A normally closed contact symbol represents a switch, relay, or contactor contact that conducts current in its unactuated, resting state. When the device is triggered (e.g., a coil is energized or a button is pressed), the contact opens, breaking the circuit. In NEMA (US) schematics, it is drawn as two parallel lines with a diagonal slash; in IEC (global) schematics, it is a single moving line intersecting a fixed point with a diagonal slash.
The Complete NC Contact Symbol Reference Table
Use this reference to identify the specific flavor of normally closed (NC) contact you are looking at on a schematic. The table below covers standard switches, pushbuttons, and time-delay variants across the two dominant global standards.
| Component Type | NEMA Symbol Description (US/Canada) | IEC Symbol Description (EU/Global) | Typical Application |
|---|---|---|---|
| Standard NC Switch | Two parallel vertical lines crossed by a diagonal slash. No external actuator drawn. | A single vertical line (moving contact) resting against a fixed point, crossed by a diagonal slash. | Limit switches, pressure switches, or disconnect interlocks. |
| NC Pushbutton | Parallel lines with a diagonal slash, topped by a horizontal line and a downward-pointing arrow (manual actuator). | Single line with a slash, topped by a horizontal line and a downward arrow. | Stop buttons, Emergency Stops (E-Stops), and interlock reset buttons. |
| Relay/Contactor NC Contact | Parallel lines with a slash, linked via a dashed line to the controlling coil. | Single line with a slash, linked via a dashed line to the controlling coil. | Electrical interlocks (preventing two contactors from closing simultaneously). |
| Time-Delay NC (Off-Delay) | Standard NC symbol with an arrow pointing away from the contact intersection. | Standard NC symbol with an arrow pointing away from the contact intersection. | Motor coasting circuits; cooling fans that run after the main drive stops. |
| Time-Delay NC (On-Delay) | Standard NC symbol with an arrow pointing toward the contact intersection. | Standard NC symbol with an arrow pointing toward the contact intersection. | Soft-start bypass circuits; delayed alarm triggering. |
Regional Standards: NEMA vs. IEC vs. Legacy UK
Interpreting a schematic correctly requires knowing which standard the original engineer used. Mixing up NEMA and IEC conventions is a primary cause of wiring errors in imported machinery.
- NEMA (National Electrical Manufacturers Association): Dominant in the US and Canada. NEMA uses two parallel lines to represent all contacts (both Normally Open and Normally Closed). The only difference is the diagonal slash for NC. This can make complex schematics look cluttered, as every contact looks like a small ladder rung. Refer to the NEMA electrical control diagrams standard for exact drafting rules.
- IEC (International Electrotechnical Commission): The global standard (IEC 60617), dominant in Europe, Asia, and increasingly in modern US OEM panels. IEC uses a single line for the moving contact. A Normally Open (NO) contact is drawn with a gap; an NC contact is drawn with the line touching the fixed point and crossed by a slash. IEC symbols are generally more compact and visually distinct. See the IEC graphical symbols database for official vector definitions.
- Legacy UK (BS 3939): If you are troubleshooting a panel in the UK built before the 1990s, you may encounter BS 3939. It is visually similar to IEC (single-line contacts) but uses different actuator representations and older numbering conventions. Treat them as IEC for the contact itself, but verify the actuator mechanics physically.
Rows People Get Wrong (and How to Avoid Them)
When reading the reference table above, technicians frequently misinterpret three specific symbol variations, leading to incorrect part orders or dangerous wiring faults.
People frequently wire an "Off-Delay" NC contact (arrow pointing away) as a standard NC contact. If you do this on a cooling fan circuit, the fan will stop immediately when the machine powers down, rather than running for the programmed 5-minute cooldown. Rule of thumb: The arrow points in the direction of the delay action.
Mistake 2: Ignoring the Actuator (The Top Half)
The bottom half of the symbol (the parallel lines or single line with a slash) only tells you the contact is NC. The top half tells you what opens it. A common error is ordering a standard NC pushbutton when the schematic actually shows an NC limit switch (indicated by a roller or lever actuator symbol above the contact). The electrical behavior is identical, but the physical mounting and activation method are completely different.
