The symbol for a normally open (NO) switch in North American (NEMA/NEC) schematics is a gap between two horizontal contact points with a hinged actuator line resting above the gap, indicating the circuit is broken until physical force closes it. In international (IEC) standards, it is represented by two parallel horizontal terminal lines with a diagonal bridging line that does not touch the top terminal. In both conventions, the symbol is drawn in the de-energized, at-rest state.
Because switch symbols vary wildly depending on whether you are looking at a US-manufactured motor control center or an imported European PLC panel, relying on a single standard will lead to miswired circuits. Below is the complete reference for identifying NO, NC, and complex switch configurations on the bench or jobsite.
The Complete Switch Symbol Reference Table
Use this table to cross-reference the geometry of the symbol on your schematic with its physical real-world application. Note that IEC standards rely heavily on alphanumeric terminal tagging, while NEMA relies on visual geometry and descriptive device tags (like CR1 or M1).
| Switch Type | NEMA / NEC Visual Geometry | IEC Visual Geometry & Terminal Tags | Practical Application & Real-World Use |
|---|---|---|---|
| SPST Normally Open (NO) | Gap between two dots; hinged line above resting open. | Two parallel lines; diagonal line resting open. Tags: 13 and 14. | Start buttons, safety interlocks, basic lighting toggles. Passes current only when actuated. |
| SPST Normally Closed (NC) | Two dots bridged by a hinged line; a small wedge intersects the line. | Two parallel lines; diagonal line resting closed, crossed by a perpendicular tick. Tags: 11 and 12. | Stop buttons, emergency stops, thermal overloads. Breaks current when actuated. |
| Pushbutton NO (Momentary) | NO symbol with a dashed vertical line and a horizontal 'T' actuator above it. | NO symbol with a dashed vertical actuator line ending in a flat bar. Tags: 13 and 14. | Jog buttons, horn buttons, reset switches. Returns to open state when finger is removed. |
| Pushbutton NC (Momentary) | NC symbol with a dashed vertical line and 'T' actuator. | NC symbol with dashed vertical actuator. Tags: 11 and 12. | E-Stop circuits, safety door interlocks. Failsafe design: broken wire mimics a button press. |
| Limit Switch NO | NO symbol with a lever and roller wheel drawn at the actuator end. | NO symbol with a lever/roller. Tags: 13 and 14. | Conveyor end-of-travel stops, garage door limits, CNC machine homing. |
| Limit Switch NC | NC symbol with a lever and roller wheel drawn at the actuator end. | NC symbol with a lever/roller. Tags: 11 and 12. | Over-travel safety limits, elevator shaft top/bottom limits. |
| SPDT (Form C) | Single hinged line pivoting between two separate contact points (one NO, one NC). | Single pivoting line between two terminals. Tags: Common is 14, NC is 11, NO is 12. | Reversing motor controls, selector switches (Auto/Manual), HVAC changeover thermostats. |
Regional Standard Variants: NEMA vs. IEC vs. Old UK
Reading a schematic from a different region is like reading a map in a foreign language. The underlying physics are identical, but the drafting conventions change how you trace the logic. Here is how the three major standards handle switch representation.
| Standard | Region | Drafting Philosophy | Key Identifying Feature for NO Switches |
|---|---|---|---|
| NEMA / ANSI | USA, Canada, Mexico | Physical layout and ladder logic readability. Drawn to mimic the physical 'ladder' rungs of a control panel. | Relies on visual geometry (the gap and the hinged line). Device tags (e.g., 1CR, 2M) are used to link coil states to switch states across different rungs. |
| IEC 60617 | Europe, UK, Australia, Global | Functional logic and modularity. Drawn to show the logical flow of signals rather than physical panel layout. | Relies heavily on alphanumeric terminal tags. A NO contact is universally identified by the numbers 13 and 14, regardless of how the symbol is drawn on the page. |
| BS 3939 (Old UK) | Legacy UK / Commonwealth | Graphical representation of the physical mechanism. Highly detailed but cluttered. | Largely obsolete but found in pre-1990s industrial plants. Uses specific hatch marks and mechanical linkage lines that modern IEC diagrams omit. |
According to the All About Circuits reference guide on wiring symbols, the shift from NEMA to IEC in global manufacturing means modern US electricians must be fluent in IEC terminal numbering (13/14 for NO) to troubleshoot imported PLCs and VFDs.
