When reading a control schematic, the standard symbol for a relay coil is a simple rectangle (IEC standard) or a circle labeled 'CR' (NEMA standard). Contacts are drawn as standard SPST or SPDT switch symbols, linked to the coil by a dashed mechanical line. However, translating these schematic symbols to physical terminal pins on the bench is where most builders and technicians make costly wiring errors. Below is the direct translation from paper to physical hardware.
The Master Relay Symbol & Pinout Reference Table
This table bridges the gap between schematic symbols and the physical pins you will terminate with your ferrules and ring terminals. It covers the most common electromechanical relay (EMR) configurations found in industrial and residential control panels.
| Component | IEC 60617 Symbol | NEMA / IEEE 315 Symbol | Physical Pin Label (IEC) | Practical Meaning & Bench Test |
|---|---|---|---|---|
| Relay Coil | Rectangle (often with K1, K2) | Circle with 'CR' or 'K' inside | A1 (+/Hot), A2 (-/Neutral) | The electromagnet. Measure across A1/A2 with a multimeter; expect 50Ω to 1000Ω depending on voltage. 'OL' means a burnt coil. |
| Normally Open (NO) Contact | Two parallel lines, one angled away | Two parallel lines, one angled away | 13 (Common), 14 (NO) | Open when unpowered. Bench test: Unpowered reads 'OL'. Powered reads < 0.1Ω. |
| Normally Closed (NC) Contact | Two parallel lines, one angled across | Two parallel lines, one angled across | 11 (Common), 12 (NC) | Closed when unpowered. Bench test: Unpowered reads < 0.1Ω. Powered reads 'OL'. |
| Changeover (CO / Form C) | Single pole switching between two throws | Single pole switching between two throws | 11 (Common), 12 (NC), 14 (NO) | A 3-pin block. Common (11) routes to 12 at rest, and snaps to 14 when the coil energizes. |
| Time-Delay ON (TON) | Rectangle with a small 'x' box on the bottom edge | Circle with 'TR' and an ON-delay arrow | A1, A2, 15/18 (NO delay), 16/18 (NC delay) | Contacts change state only after a set time after the coil receives power. |
Regional Variants: NEMA (US) vs IEC (Global) vs Old UK (BS)
A common misconception is that the US National Electrical Code (NEC) dictates schematic symbols. It does not. The NEC (NFPA 70) governs physical wiring methods, ampacity, and safety. Schematic symbols in North America are actually governed by NEMA ICS 19 and IEEE 315. Here is how the regional standards diverge on the page and in the panel:
- NEMA (North America): Schematics are drawn in 'ladder logic' format. Power rails are vertical lines on the left (L1) and right (L2). Relays are drawn as circles (e.g., CR1, CR2). Physical relays in NEMA panels are typically larger, octal-pin (8-pin or 11-pin) plug-in types like the classic Allen-Bradley 700-HA series.
- IEC 60617 (Europe, Asia, Global): Schematics are drawn with horizontal power rails at the top and bottom. Relays are drawn as rectangles (e.g., K1, K2). Physical hardware favors DIN-rail mounted, multi-pole relays with clear alphanumeric pin stamps (A1/A2 for coils, 13/14 for contacts). The Electrical Engineering Portal's guide on IEC vs NEMA provides an excellent visual breakdown of these drafting differences.
- Old UK (BS 3939): Obsolete since the late 1990s, but you will still encounter it when retrofitting legacy British machinery. It used unique graphical symbols (like a rectangle with a diagonal slash for a coil) and non-standardized pin numbering. If you are working on a pre-2000 UK panel, assume nothing—trace every wire with a tone generator and multimeter.
Rows People Get Wrong (And How to Fix Them)
Even experienced technicians misinterpret specific relay symbols and pinouts. Watch out for these three critical errors:
In IEC standard 8-pin and 14-pin relays, the first digit indicates the contact block number (1, 2, 3, 4), and the second digit indicates the function. x1 and x2 are always NC. x3 and x4 are always NO. Wiring a safety interlock to pin 14 (NO) instead of 12 (NC) means the machine will run when the safety guard is open.
