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  • PLC wiring basics

    Hi,

    I have 120VAC coming in to an enclosure, live, neutral, and ground; and I am using a DC power supply to convert the AC power to DC to power my PLC and other 24VDC components.

    The PLC has a place for a ground wire, but the only ground I have coming into the PLC is the 120VAC. I have terminated the ground wire onto some terminal blocks so I can supply the ground to multiple devices, but I am nervous to plug an AC ground wire into a PLC that is powered by DC power.

    Do I just wire that AC ground wire directly to the G terminal of the PLC? The instructions specify the PLC must be grounded to earth, which I know is the same ground coming in through my three pronged AC power cord.

    Just don't want my PLC exploding because I run something AC to it.

    Thanks for your help!



  • #2
    Assuming you're using steel DIN rail, run the incoming 120VAC ground wire to a GROUNDING terminal block (special type that have metal fingers that clip onto the rail) and then run all ground terminals from other devices (power supply, PLC, network switch, etc..) to the same type of grounding terminal block. Some will also run the DC negative to ground as well but I never have. It may or may not help if you run into a noise issue with anything.

    Here's a grounding terminal:

    https://www.automationdirect.com/adc...ks/kn-g12sp-10

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    • #3
      Adisharr has it right. We show the proper method on page 1-11 of the BRX user manual: https://cdn.automationdirect.com/sta...xuserm/ch1.pdf
      If you have an urgent issue, please contact AutomationDirect's Technical Support team.

      AutomationDirect.com Technical Support: 1(800) 633-0405 or (770) 844-4200 Email Tech Support

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      • #4
        Ground wires should "never" be conducting any electrical current in circuits anyway. It is there mainly to provide voltage reference. There are some types of electrical circuits that do intentionally run small bits of current through the ground wires though.

        DC negative is essentially a ground itself in the majority of DC voltage systems like those PLCs use. Instead of +24v and -24v with a 0v ground reference, it is simply +24v and 0v. That 0v is not always the same thing as normal ground, it is usually more akin to neutral wire on an AC line. However some systems do require 0v DC be tied to panel ground, and in many AC systems neutral is also tied to ground at either the main subpanel or the transformer itself. This doesnt mean you can just use ground wires for your AC neutral or your 0v DC, and ground is still meant to not carry any current itself. They may or may not be tied together at some point, but the actual wire purpose is different. That is why using proper wire color codes is important as well, so you know what wires are for what purpose and what type of voltage they will most likely be carrying.
        Last edited by MikeN; 11-29-2018, 09:58 AM.

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