The standard CEE plugs and sockets according to EN 60309 respectively IEC 60309 are designed in their main dimensions in a way that plugs and sockets with the same rated currents, nominal working voltage ranges, the same number of poles and frequency of different producers are compatible.
In order to prevent the insertion of plugs and sockets of different voltages and frequencies, 12 positions of the earthing contact are assigned to the polarizing slot of the skirt of a socket.
The number with the following letter "h" indicates the position of the earth contact tube, comparing the frontside of the socket or connector with the face of a clock. The keyway is situated at 6 o'clock (please see International Rating-SERIE I section).