How to connect a two-phase flow heater to an RCD?

How to connect a two-phase flow heater to an RCD?

The instantaneous water heater Stiebel Eltron Eil 7 plus (6.5 kV), but it has three terminals (2 phases + ground), but there is no zero. How to connect it to a difavtomat or RCD and what denomination to choose?

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