The standard symbol for a push button switch depends on your regional drafting standard, but universally it consists of a contact symbol (Normally Open or Normally Closed) intersected by a mechanical actuator line ending in a flat, extended, or mushroom head. In modern global schematics, the IEC 60617 abstract line style is the default, while North American legacy panels still heavily feature NEMA ICS 19 literal linkage drawings.

The Master Push Button Switch Symbol Reference Table

Before tracing wires or ordering replacements, identify the exact switch behavior from the schematic. This table maps the symbol to its physical terminal markings (stamped directly on the contact block) and its real-world function.

Switch Type IEC 60617 Symbol NEMA / ANSI Symbol Terminal Pins (IEC) Practical Meaning & Application
Momentary NO (Normally Open) Parallel lines separated by gap; actuator line with flat bar Dotted mechanical linkage to open contacts 13, 14 Circuit is open until pressed. Used for 'Start', 'Reset', or 'Jog' functions.
Momentary NC (Normally Closed) Parallel lines crossed by an 'X'; actuator line with flat bar Dotted mechanical linkage to closed contacts 21, 22 Circuit is closed until pressed. Used for 'Stop' or interlock safety loops.
Maintained (Latching) NO NO symbol with a small mechanical hook/latch on the actuator line Toggle/latch mechanism shown on linkage 13, 14 Stays closed after release. Press again to open. Used for 'Power On' or mode selects.
Maintained (Latching) NC NC symbol with a mechanical hook/latch on the actuator line Toggle/latch mechanism shown on linkage 21, 22 Stays open after release. Press again to close. Used for 'Master Disable' or E-Stop alternatives.
Mushroom E-Stop (NC Latching) NC symbol with a mushroom head actuator and twist-to-release latch indicator Mushroom head with explicit mechanical latch block 21, 22 (often multi-pole: 21/22 and 31/32) Breaks circuit and mechanically locks. Requires twist or pull to reset. Mandatory for emergency stops.

Regional Variants: IEC vs. NEMA vs. Legacy UK

When troubleshooting a machine imported from overseas or maintaining a 40-year-old domestic plant, you will encounter conflicting symbol dialects. Knowing which standard applies to your panel prevents miswiring.

Rule of Thumb: If the panel was built after 2005 outside of North America, or by a modern global OEM, it uses IEC 60617 standards. If it is a legacy North American machine built before the widespread adoption of global PLCs, expect NEMA ICS 19 symbols.
  • IEC 60617 (Global/European): Uses abstract, minimalist geometry. The actuator is a simple straight line ending in a perpendicular bar. Contacts are purely electrical representations without mechanical context. This is the modern standard for PLC I/O diagrams.
  • NEMA ICS 19 (North America): Uses literal, mechanical representations. The actuator is drawn as a dotted or dashed line showing the physical linkage pushing the contact arm. It is visually busier but easier for novice technicians to visualize the physical movement.
  • Legacy UK (BS 3939): Superseded by IEC, but still found in older British infrastructure. It used solid lines with specific arrowheads to denote actuation direction. If you see BS 3939, treat the contact logic identically to IEC, but verify the actuator arrow direction carefully.

The 'Rows People Get Wrong' Notes

Misreading a push button symbol leads to short circuits or, worse, defeated safety interlocks. Here are the specific schematic rows and details that trip up even experienced wiremen:

  • The 'X' on NC Contacts: In the IEC NC symbol, the moving contact line crosses the stationary line, forming an 'X'. Many beginners think this indicates a broken wire or a fault. It does not. It simply indicates that the moving element physically crosses the stationary contact point to break the circuit.
  • Missing the Latch Hook: The difference between a momentary and maintained switch is a tiny mechanical 'hook' or 'catch' drawn on the actuator line. If you miss this 2mm detail on a schematic, you will wire a momentary push button into a circuit that requires a latching switch, causing the machine to shut off the second the operator releases the button.
  • Confusing Terminal 13/14 with L1/L2: The numbers 13 and 14 on a contact block do not mean Line 1 and Line 2 (or Hot/Neutral). They are strictly IEC standard designations for a Normally Open contact pair. Similarly, 21/22 designates NC. Never assume pin 13 is the 'input' and 14 is the 'output'; they are bidirectional electrical contacts.
  • E-Stop Multi-Pole Assumptions: A schematic might only show one NC symbol for an E-Stop, but the physical industrial E-Stop button often contains two or three stacked NC contact blocks (e.g., 21/22 and 31/32) for redundant safety relay wiring. Always count the physical blocks on the DIN rail, not just the symbols on the page.

Safe Interpretation When Markings Are Faded or Missing

On older panels, the physical legend plate showing 'NO' or 'NC' is often rubbed off by years of grease and operator thumbs. Do not guess based on the button color (Red is usually NC/Stop, Green is usually NO/Start, but this is not guaranteed).

SAFETY WARNING: Never probe for continuity on a live control circuit. De-energize the panel, apply Lockout/Tagout (LOTO), and verify dead with a known-working CAT III or CAT IV multimeter before touching the terminals.

The Multimeter Mapping Procedure:

  1. Set your multimeter to the Continuity or low-Ohms setting.
  2. Identify the common terminal (often marked 'C' or 'COM' on older switches, or just test all pairs).
  3. Place probes on two terminals. If the meter beeps (reads < 1 ohm) without pressing the button, you have found the NC pair (21/22).
  4. Press and hold the button. If the beep stops (reads OL or infinite ohms), it is confirmed NC.
  5. If the meter is silent (OL) until you press the button, and then beeps, you have found the NO pair (13/14).
  6. Test for latching: Press the button and release it. If continuity remains after release, it is a maintained/latching switch.

Decision Path: Selecting the Right Push Button for Your Panel

When replacing a broken switch or building a new control enclosure, use this decision matrix to terminate your part selection. Do not mix 16mm and 22mm formats in the same panel.

Condition / Environment If True, Select... Concrete Part Recommendation
Standard indoor control panel, clean environment, cost-sensitive. 22mm Plastic Body, IP65 rated. Buy: Schneider Electric Harmony XB4 (Plastic) series. E.g., XB4BA31 (Green NO).
Heavy industrial, washdown, oil mist, or high-impact environment. 22mm Metal Body, IP66/IP67 rated. Buy: Schneider Electric Harmony XB5 (Metal) or Siemens 3SU1 metal series.
Space-constrained HMI enclosure or small electronics box. 16mm Plastic Body. Buy: Schneider Harmony XB6 or IDEC A6 series.
Requires Emergency Stop function (Mandatory Safety). 40mm Mushroom Head, Twist-to-Release, NC Latching. Buy: XB4ES842 (22mm mount, 40mm red mushroom, 2x NC blocks).

Final Default Pick: If you are building a general-purpose 120V/24V DC control panel and are unsure which line to pick, standardize on the Schneider Electric Harmony XB4 22mm plastic series. It offers the best balance of global availability, modular contact blocks (ZB2 series), and clear IEC terminal stamping, ensuring your physical wiring will perfectly match your IEC 60617 schematics.