Test coverage of the safety instrumented system

Diagnostic test and proof test all can detect the dangerous failure of the safety instrumented system. The coverage of the two kinds of test is the main parameter for the assessment of safety instrumented system performance. The diagnostic coverage of independed failure and depended failure are been differentiated by studying the efficiency of the comparison diagnostic test. Then, the more reasonable model for PFD_avg of the loo2D cofiguration is builded. The assumption that the proof test coverage is perfect is unreasonable. In fact, the proof test is imperfect. The probability of failure for a safety function is optimized by introducing the proof test coverage.