The problem with mechanical safety devices is that eventually they fail to perform their critical function. Unless they are regularly tested and maintained they are sure to fail sooner than later and even if they 'are' regularly tested and maintained they are sure to fail eventually! Actuators of all types are prone to failure ... even the most expensive actuators with huge development investment, research and testing can fail ... remember the subsea BOP in the gulf of mexico last year! ... it failed! You can routinely test an actuator successfully for years and have it pass critical function testing each time only to have it fail when you need it most!
