A pushbutton symbol on a schematic defines the switch’s resting electrical state (Normal Open or Normal Closed), its mechanical actuation method (momentary or latching), and its safety classification. Misreading these symbols leads to bought-wrong-parts delays or, worse, safety relay failures on the shop floor. Below is the direct reference data you need to translate schematic symbols into physical hardware, standardized across global and regional formats.

The Complete Pushbutton Symbol Reference Table

Read this table left-to-right: identify the symbol on your print, check the standard your facility uses, and note the physical behavior. 'Normal' refers to the unactuated, resting state of the switch when no human finger or machine actuator is pressing it.

Switch Type Schematic Behavior IEC 60617 Designation NEMA / ANSI Designation Physical Action
Momentary NO Closes circuit only while pressed S001 (Standard NO) PB-NO (Normally Open) Spring return to open
Momentary NC Opens circuit only while pressed S002 (Standard NC) PB-NC (Normally Closed) Spring return to closed
Latching NO Closes circuit, stays closed until pulled S005 (Maintained NO) PB-LNO (Latching NO) Push to lock, pull/twist to release
Latching NC Opens circuit, stays open until pulled S006 (Maintained NC) PB-LNC (Latching NC) Push to lock, pull/twist to release
E-Stop (Mushroom) Opens circuit, mechanically locks open S008 (Twist/Pull release NC) PB-ES (Emergency Stop) Push to lock, twist to release
Illuminated NO Closes circuit, includes integral lamp S012 (Illuminated NO) PB-IL-NO (Lighted NO) Spring return, LED/Incandescent
Dual Contact (1NO/1NC) Simultaneously opens NC and closes NO S016 (Changeover/1NO-1NC) PB-1NO1NC (Form C) Single press actuates both blocks

Regional Standards: IEC 60617 vs. NEMA vs. Legacy UK

The pushbutton symbol you see depends entirely on when and where the control panel was designed. Applying the wrong standard's logic to a physical panel is a primary cause of wiring errors during retrofits.

  • IEC 60617 (Global Standard): Used in Europe, Asia, and modern North American facilities. Symbols are minimalist, relying on standardized alphanumeric codes (like S001 for a standard switch) and simple line-and-gap graphics. The IEC standard strictly separates the actuator symbol (the button head) from the contact symbol (the electrical state), connected by a dashed mechanical linkage line.
  • NEMA / ANSI (North America): Dominates legacy US and Canadian industrial prints. NEMA symbols are more pictorial. A pushbutton is often drawn as a literal button cap over a set of contacts. NEMA heavily utilizes letter designations inside the symbol (e.g., 'PB' for pushbutton, 'M' for motor starter).
  • Legacy UK (BS 3939): Obsolete since the 1990s but still found in older British manufacturing plants. It used a mix of pictorial and abstract symbols that often confused the actuator and contact blocks. If you are working on a UK panel built before 1995, assume BS 3939 and verify every contact with a meter.
Bench Tip: When tracing IEC schematics, the dashed line connecting the pushbutton actuator to the contact block is called the 'mechanical interlock' or 'linkage' line. If a single pushbutton actuates three separate contact blocks scattered across different pages of a 50-page schematic, the IEC standard uses cross-reference tags (e.g., '-S1 / 13-14') rather than drawing physical lines across pages.

The 'Rows People Get Wrong' Trap

When reading pushbutton schematics, two specific misinterpretations cause 90% of field troubleshooting headaches. Do not gloss over these rows.

1. The E-Stop vs. Standard NC Confusion

On a quick glance, a momentary NC pushbutton and an Emergency Stop (E-Stop) pushbutton both show a closed contact gap with a line above it. However, an E-Stop symbol in IEC 60617 includes a specific actuator symbol (a mushroom head) and a latching indicator (a small hook or twist arrow).

