FHE. Frequency of hazardous events and related risks. Identification of the frequency categories of the so-called risky events: “Incredible” (a non-credible event, so extremely unlikely to happen, once every 114,000 years or more, that it can be assumed that there is no probable danger of occurrence), “Improbable” (unlikely, but possible even if exceptionally, once every 1140 years – 114,000 years), “Remote” (remote, but possible once every 12 years – 1,140 years), “Occasional” (occasional, probable that often happens, once every 14 months – 12 years), “Probable” (which happens several times in the life of a system, at intervals of 42 days – 14 months), “Frequent” (frequent, continuous, once every 42 days or less ). See Preliminary Hazard Analysis. A system has SIL 4 characteristic if it is such that it can cause an “incredible” FHE
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