the Classic Reliability Block Diagram Methodology and Constructs
Reliability block diagrams (RBDs) have been around for a long time, and have been widely used to model systems. A reliability block diagram is a graphical representation of how the components
of a system are reliability-wise connected. As with any approach or methodology, reliability block diagrams have their advantages as well as disadvantages compared to competing methods. Some of
these disadvantages are rooted in the basic elements and constructs used in a reliability block diagram.
The constructs in the reliability block diagram methodology have not changed since its inception. This is unlike many other techniques/methodologies that have gone through multiple revisions and
changes to improve upon their original capabilities (e.g., consider the improvements to the HTML markup language as the methodology has matured). In order to address some of the current
inadequacies in reliability block diagrams, perhaps it is time to revise the standard to make the methodology easier to use and also more effective. This article reviews some of the existing
techniques and introduces some new constructs to the tried and true reliability block diagram approach.
Series, Parallel and Series Parallel Combination Configurations
The simplest and most elementary configurations of an RBD are the series and parallel configurations. Items placed in series must all work for the system to work, as shown in Figure 1, where the
system fails if either A, B or C fails. Items placed in parallel are considered to be redundant, as shown in Figure 2, where either D or B can fail and the system will continue to function. The
concept can be expanded further, as shown in Figure 3, with combinations of series and parallel configurations in the same diagram. These elementary configurations form the basis of the
reliability block diagram constructs.
Figure 1: Series Configuration
Figure 2: Parallel Configuration
Figure 3: Combination of Series and Parallel Configurations
If one takes the approach a step further, “Complex” block diagrams can be created. “Complex” diagrams cannot be expressed as a simple combination of series and parallel blocks (such as the
diagram in Figure 3) and thus require a more advanced analytical treatment. A network is a good example of system requiring a “Complex” reliability block diagram and Figure 4 illustrates an
example of this RBD type.
Figure 4: Complex System Configuration
The creation of reliability block diagrams with series, parallel, combination and complex configurations required the use of blocks and lines only. To extend the functionality of a block diagram,
one needs to introduce some additional elements to the “tool kit.” One such element is the
k-out-of-n node, which allows the analyst to specify an alternative form of redundancy known as
k-out-of-n redundancy. A k-out-of-n node can have n paths leading into it, and requires that
k of those n paths must function for the system to function. Figures 5 and 6 present RBDs that
incorporate such nodes. The configuration in Figure 5 includes a k-out-of-n node where either B, C or D must operate for the system to function but any two of the other items may fail without
causing system failure. With the traditional reliability block diagram methodology,
k-out-of-n redundancy could have been specified for units drawn in parallel. However, with the introduction of
the node element in the diagram, one can specify such redundancy for complex configurations, such as the one shown in Figure 6.
Figure 5: k-out-of-n Node Configuration
Figure 6: Complex Configuration with 2-out-of-5 Node
A Standby Container can be used to represent items configured with standby redundancy. Standby redundancy configurations consist of items that are inactive and available to be called into service
when/if the active item fails (i.e., on standby). A container block, with other blocks inside, is utilized 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
7 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 7: Configuration with Standby Container
Load Share Containers
The container concept can be expanded to also represent load sharing configurations. As the name implies, load sharing configurations consist of a components that are in load sharing redundancy.
Units in load sharing redundancy exhibit different failure characteristics when one or more fail. In Figure 8,
units 1, 2 and 3 are in a load share container 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.
Figure 8: Configuration with Load Share Container
In addition to Standby and Load Share containers, other new types of blocks can be used to increase the versatility of reliability block diagram constructions and facilitate more rapid creation of
diagrams that are easier to read. These include Subdiagram Blocks, Multi Blocks and Mirror Blocks, which are described next.
Subdiagram Blocks to Represent Inheritance
A Subdiagram Block inherits some or all of its properties from another block diagram. This allows the analyst to maintain separate diagrams for portions of a system and incorporate those diagrams
as components of another diagram. With this technique, it is possible to generate and analyze extremely complex diagrams representing the behavior of many subsystems, subsubsystems etc. in a
manageable way. In Figure 9, Subdiagram Block A in the top diagram represents the series configuration of the subsystem reflected in the middle diagram, while Subdiagram Block G in the middle
diagram represents the series configuration of the subsubsystem in the bottom diagram.
Figure 9: Subdiagram Blocks to Represent Inheritance
Multi Blocks to Save Time and Space
By using Multi Blocks, a single block can represent multiple identical blocks in series or in parallel configuration. This technique is simply a way to save time when creating the RBD and to save
space within the diagram. Each item represented by a Multi Block is a separate entity with identical reliability characteristics to the others. However, each item is not rendered individually
within the diagram. In other words, if the RBD contains a Multi Block that represents three identical components in a series configuration, then each of those components fails according to the
same failure distribution but each component may fail at different times. Because the items are arranged reliability-wise in series, if one of those components fails, then the Multi Block fails.
It is also possible to define a Multi Block with multiple identical components arranged reliability-wise in parallel or k-out-of-n redundancy.
Mirror Blocks to Simulate Bi-Directional Paths
While Multi Blocks allow the analyst to represent multiple items with a single block in the RBD, Mirror Blocks can be used to represent a single item with more than one block placed in multiple
locations within the diagram. Mirror Blocks can be used to simulate bi-directional paths within a diagram. For example, in a reliability block diagram for a communications system where the lines
can operate in two directions, the use of Mirror Blocks will facilitate realistic simulations for the system maintainability and availability. It may also be appropriate to use this type of block
if the component performs more than one function and the failure to perform each function has a different reliability-wise impact on the system.
As this article demonstrates, it is possible to update and expand the techniques available for reliability block diagrams in order to increase the effectiveness of the analysis tool and also
improve the ease of creation and appearance of the diagrams. The proposed enhancements to the methodology include complex configurations, k-out-of-n nodes, Standby and Load Sharing containers,
Subdiagram Blocks to represent inheritance from other diagrams, Multi Blocks to represent multiple identical blocks and Mirror Blocks to represent the same block in multiple locations within the
diagram. ReliaSoft’s BlockSim 6 software supports all of the standard RBD techniques as well as the enhancements discussed in this article. On the Web at