A block diagram visually represents the portions of the system to model. Because reliability prediction software tools assume that all components in a system are in series, they cannot be used to analyze a system with redundant components. However, you can use a tool for evaluating block diagrams to model everything from simple series-parallel configurations to complex networks. Such tools provide both drawing and calculation capabilities.

Constructing a block diagram consists of inserting a block for each portion of the system to analyze and then connecting all of these blocks to represent how they are configured. After failure and repair data is specified for the blocks in the diagram, sophisticated mathematical algorithms and/or a Monte Carlo simulation engine are used to calculate many different reliability measures, including failure rate, MTBF, reliability, availability, and more.

If you later change the configuration of the blocks in the diagram, you can recalculate the diagram to see how the results change. Thus, using such a tool provides an efficient and effective way to compare various configurations to find the best overall system design.

For example, you can determine how the use of parallel redundancy and standby redundancy can improve the reliability and availability of your design. If a block in a diagram has a high failure rate, you might change its configuration to a 1- out-of -2 parallel redundancy. The calculation results for this modified diagram would then show how much system reliability and availability improve. If improvements are significant, you might decide to change the system design.

If a specified level of reliability must be obtained, you can determine how redundancy can help you to achieve this level. If cost information is also supplied, you can see how adding redundancy increases system cost. In most cases, a point is reached where adding more redundancy is not worth the cost of the reliability improvement. This point varies, depending on system requirements.