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Hello

I think, that there is made a mistake by  'Rated impulse-withstand voltage (Uimp)'

For industrial devices we can really speak about Uimp=8kV, if we speak about overvoltage category IV and nominal voltage 690V. 
But domestic devices standard do not know voltages over 3x400 V and highest overvoltage category there is III. So - correct 'usual' Uimp by those installation is 4 kV.  Some (selektive) Main CB-s are rather exceptions of that rule.

BR

trebla