Electric symbols are the universal shorthand for circuit design, but "universal" is a misnomer. Depending on whether you are reading a modern European PLC manual or a 1980s American motor control center drawing, the glyphs change entirely. The two dominant standards governing these schematics are IEC 60617 (International) and IEEE 315 / NEMA (North American industrial). Misinterpreting a symbol can lead to catastrophic wiring faults or destroyed components. Below is the master reference chart to decode both standards.

Standard Electric Symbols Reference Chart

Component IEC 60617 Symbol Description IEEE 315 / NEMA Symbol Description Practical Meaning & Application
Resistor Empty rectangle (inline) Zigzag line Limits current flow. IEC uses the rectangle for all passive impedance; NEMA reserves zigzag for resistors and uses rectangles for relays/coils.
Capacitor (Non-Polarized) Two parallel vertical lines Two parallel vertical lines Stores electrical charge. Used in AC coupling, filtering, and snubber circuits where polarity does not exist.
Capacitor (Polarized) One straight line, one curved line (with + sign) One straight line, one curved line (with + sign) Electrolytic/Tantalum capacitor. The curved plate denotes the negative (outer foil) terminal. Reversing polarity risks violent venting.
Inductor / Coil Series of half-circles (loops) Series of touching loops or a rectangle (IEC style often adopted in modern NEMA) Opposes changes in current. Used in chokes, transformers, and relay coils. NEMA often adds a diagonal arrow for variable inductors.
Diode Triangle pointing to a vertical line Triangle pointing to a vertical line Allows current in one direction. The triangle points in the direction of conventional forward current flow (anode to cathode).
Normally Open (NO) Contact Two parallel lines with a gap, bridged by a diagonal slash Similar, but often drawn with a distinct hinge dot Switch or relay contact that is open when de-energized. Closes to complete the circuit when the coil is activated.
Normally Closed (NC) Contact Two parallel lines overlapping, crossed by a diagonal slash Overlapping lines with a hinge dot and cross slash Switch or relay contact that is closed when de-energized. Opens to break the circuit when the coil is activated.
Earth Ground Three decreasing horizontal lines beneath a vertical stake Identical three-line ground symbol Physical connection to the earth via a grounding rod. Provides a safe path for fault currents to trip the breaker.

For a deeper dive into specific logic gate and semiconductor symbols, the All About Circuits Electrical Symbol Reference provides an excellent supplemental database.

Regional Variants: IEC 60617 vs IEEE/NEMA vs Old UK

Knowing which standard applies to your region—and more importantly, which standard the original equipment manufacturer (OEM) used—is critical for safe interpretation.

  • IEC 60617 (International / Modern Global): The dominant standard in Europe, Asia, and modern globalized machinery. IEC favors geometric abstraction. Resistors are rectangles, and circuit breakers are represented by a simple line with a manual toggle mark. IEC schematics also heavily rely on alphanumeric reference designators (e.g., Q1 for breakers, K1 for contactors, F1 for fuses) rather than drawing pictorial representations of the devices.
  • IEEE 315 / NEMA (North America): The legacy and industrial standard in the US and Canada. NEMA symbols are more pictorial. A resistor is a zigzag (resembling the old carbon composition physical build), and a thermal overload relay is drawn with a distinct heater element loop. If you are troubleshooting a legacy US manufacturing panel built before 2010, you will almost exclusively encounter NEMA symbols.
  • Old UK (BS 3939 / Pre-2004 BS 7671): Older British schematics often mixed IEC logic symbols with unique mechanical switch representations. While the UK has fully harmonized with IEC for modern designs, maintenance electricians working in older UK infrastructure will still find archaic symbols for crossbar switches and specific telegraph-style grounds. Always check the drawing's title block for the governing BS or IEC standard revision year.

Rows People Get Wrong & Faded Marking Protocols

Even experienced bench technicians and journeyman electricians misread specific symbols under pressure. Here are the most common traps and how to handle degraded documentation.

