Reliability Block Diagrams
A model for Reliability, Safety and Maintainability Analysis

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

What - Reliability Block Diagrams (RBD) are used for modeling the exact propagation of a failure. These models are similar to fault trees except they map exactly to the physical architecture of a design. Whereas Fault Trees look more for combinations of events that lead to a problem, an RBD models how each failure mode affects other (usually phyiscally connected) components.

Why - RBDs provide the reliability engineer with a highly detailed view of a subsystem so that a design can be improved in terms of redundancy, safety, etc. This makes them useful for supporting a wide range of reliability engineering tasks.

When - RBDs can be developed and support throughout the entire design process, however, they take a fair amount of work to keep up to date as the design matures. This is due to the fact that early RBDs tend to represent functional loss, while later RBDs tend to represent failure mode propagation. The transition between the two occurs throughout the development process. While this effort is acceptable and even expected for the reliability engineer, it represents an unacceptable overhead as a diagnostic approach. For this reason, Functional Models and Hybrid models were created

Related Links - Fault Trees