How to Read Industrial Control System Wiring Diagrams

I write a lot about the PLC side of industrial automation, but it’s also fundamental to have a good foundation in the electrical side of things.

First of all, most modern (North American) industrial control system wiring diagrams have a relatively common numbering scheme, and once you understand the scheme, it makes it fairly easy to navigate the wiring diagram (commonly called a “print set”).

Let’s start with the page and line numbering. Most multi-page wiring diagrams use a two digit page number (page 1 is “01”). In the rare, but possible, event that you end up with over 99 pages, some diagrams will just add 3 digit page numbers (starting at “100”), but if there was any forethought, many designers will divide their wiring diagrams into sections, giving each section a letter (let’s say “A” for the header material, “B” for power distribution, “C” for safety circuits, etc.). Within each section, you can re-start the page numbering at “01”. This has the added bonus of letting you insert more pages into one section without messing up the page numbering.

Within a page, you’ll typically see line numbers down the left side (and frequently continuing down the middle if you don’t need the whole width of the page for your circuits). These numbers will start with the two digit page number, followed by a two digit line number. Typically these start at zero, and increment by twos:

  • 1000
  • 1002
  • 1004
  • 1006
  • … and so on

Now, devices (like pushbuttons, power supplies, etc.) usually have a device ID based on the four digit line number where they are shown in the wiring diagram, possibly with a prefix or suffix noting the device type. So, if you have a pushbutton on line 2040, the device ID might be PB2040 or 2040PB. The device ID should also be attached to the device itself, normally with an indelible etched label (lamacoid). Therefore, if you find a device in the field, you should be able to find its location in the wiring diagram. (Finding the wiring diagram, of course, is often the more difficult task.)

Wires are numbered similarly. The wire number is typically based on the four digit line number where the wire starts, plus one extra digit or letter in case you have more than one wire number starting on the same line. So, the wire numbers for 2 wires starting on line 1004 might be 10041 and 10042.

It’s typical for wires to connect to devices that are on other pages (it’s extremely common, in fact). In that case, you’ll see off-page connectors. The shape of these vary based on whose standard was used for the wiring diagram, but they’re typically rectangles or hexagons. In either case, inside the shape will be the four digit line reference number where the wire continues. The other end of the off-page connector (on the other page) also has an off-page connector in the opposite direction. Note that you frequently see connectors from one place on a page to another place on the same page, if it happens to improve readability.

That’s all you really need to know to find devices and follow wires in a wiring diagram. Now, to understand the components in an industrial control system, that’s going to take longer than a blog post. For a great introduction, I recommend the book Industrial Motor Control by Stephen Herman. Google books has a great preview if you want to check it out.

4 thoughts on “How to Read Industrial Control System Wiring Diagrams

  1. Marshall

    “First of all, most modern (North American)…” And for EU based equipment IEC 61346 the standard to read up on. Personally I find both systems have there pros and cons. The N. American style is very basic and simple. For example from the output of the PLC there is a line directly to the actuator. In a IEC based drawing this could result in jumping though several pages as every connector, terminal box and wire used is documented. For a programmer the N. American style is quick and easy to use as we generally don’t care about the connection hardware when developing a program. From the electrical side, while at first the IEC based drawings can be overwhelming in a troubleshooting situation having all connections documented reduces down time as less time is spent tracing wires.

    The IEC 61346 standard is modular based I don’t see the needs to explain the benefits with that concept.

  2. Scott Whitlock Post author

    @Marshall – good points. I’ve only run into EU-style drawings a couple of times, and found them difficult. You need someone to explain them to you. I did have a paper on how to read them one time, but I’ve lost it unfortunately. Do you know of a good online tutorial?

  3. Jeremy

    @marshall, @scott whitlock – did either of you find a good online class, or in room class for reading EU style drawings? I have been getting more and more of their drawings and it is difficult to navigate.

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