Standard electrical schematic symbols fall into two dominant global frameworks: IEEE 315 (ANSI/NEMA, used primarily in North America) and IEC 60617 (used in Europe and most international markets). Misinterpreting a symbol from the wrong standard can lead to miswired control circuits or blown components. Below is the direct reference mapping you need for the bench or the jobsite.
The Master Symbol Reference Table
This table maps the most common components across the two major standards. Keep this bookmarked when reading imported equipment manuals or legacy domestic prints.
| Component | IEEE 315 (ANSI/NEMA) | IEC 60617 | Bench & Practice Note |
|---|---|---|---|
| Resistor | Zigzag line | Empty rectangle | IEC variable resistors add a diagonal arrow through the rectangle. IEEE adds an arrow pointing at the zigzag. |
| Capacitor (Non-Polarized) | Two parallel straight lines | Two parallel straight lines | Identical in both. Ensure the gap between lines is clear; if bridged, it is a short, not a capacitor. |
| Capacitor (Polarized) | One straight line, one curved line (+ on straight side) | Two straight lines, with a '+' sign on the anode side | Critical difference: IEC does not use the curved plate. Look for the '+' polarity marker on IEC prints. |
| Inductor / Coil | Series of loops or humps (usually 3 or 4) | Series of half-circles or full circles | Do not confuse with a relay coil. Inductors are in-line with the power path; relay coils branch to a control voltage. |
| Relay / Contactor Coil | Circle or rectangle, sometimes with diagonal hash lines | Rectangle, often labeled with a function letter (e.g., 'K' for relay, 'KM' for contactor) | IEC relies heavily on alphanumeric designators (K1, K2) next to a plain rectangle to define the coil type. |
| NO Contact (Normally Open) | Gap with a hinged bridge that swings up/away | Gap with a slanted line pointing up-right | When energized, the bridge closes. In IEC, the line does not touch the fixed pin when de-energized. |
| NC Contact (Normally Closed) | Hinged bridge resting on the fixed pin, swinging away when actuated | Slanted line pointing down-right, intersecting/touching the fixed pin | IEC NC contacts feature a distinct intersection 'X' or slash through the fixed pin line. |
| Ground / Earth | Three descending horizontal lines (or a solid triangle pointing down) | Three descending lines, or an empty downward-pointing triangle | Distinguish between signal ground (often a simple line or triangle) and safety earth (the 3-line fork). |
Rows People Get Wrong (And How to Read Faded Prints)
Even with a perfect reference table, real-world schematics are rarely perfect. Coffee stains, sun fading, and poor toner contrast on 1980s dot-matrix prints routinely obscure critical details. Here is how to safely interpret ambiguous symbols without guessing.
The Capacitor vs. Battery Trap
In IEEE 315, a battery is represented by alternating long and short parallel lines (the long line is the positive terminal). A non-polarized capacitor is two equal-length parallel lines. On a heavily faded print, the 'short' line of a battery cell can fade to look identical to the 'long' line, making a 4-cell battery pack look like a bank of four capacitors. The fix: Trace the node. If the component bridges the main DC bus and ground, it is almost certainly a filter capacitor bank. If it is the primary source feeding a low-voltage control board, it is a battery. Verify with a multimeter: a battery will read a steady DC voltage (e.g., 12.4V for a lead-acid); a capacitor will read 0V once discharged and will show a momentary continuity spike when your DMM probes are applied in resistance mode.
NO vs. NC Contacts on Faded IEC Prints
This is the most dangerous misread in industrial controls. In the IEC standard, a Normally Closed (NC) contact is drawn with a slanted line intersecting the fixed pin. If a coffee stain or toner smear obscures that intersection point, it looks exactly like a Normally Open (NO) contact (which just has a gap). Wiring a safety interlock or an emergency stop circuit based on a misread NC contact can result in a machine failing to stop when the E-Stop is pressed.
Never assume a contact state from a degraded print. De-energize the control circuit, lock out/tag out the main breaker, and use your DMM in continuity mode. Place probes across the physical contact terminals. If it reads < 1 ohm with the actuator unpressed, it is NC. If it reads OL (Open Line), it is NO. Always verify the physical hardware against the schematic before re-energizing.
Inductors vs. Relay Coils
Both use loop or circle symbols. The confusion arises when tracing power paths. An inductor (choke) is placed in series with the load to filter high-frequency noise or limit current. A relay coil is placed in parallel across a control voltage (e.g., 24VDC to GND) to actuate a magnetic field. If the symbol bridges Line and Neutral directly, it is an inductor or transformer winding. If it bridges a control voltage and a switching transistor or mechanical switch, it is a relay coil.
Regional Standards: Which One Applies to You?
Knowing which standard you are looking at dictates how you read the entire document. Equipment imported from Germany will use IEC; a control panel built in Ohio will use IEEE/NEMA. Mixing them on a single drawing is a drafting error, but it happens frequently in modified legacy systems.
IEEE 315 / ANSI Y32.2 / NEMA (North America)
Dominant in the United States, Canada, and Mexico. This standard favors pictorial representations—the zigzag resistor literally looks like a wire wrapped around a form, and the hinged switch looks like a physical knife switch. If you are working on residential wiring, commercial HVAC, or domestic industrial panels (like Allen-Bradley MCCs), you will primarily encounter IEEE symbols. For a comprehensive breakdown of North American drafting practices, the NEMA standards directory remains the authoritative baseline for industrial control nomenclature.
IEC 60617 (International / Europe / Global)
Dominant in the EU, UK, Australia, and most of Asia. The IEC standard abandoned pictorial shapes in favor of geometric abstraction. A resistor is just a box; a coil is just a box. The differentiation relies entirely on alphanumeric designators (e.g., 'R' for resistor, 'C' for capacitor, 'K' for relay) printed next to the symbol. If you are troubleshooting a Siemens PLC cabinet or a Schneider Electric VFD, you must read the letter designators, not just the shapes. The All About Circuits reference guide provides an excellent cross-walk for hobbyists transitioning from IEEE to IEC layouts.
Legacy UK (BS 3939) and Obsolete Variants
If you are retrofitting a facility in the UK or Commonwealth nations built before the late 1980s, you may encounter BS 3939. This standard was largely harmonized with IEC, but older prints feature unique quirks, such as drawing resistors as empty rectangles but using a distinct 'bow-tie' symbol for certain transformers. If you find a symbol that matches neither IEEE nor modern IEC, check the title block of the schematic for the drafting standard revision year. If it predates 1990, treat every unfamiliar symbol as a black box and trace its electrical behavior with a meter rather than relying on visual identification.
