A reliability block diagram is a model-based analysis methodology to check or demonstrate the reliability of systems. A system under investigation is dissected into blocks which reflect the system performance or behaviour as part of the analysis. It thus shows the logical links between the elements which are necessary to satisfy the requirements of a system. The elements which are not relevant to satisfying the requirements are not modelled. For example, when examining a system in terms of achieving a safety objective, only the respective safety-relevant system components are investigated.
The modelled blocks have the characteristic of being able to show an element only in two modes: functional or failure. The possible reasons for a failure are not graphically represented. However, during the course of the calculation various causes may be considered.
Based on a completely modelled reliability block diagram it can become quickly evident which system elements are essential to satisfy its requirements and which elements may fail without having any effect on the requirements.