The most commonly used gates for representing failure logic paths between various fault tree levels are static gates. They are ‘static’ because the order of occurrence of the inputs does not matter. In other words, the underlying occurrence of events is not sequence-dependent. Only the occurrence of the input event is considered, not its temporal relation, or the order of events in time, to the occurrence of other events.

For all static gates, the output can be the top event or any intermediate event. The input events can be basic events, intermediate events (outputs of other gates), or combinations of both.

The following topics describe the static gates that are most commonly used to represent the failure logic between successive levels of a tree: