Industrial Control Panel - Ground Bus

[designer82]

Industrial control panels housing plc's, I/O modules etc.

There is a safety ground bus on these panels.

Should there also be an isolated instrument ground bus?

 

[petersonra]

Industrial control panels housing plc's, I/O modules etc.

There is a safety ground bus on these panels.

Should there also be an isolated instrument ground bus?

You can have as many ground busses as you want.

However, they all need to be connected together at some point. Not necessarily in the control panel.

I am of the opinion that paragraph 14.1 of UL 508a prohibits an "isolated" ground bus, but it might be read otherwise.

 

[tom baker]

What would the isolated instrument ground bus connect to outside if the panel and what would it’s function be?

 

[drktmplr12]

an isolated "ground" isn't a ground. it's just a floating conductor. ground means its a path to earth. you want the path to earth to have as little disturbance as possible.

if you want an instrument ground for shields, etc.- stand the ground bus off on insulators then connect that bus at one end to the panel's main ground bus with a #6 wire or something like that. connect nothing else to it except instruments and other low voltage devices which you want to use the same reference. then have a separate, more typical, ground for SPD ground conductors and panel grounds and connect that to the panel ground.

if you want to truly isolate a system, float the (-) terminal of the DC power supply. that can get you into trouble elsewhere though