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The system breakdown structure was introduced briefly in Chapter 3 as part of step 2.1 of the risk assessment process. The system elements (i.e. subsystems, subsubsystems, and so on, down to the component level) may be organized as a system breakdown structure as shown in Figure 4.1 , where the system is split into three levels. The levels of the hierarchy are called indenture levels, where the first level is called indenture level 1, the next, indenture level 2, and so on. IEV defines indenture level as the “level of subdivision within a system hierarchy” (IEV 192‐01‐05). The number of levels required depends on the size of the system and the objectives of the risk assessment. The various subsystems may have different numbers of levels.

Figure 4.1 System breakdown structure (simplified).

Figure 4.1 shows a breakdown structure where the physical system is broken down into subsystems and components. An alternative approach is to consider the system functions and to break each function down into subfunctions and actions. A functional breakdown structure may often be more useful than a hardware breakdown structure as a starting point for risk and reliability studies.

The causal structure for a system failure or a system accident may also be represented as a hierarchical structure starting from the system failure/accident. Indenture level 2 represents the direct causes of the failure/accident, whereas indenture level 3 represents the direct causes leading to each of the causes at indenture level 2, and so on. The obtained structure represents a hierarchical structure of the causes of the system failure/accident.