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Wikipedia

Safety-critical system

January 06, 2023

Not to be confused with Critical system.

A safety-critical system (SCS)[2] or life-critical system is a system whose failure or malfunction may result in one (or more) of the following outcomes:[3][4]

  • death or serious injury to people
  • loss or severe damage to equipment/property
  • environmental harm
Examples[1] of safety-critical systems. From left to right, top to bottom: the glass cockpit of a C-141, a pacemaker, the Space Shuttle and the control room of a nuclear power plant.

A safety-related system (or sometimes safety-involved system) comprises everything (hardware, software, and human aspects) needed to perform one or more safety functions, in which failure would cause a significant increase in the safety risk for the people or environment involved.[5] Safety-related systems are those that do not have full responsibility for controlling hazards such as loss of life, severe injury or severe environmental damage. The malfunction of a safety-involved system would only be that hazardous in conjunction with the failure of other systems or human error. Some safety organizations provide guidance on safety-related systems, for example the Health and Safety Executive (HSE) in the United Kingdom.[6]

Risks of this sort are usually managed with the methods and tools of safety engineering. A safety-critical system is designed to lose less than one life per billion (109) hours of operation.[7][8] Typical design methods include probabilistic risk assessment, a method that combines failure mode and effects analysis (FMEA) with fault tree analysis. Safety-critical systems are increasingly computer-based.

Reliability regimes

Several reliability regimes for safety-critical systems exist:

  • Fail-operational systems continue to operate when their control systems fail. Examples of these include elevators, the gas thermostats in most home furnaces, and passively safe nuclear reactors. Fail-operational mode is sometimes unsafe. Nuclear weapons launch-on-loss-of-communications was rejected as a control system for the U.S. nuclear forces because it is fail-operational: a loss of communications would cause launch, so this mode of operation was considered too risky. This is contrasted with the fail-deadly behavior of the Perimeter system built during the Soviet era.[9]
  • Fail-soft systems are able to continue operating on an interim basis with reduced efficiency in case of failure.[10] Most spare tires are an example of this: They usually come with certain restrictions (e.g. a speed restriction) and lead to lower fuel economy. Another example is the "Safe Mode" found in most Windows operating systems.
  • Fail-safe systems become safe when they cannot operate. Many medical systems fall into this category. For example, an infusion pump can fail, and as long as it alerts the nurse and ceases pumping, it will not threaten the loss of life because its safety interval is long enough to permit a human response. In a similar vein, an industrial or domestic burner controller can fail, but must fail in a safe mode (i.e. turn combustion off when they detect faults). Famously, nuclear weapon systems that launch-on-command are fail-safe, because if the communications systems fail, launch cannot be commanded. Railway signaling is designed to be fail-safe.
  • Fail-secure systems maintain maximum security when they cannot operate. For example, while fail-safe electronic doors unlock during power failures, fail-secure ones will lock, keeping an area secure.
  • Fail-Passive systems continue to operate in the event of a system failure. An example includes an aircraft autopilot. In the event of a failure, the aircraft would remain in a controllable state and allow the pilot to take over and complete the journey and perform a safe landing.
  • Fault-tolerant systems avoid service failure when faults are introduced to the system. An example may include control systems for ordinary nuclear reactors. The normal method to tolerate faults is to have several computers continually test the parts of a system, and switch on hot spares for failing subsystems. As long as faulty subsystems are replaced or repaired at normal maintenance intervals, these systems are considered safe. The computers, power supplies and control terminals used by human beings must all be duplicated in these systems in some fashion.

Software engineering for safety-critical systems

Software engineering for safety-critical systems is particularly difficult. There are three aspects which can be applied to aid the engineering software for life-critical systems. First is process engineering and management. Secondly, selecting the appropriate tools and environment for the system. This allows the system developer to effectively test the system by emulation and observe its effectiveness. Thirdly, address any legal and regulatory requirements, such as FAA requirements for aviation. By setting a standard for which a system is required to be developed under, it forces the designers to stick to the requirements. The avionics industry has succeeded in producing standard methods for producing life-critical avionics software. Similar standards exist for industry, in general, (IEC 61508) and automotive (ISO 26262), medical (IEC 62304) and nuclear (IEC 61513) industries specifically. The standard approach is to carefully code, inspect, document, test, verify and analyze the system. Another approach is to certify a production system, a compiler, and then generate the system's code from specifications. Another approach uses formal methods to generate proofs that the code meets requirements.[11] All of these approaches improve the software quality in safety-critical systems by testing or eliminating manual steps in the development process, because people make mistakes, and these mistakes are the most common cause of potential life-threatening errors.

