During the last years big effort was devoted to increase the electrostatic deflectors’ reliability; this provided a better comprehension of the most significant effects concerning their working conditions. Deflectors were checked during the normal operation of the K800 Superconducting Cyclotron (CS) at LNS, at the operating pressure of 1 10-6mbar and a magnetic field of 3.5 T, the maximum cathodes voltage was – 60kV (120 kV/cm). The maximum extracted beam power was, up to now, 100 W; it is foreseen to extract up to 500 W. In this contribution we present the study, the tests and the design of a new water cooled electrostatic deflector. Particular effort was applied to optimise the beam extraction efficiency, the thermal dissipation, and the mechanical stability. In particularly we implemented new insulators, new anodised aluminium cathodes, new Ta septum, new voltage and water feedthroughs and a more efficient cooling system. All these improvements were performed to increase the mean time between failure and the beam current stability.