VFD interference problem

[cropwell]

Does your building have an earth supplied by the electric company or do you use earth rods in the ground. Are you still using the plastic control box? Pictures would help.

When you say the motors move a few steps when you turn on the vfd are they enabled and the motors locked when this happens?

I don't know the electrical safety regulations in Belgium, but I would guess they are much the same as in UK. I would expect the earth to be strapped to the plumbing although this is not necessarily a good ground contact with plastic pipes .