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I'm putting together the controls for a CNC router table I'm building. In the enclosure I've got some DIN rail power supplies, contactors, and terminal blocks.

Also in the enclosure is the VFD for the spindle and the computer interface electronics.

The power supplies supply +24 V (computer interface, sensors, and fans) and +75 V (servo power). Should the negative (0 V) rails be tied together and to ground (and the chassis)? I don't mean using the chassis as a conductor, there are return lines for everything. But should they be tied together?

Is there a typical practice for this in industrial control systems? I'm inclined to tie my 0 V rails together (so everyone’s referenced to the same voltage), but not tie that to chassis ground.

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Preferably all your GNDs should be connected at one point and one point only. Near to your power supply is best.

You don't need to connect them to the chassis. If you wanted your DC- to match your Earth, you should connect it at the power supply to the Earth wire. Doing so can help with noise, but it means your DC side is no longer isolated.

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  • \$\begingroup\$ And is there any reason to isolate the DC supplies from one another? For example, if I keep the 75 V supply isolated from the 24 V supply by not tying the 0 V rails together, would that help mitigate noise from the servos (if any)? \$\endgroup\$
    – Rick
    Commented Jun 7, 2019 at 7:02
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    \$\begingroup\$ I've also seen connecting the GND to the earth making the noise worse. I'm not sure if there is a clear best practice for this thing, to me it seems to be highly dependent on the system and only testing will reveal how you get the least noise. \$\endgroup\$
    – TemeV
    Commented Jun 7, 2019 at 7:14
  • \$\begingroup\$ @Rick How are the servos controlled? Does the 75V go through relays, mosfets, or? Are the circuits from the different supplies completely isolated from each other? \$\endgroup\$
    – hekete
    Commented Jun 7, 2019 at 14:30
  • \$\begingroup\$ @TemeV connecting digital GND to earth is a bit complicated. Since you're dealing with EMI, ESD, RFI, etc. People seem to have a whole bunch of different ideas about what is the best practice.. \$\endgroup\$
    – hekete
    Commented Jun 7, 2019 at 14:33
  • \$\begingroup\$ @hekete Yeah, the different ideas is one thing that makes it really complicated. And rarely I hear a proper rationale behind the idea. \$\endgroup\$
    – TemeV
    Commented Jun 7, 2019 at 16:34

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