Power supply units and powersupplies receive their mains voltage via the mains plug. In Germany, the Schuko plug is a three- pin plug whose two pins are used to connect the mains phase (L) and the neutral conductor (N) and whose contact surfaces on the sides are connected to the protective earth( PE).
Accordingly, the device coupling also has three contacts, of which the middle one must be used for the protective earth conductor.
In addition to the Schuko plug, there is also the two-pin Euro plug, which does not have a protective contact. The two contacts of the Euro plug are for the phase conductor and the neutral conductor. It is only used if the device offers sufficient protection against contact and there is no danger of the protective conductor coming into contact with the housing.