Common Mode Failure 6
Common Mode Failure 6 : The failure of components in the same manner. Quantification of Event Consequences: There are three stages in the quantification of the possible consequences of a hazardous event. The first stage requires a model for the attenuation of the damage causing effect (e.g. toxic concentration, explosion overpressure of thermal radiation) over time and distance. The second stage utilises knowledge of the critical levels of exposure (e.g. a dose-effect relationship) to obtain a relationship between degree of damage and distance, often known as the Hazard Range. This may be a maximum distance for a particular level of damage (e.g. fatal injury) or may be a relationship between distance and probability or degree of damage or injury. Vulnerability Model is a term used to describe the mathematical models adopted to combine these two stages of the quantification of event consequences. There may be different outcomes from a hazardous event depending on the prevailing circumstances. In the third stage of consequence quantification, the results of the vulnerability model are applied to the particular case under consideration (e.g. plant layout, personnel or population distribution) with probabilities allocated to variable factors such as wind direction, weather conditions and occupancy. The result is a relationship between probability (conditional on the occurrence of the hazardous event) and the extent of detriment, usually expressed as the number of people suffering a specified degree of harm. The product of the vent frequencies and the conditional probabilities gives the frequency at which numbers of people would be harmed by the event. Summation of the frequencies for particular numbers of people over all events gives the overall relationship between the number of people affected and the frequency, i.e. the societal risk. This is often expressed as an F-N Curve showing the cumulative frequency at which N or more people are affected. This presentation is adopted, as it is not particularly useful to state the chance of killing exactly 10 people rather than 9 or 11. Besides giving a misleading impression of the accuracy of such a calculation, it is actually the chance of all accidents larger than certain sizes, which is usually of interest. The frequency, at which an individual at a particular location would be harmed by an event, is obtained from the product of the event frequency and the conditional probability for the specified degree of harm at that location. Where more than one event has the potential to harm the individual, the individual risk is obtained by summation over all such events
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