Supply Side bonding jumper

[RBlais13]

I have a 13.8-4.16kV transformer (D-Y). The neutral is impedance grounded (400A @ 10 seconds). The transformer feeds a lineup of switchgear. According to 250.30(A)(1), Exception 2 of the NEC, is worded such that a wire type bonding jumper from the transformer enclosure to the "Incomming Main" of the 4.16kV switchgear wouldn't be required.

Because I have an impedance grounded transformer, I don't know that 250.30(A)(2) applies at all.

Backing away from the code, the goal here is to provide a path back to the source for ground fault current. The path should be able to carry the current long enough for the relaying to actuate. Since the ground current is limited to 400A, I would think that the 5kV shields on the lowside feeder cables should suffice? Do I really need to run a wire type supply side bonding jumper with the 4160 circuit conductors from the outdoor transformer to the switchgear building?

In addition to this. We have a ground loop around the outdoor transformer that will be tied to the ground loop outside the switchgear building. I'm thinking that this makes this system NOT separately derived.

 

[melspuds]

I take it that the transformer Hi side has relay protection to drop out the 13.8kV when you have a ground fault on the 4160 side. Is the impedance grd at the transformer or 4160 switchgear main?
NEC ART. 250.30 applies to low voltage systems, need to go by 250.180 ( systems > 1000 volts). See 250.187 about Impedance grd neutral systems.
See ART. 250.66(C) and 250.52(A)(4) grd rings. The code is very lack on grd rings unfortunately, it depends on the AHJ. Some say that a ground ring should be only used for equipment grounding only but there times that a grd ring will be the only good method to use. Check with the AHJ.
Using cable shields for the ground return path, is not a good idea in IMO. The cost of a grounding conductor is not much when you could damage a cable with a high current ground fault. MV cables are not cheap compared to a single grounding conductor.OMI
Hopes this helps and good luck,
Dave