The push button normally open symbol represents a momentary switch that remains non-conducting (open) at rest and completes the circuit only when physically actuated. In electrical schematics, the globally dominant IEC 60617 standard draws this as a line break with a perpendicular actuator bar, strictly assigning terminals 13 and 14 for the contact block. In North America, the NEMA standard often depicts it as a gap with a cross or 'bowtie' actuator, though terminal numbering relies more on manufacturer-specific layouts or ladder logic line references.
Push Button Normally Open Symbol Reference Table
Use this reference table to identify the correct schematic symbol, terminal designations, and physical button color codes based on the standard governing your control panel. This data applies to standard 16mm, 22mm, and 30mm industrial pilot devices (e.g., Siemens 3SU1, Schneider Harmony XB5, Allen-Bradley 800T).
| Standard Body | Schematic Symbol Graphic | Terminal Designations | Standard Button Color | Primary Region / Application |
|---|---|---|---|---|
| IEC 60617 | Line gap with perpendicular actuator bar (T-shape) | 13 (Input) / 14 (Output) | Green (Start), Black (General) | Global, EU, Modern US Panels |
| NEMA ICS-19 | Line gap with cross/bowtie actuator or simple wedge | Varies (Often 3/4 or L1/L2) | Green (Start), Black (General) | North America (US/Canada) |
| JIC (Legacy) | Similar to NEMA, often with a distinct 'X' over the gap | Sequential ladder numbering | Green (Start) | Legacy US Automotive/Machinery |
| BS 3939 (Old UK) | Two parallel lines broken by a bridging actuator | A1 / A2 or custom | Green (Start) | Pre-1990s UK Installations |
| Maintained (Latching) NO | IEC NO symbol with a small mechanical latch hook | 13 / 14 (IEC) | Varies (Often Red/Green) | Global (Alternating Motor Controls) |
According to standard electronic symbol conventions, the defining feature of the 'normally open' (NO) state is the physical gap in the contact line. If the actuator line touches the contacts at rest, it is normally closed (NC).
Regional Standards: IEC vs NEMA vs Legacy Formats
Knowing which standard applies to your region is critical for troubleshooting and designing control circuits. Misinterpreting a NEMA symbol as an IEC symbol can lead to incorrect terminal wiring, especially when adding auxiliary contact blocks.
IEC 60617 and IEC 60947-5-1 (The Global Baseline)
The International Electrotechnical Commission (IEC) standardizes not just the drawing, but the physical terminal numbering. Under IEC 60947-5-1, a normally open contact block must be wired to terminals 13 and 14. If you are looking at a Schneider TeSys or Siemens contactor block and see '13' and '14' stamped into the plastic, you are looking at a NO contact, regardless of what the faded schematic on the cabinet door says. IEC also strictly dictates that a green push button must be used for 'Start' (actuating a NO seal-in circuit).
NEMA and JIC (North American Practice)
The National Electrical Manufacturers Association (NEMA) and the legacy Joint Industry Conference (JIC) standards focus heavily on ladder logic readability. While NEMA symbols for the push button normally open symbol are visually distinct (often using a cross-hatch or bowtie actuator), NEMA does not rigidly enforce the '13/14' terminal numbering rule. Instead, North American panels often wire NO contacts to terminals 3 and 4 on specific relay blocks, or simply label them according to the wire number on the schematic (e.g., Wire 104 to Wire 105). When reading All About Circuits schematic references, you will frequently see NEMA symbols paired with ladder rung numbers rather than physical device pinouts.
BS 3939 (Old UK Standard)
If you are retrofitting machinery in older British facilities, you may encounter BS 3939 symbols. These look remarkably different, using parallel lines and distinct bridging mechanisms. Always verify these with a multimeter, as the physical contact blocks may have been swapped to modern IEC equivalents during previous maintenance cycles, leaving the original BS 3939 schematic dangerously inaccurate.
Rows and Symbols People Get Wrong
Even experienced technicians misread specific variations of the push button normally open symbol. Here are the most common schematic traps and how to avoid them.
Confusing Momentary NO with Maintained (Latching) NO
A standard NO push button is momentary—it springs back when you release it. However, schematics for alternating motor controls or emergency override circuits often use a maintained (latching) NO button. The symbol for a maintained NO button includes a small mechanical hook or latch drawn next to the actuator bar. If you wire a standard momentary button into a circuit designed for a maintained symbol, the machine will only run while the operator physically holds the button down.
The 'X' vs. The 'Gap' (NEMA NC vs NO)
In older NEMA and JIC drawings, a normally closed (NC) push button is drawn with the contacts touching, and the actuator pushing them apart. A common mistake is misreading a poorly photocopied or faded NEMA NO symbol (which has a gap and a cross actuator) as an NC symbol because the 'cross' looks like intersecting contacts. Rule of thumb: if the main current path line has a visible break in it at rest, it is NO. If the line is continuous, it is NC.
Delayed Action NO Symbols
Some control circuits use time-delay relays triggered by push buttons. A push button normally open symbol with a small arrow pointing away from the actuator indicates a delayed break (it closes instantly, but opens slowly). An arrow pointing toward the contacts indicates a delayed make. Ignoring these small arrows during troubleshooting will lead you to replace perfectly good contact blocks, assuming they are 'sticking' when they are actually functioning as designed.
Safe Interpretation and Field Verification for Faded Markings
Schematics taped to the inside of control panel doors fade, get torn, or are simply never updated after field modifications. When the push button normally open symbol on the diagram no longer matches the physical reality of the panel, you must rely on field verification.
The Multimeter Continuity Test
When terminal markings (like 13/14) are obscured by grime, paint, or heat damage, use a digital multimeter (such as a Fluke 117 or 87V) set to the continuity/diode test mode to definitively identify a NO contact block.
- Isolate the block: If possible, pull one wire off the suspected NO terminal to prevent parallel circuit paths from giving a false continuity reading.
- Test at rest: Place your probes on the two terminals of the contact block. The meter should read 'OL' (Open Loop) or infinite resistance. If it beeps or reads near 0.0 ohms, this is a Normally Closed (NC) block, not NO.
- Test actuated: While keeping the probes firmly on the terminals, press the push button face or manually depress the contact block actuator with an insulated screwdriver. The meter should now beep or read less than 1.0 ohms.
- Release: Let go of the button. The meter must immediately return to 'OL'. If it stays at 0.0 ohms, the mechanical spring has failed, or the contacts have welded together due to a past short-circuit event.
By combining a solid understanding of the proper multimeter continuity testing techniques with the regional symbol tables above, you can safely and accurately troubleshoot any industrial control circuit, regardless of the schematic's condition or origin.