The physical reality: A standard NC pushbutton uses standard snap-action contacts. An E-Stop must use positive-break (forced-open) contacts compliant with EN ISO 13850. If you wire a standard NC block to a safety relay expecting an E-Stop, and the contact welds shut during a fault, the safety relay will not drop out, and the machine will not stop. Always verify the physical block has a red 'positive break' indicator mark on the side.

2. DPDT vs. Two Separate Contact Blocks

A schematic showing a pushbutton with one NO and one NC contact is often misread as a single DPDT (Double Pole, Double Throw) switch. In industrial 22mm/30mm pushbutton architecture (like the Schneider Harmony or Siemens SIRIUS lines), there is no single DPDT contact block. Instead, the physical button actuator pushes a cam that simultaneously triggers two physically separate, electrically isolated single-contact blocks stacked behind the panel. This distinction matters for clearance and wiring gauge routing in the enclosure.

Reading Faded or Unmarked Industrial Pushbuttons Safely

In older panels, thermal cycling and UV exposure melt the silk-screened NO/NC markings off the contact blocks. Never guess the state of an unmarked block based on its wire color or position.

Safety Warning: Before testing unmarked contacts, de-energize the control circuit, lock out the main disconnect, and verify dead with a Category III or IV rated voltage tester. Testing continuity on a live 120VAC/240VAC control circuit will destroy your multimeter's internal fuse and risks arc flash.

The Verification Procedure:

  1. Set your multimeter (e.g., Fluke 117 or 87V) to Continuity mode (the acoustic beep setting, typically triggering at < 40 ohms).
  2. Touch the probes to the two terminals of the unmarked contact block. The meter should beep (NC) or remain silent (NO).
  3. Physically press the pushbutton actuator with your finger or a non-conductive probe.
  4. The state must invert. If it beeped before, it must stop. If it was silent, it must beep.
  5. If the meter shows partial continuity (e.g., 15-30 ohms) instead of a hard zero/beeep, the contacts are carbon-tracked or pitted. Replace the block; do not trust it for logic control.

Decision Tree: Schematic Symbol to Physical Part Selection

Use this decision matrix to translate the pushbutton symbol on your print into a concrete, orderable part number for standard 22mm industrial control panels. This assumes a standard 24VDC or 120VAC control voltage environment.

If Your Schematic Shows... And The Application Is... Then Order This Exact Part (22mm Metal/Plastic)
Standard Momentary NO (Spring Return) Standard 'Start' or 'Reset' function Schneider XB4BA31 (Metal, Green, 1NO) or XB5AA31 (Plastic)
Standard Momentary NC (Spring Return) Standard 'Stop' function (Non-Safety) Schneider XB4BA42 (Metal, Red, 1NC)
E-Stop Mushroom Head with Twist-Release Arrow Safety circuit, EN ISO 13850 compliance required Siemens 3SU1150-0AA40-1AA0 (40mm Mushroom, Twist Release, 1NC Positive Break)
Latching/Maintained NO (Pull to Release) Selector switch alternative, stay-put 'Run' command Eaton FAZ-XB31 (Maintained NO, requires separate pull-to-release actuator head)
Illuminated Momentary NO with LED indicator Push-to-test or status-acknowledge button Schneider XB4BVB3 (Metal, Green LED 24V AC/DC, 1NO)
Dual Contact (1NO + 1NC on single actuator) Interlock bypass or state-toggle requiring both signals Schneider XB4BA3312 (Wait, standard is stacking: Order XB4BA31 + ZB4BZ101 (Add-on NC block) to build a 1NO/1NC stack).

When building dual-contact stacks (the last row), remember that the physical depth of the enclosure matters. A standard 22mm pushbutton with two stacked contact blocks requires a minimum of 65mm (2.56 inches) of clear depth behind the panel face. If you are retrofitting into a shallow NEMA 4X fiberglass enclosure, measure the internal depth before ordering multi-block stacks, or switch to a compact 16mm line like the Schneider Harmony XB7.