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I am currently designing a control system. I have to provide two separate DC voltages to my PLC and network switch.

I want to know what the correct practice is for grounding on the DC side. If I have two different DC power supplies, must the ground between the power supplies be connected or not?

I do not understand why they have to be connected within a control system or when. If any help can be given it will be very much appreciated.

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  • \$\begingroup\$ Hi Feri, Polarity errors have been corrected. \$\endgroup\$
    – vu2nan
    Feb 3, 2023 at 13:23

2 Answers 2

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The key things you need to understand are:

  1. Whether the PSUs are internally isolated or grounded
  2. If isolated, whether there are any AC coupled transients from the mains side (many isolated SMPSs, have enough capacitive coupling that you can get 240V at ~10uA relative to ground)
  3. How/if the PLC connects to the network switch, and whether these connections are galvanically isolated.

It might be hard/impossible to definitively determine the answers to these questions, and the potential stray currents might play havoc with reliable system operation.

This is why cross-connecting the DC negatives to give a shared reference is such a common approach, but it may or may not be needed.

If you want to really see what is going on, take a two channel scope that can do "difference", then connect the tip of one probe to one negative and the other tip to the other negative (leave the shield clips unconnected). The signal you see is what you are minimizing by the cross-connect...

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  • \$\begingroup\$ If you do add common ground, the other thing to be aware of is "ground loops". The classic example is wiring which led to hum on old style separate element HiFi systems. In a "star" grounded system there is a single ground point and no grounding between elements other than that. That's fine. If you have multiple cross-connects, you can get "ground loops", where varying magnetic field in the loop lead to hard to avoid induced EMF. Either star ground, or have lots of short ground connects minimizing loop area, don't have 2 connections with a large loop area.... \$\endgroup\$
    – colintd
    Feb 3, 2023 at 13:42
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I don't see any grounds on the PWS outputs-. Need that to meet code. If you tie each supply out- directly to the main PE ground instead of each other your commons are now tied also. You can run each supply common separately to the remote load returns for that supply (different wire numbers), knowing they are the same potential with no worries about PWS1 return current going to PWS2 & vica-versa so no ground/common return loops. NEC, NFPA 79 and EN 60204 have some decent examples and explanations on this.

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  • \$\begingroup\$ am I understanding correctly that you are referring to this kind of connection? wiring diagram If possible can you please direct me to any possible examples relating to these as I am unable to find specific examples with regard to the ground loops that are referred to in your answer? \$\endgroup\$
    – Feri
    Feb 4, 2023 at 8:04

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