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A Reliability Block Diagram (RBD) illustrates the state of a specific function in a system with several elements. The diagram is made up of functional blocks represented as blocks and connected by lines. Reliability Block Diagram (RBD) has a single starting point (A) and single ending point (D), as shown in the following figure: RBDs are also known as dependence diagrams (DDs). RBDs are represented by series or parallel connections of blocks. Parallel blocks refer to redundant subsystems or components that contribute to a lower failure rate. Each block represents a component of the system with a high failure rate. The RBD will indicate the type of redundancy in the parallel path. For a group of parallel blocks to succeed, two out of three components would have to be successful. On the other hand, any failure along a series path causes the entire series path to fail. A RBD may be drawn using switches in place of blocks, with a closed switch representing a working component and an open switch representing a failed component. As long as there is a path through the network of switches from beginning to end, the system still works. DMADV stands for Define, Measure, Analyse, Design and Verify. All five phases can be addressed with RBD. At the define stage, to identify which system needs further analysis using RBD, at the measure stage, to study the opportunity for failure rate at each subsystem, at analyse stage, to quantify the probability of failure rate from study and historical data analysis, then at the design stage, to indicate which system requires modification or updating and at verify stage, to ensure that the right subsystem is affected by the action. In this way, RBD can be used as a tool to keep the team focused on the right problem throughout the entire process.

The Reliability Block Diagram is used for industrial and commercial power systems. It is a graphical presentation of a system diagram based on reliability or functional logic; i.e., connecting subsystems or components according to their function or reliability relationship. The significance of RBD is that it is easy to read. It is easily understood by customers who purchase the critical power systems, by the people who sell the systems, by engineers who design and test the systems, and by managers who make decisions on the systems. With knowledge of the system design, engineers can easily construct, verify, and modify the RBD, and also communicate with those of different functions. It is generally drawn from top-down, left-right approach, depicting the system element functions. The 3 types of RBD are Series, Parallel and Combination of Series & Parallel elements. An example of Reliability block diagram Reliability is typically calculated as below: Reliability = e-λt λ (lambda) = Failure rate = 1/MTBF t = time in cycles, hours, miles, etc. e = natural logarithm = 2.71828 RBD can be used in the following phases of DMADV: Analyze Phase - To identify the to identify potential areas of poor reliability and where improvements can be made to lower the failure rates for the equipment Design Phase - To construct / design the system & subsystems thus improving the reliability from the functional perspective Verify Phase - To check if the newly designed system / subsystem is able to eliminate the potential bottlenecks identified with respect to reliability are improved. This provides a useful benchmark to refer to when looking back at how the system used to perform, and whether the system’s current performance is as expected and is based on the model created.