Thank you for your reply.
To start with, I'm designing this project, commissioned by a company. This means, if this design gets through, people with the right competences will be working on these electric circuits (not me).
The purpose of the 230V circuitry is to power electric motors via a motor controller (frequency drive). For our design, AC motors seem to be better.
Double isolation is indeed an option, but frequency drives don't work well without earthing because the earthing act as a filter.
Do you suggest to change our design and use DC motors instead?