Indenture level

Assembly drawings—documents the relationship of a combination of parts and subassemblies required to form the next higher indenture level of equipment or system. 

The scope is easily defined if an analytical tree is used as a feeder document (Chapter 10). The indenture levels can be identified by specifying the number of tiers of the tree to be included in the analysis (limits of resolution). A functional FMEA includes upper and/or middle tiers only a hardware FMEA includes the entire tree. 

If an analytical tree is not used, in addition to defining the system and clarifying the scope in terms of breadth (limits of resolution) and depth (indenture levels), a block diagram or other project description document may be required. 

Column 3—Effects on Other Components. List the impact of each failure listed in column 2 on other components. As in column 1, components may also include subassemblies, assemblies, or subsystems, especially if a functional FMEA is being performed. List those components that will physically be damaged and those whose functions may be adversely affected, which are generally those located adjacent to or near the failed component physically and/or operationally. They are generally in the same or the next higher indenture level or tier. 

The first step in performing a fault tree analysis is to collect the appropriate project description documents, existing hazard analyses, and guidance documents and carefully review them to determine the limits, scope, and ground rules for the FTA.This review includes defining the system to be analyzed, the depth or indenture levels to be included in the effort, and, of course, the nature of the undesired event or failure to be studied. 

End effect The consequence a failure mode has upon the operation, function, or status at the highest indenture level. 

Indenture levels An identifier for system level. Fevels identify or describe the relative complexity of an assembly or function. Complexity increases as it is closure one to failure point. 

Conditions that constitute part and full system failure should also be determined. The system indenture levels must be identified to complete the FMECA. 

Construct functional block diagrams that indicate how the different system indenture levels are related. 

Assess each functional block (at the block level) and determine if its failure would affect the rest of the system. If it would not, then ignore the block. If its failure would affect the rest of the system, go down another indenture level and perform the following scheme. Continue down to the level of relevance. 

Figure 2.20 depicts the FMEA concept. The subsystem being analyzed is divided into its relevant indenture levels, such as Unit 1, Unit 2, Unit 3. Each unit is then further subdivided into its basic items. Each item is listed down... 

The structural FMEA is performed on hardware and focuses on potential hardware failure modes. The hardware can be at any hardware indenture level for the analysis subsystem, unit, assembly, or part (component). The structural approach tends to be a detailed analysis at the component level. 

The functional FMEA is performed on functions. The functions can be at any functional indenture level for the analysis system, subsystem, unit, or assembly. This approach focuses on ways in which functional objectives of a system go unsatisfied or are erroneous. The functional approach is also applicable to the evaluation of software through the evaluation of required software functions. The functional approach tends to be more of a system-level analysis. 

Level of assembly refers to the natural hierarchy of elements within a system. A system is composed of subsystems, assemblies, subassemblies, components, and parts, all of which can be categorized by level of assembly or system hierarchy. Level of assembly is a decomposition of a system into a hierarchy of levels that incrementally reduce the complexity description of the system. Levels of assembly are typically used for describing development, analysis, and test configurations of a system. This type of system breakdown is also referred to as system indenture levels and can also be part of a system work breakdown structure (WBS). 

The following is a typical breakdown of successive indenture level groupings in the system hierarchy ... 

Once all criticality numbers of the item under all severity classes have been obtained, a criticality matrix can be constructed which provides a means of comparing the item to all others. Such a matrix display shows the distributions of criticality of the failure modes of the item and provides a tool for assigning priority for corrective action. Criticality analysis can be performed at different indenture levels. Information produced at low indenture levels may be used for criticality analysis at a hi er indenture level. Failure modes can also be prioritised for possible corrective action. This can be achieved by calculating the Risk Priority Number (RPN) associated with each failure mode. This will be studied in detail in Chapter 7. 

In the hazard identification phase, the combined experience and insight of engineers is required to systematically identify all potential failure events at each required indenture level with a view to assessing their influences on system safety and performance. This is achieved using brainstorming techniques. The hazard identification phase can be further broken down into several steps as follows ... 

Define the system to be analyzed. A System Design Document should provide a complete system definition, which includes identification of internal and interface functions, expected performance at all system and subsystem indenture levels, system restraints, failure definitions, operational tasks, environmental profiles, equipment utilization, and the functions and outputs of each item. 

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