Hi Thierry,
I’m in the US, so am on the same type of grid you are.
200 Amp service is quite oftem very misunderstood. More info here:
Your 18 kW heater draws ~75 Amps, so you might be better served by a 100A:333mV CT.
(150A:333mV CTs are also available) The downside is finding one with a large enough wire-window to accommodate your Service Entrance Wires.
Info on said CTs here:
https://learn.openenergymonitor.org/electricity-monitoring/ac-power-theory/use-in-north-america
The table is about half-way down the page.