Conceptual frameworks for dependability and safety of a system

Dependability and safety of a system are two topical subjects, which find the proper assessment into the conceptual frameworks of the dependability theory and the functional safety standards. The purpose of this paper is to explicate the two frameworks, showing how one framework 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 sets the behavioral requirements for a system in case of fault but does not deal with the characteristics of the systems used to achieve the requirements. 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 installed aboard trains is analyzed from the dependability and functional safety viewpoints