The standard push button symbol consists of a break in a horizontal line (representing the wire) bridged by a movable contact. In the global IEC 60617 standard, this is drawn as a simple perpendicular line with a horizontal bar; in the North American NEMA standard, it includes a dashed mechanical linkage line. The exact symbol changes based on whether the contacts are Normally Open (NO), Normally Closed (NC), momentary, or maintained (latching). If you are reading a schematic today, assume IEC unless the panel was built in North America before 2000.
The Complete Push Button Symbol Reference Table
Use this table to decode the symbols on your schematic. 'Normal' state always refers to the physical switch at rest (unpressed).
| Switch Type | IEC 60617 Visual Description | NEMA / ANSI Visual Description | Contact State (Resting) | Typical Application |
|---|---|---|---|---|
| Momentary NO | Horizontal line gap, bridged by a top bar with a vertical actuator line pointing down. | Similar to IEC, but includes a dashed line linking the actuator to the contact bar. | Open (No continuity) | Start buttons, horn triggers, jog functions. |
| Momentary NC | Horizontal line gap, bridged by a top bar that overlaps the gap, with an upward actuator line. | Dashed mechanical linkage with an overlapping contact bar. | Closed (Continuity) | Stop buttons, interlocks, limit switches. |
| Maintained NO | Momentary NO symbol, plus a small mechanical latch hook on the actuator line. | Dashed linkage with a latch hook mechanism drawn on the actuator. | Open (until latched) | Selector switches, toggle power controls. |
| Maintained NC | Momentary NC symbol, plus a mechanical latch hook. | Dashed linkage with overlapping bar and latch hook. | Closed (until latched) | Safety bypass toggles, continuous run modes. |
| Mushroom E-Stop | NC symbol with a wide, flat 'mushroom' cap drawn on top of the actuator line. Often includes a twist-to-release latch symbol. | NC symbol with a distinct wide-cap profile and dashed mechanical linkage. | Closed (Continuity) | Emergency stop circuits, Category 0/1 safety stops. |
Regional Standard Variants: IEC vs NEMA vs Legacy UK
Knowing which standard your schematic follows prevents catastrophic miswiring. A symbol that looks like a simple disconnect in one standard might imply a mechanical latch in another.
- IEC 60617 (Global Standard): Used in Europe, Asia, and modern North American panels. It focuses on the electrical function rather than the mechanical construction. The symbols are cleaner and take up less space on dense PLC I/O drawings.
- NEMA ICS 19 / ANSI JIC (North America): Dominant in US/Canadian industrial panels built before the late 1990s. These symbols explicitly draw the mechanical linkage (dashed lines) and often show the physical arrangement of contact blocks on the back of the switch.
- Legacy UK (BS 3939): Largely obsolete since the UK harmonized with IEC standards, but you will still encounter BS 3939 symbols when retrofitting pre-1990s British machinery. It uses a distinct 'cross' notation for NC contacts that differs from both modern IEC and NEMA.
For a deeper visual breakdown of these historical and modern differences, the All About Circuits switch reference guide provides an excellent side-by-side comparison of NEMA and IEC drafting styles.
Rows People Get Wrong (And How to Fix Them)
When troubleshooting a control board, misinterpreting the symbol leads to chasing phantom faults. Here are the three most common errors bench technicians and DIYers make:
1. Confusing 'Normal' with 'Usual Operating State'
'Normal' in schematic terminology strictly means unactuated (the button is not being pressed, and no external force is applied). A 'Normally Closed' (NC) stop button passes current during normal machine operation. People often assume NC means 'the circuit is usually closed when the machine is running,' which happens to be true here, but for a limit switch, the 'normal' state is when the machine arm is not pressing the switch. Always trace symbols based on the unactuated resting state.
2. Misidentifying E-Stop Positive-Opening Contacts
An Emergency Stop mushroom button is not just a standard NC push button. According to Electrical Engineering Portal safety guidelines, E-stop symbols imply positive-opening NC contacts. This means the mechanical linkage physically forces the contacts apart, even if they have welded shut from a previous fault. If you replace an E-stop with a standard NC momentary switch, you violate safety codes (like NFPA 79 or IEC 60204-1) because a welded contact will prevent the machine from stopping.
3. Ignoring the Latch/Twist-Release Notation
If an IEC symbol features a small curved arrow next to the mushroom cap, it dictates a 'twist-to-release' mechanism. If you install a 'pull-to-release' or 'push-to-release' physical button in its place, the physical housing will not match the schematic's safety intent, potentially causing operators to panic during an emergency when the button doesn't release as expected.
Safe Interpretation When Schematics Are Faded or Missing
When you inherit a machine with a sun-bleached, oil-stained schematic where the push button symbols are unreadable, you must map the physical switch blocks manually.
- Isolate and Verify Dead: Shut off the main disconnect. Use a non-contact voltage tester, followed by a multimeter on AC voltage to confirm 0V across the DC power supply output and AC control lines.
- Identify the Common (C) Terminal: Set your multimeter to continuity (the diode/beep setting). Locate the terminal marked 'C', 'COM', or 'Z' (on some legacy European blocks). This is your reference point.
- Map the NO Contact: Place one probe on COM and the other on the terminal marked 'NO' or '13/14'. With the button unpressed, the meter should read 'OL' (Over Limit / Open). Press the button; the meter should beep and read < 1 ohm.
- Map the NC Contact: Move the probe to 'NC' or '11/12'. Unpressed, it should read < 1 ohm. Pressed, it should read 'OL'.
- Draft Your Own Legend: Tape a piece of masking tape to the inside of the panel door and draw the verified IEC symbols based on your multimeter readings. This becomes your new source of truth.
Decision Path: Selecting the Right Physical Switch
When replacing a broken push button or designing a new control panel, do not just buy a generic 'push button switch.' Use this decision tree to select the exact industrial part number that matches your schematic symbol and safety requirements.
| Application Requirement | If True... | Concrete Part Pick (22mm Mounting) |
|---|---|---|
| Is this an Emergency Stop function? | YES → You must use a twist-release mushroom head with positive-opening NC contacts. Red cap, yellow background required. | Schneider Harmony XB4BS8442 (Metal bezel, 1 NC + 1 NO, twist-release E-Stop). |
| Is this a standard Start/Run command? | YES → Use a Momentary NO flush button. Green cap is standard for 'Start'. | Schneider Harmony XB4BA31 (Plastic bezel, 1 NO contact, Green flush cap). |
| Is this a standard Stop command (Non-Emergency)? | YES → Use a Momentary NC flush button. Black or Red cap. (Do NOT use an E-stop mushroom for a standard cycle stop). | Schneider Harmony XB4BA42 (Plastic bezel, 1 NC contact, Red flush cap). |
| Does the operator need to select between Manual/Auto modes? | YES → You need a Maintained (Selector) switch, not a push button. 2-position or 3-position. | Schneider Harmony XB4BD25 (2-position maintained selector, 1 NO + 1 NC). |
For modern PLC-controlled panels, stick to the 22mm mounting standard (like the Schneider XB4 metal or XB5 plastic series, or the Siemens SIRIUS 3SU1 line). While 16mm switches exist for tight spaces, 22mm remains the global industrial baseline, ensuring you can source replacement contact blocks and actuator heads locally without waiting for specialized shipping.
