The standard IEC electrical symbol for a relay coil is a simple rectangle, while legacy North American NEMA and JIC schematics use a circle or semicircle. Because relay symbols dictate how control logic isolates high-voltage loads from low-voltage triggers, misreading a single contact variant can cause a dead short or defeat a critical safety interlock. Below is the definitive reference for identifying, interpreting, and tracing relay symbols on the bench and in the panel.
The Master Electrical Symbol for Relay Reference Table
Before troubleshooting or wiring a control panel, you need to know exactly what the schematic is asking for. The table below maps the most common relay symbols across the two dominant global standards. Reference designators (RefDes) typically use 'K' or 'CR' for control relays, and 'KA' for auxiliary relays.
| Component | IEC 60617 Symbol Description | NEMA / JIC Symbol Description | Common RefDes | Practical Meaning on the Bench |
|---|---|---|---|---|
| Standard Coil | Rectangle (often with K1, K2 inside) | Circle or Semicircle (CR, M inside) | K, CR, KA | The electromagnetic trigger. Apply rated voltage (e.g., 24VDC) to pull in the contacts. |
| Latching Coil | Rectangle with a mechanical latch hash mark or two opposing arrows | Circle with a small 'L' or latch box attached | KL, KRL | Requires only a momentary pulse to change state. Stays latched when power is removed. |
| Time-Delay (On-Delay) | Rectangle with an inward-pointing arrow or 'umbrella' on the coil line | Circle with a clock symbol or 'ON' text | KTD, TR | Contacts change state only after a set time once the coil is energized. |
| NO Contact (Form A) | Two parallel lines with a diagonal movable contact line resting open | Two parallel lines with a diagonal line resting open (often with a small 'NO') | K1-13/14 | Normally Open. Closes the circuit only when the coil is energized. |
| NC Contact (Form B) | Two parallel lines with a diagonal movable contact line resting closed (crossed) | Two parallel lines with a diagonal line resting closed (often with a small 'NC') | K1-11/12 | Normally Closed. Opens the circuit when the coil is energized. Critical for E-Stops. |
| Changeover (Form C) | Combination of NO and NC sharing a common pivot point | Combination of NO and NC sharing a common pivot point | K1-11/12/14 | Single Pole Double Throw (SPDT). Breaks one circuit while making another. |
| Solid State Relay (SSR) | Rectangle containing an optocoupler symbol (LED and phototransistor) | Rectangle with a diode/LED symbol inside | SSR, KSS | No moving parts. Input side is usually low-voltage DC; output side switches AC/DC loads. |
Regional Standard Variants: IEC vs. NEMA vs. JIC
If you are working on modern equipment or designing a new panel, you will almost exclusively encounter IEC 60617 symbols. The IEC standard uses a strict rectangular box for all coils and separates the coil drawing from the contact drawing, linking them only by the Reference Designator (e.g., K1). This 'exploded' schematic style keeps wiring diagrams clean, but it forces you to trace the K1 tag across multiple pages to find where the contacts actually live.
In North American heavy industry, you will still run into legacy NEMA and JIC (Joint Industry Council) prints. NEMA uses circles for coils and often draws the contacts physically attached to the coil symbol in a 'ladder' format. While NFPA 79 (the Electrical Standard for Industrial Machinery) has largely harmonized with IEC practices for new builds, maintenance electricians in US plants must still read 40-year-old JIC blueprints where a semicircle denotes a relay and a full circle denotes a motor starter.
When reading IEC symbols for Solid State Relays, the internal optocoupler symbol clearly marks the input side (the LED) and the output side (the TRIAC or MOSFET). A common bench mistake is treating the SSR input like a standard mechanical coil and applying 120VAC to it. The internal LED will instantly pop. Always verify the input voltage rating (usually 3-32VDC) before wiring the control side.
The 'Rows People Get Wrong' and Faded Marking Protocols
Even experienced makers trip over specific relay symbol variants. Here are the rows from the master table that cause the most field errors, followed by a protocol for when the schematic print is degraded.
