An electrical contact symbol represents the physical state and switching logic of a relay, contactor, or pushbutton in a schematic. Because a misread symbol can lead to dead shorts or defeated safety interlocks, knowing exactly how your region draws a Normally Open (NO) versus a Normally Closed (NC) contact is non-negotiable bench and jobsite knowledge. Below is the definitive reference for reading these symbols across the two dominant global standards.
Master Electrical Contact Symbol Reference Table
The table below maps the most common contact configurations to their visual representations in both North American (NEMA) and International (IEC) standards. Use this as your primary lookup when tracing control circuits.
| Contact Type / Form | NEMA Symbol Description (US/NEC) | IEC Symbol Description (Global) | Practical Application & Behavior |
|---|---|---|---|
| Normally Open (NO) Form A |
Two parallel lines with a gap, bridged by an angled movable contact line. | Two parallel lines with a gap, bridged by a straight diagonal slash (/). | Start buttons, enable circuits. Passes current only when the coil is energized or the button is pressed. |
| Normally Closed (NC) Form B |
Two parallel lines touching a continuous crossbar, broken by an angled movable line. | Two parallel lines bridged by a diagonal slash, crossed by a second diagonal line (X). | Stop buttons, E-stops, limit switches. Passes current at rest; breaks circuit when actuated. |
| Changeover (SPDT) Form C |
A single movable line pivoting between two fixed contacts (one NO, one NC). | A single diagonal slash pivoting between two fixed points, often with a small triangle indicating the common terminal. | Direction reversing, interlocks. The 'Common' (C) terminal switches between NO and NC states. |
| Timed Close (NO-TC) On-Delay |
Standard NO symbol with an upward-pointing arrow or a small 'clock' box on the actuator. | Standard IEC NO slash with a small vertical line and arrow pointing up (⏐↑). | Motor star-delta transitions. Contact closes only after a set time delay once the coil is energized. |
| Timed Open (NC-TO) On-Delay |
Standard NC symbol with a downward-pointing arrow on the actuator. | Standard IEC NC slash/cross with a vertical line and arrow pointing down (⏐↓). | Braking circuits, cooling fan run-on. Contact opens only after a set time delay once the coil is energized. |
| Double Throw (DPDT) | Two Form C contacts mechanically linked by a dashed line. | Two Form C pivots mechanically linked by a dashed line. | Motor reversing contactors. Switches two independent circuits simultaneously with one coil pulse. |
NEMA vs. IEC: Regional Standard Variants
The philosophical difference between NEMA (National Electrical Manufacturers Association) and IEC (International Electrotechnical Commission) symbols dictates how you read a print. NEMA symbols, governed largely by NFPA 79 and NEMA ICS 19 standards, are highly descriptive. They often draw the physical mechanical linkage explicitly, making it easier for a novice to visualize the physical switch. IEC symbols, standardized under IEC 60617, are minimalist and geometric. They prioritize drawing speed and compact schematic layouts over physical resemblance.
A major point of divergence is the mechanical linkage. In NEMA, a dashed line physically connects a relay coil to its contacts. In IEC, the coil and contacts are often drawn on completely different pages of the schematic, linked only by an alphanumeric reference designator (e.g., Coil 'K1' on page 4 actuates contact 'K1' on page 12). If you are used to NEMA, IEC prints will initially look like the contacts are floating with no actuator.
The 'Rows People Get Wrong' Guide
Even experienced electricians and panel builders misinterpret specific rows in the reference table. Here are the most common field mistakes:
- Confusing Timed-Close (TC) with standard NO: On a time-delay relay, a 'Timed Close' contact is physically open at rest, just like a standard NO. However, if you wire it into an E-stop chain assuming it will drop out immediately when power is cut, you will fail. TC contacts remain closed until the time delay expires after power is removed (if it's an off-delay variant). Always verify if the timer is On-Delay or Off-Delay by checking the arrow direction or the manufacturer's timing diagram.
