Do you need the ground wire for the welding machine?
The welding machine shell needs the earthing protection, in order to avoid welding operator to get an electric shock, cause danger.
All electrical appliances with metal housings must be grounded for personal safety.
The butt welding machine is a kind of resistance welding method which uses two electrodes to clamp the workpiece, and after a short time and a large current discharge, the part is welded to form a nugget. There is no need for ground wire and welding machine to form loop.