Rows People Get Wrong (and How to Read Faded Diagrams)
Even experienced journeyman electricians and bench technicians misinterpret specific switch symbols, especially when dealing with legacy schematics, faded blueprints, or hasty CAD drawings. Here are the most common pitfalls and how to resolve them safely.
Mistake 1: Confusing a NO Pushbutton with a Maintained Toggle
If the actuator line above the switch gap has a small 'latch' or mechanical catch symbol (often a small hooked line or a cross-hatch), it is a maintained switch (like a standard wall light switch or an E-Stop twist-to-release). If it has a simple dashed line with a flat bar, it is a momentary pushbutton. Wiring a maintained switch in place of a momentary jog button will cause a motor to run continuously, potentially destroying the mechanical load.
Mistake 2: Misinterpreting the 'X' on a Limit Switch
On NEMA diagrams, an 'X' drawn on the actuator arm of a limit switch does not mean the switch is broken or crossed. It specifically denotes a roller actuator. A simple straight line denotes a lever arm, and a 'W' denotes a wobble stick. Assuming an 'X' means a fault will lead you to replace a perfectly good $40 Allen-Bradley 802T limit switch.
Safe Interpretation When Markings are Faded or Missing
If you are troubleshooting a 480V motor control circuit and the schematic is faded, grease-stained, or completely missing, never guess based on wire colors or physical switch appearance. A green button is usually NO (Start), but on imported machinery, color codes may not follow NFPA 79.
- De-energize the circuit. Lock out and tag out (LOTO) the main disconnect.
- Verify dead with a Category III or IV rated voltage tester (e.g., Fluke T6-1000).
- Set your digital multimeter to Continuity mode (the diode/audio symbol).
- Place probes across the switch terminals. If the meter reads OL (Open Loop) at rest, it is a Normally Open (NO) switch. If it reads less than 1.0 Ω and beeps, it is Normally Closed (NC).
- Actuate the switch manually. The states must invert. If they do not, the internal contacts are welded or the spring is broken.
For a deeper dive on multimeter techniques, refer to the Fluke guide on testing continuity.
Frequently Asked Questions
What is the difference between normally open and normally closed symbols?
The fundamental difference lies in the state of the contacts when no external force is applied. A Normally Open (NO) symbol shows a physical gap between the contacts, meaning no current flows at rest. A Normally Closed (NC) symbol shows the contacts bridged, often with a small perpendicular line or wedge intersecting the bridge, meaning current flows freely until the switch is pressed or actuated. In safety circuits, NC symbols are preferred because a broken wire mimics a switch press, safely shutting down the machine (fail-safe design).
How do I read a normally open switch symbol on a complex PLC ladder diagram?
In PLC ladder logic (which follows NEMA-style conventions), a Normally Open contact is represented by two parallel vertical lines (resembling a capacitor symbol: | | ). This is called an 'Examine If Closed' (XIC) instruction. When the physical NO switch is pressed, or the internal software bit goes HIGH, the | | symbol becomes 'true' and allows logic power to flow to the output coil on the right side of the rung. Conversely, a Normally Closed physical switch is represented by | / | (Examine If Open).
What does the IEC alphanumeric tag '13 and 14' mean for a normally open contact?
Under IEC 60617 and related international electrical symbol standards, terminals are numbered by function rather than sequential pinout. The first digit represents the sequence number of the contact on that specific device (e.g., 1, 2, 3 for the first, second, and third NO contacts on a single contactor). The second digit represents the function: 3 and 4 always denote a Normally Open (NO) contact, while 1 and 2 always denote a Normally Closed (NC) contact. Therefore, 13/14 is the first NO contact, and 23/24 is the second NO contact on the same relay.
Is a normally open symbol drawn in the energized or de-energized state?
All standard electrical schematics (both NEMA and IEC) draw switches, relays, and contactors in their de-energized, at-rest, shelf state. A Normally Open switch is drawn open because, sitting on a shelf with no power applied and no finger pressing it, the internal spring holds the contacts apart. If a schematic requires you to understand the circuit in an energized state, it will explicitly state 'Drawn in Energized State' in the title block, but this is exceptionally rare and considered bad drafting practice.