Rows People Get Wrong
- Time-Delay Arrows: In IEC, an arrow pointing toward the contact line means 'On-Delay' (delays when energizing). An arrow pointing away from the contact line means 'Off-Delay' (delays when de-energizing, then snaps back immediately when re-energized). Swapping these in a motor braking circuit will cause mechanical shock.
- Latching vs. Standard: A latching relay symbol often includes a small diagonal hash mark inside the coil box. If you replace a latching relay with a standard non-latching relay without updating the PLC logic to hold the output high, the circuit will drop out the moment the PLC scan cycle ends.
- Forced-Guided Contacts: Safety relays (like the Pilz PNOZ series) use a special symbol where the NO and NC contacts are linked by a rigid mechanical line. This indicates 'forced-guided' or 'positively guided' contacts, meaning the NO and NC contacts can never be closed at the same time. Never substitute a standard relay for a forced-guided symbol in a safety interlock loop.
Safe Interpretation When Markings are Faded or Missing
When you inherit an old control panel and the schematic is sun-faded, oil-stained, or missing entirely, never guess the wiring based on wire colors. Follow this strict bench protocol:
- De-energize and LOTO: Kill the main disconnect and apply Lockout/Tagout. Verify dead with a CAT III/IV meter.
- Identify the Coil: Look for terminals marked A1 and A2 (IEC standard) or 2 and 7 (Octal base). Set your multimeter to Ohms. A healthy 24VDC coil will typically read between 150Ω and 400Ω. A 120VAC coil will read much higher (2kΩ to 10kΩ). If it reads OL (infinite), the coil is burnt open. If it reads near 0Ω, it is shorted.
- Map the Contacts: Switch your meter to Continuity (the beep setting). Manually press the relay's test button (or push the armature with a non-conductive tool). Find the common pin (usually 11, 21, 31 on DIN relays) and trace which pin beeps before you press (NC: 12, 22, 32) and which beeps after you press (NO: 14, 24, 34).
- Label Physically: Use a Brady or Brother label maker to physically tag the relay base with the confirmed pinout before re-energizing.
Matching Schematic Symbols to Physical Bench Relays
Abstract symbols only get you so far; you eventually have to wire physical hardware. Here is how the schematic symbols translate to the pinouts of the two most common relay families found in industrial and maker environments.
| Relay Model | Type | Coil Pins | Contact Pins (Pole 1) | Contact Pins (Pole 2) |
|---|---|---|---|---|
| Omron G2R-2-SND | DPDT (Form C x2), 10A, DIN | A1 (+), A2 (-) | 11 (COM), 12 (NC), 14 (NO) | 21 (COM), 22 (NC), 24 (NO) |
| Finder 40.52 | DPDT (Form C x2), 8A, PCB/DIN | 2, 7 | 1 (COM), 4 (NC), 3 (NO) | 8 (COM), 5 (NC), 6 (NO) |
| Schneider RXM4AB2 | 4PDT (Form C x4), 6A, DIN | A1, A2 | 11/12/14 | 21/22/24, 31/32/34, 41/42/44 |
Pro-Tip on Sockets: When wiring an Omron G2R or Schneider RXM series relay, always wire the socket base, not the relay itself. The symbols on the schematic map to the socket terminal numbers. If you wire the relay body directly and later need to swap a burnt-out coil, you will have to undo every screw. Furthermore, never mix up 8-pin Octal sockets (where pins 2 and 7 are the coil) with 10-pin or 14-pin DIN sockets. Forcing an 8-pin relay into a mismatched socket will short the coil directly across your NO contacts.
Understanding the electrical symbol for a relay is about more than just passing an exam; it is the foundational language of control logic. Whether you are reading a pristine IEC 60617 PDF on a tablet or squinting at a 1980s NEMA blueprint on a factory wall, knowing exactly what the coil and contact symbols demand will keep your circuits functional and your bench safe.
