Overspeed Protective Device Fo

steam and gas turbines are fitted with a shutdown system to prevent damage to the machine.  In the event the speed governor fails to control the speed, the overspeed device actuates to shut down the machine. When shaft speed exceeds a desired safe level, generally 10% overspeed, a latching device or oil dump mechanism is actuated to close a special emergency stop valve. This system is totally independent of the governor.
Mechanical overspeed protective device
A trip pin or plunger is mounted in the turbine shaft with its center of gravity slightly off center.  In the event the speed regulating governor fails to control the speed, the unbalanced overcomes a spring force at a preset trip speed. As plunger it moves outward, it strikes the trip-lever, causing release of a spring dump valve that releases the trip circuit oil pressure. This unbalances a piston-spring combination and causes the trip and throttle valve to slam shut by the force of a spring and the steam pressure above the valve disk. 
A few high-speed machines use a weighted disk and a dished washer to accomplish the tripping action. The remainder of the action is identical.