When decoding a symbol push button switch on a schematic or wiring a physical control panel, the graphic representation tells you exactly how the circuit behaves before you ever strip a wire. A Normally Open (NO) momentary symbol means the circuit is dead until pressed; a Normally Closed (NC) symbol means current flows until interrupted. Misinterpreting these symbols—or ignoring the terminal numbering standards tied to them—is the leading cause of dead control circuits and unsafe emergency stop wiring.
The Core Push Button Switch Symbol Reference
The table below maps the most common schematic symbols to their physical contact states and IEC terminal designations. Keep this on your bench when wiring 22mm industrial control panels.
| Switch Type | Schematic Symbol Shape | Contact State (Resting) | IEC Terminal Numbers | Common Application |
|---|---|---|---|---|
| NO Momentary | Line gap with angled pusher, no latch hook | Open (No flow) | 13 and 14 | Motor START, Horn, Jog |
| NC Momentary | Continuous line with angled pusher breaking it | Closed (Flowing) | 21 and 22 | Motor STOP, Interlocks |
| E-Stop (Mushroom) | NC symbol with a mushroom head and twist/pull release indicator | Closed (Flowing) | 21 and 22 (Forced-open) | Emergency circuit interruption |
| NO Latching (Maintained) | Line gap with pusher and a small mechanical latch hook | Open (No flow) | 13 and 14 | Panel lighting, continuous run toggles |
| NC Latching (Maintained) | Continuous line broken by pusher with a latch hook | Closed (Flowing) | 21 and 22 | Main control power isolation |
| Double-Throw (Form C) | Common line switching between two paths (one NO, one NC) | Flows to NC path | 11 (Com), 12 (NC), 14 (NO) | Directional control, transfer switching |
Regional Standards: IEC 60617 vs. ANSI/NFPA vs. Legacy UK
Schematic symbols and physical button colors are governed by different standards depending on your region and the machinery's origin. While the fundamental physics remain the same, drawing conventions and safety color codes vary.
Never use a red button for a standard STOP function if it is not an Emergency Stop. According to NFPA 79 (Electrical Standard for Industrial Machinery), red is strictly reserved for emergency or abnormal intervention. Standard STOP should be black, white, or grey.
| Feature | IEC (Global / EU / Modern US) | ANSI / NEMA / NFPA (US Industrial) | Legacy UK (BS 3939 - Obsolete) |
|---|---|---|---|
| NO Symbol Drawing | Angled pusher, simple line gap | Similar, but actuator line often drawn perpendicular to contacts | Similar to IEC, but older drafts used a distinct semicircle pusher |
| START Button Color | Green (IEC 60073) | Green, White, or Black (NFPA 79) | Green (BS 3939) |
| STOP Button Color | Red (IEC 60073) | Red, Black, White, or Grey (NFPA 79) | Red (BS 3939) |
| E-Stop Symbol | Mushroom head, twist/pull release arrow | Mushroom head, often explicitly labeled 'E-STOP' | Historically lacked distinct symbol; relied on red mushroom physical trait |
Source reference: For deep-dive schematic drafting rules, refer to the IEC 60617 standard database or the NEMA ICS 19 standard for industrial control diagrams.
Rows People Get Wrong: Schematic & Wiring Traps
Even experienced bench builders make mistakes when translating these symbols to physical 22mm or 30mm switch stacks. Here are the most common failure modes:
- The E-Stop 'Silent Failure' Trap: The E-Stop symbol implies a Normally Closed (NC) contact. Amateurs sometimes wire an E-Stop using an NO contact block and a shunt relay, reasoning that "pressing it will break the shunt." Do not do this. If a wire breaks in an NO E-Stop circuit, the machine will not stop when you hit the button. E-Stops must use NC contacts with positive-opening (forced guide) mechanics (per IEC 60947-5-5) so that even if the contacts weld together, the physical pusher forces them apart.
- Momentary vs. Latching Confusion: A latching (maintained) symbol has a small hook or detent drawn near the actuator line. If you wire a standard momentary switch (like a Schneider Harmony XB4 spring-return block) where a latching symbol is specified, the motor or light will only operate while the operator physically holds the button down.
- Ignoring Terminal Numbers: Schematics often just say "NO" or "NC" without numbers. If you are using standard industrial contact blocks, you must wire NO to 13 and 14, and NC to 21 and 22. Wiring a 24VDC control signal to terminals 11 and 12 on a standard block will result in an open circuit, as those terminals belong to a different internal pole configuration.
Safe Interpretation When Markings Are Faded or Missing
On legacy panels or surplus equipment, the physical legend plates fade, and the contact blocks get covered in grease. Never guess the state of an unknown push button switch based on its physical appearance alone. Follow this verification path:
Turn off the main control breaker. Use a non-contact voltage tester and a multimeter to verify the circuit is dead before removing any panel covers or touching terminals.
- Set your multimeter to Continuity (or Ohms). A Fluke 117 or similar CAT III meter is ideal.
- Identify the terminals. Look for stamped numbers on the metal contact block. If you see 13 and 14, it is an NO block. If you see 21 and 22, it is an NC block.
- Test the resting state. Place probes on the two terminals. If the meter beeps (reads < 1 ohm) without pressing the button, it is NC. If it reads OL (open loop), it is NO.
- Test the actuated state. Press and hold the button. The states should invert. If they do not invert, the internal spring is broken or the contacts are welded—replace the block immediately.
- Check for E-Stop mechanics. If it is a red mushroom head, twist it to see if it pops out. If it lacks a positive-opening mechanical linkage (you can't feel the heavy physical cam forcing the contacts apart), it is not a true safety E-Stop, regardless of its color.
Decision Tree: Picking the Right Switch and Symbol
Use this decision matrix to terminate your design process and select the exact physical component and schematic symbol for your build. These picks assume a standard 22mm industrial panel cutout.
| Application Condition | Required Symbol & Color | Concrete Part Pick (22mm) |
|---|---|---|
| Standard Machine START (Must require continuous press or separate seal-in circuit) |
NO Momentary Green Bezel |
Schneider XB4BA31 (Complete unit: Green flush, 1NO contact block) |
| Standard Machine STOP (Interrupts control power safely) |
NC Momentary Red or Black Bezel |
Schneider XB4BA42 (Complete unit: Red flush, 1NC contact block) |
| Emergency Stop (Must lock out, fail-safe, positive opening) |
NC E-Stop (Mushroom) Red/Yellow background |
Schneider XB4BS844 (Twist-release, 2NC forced-open blocks) |
| Panel Light / Heater Toggle (Must stay on without operator holding it) |
NO or NC Latching White or Black Bezel |
Eaton M22-DL-W + M22-A-K11 (White latching head + 1NO/1NC block) |
| Motor Reversing / Jog (Needs to trigger two different contactors) |
Double-Throw (Form C) Black Bezel |
Siemens 3SU1150-0AB40-0BA0 (Black momentary, 1NO + 1NC blocks) |
If your schematic shows a NO momentary START button but the motor needs to stay running after you let go, the symbol itself doesn't change. Instead, the schematic will show a parallel NO contact from the main contactor (the "seal-in" or "holding" contact) wired across the START button terminals (13 and 14). Always look for this parallel path in the schematic before assuming the switch itself is latching.
