The standard symbol for a relay coil depends entirely on the drafting standard used in your region. Under the global IEC 60617 standard, the symbol for a general relay coil is a simple rectangle labeled with the letter 'K' (or 'KA' for auxiliary relays). In North America, the ANSI/IEEE 315 and NEMA ICS 2 standards dictate using a circle or a rectangle labeled 'CR' (Control Relay). If you are looking at a physical 8-pin or 14-pin industrial relay, the coil pins are almost universally designated as 13 and 14 (IEC) or A1 and A2 (DIN/IEC terminal markings).
The Complete Relay Coil Symbol Reference Table
Before tracing wires or ordering replacements, match the symbol on your schematic to the table below. This covers the four most common relay coil types across the major global drafting standards.
| Relay Type | IEC 60617 Symbol & Designation | ANSI/IEEE 315 / NEMA Symbol | Standard Physical Pin Labels |
|---|---|---|---|
| General Electromechanical (DC) | Rectangle with solid line inside; labeled 'K' | Circle or Rectangle with 'CR' | A1 (+), A2 (-) or 13, 14 |
| General Electromechanical (AC) | Rectangle with solid line over dashed line; labeled 'K' | Circle or Rectangle with 'CR' | A1, A2 (polarity ignored) |
| Latching (Bistable) Relay | Rectangle with mechanical latch box attached; 'KL' | Circle with 'CR' and latch indicator | Set/Reset coils (varies by mfr) |
| Time-Delay (On-Delay) | Rectangle with 'X' or clock symbol; 'KT' | Circle with 'TR' or 'TD' | A1, A2 |
| Solid-State Relay (SSR) Input | Rectangle with optocoupler symbol inside | Rectangle with LED/diode symbol | 3 (+), 4 (-) or IN+, IN- |
Regional Standards: Which One Applies to You?
Misinterpreting a regional symbol variant is the most common cause of wiring errors when maintaining imported machinery or working on legacy panels.
- IEC 60617 (Global, EU, UK, Australia, Modern US): Uses rectangular blocks for almost all components. If you are working on a panel built after 2005, or any equipment imported from Europe or Asia, you will see IEC symbols. The UK officially retired its old BS 3939 standard in favor of IEC harmonization decades ago, though you may still see BS 3939 circles on panels built before 1990.
- NEMA ICS 2 (North American Industrial): NEMA dominates heavy industrial control panels in the US and Canada (e.g., motor control centers). NEMA strictly uses circles for relay coils, contactors, and timers. If you see a circle with 'CR' inside, it is a NEMA control relay coil.
- ANSI/IEEE 315 (North American Electronic/Power): Used primarily in power distribution single-line diagrams and PCB-level schematics. It often overlaps with NEMA but uses more specific function letters (like 'K' for relays in power protection schemes, per ANSI/IEEE C37.2).
The 'Rows People Get Wrong' Notes
Even experienced technicians misread specific relay coil variations. Here are the three most dangerous schematic misinterpretations:
1. The AC vs. DC Hash Mark Trap
In IEC schematics, a DC coil is represented by a rectangle with a single solid straight line inside or beneath it. An AC coil features a solid line with a dashed line directly below it (the AC hash mark). If you ignore this detail and apply 24VAC from a control transformer to a 24VDC relay coil, the coil will lack the necessary shading ring to prevent AC zero-crossing chatter. The relay will hum violently, draw excessive current, and burn out within minutes.
2. Confusing the Coil with the Contact
A relay coil consumes power to create a magnetic field; a relay contact switches the load. In IEC, the coil is a rectangle, while the contacts are drawn as separate, disconnected symbols (a line with a diagonal slash for Normally Open, or overlapping lines for Normally Closed) linked only by a dashed mechanical line or a shared alphanumeric tag (e.g., K1 coil operates K1-13/14 contact). Never wire load current through the coil symbol's pins.
3. Time-Delay 'TR' vs. Transformer Symbols
In older ANSI/NEMA diagrams, a time-delay relay coil is a circle labeled 'TR' (Time Relay). However, a transformer is also represented by overlapping circles. The differentiator is the label and the connection lines: a transformer will have distinct primary and secondary winding taps (e.g., H1-H2, X1-X2), whereas a 'TR' coil will only have two control power connections and will be linked via dashed lines to its timed contacts.
Decision Path: Identifying an Unknown or Faded Relay Coil
When you are troubleshooting a machine with a missing schematic, or the physical relay's label has been baked off by years of panel heat, use this decision path to safely identify the coil pins and voltage rating.
| Condition / Observation | Action / Measurement | Conclusion & Next Step |
|---|---|---|
| Pins are labeled A1 and A2 (or 13 and 14) | Measure resistance across A1 and A2. | These are the coil pins. Proceed to voltage identification below. |
| Resistance reads between 150Ω and 1,200Ω | Check the physical size and wire gauge of the coil winding if visible. | Likely a 12VDC or 24VDC electromechanical coil. (e.g., 24VDC Omron MY2N reads ~650Ω). |
| Resistance reads between 2,000Ω and 10,000Ω | Look for an AC hash mark on the schematic or a bridge rectifier nearby. | Likely a 120VAC or 230VAC coil. Do not apply DC to this coil. |
| Resistance reads infinite (OL) in both directions | Switch multimeter to Diode Test mode and probe pins. | If it reads a ~1.2V to 1.5V forward voltage drop, this is a Solid State Relay (SSR) LED input, not an electromagnetic coil. |
| Relay has 3 coil pins (e.g., Common, Set, Reset) | Measure resistance from Common to Set, then Common to Reset. | This is a dual-coil latching relay. You must pulse voltage; continuous voltage will destroy the coil. |
Concrete Component Picks & Wiring Defaults
If your decision path leads to a dead, unmarked relay that you need to replace, do not guess. Default to industry-standard, globally available part numbers that match the IEC pinouts (13/14 for coil, 9-10-11-12 for contacts) to ensure they plug into standard DIN rail sockets.
- Standard 24VDC Industrial Workhorse: Omron MY2N-D2 DC24 (approx. $6 - $9 USD). This is a DPDT (Double Pole, Double Throw) relay with a built-in flyback diode. The 'D2' suffix is critical—it means the diode is integrated, saving you from having to wire a separate 1N4007 diode across A1 and A2 to protect your PLC output transistors from inductive kickback.
- Standard 120VAC Panel Replacement: Schneider Electric RXM2AB2BD (approx. $8 - $12 USD). A highly reliable DPDT relay with a clear test button and mechanical flag indicator, ideal for AC control circuits where visual confirmation of the coil's physical state is required.
- High-Vibration / PCB Mount: Finder 40.52.9.024.0000 (approx. $4 - $6 USD). If you are repairing a PCB rather than a DIN panel, this 24VDC coil relay uses a direct-solder footprint that withstands industrial vibration far better than socketed alternatives.
For further reading on relay contact configurations and coil protection circuits, refer to the comprehensive guides at All About Circuits and the electromechanical breakdowns at Electronics Tutorials. Always verify the exact coil resistance and pickup voltage in the manufacturer's datasheet before energizing a replacement circuit.
