Push button symbols translate physical switch mechanics into schematic logic. Whether you are reading a legacy NEMA panel drawing or a modern IEC 60617 PLC schematic, misinterpreting a single actuator line can lead to dead shorts, defeated safety interlocks, or blown fuses. Below is the definitive reference for identifying, interpreting, and verifying push button symbols on the bench and in the field.
The Master Push Button Symbol Reference Table
The table below maps the most common push button functions to their schematic geometries. Because schematic drawing software (like AutoCAD Electrical or EPLAN) renders these slightly differently, we focus on the defining geometric features rather than exact pixel-perfect line weights.
| Function / Description | IEC 60617 Geometry | NEMA / NFPA 79 Geometry | Normal State & Mechanical Reality | Common Real-World Part Example |
|---|---|---|---|---|
| Normally Open (NO) Momentary | Parallel vertical lines with a diagonal actuator line ending in a flat horizontal bar. | Similar to IEC, but the actuator often features a small circle or square representing the physical button head. | Open (OL). Closes only while physical pressure is applied. Spring returns. | Schneider ZB4BZ101 (XB4 series) or Siemens 3SU1050-4AA00-0AA0. |
| Normally Closed (NC) Momentary | Parallel vertical lines with a diagonal line crossing through them (slash), topped with a flat bar. | Crossed lines with a literal button-head symbol on the actuator shaft. | Closed (<1Ω). Opens only while physical pressure is applied. Spring returns. | Schneider ZB4BZ102 or Eaton FAZ-XL11. |
| E-Stop (Mushroom, Latching) | NC symbol with a mechanical latch indicator (often a small interlocking hook or crossbar on the actuator). | NC symbol with a prominent mushroom-head graphic and a latch indicator. | Closed. When pressed, it opens and mechanically locks. Requires twist or pull to reset. | Siemens 3SU1150-1HB20-1FA0 (Twist-release, 40mm red mushroom). |
| Delayed Make (NO) | NO symbol with a small upward-pointing arrow or a dashpot (piston) symbol on the actuator. | NO symbol with a mechanical delay hash mark on the moving contact. | Open. When pressed, there is a mechanical or pneumatic delay (e.g., 50-500ms) before contacts close. | Custom pneumatic dashpot attachments on heavy-duty NEMA starters. |
| Delayed Break (NC) | NC symbol with a downward-pointing arrow or dashpot symbol. | NC symbol with delay hash marks. | Closed. When pressed, the contacts remain closed for a brief delay before opening. | Used in motor braking circuits to keep dynamic braking engaged until the coil field collapses. |
| Two-Circuit (NO/NC Dual) | A single actuator line mechanically linked (dashed line) to both an NO and an NC contact symbol. | Stacked NO and NC symbols sharing a single vertical actuator shaft. | One block is Open, one is Closed. Pressing toggles both simultaneously. | Standard 3-stack contact blocks on a single Harmony XB5 operator. |
| Key-Operated NO | NO symbol with a key symbol (circle with a diagonal slot) on the actuator. | NO symbol with a literal key graphic. | Open. Requires a physical key to actuate. Often maintained (stays closed) until key is turned back. | Panel override switches for PLC manual/auto mode selection. |
Regional Standards: IEC vs. NEMA vs. Legacy UK
The standard that applies to your region dictates not just how the symbol is drawn, but the physical dimensions and testing requirements of the hardware behind the panel.
- IEC 60617 (International / Europe / Modern Global): The IEC standard favors abstract, minimalist geometry. An actuator is just a line; a button head is rarely drawn. This keeps schematics clean but requires the reader to memorize the abstract modifiers (like the dashpot for delays). Hardware follows IEC 60947-5-1, standardizing on 22mm and 16mm mounting diameters.
- NEMA / NFPA 79 (North America): NEMA schematics are more literal. If it is a mushroom-head E-stop, the schematic will often draw a mushroom head. NEMA hardware traditionally uses larger 30mm mounting holes and heavier, silver-alloy contact blocks designed for higher inrush currents without external contactors.
