Risk Assessment
What is Fault Tree Analysis? A Deductive Risk Assessment Method
Vinit Pandey · Published 4 August 2026
In short: Fault Tree Analysis (FTA) is a deductive, top-down technique that models the logical combinations of equipment failures and human errors that could lead to a defined top event, used to calculate that event's overall frequency.
Key takeaways
- FTA works backward from a defined top event to the combinations of failures that could cause it
- AND gates represent redundancy (all events needed); OR gates represent single points of vulnerability
- Basic event probabilities combine through the tree's logic to calculate overall top event frequency
- FTA suits scenarios where causation involves complex failure combinations, not a simple frequency lookup
FTA starts from a defined top event — such as loss of containment from a specific vessel — and works backward, identifying the combinations of basic equipment failures, human errors, or external events that could cause it, connected through logical AND and OR gates.
An AND gate in a fault tree means all connected basic events must occur together for the top event to result, typically representing redundant protection; an OR gate means any one of the connected events is sufficient, typically representing a single point of vulnerability.
Basic event failure probabilities, drawn from equipment reliability data, are combined through the tree's logic structure to calculate an overall frequency for the top event — a frequency that then feeds into LOPA or QRA as the initiating event or scenario frequency.
FTA is particularly useful where a scenario's causation involves complex combinations of failures that a simple initiating-event-frequency lookup would not adequately capture, such as failures requiring simultaneous instrument and human error.