Position of protective contact and auxiliary lug

To ensure that a plug cannot be inserted into a socket with a different voltage or frequency, the positions of the protective contacts relative to the keyway of the socket collar have been assigned to certain voltages and frequencies. A groove in the connector collar and a corresponding lug on the plug prevent hazardous confusion between the systems.

The illustration shows the position of the protective contact of a socket (front view) with rated operating voltages above 50 V. Plugs and inlets are mirror-inverted.