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3. BLOCK DIAGRAMMING AND RELIABILTY ALLOCATION. 3.1 Overview. For a given system, models are often used to describe the relationship between the system components in order to determine the reliability of the system as a whole. A common and useful way to model these interconnected relationships is the utilization of Reliability Block Diagrams (RBD). An RBD is a success-oriented network drawing and calculation tool used to model specific functions of complex systems by using a series of images (blocks). When used to model a system, each component within the system is represented by a block and the connections between the blocks are used to indicate that each component is properly performing its intended function. As seen from Figure 1, if a connection exists between the two end points (a,b) of the diagram, it is said that the system is performing its intended function or that some specified failure mode is not occurring. Once the blocks are configured in the proper form the hazard rate (instantaneous failure rate), mean time between failures (MTBF), mean time to repair (MTTR), reliability, and availability of the system can be calculated. Component 1 Component 2 Component 3 Component 4 Component 5 Component 6 Figure 1. Reliability Block Diagram. RBD can take a number of different forms allowing for testing for different item configurations and system loads. The basic forms of RBD include components placed in series, parallel, stand-by, load-sharing or complex. As seen in Figure 2, an RBD in series is represented by each block placed in a straight line sequence. As with any RBD, as long as a single connection between the end points exists, the system is performing its intended function. Component 1 Component 2 Component 3 Figure 2. RBD in a Series Configuration. In a series configuration, if any single component fails, it would result in the end points being disconnected and the system failing to perform its function. Thus, for this system to perform its intended mission, every component must succeed during the mission time. An RBD with components in a parallel configuration is represented by Figure 3. In this configuration, as long as at least one block is operational, the system is performing its intended function. Parallel configurations represent active redundant systems where if one component fails a back-up is already in place. 4

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