Learn in the textbookReliability Design
DefinitionFail-safe (フェールセーフ) design moves a system to an action on the safe side when something goes wrong, whether from a failure or an operating mistake, so the damage does not spread.
ExplanationAt the design stage, engineers decide which behavior carries the least danger when something goes wrong and make the system fall back to it. In most cases the safe side means stopping the machine or cutting off its output.
AnalogyA level-crossing barrier built to drop under its own weight when the power goes out will, even when it breaks, end up blocking the road.
Memory hookFail plus safe: when it fails, it fails toward safety.