AC and DC grounds

[redeemer 0]

Hi everybody, I was wondering: if in a control panel I have both AC and DC, which is better, to connect both grounds together, or leave them completely seperate? Does connecting the two grounds cause any problems to either network? Thanks, Fadi Mansour

[RMA 0]

I would leave them separate. The chance that they are brought together somewhere else outside of the panel is pretty high. If you then join them in the panel as well, you can finish up with ground loops, which under some circumstances can seriously disturb your analogue signals. Edited 13 Dec 2004 by RMA

[Pierre 1]

I personnaly never connect any DC supply to ground. It does carry more noise but it has saved a few CPU. When you put 120Vac to a 24 Vdc input and that is an isolated input nothing will happen IF the common dc supply IS NOT grounded. So I am never afraid to "play" live with the dc signals. If I short it... well its not a short :)

[KinK 0]

Just a Low resistance load right

[panic mode 246]

you will quickly change your mind first time you touch 24VDC of the PSU that still runs but has broken (melted) insulation primary/secondary (DC side not floating anymore, it's riding on AC wave). ------------------------------------------------------- still trying hard to kill my first CPU

[Pierre 1]

And of course you have never touched any live wires. And of course you have never worked on live wires. And of course you have never shorted a live wire. And of course you have never given any answers to the original question. Blablabla.

[Snerkel 0]

I always tie 24v to earth. I would rather be aware of a fault caused by a short/blown fuse than to have to trace what could be multiple faults, and also could cause damage to machinery or persons. Image you have 24v floating with respect to earth. You have a proximity switch with a fault that brings the switch wire in to contact with earth... you have no indication of this. Then another fault occurs, a 24v valve gets an earth fault, again on its own this would not manifest itself. Together the results would be:- When the proximity switch turns on the valve turns on. When the valve turns on the input for the proximity switch shows as on. The fault is going to be hard to track and could be potentially fatal.

[panic mode 246]

I tought i did... I always ground DC end of every PSU and I still didn't kill any PLCs by doing so.

[Lancie1 0]

It is also the local law to ground all electrical devices, as implemented by the US National ELectrical Code in most places in the States....