Application conditions for lethality analysis (CA)

Fault type and impact analysis (FMEA) is a method that employs system segmentation; it involves dividing the system into subsystems or components as needed, and then analyzing individually the various potential fault types and causes for each subsystem or component, as well as the impacts these faults have on the subsystems and the entire system. Measures are then established to prevent or eliminate such faults. Some of these fault types that may cause casualties or significant property damage are taken for further analysis to determine the probability and severity of fatal outcomes, a process known as Criticality Analysis (CA). Fault type and impact analysis are used to identify hazards qualitatively, while lethality analysis conducts a quantitative analysis of faults with fatal consequences. Combining the two is known as Failure Mode, Effects, and Criticality Analysis (FMECA). (Safety Management Exchange – www.riskmw.com) A failure refers to a situation in which a component, subsystem, or system fails to meet the requirements specified in its design during operation, as a result of which it is unable to carry out its designated tasks or performs them inadequately. The fault type is the state in which the fault appears. For example, a valve failure can have four types: internal leakage, external leakage, failure to open, and failure to close properly. 　　Fault type and impact analysis are primarily used for the secure design of systems. As is well known, if new engineering projects or mechanical equipment are not properly designed from the outset, they may develop weak points or potential sources of accidents, resulting in frequent malfunctions or accidents during operation. Fault type and impact analysis involves identifying the possible faults in each component one by one; as a result, hazard identification is carried out in a thorough and comprehensive manner, providing a solid basis for safe design and thereby ensuring the reliability of the system. 　　This method has been used in aircraft design in the United States since 1957. The NASA and the Army also require contractors to provide FMECA documents when issuing tenders for engineering projects. Currently, this method has been widely applied in industries such as nuclear power plants, power generation, machinery, and instrumentation. 　　Severity level of fault type – Qualitative grades: Based on the severity of the consequences that a fault may cause (i.e., the impact of the fault type), the following qualitative grades can be used: (1) Safe (Level 1): No action is required; (2) Critical (Level 2): It may cause minor injuries and damage; actions should be taken; (3) Dangerous (Level 3): It can result in casualties and system damage; immediate action is necessary; (4) Destructive (Level 4): It can lead to catastrophic failures; it must be resolved immediately.

Lethality analysis is relatively simple and not widely used nowadays; the main reason is that it cannot take human factors into account

Under what circumstances is the lethality analysis (CA), a qualitative assessment for evaluating the level of danger and hazard, applied? It would be better to provide examples; thank you. When the Criticality Analysis method (a probability-based calculation) is applied in this approach, it is called the Failure Mode, Effects and Criticality Analysis method, abbreviated as FMECA. It can determine the probability of failure for each component in the system, and quantitatively describe the impact of those failures. The advantage of this method is its simple formatting, but the disadvantage is its lack of logic; it is difficult to analyze the influence between various elements, and analysis becomes more challenging when two or more elements fail simultaneously.

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