- BS 3939 (Legacy UK): Superseded by IEC adoption, but still found in older British manufacturing plants. BS 3939 used a confusing hybrid of literal and abstract symbols, often drawing the physical switch enclosure in the schematic. If you encounter a drawing where the switch symbol looks like a cross-section of the actual physical device, you are likely looking at legacy BS 3939.
Never trust a faded panel engraving, a painted-over button head, or a missing physical label to determine if a switch is NO or NC. In harsh industrial environments, UV exposure and chemical washdowns destroy surface markings within a few years. Furthermore, maintenance technicians frequently swap NO and NC contact blocks behind the panel without updating the front engraving. Always de-energize the panel, lock out the main disconnect, and verify the switch state physically with a multimeter before assuming the schematic matches the physical reality.
The 'Rows People Get Wrong' Troubleshooting Notes
When troubleshooting control circuits or designing PLC I/O diagrams, these three symbol misinterpretations cause the most downtime and blown fuses.
1. Confusing Delayed Make with Delayed Break
A 'Delayed Make' NO contact does not close immediately when you press the button; it waits for the dashpot to cycle. A 'Delayed Break' NC contact stays closed for a fraction of a second after you press the button. A common mistake is using a Delayed Break symbol in a safety interlock circuit where immediate opening is required by OSHA machine guarding standards. Safety circuits must use standard, instant-break NC contacts or specialized force-guided safety relays, never pneumatic delays.
2. Assuming E-Stop is Just a 'Big NC Button'
The E-Stop symbol includes a mechanical latch indicator for a reason. According to IEC 60947-5-5, an E-stop device must feature a positive-break mechanism and a mechanical latching feature that prevents the contacts from closing again simply by releasing the button. It requires a deliberate reset action (twist, pull, or key). If you wire a standard momentary NC push button (without the latch) in place of an E-stop, the machine will restart the moment the operator's hand slips off the button, creating a severe crushing or electrocution hazard.
3. Illuminated Push Buttons: Mixing Lamp and Contact Circuits
An illuminated push button symbol shows both a switch contact and a lamp (circle with an X or rays). A classic rookie mistake on the bench is wiring the 24VDC LED indicator module in series with the motor contactor coil to 'save a wire'. The LED module draws roughly 10-15mA, while the contactor coil requires 50-200mA to pull in. The contactor will chatter, overheat, and fail. The lamp circuit and the switch contact circuit must always be wired as completely separate parallel branches, even if they share the same physical 22mm mounting hole.
Practical Wiring and Verification on the Bench
When you have a physical push button assembly (like a Schneider Harmony XB4 metal or XB5 plastic series) and the schematic is missing or illegible, you must map the contacts manually.
- Isolate and De-energize: Turn off the control circuit breaker. Verify zero voltage using a CAT III rated multimeter (e.g., Fluke 117) across the line and load terminals.
- Set Meter to Continuity/Ohms: Use the continuity setting with the audible beep. Ensure your leads read less than 0.2Ω when shorted together to account for test lead resistance.
- Probe the Contact Blocks: Look behind the operator. You will see terminal screws marked '13/14' (Standard IEC designation for NO contacts) and '21/22' (Standard IEC designation for NC contacts). NEMA blocks may simply be labeled NO and NC.
- Test the Unpressed State: Place probes on 13 and 14. The meter should read 'OL' (Open Loop). Place probes on 21 and 22. The meter should read <0.5Ω and beep.
- Test the Pressed State: Have an assistant press and hold the button (or use a non-conductive zip tie to hold the actuator down). The 13/14 terminals should now beep (<0.5Ω), and the 21/22 terminals should read 'OL'.
- Check for Mechanical Binding: Release the button. Listen for the sharp 'click' of the return spring. If the button feels mushy or the continuity state doesn't instantly revert, the contact block carrier is likely cracked or the actuator is binding against the panel knockout hole—a very common issue when panels are punched with the wrong size die.
By cross-referencing the physical terminal markings (13/14 vs 21/22) with the geometric symbols on the schematic, you eliminate the guesswork and ensure your control logic matches the physical hardware.
