A turbine overspeed accident can be caused by a lightning-induced power surge, a fouled pilot valve, an electrical fault, operator failure, or any of a few dozen other problems. Turbines are equipped with shutoff systems that are designed to automatically stop them if they exceed the design speed. These systems are built to cut the energy supply to the turbine so that it will coast to an orderly stop. Such a stop is usually referred to as a “trip” by operating staff. - http://www.plantservices.com/articles/2012/12-overspeed-fail...