The formal methods:
Allow to define mathematically and rigorously the properties of operation of a system. Formal methods may apply to any stage of development of a project, as well from the specification phase to the phase of implementation.
The operational safety:
aims to ensure, with a certain level of confidence, the impossibility of dangerous behavior of a system or equipment. In other words, this «insurance for the proper functioning » requires clear and strict operation control of the system or equipment.
Thus, formal methods, highly recommended at the beginning of the development cycle of a system, like in the stages of specification of the safety properties of a system with high integrity level (SIL4 for EN 61508 and SIL3 SIL4 to EN 50128), make the process of operational safety with safety requirements specified in a structured way clear, precise, unambiguous, verifiable, testable, maintainable, and free of ambiguous terms or description and / or likely to be misunderstood by the users of the document to all stages of development.