NOT logic gate are static gates that indicate how the lack of an event's occurrence can cause the top event to occur. NOT logic includes NOT, NOR, NAND, and XOR gates. Because the presence of a NOT logic gate typically results in a noncoherent rather than coherent fault tree, NOT logic gates are sometimes referred to as noncoherent gates.

In a coherent tree, each component in the system is relevant, and the structure function is monotonically increasing. A tree that contains only AND gates, OR gates, and/or independent events is always coherent. When a NOT logic gate (or a disjoint or dependent event) is introduced into a tree, it is likely to become noncoherent.

A tree is noncoherent when both component failures and successes (positive and negative events) can cause the top event to occur. For example, system failure might occur due to the recovery of a failed component. Alternatively, during system failure, the failure of an additional component may bring the system to a good state. If the NOT logic can be eliminated from the tree, the tree is coherent. If the NOT logic cannot be eliminated from the tree, the tree is noncoherent.

The following topics describe the NOT logic gates: