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Fault tolerance and common-cause failures

No reproduction or networking permitted without license from IHS [Pg.168]

Although physical separation can reduce the effects of common-mode failures, the owner/operator should also evaluate the likelihood that these common-mode failures occur at the same time. Only simultaneous common-mode failures are of concern. Typically, electronic component stress results in failures at different times, often separated in time by months or years. In these situations, the advantages of physical separation may be outweighed by the economic advantages of installing the redundant hardware in the same physical location or cabinet. [Pg.169]

In assessing overall process risk, it is often more important to ensure separation and diversity between the BPCS and the SIS than it is to ensure diversity in the selection of SIF devices. The use of diverse SIF devices can lead to maintenance problems and human error due to the inherent differences between the diverse parts of the SIF. [Pg.169]

Redundancy is applicable to both hardware and software. It can be implemented using identical or diverse devices. When assessing the integrity of an SIF, the potential for common-cause faults should be considered. Elimination or reduction of the common fault source is a very effective means to reduce the potential for failure. In addition, diverse redundancy can be employed to reduce the potential for common-cause faults. Some examples of common-cause faults were given in Annex G.7, such as  [Pg.169]

Redundant devices using different technologies may also decrease common-cause susceptibility if the redundant units respond differently to a common stress. This can be accomplished by designing redundant subsystems using diverse technology. Many think that using different manufacturers provides diversity. This is not true if both devices respond similarly to the same stress. Diversity works only if the redundant devices respond differently to a common stress. [Pg.169]


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