Analysts who are new to using block diagrams commonly ask what level of the system the blocks in the diagram should represent. You have complete control over the configuration of the diagram. You may prefer to represent individual components in the diagram for completeness. Or, you may prefer to represent only higher level assemblies.

One common rule of thumb is to use blocks to represent the levels of your system that include redundancies. Because one of the main purposes of this type of diagram is to calculate the reliability and availability of a system with redundant components, it is often useful to analyze the system at the level where redundancies are found.

The reason for this is that if no redundancies exist at the chosen level (at the part level, for instance), the calculation of reliability and availability is based on a series configuration, assuming an exponential distribution. The failure rate for a system in a series configuration is always the sum of the failure rates for its components. Thus, you can simplify your diagram and save time by representing all of the lowest-level series items with one higher-level block.