Towards a Unifying View of Fault Propagation Analyses and Notations

The design of complex systems requires a careful consideration of the possible hazards and failure conditions that may affect system functions, possibly compromising system reliability and safety. Complex systems must be able to detect components faults and isolate them before they can propagate and cause system failures. To this aim, Preliminary Safety Assessment analyzes failure conditions and allocate safety requirements to components and subsystems, based on a candidate system architecture. A modern way to conduct this analysis is via the use of fault propagation models, i.e. formal representations linking the occurrence of basic faults to their effects on other components and subsystems. Examples of such models include Timed Failure Propagation Graphs (TFPG), Finite Degradation Models (FDM) and Propagation Graphs over Finite Degradation Structures (PGFDS). In this paper, we generalize previous models for fault propagation. We define a general formalism, called Unifying Propagation Graphs (UPG) which encompasses, and is strictly more expressive of, previous notations, and we formally define its syntax and semantics. We discuss the integration of UPG into the xSAP safety analysis platform, and the generalization of existing routines for fault propagation analysis to the complete fragment of UPG. Finally, as a first contribution, we extend the existing engine for computation of minimal cut sets of PGFDS to support interval timings, and we experimentally evaluate its performance.