SESKStuff Everyone Should Know
SESK: Stuff Everyone Should Know · sesk.info/eng-redundancy-common-cause

Why can two redundant components still fail together?

A common cause can defeat both channels.

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  1. Redundancy guarantees perfection
  2. Both may share one vulnerability ✔
  3. Backups always fail one at a time

Why: Redundancy helps only when backup paths are sufficiently independent. Shared power, location, software, environment, or contamination can create a common-cause failure that disables every copy.

Huh, didn't know that: NASA safety rules require that redundancy in safety-critical functions be verified under operational conditions.

The dad joke
The backup brought a backup, then checked they did not share one plug.
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References

Answer: Both may share one vulnerability. Redundancy helps only when backup paths are sufficiently independent. Shared power, location, software, environment, or contamination can create a common-cause failure that disables every copy.

  1. NASA Procedural Requirements 8715.3, Chapter 1 nodis3.gsfc.nasa.gov/displayCA.cfm?Internal_ID=N_PR_8715_0003_&page_name=Chapter1
    Use of redundancy shall include a verifiable requirement that common cause failures (e.g., contamination, close proximity) do not invalidate the failure tolerance
    Checked 2026-10-10.

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