Reliability Block Diagram (RBD)

A reliability block diagram is a graphical model of how the reliability of a system depends on its components, with blocks arranged in series, parallel or more complex configurations according to which components must work for the system to work.

Each block represents a component or subsystem with a known reliability or failure distribution. In a series arrangement every block must work, so the system reliability is the product of the block reliabilities and is lower than that of the weakest block. In a parallel, or redundant, arrangement the system works if at least one block works, so the system unreliability is the product of the block unreliabilities. K-out-of-n arrangements require a minimum number of identical blocks to work, and standby arrangements model spare units that switch in on failure.

RBDs are used during design to compare architectures, decide where redundancy is worthwhile, allocate reliability targets to subsystems and estimate system availability when combined with repair data. They are common in power systems, process plants, data centres, telecommunications, aerospace and production line design, where a line with no buffers between stations behaves as a series system.

Simple RBD calculations assume that block failures are independent, an assumption that common-cause failures, shared utilities and shared maintenance resources can violate. The diagram shows success logic, not physical connection, so two physically parallel pumps form a series arrangement if both are needed. Complex systems with repair and dependencies are often analysed with Markov models or simulation. IEC 61078 covers reliability block diagrams.

Key points

Models system reliability from component blocks in series and parallel.
Series reliability is the product of the block reliabilities.
Parallel redundancy improves reliability only if failures are independent.
IEC 61078 covers reliability block diagrams.

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Related terms

Terms that refer to Reliability Block Diagram (RBD)

Failure Mode, Effects and Criticality Analysis (FMECA)Quality, Reliability & Maintenance