An actuator assembly for a pneumatic trip system, the actuator assembly comprising: a cylinder; a piston configured to be connected to a trip valve, the piston being movably disposed within the cylinder between a first position corresponding to an open position of the trip valve and a second position corresponding to a closed position of the trip valve, and the piston dividing the cylinder into a first chamber and a second chamber; a biasing element disposed in the second chamber of the cylinder, the biasing element engaging the piston to bias piston toward the second position; wherein the cylinder includes a first port for placing the first chamber in communication with an exterior of the cylinder such that the first chamber of the cylinder can be pressurized to cause the piston to remain in the first position, and wherein the actuator assembly further comprises a pneumatic mechanism in communication with the first chamber of the cylinder via the first port and configured to pressurize the first chamber of the cylinder to hold the piston in the first position and to release pressure from the first chamber of the cylinder to allow the piston to move to the second position under the bias of the biasing element, characterized in that the actuator assembly comprises a pneumatic circuit in communication with the second chamber of the cylinder, wherein the cylinder includes a second port for placing the second chamber in communication with the pneumatic circuit, wherein the pneumatic circuit is configured to pressurize the second chamber of the cylinder to actuate the piston to move to an intermediate position between the first position and the second position corresponding to an exercised position of the trip valve while the first chamber is pressurized by the pneumatic mechanism, and wherein the pneumatic mechanism is a Pneumatic Overspeed Trip System and the Pneumatic Overspeed Trip System is configured to be operated to allow the piston to move to the second position independent of the pneumatic circuit. a cylinder; a piston configured to be connected to a trip valve, the piston being movably disposed within the cylinder between a first position corresponding to an open position of the trip valve and a second position corresponding to a closed position of the trip valve, and the piston dividing the cylinder into a first chamber and a second chamber; a biasing element disposed in the second chamber of the cylinder, the biasing element engaging the piston to bias piston toward the second position; wherein the cylinder includes a first port for placing the first chamber in communication with an exterior of the cylinder such that the first chamber of the cylinder can be pressurized to cause the piston to remain in the first position, and wherein the actuator assembly further comprises a pneumatic mechanism in communication with the first chamber of the cylinder via the first port and configured to pressurize the first chamber of the cylinder to hold the piston in the first position and to release pressure from the first chamber of the cylinder to allow the piston to move to the second position under the bias of the biasing element, characterized in that the actuator assembly comprises a pneumatic circuit in communication with the second chamber of the cylinder, wherein the cylinder includes a second port for placing the second chamber in communication with the pneumatic circuit, wherein the pneumatic circuit is configured to pressurize the second chamber of the cylinder to actuate the piston to move to an intermediate position between the first position and the second position corresponding to an exercised position of the trip valve while the first chamber is pressurized by the pneumatic mechanism, and wherein the pneumatic mechanism is a Pneumatic Overspeed Trip System and the Pneumatic Overspeed Trip System is configured to be operated to allow the piston to move to the second position independent of the pneumatic circuit.