The standard normally open (NO) push button symbol represents a switch that blocks current flow in its resting state and completes the circuit only when the actuator is physically depressed. In North American NEMA schematics, it is drawn as two parallel vertical lines (the stationary contacts) intersected by a perpendicular horizontal bridge. In international IEC 60617 drawings, it appears as a broken horizontal line with a movable bridge suspended above the gap. Regardless of the standard, the core function remains identical: no continuity until the button is pushed.
The Master Reference Table: Push Button Symbols & Contacts
Before tracing wires or programming logic, confirm exactly which variant of the normally open push button symbol you are looking at. The table below maps the visual symbol to its physical behavior and standard terminal numbering.
| Symbol Variant | Visual Description (IEC / NEMA) | Physical Behavior | Standard IEC Terminals | Standard NEMA Terminals |
|---|---|---|---|---|
| NO Momentary | Gap with bridge above (IEC) / Parallel lines with crossbar (NEMA) | Circuit closes only while physically held down. Spring returns to open. | 13 & 14 | 3 & 4 |
| NO Maintained | Standard NO symbol with a mechanical latch/detent indicator (small hook or cross) | Circuit closes on first press and stays closed. Opens on second press. | 13 & 14 (Latch) | 3 & 4 (Latch) |
| NO Time-Delay | Standard NO symbol with an upward-pointing arrow or timer box attached to the actuator | Contacts close only after the button is held for a set duration (e.g., 3 seconds). | 67 & 68 (Delayed) | N/A (Usually relay) |
| 1NO + 1NC Combo | Two symbols stacked vertically on the same actuator shaft line; one open, one closed | Pressing the button simultaneously opens one circuit and closes the other. | 13/14 (NO), 21/22 (NC) | 3/4 (NO), 1/2 (NC) |
Regional Standards: Which Symbol Applies to Your Panel?
A common point of failure for junior technicians and DIYers is misreading a schematic because they assume their regional standard is universal. The symbol for a normally open push button changes depending on the governing electrical body that drafted the prints.
| Standard | Region of Dominance | Visual Style | Key Reference |
|---|---|---|---|
| NEMA (ANSI) | United States, Canada, Mexico | Ladder-style vertical power rails. Contacts are drawn horizontally between rails. The NO symbol uses distinct parallel lines for contacts. | All About Circuits: NEMA Symbols |
| IEC 60617 | Europe, Asia, Australia, Global Export | Horizontal or vertical flow. The NO symbol is a literal gap in a single wire line with a bridge. Focuses on function over physical layout. | Electrical Technology: IEC Symbols |
| BS 3939 (Legacy) | United Kingdom (Pre-1990s panels) | Similar to IEC but often uses older cross-hatching or distinct actuator dashes. Rarely used in new designs but prevalent in retrofits. | Historical BS Standards Archive |
Which one should you use? If you are wiring a new control panel in North America using Allen-Bradley or Eaton components, draft your schematics in NEMA format. If you are building a system using Schneider Electric, Siemens, or ABB components for international deployment, strictly adhere to IEC 60617. Mixing the two on a single drawing is a fast track to wiring errors.
Rows People Get Wrong: Faded Labels & Contact Confusion
The most dangerous mistake with push button contact blocks (like those on a Schneider Harmony XB4 or Eaton RMQ-Titan series) is assuming the physical position of the block dictates its logic. Just because a contact block is mounted on the left side of the operator does not mean it is Normally Open.
Industrial environments destroy ink. If the 'NO' or 'NC' paint is rubbed off the contact block, or the schematic symbol is illegible on a 30-year-old machine print, never guess based on wire color or terminal position. A green wire is not guaranteed to be a NO start circuit. To safely interpret an unmarked block:
- Lock out and tag out (LOTO) the panel. Verify zero energy with a known-working CAT III/IV meter.
- Set your multimeter to continuity mode (the beep/diode setting).
- Place probes on the two screw terminals of the suspect block.
- If it beeps without pressing the button: It is Normally Closed (NC).
- If it is silent, and only beeps when you press the button: It is Normally Open (NO).
Another frequent error involves terminal numbering. On IEC-standard contact blocks, 13 and 14 are strictly reserved for Normally Open contacts, while 21 and 22 are Normally Closed. If you are tracing a wire labeled '14' on an IEC print, you know with 100% certainty it originates from a NO contact, even if you cannot see the switch itself. NEMA uses 3 and 4 for NO, and 1 and 2 for NC.
Frequently Asked Questions
What does a normally open push button symbol look like on a PLC ladder diagram?
In PLC programming environments like Allen-Bradley Studio 5000 or Siemens TIA Portal, the physical schematic symbol is replaced by ladder logic instructions. A physical NO push button wired to a PLC input is represented by the Examine If Closed (XIC) instruction, which looks like two vertical parallel lines with a gap between them: --| |--. When the physical button is pressed, 24VDC hits the input card, the PLC bit goes high, and the --| |-- instruction 'closes' in the software, allowing logic to flow to the output coil.
How to test a normally open push button symbol circuit with a multimeter?
Testing the live circuit requires a different approach than testing the isolated block. If the machine is powered and you need to verify the NO push button is actually passing voltage when pressed:
- Set your multimeter to AC or DC Voltage, matching the control circuit (usually 24VDC or 120VAC).
- Place your black (common) probe on a known good ground or the negative DC bus.
- Place your red probe on the line side (terminal 13 or 3) of the push button. You should read full control voltage at all times.
- Move the red probe to the load side (terminal 14 or 4). With the button released, you should read 0V. Press the button; the meter should immediately jump to full control voltage.
Why does my normally open push button symbol show two contact bars?
If you see a normally open push button symbol with a second, identical set of contacts drawn directly below it on the schematic, you are looking at a 2NO (Two Normally Open) contact block assembly. This is common in safety circuits or high-reliability motor starters. Pressing the single physical button mechanically closes both independent sets of contacts simultaneously. One set might be wired to a PLC input for logic monitoring, while the second set is wired in series with a main contactor coil for direct hardware actuation, providing redundancy.
