A reliability block diagram allows the engineer to identify single points of failure, make numerical allocations, evaluate redundant designs, show the series-parallel relationships between the components of the design, and summarize all factors affecting system reliability. If the failure rates of the individual components of the design are known, then the overall probability of success (or conversely, failure) over time can be calculated. Reliability block diagrams of this type do not usually consider the severity or consequence of the failure.
In order to account for the consequence of the failure, a couple of approaches can be considered. Using a reliability block diagram construct, the engineer must identify the failure modes that cause the failure effect of concern, partition the total failure rate for each block in the model into failure mode failure rates using a failure mode ratio or knowledge about the design, and only include the resulting failure mode failure rates in the overall probability of success calculation. A second, and probably more preferred, approach would be to construct and evaluate fault trees for each event of concern.