U-Line® Factory Sealed 20 A Plugs and Receptacles are used with stationary or portable electrically operated devices in hazardous locations. The ECP Interchanger plug is designed with built-in switch contacts which activate when the plug is twisted into the receptacle. This design allows the ECP plug to fit into standard NEMA 5-20R and 6-20R receptacles, but does not allow standard plugs to be used with the receptacle, ensuring that only safe connections can be made in a hazardous location.