Assessing the effects of diagnostic failures on safety-related control systems

Diagnosis is implemented within safety-related control systems in order to reveal internal faults that could lead to a loss of the safety function, thus reducing the resulting values of the probability of failure on demand (PFD) and probability of failure per hour (PFH) of such systems. The question to be discussed in this paper is how to assess the effects of failures of the diagnostic facilities themselves. In order to investigate this effect, we propose to consider a state-transition representation of the system, as reliability block diagrams are not well suited to reflect the changing properties when faults occur and affect the diagnostic capabilities of the system. We also discuss the role of common causes leading to the simultaneous failure of the control system and its diagnosis and their mathematical representation.