Isolated Ground Options
OK, not sure if I have ever actually posted anything here or not…been a fan for years. I am an electrical engineer – been doing this about 15 years. Feel free to take your shots.
I have an isolated ground question…which I have read many, many threads about.
The AV designer on a project I am working on in Georgia insists on IG receptacles for their equipment. The contractor installed the IG receptacles AND installed the separate IG and EG conductors from each IG receptacle back to the serving panelboard. That panelboard has a separate isolated ground bar onto which the IG conductors from these branch circuits were landed.
Here is where it gets interesting. This branch circuit panelboard (100 amps, 208v, 3 phase) is served from a 600 amp, 208v, 3 phase distribution panelboard in another part of the building. The building service is 480/277 volts, 3 phase so this distribution panel is fed by a dry type transformer. The contractor did not include a separate IG conductor in with the feeder conductors from the distribution panel to the downstream branch panel. This feeder is installed in RMC and includes an insulated EGC. The feeder conductors are now installed and connected.
I would normally take a separate insulated ground conductor all the way back to the dry type transformer XO bushing. It would route through and not be bonded to the distribution panel as allowed by the NEC. Since this was not done the way I see it the contractor has two options available (without physically pulling out the feeder conductors and pulling in a separate ground conductor) for this installation to comply with NEC:
Option 1- Use the existing insulated equipment ground conductor installed with the feeder as an IG conductor. This would require installing an isolated ground bar in the distribution panelboard. The equipment grounding conductor installed with the feeder serving the downstream panel with IG loads would be terminated on this IG ground bar. The RMC would then become the equipment grounding conductor.
Option 2 – Just bond the isolated ground bar to the standard panelboard ground bar in the branch circuit panelboard. I realize this defeats the purpose of the IG ground bar…but I am not sold on the effectiveness of IG anyway. I can tell the AV guy that we have IG outlets and separate insulated grounding conductors, isolated from the metallic box/conduit system back to the serving panel. That much is true.
Timothy W. Trotter, PE, RCDD