Mistake 3: Form C (SPDT) Terminal Confusion
On standard 8-pin "ice cube" relays (like the widely used Omron LY2N or Schneider RXM), the NC contact does not have its own dedicated pair of isolated terminals. It shares a Common (COM) terminal with the NO contact. Technicians often look for a dedicated "NC1" and "NC2" on the pinout diagram, not realizing they must wire between the COM pin (e.g., Pin 1) and the NC pin (e.g., Pin 4).
Safe Interpretation: Testing Faded or Missing Markings
In the field, schematics are often missing, faded, or inaccurate due to undocumented field modifications. Never assume a contact is NC based on a guess. Use a digital multimeter (DMM) like a Fluke 87V or 117 to verify the state safely.
- De-energize and Lock Out: Turn off the main disconnect and apply your Lockout/Tagout (LOTO) device. Verify the circuit is dead by testing a known live source, then the target circuit, then the known live source again (Live-Dead-Live test).
- Identify Standard Terminals: If the physical markings are worn off, rely on IEC standard terminal numbering. On a standard 3-pole contactor, the auxiliary NC contacts are typically 21 and 22. On a thermal overload relay, the NC trip contact is 95 and 96. (Conversely, NO contacts are 13-14 and 97-98).
- Test Resting State: Set your DMM to continuity or resistance (Ω). Place probes across the suspected NC terminals. You should read < 1 ohm (and hear a continuity beep). If you read OL (Overload) or infinite resistance in the resting state, it is a Normally Open contact, or the contact is damaged/welded open.
- Test Actuated State: Manually press the contactor armature or pushbutton. The resistance should immediately jump to OL (infinite). If it stays near 0 ohms, the contacts are welded together—a critical failure mode that requires immediate component replacement.
For deeper diagnostic techniques on industrial relays, refer to the Fluke guide on testing relays.
Frequently Asked Questions
What does a normally closed contact symbol look like on a PLC wiring diagram?
On the physical wiring diagram, the PLC input (e.g., I:0/0) is shown connected to a standard NC pushbutton symbol. However, this creates a notorious "double negative" logic trap in the PLC software. Because the physical NC button passes 24V to the PLC input when resting, the PLC sees a logical TRUE (1) when the button is NOT pressed. Therefore, in the ladder logic code, the programmer must use a Normally Open (Examine If Closed / XIC) software instruction to represent the physical NC stop button. If you use a software NC instruction (Examine If Open / XIO), the machine will never run.
How do I wire a normally closed push button for a hardwired emergency stop circuit?
Emergency Stops (E-Stops) must always use NC contacts. This is a fundamental fail-safe principle: if a wire breaks, a terminal vibrates loose, or the circuit loses power, the continuity is broken, which the safety relay interprets exactly the same as someone pressing the E-Stop button. To wire it, connect the 24V supply to one side of the NC E-Stop contact, and wire the other side to the input channel (e.g., S11/S12) of a dedicated safety relay (like a Pilz PNOZ or Allen-Bradley MSR series). Never wire an E-Stop directly to a standard PLC input or a basic contactor coil without a safety relay intermediary, as this defeats diagnostic wire-break detection.
Why did my motor starter trip immediately when I wired the NC auxiliary contact in series?
This usually happens when a technician confuses the contactor's auxiliary NC contact (terminals 21-22) with the overload relay's trip NC contact (terminals 95-96). If you wire the auxiliary NC contact (21-22) in series with the contactor's own coil to create a holding circuit, the moment the contactor pulls in, the NC contact opens, instantly dropping power to the coil. The contactor will chatter violently or fail to latch. For a standard 3-wire start/stop holding circuit, you must wire the NO auxiliary contact (13-14) in parallel with the Start button, and place the overload NC trip contact (95-96) in series with the coil to break the circuit only during a thermal fault.
