Using an H.F. start plasma unit should not be a problem if you get your grounding correctly installed. As you say shielded cables will be a requirement. You will need to get your grounding cables back to a star point and connected to an earthing rod would be a good idea but just make sure your earthrod is installed in a position where the soil does not dry out too much.
Try to avoid any parallel paths for your grounding cables.

Regards

Ian