Reliability Block Diagram (RBD)

The Reliability Block Diagram (RBD) is used to identify potential areas of poor reliability and where improvements can be made to lower the failure rates for the equipment. This method can be used in both the design and operational phase to identify poor reliability and provide targeted improvements.

The RBD shows the logical connections of components within a piece of equipment. It is not necessarily the schematic diagram of the equipment, but the functional components of the system. The equipment is made up of multiple components/systems in series, parallel and a combination of the two. These components/systems and configuration of them provides us with the inherent reliability of the equipment. The RBD analysis consists of reducing the system to simple series and parallel blocks which can be analyzed using the appropriate Reliability formula. Reliability typically utilizes three main formulas;

  • Reliability;
 Reliability(Mission Time) = e-λt
 λ (lambda) = Failure rate = 1/MTBF
 t = mission time in cycles, hours, miles, etc.
 e = natural logarithm = 2.71828
  • Reliability(System) = R1 x R2 x R3 x R4 x ….RN
  • Reliability(Active Redundant Parallel System) = 1 – (1 – R1)(1 – R2)