The push button schematic symbol denotes a mechanically actuated, momentary or latching switch in a circuit diagram. The default state is "Normal" (unpressed). A Normal Open (NO) symbol shows a gap in the circuit line, while a Normal Closed (NC) symbol shows a continuous line interrupted by a diagonal slash or bridge. Below is the definitive reference for identifying, interpreting, and selecting the correct switch for your panel or PCB.
The Complete Push Button Schematic Symbol Reference Table
Use this spec-sheet-table to instantly identify the switch type on your schematic. This table covers both momentary (springs back when released) and maintained (latches in place) actions.
| Switch Type | NEMA Symbol (US/Canada) | IEC Symbol (Global/EU) | Action & State | Common Use Case |
|---|---|---|---|---|
| NO Momentary | Gap with flat horizontal bar above | Gap with angled bar above | Open circuit until pressed | Start buttons, doorbells, triggers |
| NC Momentary | Continuous line with bridge above | Continuous line with diagonal slash | Closed circuit until pressed | Stop buttons, E-Stops, interlocks |
| NO Maintained | NO symbol + mechanical latch hook | NO symbol + mechanical latch hook | Stays closed after pressing | Power toggle, mode selection |
| NC Maintained | NC symbol + mechanical latch hook | NC symbol + mechanical latch hook | Stays open after pressing | Safety lockouts, mute circuits |
| Multi-Pole (e.g., 1NO+1NC) | Two symbols linked by dashed line | Two symbols linked by dashed line | One button actuates both contacts | Motor reversing, dual-signal control |
| Illuminated Push Button | Switch symbol + circle with light cross | Switch symbol + circle with light arrow | Switch + integrated LED indicator | Status feedback, alarm acknowledgement |
Rows People Get Wrong (and How to Fix Them)
Even experienced technicians misinterpret specific rows in the reference table above. Here are the most common schematic reading errors and how to correct them on the bench.
1. The "Normal" State Confusion
The most frequent error is assuming "Normal" means "what the machine usually does when running." In electrical theory, Normal means the unactuated state—the physical position of the switch when no human finger or mechanical force is pressing it. An Emergency Stop (E-Stop) button uses an NC symbol because its normal, unpressed state allows current to flow to the safety contactor. When you slap the E-Stop, you open the circuit. If you wire an E-Stop as NO, a broken wire will silently disable the safety circuit, leading to catastrophic failure.
2. The Dashed Mechanical Linkage
When you see two separate switch symbols connected by a dashed or dotted vertical line, it does not mean two separate buttons. It indicates a single physical push button with multiple contact blocks (e.g., a 1NO-1NC block). Pressing the single actuator moves both contacts simultaneously. Missing this on a schematic leads to buying two separate 22mm switches instead of one switch with stacked contact blocks.
3. The IEC Slash vs. The NEMA Bridge
For NC momentary switches, the IEC 60617 standard uses a diagonal slash that physically cuts through the horizontal wire line. NEMA ICS 19 often depicts it as a bridge or loop over the line. CAD software sometimes defaults to one or the other, causing confusion when reading printed legacy schematics. Both mean exactly the same thing: the circuit is closed until the actuator breaks it.
Regional Standards: IEC vs. NEMA vs. Old UK
Your region and the origin of the machinery dictate which standard the schematic will follow. Mixing them up can lead to incorrect panel layouts.
- IEC 60617 (Global/Europe/Modern US): The dominant global standard. Symbols are minimalist, using thin lines, diagonal slashes for NC, and simple angles for actuators. If you are designing a new panel in 2026, use IEC.
- NEMA ICS 19 / JIC (North America): Historically dominant in the US and Canada. Symbols are heavier, using specific bridge shapes for NC contacts and distinct geometric shapes for actuators. You will see this on legacy US manufacturing equipment and older Allen-Bradley schematics.
- Old UK (BS 3939): Largely obsolete but still found in legacy British infrastructure and older marine panels. It used a distinct "bow" or "cup" shape for the push button actuator. If you encounter this, treat it as functionally identical to IEC, but plan to redraw the schematic during your next panel retrofit.
Faded Markings? Safe Interpretation and Testing
On the jobsite, you will frequently encounter physical switches with faded terminal markings (NO/NC or 13-14/21-22) or schematics where the ink has degraded. Never guess. Use a digital multimeter to definitively identify the switch state.
Follow this exact procedure using a meter like the Fluke 117 or 87V in continuity mode:
- Isolate: Turn off the control power breaker and lock it out (LOTO).
- Verify Dead: Use the meter's voltage setting to confirm 0V across the switch terminals.
- Switch to Continuity: Set the dial to the continuity/diode symbol (the one that beeps).
- Test Unpressed: Place probes on the common (C) and the suspected NO/NC terminals. If the meter beeps without you touching the button, it is NC.
- Test Pressed: If there is no beep unpressed, hold the probes in place and press the button. If it beeps only while held down, it is NO.
- Check Multi-Pole: If the switch has 4 terminals (e.g., 13/14 and 21/22), test both pairs. A standard 1NO-1NC block will beep on one pair unpressed, and the other pair only when pressed.
For terminal numbering context: IEC standard numbering dictates that 13-14 is NO, and 21-22 is NC. NEMA often simply stamps "NO" and "NC" on the contact blocks.
Decision Tree: Picking the Exact Switch for Your Circuit
Stop guessing which physical component matches your schematic. Use this decision-tree-table to terminate your selection process with a concrete part number.
| Application Requirement | Schematic Symbol Needed | Contact Logic | Concrete Part Pick (22mm Industrial) |
|---|---|---|---|
| Machine Start / Cycle Trigger | NO Momentary (IEC or NEMA) | Closes on press, opens on release | Schneider XB4BA31 (Green, 1NO) |
| Machine Stop / Safe Halt | NC Momentary (IEC or NEMA) | Opens on press, closes on release | Schneider XB4BA42 (Red, 1NC) |
| Emergency Stop (Safety Rated) | NC Momentary with forced-open mechanism | Opens on press, mechanical twist-to-release | Schneider XB4BS8442 (Red E-Stop, 2NC) |
| Toggle Power / Maintain State | NO or NC Maintained (Latching) | Stays in actuated state until pressed again | Siemens 3SU1050-0AB10-0AA0 (Latching NO) |
| Motor Forward/Reverse Interlock | Multi-Pole (1NO + 1NC linked) | Breaks one circuit while making the other | Eaton FAZ-XHI11 (or equivalent 1NO-1NC block) |
The Default Recommendation
If you are building a general-purpose DIY control panel, prototyping an Arduino/ESP32 industrial interface, or replacing an unmarked switch on a legacy board and need a safe baseline: buy a 22mm NO Momentary push button (Schneider Harmony XB4BA31 or Eaton XT series). NO momentary switches are the universal standard for logic triggers, PLC inputs, and microcontroller GPIO pull-down circuits. They fail safe (open circuit) if a wire breaks, preventing accidental machine starts. For the physical switch body, always choose the 22mm metal bezel (XB4) over the plastic bezel (XB5) for any application involving vibration or heavy use; the metal locking collar prevents the switch from rotating in the panel cutout over time.
