Units in load sharing redundancy exhibit different failure characteristics when one or more fail. In Figure 4.20, blocks 1, 2 and 3 are in a load sharing container in BlockSim and have their own failure characteristics. All three must fail for the container to fail. However, as individual items fail, the failure characteristics of the remaining units change since they now have to carry a higher load to compensate for the failed ones. The failure characteristics of each block in a load sharing container are defined using both a life distribution and a life-stress relationship that describes how the life distribution changes as the load changes.
Standby redundancy configurations consist of items that are inactive and available to be called into service when/if an active item fails (i.e. the items are on standby). Within BlockSim, a container block with other blocks inside is used to better achieve and streamline the representation and analysis of standby configurations. The container serves a dual purpose. The first purpose is to clearly delineate and define the standby relationships between the active unit(s) and standby unit(s). The second purpose is to serve as the manager of the switching process. For this purpose, the container can be defined with its own probabilities of successfully activating standby units when needed. Figure 4.21 includes a standby container with three items in standby configuration where one component is active while the other two components are idle. One block within the container must be operating or, because the container block is part of a series configuration, the system will fail.
Figure 4.20: An RBD with a load sharing container.
Figure 4.21: An RBD with a standby container.
Because both of these concepts are better understood when time-dependency is considered, they are addressed in more detail in the Time-Dependent System Reliability (Analytical) section of this on-line reference.
See Also:
RBDs and Analytical System Reliability
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