The standard normally open (NO) push button circuit symbol is a break in the conductor line with a perpendicular actuator bar and a dashed return spring, but its exact geometry depends entirely on whether you are reading IEC (global/European) or ANSI/IEEE (North American) schematics. Misinterpreting these symbols or confusing the 'normal' shelf state with the machine's running state is one of the most common causes of control wiring faults. Below is the definitive reference for identifying, specifying, and troubleshooting push button symbols across global standards.
The Complete Push Button Symbol Reference Table
This table maps the most common push button configurations to their visual descriptions and standard terminal designations. Keep this handy when tracing control circuits on a machine panel.
| Switch Type | IEC 60617 Symbol Description | ANSI/IEEE Y32.2 Symbol Description | Standard Terminal Numbers (IEC) | Typical Use Case |
|---|---|---|---|---|
| Normally Open (NO) | Gap in line, perpendicular actuator, dashed return line. | Break in line, bridging actuator angled away from contacts. | 13 / 14 | Start buttons, jog controls, PLC inputs. |
| Normally Closed (NC) | Continuous line, perpendicular actuator holding contacts open, dashed return. | Continuous line, bridging actuator holding contacts apart. | 21 / 22 | Stop buttons, interlock loops, safety chains. |
| Dual NO/NC | Combined NO and NC symbols sharing a single actuator line. | Combined break and continuous line with single actuator. | 13/14 (NO), 21/22 (NC) | Interlocking motor reversals, status feedback. |
| E-Stop (Mushroom) | NO/NC symbol with a specific arrow pointing to a mushroom-head indicator. | Standard NC/NO with a specialized E-stop actuator flag. | 11/12, 21/22 (NC) | Emergency machine halting, safety relay inputs. |
| Illuminated | Standard NO/NC symbol with a small circle or lamp symbol integrated into the actuator. | Standard symbol with an adjacent lamp indicator. | 13/14 (Switch), X1/X2 (Lamp) | Start/Stop with visual confirmation, fault indicators. |
| Key-Operated | Standard symbol with a key symbol (flag with a circle) on the actuator line. | Standard symbol with a keyhole graphic on the actuator. | 13/14 or 21/22 | Mode selection, lockout/tagout physical security. |
Regional Standards: IEC vs. ANSI vs. Legacy UK
Before you start tracing wires, you must identify which drafting standard the schematic follows. Mixing up IEC and ANSI visual conventions can lead to catastrophic wiring errors, especially in safety circuits.
- IEC 60617 (International / Europe / Australia): This is the dominant global standard for industrial automation and PLC schematics. It relies heavily on standardized terminal numbering (e.g., 13/14 for NO) and uses clean, orthogonal lines. If you are working with Siemens, Schneider Electric, or ABB components, your drawings will almost certainly follow IEC.
- ANSI/IEEE Y32.2 / NEMA (North America): Common in older US industrial panels, HVAC controls, and residential/commercial architectural drawings. It uses more illustrative symbols (like drawing the actual physical shape of a limit switch or push button) rather than the abstract functional blocks of IEC. NEMA standardizes the physical dimensions (like 22mm vs 30mm operator stations) but defers to IEEE 315 for the schematic symbols.
- Legacy UK (BS 3939): Superseded by the IEC equivalent in the 1990s, but you will still encounter BS 3939 symbols if you are retrofitting or troubleshooting older British machinery. It shares many similarities with IEC but uses different hatching and actuator representations for maintained vs. momentary switches.
Which applies to you? If you are in the US doing residential or commercial building wiring, expect ANSI/NEMA. If you are anywhere in the world working on industrial automation, PLCs, or modern motor control centers (MCCs), expect IEC 60617. For a deeper dive into standardized schematic conventions, the All About Circuits reference library provides an excellent breakdown of these regional differences.
Rows People Get Wrong (And How to Avoid Wiring Disasters)
Even experienced technicians misread specific rows in the reference table above. Here are the most common pitfalls and how to avoid them.
