A New Component Concept for Fault Trees

The decomposition of complex systems into manageable parts is an essential principle when dealing with complex technical systems. However, many safety and reliability modelling techniques do not support hierarchical decomposition in the desired way. Fault Tree Analysis (FTA) offers decomposition into modules, a breakdown with regard to the hierarchy of failure influences rather than to the system architecture. In this paper we propose a compositional extension of the FTA technique. Each technical component is represented by an extended Fault Tree. Besides the internal basic events and gates, each component can have input and output ports. By connecting these ports, components can be integrated into a higher-level system model. All components can be developed independently and stored in separate files or component libraries. Mathematically, each Component Fault Tree represents a logical function from its input ports and internal events to its output ports. As in traditional FTA, both qualitative and quantitative analyses are possible. Known algorithms e.g. based on Binary Decision Diagrams (BDDs) can still be applied. The Windows based safety analysis tool UWG3 has been developed to prove this concept in practice. It allows creating component libraries in an exchangeable XML format. We have carried out some case studies in order to show that the new concept improves clearness and intuitive modelling while maintaining the same results as traditional FTA.