The standard symbol of push button switch components in electrical schematics depends entirely on the switch's resting state (Normally Open vs. Normally Closed) and the governing drafting standard (IEC 60617 vs. NEMA/ANSI IEEE 315). Misreading these symbols on a ladder diagram or control panel print can lead to short circuits, defeated safety interlocks, or damaged motor starters. Below is the exact reference data you need to read wiring prints without guessing.

Core Push Button Switch Symbols & Contact Ratings
Switch Type IEC 60617 Symbol Geometry NEMA/ANSI Symbol Geometry Typical Application Standard Contact Rating
Normally Open (NO) Two parallel vertical lines (terminals) with a diagonal crossbar and an upward-pointing arrow indicating momentary action. Two parallel lines with a small wedge or 'V' shape pointing toward the gap, actuated by a dashed line. Start buttons, momentary triggers, reset switches. 10A @ 250VAC (e.g., Schneider XB5)
Normally Closed (NC) Two parallel vertical lines bridged by a solid horizontal line (the closed contact), crossed by a diagonal actuator line. Two parallel lines bridged by a solid line, with a wedge pointing away from the bridge. Stop buttons, safety interlocks, limit switch overrides. 10A @ 250VAC (6A @ 24VDC inductive)
Double (1NO + 1NC) A single actuator line branching to two separate contact sets (one NO, one NC) drawn vertically aligned. Similar to IEC, but often drawn with the NO and NC contacts side-by-side on the same horizontal rung. Directional control, interposing relays, state indication. 10A per pole, 600VAC max
Illuminated NO Standard NO symbol enclosed in a circle, with a small internal 'X' or cross denoting the integrated pilot lamp. Standard NEMA NO symbol with a circle and internal cross adjacent to the contact block. Status confirmation buttons (e.g., 'Run' indicator). Lamp: 24V AC/DC LED, 20mA
Mushroom E-Stop (NC Latching) Standard NC symbol, but the actuator line features a mechanical latch icon (a small hook or locking bar) and a mushroom head. NC contact with a specific 'maintained' or 'latching' mechanical symbol (often a jagged line or lock) on the actuator. Emergency stops, main disconnect triggers, safety circuits. 3A @ 250VAC (Direct opening action)

Decoding the Standard Symbols in Practice

When you look at a schematic, the symbol tells you the electrical resting state of the switch, not necessarily its physical appearance. A 'Normally Open' (NO) symbol means that when no one is touching the button, the circuit is broken. Current only flows during the exact moment the operator holds the button down.

Consider a standard 24V DC motor control circuit. The 'Start' button is drawn with the NO symbol. When pressed, it energizes a contactor coil. However, because the NO switch is momentary, releasing the button would immediately drop power to the coil. This is why schematics pair the NO push button symbol with a parallel 'seal-in' or 'holding' contact from the main contactor. Once the contactor pulls in, its auxiliary contact bypasses the push button, keeping the circuit energized until the NC 'Stop' button breaks the line.

Safety Note on DC vs AC Ratings: Notice the rating drop in the table for DC inductive loads (6A at 24VDC vs 10A at 250VAC). DC current does not have a natural zero-crossing to extinguish arcs. If you use a standard 10A AC-rated push button to switch a 24V DC solenoid directly without an interposing relay, the internal contacts will pit and weld shut within a few hundred cycles. Always use the switch to trigger a relay or contactor coil for heavy inductive loads.

Regional & Standard Variants (IEC vs. NEMA vs. Legacy)

Which standard applies to your region dictates how the symbols are drawn on the prints you are reading. While the underlying physics are identical, the visual language differs significantly.

Standard Region of Dominance Visual Philosophy Key Identifier on Prints
IEC 60617 EU, UK, Australia, Global Export Abstract, geometric. Uses strict vertical lines for terminals and diagonal lines for actuators. Component designators use letters like 'S1', 'S2' for switches, and 'K1' for contactors.
NEMA / ANSI/IEEE 315 USA, Canada, Mexico More literal and historical. Uses wedges, dashes, and specific geometric shapes to denote switch types. Component designators often use 'PB' (Push Button), 'CR' (Control Relay), and 'M' (Motor Starter).
BS 3939 (Legacy UK) Older UK installations (Pre-1990s) Very similar to IEC, but often drew the actuator line horizontally rather than diagonally. Often found on faded, decades-old panel prints in older British manufacturing plants.

If you are working on a modern PLC-controlled panel built in 2026, it will almost certainly use IEC 60617 nomenclature, even in North America, as global PLC software platforms (like Siemens TIA Portal or Rockwell Studio 5000) default to IEC-style ladder logic symbols. However, if you are troubleshooting a legacy 480V motor control center (MCC) in the US Midwest, you will need to read NEMA/ANSI prints.

The 'Rows People Get Wrong' & Faded Marking Protocols

Even experienced technicians misinterpret specific rows in the symbol table, leading to dangerous wiring errors. Here are the most common pitfalls and how to resolve them.

Mistake 1: Confusing Standard NC with E-Stop (Row 5)

The most dangerous error in control wiring is treating an Emergency Stop (E-Stop) as a standard Normally Closed momentary switch. A standard NC push button (Row 2) is momentary—it springs back when released. An E-Stop (Row 5) is mechanically latching. When you slam a mushroom-head E-Stop, it locks in the open position and must be manually twisted or pulled to reset. Furthermore, true E-Stop contacts feature 'direct opening action' (force-guided contacts) to ensure the circuit breaks even if the contacts are welded. If a schematic shows a standard NC symbol for an E-Stop, the print is wrong; it must show the mechanical latch icon to indicate the maintained state.

Mistake 2: Misunderstanding Illuminated Switches (Row 4)

When beginners see the illuminated NO symbol (a circle with an 'X'), they often assume the pilot lamp is wired in series with the switch contacts. In practice, the lamp and the switch contacts are entirely separate circuits housed in the same physical bezel. The lamp is wired to a 24V pilot light circuit to indicate machine state, while the NO contacts switch a completely different logic line. Never wire the load through the lamp circuit.

Safe Interpretation When Markings are Faded or Missing

On older panels, the physical switch markings (like 'NO' or 'NC' printed on the contact blocks) fade, or the original schematic is lost entirely. Never guess the symbol based on the button's color (e.g., assuming green is always NO and red is always NC). Instead, map the physical switch to its schematic symbol using a multimeter.

  1. De-energize the panel. Lock out and tag out (LOTO) the main disconnect. Verify dead with a known-working voltage tester.
  2. Set your multimeter to continuity. Use the continuity/diode setting (the symbol with the sound wave). A standard Fluke 117 or similar meter will beep and display a value close to 0.00.
  3. Probe the terminals. IEC standard numbering dictates that 13/14 are NO contacts, and 21/22 are NC contacts. Place your probes across 13 and 14.
  4. Actuate the switch. Press the button. If the meter reads < 1 ohm and beeps only while pressed, you have confirmed a Normally Open (NO) contact. If the meter beeps when unpressed, and goes to 'OL' (Open Loop) when pressed, you are on the Normally Closed (NC) terminals.

By relying on direct continuity measurement rather than faded silk-screening or button color, you guarantee your physical wiring matches the logical intent of the schematic, ensuring safe and predictable machine operation.