Slide 1
Two copies can improve availability
Redundancy adds another path or component so a function can survive a failure. The benefit depends on how independently those paths can fail.
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Two units sharing an essential power supply still depend on that supply. Counting boxes therefore gives less information than tracing the complete function from input to output.
Slide 2
One environment can defeat many units
A fire, flood, contamination event or excessive temperature can affect several components together. Their failures cannot always be treated as unrelated coincidences.
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A reliability calculation that multiplies independent probabilities may then be misleading. The model must represent the shared exposure rather than assume independence merely because the hardware is duplicated.
Slide 3
Identical designs can share a defect
Two units built to the same design may contain the same latent weakness. A particular input or operating condition can trigger both.
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Design diversity can reduce some shared weaknesses, but different implementations can still share requirements errors or environmental limits. Diversity needs a reasoned account of what it actually separates.
Slide 4
Separation must be functional
Physical spacing, independent supplies and protected routes can reduce shared vulnerabilities. The useful separation depends on the event being considered.
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Two cables routed through one fire zone remain exposed together even if their endpoints are separate. Engineers examine the entire route, including interfaces that reconnect supposedly independent paths.
Slide 5
A spare must be able to take over
A redundant component is useful only if faults are detected and the surviving path can perform the required function. Switching logic and maintenance become part of the design.
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A dormant spare may contain an undetected fault. Periodic checks and suitable diagnostics help reveal that weakness before another failure makes the spare essential.
Sources
- Common Cause Failures Dominate and Defeat Redundancy
NASA Technical Reports Server
- Systems Engineering Handbook: Fundamentals
NASA