Fault Trees
A model for Reliability and Safety Analysis

Areas of Applicability: (comparison chart)
 Diagnostics / Prognostics  System Engineering  Maintainability
 Testability  Reliability  Life Cycle Cost
 Design-for-Test  Safety  Other

What - Fault Tree models take low level failures and propagate them out towards system-level and observable behavior. They can include complex events as well as actual physical failures, and often use both AND and OR logic symbols. Each node in a fault tree can be assigned a failure probability so that the probability of the higest level effect can be computed.

Why - Fault Trees are commonly used for safety analysis in large systems such as nuclear reactors, space vehicles, processing facilities, etc. They are useful because they can be easily created in the conceptual phase in order to build in some safety considerations from the beginning.

When - When performed in the concept phase, Fault Trees are largely based on similarity analysis, or as a mockup. Often, the failures are flipped into losses of functions for lack of failure mode data at the early phases. When performed later, they tend to be analyses based on actual designs and implementation plans, as a proof of the existance of safety factors and remediation capabilities

When - Fault Trees first began appearing in the 1960's, coincidentally with classic Dependency Modeling. Over the years, they have grown in their applications to include complex logic, hierarchy and sophisticated probability calculations.


Related Links - Reliability Block Diagrams