The Rows People Get Wrong

  1. NO vs. NC Contact Slash Direction: The difference between a Normally Open and Normally Closed contact on a schematic is incredibly subtle. On an NO contact, the diagonal actuator slash rests outside or below the parallel contact lines, indicating it must move to bridge the gap. On an NC contact, the slash crosses through the overlapping lines, indicating it must move away to break the connection. Miswiring a safety interlock because you misread this slash can defeat a machine guard.
  2. Polarized Capacitor Curvature: The curved line on a polarized capacitor symbol is not just an artistic choice; it represents the physical outer foil of an electrolytic capacitor, which is tied to the negative terminal. If a schematic is printed poorly and the curve looks straight, you might install a 400V DC bus capacitor backward, resulting in a dielectric failure and explosive venting.
  3. Ground vs. Chassis vs. Signal: Earth ground (three horizontal lines) connects to dirt. Chassis ground (a horizontal line with three diagonal lines underneath) connects to the metal enclosure. Signal ground (a solid downward-pointing triangle) is the 0V reference for low-voltage logic. Tying a noisy VFD chassis ground to a sensitive PLC signal ground symbol node will cause erratic communication faults.

Safe Interpretation When Markings Are Faded or Missing

When dealing with 30-year-old industrial panels, UV exposure and oil mist often fade schematic prints until the symbol distinctions vanish. Do not guess based on context clues.

⚠️ SAFETY WARNING: Never trace or probe a faded mains-voltage schematic while the panel is energized. De-energize the main disconnect, apply a lockout/tagout (LOTO) device, and verify dead with a tested CAT III/IV multimeter before opening the enclosure.

The Protocol:
First, locate the alphanumeric reference designators (e.g., 1CR for Control Relay 1 in NEMA, or K1 in IEC). Even if the coil symbol is faded into a smudge, the designator tells you exactly what the component is. Second, use your multimeter in continuity mode to physically trace the wiring from the component's terminal block back to the PLC I/O or power supply. Finally, cross-reference the physical part number stamped on the component's casing (e.g., an Allen-Bradley 100-C09 or a Schneider LC1D09) with the manufacturer's modern datasheet to confirm its internal contact layout (NO/NC).

Electric Symbols FAQ

What is the difference between IEC and NEMA electric symbols?

The primary difference lies in abstraction versus pictorial representation. IEC 60617 symbols are highly abstracted and geometric (e.g., a simple rectangle for a resistor or a generic switch symbol with a functional label for a breaker). NEMA / IEEE 315 symbols are more pictorial, attempting to mimic the physical appearance or internal mechanism of the component (e.g., a zigzag for a resistor, or a specific heater loop for a thermal overload). Furthermore, IEC schematics separate the power circuit and the control logic into distinct, highly structured vertical and horizontal rails, while older NEMA drawings often mix them in a ladder-logic format.

How do you read a normally open vs normally closed contact symbol?

Look at the relationship between the diagonal actuator line and the two parallel contact lines. If the two parallel lines have a visible gap between them and the diagonal line sits adjacent to the gap, it is Normally Open (NO)—meaning the circuit is broken until the relay coil pulls the diagonal line to bridge the gap. If the two parallel lines overlap each other and the diagonal line crosses directly through them, it is Normally Closed (NC)—meaning the circuit is complete until the coil pulls the diagonal line away to break the overlap.

Why are there three different ground symbols on my schematic?

Engineers use three distinct ground symbols to separate different electrical domains and prevent noise or shock hazards. Earth Ground (three decreasing horizontal lines) is the safety ground tied to a physical copper rod in the dirt, designed to carry fault currents. Chassis Ground (a horizontal line with diagonal hash marks) represents a connection to the metal frame or enclosure of the device, used for shielding and static dissipation. Signal Ground (a downward-pointing triangle) is the 0V DC reference point for sensitive logic circuits. Keeping these isolated prevents high-current motor noise from corrupting low-voltage microcontroller signals.