Safety-critical systems are computer systems whose failure could result in injury, death, or environmental damage. Examples include aircraft control systems, nuclear power plant controls, medical devices like pacemakers, and railway signaling systems. These systems require high integrity to avoid hazards and ensure safety. Techniques like developing diverse redundant systems can improve safety by detecting and tolerating a wider range of faults.

1. Safety Critical Systems

5. SILs and Dangerous Failure Probability

7. (Old) Interlocking Systems Mechanical / Electromechanical Systems

11. Hazard Severity Level (Example) Category Id. Definition CATASTROPHIC I General : A hazard, which may cause death, system loss, or severe property or environmental damage. CRITICAL II General : A hazard, which may cause severe injury, major system, property or environmental damage. MARGINAL III General : A hazard, which may cause marginal injury, marginal system, property or environmental damage. NEGLIGIBLE IV General : A hazard, which does not cause injury, system, property or environmental damage.

12. Hazard Probability Level (Example) Level Probability [h -1 ] Definition Occurrences per year Frequent P ≥ 10 -3 may occur several times a month More than 10 Probable 10 -3 > P ≥ 10 -4 likely to occur once a year 1 to 10 Occasional 10 -4 > P ≥ 10 -5 likely to occur in the life of the system 10 -1 to 1 Remote 10 -5 > P ≥ 10 -6 unlikely but possible to occur in the life of the system 10 -2 to 10 -1 Improbable 10 -6 > P ≥ 10 -7 very unlikely to occur 10 -3 to 10 -2 Incredible P < 10 -7 extremely unlikely, if not inconceivable to occur Less than 10 -3

13. Risk Classification Scheme (Example) Hazard Severity Hazard Probability CATASTROPHIC CRITICAL MARGINAL NEGLIGIBLE Frequent A A A B Probable A A B C Occasional A B C C Remote B C C D Improbable C C D D Incredible C D D D

14. Risk Class Definition (Example) Risk Class Interpretation A Intolerable B Undesirable and shall only be accepted when risk reduction is impracticable. C Tolerable with the endorsement of the authority. D Tolerable with the endorsement of the normal project reviews.

16. Risk Tolerability Hazard Severity Probability Risk Risk Criteria Tolerable? No Risk Reduction Measures Yes

23. Layers of Diversity