Electrical Schematic Symbols in 2026: IEC 60617 & 81346
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- IEC 60617 (now IEC 60617database) standardizes graphic symbols for every component in an electrical schematic; IEC 81346-2 governs the alphanumeric reference designations (R, S, K, Q, M, etc.).
- The 2024–2026 maintenance cycle harmonized several symbols across IEC and IEEE/ANSI, narrowing the JIC / ladder / IEC gap in PLC I/O diagrams.
- Three things changed since 2024: structured reference designations per IEC 81346-1, single-line vs multi-line conventions per IEC 61082-1, and CAD library imports that preserve IEC naming.
- Reference designations let a Siemens 6ES7, Allen-Bradley 1756, or Mitsubishi MELSEC schematic all read the same way.
- When in doubt: send the original PDF to Moritta@KOEED.COM and we will cross-reference the part within 24 hours.
A 2026 field guide to reading electrical schematics: standardized graphic symbols, alphanumeric reference designations, and multi-brand conventions for PLC maintenance teams.
Hello fellow PLC professionals. In the 2024 edition of this note, we walked through the visual vocabulary of an electrical control schematic — the rectangles, lines, NO/NC marks, and the I1/Q1 labels. Two years later, that vocabulary has not gone away, but the standards behind it have quietly tightened. This Engineering Note refreshes the basics against the current (2026) state of IEC 60617 for graphic symbols and IEC 81346-2 for reference designations, and shows how the same diagram now reads identically on a Siemens SIMATIC, an Allen-Bradley ControlLogix, or a Mitsubishi MELSEC drawing.
Why graphic symbols still matter in 2026
Every electrical schematic is a contract between the designer and the maintenance technician. The contract has two parts: the pictorial part (what shape, what line, what mark) and the alphanumeric part (which letters, which numbers, in which order). IEC 60617 governs the pictorial side, IEC 81346 governs the alphanumeric side, and IEC 61082-1 governs how the two are laid out on the page. Together they are the reason a wireman in Stuttgart, a controls engineer in Houston, and a panel builder in Shenzhen can all open the same PDF and see the same motor contactor.
Graphic symbols (IEC 60617) — the pictorial vocabulary
The IEC 60617 database (formerly the IEC 60617 printed standard, now an online database maintained by IEC since 2012 and refreshed through 2026) is the canonical source for every shape you see on a modern schematic. The five families a maintenance engineer will see most often are:
| Symbol family | Typical IEC 60617 ID | What it represents on the drawing |
|---|---|---|
| Conductors & connections | S00001 / S00019 / S00020 | Solid line = power or signal; dashed = mechanical link; dotted = logical / sensor link. |
| Passive components | S00555 / S00557 / S00565 | Resistor, capacitor, inductor (with IEC arrowhead conventions). |
| Contacts & switching | S00265 / S00267 | NO contact (S00265) and NC contact (S00267) — the two most-used symbols in any ladder diagram. |
| Operating coils | S00336 / S00337 | Relay coil, contactor coil — rectangular box with terminal numbers. |
| PLC I/O blocks | S00915 / S01452 | Generic input/output card symbol — manufacturer-specific pinout drawn inside the rectangle. |
The practical takeaway: when you see a rectangle with a single diagonal line through it, you are looking at a normally open contact regardless of whether the schematic was produced in EPLAN, AutoCAD Electrical, or Studio 5000. The shape is owned by IEC 60617, not by any vendor.
! Watch out
IEC 60617 and the legacy JIC / NEMA symbol sets still coexist on European-built versus American-built panels. If your schematic was exported from an older RSLogix 500 project, expect a JIC NO contact (two parallel lines, no diagonal). Re-tag the drawing before you hand it to a CE-marked panel shop — or send the PDF to Moritta@KOEED.COM and we will help you identify which standard it follows.
Reference designations (IEC 81346-2) — the alphanumeric vocabulary
IEC 81346-2 (most recent revision published 2020, reaffirmed through 2026) defines a single-letter code for the kind of object on the drawing and a structured prefix for which subsystem it belongs to. The single-letter codes every maintenance engineer should keep in their back pocket are:
| Letter | Class (IEC 81346-2) | What you actually see on the drawing |
|---|---|---|
| R | Restrict / resist | Resistor, heater, brake resistor on a Yaskawa drive. |
| K | Process / close | Relay, contactor, auxiliary contactor — the heart of any ladder. |
| Q | Switch / power control | Circuit breaker, disconnector, motor starter. |
| S | Sense / switch | Limit switch, selector, proximity sensor, photo-eye from KEYENCE. |
| M | Motor / mechanical drive | Three-phase motor, servo motor, stepper. |
| A | Assembly / control device | PLC, HMI, robot controller — the entire ControlLogix chassis collapses to one A-tagged block. |
| F | Protection | Fuse, thermal overload, current-limiting protector. |
| G | Generation / supply | Power supply unit, UPS, generator. |
In practice the letters are usually prefixed by a higher-level location or function block per IEC 81346-1: an input card on the conveyor subsystem might be tagged =A1-A2-X1 for cabinet A1, subfield A2, terminal X1. This is exactly the kind of structured designation that newer EPLAN P8 and Zuken E3 templates enforce by default in 2026.
