Fault events

Other considerations for fault tree constmction 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 coimected through labeled fault events. The output from one gate is never coimected directly into another. 

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. 

The immediate causes of the TOP event are shown in the fault tree with thek relationship to the TOP event. If any one of the immediate causes results dkecdy in the TOP event, the causes are connected to the TOP event with an OR logic gate. If all the immediate causes are requked for TOP event occurrence, then the causes are connected to the TOP event with an AND logic gate. Each of the immediate causes is then treated in the same manner as the TOP event, and its immediate, necessary, and sufficient causes are determined and shown on the fault tree with the appropriate logic gate. This development continues until all intermediate fault events have been developed into thek basic causes. 

Intamiediate event A fault event that occurs because of one or more antecedent causes acting through logic gates. 

A failure modes and effects analysis delineates components, their interaction.s ith each other, and the effects of their failures on their system. A key element of fault tree analysis is the identification of related fault events that can contribute to the top event. For a quantitative evaluation, the failure modes must be clearly defined and related to a numerical database. Component failure modes should be realistically and consistently postulated within the context of system operational requirements and environmental factors. 

All component fault events can be described by one of three failure characteristics... 

PSAPACK 4,2 is an integrated fault/event tree package with capabilities for easily examining results and performing recalculations. There are two levels of processing in the package ... 

The loss of all AC power initiator includes both commercial (offsite) power and all three diesels, including the normally-running diesel. Local fault events include localized flow blockages in iho core and localized fuel faults. A lobe power imbalance initiator occurs if the individually-controlled core section powers (lobe powers)... 

This study is a good reference for the construction of fault/event trees of systems that are affected by valve performance. The valve failure modes are identified, the associated mechanisms are described in detail, and preventive measures are offered. 

Staiidind and ADL Usage Logic gale where output fault occurs only if all input faults/events occur... 

Standard and ADL Usage Logic gate where output fault occurs if at least one of the input faults/events occurs... 

INTERMEDIATE A fault event that results from the interactions of Event other fault events that are developed through... 

BASIC Event A fault event representing a component failure... 

A fault event representing a failure or error which is not examined further because information is not available or because further development is beyond the scope of the study. 

Unallowed events are those that are considered to be incredible or that, for some other reason, are not to be considered in the analysis. For example, wiring failures might be excluded from the analysis of an instrument system. Existing conditions are, for the purposes of the FTA, events or conditions considered certain to occur. The unallowed and existing conditions do not appear in the fault tree, but their effects must be considered in developing other fault events as the fault tree is constructed. 

A fault event that needs no further definition. 

Unavailability—The probability the fault event exists at a specified time. The percentage of time that one or more fault events prevent effective operation of a device or system. 

Examples of logic trees—fault, event, causal, and why—are discussed below in order of increasing rigor. Chapter 9 contains detailed information on developing logic trees. 

UNDEVELOPED Event The UNDEVELOPED event represents a fault event that is not examined further because information is unavailable or because its consequence is judged insignificant. In this book, the diamond with Why inside it is used to indicate events which were not developed in the examples, but should be developed in actual investigations. 

Alluvial fans cut by tectonic faults can also yield information on the frequency of earthquakes. An offset debris flow fan in Owens Valley (California) was dated with °Be by Bierman et al. (1995). Based on boulder ages from different parts of the flow and the record of three faulting events, the authors deduced earthquake recurrence intervals of 5800-8000 years for that location. Zreda and Noller (1998) measured C1 in a bedrock fault scarp at Hebgen Lake (Montana) and were able to extract the ages of six prehistoric... 

A fault event representing a component failure or human error that requires no further development. A basic event is the lowest level of resolution in a fault tree. 

A fault event representing a condition or event that is assumed to exist either as a boundary condition for the fault tree or because the event always occurs unless a failure takes place. 

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... 

A fault tree is a model that graphically and logically represents the various combinations of possible events, both fault and normal, occurring in a system that lead to the top event. A fault event is an abnormal system state. A normal event is an event that is expected to occur. The term event denotes a dynamic change of state that occurs in a system element. System elements include hardware, software, human, and environmental factors. Details about the construction of fault trees can be found in Henley et al. (1992). 

The growth of a defect into what becomes a fault or a faulty component really depends on many factors, which is predominantly the type of corrosion that is progressing. In the fault-tree analysis context, the fault event of a component is defined as a state transition from the normal state to a faulty state of that component. These state transitions are irreversible, which means that a faulty state does not return to the intended state even if the influences that caused the fault event in the first place disappear. 

E-17 Two loop natural circulation heat rejection from initial three loop operation E-18 Two loop natural circulation E-19 Loss of flow in two sodium loops D. Faulted Events F-1 (Deleted)... 

Note The FTA is primarily a graphical method using logic gates and fault events to model the cause— effect relationship in causing an undesired event This gr cal method can be translated into a mathematical model to compute failure probabiUties mid system importmice measures [Eiicson, 2005, Chapter 11]. This quantitative iproach provides more useful results, but requires mo e time (e.g. gathering of component failure rate data) mid expmenced personnel. 

Detailed investigation of the most important scenarios identified in step 1 Creation of fault event trees Quantification of probabilities and consequences focus on safety-related measures and probability for loss of life Detailed investigation of the most important scenarios identified in step 1 Creation of fault event trees Quantification of probabilities and consequences, focus on economic indteators Economic loss given disruption, short and long term sur tvability of s)stem, which stakeholder is responsible for loss and which stallholder incurs loss for consequences... 

Output (fault) event, Output (fault) event... 

Step 2 Identify the undesirable event (i.e., the top fault event) to be investigated (e.g., system failure). 

Step 3 Determine all types of possible potential causes that can result in the occurrence of the top fault event by using fault tree symbols, such as shown in Figure 4.4, and the logic tree format. 

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