Per below I'm not clear by the "ground bushings connected to ground bus by mean of cooper wire". There is never a separate ground wire that goes from a panel to a ground bar.
Equipment Ground detail: All conduits entering service or distribution switchboards, distribution or power panels,
[FONT=Arial,Arial Narrow][FONT=Arial,Arial Narrow]transformers, PDU’s, and ATS shall be provided with grounding bushings, connected to the respective equipment [/FONT][/FONT]
ground bus by means of an insulated copper wire (sized as per code requirements). All conduits entering cable
support boxes, pull boxes and junction boxes shall be provided with (2) grounding locknuts (1 inside 1 outside the enclosure). All non-[FONT=Arial,Arial Narrow][FONT=Arial,Arial Narrow]conductive paint or coating’ shall be removed from the enclosure at the threads, contact points [/FONT][/FONT]and contact surfaces unless the fitting provided is designed to make such removal unnecessary.