Overspeed trip

An overspeed trip is the independent safety device that shuts an engine down when speed rises past a set limit, typically 110 to 115 percent of rated, protecting against the destructive runaway that follows a governor failure or a sudden loss of load. Mechanical trips use a spring-loaded bolt or flyweight that flies out at the trip speed and releases the fuel-rack linkage or a fuel-shutoff valve; electronic systems use a speed pickup and a solenoid that cuts fuel. Class rules require the trip to be separate from the speed governor so a single fault cannot disable both. On a generator engine, sudden breaker opening removes load and is the classic overspeed trigger.