The standard TP and STP range of safety switches are designed to ensure that guard devices cannot be opened with machines in operation, and remain closed until all potentially hazardous moving parts have become static. The switches are provided as an option with a manual emergency release mechanism located at the rear of the switch housing. To prevent the unintentional locking-in of personnel within an individual machine or machining-cell enclosure, however, adaptations of these switches can be provided which incorporate an integrated bistable solenoid, operated from a separate supply voltage, which provides both actuation and condition monitoring of the guard locking. With varying combinations of the mains power and control voltage being On or Off either collectively or separately, the necessary conditions of Open/Closed safety devices with locking mechanisms Activated/Deactivated can be achieved. This ensures appropriate safeguards for personnel, together with the ability of obtaining exit from machine enclosures.