A relay schematic symbol translates a physical electromagnetic coil and its mechanically linked switch contacts into a standardized 2D drawing. The exact symbol you read depends entirely on your region and industry: IEC 60617 dominates Europe and global automation, IEEE/ANSI 315 governs North American commercial electronics, and NEMA ICS 19 rules US heavy industrial control panels. Below is the direct master reference for identifying these components on the bench or in the field.
Master Relay Schematic Symbol Reference Table
Use this table to cross-reference the drawing on your schematic with the physical relay in your panel. All contact states represented in these symbols denote the de-energized (shelf) state of the relay.
| Relay Type / Function | IEC 60617 Symbol Description | ANSI/IEEE 315 Symbol Description | NEMA ICS 19 Standard | Common Physical Equivalents |
|---|---|---|---|---|
| Standard SPDT (Form C) | Rectangle coil box; contact drawn as a pivot arm resting on the NC pin, with a gap to the NO pin. | Circle or rectangle coil; switch blade drawn physically touching the NC contact dot. | Letter 'CR' for control relay; contacts drawn as standard switch with 'C', 'NO', 'NC' labels. | Omron MY4N, Finder 55.34, Schneider Zelio RXM |
| Time-Delay On-Delay | Rectangle coil box with a solid black arrow pointing up (towards the armature) inside the box. | Coil circle with a 'clock' symbol or 'TD' prefix; contact blade has a downward-pointing arrow. | 'TR' prefix for timer relay; contacts feature an arrow indicating delay direction on energization. | Macromatic TD-8501, Omron H3Y, Eaton ET4 |
| Latching (Magnetic) | Rectangle coil box with a small 'U' shaped permanent magnet symbol adjacent to the armature. | Coil circle with a diagonal line or 'L' prefix; sometimes shown as two opposing coils (set/reset). | 'L' or 'M' prefix; dual coils explicitly drawn if bistable electrical latching is used. | Finder 60.13, Panasonic ALDP, TE Connectivity IM |
| Solid State Relay (SSR) | Input diode/LED symbol optically coupled (dashed line) to an output thyristor/triac symbol. | Optocoupler triangle/LED pointing at a phototransistor or SCR gate symbol. | Rarely standardized in legacy NEMA; usually drawn as a box with 'SSR' and input/output terminals. | Omron G3NA, Crydom D1240, Carlo Gavazzi RJ50 |
| Thermal Overload Element | Heating element box with a bimetallic strip symbol (curved line) mechanically linked to a trip contact. | Heater coil (zigzag) in series with the motor line, mechanically linked to an 'OL' contact. | 'OL' prefix; heater elements drawn as overlapping loops in the power circuit, trip contact in control. | Schneider TeSys LRD, ABB TA25DU, Eaton PKZM0 |
Regional Standards & 'Rows People Get Wrong'
Reading a schematic from a different region or era is a primary cause of wiring errors. Here is how to determine which standard applies to you, followed by the specific symbols that cause the most misinterpretations on the workbench.
Which Standard Applies to Your Region?
- Europe, UK, Australia, and Global Automation: IEC 60617 is the law of the land. You will see rectangular coils and strictly orthogonal contact lines. Refer to the IEC official symbol database for exact vector definitions.
- North American Commercial & Electronics: IEEE/ANSI 315 (and the older MIL-STD-806) uses circular coils and more illustrative, curved switch blades.
- US Heavy Industrial & Motor Control: NEMA ICS 19 governs motor control centers (MCCs) and heavy machinery. It relies heavily on letter designations (CR, TR, OL) rather than purely graphical shapes. See the NEMA electrical diagrams standard for historical context.
Rows People Get Wrong (And How to Fix Them)
- The IEC SPST-NO vs. SPST-NC Slash: In IEC 60617, a Normally Open (Form A) contact is drawn with a diagonal slash that does not touch the fixed contact point. A Normally Closed (Form B) contact has a slash that crosses over the fixed point. Under poor lighting or on faded blueprints, the overlap is easily missed, leading to dead circuits or safety interlock failures.
- Time-Delay Arrow Direction: On ANSI/IEEE schematics, an arrow pointing away from the contact pivot indicates an On-Delay (timing starts when coil energizes). An arrow pointing toward the pivot indicates an Off-Delay (timing starts when coil de-energizes). People routinely swap these when replacing a failed Macromatic timer.
- SSR vs. Electromechanical Confusion: If the symbol lacks a physical moving armature line and instead shows a dashed optical isolation line, it is an SSR. Swapping an SSR for an electromechanical relay without checking the schematic symbol will result in leakage current issues, as SSRs never achieve true mechanical open-circuit isolation.
Safe Interpretation When Markings Are Faded or Missing
In legacy industrial panels, the schematic taped to the inside of the door is often faded, oil-stained, or missing entirely. Furthermore, the physical relay label (e.g., on a 20-year-old Omron LY2N) might be completely rubbed off. Here is the exact bench procedure to safely identify and verify an unknown relay without relying on the schematic symbol.
Step 1: Identify the Coil Pins
For standard 8-pin octal relays (like the ubiquitous Schneider 8501CO or Omron MY2), the coil is almost universally on pins 2 and 7. For 11-pin relays, the coil is on pins 2, 7, and 10 (depending on the specific manufacturer's socket, always verify). For PCB or DIN-rail relays (like Finder 55-series), look for pins marked A1 and A2.
Step 2: Trace the Contacts with a Multimeter
Set your multimeter (e.g., Fluke 87V) to continuity mode. With the relay completely removed from the socket and de-energized:
- Find the Common (COM) pin by probing until you find a pin that shows continuity to one other pin (NC) and open-circuit (OL) to a third pin (NO).
- In an 8-pin octal, you will have two identical sets: Set 1 (COM=1, NC=4, NO=3) and Set 2 (COM=8, NC=5, NO=6).
Step 3: Energize and Re-Test
Check the coil resistance. A 24VDC relay coil typically reads between 600Ω and 800Ω. A 120VAC coil will read much higher (often 3kΩ to 5kΩ). Apply the suspected rated voltage to the coil pins. You should hear a distinct mechanical click. Re-test continuity: the COM pin should now show continuity to the NO pin, and OL to the NC pin.
Quick Verification Checklist for Panel Builders
Before closing the panel door and energizing the main breaker, run through this physical verification checklist to ensure your wiring matches the schematic symbols:
- Terminal Torque: Verify socket terminal screws are tightened to spec. For standard DIN-rail relay sockets, this is typically 0.8 Nm to 1.2 Nm. Loose A2 connections cause coil chatter and contact arcing.
- Diode Orientation: If the schematic symbol includes a flyback diode across a DC coil (common in PLC output cards), ensure the physical diode band points toward the positive supply. A reversed diode will short the PLC output and blow the internal trace.
- Contact Material Check: If the schematic specifies a low-level signal relay (gold-flashed contacts for <5V / 10mA signals), do not substitute a standard silver-alloy power relay. Silver oxide requires higher wetting current to break through, which will cause open-circuit failures in logic-level PLC inputs.
For deeper study on relay contact forms and material science, refer to the comprehensive relay switching circuit guide on Electronics Tutorials. Always defer to the specific manufacturer's datasheet (e.g., Omron, Finder, TE Connectivity) for exact pinouts, as regional variations in socket wiring do exist.
