Al is (as usual) correct on every point.
Your common ground point should be a copper rail situated where the cables enter the cabinet. All shielded cables must be stripped to expose their shield all the way around, and clamped to this rail immediately upon entering the cabinet. The clamps used must assure contact all around the cable shield. This rail must be connected to the cabinet backplate, the cabinet itself and don't forget the door and the gland panel. The dimension of the rail depends on the currents involved, but I use 20x5mm. If you run the ground from the end of the shield and back to a common ground point, this ground wire will emit noise inside the cabinet, and it will be picked up by other wires and even ride back on the outside of the shield and back out of your cabinet. VFD's have harmonics very high up in frequency, and even a short piece of wire becomes an excellent aerial.
If heaven forbid you have a cable too long and need to coil it up, do it outside the cabinet, and as a bifilar coil.  |