A three phase WYE connected system with 230VAC from any phase to the neutral connection is 400VAC phase to phase. The Phase to phase voltage is going to be 1.732 (square root of 3) times the phase to neutral voltage on any three phase WYE system.
If you disconnect the neutral line, you only have the full phase to phase voltage available of the WYE connection, which, again in your case, is 400VAC.