They are designed to provide a common ground point for grounding using protective earth in an ESD protected area (EPA).
The plug fits into the mains supply socket, making a connection with the earth conductor only. In place of the live and neutral pins are moulded insulating plastic pins to allow positive location in the socket. Connectors on the front of the plug are available for connection via ground cords to the various elements of the EPA. Each element is held at a common potential. A resistance of sufficient resistance to limit current to less than 0.0005 amperes (0.5 milliamps), at the highest voltage that may be encountered, should be incorporated.