I am assisting in moving a machine shop into a new to us building. The building service is 1200A 240V Delta with a center-tapped phase.
All subpanels located on perimeter walls have only 4 conductors run to them. The 3 phases and the neutral. I am concerned about a lack of grounded Earth conductor. All of the subpanel feeders appear to be run thru EMT buried in or below the concrete slab floor. The building is corrugated metal on a steel I beam frame. The bases of the I Beams are sitting on the foundation walls. I do not see any bonding jumpers from the steel to earth, and only at the building's main CB do I find some sort of bonding jumper.
ASSUMING the neutral is bonded to the building steel, is it OK to install the listed ground bars for the subpanels and bond them to the EMT coming out of the concrete and also to the building steel?
I have been told that the job will not be inspected. Never the less, I am pushing to do the job right.
The EMT is being used as the required equipment grounding conductor for the feeders to the sub-panels. Installing an equipment ground bus to the cabinet should suffice. You could install a grounding bushing or a ground clamp to the EMT & connect it to the equipment ground bus if you want.
I don't believe that connecting each sub-panels equipment ground bus to the building steel will have much benefit.
Few things are harder to put up with than the annoyance of a good example.
Our AHJ would have a fit: EMT in the slab! Be it setscrew or compression, I have yet to see an EMT coupling I would trust to carry fault current having been exposed to wet soil. The high threshholds of OCPDs in motor circuits, the nature of the usage -- craftsmen stomping around with metal in their hands, in a metal building with metal encased motors sitting on a semi-dielectric pad.... I would want separate bonding conductors. My AHJ would insist on abandoning the old work entirely. End of story. ( I had the 'good fortune' to step in as foreman on just such a 'red tag'. )
Re: Center-tapped 240V delta subpanel GEC#88684 07/08/0409:19 AM07/08/0409:19 AM
358.10 Uses Permitted. (A) Exposed and Concealed. The use of EMT shall be permitted for both exposed and concealed work. (B) Corrosion Protection. Ferrous or nonferrous EMT, elbows, couplings, and fittings shall be permitted to be installed in concrete, in direct contact with the earth, or in areas subject to severe corrosive influences where protected by corrosion protection and judged suitable for the condition.
Ryan Jackson, Salt Lake City
Re: Center-tapped 240V delta subpanel GEC#88685 07/08/0410:14 AM07/08/0410:14 AM
Well its been almost a year, and we finally moved the rest of the company into this building.
I still have concerns about the ability of the grounding paths to handle any significant fault currents.
My thought is to test the resistance of the ground paths by using a welder to supply a controlled current of 100-200 Amps for a few seconds between the subpanel ground bar and the building's service bonding location and then measuring the voltage drop between the subpanel ground bar and the building's central ground.
Obviously this would have to happen with the subpanel deenergized, along with suitable Kelvin connections to ensure we are only measuring the voltage drop along the ground path and not the v drop across the welder to ground bar connection. Also we would need enough people to look for unexpected arcs or excessive heating across any high resistance ground paths.
Assuming I can get permission to perform these tests, what would be an acceptable range of voltage drops?