- Miswiring the Form C (SPDT) Common Terminal: People often treat the three terminals of a Form C contact as three independent points. The center terminal is the Common (C) pivot. Power must enter (or leave) through C. If you wire Line to the NO terminal and Load to the NC terminal without utilizing C, the circuit will never function.
- Ignoring the Actuator Symbol on Pressure/Temperature Switches: A pressure switch uses the exact same electrical contact symbol (NO or NC) as a pushbutton. The difference is the actuator symbol attached to it. A pushbutton has a manual actuator line; a pressure switch has a zigzag or arrow pointing into a circle. If you ignore the actuator symbol, you might troubleshoot the electrical contact with your finger instead of checking the system pressure.
Safe Interpretation for Faded or Missing Schematics
When you are retrofitting an old panel and the schematic is missing, or the printed markings on a physical contactor (like an Eaton XTCE or legacy NEMA block) are faded beyond legibility, you must safely identify the contacts using a multimeter. Never guess based on physical proximity.
- De-energize and Lockout: Shut off the main breaker, apply your LOTO (Lockout/Tagout) device, and verify zero energy with a proven voltage tester (like a Fluke 117 or 1000V CAT III rated tester).
- Identify the Coil: Set your DMM to resistance (Ohms). Probe the suspected coil terminals (marked A1/A2 on IEC, or simply two isolated pins on NEMA). You should read a specific resistance (typically 10 to 500 ohms depending on voltage and size). If it reads infinite (OL), the coil is open; if it reads 0.1 ohms, it is shorted.
- Map the Contacts via Continuity: Switch your DMM to continuity mode (the beeper). Place probes across the suspected contact terminals.
- If it beeps at rest, it is NC.
- If it is silent at rest, use a small insulated tool or your finger to manually depress the relay armature or contactor crossbar. If it beeps when depressed, it is NO.
- If it beeps at rest, and when depressed the beep stops on one pin but moves to an adjacent pin, you have found the Common (C), NC, and NO terminals of a Form C block.
Frequently Asked Questions
What does the electrical contact symbol for a relay look like compared to a manual switch?
The core contact symbol (the NO or NC lines) is identical for both. The difference lies in the actuator. A manual switch symbol features a simple lever or pushbutton actuator line. A relay electrical contact symbol features a small rectangle (representing the electromagnetic coil) attached to the contact via a dashed mechanical linkage line (in NEMA) or shares an alphanumeric reference tag like 'K1' (in IEC). The contact itself doesn't 'know' whether a human finger or a magnetic field is moving it.
How do I read an electrical contact symbol with a diagonal line through it?
If you are looking at an IEC schematic, a diagonal slash (/) bridging two parallel lines represents a Normally Open (NO) contact. If there is a second diagonal line crossing the first one to form an 'X' or a cross, it represents a Normally Closed (NC) contact. The diagonal line represents the movable physical contact arm inside the switch mechanism.
What is the difference between a momentary and maintained electrical contact symbol?
A momentary contact (like a standard doorbell or start button) is drawn with a standard NO or NC symbol and a simple manual actuator. A maintained contact (like a toggle switch or selector switch that stays in position) is drawn with the same contact lines, but the actuator symbol includes a small 'latch' mechanism, a detent symbol (a small zigzag or notch), or a dotted line indicating it stays in the thrown position until manually reset.
Why do some electrical contact symbols show a small rectangle next to the switch line?
That small rectangle represents the actuating coil of a relay or contactor. In NEMA schematics, it is often drawn directly adjacent to the contacts it controls to show the mechanical linkage. In IEC schematics, the rectangle represents the coil on the power/control page, while the contacts are drawn elsewhere. If the rectangle has a diagonal line through it, it indicates a relay coil with a built-in suppression diode or RC snubber, common in DC control circuits to prevent inductive voltage spikes from damaging PLC outputs.
