The universal symbol for a normally open (NO) contact is a pair of parallel lines separated by a gap, intersected by an actuator line. However, the exact geometry changes based on your region. In North American NEMA schematics, the contact gap is drawn vertically with a horizontal actuator. In international IEC 60617 schematics, the gap is horizontal and the actuator is diagonal. "Normal" always refers to the de-energized, unactuated, resting state of the device.
The Complete Reference Table: Symbols for Normally Open Contacts
Before wiring a panel or troubleshooting a ladder logic diagram, you must identify which drafting standard the original engineer used. Mixing up NEMA and IEC symbols can lead to miswired safety circuits or failed interlocks. The table below maps the visual geometry, terminal designators, and real-world component equivalents for normally open contacts across global standards.
| Standard / Region | Contact Type | Visual Geometry & Symbol | Terminal Designators | Typical Component Example |
|---|---|---|---|---|
| NEMA (US / Canada) | Standard NO | Vertical parallel gap, horizontal actuator line crossing it | Manufacturer specific (e.g., 13/14 or A/B) | Allen-Bradley 100-C Contactor |
| IEC 60617 (Global / EU) | Standard NO | Horizontal parallel gap, diagonal actuator line | 13 (Input) / 14 (Output) | Schneider Electric TeSys D |
| NEMA (US / Canada) | NO Time-Delay (On-Delay) | Vertical gap, horizontal actuator with a left-pointing arrow | Manufacturer specific | Eaton C440 Motor Management Relay |
| IEC 60617 (Global / EU) | NO Time-Delay (On-Delay) | Horizontal gap, diagonal actuator with left cross-hatch | 67 (Input) / 68 (Output) | Siemens SIRIUS 3RU2 Timer |
| Old UK (BS 3939 Legacy) | Legacy NO | Circle with a diagonal cross-line, or simple unadorned gap | N/A (Pre-standardization) | Legacy GEC Motor Starters (Pre-1990) |
Note: For IEC 60617, the terminal numbers are strictly standardized. Odd numbers (13, 67) are the line-side inputs, and the sequential even numbers (14, 68) are the load-side outputs for NO contacts. For NC contacts, IEC uses 21/22 or 31/32.
Regional Standards: Which Symbol Applies to Your Panel?
When you open a control panel, the symbols printed on the door or drawn on the schematic inside dictate the standard. The two dominant frameworks are NEMA (National Electrical Manufacturers Association, heavily tied to ANSI Y32.2 and NFPA 70 practices) and IEC (International Electrotechnical Commission standard 60617).
NEMA / North American Practice:
In US and Canadian facilities, NEMA symbols dominate. The Rockwell Automation NEMA and IEC comparison guides highlight that NEMA symbols are highly pictorial. A normally open pushbutton looks like a physical button pressing down on a horizontal bar, which bridges a vertical gap. NEMA symbols were designed to be easily understood by maintenance mechanics on the shop floor without requiring deep electrical theory knowledge. Terminal numbering on NEMA devices is largely left to the manufacturer, though many adopt 13/14 for NO auxiliary contacts by convention.
IEC 60617 / Global Practice:
IEC symbols are minimalist and abstract. The symbol for a normally open contact is simply two short horizontal parallel lines with a gap between them, and a diagonal line resting against the top line to represent the actuator. The All About Circuits reference library notes that IEC relies heavily on standardized alphanumeric terminal designators rather than pictorial representations. If you see a 13/14 designation next to a minimalist horizontal gap, you are looking at an IEC NO contact. IEC schematics separate the control logic (ladder diagram) from the physical layout, meaning the symbol tells you the logical function, not what the physical button looks like.
Legacy UK (BS 3939):
If you are retrofitting an older facility in the UK or Commonwealth nations, you may encounter BS 3939 symbols. These often use a circle with a diagonal line to denote a switch or contact. While BS 3939 was officially superseded by BS EN 60617 (the UK adoption of IEC) decades ago, replacement panels in legacy water treatment or rail systems still occasionally feature these hybrid drawings.
Rows People Get Wrong & Faded Marking Protocols
Even experienced technicians misinterpret schematic symbols when fatigue sets in or when dealing with poorly maintained documentation. Here are the specific rows and concepts from the reference table that cause the most field errors.
Rows People Get Wrong
- Confusing NO with NC (Normally Closed): In NEMA, a NC contact has a diagonal line crossing the horizontal actuator, indicating the circuit is complete until pushed. In IEC, the diagonal actuator line physically crosses through the horizontal gap lines. If the actuator line stops before crossing the gap, it is NO. If it crosses the gap, it is NC.
- Misunderstanding "Normal": The most dangerous mistake is assuming "Normal" means the machine's running state. In electrical theory, "Normal" strictly means the de-energized, unactuated, shelf state. A fire alarm pull station uses a normally open contact. When the machine is "normal" (no fire), the contact is open. When actuated (pulled), it closes. Never size a safety circuit based on the running state; always base it on the de-energized state.
- Time-Delay Arrows: In the IEC time-delay rows, a left-pointing arrow or cross-hatch on the actuator means "On-Delay" (closes X seconds after the coil energizes). A right-pointing arrow means "Off-Delay" (opens X seconds after the coil de-energizes). Swapping these in a motor star-delta starter will cause a dead short across the line.
Safe Interpretation When Markings Are Faded or Missing
In aging panels, the schematic sticker on the inside of the door is often faded, oil-stained, or entirely missing. Furthermore, auxiliary contact blocks (like a Schneider LADN11) may have their printed terminal numbers worn away by years of thermal cycling and vibration. Never guess whether a contact is NO or NC based on wire color or physical position.
The Definitive Multimeter Protocol for Unmarked Contacts:
- Isolate the Circuit: Ensure the control circuit is de-energized and locked out.
- Set the Meter: Switch your multimeter to Continuity mode (the diode/sound wave icon) or the lowest Ohms (Ω) range.
- Zero the Probes: Touch the probe tips together. The meter should read < 0.5 ohms and beep. This confirms your leads are intact.
- Measure the Resting State: Place the probes across the two terminals of the unmarked contact block.
- If the meter reads OL (Open Loop) or infinite resistance, the contact is Normally Open (NO).
- If the meter reads < 1 ohm and beeps, the contact is Normally Closed (NC).
- Verify Actuation: Manually press the contactor armature or pushbutton with an insulated tool. The OL reading should drop to < 1 ohm (for NO), or the < 1 ohm reading should snap to OL (for NC). If the resistance fluctuates wildly or reads in the hundreds of ohms, the contact pitting is severe and the block must be replaced.
By relying on the physical continuity test rather than faded ink, you guarantee that your replacement wiring matches the actual mechanical state of the hardware, ensuring the control logic operates exactly as the original engineer intended.
