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System safety


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Accompanies video on my YouTube channel on system safety

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System safety

  1. 1. System safety System safety, 2013 Slide 1
  2. 2. Safety • Safety is a property of a system that reflects the system’s ability to operate, normally or abnormally, without danger of causing human injury or death and without damage to the system’s environment. System safety, 2013 Slide 2
  3. 3. Principal dependability properties System safety, 2013 Slide 3
  4. 4. Safety criticality • Primary safety-critical systems – Embedded software systems whose failure can cause the associated hardware to fail and directly threaten people. System safety 2013 Slide 4
  5. 5. System safety 2013 Slide 5
  6. 6. • Secondary safety-critical systems – Systems whose failure results in faults in other (socio-technical) systems, which can then have safety consequences. – For example, a medical records system is safetycritical as failure may lead to inappropriate treatment being prescribed. System safety 2013 Slide 6
  7. 7. System safety 2013 Slide 7
  8. 8. Safety and reliability • Safety and reliability are related but distinct • For a system to be safe, it must generally be reliable • However, this is not enough. It must also be safe in the event of system failure System safety 2013 Slide 8
  9. 9. • Reliability is concerned with conformance to a given specification and delivery of service • Safety is concerned with ensuring system cannot cause damage irrespective of whether or not it conforms to its specification System safety 2013 Slide 9
  10. 10. • A reliable system can be unsafe if the requirements are incorrect or incomplete System safety 2013 Slide 10
  11. 11. Unsafe reliable systems The software on this plane operated ‘reliably’ yet the plane crashed- covered in a separate video System safety 2013 Slide 11
  12. 12. Unsafe reliable systems • There may be dormant faults in a system that are undetected for many years and only rarely arise. • Specification errors – If the system specification is incorrect then the system can behave as specified but still cause an accident. System safety 2013 Slide 12
  13. 13. • Hardware failures generating spurious inputs – Hard to anticipate in the specification. • Context-sensitive commands i.e. issuing the right command at the wrong time – Often the result of operator error. System safety 2013 Slide 13
  14. 14. Accident • An unplanned event or sequence of events which results in human death or injury, damage to property, or to the environment. • An overdose of insulin by a softwarecontrolled insulin injection system is an example of an accident. System safety 2013 Slide 14
  15. 15. Hazard • A condition with the potential for causing or contributing to an accident. A failure of the sensor that measures blood glucose is an example of a hazard. System safety 2013 Slide 15
  16. 16. Damage • A measure of the loss resulting from a mishap. Damage can range from many people being killed as a result of an accident to minor injury or property damage. • Damage resulting from an overdose of insulin could be serious injury or the death of the user of the insulin pump. System safety 2013 Slide 16
  17. 17. Safety critical systems development • Hazard avoidance – The system is designed so that some classes of hazard simply cannot arise. System safety 2013 Slide 17
  18. 18. • Hazard detection and removal – The system is designed so that hazards are detected and removed before they result in an accident. System safety 2013 Slide 18
  19. 19. • Damage limitation – The system includes protection features that minimise the damage that may result from an accident. System safety 2013 Slide 19
  20. 20. Summary • Safety is a property of a system that reflects the system’s ability to operate, normally or abnormally, without danger of causing human injury or death. • Safety and reliability are related but reliable systems may be unsafe. System safety 2013 Slide 20
  21. 21. Summary • Hazards are conditions that may (but need not) lead to an accident • Safety is achieved through – Hazard avoidance – Hazard detection and removal – Damage limitation System safety 2013 Slide 21