Electrical schematics that survive the field: from harness design to as-built reality

[Aior]

The schematic is a contract​

The schematic is the document an electrician reads at 3 AM in a cabinet that's been on the line for 8 years. It needs to be unambiguous, readable, and consistent with reality. Most schematics aren't. The patterns below are what we've converged on after re-drawing too many cabinets that "had documentation".

Page-level discipline[/HEADING>

• One function per page. 24 V supply on one page, motor 1 power on another, motor 1 control on a third. Don't cram three functions into one A3.
• Title block done properly. Project number, page number, page count, revision, designer, approver, date. The 1990s discipline still pays off.
• Sheet references explicit. Where a wire continues to another sheet, the reference includes sheet number AND coordinate (e.g. "/3.B5"). "See sheet 3" is not enough.
• Component IDs unique across the whole project. -K1 means one and only one contactor in the whole drawing set.

Component naming that survives​

• IEC 61346 / 81346 reference designations — letters that mean things (Q for switchgear, K for relays/contactors, F for fuses, M for motors, U for inverters/drives).
• Numbered sequentially per type, not arbitrarily.
• The same designation on the schematic, the panel layout, the BOM, and the physical label on the part. No translation table required.

A schematic with -K1 in three places and -KM01 elsewhere wastes electricians' time forever. Pick one and enforce.

Wire numbering​

We use signal-oriented wire numbering: the same signal has the same wire number from source to destination, even if it goes through terminal blocks. So +24V_A is +24V_A everywhere. Differs from project to project — pick a system and document it on the cover page.

Don't use sequential wire numbering ("wire 247"). It gives the electrician no information when they trace.

Cable schedules and harness drawings​

The schematic alone isn't enough for a panel build. The package needs:

• Cable schedule — which cable, which cores, from where to where, with cross-section and rating
• Terminal lists — which terminal block, which terminal number, which wire, which ferrule label
• Panel layout — physical placement of components, with mounting hole locations
• External wiring diagram — interconnections to the rest of the cell, sensor/actuator pinouts

The shop builds from the cable schedule and terminal list. The schematic is the reference; the build documents are the worksheet.

Tools we ship in​

• EPLAN — industry standard for industrial electrical design. Expensive, mature, what most automotive and process customers expect.
• AutoCAD Electrical — the lighter alternative. Common in smaller shops.
• KiCAD — for in-house product PCB design and simpler wiring schematics. Free, capable, growing rapidly.
• Altium — when the project is mainly PCB design and the team is already there.

For PCB-level design (custom electronics, sensor boards, controller cards), KiCAD has become genuinely competitive with Altium for our typical complexity. For panel design, EPLAN is still the answer most customers expect.

Simulation: SPICE for the analog, behavioural for the digital control​

Pre-build simulation that pays off:

• Power supply transient analysis — does the rail droop on motor inrush?
• Filter response — frequency response of input filtering, EMC mitigation
• Behavioural simulation of safety circuits — does the safety chain meet the required PL?

Simulating the entire control system is rarely worth it for industrial applications — the PLC is the simulation. Focus simulation effort on the analog edges and the safety-critical logic.

As-built vs as-designed: the documentation that's never updated​

Every commissioning has wiring changes, component swaps, parameter tweaks. The discipline that survives:

• Mark up the printed drawing during commissioning, in red pen
• Update the master drawing within a week of commissioning end
• Re-issue the drawing set as "as-built", revision A
• Commit revision A to source control / DMS

The "we'll update the drawings later" plan never executes. As-built within a week or never.

One thing we'd never repeat​

Designing the schematic in isolation from the panel builder. The 30-minute review with the panel builder before drawing freeze catches mounting issues, sizing issues, wiring path issues that would otherwise show up as build problems.

What's your schematic toolchain? Anyone fully on KiCAD for industrial panels yet?​