A relay symbol on a schematic dictates both the coil's control requirements and the contact configuration (Form A, B, or C). Because global engineering relies on different drafting standards, misreading a symbol can lead to wiring a 24VDC coil to 120VAC or shorting a critical control circuit. Below is the definitive reference for interpreting relay symbols across IEC, IEEE, and NEMA standards, followed by practical bench-testing methods for when physical markings fail.
The Master Relay Symbols Reference Chart
The table below maps the most common relay functions to their graphical representations. Note that IEC 60617 separates the coil and contacts logically, while older North American standards often group them physically.
| Function / Type | IEC 60617 (Global/EU) | IEEE 315 / NEMA (North America) | Common Part Example |
|---|---|---|---|
| Standard Coil | Rectangle (often labeled K1, KA1) | Circle or Rectangle (labeled CR1, M1) | Omron G2R-1-S (General Purpose) |
| SPST-NO (Form A) | Line gap with angled line above (open) | Two parallel lines, one angled away | Finder 38.51 (SPST-NO) |
| SPST-NC (Form B) | Line gap with angled line crossing (closed) | Two parallel lines, one angled across | Carlo Gavazzi RM1B (SPST-NC) |
| SPDT (Form C) | Wiper line touching one side, gap on other | Wiper line between two fixed contacts | Omron MY4N (4PDT, uses Form C) |
| DPDT (2 Form C) | Two SPDT symbols linked by dashed line | Two wiper lines mechanically ganged | Phoenix Contact REL-MR-24DC/21 |
| Time-Delay (ON) | Rectangle with arrow pointing UP/OUT | Circle with upward arrow inside | Macromatic TR-651 (On-Delay) |
| Time-Delay (OFF) | Rectangle with arrow pointing DOWN/IN | Circle with downward arrow inside | Finder 80.01 (Off-Delay) |
| Latching / Polarized | Rectangle with small 'M' or permanent magnet box | Circle with 'L' or polarity '+'/'-' marks | Omron G6CK (Latching) |
Regional Standards and the 'Rows People Get Wrong'
The biggest point of failure in reading relay symbols is assuming the drafting standard matches your physical location. A European machine imported to the US will use IEC logic, while a legacy US control panel will use NEMA ladder logic.
IEC 60617 vs. NEMA/IEEE 315
Under IEC 60617, the coil and its associated contacts are rarely drawn next to each other. The coil is a simple rectangle labeled 'K1'. The contacts are scattered throughout the schematic wherever they are needed in the circuit, identified by an alphanumeric tag (e.g., K1.1, K1.2) and a dashed mechanical link line. Under NEMA/IEEE 315, particularly in older ladder diagrams, the coil (drawn as a circle) and its contacts are often grouped physically on the same rung for visual tracing.
The 'Rows People Get Wrong'
- Time-Delay Arrows: The arrow direction inside the coil symbol is frequently misread. An arrow pointing away from the contact line (or up) means delay-on-energize. An arrow pointing toward the contact line (or down) means delay-on-de-energize. Reversing these in a safety interlock circuit can cause catastrophic timing failures.
- Form C (Transfer) Wipers: In IEC symbols, the center wiper line of a Form C contact must be checked for a small bridging line. Standard relays are 'break-before-make'. If the symbol shows the wiper overlapping both fixed contacts, it indicates a 'make-before-break' (MBB) contact, used to prevent circuit interruption during switching.
- Latching Coil Markers: A small rectangle with a diagonal line or an 'M' next to the coil denotes a permanent magnet latching relay. This relay requires only a momentary pulse to change state and will remain latched if power is lost. Treating it as a standard coil in PLC logic will result in continuous, unnecessary pulsing.
Safe Interpretation When Markings Are Faded or Missing
Bench technicians frequently encounter relays like the Finder 55.34 or Omron MY4N where the printed schematic on the side of the housing has been rubbed off by years of vibration and heat. Never guess the pinout based on physical layout alone. Use a digital multimeter (DMM) to map the symbol to the pins.
- Identify the Coil: Set your DMM to resistance (Ohms). Probe the suspected coil pins. A healthy 24VDC relay coil (like an Omron MY4N) will read around 650Ω. A 120VAC coil will read much higher (typically 2kΩ to 10kΩ). If you read infinite (OL), the coil is open and the relay is dead. If you read near 0Ω, the coil is shorted.
- Map the Contacts: Switch the DMM to continuity or diode-test mode. With the relay unpowered, probe the remaining pins to find the common (wiper) and normally closed (NC) contacts. They will beep or show near 0Ω.
- Verify Normally Open (NO): The pin that shows infinite resistance to the common pin is your NO contact. You can verify this by temporarily applying the rated DC voltage to the coil pins; you should hear an audible click, and the continuity should swap from the NC pin to the NO pin.
Note on Solid State Relays (SSRs): If you are testing an SSR (e.g., Omron G3NA), standard continuity testing will fail. SSRs contain internal snubber networks and optoisolators. You must test them under live load conditions or use the specific forward-voltage diode test on the input terminals, as detailed in standard solid-state relay testing procedures.
Frequently Asked Questions
What does a circle vs a rectangle mean for a relay coil symbol?
In modern schematics, a rectangle universally denotes an IEC 60617 standard relay coil. A circle typically denotes a NEMA/IEEE 315 standard coil, often seen in North American ladder logic diagrams. However, in fluid power or specialized instrumentation schematics, a circle might represent a solenoid valve rather than an electrical relay. Always check the schematic's title block legend to confirm the drafting standard being used.
How do I read the dashed lines in an IEC relay schematic?
In IEC 60617 drafting, dashed lines indicate a mechanical linkage or functional association between components that are drawn in different physical locations on the page. If you see a dashed line connecting a relay coil (K1) to a specific contact symbol, it means that contact is physically actuated by that specific coil, even if the contact is drawn three pages away in a motor starter circuit.
What is the symbol for a solid state relay (SSR)?
An SSR symbol replaces the mechanical coil rectangle with an optoisolator symbol (typically a box containing an LED and a phototransistor or phototriac symbol). The output side will show a thyristor or triac symbol rather than mechanical wiper lines. This visually warns the technician that the output has a voltage drop (usually 1V to 1.5V) and requires heat sinking, unlike the near-zero resistance of a mechanical contact.
Why are my relay contacts labeled 11, 12, 14 instead of COM, NO, NC?
This is the IEC standard numbering system for relay terminals, designed to prevent wiring errors in complex panels. The first digit indicates the pole number (1 for the first pole, 2 for the second). The second digit indicates the function: '1' and '2' are the Common and NC terminals, while '4' is the NO terminal. Therefore, '11' is Pole 1 Common, '12' is Pole 1 NC, and '14' is Pole 1 NO. For a DPDT relay, the second pole would be labeled 21, 22, and 24.
