A Method of Analysis of Fault Trees with Time Dependencies

Safety is one of the biggest concerns in the design of computer-aided control systems. In order to make the system as safe as possible a number of analysis techniques has been developed. One of them is Fault Tree Analysis. Fault tree (FT) represents causal and generalization relations between events (e.g. between hazard and its causes). However, original FT cannot express either time relations between events or times of: detection of a danger situation and protection. A new method based on systems of inequalities and equalities for analysis of FTs with time dependencies is proposed in the paper. The method can be used for analysis of protections too. FT analysis and modelling of protection using systems of inequalities and equalities will be illustrated by an example. Formal models of FT gates used in the paper have the same expressive power as Timed Petri Net (TPN) models of FT gates from the paper [5]. However, present analysis method has greater decision power than classic TPN analysis method because the present method can be applied for much greater FTs. Additionally, the present approach results in more clear final conclusions.