>
isolation trafo that I would like to connect. Primary ... 400V Secondary ... 230 V
OK, Euro-Power basics.
First: this transformer can not be connected for 230:230. However it
may be able to give an isolated 230V supply, depending how your house is wired. See first drawing.
Small houses get a single circuit, one side grounded, the other side 230V hot to ground.
Long power lines are much more efficient as 3-phase power, so for more than one house on the block, it gets cheaper to use a 3-phase transformer and connect houses to all three hot legs equally (more or less). Each house gets 230V to ground, single-phase. This directly runs 230V lamps, computers, and cookers.
When you have a big house, or a big load (industrial machinery) a single 230V line has to be very fat to carry the current. As Tony in Hellgium says, "These days we have 3 times 240 V coming in." See second drawing. (Other houses or offices may get the same 3-phase line, which means you may get crap from the welding-shop next door.)
OK, if there is 230V to ground on each leg of the 3-phase line, what is the voltage from one leg to another leg (an un-grounded load)? Not 230*2= 460, but 230*(square-root of 3)= 398V (call it 400V). Why square-root of 3? Each leg is neither in-phase nor completely out of phase with another, they are all 120 degrees apart.
Therefore if you connect the 400V side of your transformer to
two hot legs of your 3-phase 230V line, you will get 230V on the other side. If you have heavy-duty appliances, you may have seen 400V un-grounded circuits.
(In US terms, this is similar to a 240V stove that has ungrounded heating elements bridged across both hot legs of our 120V/240V split-phase power. It is more like the 208V that I get here at work where we do have 3-phase 120V power inside the building, but US homes almost never get 3-phase power.)
However I really wish you would ask an industrial electrician to guide you. If you do it wrong, it can really
bite you: electrocution, or burn the house down. In the US, a derived voltage like this has pages of Code requirements, including some non-optional Grounding requirements (which make true isolation difficult).
>
put some capacities across the mains
That rasises a point. A floating transformer can reduce common-mode garbage, but will not reject differential mode garbage. Most crap is a combination of both.