Substation neutral unbalanced current in grid

[RSChinn]

Are there standards (IEEE, NESC) that require the need for an express (implying insulated) connection between the neutral bus in a switchgear and the Xo bushing of the transformer? There are thoughts that simply connecting the switchgear neutral bus which may be carrying 100 amps or more of unbalanced load to the grounding grid and using the grid to connect to the Xo bushing is qa concern. The grounding grid connects to the fence and all the metal structures throughout the substation yard making all metallic surfaces part of the return path. Is this a safety concern for personnel touching the metal surfaces during 1) the normal flow of unbalanced current and 2) during a GPR event.