Tie PLC's DC Negative To Ground??

[Elihu]

When building small control panels w/PLC's I'm usually adding them to an existing control for a large machine. The OEM control is often an all AC relay-based system, with a 24VAC control voltage, and 240 (or 480) 3-phase powering the big motors, etc.

I mount a 24VDC supply to run my PLC, sensors, etc, and I "talk" to the OEM control through relay outputs.

I usually just float the DC negative, and everything works fine. But I think I should be tying the DC negative to the machine ground. Should I be doing this every time? Is there a down-side to tying my DC negative to ground?

Yes, the (Automation Direct) PLC's input power plug has a ground terminal (which I use) -- but this doesn't tie the negative to ground.

 

[djs]

There is no reason that I know of to ground the 24VDC power if you are not using analog inputs. If you are using analog inputs that feed off of the 24VDC supply, then grounding the power supply might reduce noise. But then again it might increase the noise. So we make it optional.

 

[danw2]

Lots of bar stool opinions here:

http://www.plctalk.net/qanda/showthread.php?t=23355&highlight=grounding+24vdc