Medium-Voltage Separately-Derived Systems

[sparkyguy545]

Given
Owner takes utility power @ Medium-Voltage and maintains their own on-side medium-voltage distribution. Proposed improvements require a new outdoor oil-filled padmount xfmr. Primary is Delta, Secondary is 480Y/277 solidly-grounded. Owner wishes to use rigid steel conduit between the outdoor xfmr and interior-mounted switchboard. We wish to install the N-G bond link in the interior switchboard so it can be inspected without opening the exterior transformer door. Only the Phase conductors and Neutral conductors are to be pulled in S.E. conduits.

Question #1
Isn't it mandatory to open (remove) the N-G link in the outdoor xfmr? If the N-G bond is installed in both the xfmr and the switchboard the GRS conduit into the building will be in parallel with the system Neutral. This means it would (incorrectly) carry approximately 1/2 of the neutral current.

Question #2
Similar question at a different site. This time, the padmount xfmr is installed and owned by the utility. How can metallic S.E. conduits be used for service-entrance? Locally, most of our utilities tell us they MUST install a N-G bond at their pad-mount transformer. Then, we install the typical N-G bond at the switchboard. Problem . . . . the metallic S.E. conduits are now in parallel with the system Neutral conductors. The conduits will (incorrectly) carry 1/2 of the neutral current.

Section References appreciated. It is recognized that all these issues will go away if no metallic conduit is utilized or direct-buried cable installed.

 

[charlie]

Question #1
. . . Isn't it mandatory to open (remove) the N-G link in the outdoor xfmr? . . .

Question #2
. . . This time, the pad-mount xfmr is installed and owned by the utility. How can metallic S.E. conduits be used for service-entrance? Locally, most of our utilities tell us they MUST install a N-G bond at their pad-mount transformer.

We have escaped these questions by prohibiting a customer to enter our pad-mounted transformers with metallic conduit. I believe the answer to the first question is no but I don't have a reference. To address the second question, the N-G bond is required by the NESC rule 092.B1.

 

[don_resqcapt19]

This is an ongoing issue with the code. The rules change at the service point. In the first case, you have a SDS and you can only have a neutral to ground point a either the transformer or the first disconnect, but not both. In the second case the code requires a bond at both the transformer and the service disconnect at the building. Per the NEC parallel grounded conductor paths are not only not a problem on the line side of the service point and are actually required in a number of cases. A couple of examples that are code required parallel paths are a common underground metal water pipe, the shield of a TV cable system, and the metal sheath of an underground phone cable.