Fault Tree AnalysisFTA

A Fault Tree Analysis — FTA — is the deductive risk method that starts at the undesired event — the top event, the loss of containment, the reactor runaway, the safety-function failure — and works backward through the AND and OR logic gates to the basic events, the equipment failures and the human errors and the common-cause failures that combine to produce it. The tree is read top-down: the top event happens when the gate below it is true, and the gate is true when its inputs are true, and the cut sets are the minimal combinations of basic events that would trigger the top event. FTA is the method that finds the single-cause path and the multi-cause path, and the cut set with one event is the single point of failure the design has to eliminate.

FTA is used in process safety, aerospace, and nuclear — the high-consequence domains where the top event is catastrophic and the analysis has to prove the probability is low enough — and it pairs with the bottom-up FMEA, which starts at the failure modes and asks what they cause, where FTA starts at the consequence and asks what causes it. The two cross-check each other: the FMEA finds the failure modes, the FTA finds whether the failure modes combine to the top event, and the LOPA takes the cut sets and assigns the independent protection layers. The discipline that makes FTA work is the basic event with a real probability — a gate populated with a guessed frequency is a tree that looks rigorous and is not — and the common-cause failure that breaks the assumed independence between two protection layers. See FMEA, bowtie, and LOPA.