A wiring diagram relay symbol represents an electromechanical or solid-state switch controlled by a coil. The standard symbol consists of a coil shape (a rectangle for IEC, a circle for legacy NEMA) and linked contact symbols (NO/NC) marked with the same device tag (e.g., K1 or CR1). The exact graphical representation depends entirely on whether your schematic follows IEC 60617, NEMA Z91.1, or ANSI Y32.2 standards. Below is the definitive reference for reading, interpreting, and troubleshooting these symbols on the bench or in the field.
The Master Wiring Diagram Relay Symbol Table
Use this table to decode the most common relay symbols you will encounter in industrial control panels, HVAC schematics, and motor control centers. The coil and its associated contacts are always linked by a shared alphanumeric designation (like K1 for IEC or CR1 for NEMA).
| Component | IEC 60617 Symbol Description | NEMA / ANSI Symbol Description | Standard Device Tag |
|---|---|---|---|
| Relay Coil | Empty rectangle. Polarity marked with + and - for DC coils. | Circle (legacy) or rectangle. Often drawn with a diagonal line for AC. | K1, KA1 / CR1 |
| Normally Open (NO) | Line with a gap; a diagonal line rests above the gap, pointing down-right. | Two parallel lines with a gap; a bridging line rests above, not touching. | K1-13/14 |
| Normally Closed (NC) | Line with a gap; a diagonal line crosses entirely through the gap. | Two parallel lines with a gap; a bridging line crosses through the gap. | K1-21/22 |
| On-Delay Timer | Rectangle with an 'X' on the left side, or an arrow pointing toward the contact. | Circle/Rectangle with 'ON' or a specific dash notation inside. | KT1, TR1 |
| Off-Delay Timer | Rectangle with an 'X' on the right side, or an arrow pointing away from the contact. | Circle/Rectangle with 'OFF' inside the coil symbol. | KT2, TR2 |
| Latching Relay | Rectangle with a small mechanical latch symbol (hook) on the side. | Circle with a mechanical latch hook or dual opposing coils. | KL1 |
Rows People Get Wrong & Faded Marking Protocols
Even experienced electricians misread specific relay symbols, leading to dangerous wiring errors or hours of troubleshooting. Here are the most common pitfalls and how to handle degraded hardware.
The Time-Delay Arrow Confusion
In IEC diagrams, a time-delay relay uses a box with an 'X' inside or an arrow next to the contact. If the 'X' is on the left (or the arrow points toward the contact line), it is an On-Delay (slow to energize). If the 'X' is on the right (arrow points away), it is an Off-Delay (slow to de-energize). Wiring an off-delay when the schematic calls for an on-delay will cause motor starters to chatter or safety interlocks to fail during E-stop resets because the contacts drop out immediately when power is cut.
Form C (SPDT) Common Pin Assumptions
Schematics show the common (C) pin as the pivot point of the NO and NC contacts. On a physical 11-pin octal relay (like an Omron MY3N), the common pins are 9, 10, and 11. Beginners often assume the 'middle' physical pin on the socket is common. This is false; the physical layout is dictated by the mold, not the schematic. Assuming the middle pin is common leads to dead shorts when wiring 120VAC loads.
Never guess coil voltage on a faded relay. Applying 120VAC to a 24VDC coil will cause an explosive failure, vaporize the internal winding, and create an arc flash hazard. If the label is illegible, use this bench protocol:
- Set your digital multimeter (DMM) to resistance (Ω).
- Probe pairs of pins. Coil pins will show a specific resistance (e.g., ~650Ω for a 24VDC coil, ~10kΩ for a 120VAC coil). NO pins read OL (open). NC pins read <1Ω.
- Identify the coil pins and apply the verified voltage via a current-limited bench supply.
- Listen for the mechanical click, then re-test continuity across the remaining pins to map the NO contacts.
Regional Standards: IEC vs. NEMA vs. Old UK
The way a relay coil and its contacts are linked on paper changes drastically depending on the region and the era of the schematic. Understanding these regional variants prevents you from missing 'invisible' connections.
| Standard | Region / Era | Coil Shape | Contact Linkage Style |
|---|---|---|---|
| IEC 60617 | Global, EU, Modern US | Rectangle | Alphanumeric Tag: Contacts are drawn anywhere on the page, linked only by a shared tag (e.g., K1). No physical lines connect the coil to the contacts. |
| NEMA Z91.1 / ANSI Y32.2 | Legacy US, Older Industrial | Circle or Rectangle | Mechanical Dashed Line: Contacts are often drawn physically attached to the coil via a dashed mechanical linkage line, or grouped tightly together in a 'ladder' format. |
| Old UK (BS 3939) | UK (Pre-1990s) | Rectangle (Heavy Lines) | Cross-Reference Grid: Used heavy lines and a grid reference system at the bottom of the coil to tell the reader which page/line the contacts were located on. |
How to tell which standard you are reading: Look at the contacts. If the NO/NC contacts are scattered across different rungs of a ladder diagram with no dashed lines connecting them to the coil, you are looking at an IEC-style schematic. You must trace the device tag (like -K14) to find all associated contacts. If you see dashed lines physically tying a contact to a circular coil, you are reading a legacy NEMA/ANSI print.
Real-World Pinouts: 8-Pin, 11-Pin, and 14-Pin Bases
Wiring diagrams show the logical flow, but when you are standing in front of a panel with a screwdriver, you need the physical pinout. The industry standard for plug-in 'ice cube' relays is the Omron MY series (or equivalents like Schneider Harmony RXM). Here is the exact pin mapping you need to memorize.
The Golden Rule of Relay Pinouts
The coil is always on the highest numbered pins, which correspond to the bottom of the physical relay when inserted into a socket. The contacts are numbered sequentially from the top down.
- 8-Pin (DPDT, e.g., Omron MY2N): Coil is on pins 7 and 8. Contact Set 1 is (1=NC, 2=NO, 3=Common). Contact Set 2 is (4=NC, 5=NO, 6=Common).
- 11-Pin (3PDT, e.g., Omron MY3N): Coil is on pins 10 and 11. (Note: Pin 2 is skipped on the physical socket to prevent misalignment). Contacts are (1,2,3), (4,5,6), and (7,8,9) where the highest number in the triplet is the Common pin.
- 14-Pin (4PDT, e.g., Omron MY4N): Coil is on pins 13 and 14. Contacts are (1,2,3), (4,5,6), (7,8,9), and (10,11,12). Again, the highest number in each triplet (3, 6, 9, 12) is the Common pin.
Solid-State Relay (SSR) Note: If your schematic shows a relay symbol with a diode or triac symbol inside the rectangle, it represents an SSR (like a Fotek SSR-25DA). SSRs do not have NC contacts, they do not click, and they leak a small amount of current (typically 1-3mA) when 'off'. Never use a standard mechanical relay pinout socket for an SSR, as the internal heat dissipation requirements and pin layouts differ entirely. Always verify the datasheet for the specific SSR part number before terminating load wires.
