Fail safe

A fail-safe in engineering is a design feature or practice that in the event of a specific type of failure, inherently responds in a way that will cause no or minimal harm to other equipment, the environment or to people.

A system's being "fail-safe" means not that failure is impossible or improbable, but rather that the system's design prevents or mitigates unsafe consequences of the system's failure. That is, if and when a "fail-safe" system "fails", it is "safe" or at least no less safe than when it was operating correctly.

Since many types of failure are possible, failure mode and effects analysis is used to examine failure situations and recommend safety design and procedures.

Some systems can never be made fail safe, as continuous availability is needed. Redundancy, fault tolerance, or recovery procedures are used for these situations (e.g. multiple independent controlled and fuel fed engines). This also makes the system less sensitive for the reliability prediction errors or quality induced uncertainty for the separate items. On the other hand, failure detection & correction and avoidance of common cause failures becomes here increasingly important to ensure system level reliability.

Examples include:

Examples include:

As well as physical devices and systems fail-safe procedures can be created so that if a procedure is not carried out or carried out incorrectly no dangerous action results. For example:

Fail-safe (foolproof) devices are also known as poka-yoke devices. Poka-yoke, a Japanese term, was coined by Shigeo Shingo, a quality expert. "Safe to fail" refers to civil engineering designs such as the Room for the River project in Netherlands and the Thames Estuary 2100 Plan which incorporate flexible adaptation strategies or climate change adaptation which provide for, and limit, damage, should severe events such as 500-year floods occur.

Fail-safe and fail-secure are distinct concepts. Fail-safe means that a device will not endanger lives or property when it fails. Fail-secure means that access or data will not fall into the wrong hands in a security failure. Sometimes the approaches suggest opposite solutions. For example, if a building catches fire, fail-safe systems would unlock doors to ensure quick escape and allow firefighters inside, while fail-secure would lock doors to prevent unauthorized access to the building.

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Wikipedia