The integration of electrical safety systems into machinery has unquestionably made workplaces safer. This is due to the ability of safety systems to detect potentially hazardous situations and reliably shut off machinery, thus protecting workers. Methodologies to help achieve this reliable safety can be found in a range of international standards.
Programmable safety controllers, utilising functional safety (as per IEC 61508), have traditionally used digital I/O exclusively, which only allows strict binary states. This is appropriate for many applications as safety conditions are often binary – a gate, for example, can be either closed or not closed, nothing else.
The input devices and logic within the controller’s program are purely binary, as are the digital outputs used to control machinery. A safety trip, such as the activation of an emergency stop switch, effects an immediate return to the safe state, where all outputs are turned off.
Once stopped, a reset operation (which is usually initiated by a reset switch) is needed to restart the machinery. (Part of the reset routine is to check if it is safe to restart i.e. the cause of the trip has been resolved).