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The tolerable level of these risks is specified as a safety requirement in the form of a target 'probability of a dangerous failure' in a given period of time, stated as a discrete SIL. Certification schemes, such as the CASS Scheme (Conformity Assessment of Safety-related Systems) are used to establish whether a device meets a particular SIL ...
Given ASIL is a relatively recent development, discussions of ASIL often compare its levels to levels defined in other well-established safety or quality management systems. In particular, the ASIL are compared to the SIL risk reduction levels defined in IEC 61508 and the Design Assurance Levels used in the context of DO-178C and DO-254. While ...
The safety integrity level (SIL) provides a target to attain for each safety function. A risk assessment effort yields a target SIL for each safety function. For any given design the achieved SIL is evaluated by three measures: 1. Systematic Capability (SC) which is a measure of design quality. Each device in the design has an SC rating.
Assessment of the risk-reduction required by the safety function, which will involve a safety integrity level (SIL) or performance level or other quantification assessment. A SIL (or PL, AgPL, ASIL) applies to an end-to-end safety function of the safety-related system, not just to a component or a part of the system.
An SIS performs a safety instrumented function (SIF).The SIS is credited with a certain measure of reliability depending on its safety integrity level (SIL).The required SIL is determined from a quantitative process hazard analysis (PHA), such as a Layers of Protection Analysis (LOPA).
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Once the SIL level is determined this specifies the required performance level of the safety systems during the operational phase of the plant. The metric for measuring the performance of a safety function is called the average probability of failure on demand (or PFD avg) and this correlates to the SIL level as follows