When you look at a relay symbol wiring diagram, you are looking at a standardized graphical language that separates the control circuit (the coil) from the load circuit (the contacts). The direct answer to reading these diagrams is recognizing that the coil symbol dictates the trigger, while the contact symbols (NO, NC, or Changeover) dictate the switched power path. However, the exact symbols and pin numbers change drastically depending on whether your region follows IEC, NEMA, or legacy standards.
The Master Relay Symbol & Pinout Reference
Before tracing any wires, you must map the schematic symbols to the physical pins on the relay base. The table below provides the direct translation between the graphical symbol you see on a standard relay logic diagram and the physical pin designations stamped on industrial relays like the Omron MY4N or Schneider RXM series.
| Function / Contact Type | IEC 60617 Symbol / Designation | NEMA / Standard Pin Numbers | Practical Meaning & Use Case |
|---|---|---|---|
| Relay Coil | Rectangle (IEC) or Circle (NEMA) | A1 / A2 (IEC) 85 / 86 (Automotive) |
The electromagnetic trigger. Applying rated voltage here generates the magnetic field to pull the contacts. Always place flyback diodes across DC coils. |
| Normally Open (NO) / Form A | Two parallel lines, one angled away | 13 / 14 (IEC) 87 (Automotive) |
Open when de-energized. Closes when the coil pulls in. Used for starting motors, latching circuits, or turning on loads. |
| Normally Closed (NC) / Form B | Two parallel lines, one angled across | 11 / 12 (IEC) 87a (Automotive) |
Closed when de-energized. Opens when the coil pulls in. Used for emergency stop interlocks, alarms, or breaking a holding circuit. |
| Changeover (CO) / Form C | Combination of NO and NC sharing a common | 9 (Common), 11 (NC), 14 (NO) | A single pole that switches between two paths. The common pin (9) connects to 11 at rest, and swings to 14 when energized. |
| Time-Delay NO (On-Delay) | NO symbol with a small 'X' or arrow below | 15 / 18 (Delayed NO) | Remains open for a set duration after the coil is energized, then closes. Used for star-delta motor starting or staggered load shedding. |
Regional Standards: IEC vs. NEMA vs. Legacy UK
A major point of failure for DIYers and junior technicians is assuming a relay symbol wiring diagram drawn in Europe uses the same conventions as one drawn in North America. The IEC standard symbols database and the North American NEMA ICS 2 standard diverge significantly in how they depict the coil and label the pins. Knowing which standard applies to your region prevents catastrophic miswiring.
| Criteria | IEC 60617 (Europe, Global, Modern AU/NZ) | NEMA ICS 2 (North America) | Legacy DIN / Old UK (Pre-Harmonization) |
|---|---|---|---|
| Coil Symbol | Rectangle (often with diagonal lines for AC) | Circle (sometimes with a 'C' inside) | Rectangle or Circle, heavily dependent on the specific draftsman's era. |
| Pin Labeling Style | Alphanumeric (A1/A2 for coil, 13/14 for NO) | Strict numeric sequencing (1, 2, 3... or 85/86) | Mixed numeric, often matching specific manufacturer catalogs rather than a unified code. |
| Contact Representation | Drawn in the de-energized (resting) state. Separated from the coil symbol physically on the page. | Often drawn grouped together in a single vertical stack or 'ladder' rung. | Drawn in resting state, but physical proximity to the coil symbol was common in older schematics. |
| Standard Voltage Context | 230V/400V AC systems; 24V DC control logic is standard. | 120V/480V AC systems; 120V AC or 24V DC control logic. | 240V/415V AC systems; 110V AC center-tapped control logic was highly prevalent. |
Which applies to you? If you are working on modern industrial equipment, commercial HVAC, or importing machinery from Europe/Asia, default to IEC. If you are troubleshooting older North American industrial panels, MCCs (Motor Control Centers), or reading schematics from US-based OEMs, default to NEMA. Always verify the legend on the first page of the schematic packet.
Rows People Get Wrong & Mapping Faded Relays
Even with a perfect relay symbol wiring diagram, physical implementation is where mistakes happen. Here are the most common misinterpretations and a bulletproof method for identifying pins when the silk-screen markings on the relay base have burned off or faded.
The Most Common Schematic Mistakes
- Confusing A1/A2 with 13/14: A1 and A2 are always the coil. 13 and 14 are always a Normally Open contact pair. Wiring 120VAC into 13/14 will send mains voltage straight into your PLC output card when the relay pulls in, frying the solid-state output.
- Ignoring the 'Break-Before-Make' Timing: On a Form C (Changeover) contact, there is a millisecond gap where the common pin is disconnected from both NO and NC during the physical swing. If your circuit cannot tolerate a 5ms power interruption, a standard mechanical relay will cause a brownout; you need a solid-state relay or a bridging capacitor.
- Missing the Flyback Diode on DC Coils: When a 24VDC coil de-energizes, the collapsing magnetic field generates a high-voltage reverse spike (often >100V). Without a 1N4007 diode wired in reverse-bias across A1 and A2, this spike will arc across the switch contacts or destroy the driving transistor.
If you are testing a relay with a 120VAC or 240VAC coil, never use your multimeter's continuity or resistance mode while the circuit is live. De-energize the panel, lock out the breaker, and verify dead with a known-working CAT III/IV meter before probing. Only use voltage mode to verify the presence of control voltage at A1/A2 during live troubleshooting.
How to Map a Relay with Faded or Missing Markings
When you pull a 14-pin relay like a Finder 55.34 or an Omron MY4 from an old panel and the pinout diagram on the side is completely illegible, do not guess. Use your multimeter to reverse-engineer the pinout using standard relay switching principles.
- Find the Coil: Set your multimeter to resistance (Ohms). Probe pairs of pins. You are looking for a specific resistance. A 24VDC coil typically reads between 65Ω and 150Ω. A 120VAC coil will read much higher, often between 3,000Ω and 10,000Ω. The two pins that show this resistance are your A1 and A2 (coil).
- Find the Commons: Switch your meter to continuity (beep mode). With the relay de-energized (unplugged), probe the remaining pins. You will find sets of pins that beep continuously. These are your NC (Normally Closed) contacts and their associated Common pins.
- Verify the NO Contacts: The pins that do not beep with the coil de-energized, but share a physical metal blade with the Common pin inside the clear plastic case, are your NO (Normally Open) contacts.
- The Energized Test: Apply the rated coil voltage to A1 and A2 using a bench power supply or safe control circuit. You will hear the physical 'click'. Re-test continuity. The previously beeping NC pairs will now be open, and the previously silent NO pairs will now beep.
By systematically applying this resistance and continuity mapping, you can confidently wire any unmarked electromechanical relay into a new control panel without relying on faded silk-screening or guessing the manufacturer's internal layout.