Examples of safety-critical systems

Infrastructure

Medicine[12]

The technology requirements can go beyond avoidance of failure, and can even facilitate medical intensive care (which deals with healing patients), and also life support (which is for stabilizing patients).

Nuclear engineering[14]

Recreation

Transport

Railway[15]

Automotive[17]

Aviation[18]

Spaceflight[19]

See also

References

  1. ^ J.C. Knight (2002). "Safety critical systems: challenges and directions". IEEE: 547–550. {{cite journal}}: Cite journal requires |journal= (help)
  2. ^ "Safety-critical system". encyclopedia.com. Retrieved 15 April 2017.
  3. ^ Sommerville, Ian (2015). (PDF). Pearson India. ISBN 978-9332582699. Archived from the original (PDF) on 2018-04-17. Retrieved 2018-04-18.
  4. ^ Sommerville, Ian (2014-07-24). . an Sommerville's book website. Archived from the original on 2019-09-16. Retrieved 18 April 2018.
  5. ^ "FAQ – Edition 2.0: E) Key concepts". IEC 61508 – Functional Safety. International Electrotechnical Commission. Retrieved 23 October 2016.
  6. ^ "Part 1: Key guidance" (PDF). Managing competence for safety-related systems. UK: Health and Safety Executive. 2007. Retrieved 23 October 2016.
  7. ^ FAA AC 25.1309-1A – System Design and Analysis
  8. ^ Bowen, Jonathan P. (April 2000). "The Ethics of Safety-Critical Systems". Communications of the ACM. 43 (4): 91–97. doi:10.1145/332051.332078. S2CID 15979368.
  9. ^ Thompson, Nicholas (2009-09-21). "Inside the Apocalyptic Soviet Doomsday Machine". WIRED.
  10. ^ "Definition fail-soft".
  11. ^ Bowen, Jonathan P.; Stavridou, Victoria (July 1993). "Safety-critical systems, formal methods and standards". Software Engineering Journal. IEE/BCS. 8 (4): 189–209. doi:10.1049/sej.1993.0025. S2CID 9756364.
  12. ^ "Medical Device Safety System Design: A Systematic Approach". mddionline.com. 2012-01-24.
  13. ^ Anderson, RJ; Smith, MF, eds. (September–December 1998). "Special Issue: Confidentiality, Privacy and Safety of Healthcare Systems". Health Informatics Journal. 4 (3–4).
  14. ^ "Safety of Nuclear Reactors". world-nuclear.org.
  15. ^ (PDF). Rtos.com. Archived from the original (PDF) on 2013-12-19. Retrieved 2016-10-23.
  16. ^ a b
  17. ^ "Safety-Critical Automotive Systems". sae.org.
  18. ^ Leanna Rierson (2013-01-07). Developing Safety-Critical Software: A Practical Guide for Aviation Software and DO-178C Compliance. ISBN 978-1-4398-1368-3.
  19. ^ "Human-Rating Requirements and Guidelinesfor Space Flight Systems" (PDF). NASA Procedures and Guidelines. June 19, 2003. NPG: 8705.2. Retrieved 2016-10-23.

External links

  • An Example of a Life-Critical System
  • Explanation of Fail Operational and Fail Passive in Avionics

