5. PLCs and safety

Dependability, which expresses the confidence attributed to the system under study, is one of the essential qualities of a PLC. Its components are reliability (ensuring continuity of service), characterized by an average operating time before failure (MTBF), maintainability (ability to be repaired), availability (being ready for use for the greatest possible fraction of the time) and safety (non-occurrence of events capable of causing damage to people and equipment). The latter, of course, is the most important because of its opposite, and often expresses the whole in the eyes of the user.

To be truly significant, these properties must apply not only to the PLC, but to the entire system, i.e. to the operating part and the application program. The PLC cannot, of course, guarantee this, but it will provide procedures and components which, by preventing or detecting...