When reading an electrical schematic, the circuit symbols relay representation changes entirely based on the drafting standard used. A relay coil is drawn as a simple rectangle in IEC 60617 (global/European) drawings, but appears as a circle in NEMA/JIC (North American industrial) ladder logic. Misinterpreting these symbols—especially on legacy panels or imported machinery—leads to miswired contacts and shorted coils. Below is the definitive reference mapping schematic symbols to physical bench relays.
| Component | IEC 60617 Symbol & Designator | NEMA / JIC Symbol & Designator | Physical Pin Mapping (e.g., Omron G2R-2 / LY2N) |
|---|---|---|---|
| Relay Coil | Rectangle labeled K1, KA1, or KM1 |
Circle labeled CR1 (Control Relay) or M (Motor Starter) |
Pins 13 (+/L1) and 14 (-/L2) on 14-pin; Pins 2 and 7 on 8-pin octal. |
| Normally Open (NO) | Gap with a diagonal line crossing it; labeled 13-14 |
Two parallel lines with a gap, labeled NO |
Pins 9 (NO) and 11 (COM) for Pole 1; Pins 4 (NO) and 6 (COM) for Pole 2. |
| Normally Closed (NC) | Overlapping lines with a diagonal cross; labeled 11-12 |
Two parallel lines with a gap and a diagonal slash, labeled NC |
Pins 10 (NC) and 11 (COM) for Pole 1; Pins 1 (NC) and 3 (COM) for Pole 2. |
| Changeover (SPDT) | Three terminals with a wiper line; labeled 11, 12, 14 |
COM, NO, and NC grouped vertically with a mechanical linkage line | Pins 9 (NO), 10 (NC), and 11 (COM). Wiper physically moves from 10 to 9. |
| Time-Delay (On-Delay) | Rectangle with t and an upward-pointing arrow (↑) |
Circle with NOT (Normally Open Timed) or TD1 |
Varies by module (e.g., Finder 80 series). Coil pins remain 13/14 or A1/A2. |
Decoding the Standards: IEC vs. NEMA vs. Legacy UK
The visual language of your schematic depends on the region and the era of the machine. Understanding which standard applies to your panel prevents catastrophic wiring errors.
IEC 60617 (International / European / Modern Global):
IEC symbols are abstract and alphanumeric. The coil is a rectangle, and contacts are drawn separately from the coil, linked only by a shared alphanumeric tag (e.g., coil K1 operates contact K1.13-14). This standard separates logical function from physical layout, making it ideal for complex PLC-driven systems but confusing for beginners tracing wires on a physical panel.
NEMA and JIC (North American Industrial):
NEMA (National Electrical Manufacturers Association) and JIC (Joint Industry Council) standards dominate US ladder logic. The coil is a circle, and contacts are drawn directly in the power rails (rungs) of the ladder. NEMA uses functional prefixes: CR for Control Relay, TR for Timing Relay, and M for Motor Contactor. The physical relationship is often easier to trace on paper, but the drawings become massive for complex logic.
Legacy UK BS 3939 (Obsolete but present):
If you are troubleshooting machinery in the UK built before the late 1980s, you may encounter BS 3939 symbols. The relay coil is drawn as a semicircle (resembling a half-moon or a dome). Warning: If you see BS 3939 symbols, the panel is over 40 years old. Expect degraded PVC wire insulation, brittle terminal blocks, and a high likelihood of asbestos-containing arc chutes in older contactors. Treat all wiring as fragile.
The "Rows People Get Wrong" & Faded Marking Protocols
Even experienced technicians make specific errors when interpreting relay symbols or dealing with degraded documentation. Here are the most common pitfalls and how to resolve them safely.
Mistake 1: Treating Changeover (SPDT) as Two Independent Contacts
The most frequent error is wiring the NO and NC terminals of a changeover relay as if they are isolated circuits. They are not. They share a single physical wiper (the COM terminal). If you feed 120VAC into the COM terminal (Pin 11), both the NC (Pin 10) and NO (Pin 9) terminals will carry 120VAC when switched. If your downstream circuit on the NC side is only rated for 24VDC, you will destroy it the moment the relay de-energizes.
Mistake 2: Misreading Time-Delay Arrows
In IEC schematics, the direction of the arrow inside the coil symbol dictates the timing behavior. An arrow pointing up (↑) indicates an On-Delay (TON): the contacts switch only after the timer expires. An arrow pointing down (↓) indicates an Off-Delay (TOF): the contacts switch immediately on energization, but delay returning to their normal state when power is removed. Reversing these in safety-interlock circuits can cause premature machine startup.
Never guess pinouts on a relay if the schematic is faded, missing, or uses an unrecognizable legacy symbol. Mains voltage panels can be lethal.
- De-energize and Lock Out: Turn off the main breaker and apply a LOTO (Lockout/Tagout) device.
- Verify Dead: Use a CAT III or CAT IV rated multimeter (e.g., Fluke 87V) to verify zero voltage across the coil pins and contact terminals.
- Map the Coil: Switch your meter to continuity mode (with the audible beep). On a standard 14-pin relay (like the Omron G2R series), probe pins 13 and 14. You should read between 600Ω and 1200Ω for a 24VDC coil, or a dead short (near 0Ω) if the internal thermal fuse is blown.
- Find the Wiper (COM): Probe the remaining pins. The COM pin will show continuity to one adjacent pin (NC) when de-energized, and to the other (NO) when you manually press the relay's test button with an insulated tool.
Translating Schematics to Physical Bench Relays
Schematics show logical current flow; physical wiring requires managing spatial constraints. A major disconnect between the circuit symbols relay diagram and the actual bench build is the omission of protective components in the schematic.
The Flyback Diode Omission:
Most basic IEC and NEMA schematics do not draw a flyback (freewheeling) diode across the DC relay coil. However, if you are driving a 24VDC relay coil from a PLC transistor output, an Arduino GPIO (via a driver), or a BJT, the diode is mandatory. When the coil de-energizes, the collapsing magnetic field generates an inductive voltage spike (often exceeding 100V) that will instantly destroy the driving semiconductor. Always solder or wire a 1N4007 diode in reverse bias across the coil (cathode stripe to the positive supply pin 13, anode to pin 14). For AC coils, use an RC snubber network (e.g., 100Ω resistor in series with a 0.1µF X2-rated capacitor) instead of a diode.
Physical Pin Grouping vs. Logical Flow:
When wiring a 4PDT (4-pole double-throw) 14-pin relay like the Finder 55.34 series, the schematic will draw the four poles in completely different parts of the circuit (e.g., Pole 1 switches a 24VDC logic signal, Pole 4 switches a 120VAC indicator light). Physically, pins 9/10/11 are right next to each other on the socket. To avoid "spaghetti wiring" and accidental cross-voltage shorts, use physical partition barriers on the DIN rail socket between the low-voltage logic poles and the high-voltage AC poles. Never rely solely on the wire color code to prevent a 120VAC fault from backfeeding into a 24VDC PLC input if a wire pulls loose from the terminal.
For further reading on standardizing your control panel documentation, refer to the NFPA Codes and Standards for NEC-compliant wire color practices, and consult the Electronics Tutorials relay guide for deep-dives into solid-state relay (SSR) symbol variations, which use optocoupler symbols rather than mechanical coil representations.