January 06, 2023
safety, critical, system, confused, with, critical, system, this, article, needs, additional, citations, verification, please, help, improve, this, article, adding, citations, reliable, sources, unsourced, material, challenged, removed, find, sources, news, ne. Not to be confused with Critical system This article needs additional citations for verification Please help improve this article by adding citations to reliable sources Unsourced material may be challenged and removed Find sources Safety critical system news newspapers books scholar JSTOR April 2009 Learn how and when to remove this template message A safety critical system SCS 2 or life critical system is a system whose failure or malfunction may result in one or more of the following outcomes 3 4 death or serious injury to people loss or severe damage to equipment property environmental harmExamples 1 of safety critical systems From left to right top to bottom the glass cockpit of a C 141 a pacemaker the Space Shuttle and the control room of a nuclear power plant A safety related system or sometimes safety involved system comprises everything hardware software and human aspects needed to perform one or more safety functions in which failure would cause a significant increase in the safety risk for the people or environment involved 5 Safety related systems are those that do not have full responsibility for controlling hazards such as loss of life severe injury or severe environmental damage The malfunction of a safety involved system would only be that hazardous in conjunction with the failure of other systems or human error Some safety organizations provide guidance on safety related systems for example the Health and Safety Executive HSE in the United Kingdom 6 Risks of this sort are usually managed with the methods and tools of safety engineering A safety critical system is designed to lose less than one life per billion 109 hours of operation 7 8 Typical design methods include probabilistic risk assessment a method that combines failure mode and effects analysis FMEA with fault tree analysis Safety critical systems are increasingly computer based Contents 1 Reliability regimes 2 Software engineering for safety critical systems 3 Examples of safety critical systems 3 1 Infrastructure 3 2 Medicine 12 3 3 Nuclear engineering 14 3 4 Recreation 3 5 Transport 3 5 1 Railway 15 3 5 2 Automotive 17 3 5 3 Aviation 18 3 5 4 Spaceflight 19 4 See also 5 References 6 External linksReliability regimes EditSeveral reliability regimes for safety critical systems exist Fail operational systems continue to operate when their control systems fail Examples of these include elevators the gas thermostats in most home furnaces and passively safe nuclear reactors Fail operational mode is sometimes unsafe Nuclear weapons launch on loss of communications was rejected as a control system for the U S nuclear forces because it is fail operational a loss of communications would cause launch so this mode of operation was considered too risky This is contrasted with the fail deadly behavior of the Perimeter system built during the Soviet era 9 Fail soft systems are able to continue operating on an interim basis with reduced efficiency in case of failure 10 Most spare tires are an example of this They usually come with certain restrictions e g a speed restriction and lead to lower fuel economy Another example is the Safe Mode found in most Windows operating systems Fail safe systems become safe when they cannot operate Many medical systems fall into this category For example an infusion pump can fail and as long as it alerts the nurse and ceases pumping it will not threaten the loss of life because its safety interval is long enough to permit a human response In a similar vein an industrial or domestic burner controller can fail but must fail in a safe mode i e turn combustion off when they detect faults Famously nuclear weapon systems that launch on command are fail safe because if the communications systems fail launch cannot be commanded Railway signaling is designed to be fail safe Fail secure systems maintain maximum security when they cannot operate For example while fail safe electronic doors unlock during power failures fail secure ones will lock keeping an area secure Fail Passive systems continue to operate in the event of a system failure An example includes an aircraft autopilot In the event of a failure the aircraft would remain in a controllable state and allow the pilot to take over and complete the journey and perform a safe landing Fault tolerant systems avoid service failure when faults are introduced to the system An example may include control systems for ordinary nuclear reactors The normal method to tolerate faults is to have several computers continually test the parts of a system and switch on hot spares for failing subsystems As long as faulty subsystems are replaced or repaired at normal maintenance intervals these systems are considered safe The computers power supplies and control terminals used by human beings must all be duplicated in these systems in some fashion Software engineering for safety critical systems EditSoftware engineering for safety critical systems is particularly difficult There are three aspects which can be applied to aid the engineering software for life critical systems First is process engineering and management Secondly selecting the appropriate tools and environment for the system This allows the system developer to effectively test the system by emulation and observe its effectiveness Thirdly address any legal and regulatory requirements such as FAA requirements for aviation By setting a standard for which a system is required to be developed under it forces the designers to stick to the requirements The avionics industry has succeeded in producing standard methods for producing life critical avionics software Similar standards exist for industry in