Why: The tool aids the design process, shows weak links that cause failures, and in the critical legs of the trees helps to define maintenance strategies for which pieces of equipment and processes should be defended with the greatest maintenance vigor to prevent "Murphy" from shutting down the process or causing serious safety issues. The technique provides a graphical aid for the analysis and it allows many failure modes including common cause failures. Results from a FTA is usually more pessimistic that other analysis tools such as RBDs as you can see from a study of the Space Shuttle reliability analysis where each system is studied by multiple reliability tools because of the high cost/profile of failures.

When: FTA is widely used in the design phase of nuclear power plants, subsea control and distribution systems, and for oversight studies in layers of protection studies for process safety and loss control in chemical plants and refineries so as to prevent accidents and control the costs of risks. The technique is helpful for identifying critical fault paths, observing vague failure combinations before they occur in reality, comparing alternate designs for safety, and setting a methodology to provide management with a tool to evaluate the overall hazards in a system and avoid single sources of critical failures. Finally when thinking top down about failures and where/how they can occur, the methodology gives a diagram for setting maintenance strategies for protecting key pieces of equipment/processes to prevent failures.

Where: FTA is helpful for defining potential event sequences and potential incidents, evaluating the incident consequences of outcomes, and estimating the risks of events occurring. FTAs work in the design room and on the operating floor where first hand knowledge has been gained for preventing failures.

These definitions are written by H. Paul Barringer and are also posted on his web site at www.barringer1.com

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