown fuses, or defeated safety interlocks. Watch out for these specific pitfalls:

  • Confusing 11/12 (NC) with 13/14 (NO): The most common bench mistake. On a standard 22mm or 16mm contact block (like a Schneider Harmony or Siemens SIRIUS), terminals 11 and 12 are Always NC. Terminals 13 and 14 are Always NO. If you wire a stop button to 13/14, the machine will only run while the button is held down.
  • Assuming E-Stop Blocks are Standard NC: An emergency stop requires a 'direct opening action' contact block. A standard NC block relies on spring pressure to close; if the spring fails or the contacts weld, the E-stop fails. True E-stop NC blocks use a physical cam to force the contacts apart. Look for the red block housing or the specific mechanical latch symbol on the schematic.
  • Ignoring the 'Slash' Direction on Limit Switches: While not a push button, limit switches use similar NC/NO slash logic. A slash resting on a dot is NC. A slash resting away from a dot (or resting on a blank space) is NO. Don't let the actuator type (roller, plunger) distract you from the contact state.

Safe Interpretation When Markings Are Faded or Missing

On the jobsite, you will frequently encounter unbranded import buttons, sun-baked outdoor enclosures, or grease-covered panels where the NC, 11, and 12 silkscreen markings have completely worn off. Never guess the pinout based on physical position.

WARNING: Mains and control voltages (>50V AC) can be lethal. Always perform a Lockout/Tagout (LOTO) procedure, de-energize the panel, and verify dead with a known-working CAT III/IV multimeter before touching terminal screws.

The Definitive Multimeter Verification Sequence:

  1. Isolate: Ensure the circuit is de-energized and locked out.
  2. Set Meter: Switch your multimeter to Continuity mode (the diode/sound wave icon).
  3. Probe Terminals: Place probes on two adjacent screw terminals. If the meter reads < 1 ohm (or beeps) while the button is unpressed, you have found the NC pair.
  4. Actuate: Press the button fully. The continuity must break (meter reads OL or infinite). If it still beeps while pressed, you are probing a common bus or the contacts are welded shut—discard the switch.
  5. Verify NO: Move to the other pair of terminals. They should read OL when unpressed, and < 1 ohm when pressed.

For safety-critical applications like emergency stops, ISO 13850 mandates that the NC contact must be a positive-break design. If you are replacing a faded E-stop button, do not use a standard NC block; you must source a block specifically rated for direct opening action.

Frequently Asked Questions

What is the difference between an NC push button symbol and an NO symbol?

The core difference lies in the resting state and the graphical representation. An NC (Normally Closed) push button symbol shows the movable contact resting on the fixed contact (indicated by a solid dot in IEC), meaning current flows when the button is untouched. An NO (Normally Open) symbol shows a gap between the movable and fixed contacts, meaning current only flows when the button is physically pressed. On physical hardware, NC uses terminals 11/12, while NO uses terminals 13/14.

How do I wire an NC push button symbol for an emergency stop circuit?

Emergency stop circuits must be wired in a fail-safe series configuration. You wire the line voltage into terminal 11 of the E-stop NC block, and daisy-chain out of terminal 12 to the next safety device or the master control relay (MCR) coil. If the wire breaks, the terminal loosens, or the button is pressed, the circuit opens and drops power to the MCR. Never wire an E-stop in parallel, and never use an NO contact for the primary safety break, as a broken wire would silently defeat the safety function.

Why does my NC push button symbol have a dashed line next to it?

A dashed or dotted line adjacent to the NC push button symbol typically indicates a mechanical interlock, a secondary guided contact, or a specific actuator type (like a key-release or twist-to-reset mechanism). In IEC schematics, a dashed line connecting two switch symbols means they are physically ganged together on the same shaft or button assembly. If the dashed line points to a small box, it may indicate an integrated LED indicator circuit tied to that specific button.

Can I use an NO push button in place of an NC push button for safety circuits?

No. Using a Normally Open (NO) button for a safety or stop circuit violates fundamental fail-safe design principles and NEMA/IEC safety guidelines. If you use an NO button to trigger a safety relay, a severed wire, a corroded terminal, or a failed contact spring will prevent the button from working when you need it most. NC circuits are 'supervised'—if the wire breaks, the machine stops immediately, alerting you to the fault before a hazardous situation occurs.