Fault-Tree Construction

FTA begins with the statement of an undesired event, that is, failed state of a system. To perform a meaningful analysis, the following three basic types of system information are usually needed  

Component operating characteristics and failure modes A description of how the output states of each component are influenced by the input states and internal operational modes of the component. 

System chart A description of how the components are interconnected. A functional layout diagram of the system must show all functional interconnections of the components. 

System boundary conditions These define the situation for which the fault tree is to be drawn. 

To complete the constraction of a fault tree for a complicated system, it is necessary first to understand how the system works. This can be achieved by studying the blue prints of the system (which will reflect the interconnections of components within the system). In practice, all basic events are taken to be statistically independent unless they are common cause failures. Construction of an FT is very susceptible to the subjectivity of the analyst. Some analysts may perceive the logical relationships between the top event and the basic events of a system differently. Therefore, once the constmction of the tree has been completed, it should be reviewed for accuracy, completeness and checked for omission and oversight. This validation process is essential to produce a more useful FT by which system weakness and strength can be identified. 

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Fault Tree Construction. Eault tree constmction begins at the TOP event and proceeds, level by level, until all fault events have been developed to their basic contributing causes (BASIC events). The analyst begins with the TOP event and, for the next level, determines the immediate. 

Before the analyst begins fault tree construction, it is necessary to document and understand System design,... 

It is illustrated in Section 3.4.4 by tracing the paths for leaking engine compression and applied to fault tree construction for the FFTF reactor Fullwood and Erdmann, 1974). The method involves writing Boolean equations for all paths whereby hazardous material may be released. It is primarily useful for enumerating release paths, but not for what started the release It was used to enumerate the possible paths for stealing nuclear bomb material from a facility. 

Fullwood, R. and R. C. Erdman, 1974, On the Use of Leak Path Analysis in Fault Tree Construction for Fast Reactor Safety, CONF-74040I-P3. 

Fussell, J. B. 1975, Computer Aided Fault Tree Construction for Electrical System and Fault Tree Analysis, SIAM, Philadelphia, PA, p 37. 

The fault tree is a graphic representation of the relationships between basic events and the selected top event. Table 4.24 presents the standard symbols used in fault tree construction to show these relationships. 

Other considerations for fault tree construction are (/) assume that faults propagate through normally operating equipment. Never assume that a fault is stopped by the miraculous failure of another piece of equipment. (2) Gates are connected through labeled fault events. The output from one gate is never connected directly into another. 

CONSTRUCTING THE FAULT TREE. Fault tree construction begins at the top event and proceeds, level by level, until all fault events have been traced to their basic contributing events or basic events. The analysis starts with a review of system requirements, function, design, environment, and other factors to determine the conditions, events, and failures that could contribute to an occurrence of the undesired top event. The top event is then defined in terms of sub-top events, i.e., events that describe the specific "whens and wheres" of the hazard in the top event. Next, the analysts examine the sub-top events and determine the immediate, necessary, and sufficient causes that result in each of these events. Normally, these are not basic causes, but are intermediate faults that require further development. For each intermediate fault, the causes are determined and shown on the fault tree with the appropriate logic gate. The analysts follow this process until all intermediate faults have... 

Numerous symbols are used in the construction of a basic fault tree. These symbols, sometimes referred to as fundamental logic symbols, provide the analyst with a pictorial representation of the event and how it interacts with other events on the tree. Figure 12.2 shows the basic symbols used during the FTA process. Once the reader has a general understanding of these symbols and their use, as described below, fault tree construction will be greatly facilitated. 

Fault tree development and evaluation Genetal, system familiarization, fault tree developments, fault tree construction, and failure rates in FTA. 

Figure 12.2 shows the basic symbols used during the FTA process. Once the reader has a general understanding of these symbols and their use, as described below, fault tree construction will be greatly facilitated. 

Andrews, J. D., Morgan, J. M. (1986). Application of digraph method of fault tree construction to process plant. Reliable Engineering 14, 85. 

EXAMPLE OF A FAULT TREE CONSTRUCTION OF A MOTOR-PUMP PRESSURE SYSTEM... 

The fault tree construction proceeds by determining the failures that lead to the primary event failures. The construction of the fault tree continues until all the basic events that influence the Top Event are evaluated. Ideally, all logic branches in the fault tree are developed to the point that they terminate in basic events. 

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