EMC implementation - Cabinet cabling
From Electrical Installation Guide
Each cabinet must be equipped with an earthing bar or a ground reference metal sheet. All shielded cables and external protection circuits must be connected to this point. Anyone of the cabinet metal sheets or the DIN rail can be used as the ground reference.
Plastic cabinets are not recommended. In this case, the DIN rail must be used as ground reference.
Fig. R20: The protected device must be connected to the surge-arrestor terminals