All overspeed governors and trips operate on a spring-loaded centrifugal governor element. In overspeed devices, the spring tension is great enough to overbalance the centrifugal force of the weights until the engine speed rises above the desired maximum. When the speed setting of the governor is reached, the centrifugal force over-comes the spring tension and operates the mechanism that stops or limits the fuel or air supply.

When a governor serves as a safety device, the fuel or air control mechanism is operated by the centrifugal force either directly, as in a mechanical governor, or indirectly, as in a hydraulic governor. In an overspeed trip, the shutoff control is operated by a power spring. The spring is placed under tension when the trip is manually set and is held in place by a latch. If the maximum speed limit is exceeded, a spring-loaded centrifugal weight will move out and trip the latch, allowing the power spring to operate the shutoff mechanism. NOTE: If the engine overspeeds and exceeds its rated rpm trip setting, internal inspection of the engine must be accomplished before the engine is restarted.

Overspeed safety devices must always be operative and must never be disconnected for any reason while the engine is operating. All overspeed safety devices should be tested under the Planned Maintenance System (PMS).