Requires some type of model even if only in head of

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Unformatted text preview: ˜ ž ¨ ¥ ˜ ¡ Marginal ‚ € } ~ } ” € … III ‚ Š ƒ ‘ ‚ Š € Negligible ¬ ¥ ¡  ¤  £ ˜ ¢ © š › š ¢ › ˜ IV A Frequent 10 5 3 1 B Probable 11 6 4 2 C Occasional 8 12 6 3 D Remote 12 10 7 4 E F Improbable Impossible 12 12 12 9 12 12 12 12 Hazard Causal Analysis Used to refine the high−level safety constraints into more detailed constraints. Requires some type of model (even if only in head of analyst) Almost always involves some type of search through the system design (model) for states or conditions that could lead to system hazards. Top−down Bottom−up Forward Backward Forward vs. Backward Search Initiating Events A B C D Final States W X Y Z nonhazard HAZARD nonhazard nonhazard Initiating Events A B C D Final States W X Y Z nonhazard HAZARD nonhazard nonhazard Forward Search Backward Search · Ë · ½ À Ê » Å » ¹ º É ¹ È Ç ¸ Å · Æ µ Å ¶ µ à ´ c  µ Á · À ¿ · Ë · ¾ À Ê » Å » ¹ º É ¹ È Ç ¸ Å · Æ µ Å ¶ µ à ´ c  µ Á · À ¿ FTA and Software Appropriate for qualitative analyses, not quantitative ones System fault trees helpful in identifying potentially hazardous software behavior. Can use to refine system design constraints. FTA can be used to verify code. Identifies any paths from inputs to hazardous outputs or provides some assurance they don’t exist. Not looking for failures but incorrect paths (functions) Fault Tree Example Explosion and Pressure too high Relief valve 1 does no...
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