If you are reading a US-based schematic, you are looking at IEEE 315 / NEMA electric drawing symbols. If you are reading a European or modern global PLC manual, you are looking at IEC 60617. Assuming these two dialects are interchangeable is the fastest way to wire a normally-open contact as a normally-closed one, potentially causing a machine to start unexpectedly. Below is the definitive reference chart to keep on your bench, followed by the regional rules and the specific symbols that trip up even experienced technicians.
The Master Electric Drawing Symbols Reference Chart
The table below maps the most critical components across the two dominant standards. Because we cannot draw the symbols directly in text, the descriptions detail the exact geometric shapes you will see on the schematic.
| Component | IEEE 315 / NEMA (US/Canada) | IEC 60617 (EU/Global) | Practical Circuit Function |
|---|---|---|---|
| Resistor | Zig-zag line (jagged peaks) | Empty rectangle (longer than wide) | Limits current flow; dissipates power as heat. Value usually noted in ohms (Ω) adjacent to the symbol. |
| Capacitor (Non-Polarized) | Two parallel straight lines separated by a gap | Two parallel straight lines separated by a gap | Stores electrical energy in an electric field. Used for filtering, decoupling, and AC coupling. |
| Capacitor (Polarized) | One straight line, one curved line (curved is negative), '+' on straight side | Two parallel lines, one thicker than the other, or standard lines with a '+' sign | Electrolytic/Tantalum capacitor. Reversing polarity can cause catastrophic venting or explosion. |
| Inductor / Coil | Series of connected semi-circles (loops) | Series of connected semi-circles OR a rectangle with a diagonal line | Stores energy in a magnetic field. Resists changes in current. Used in filters and relay coils. |
| Relay / Contactor Coil | Circle or rectangle with 'CR' (Control Relay) or 'M' (Motor Starter) inside | Rectangle with a diagonal line inside, or a circle with a specific letter code (e.g., 'K') | The electromagnetic actuator. When energized, it pulls in the mechanical contacts mapped elsewhere on the drawing. |
| Normally Open (NO) Contact | Two horizontal lines with a diagonal slash crossing them | Two horizontal lines with a diagonal line resting on the bottom one (hinge style) | Open circuit when de-energized. Closes to pass current only when the associated coil is energized. |
| Normally Closed (NC) Contact | Two horizontal lines with a diagonal slash and a small perpendicular tick mark | Two horizontal lines with a diagonal line crossing them and a small perpendicular tick | Closed circuit when de-energized. Opens to break current when the associated coil is energized. |
| Transformer (2-Winding) | Two sets of inductor loops facing each other, sometimes with parallel lines between | Two overlapping circles, or two inductor loops with parallel lines between | Steps AC voltage up or down via magnetic induction. Provides galvanic isolation between primary and secondary. |
Regional Standards: IEEE 315 vs IEC 60617 vs Legacy UK
Before you trace a single wire, you must identify which standard the draftsperson used. Mixing them up leads to misidentified contacts and blown fuses.
- IEEE 315 / NEMA (North America): The default for US and Canadian industrial panels, residential wiring diagrams, and legacy HVAC systems. NEMA symbols tend to be more pictorial (e.g., the zig-zag resistor looks like the physical wire-wound resistors of the 1950s). Reference the IEEE 315 standard documentation for the exact geometric tolerances.
- IEC 60617 (Europe / Global / Modern PLCs): The international standard. IEC symbols are highly abstract and modular. A relay coil is just a rectangle; the function is defined by the alphanumeric prefix (e.g., 'K' for relay, 'Q' for breaker). Most modern PLC manuals from Siemens, Beckhoff, and Schneider Electric use IEC.
- BS 3939 (Legacy UK): Largely superseded by IEC 60617 in the UK, but you will still encounter BS 3939 symbols in older British machinery and marine installations. It shares some DNA with early NEMA but uses unique grounding and switch notations.
Which applies to you? If the schematic has a NEMA enclosure rating (e.g., NEMA 12, NEMA 4X), expect IEEE 315 symbols. If it references IP ratings (e.g., IP65) and metric wire sizes (mm²), you are looking at an IEC 60617 drawing.
Rows People Get Wrong on the Bench
When troubleshooting under time pressure, technicians frequently misread specific electric drawing symbols. Here are the most common culprits and how to verify them.
1. Normally Open (NO) vs. Normally Closed (NC) Contacts
In IEC drawings, the NO and NC symbols are essentially mirror images of each other, differentiated only by a tiny perpendicular tick mark on the NC diagonal line. When a schematic is printed on a low-resolution plotter, that tick mark disappears. The Fix: Never trust the print blindly. Put your multimeter in continuity mode (with power locked out) and probe the physical contact block. If it beeps with the coil de-energized, it is NC, regardless of what the faded paper says.
2. Polarized vs. Non-Polarized Capacitors
In IEEE 315, the curved plate on a capacitor symbol always denotes the negative terminal. A common mistake is assuming the curved plate represents the physical curve of a ceramic disc capacitor (which is non-polarized). The Fix: If the symbol has a curved plate, it is an electrolytic or tantalum capacitor. Verify the physical component on the PCB. If you are replacing it, the stripe on the physical can indicates the negative lead, which must match the curved plate on the schematic.
3. Ground (Earth) vs. Chassis vs. DC Common
Schematics use three distinct ground symbols: Earth ground (a vertical line with three descending horizontal lines), Chassis ground (a vertical line intersecting three diagonal lines), and Signal/DC common (an open triangle pointing down). The Fix: Do not tie DC common to Earth ground unless the schematic explicitly shows a bond. Tying a floating 24V DC common to an earthed chassis can cause immediate ground faults and trip the power supply's internal protection.
Safe Interpretation for Faded or Missing Schematics
When you open a control panel from the 1980s, the schematic is often sun-faded, torn, or missing entirely. Here is the professional workflow for safely interpreting the circuit when the electric drawing symbols are illegible:
- Identify the Coil Voltage First: Look at the physical transformer and the coil labels on the contactors (e.g., 'A1-A2 120VAC'). This tells you the control circuit voltage, allowing you to set your multimeter to the correct AC or DC range.
- Trace the Wire Gauge and Color: In North American legacy panels, 14 AWG blue wire is typically 120VAC control; 18 AWG yellow or red is often 24VDC. Use the wire physical characteristics to reconstruct the schematic's power rails.
- Verify Coil Resistance: If a symbol is faded and you cannot tell if a component is a relay coil or a motor starter coil, remove the wires and measure resistance. A 120V AC control relay coil typically reads between 10 and 50 ohms. A large motor starter coil might read much lower, while a 24V DC instrument relay will read significantly higher (100+ ohms).
- Map the Auxiliary Contacts: Physically trace the mechanical linkages. If a main contactor pulls in, watch which side-mounted auxiliary blocks move with it. Map these physical blocks to the wire numbers on the terminal strips to reconstruct the logic flow that the missing schematic used to show.
For a deeper dive into standardizing your own facility's documentation, refer to the NEMA electrical drawing guidelines or consult the All About Circuits schematic reference for cross-referencing obscure legacy symbols. Always prioritize physical verification over paper assumptions when working on live or recently de-energized systems.
