In fault tree analysis, which logic gate is used to represent that any one of several failures can cause the top event?

• A. AND
• B. OR
• C. NAND
• D. NOR

Answer: (B)

In fault tree analysis, the gate that represents that any one of several failures can cause the top event is the OR gate. It captures the idea of logical disjunction: the top event occurs if at least one of its input failures happens. For example, if the top event is a system outage, and there are several possible root causes—power loss, pump failure, or sensor fault—any one of these being true will bring the top event true, so you link them with an OR.

An AND gate would require all listed failures to occur before the top event happens, which is a different relationship. NAND and NOR are negations of AND and OR and are not the standard way to express “any one” of several causes in fault trees.