FTA: Fault Tree Analysis

FTA is one of the important investigation tool.

  1. Introduction 
  • Fault tree analysis (FTA) is concerned with the identification and analysis of conditions and factors that cause or may potentially cause or contribute to the occurrence of a defined top event.
  • With FTA this event is usually seizure or degradation of system performance, safety or other important operational attributes, while with STA (success tree analysis) this event is the attribute describing the success.
  • FTA is often applied to the safety analysis of systems (such as transportation systems, power plants, or any other systems that might require evaluation of safety of their operation).
  • Fault tree analysis can be also used for availability and maintainability analysis.
  • There are two approaches to FTA.

a. Qualitative Approach

  • Where the probability of events and their contributing factors, – input events – or their frequency of occurrence is not addressed.
  • This approach is a detailed analysis of events/faults and is known as a qualitative or traditional FTA.
  • It is largely used in nuclear industry applications and many other instances where the potential causes or faults are sought out, without interest in their likelihood of occurrence.
  • At times, some events in the traditional FTA are investigated quantitatively, but these calculations are disassociated with any overall reliability concepts, in which case, no attempt to calculate overall reliability using FTA is made.

b. Quantitative Approach,

  • Where a detailed FTA models an entire product, process or system, and the vast majority of the basic events, whether faults or events, has a probability of occurrence determined by analysis or test.
  • In this case, the final result is the probability of occurrence of a top event representing reliability or probability of fault or a failure.

2. Terms And Definitions

  • In fault tree methodology and applications, many terms are used to better explain the intent of analysis or the thought process behind such analysis.  (See in Table)

3. Symbols

  • The graphical representation of a fault tree requires that symbols, identifiers and labels be used in a consistent manner. Symbols describing fault tree events vary with user preferences and software packages, when used. (See in Table)

4. Fault Tree Description And Structure

  • Several analytical methods of dependability analysis are available, of which fault tree analysis (FTA) is one.
  • The purpose of each method and their individual or combined applicability in evaluating the flow of events or states that would be the cause of an outcome, or reliability and availability of a given system or component should be examined by the analyst before starting FTA.
  • Consideration should be given to the advantages and disadvantages of each method and their respective products, data required to perform the analysis, complexity of analysis and other factors.
  • A fault tree is an organized graphical representation of the conditions or other factors causing or contributing to the occurrence of a defined outcome, referred to as the “top event”.
  • When the outcome is a success, then the fault tree becomes a success tree, where the input events are those that contribute to the top success event.
  • The representation of a fault tree is in a form that can be clearly understood, analyzed and, as necessary, rearranged to facilitate the identification of:
  • Factors affecting the investigated top event as it is carried out in most of the traditional fault tree analyses;
  • Factors affecting the reliability and performance characteristics of the system, when the FTA technique is used for reliability analysis, for example design deficiencies, environmental or operational stresses, component failure modes, operator mistakes, software faults;
  • Events affecting more than one functional component, which could cancel the benefits of specific redundancies or affect two or more parts of a product that may otherwise seem operationally unrelated or independent (common cause events).
  • Fault tree analysis is a deductive (top-down) method of analysis aimed at pinpointing the causes or combinations of causes that can lead to the defined top event.
  • The analysis can be qualitative or quantitative, depending on the scope of the analyses.
  • A quantitative FTA can be used when the probabilities of primary events are known. Probabilities of occurrence of all intermediate events and the top event (outcome) can then be calculated in accordance with the model.
  • Also, the quantitative FTA is very useful in reliability analysis of a product or a system in its development. FTA can be used for analysis of systems with complex interactions between sub-systems including software/hardware interactions.

5. Fault Tree Graphical Description and Structure

Components of a fault tree are as follows:

  • Gates:

-Symbols showing the logical relationship between input events and the output event

-Static gates – outcome not dependent on the order of occurrence of inputs,

-Dynamic gates – outcome dependent on the order of occurrence of inputs.

  • Events

-Lowest level of inputs in a fault tree.

6. Fault Tree Development And Evaluation

  • Development of a fault tree starts with the definition of the top event.
  • Development of a fault tree in its traditional application, or for the system reliability and the failure mode analysis, is a deductive method where the analysis starts from the top undesired event as it is defined for the scope of analysis.
  • Once developed to the intended extent, the fault tree becomes a graphical representation of all events that either by themselves or in conjunction with other events contribute to the occurrence of the top event.

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