I would suggest you get a pro to do this for you. 230v and water together can make for a nasty surprise. If you connect the earth wrong, you could potentially have every metal pipe and everything connected to that become live without the circuit tripping. I was told it is an unpleasant experience. Admittedly not from the horse’s mouth but from his widow.
Diy is great but you really do not seem to be able to judge whether it is safe or not. My guess would be that you have not wired it up correctly or set the thermostat wrong.
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