Fault trees provides useful information about the likelihood of a failure occurring and the means by which this failure could occur. While a fault tree is often constructed after a preliminary hazard analysis or a FMEA (failure modes and effects analysis), neither of these analyses is a prerequisite. You can construct fault trees at any stage of a design. The results of a fault tree can help you to focus and refine efforts to improve system safety.

Fault trees provide a procedure for determining the various combinations of hardware and software failures and human errors that can result in the occurrence of a specified undesired event at the system level. The undesired event to evaluate is referred to as the top event. It is generally some type of failure or malfunction. The analysis begins with clearly defining the top event and then determining the various combinations of events that can lead to the occurrence of the top event. Fault trees employ a deductive or top-down approach, which is in contrast to the inductive or bottom-up approach that FMEAs employ.

Fault trees were developed in 1962 for the U.S. Air Force by Bell Telephone Laboratories for use with the Minuteman missile system. They were later adopted and extensively applied by the Boeing Company. Today, fault trees are widely used to investigate the reliability and safety of complex and large systems for diagnostic applications. Additionally, they are applied extensively in the nuclear and aerospace industries due to concern for human safety.

A fault tree provides a compact, graphical, and intuitive method for failure analysis. In the tree, convenient symbols represent the various combination of events and failure logic that can cause the top event to occur. Because a fault tree provides a visual representation of the structure of the system, it often simplifies explanations as to how a particular failure can occur.

The goal of a fault tree is to evaluate the probability of the occurrence of the top event. Once the tree is constructed, this probability can be evaluated using analytical or statistical methods. Typically, system quantitative reliability and maintainability information such as failure probability, failure rate, or repair rate, is used to determine the probability of occurrence of the top event.