Captive key systems do not provide full isolation of the power source, but may provide limited temporary access under controlled conditions.
Heron of Alexandria in his study listed the five mechanisms as the lever, windlass, pulley, wedge, and screw, which were known to set a load in a motion. Flemish engineer Simon Stevin derived the mechanical advantages of the inclined plane that was merged with the simple machines. Leonardo da Vinci also contributed to the study of machines by discovering the classic rules of sliding friction.
Interlock guarding occurs when the act of moving the guard (opening, sliding or removing) to allow access, stops the action of the hazardous mechanism. Interlock guarding works by: • disconnecting the drive mechanism mechanically (e.g. applying a brake or disengaging a clutch or geared mechanism) • isolating the power source of the drive mechanism (e.g. stopping the motor) • a combination of mechanical and power disconnection.
Emergency retrieval plans should allow for immediate local response in safely retrieving people to avoid fatalities.