Additional grounding is required
In the event of a grid shutdown by your distribution grid operator (e.g. for maintenance work), the PEN conductor can be interrupted if the corresponding line conductors are disconnected.
In that case, the Backup System will not have a ground connection, which is not permitted from a functional or safety-related perspective. Furthermore, the additional grounding prevents an increase of the PEN conductor’s voltage in the power distribution grid in stand-alone grid operation.