A Comparative Explication of the Dependability Theory and the Functional Safety Standards

Dependability and safety of a system are topical subjects, which have found the proper assessment in two formal corpuses: the dependability theory and the functional safety standards. The purpose of this paper is to explicate the two corpuses, showing how one corpus supplements the other one. At first the dependability theory is introduced and the related concepts of threatens, attributes and techniques are presented. Regarding the safety, the dependability theory limits itself to the enunciation of principles and axioms without giving considerations to the problems of their applications to an industrial system. This gap is filled up by the standards on the functional safety, which are introduced in the second part of the paper together with the concepts of risk, safety function and safety integrity level. Finally, as a case of study, the power converter supplying the air brake system aboard train is examined from the dependability and functional safety viewpoints.