Relay symbols differ wildly depending on whether you are looking at a North American (NEMA/JIC) ladder diagram or an International (IEC 60617) schematic. The direct answer for modern global panels: the coil is designated by A1/A2, normally open (NO) contacts end in 3/4 (like 13/14), and normally closed (NC) contacts end in 1/2 (like 21/22). If you are working on legacy US industrial equipment, you will instead see circular coil symbols labeled with device prefixes like CR1 or M1.
Below is the definitive bench reference for mapping these symbols to physical pins on standard 8-pin, 11-pin, 14-pin, and automotive relays.
Master Relay Symbol & Pinout Reference Table
This table maps the schematic symbols to the physical pin numbers you will find on the base of common industrial relays (like the Omron MY4N or Finder 60.13) and standard automotive ISO 7588 relays.
| Component / Function | IEC 60617 Designation | NEMA / ANSI Designation | 14-Pin / 11-Pin / 8-Pin Pins | Automotive (ISO 7588) Pins | Practical Meaning on the Bench |
|---|---|---|---|---|---|
| Relay Coil | Rectangle with A1 / A2 | Circle or Rectangle with CR (Control Relay) | 13 & 14 (14-pin); 2 & 7 (8-pin) | 85 & 86 | The electromagnet. Apply your control voltage here. Polarity matters only if a flyback diode is built-in. |
| NO Contact (Form A) | Two parallel lines, top one angled. Labeled 13 / 14 | Two parallel lines with a gap, labeled NO | 9 & 13 (Pole 1); 3 & 5 (Automotive) | 30 & 87 | Open when coil is off. Closes to pass load current when coil is energized. |
| NC Contact (Form B) | Two parallel lines, top one straight with diagonal slash. Labeled 21 / 22 | Two parallel lines touching, labeled NC | 10 & 14 (Pole 1); 4 & 6 (Automotive) | 30 & 87a | Closed when coil is off. Opens to break load current when coil is energized. |
| Changeover (Form C) | Common line with two throws. Labeled 11 / 12 / 14 | Single pole double throw (SPDT) symbol | 9, 1, 5 (Pole 1 on 8-pin) | 30, 87a, 87 | Common (11/30) switches between NC (12/87a) and NO (14/87). |
| Timed NO (On-Delay) | NO symbol with an X below the angled line | NO symbol with a timer block and TON | N/A (Usually dedicated timer modules) | N/A | Contact closes only after a set time delay once the coil is energized. |
Regional Standard Variants: Which Standard Applies to You?
Before you start tracing wires, you must identify which drafting standard the original engineer used. Misinterpreting a NEMA ladder diagram as an IEC schematic is a fast way to short out a control transformer.
| Standard | Region / Era | Coil Symbol Style | Contact Naming Convention | When You Will Encounter It |
|---|---|---|---|---|
| IEC 60617 | Global, EU, UK, AU (Modern) | Rectangle (A1/A2) | Alphanumeric function digits (13/14, 21/22) | 90% of new PLC panels, modern HVAC, DIN-rail mount equipment. |
| NEMA / ANSI / JIC | North America (US/Canada) | Circle or Box (CR1, 1CR) | Descriptive letters or ladder rung numbers | Legacy US manufacturing plants, older motor control centers (MCCs). |
| BS 3939 | Old UK (Pre-1990s) | Box with a striking line | Graphical representations, often unnumbered | Retrofit projects in older British facilities. Superseded by IEC. |
The IEC 'Function Digit' Rule Explained
The most powerful tool for reading modern IEC relay schematics is understanding the second digit of the contact designation. The first digit simply tells you which pole you are looking at (1, 2, 3, or 4 on a 4PDT relay). The second digit tells you the function:
- 1 and 2 = Normally Closed (NC) circuit. (e.g., 11 and 12, or 21 and 22).
- 3 and 4 = Normally Open (NO) circuit. (e.g., 13 and 14, or 43 and 44).
- 5 and 6 = Changeover / Form C circuit. (e.g., 15 and 16).
- 7 and 8 = Special function (used for timed contacts or solid-state outputs).
If you see 43 and 44 on a diagram, you instantly know it is the NO contact on the 4th pole of the relay, even if the physical relay is buried deep inside a panel and you can only see the wire ferrules.
The 'Rows People Get Wrong' Notes & Faded Markings
Even experienced technicians make specific, repeatable errors when wiring relays. Here are the most common pitfalls and how to recover when the physical markings on your relay socket are worn off.
Common Wiring Mistakes
- Automotive 87 vs. 87a Confusion: On a standard 5-pin Bosch-style automotive relay, pin 87 is your NO output, and pin 87a is your NC output. People frequently wire the load to 87a, wondering why their fuel pump or cooling fan runs constantly when the ignition is on, and shuts off when the ECU triggers the relay.
- Assuming DC Polarity Doesn't Matter on the Coil: While a raw mechanical coil will pull in regardless of DC polarity, many modern 14-pin relays (like the Omron MY4N-D2) have a built-in flyback diode or an LED indicator. If you wire A1 (+) and A2 (-) backward on a diode-protected relay, you will dead-short the control circuit and blow your PLC output fuse the millisecond the PLC tries to turn it off.
- Overlooking the 8-Pin Octal Pinout Shift: An 8-pin octal relay (like a classic tube-style or flat-blade) does not use the 13/14 IEC numbering on the physical pins. The physical pins are usually numbered 1 through 8. Coil is 2 and 7. Pole 1 common is 1, NC is 4, NO is 3. Always check the physical diagram printed on the relay casing, not just the IEC schematic.
If you are troubleshooting a legacy panel where the relay socket markings are faded, painted over, or missing, never probe with a live multimeter if the circuit involves mains voltage (>50V AC / >120V DC). Arc flash and lethal shock are severe risks. De-energize the panel, lock out the main breaker, and verify dead with a known-good CAT III/IV meter before performing continuity checks.
How to Map an Unmarked Relay Socket with a Multimeter
When you are staring at a bare, unmarked 14-pin socket and need to figure out which pins are the coil and which are the contacts, use this bench procedure:
- Isolate the Socket: Ensure the relay is unplugged and the socket is completely de-energized.
- Find the Coil via Resistance: Set your multimeter to Ohms (Ω). Probe the pins. You are looking for a specific resistance range based on the expected coil voltage.
- 12V DC Coil: Expect roughly 40Ω to 100Ω (e.g., a 12V Finder 60.13 is ~73Ω).
- 24V DC Coil: Expect roughly 250Ω to 700Ω (e.g., an Omron MY4N 24VDC is ~650Ω).
- 120V AC Coil: Expect roughly 2,500Ω to 4,000Ω (2.5k - 4kΩ).
- 240V AC Coil: Expect roughly 8,000Ω to 12,000Ω (8k - 12kΩ).
- Map the Poles via Continuity: Switch your meter to Continuity mode (the beep setting). Find a group of three pins where one pin (the Common) shows continuity to a second pin (NC), but shows an open circuit (OL) to the third pin (NO). That third pin will only show continuity to the Common when you manually press the relay's mechanical test button with a non-conductive tool.
- Verify Solid State Relays (SSRs): If you suspect it is an SSR rather than a mechanical relay, standard continuity testing will fail. SSRs require a DC control voltage to switch the internal TRIAC or MOSFET. Set your meter to Diode Test mode; the control pins should show a forward voltage drop of ~1.2V to 1.5V in one direction and OL in the reverse direction.
By understanding the underlying logic of the IEC function digits and the physical resistance characteristics of relay coils, you can decode any automotive or industrial relay diagram, regardless of how poorly the original panel was labeled.
