When the regulating system fails, the rotating speed of the steam turbine possbily will rise suddenly and the stress of the rotor parts will reach the unallowed value, thus the serious accident of equipment damage occurs. For this reason, the steam turbine is provided with overspeed protective device.  When the rotating speed of the steam turbine reaches 110% - 112% of the rated speed, the overspeed protective device will automatically shut off the main steam valve and the steam regulating valve for emergency shutdown.