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.[1][2]

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.[3]

Examples

Mechanical or physical

Globe control valve with pneumatic diaphragm actuator. Such a valve can be designed to fail to safety using spring pressure if the actuating air is lost.

Examples include:

Railway semaphore signals. "Stop" or "caution" is a horizontal arm, "Safe to go" is 45 degrees upwards, so failure of the actuating cable fails the arm to safety under gravity.

Electrical or electronic

Examples include:

Procedural safety

An aircraft lights its afterburners to maintain full power during an arrested landing aboard an aircraft carrier. If the arrested landing fails, the plane can safely take off again.

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:

Other terminology

Fail-safe (foolproof) devices are also known as poka-yoke devices. Poka-yoke, a Japanese term, was coined by Shigeo Shingo, a quality expert.[10][11] "Safe to fail" refers to civil engineering designs such as the Room for the River project in Netherlands and the Thames Estuary 2100 Plan[12][13] which incorporate flexible adaptation strategies or climate change adaptation which provide for, and limit, damage, should severe events such as 500-year floods occur.[14]

Fail safe and fail secure

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, also called fail-closed, 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.

The opposite of fail-closed is called fail-open.

Fail Active Operational

Fail active operational can be installed on systems (such as aircraft AutoLand systems) that have a high degree of redundancy so that a single failure of any part of the system can be tolerated (fail active operational) and a second failure can be detected – at which point the system will turn itself off (uncouple, fail passive). One way of accomplishing this is to have "three of everything."

See also

Look up fail-safe in Wiktionary, the free dictionary.

References

  1. "Fail-safe". AudioEnglich.net. Accessed 2009.12.31
  2. e.g., David B. Rutherford, Jr., "What Do You Mean — It's Fail-Safe?": Evaluating Fail-Safety in Processor-Based Vital Control Systems. 1990 Rapid Transit Conference
  3. Bornschlegl, Susanne (2012). Ready for SIL 4: Modular Computers for Safety-Critical Mobile Applications (pdf). MEN Mikro Elektronik. Retrieved 2015-09-21.
  4. "What is a Unidirectional Rotating Bezel". Retrieved 9 May 2013.
  5. Bornschlegl, Susanne (2012). Ready for SIL 4: Modular Computers for Safety-Critical Mobile Applications (pdf). MEN Mikro Elektronik. Retrieved 2015-09-21.
  6. http://www.obd-codes.com/p2138
  7. Manual on Uniform Traffic Control Devices, Federal Highway Administration, 2003
  8. "When Failure Is Not an Option: The Evolution of Fail-Safe Actuators". KMC Controls. Archived from the original on 1 July 2016. Retrieved 30 October 2015.
  9. Harris, Tom. "How Aircraft Carriers Work". HowStuffWorks, Inc. Retrieved 2007-10-20.
  10. Shingo, Shigeo; Andrew P. Dillon (1989). A study of the Toyota production system from an industrial engineering viewpoint. Portland, Oregon: Productivity Press. p. 22. ISBN 0-915299-17-8. OCLC 19740349
  11. John R. Grout, Brian T. Downs. "A Brief Tutorial on Mistake-proofing, Poka-Yoke, and ZQC", MistakeProofing.com
  12. "Thames Estuary 2100 Plan" (PDF). UK Environment Agency. November 2012. Archived from the original (PDF) on 2012-12-10. Retrieved March 20, 2013.
  13. "Thames Estuary 2100 (TE2100)". UK Environment Agency. Retrieved March 20, 2013.
  14. Jennifer Weeks (March 20, 2013). "Adaptation expert Paul Kirshen proposes a new paradigm for civil engineers: 'safe to fail,' not 'fail safe'". The Daily Climate. Retrieved March 20, 2013.
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