Abstract
In order to gain an improvement of safety or availability measures of the safety-relevant control system through employment of redundancy a redundant system has to comply with the requirement on independence of redundant parts. If the requirements on the independence of redundant parts are unfulfilled, then a common-cause failure can directly cause a hazardous state on a system level through its effects on multiple redundant parts. Identification of sources and quantification of the common-cause failure parameters has been proved to be a formidable task. The latter problem, including other major safety-affecting factors lays in the focus of this paper. Our proposed technical safety analysis concept is extended, so now it partially covers elusive problems related to the common-cause failures.
Highlights
Nowadays it is rather common that a control system is used to perform its designated control functions, and safety related functions
Availability cannot be seen as directly related to safety; it can significantly influence the safety of a safety-related control system (SRCS), especially when the SRCS operates in the high-demand mode or continuous mode of operation
In the quantitative safety analysis these parameters include a definition of a system, its boundaries and architecture, identification of non-safety-related system elements, reliability measures of the system elements, and diagnostic properties
Summary
In the quantitative safety analysis these parameters include a definition of a system, its boundaries and architecture, identification of non-safety-related system elements (that can be excluded from the analysis), reliability measures of the system elements (e.g. failure rate λ, recovery rate μ), and diagnostic properties (diagnostic coverage coefficient c, time to detect a failure tD). The most common combinatorial methods are reliability block diagram method (RBD) and (static) failure tree analysis (FTA). These methods are recommended by many standards (e.g. IEC 61508) and can be adopted in the safety analysis. The common state-oriented methods include Continuous time Markov Chains (CTMC), [3], Petri nets [4], and various formal language-based methods. These methods do not produce analytical solution, so the employment of the numerical method like Monte Carlo is necessary
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