A system disconnector for physically disconnecting an upstream liquid system from a downstream liquid system by means of an outlet valve (35;135) depending on the pressure difference between the upstream and the downstream liquid system comprises an upstream backflow preventer (48; 148), a downstream backflow preventer (88), and an outlet valve body in the form of a piston (26; 126) arranged between the backflow preventers (48, 88). The pressure difference at the outlet valve body (26; 126) between the inlet pressure and the medium pressure between the backflow preventers counteracts a biasing spring (82; 182) acting in the opening direction of the outlet valve body. An actuating member (64; 164), which is exposed to the inlet pressure on one side and to the medium pressure on the other side is adapted to move between two stops (54, 46; 171, 146) against the power of a spring (86; 182) to compensate for small changes in inlet pressure and prevent dripping of the valve.