Redundancy
Section 06: System Safety & Functional Safety
Definition
The provision of more than one means (item, function, or pathway) for accomplishing a given function, such that the failure of one means does not result in the loss of the function. Redundancy can be active (all redundant elements operating simultaneously, as in dual flight computers both processing commands) or standby (a backup element activated only upon failure of the primary, as in a standby hydraulic pump). The effectiveness of redundancy in meeting safety objectives depends on the independence of the redundant elements, the detection and switching mechanisms, and the coverage of failure modes.
Where This Shows Up
Redundancy is the primary architectural technique for meeting quantitative safety objectives. An OR gate in a fault tree becomes an AND gate when redundancy with independence is introduced, dramatically reducing the probability of the top event. However, redundancy alone is insufficient: the independence of redundant elements must be assured through physical separation, electrical isolation, and potentially dissimilar design.
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