24VDC negative ground connection

no body

New User
Location
Edmonton
Occupation
electrical field service tech
I currently went to a site to troubleshoot some signal spike issue on their gas plant. What I found is three ground bar in the PLC cabinet. equipemnt ground bar, DC negative ground bar, and isolated instrument ground bar. and they put a jumper to jump negative ground bar to equipment ground bar. I was wondering shouldn't be jump DC negative ground bar to isolated instr ground bar to avoid nosie for the 24vdc signal more make sense. Please let me know where the negative ground bar should connected to?
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Generally ul508a requires the DC control power be grounded so this is probably intentional and correct.

The instrument ground bus is for shields. It does not serve any useful purpose but makes people feel better.
 

epelectric33

Member
Location
SD
Occupation
Electrical Consulting/Inspection/Master
Generally ul508a requires the DC control power be grounded so this is probably intentional and correct.

The instrument ground bus is for shields. It does not serve any useful purpose but makes people feel better.
The funny part is that it is still required for the "isolated ground" still be connected to the system grounding servicing the facility (by a smaller conductor so that it has some resistance).
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
The funny part is that it is still required for the "isolated ground" still be connected to the system grounding servicing the facility (by a smaller conductor so that it has some resistance).
Most cases where I have seen this arrangement they just run a wire between the shield grounding bus and the equipment grounding bus. I've never actually seen a case where they have a separate isolated shield bus. It used to be really common in specs though.
 

epelectric33

Member
Location
SD
Occupation
Electrical Consulting/Inspection/Master
Most cases where I have seen this arrangement they just run a wire between the shield grounding bus and the equipment grounding bus. I've never actually seen a case where they have a separate isolated shield bus. It used to be really common in specs though.
I have seen that as well. I have also seen where they will make you install a ground rod triad and bring that to a separate bus but also make you do the small jumper to the equipment grounding bus. I remember this came out when computers and PLCs really started getting popular, but the real problem was that the grounding/bonding system was installed improperly to begin with and surge or lightning would pop them all the time so this was there so called uneducated solution. This system does help with "noise" on instrumentation and such from my experience, but so does a solidly grounded system that is installed correctly.
 
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