Fault trees

See also: Probability rules and diagrams introduction, Event trees

Fault trees start from the reverse approach to an event tree. An event tree looks forward from a starting point and considers the possible future outcomes. A fault tree starts with the outcome and looks at the ways it could have arisen. A fault tree is therefore constructed from the right with the outcome and moves to the left with the possible immediate events that could have made that outcome arise, and continues backwards with the possible events that could have made the first set of events arise, etc.

The main difficulty with event trees is the identification of the failure modes of a system. The analyst must have a good knowledge of the system first. A further difficulty is that fault tree analysis uses Boolean logic (the combination of ON and OFF with functions like AND, OR, NAND and their compliments), which doesn't take into account the partial failure of systems (e.g. reduced performance of a pump), although some effort has gone into making necessary corrections.

Read on: Probability parameters introduction

 

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