When reading a schematic or tracing a control panel, misidentifying a single electric scheme symbol can lead to a dead short, a miswired motor starter, or a fried microcontroller. Below is the direct reference data you need, followed by regional differences and field troubleshooting protocols.
Master Reference Table: Common Electric Scheme Symbols
The following table maps the most frequently encountered components to their respective international and North American standard representations. Use this as your bench-side cheat sheet.
| Component | IEC 60617 Symbol Description | NEMA / ANSI (IEEE 315) Description | Practical Meaning & Application |
|---|---|---|---|
| Resistor | Empty rectangle | Zigzag line | Limits current flow. In IEC, look for the 'R' designator to differentiate from coils. |
| Capacitor (Non-Polarized) | Two parallel lines | Two parallel lines (one curved for electrolytic) | Stores electrostatic energy. Used for filtering, decoupling, and AC coupling. |
| Capacitor (Polarized) | One straight line, one curved line with '+' sign | One straight line, one curved line with '+' sign | Electrolytic/Tantalum. Must observe polarity; reverse bias causes catastrophic failure. |
| Inductor / Coil | Series of half-circles (loops) | Series of half-circles (loops) | Stores magnetic energy. Used in chokes, filters, and relay coils. |
| Diode | Triangle pointing to a line | Triangle pointing to a line | Allows current flow in one direction. The line represents the cathode (N-side). |
| NPN Transistor | Circle with base, collector, emitter; arrow on emitter points OUT | Circle with base, collector, emitter; arrow on emitter points OUT | Current-controlled switch. 'Not Pointing iN' is the common mnemonic for NPN. |
| Relay / Contactor Coil | Rectangle (often with 'K' or 'KM' designator) | Circle or rectangle with specific NEMA device number (e.g., 'M' for motor starter) | The electromagnetic actuator. Energizing this pulls in the mechanical contacts. |
| Normally Open (NO) Contact | Two parallel lines with a diagonal bridging line (gap visible) | Two parallel lines with a diagonal bridging line (gap visible) | Passes current only when the associated coil is energized or actuator is pressed. |
| Normally Closed (NC) Contact | NO symbol with a diagonal slash through the bridge | NO symbol with a diagonal slash through the bridge | Passes current at rest; breaks the circuit when the coil is energized. |
| Thermal Overload Relay | Rectangle with a bimetallic strip symbol and 'F' designator | Heater element symbol (box with loop) in series with NC contact | Protects motors from overcurrent by tripping a mechanical link when heat builds up. |
- IEC Relay Coil vs. IEC Resistor: Both are drawn as simple rectangles in IEC 60617. Beginners frequently wire a 24VDC control signal into a power resistor because they missed the designator letter. Always check the prefix: 'R' is a resistor, 'K' is a relay, 'KM' is a contactor.
- Polarized vs. Non-Polarized Capacitors: In a rush, builders often grab a polarized electrolytic capacitor for an AC-coupling or snubber circuit because they ignore the curved plate or the '+' sign in the schematic. Applying reverse voltage to an electrolytic cap will cause it to vent or explode.
- Ground vs. Neutral: A ground symbol (three descending horizontal lines or a downward-pointing triangle) is a safety earth or signal reference. A neutral symbol (often a circle with 'N' or a specific bus bar line) is a current-carrying conductor. Never swap them on a schematic; doing so on the bench will trip your GFCI or RCD instantly.
Regional Standard Variants: IEC 60617 vs NEMA vs Legacy
Electric scheme symbols are not universally identical. The standard you encounter depends heavily on the origin of the equipment and the era in which it was designed.
IEC 60617 (International Electrotechnical Commission): This is the modern global standard, dominant in Europe, Asia, and increasingly in North American OEM equipment. IEC favors abstract, geometric shapes. For example, all coils, resistors, and fuses are based on rectangles or simple lines, relying heavily on alphanumeric designators (like Q1 for a breaker or K1 for a relay) to define the component's function.
