High Integrity Pressure Protecting System (HIPPS 1) for a fluid line, wherein the HIPPS has a. an ON/OFF-valve (2, 28, 33), with a housing, wherein the housing has an inlet and an outlet, and a conduit for passage of a fluid from the inlet to the outlet, and with a closure member (14, 30, 35), that is axially movable in a direction from the inlet to the outlet from an open position to a closing position, wherein in the open position the closure member (14, 30, 35) opens the conduit for allowing the passage, and in the closing position the closure member (14, 30, 35) closes the conduit for inhibiting the passage, and b. a hydraulic actuator (3, 32, 34), having a control element (23, 41) that is kinematically fixed to the closure member (14, 30, 35), and c. a switchable hydraulic supply unit, which in an active state supplies a control face of the control element (23, 41) with a control pressure, and in a passive state releases the control pressure from the control face, d. a sensor measuring a downstream pressure of the fluid, downstream of the outlet, e. and a logic solver i. that compares the downstream pressure to a critical value, ii. that switches the hydraulic supply unit to the active state for allowing the closure member (14, 30, 35) to move into, or to stay in the open position, whenever the downstream pressure is below the critical value, and iii. that switches the hydraulic supply unit to the passive state for allowing the closure member (14, 30, 35) to stay in the closing position, whenever the downstream pressure exceeds the critical value, characterized in that an opposite face of the control element (23, 41), opposite to the control face is exposed to the downstream pressure of the fluid, so that the downstream pressure of the fluid pushes the closure member (14, 30, 35) into the closing position. a. an ON/OFF-valve (2, 28, 33), with a housing, wherein the housing has an inlet and an outlet, and a conduit for passage of a fluid from the inlet to the outlet, and with a closure member (14, 30, 35), that is axially movable in a direction from the inlet to the outlet from an open position to a closing position, wherein in the open position the closure member (14, 30, 35) opens the conduit for allowing the passage, and in the closing position the closure member (14, 30, 35) closes the conduit for inhibiting the passage, and b. a hydraulic actuator (3, 32, 34), having a control element (23, 41) that is kinematically fixed to the closure member (14, 30, 35), and c. a switchable hydraulic supply unit, which in an active state supplies a control face of the control element (23, 41) with a control pressure, and in a passive state releases the control pressure from the control face, d. a sensor measuring a downstream pressure of the fluid, downstream of the outlet, e. and a logic solver i. that compares the downstream pressure to a critical value, ii. that switches the hydraulic supply unit to the active state for allowing the closure member (14, 30, 35) to move into, or to stay in the open position, whenever the downstream pressure is below the critical value, and iii. that switches the hydraulic supply unit to the passive state for allowing the closure member (14, 30, 35) to stay in the closing position, whenever the downstream pressure exceeds the critical value, i. that compares the downstream pressure to a critical value, ii. that switches the hydraulic supply unit to the active state for allowing the closure member (14, 30, 35) to move into, or to stay in the open position, whenever the downstream pressure is below the critical value, and iii. that switches the hydraulic supply unit to the passive state for allowing the closure member (14, 30, 35) to stay in the closing position, whenever the downstream pressure exceeds the critical value,