It can be insulating, or a suitable metal ... selection of this would be in line with the enclosure itself to prevent electrolytic  corrosion.

For controlgear or switchgear panels, these are out of scope of BS 7671, and BS EN IEC 61439 series is a common standard, perhaps with BS EN 60204-1 if it involves machinery as defined in the Supply of Machinery (Safety) Regulations.

It can be insulating, or a suitable metal ... selection of this would be in line with the enclosure itself to prevent electrolytic  corrosion.

For controlgear or switchgear panels, these are out of scope of BS 7671, and BS EN IEC 61439 series is a common standard, perhaps with BS EN 60204-1 if it involves machinery as defined in the Supply of Machinery (Safety) Regulations.