System safety concepts hazard severity

Use of a variety of system safety concepts and tools, such as the order of precedence for hazard reduction, the hazard severity and probability tables, and... 

Use of a variety of system safety concepts and tools, such as the order of precedence for hazard reduction, the hazard severity and probability tables, and the hazard risk matrix, will assist the analyst in determining the appropriate risk assessment code to assign to a particular hazard risk. The RAC will prioritize for management the specific level of risk associated with a specific, identified hazard concern. 

For this paper we treat hazard assessment as a combination of two interrelated concepts hazard identification, in which the possible hazardous events at the system boundary are discovered, and hazard analysis, in which the likelihood, consequences and severity of the events are determined. The hazard identification process is based on a model of the way in which parts of a system may deviate fi om their intended behaviour. Examples of such analysis include Hazard and Operability Studies (HAZOP, Kletz 1992), Fault Propagation and Transformation Calculus (Wallace 2005), Function Failure Analysis (SAE 1996) and Failure Modes and Effects Analysis (Villemeur 1992). Some analysis approaches start with possible deviations and determine likely undesired outcomes (so-called inductive approaches) while others start with a particular unwanted event and try to determine possible causes (so-called deductive approaches). The overall goal may be safety analysis, to assess the safety of a proposed system (a design, a model or an actual product) or accident analysis, to determine the likely causes of an incident that has occurred. 

Risk is an intangible quality it does not have physical or material substance (a mishap does, but not risk). It is a future value concept with some quantifiable metrics, likelihood and severity, which characterize the future event. Risk can be thought of as the net present value of a future event. In system safety, risk is a measure of the future event, where the event is an expected mishap. Risk likelihood can be characterized in terms of probability, frequency, or qualitative criteria, while risk severity can be characterized in terms of death, injury, damage, dollar loss, and so on. Future safety events can only be identified as a hazard, which means that safety risk is the metric characterizing the amount of danger presented by a hazard. Recognizing that a hazard is the precursor (or blueprint) to a mishap, safety risk is the common denominator between the hazard and a mishap, and also the measure of the relative threat presented by a hazard. 

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