Editing Safety engineering (section)

====Fault tree analysis====
{{Main|Fault tree analysis}}
Fault tree analysis (FTA) is a top-down, [[deductive reasoning|deductive]] analytical method.  In FTA, initiating primary events such as component failures, human errors, and external events are traced through [[Boolean logic]] gates to an undesired top event such as an aircraft crash or nuclear reactor core melt. The intent is to identify ways to make top events less probable, and verify that safety goals have been achieved.

[[File:Fault tree.svg|thumb|A fault tree diagram]]

Fault trees are a logical inverse of success trees, and may be obtained by applying [[de Morgan's laws|de Morgan's theorem]] to success trees (which are directly related to [[reliability block diagram]]s).

FTA may be qualitative or quantitative. When failure and event probabilities are unknown, qualitative fault trees may be analyzed for minimal cut sets. For example, if any minimal cut set contains a single base event, then the top event may be caused by a single failure. Quantitative FTA is used to compute top event probability, and usually requires computer software such as CAFTA from the [[Electric Power Research Institute]] or [[SAPHIRE]] from the [[Idaho National Laboratory]].

Some industries use both fault trees and [[event tree]]s. An event tree starts from an undesired initiator (loss of critical supply, component failure etc.) and follows possible further system events through to a series of final consequences. As each new event is considered, a new node on the tree is added with a split of probabilities of taking either branch. The probabilities of a range of "top events" arising from the initial event can then be seen.