2. Assuming the Dashed Line Means Mechanical Linkage on the Physical Relay
On a schematic, a dashed line connecting a coil to three separate contact switches implies they all move together. Beginners often look at a physical 4-pole relay and try to find a physical metal bar linking the contacts. There isn't one. The linkage is purely magnetic inside the relay's core; the dashed line is just schematic logic.
3. Applying EMR Symbols to Solid State Relays (SSRs)
An SSR does not have a physical coil or moving contacts. If your schematic shows an optocoupler symbol (an LED pointing at a phototransistor) inside a box, you are looking at an SSR. Do not attempt to measure the 'coil' resistance of an SSR with a standard multimeter; the internal LED driver circuit will yield confusing, non-linear resistance readings. Apply the control voltage and measure the output side for continuity.
Safe Interpretation When Markings Are Faded or Missing
Relays in harsh environments (high heat, chemical exposure, or UV light) often lose their printed pin diagrams. Never guess the pinout based on physical proximity. Follow this bench procedure to safely map an unmarked electromechanical relay:
- De-energize and Isolate: Remove the relay from its socket. Never probe a relay while it is seated in a live panel.
- Identify the Coil: Set your multimeter to resistance (Ohms). Probe pairs of pins. The coil will typically read between 50Ω (for low-voltage DC coils) and 10,000Ω (for high-voltage AC coils). If you read 'OL' (Open Line) across the suspected coil pins, the internal winding is burnt and the relay is trash.
- Identify the Common (C) Pin: Switch your meter to Continuity/Diode mode. Find the pin that shows continuity (< 0.1Ω) to one adjacent pin, but reads 'OL' to another adjacent pin in the same block. That center pin is your Common.
- Map NO and NC: The pin showing continuity to Common while unpowered is Normally Closed (NC). The pin showing 'OL' to Common while unpowered is Normally Open (NO).
Decision Path: Which Relay Standard and Part to Pick
Stop guessing which relay to order for your next panel build or repair. Use this decision tree to select the exact standard and part number you need.
| If Your Application Is... | Then Choose This Standard | Buy This Exact Part Number | Why This Part Wins |
|---|---|---|---|
| A new US-based industrial control panel using NEMA ladder logic drafting. | NEMA (Octal/11-pin plug-in) | Omron MY2N-D2 DC24 (with PYF14A socket) | Industry standard in North America. Features a built-in LED indicator and a mechanical test button for troubleshooting without power. |
| A global export machine, IEC schematic, or modern DIN-rail automation panel. | IEC (14-pin, alphanumeric pins) | Schneider Electric Zelio RXM4AB2BD (with RXZE2S108M socket) | 4 CO contacts, clearly stamped A1/A2 and 11-14 pins. The socket accepts either screw or spring terminals, speeding up IEC panel wiring. |
| A compact PCB mount or tight residential HVAC control board replacement. | IEC / PCB Mount | Finder 55.34.9.024.0040 | 4-pole PCB relay, 7A per contact. Extremely reliable in high-vibration environments where DIN-rail sockets would shake loose. |
| Switching high DC loads (like a 48V solar bank or LiFePO4 BMS) where arcing is a risk. | None (Use SSR) | Crydom D2425 (Solid State Relay) | EMR contacts will pit and weld shut under high DC loads. This SSR handles 25A at 24-280VAC, but for DC specific loads, look at the Crydom D1225 DC output series. |
By matching your schematic standard to the correct physical pinout and selecting a purpose-built part number, you eliminate the most common point of failure in control wiring: the termination. For deeper hardware specifications and lifecycle ratings, always cross-reference your selection with the manufacturer's documentation, such as Omron's Relay Precautions and Application Guide, before finalizing your bill of materials.
