The test plug is used for the safe checking of the phase rotation direction at CEE sockets. According to VDE 0100 § 31b 5, three-phase sockets must be connected in such a way that, when viewed from the front of the socket, a clockwise rotation field is produced. The test plug differs from a standard plug by its transparent housing. It indicates clockwise or counterclockwise rotation fields, or the absence of a phase, through three indicator lights.