The most misunderstood concept in control wiring is the word 'Normal'. In a push button circuit symbol, 'Normal' means the shelf state (the physical state of the switch when no human finger is pressing it and no external force is applied). It does not mean the normal running state of the machine. For example, a motor 'Stop' button is wired using a Normally Closed (NC) switch. The machine runs 'normally' through that closed contact, but the switch itself is in its 'normal' (unpressed) state.
Mistake 1: Confusing Momentary with Maintained (Latching)
A standard push button is momentary—it returns to its normal state when you release it, indicated by the dashed return spring line in the symbol. A maintained (or latching) push button stays in the actuated state until you press it again (like a standard household light switch or an E-stop that twists to release). The symbol for a maintained switch replaces the dashed return spring with a small mechanical detent or hook symbol on the actuator line. Wiring a latching switch where a momentary one is specified will cause your PLC logic to lock up or your motor starter to remain engaged.
Mistake 2: Misidentifying the E-Stop Symbol
Do not confuse a standard NC push button with an Emergency Stop symbol. The IEC E-Stop symbol explicitly includes a mushroom-head indicator and often implies a positive-break contact mechanism. Physically, an E-stop uses a specialized contact block (like the Schneider Harmony XB5 series) that mechanically forces the NC contacts open even if the internal spring fails or the contacts weld together. Never substitute a standard NC push button for an E-stop in a safety circuit.
Mistake 3: Safe Interpretation of Faded or Missing Markings
On older panels, the terminal markings (13/14 or 21/22) on the physical contact blocks often fade, get painted over, or melt. Never guess the wiring based on the physical position of the block.
- De-energize the panel: Lock out and tag out (LOTO) the main disconnect. Verify zero voltage with a CAT III or CAT IV multimeter.
- Set your meter to continuity (Ω / diode symbol).
- Probe the terminals: Place your probes across the two terminals of a single pole.
- Actuate the button: If the meter beeps (reads < 1 ohm) only when the button is pressed, it is a Normally Open (NO) contact (Terminals 13/14). If it beeps when released and goes open when pressed, it is Normally Closed (NC) (Terminals 21/22).
For more on industrial switch classifications and safety standards, refer to the Electrical Engineering Portal's guide on industrial symbols.
Push Button Circuit Symbol FAQ
What does the dashed line mean on a push button circuit symbol?
The dashed line extending from the actuator bar represents the mechanical return spring. It visually communicates that the switch is 'momentary'—meaning it will automatically return to its unactuated (normal) shelf state the moment the operator removes their finger. If the dashed line is absent and replaced by a solid mechanical latch symbol, the switch is 'maintained' and will stay in the actuated position until physically reset.
How do I wire a push button if the NO and NC markings are faded off the switch block?
Never rely on visual guessing or the physical orientation of the contact block on the DIN rail. With the circuit completely de-energized and verified dead, use a digital multimeter set to the continuity or resistance setting. Probe the two terminals of the contact block. Press the push button. If the circuit closes (meter beeps or reads near 0 ohms) only while pressed, you are looking at the NO terminals (typically 13 and 14). If the circuit opens (meter reads OL or infinite) only while pressed, you are on the NC terminals (typically 21 and 22).
Is there a specific circuit symbol for a latching push button?
Yes. While a standard push button uses a dashed line to indicate a spring return, a latching (or maintained) push button symbol replaces that dashed line with a small hook, detent, or interlocking mechanical graphic attached to the actuator stem. This indicates that the switch mechanically locks into the actuated state and requires a second press, a twist, or a pull to return to the normal state. E-stops are a specialized form of latching switches but use their own distinct mushroom-head symbol.
Why do PLC schematics use different push button symbols than residential wiring?
PLC and industrial automation schematics almost universally follow the IEC 60617 standard, which prioritizes functional logic, standardized terminal numbering (13/14 for NO, 21/22 for NC), and clean orthogonal lines for complex ladder logic. Residential and commercial architectural wiring in North America follows ANSI/IEEE and NEMA conventions, which prioritize physical representation and spatial layout for electricians pulling wire in the field. The IEC symbols are optimized for control engineers reading logic flow; ANSI symbols are optimized for tradespeople reading physical installation plans.
