Qualitative Analysis of State/Event Fault Trees Based on Interface Automata

State/Event Fault Tree (SEFT) can be used for safety modeling and assessment. However, SEFT does not provide adequate semantics for analyzing the minimal scenarios leading to system failures. In this paper, we propose a novel qualitative analysis method for SEFT based on interface automata. Firstly, we propose the concept of guarded interface automata by adding guards on interface automata transitions. Based on this model, we can describe the triggers and guards of SEFT simultaneously. Then, a weak bisimilarity operation is defined to alleviate the state space explosion problem. Based on the proposed guarded interface automata and the weak bisimilarity operation, the semantics of SEFT can be precisely determined. After that, a qualitative analysis process is presented on the basis of the formal semantics of SEFT, and the analyzing result is the minimal cut sequence set representing the causes of system failures. Finally, a fire protection system case study is illustrated step by step to demonstrate the effectiveness of our method.