Reliability Block Diagrams (RBDs)
A complex system will require a large number of RBDs to describe it, and the first step is to develop an RBD at the system level as follows:
• By reference to the data assembled during System Definition, specify the functions of the system and the operating states (e.g., standby, full power, etc.).
• Specify the minimum requirements for the system to operate successfully in terms of the functions of the system.
• Draw a system RBD in terms of the system functions.
• Specify the sub-systems that are required to perform the system functions.
• Draw a system RBD in terms of the sub-systems and simplify as necessary.
Once an RBD has been constructed to show the reliability dependencies at the system level, a similar procedure should be followed to construct RBDs for each sub-system at successive levels of assembly down to the level at which reliabilities, or failure rates, can be estimated from the component/part data. This process is illustrated in Figure 4-2.
RBDs must always be as explicit as possible and should contain all pertinent information. This may not always be possible simply by the arrangement of blocks and interconnecting lines. Appropriate notes should be made on the diagrams as necessary. For example:
• Types of redundancy should be described, when not evident from the diagram.
• If the failure of a redundant element degrades performance or places additional stress on items in alternative paths, this should be noted.
• If the operating or maintenance conditions appropriate to a particular block are different from associated blocks (see Operational Duty Cycle), this should be highlighted (e.g., an item which may be replaced or repaired during the operational period).
The overall aim must be to record all data that may influence the reliability analysis and calculations.
The following points should be noted when constructing RBDs at system, sub-system and lower assembly levels:
• More than one RBD may be necessary to depict differing operational objectives or alternative functional modes.
• Elements of an RBD should contain only items that have the same operational duty cycle.
• When constructed to its lowest level, the blocks comprising an RBD should contain only series equivalent elements or have known reliability characteristics established from previous analysis.
• If an element has more than one failure mode, separate RBDs must be drawn using each failure mode.
• When functional relationships between elements cannot be represented by straightforward series, active redundant or standby redundant configurations, the group of elements concerned must be isolated and highlighted for special consideration. In general, reliability evaluation of such groups can be made using Bayes Theorem.
When developing RBDs, it is not always a matter of simple inspection to determine the conditions that represent successful operation of a system (the system up states), or alternatively the system down states. In these cases, techniques such as Truth Tables or Boolean algebra should be used as described in Reference 1. For additional information, see the Bibliography.