What changed since 2024
Three things shifted between the original 2024 article and this 2026 update:
-
Reference designations got structured. IEC 81346-1 is now widely used at the cabinet level (e.g.
=A1+B2-K3for “site A, line B, contactor K3”), not just as the legacy IEC 60750 single-letter tag. - Single-line diagrams are now drawn first. IEC 61082-1 (2024 amendment) explicitly recommends producing a single-line overview before the multi-line circuit diagram; modern CAD templates default to this workflow.
- CAD libraries import the IEC 60617 database by name. EPLAN, Zuken E3, AutoCAD Electrical 2025+, and SEE Electrical all consume the IEC 60617database; symbols are stamped with their S00xxx ID, so you can search “S00265” and get every NO contact in the project.
> Tip
If you maintain older Mitsubishi FX or Allen-Bradley SLC 500 drawings, re-export them through the manufacturer’s current CAD plugin. You will pick up the new S-prefix IEC 60617 IDs and the IEC 81346 reference designations for free.
Reading a PLC I/O diagram across brands
The real value of all this standardization is that the I/O sheet for a Siemens S7-1500, an Allen-Bradley 1756 rack, or a Mitsubishi iQ-R system now uses the same IEC 81346 letter codes. The map below shows the typical translation that a maintenance engineer needs to memorize:
| Function | Siemens (SIMATIC) | Allen-Bradley (Logix) | IEC 81346 letter |
|---|---|---|---|
| Digital input module | 6ES7 321 (SM 321) | 1756-IB16IF | A / S |
| Digital output module | 6ES7 322 (SM 322) | 1756-OB16E | A / Q |
| Analog input module | 6ES7 331 (SM 331) | 1756-IF8H | A / S |
| Power supply | 6ES7 307 (PS 307) | 1756-PB72 | G |
| CPU / controller | 6ES7 516 (CPU 1516) | 1756-L73 | A |
Whether the tag reads I1.0, Local:1:I.Data.0, or X0, the symbol on the drawing still follows IEC 60617. The letter prefix is what tells you which subsystem the channel belongs to. Use our PLC Error Code Database or the PLC Hex to Float Converter when the signal scaling crosses IEEE 754 territory.
Sourcing the modules on the drawing
The shortest path from a schematic back to a part number is to look at the IEC 81346 letter, find the manufacturer prefix on the BOM, and match it against your vendor. For modules that are end-of-life or hard to find — common on older S7-300 and SLC 500 plants — send the BOM (or even the PDF of the schematic) to Moritta@KOEED.COM. We keep active and EOL stock for the same nine brands our schematic tags cover, and we will get back to you within 24 hours.
Need a module from this schematic?
Send the BOM (or a PDF of the drawing) to Moritta@KOEED.COM. Active stock, EOL stock, and cross-references — all in one quote within 24 hours.
Send My BOM →Frequently asked questions
Is IEC 60617 the same as the old printed standard?
The content is the same; the format moved to the IEC 60617database (online) after the 2012 restructuring. The 2020 consolidation remains the 2026 reference.
Do Siemens and Allen-Bradley use IEC 81346 reference designations?
Both vendors expose IEC 81346 prefixes in their CAD libraries, but legacy projects still use IEC 60750 / vendor-specific tags. Re-tag during migration.
What is the difference between NO and NC contact symbols?
IEC 60617 S00265 (NO) shows two parallel lines; S00267 (NC) adds a diagonal slash. The line above or below a letter is a legacy NEMA / JIC convention.
Can I still use JIC / NEMA symbols on a 2026 schematic?
You can, but only inside North America. CE-marked panels shipped to the EU are expected to use IEC 60617. Most CAD tools offer a project-wide toggle.
Where can I cross-reference an EOL module on an old drawing?
Send the original PDF and BOM to Moritta@KOEED.COM. We stock legacy and current parts for the nine brands on the schematic; reply within 24 hours.
Which CAD libraries carry the IEC 60617 symbols?
EPLAN, Zuken E3, AutoCAD Electrical 2025+, SEE Electrical, and SolidWorks Electrical import the IEC 60617database; symbols carry their S00xxx ID for search.