The term Fail-safe is used to describe:

1. A device which, if (or when) it fails, fails in a way that will cause no harm or at least a minimum of harm to other devices or danger to personnel. Examples include:
   • The safety glass used in modern automobile windows which is designed to shatter into very small pieces rather than in the long jagged fragments created when common window glass breaks.
2. An operation which ensures that a failure of equipment, process, or system does not propagate beyond the immediate environs of the failing entity.
3. The automatic protection of programs and/or processing systems when a hardware or software failure is detected in a system.
4. A control operation or function that prevents improper system functioning or catastrophic degradation in the event of circuit malfunction or operator error.
5. A system which has been structured such that it cannot fail (or that the probability of such failure is extremely low) to accomplish its assigned mission regardless of environmental factors. Examples include:
   • The hardening of a nuclear missile bunker or the dispersion of nuclear bombers to multiple secret locations

See also

• Safety engineering
• Air safety
• Fail deadly
• Safe-life design
• Damage tolerant design
• Reliability

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"Fail-Safe" or "Fail Safe" is also the title of a novel, movie and made-for-television play about a possible accidental nuclear war.