NEMA and ANSI/IEEE 315 (North America): Legacy North American industrial panels heavily utilize NEMA symbols, which are more pictorial. A resistor is a zigzag line, a heater element is a distinct looped box, and device functions are often denoted by standard NEMA numbers (e.g., '52' for an AC circuit breaker, '49' for a thermal overload). If you are troubleshooting an older US-based motor control center (MCC), you must use a NEMA reference chart, as IEC symbols will lead to misinterpretation.
Legacy UK (BS 3939): If you are working on British equipment installed before the 1980s, you may encounter BS 3939 symbols. These are largely obsolete but still present in legacy infrastructure. A key difference is the representation of transformers and relays, which often used intricate internal wiring diagrams rather than the simplified black-box rectangles of modern IEC.
Safe Interpretation When Markings Are Faded or Missing
On the jobsite or at the repair bench, you will frequently encounter control panels where UV exposure, heat, or oil has destroyed the silkscreen, wire labels, and component stamps. When the electric scheme symbols on the physical board are unreadable, you must rely on electrical measurement to identify the component.
Distinguishing a Relay Coil from a Shunt Resistor: Visually, a faded cylindrical wirewound component could be either. Set your multimeter to resistance (Ohms). A standard 24VDC ice-cube relay coil will typically read between 600 and 800 ohms. A current shunt resistor will read in the milliohm range (often < 1 ohm). A thermal fuse will read near 0 ohms if intact, or infinite (OL) if blown.
Identifying Solid-State vs. Mechanical Relays: If the symbol for a relay is scratched off the PCB, use your multimeter's diode test mode. Probe the input terminals. A solid-state relay (SSR) input contains an internal LED; it will show a forward voltage drop of roughly 1.1V to 1.4V. A mechanical relay coil will not behave like a diode and will simply show its DC resistance.
Tracing NO vs NC Contacts: If you cannot read the contactor diagram, set your meter to continuity. With the coil de-energized (power locked out and verified dead), probe the auxiliary contact terminals. The pair that beeps is your Normally Closed (NC) path; the pair that reads 'OL' is your Normally Open (NO) path. Physically press the contactor plunger with an insulated tool to verify the states swap.
Electric Scheme Symbols FAQ
What is the difference between a neutral and a ground symbol in electric schemes?
In schematics, a ground symbol (earth) represents a non-current-carrying safety path connected to the physical earth, depicted as three descending horizontal lines or a downward triangle. A neutral symbol represents the grounded current-carrying conductor, usually depicted as a bus line marked with 'N' or a specific wire color code (white/grey in NEC regions, blue in IEC regions). Ground is for fault clearing; neutral is for completing the 120V/230V circuit.
Why do some schematics use a circle for a relay coil while others use a rectangle?
This is a regional standard difference. The IEC 60617 standard uses a simple rectangle for a relay coil, relying on the adjacent letter 'K' to identify it. The older NEMA and ANSI/IEEE 315 standards often use a circle, or a rectangle with a specific internal loop symbol, to visually distinguish a coil from a resistor. Always check the title block of the schematic to see which standard the drafter used.
How do I identify a solid-state relay (SSR) symbol versus a mechanical relay?
A mechanical relay is drawn as a standard coil and contact set. A solid-state relay symbol typically includes a rectangle (representing the internal optocoupler and triac/MOSFET) with an input side showing a diode/LED symbol and an output side showing a switch symbol with a semiconductor arrow or a triac symbol. If you see an optocoupler symbol integrated into the switch block, you are looking at an SSR.
Are electric scheme symbols universal across all CAD software like AutoCAD Electrical and EPLAN?
No, the software defaults to the standard you select during project setup. EPLAN natively defaults to IEC 60617 and is the standard for European panel builders. AutoCAD Electrical can generate both, but many North American legacy libraries default to NEMA/JIC symbols. If you open an EPLAN IEC file in a viewer configured for NEMA, the software will not automatically translate the rectangles into zigzag resistors; it will simply display the IEC geometry. Always verify the project standard in the CAD title block before reading the prints.
