Abstract: An apparatus for preventing a turbine from exceeding a predetermined value of a monitored physical magnitude, the apparatus including an over-center toggle lever (3) pivotally mounted on a pivot point (6) on a stationary frame (7), the lever being capable of occupying a tripped position or a released position. In the cocked position, one arm (4) of the lever (3) presses against a stop (8) provided with a unit (20) which controls steam admission units to the turbine, the lever (3) moves away from the stop, steam is cut off, while the end (14) of the other arm (5) is pivotally mounted on the end of a connecting rod (10) which is pivotally mounted at its other end on a pin (11) which is integral with a piston (12) which a spring (16) pushes away. A mass (2) strikes the lever (3) which moves to the tripped position if the monitored magnitude exceeds the predetermined value.