general IEC 61508 and automotive ISO 26262 medical IEC 62304 and nuclear IEC 61513 industries specifically The standard approach is to carefully code inspect document test verify and analyze the system Another approach is to certify a production system a compiler and then generate the system s code from specifications Another approach uses formal methods to generate proofs that the code meets requirements 11 All of these approaches improve the software quality in safety critical systems by testing or eliminating manual steps in the development process because people make mistakes and these mistakes are the most common cause of potential life threatening errors Examples of safety critical systems EditInfrastructure Edit Circuit breaker Emergency services dispatch systems Electricity generation transmission and distribution Fire alarm Fire sprinkler Fuse electrical Fuse hydraulic Life support systems TelecommunicationsMedicine 12 Edit The technology requirements can go beyond avoidance of failure and can even facilitate medical intensive care which deals with healing patients and also life support which is for stabilizing patients Heart lung machines Mechanical ventilation systems Infusion pumps and Insulin pumps Radiation therapy machines Robotic surgery machines Defibrillator machines Pacemaker devices Dialysis machines Devices that electronically monitor vital functions electrography especially electrocardiography ECG or EKG and electroencephalography EEG Medical imaging devices X ray computerized tomography CT or CAT different magnetic resonance imaging MRI techniques positron emission tomography PET Even healthcare information systems have significant safety implications 13 Nuclear engineering 14 Edit Nuclear reactor control systemsRecreation Edit Amusement rides Climbing equipment Parachutes Scuba equipment Diving rebreather Dive computer depending on use Transport Edit Railway 15 Edit Railway signalling and control systems Platform detection to control train doors 16 Automatic train stop 16 Automotive 17 Edit Airbag systems Braking systems Seat belts Power Steering systems Advanced driver assistance systems Electronic throttle control Battery management system for hybrids and electric vehicles Electric park brake Shift by wire systems Drive by wire systems Park by wireAviation 18 Edit Air traffic control systems Avionics particularly fly by wire systems Radio navigation RAIM Engine control systems Aircrew life support systems Flight planning to determine fuel requirements for a flightSpaceflight 19 Edit Human spaceflight vehicles Rocket range launch safety systems Launch vehicle safety Crew rescue systems Crew transfer systemsSee also EditSafety Critical Systems Club Mission critical Factor critical to the operation of an organization Reliability engineering Sub discipline of systems engineering that emphasizes dependability Redundancy engineering Duplication of critical components to increase reliability of a system Factor of safety System strength beyond intended load Nuclear reactor Device used to initiate and control a nuclear chain reaction Biomedical engineering Application of engineering principles and design concepts to medicine and biology SAPHIRE Systems Analysis Programs for Hands on Integrated Reliability Evaluations risk analysis software Formal methods Mathematical program specification intended to allow correctness proofs including algorithmically Therac 25 Radiotherapy machine involved in six accidents Zonal Safety Analysis High integrity software Real time computingReferences Edit J C Knight 2002 Safety critical systems challenges and directions IEEE 547 550 a href Template Cite journal html title Template Cite journal cite journal a Cite journal requires journal help Safety critical system encyclopedia com Retrieved 15 April 2017 Sommerville Ian 2015 Software Engineering PDF Pearson India ISBN 978 9332582699 Archived from the original PDF on 2018 04 17 Retrieved 2018 04 18 Sommerville Ian 2014 07 24 Critical systems an Sommerville s book website Archived from the original on 2019 09 16 Retrieved 18 April 2018 FAQ Edition 2 0 E Key concepts IEC 61508 Functional Safety International Electrotechnical Commission Retrieved 23 October 2016 Part 1 Key guidance PDF Managing competence for safety related systems UK Health and Safety Executive 2007 Retrieved 23 October 2016 FAA AC 25 1309 1A System Design and Analysis Bowen Jonathan P April 2000 The Ethics of Safety Critical Systems Communications of the ACM 43 4 91 97 doi 10 1145 332051 332078 S2CID 15979368 Thompson Nicholas 2009 09 21 Inside the Apocalyptic Soviet Doomsday Machine WIRED Definition fail soft Bowen Jonathan P Stavridou Victoria July 1993 Safety critical systems formal methods and standards Software Engineering Journal IEE BCS 8 4 189 209 doi 10 1049 sej 1993 0025 S2CID 9756364 Medical Device Safety System Design A Systematic Approach mddionline com 2012 01 24 Anderson RJ Smith MF eds September December 1998 Special Issue Confidentiality Privacy and Safety of Healthcare Systems Health Informatics Journal 4 3 4 Safety of Nuclear Reactors world nuclear org Safety Critical Systems in Rail Transportation PDF Rtos com Archived from the original PDF on 2013 12 19 Retrieved 2016 10 23 a b Wayback Machine Safety Critical Automotive Systems sae org Leanna Rierson 2013 01 07 Developing Safety Critical Software A Practical Guide for Aviation Software and DO 178C Compliance ISBN 978 1 4398 1368 3 Human Rating Requirements and Guidelinesfor Space Flight Systems PDF NASA Procedures and Guidelines June 19 2003 NPG 8705 2 Retrieved 2016 10 23 External links EditAn Example of a Life Critical System Safety critical systems Virtual Library Explanation of Fail Operational and Fail Passive in Avionics Retrieved from https en wikipedia org w index php title Safety critical system amp oldid 1112933768, wikipedia, wiki, book, books, library,

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