January 2017 – Fault Tree Analysis Example

This subject guide provides a brief overview of the basic concepts in fault tree analysis as it applies to system reliability along with a fault tree analysis example. 

As leading reliability professionals we are asked to conduct Fault Tree Analyses on mission and safety critical products, especially where failures can have major consequences. 

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What is Fault Tree Analysis?
Fault Tree Analysis (FTA) is a failure analysis in which an undesired event of a system is analyzed using Boolean logic to combine a series of lower-level events. FTA is a top-down analysis that helps determine the probability of occurrence for an undesirable event. The analysis creates a visual record showing the logical relationships between events and failures that lead to the undesirable event. It easily presents the results of your analysis and pinpoints weaknesses in the design. 

How is it Useful?
A Fault Tree Analysis looks for various ways that individual components, or groups of components, would have to fail in order to produce undesirable events. Analysis results for each event are presented in a tree-like diagram using logic symbols to show how dependencies among components contribute to the undesirable event at the top-level. In this way, FTA provides valuable information regarding the robustness of the design and shows failure probabilities at each level, from components to the undesirable event. 

Methodology
An undesired effect is taken as the top event of a tree of logic. One fault tree is prepared for each undesired event. Then, each situation that could cause that effect is added to the tree as series of logic expressions. The first step of performing an FTA is to identify undesirable events for the system under analysis. This is accomplished by a Functional Hazard Analysis. Once the undesired events are identified, each cause and its associated probability are identified and added to the tree. Full knowledge of the system is very important to not miss any causes affecting the undesired event. 

Drawing Fault Trees: Gates & Events
Gate symbols represent results of interactions among contributing failure events and can vary among tools. Basic gates used to construct the Fault Tree can be seen below:Fault Tree Analysis ExampleFTA Gate Notes
Fault Tree probabilities can be computed by simple arithmetic only if basic events (component failures without lower level contributors) are independent. Independence is determined by ensuring the failure of one basic event has no effect on any other and groups of basic events cannot fail from common causes such as shock. For independent basic events with very small failure rates, typically found in electronic components, an AND gate output probability can be computed as the product of its input failure probabilities, and an OR gate output probability can be computed as the sum of its input failure probabilities. 

Below is a Basic Fault Tree Analysis Example Structure:Fault Tree Analysis ExampleDeeper Dive
One situation Omnicon engineers came across involved a complex fault tree development in which several basic events (e.g., flight computer failure) were scattered among many subsystem fault trees, all of which led to the loss of the aircraft at the top-level. Although the top level failure probability computed by the analysis tool appeared reasonable, it was wrong because the tool did not account for multiple appearances of certain basic events. Computing fault tree probabilities in this situation is very difficult because they require statistical or other techniques rather than straightforward multiplication or addition. It is important to keep in mind that more advanced analysis techniques are required when the fault tree cannot be rearranged to eliminate multiple appearances of basic events, or if common causes contribute to two or more basic event failures. 

Learn More – Why Omnicon?
The Omnicon Group is a recognized leader in conducting Fault Tree Analyses. We work with you to tailor any type of required reliability analysis to your needs and provide results that are most useful to you. Reliability has been our number one priority since our start in 1984. The Omnicon Group is proud of its many awards and achievements in reliability, maintainability, and testability engineering. We have more engineers named Reliability Engineer of the Year than any other company in the world. Many companies rely on Omnicon’s engineering to improve the quality of their hardware, software and system designs working with our customers to create innovative and reliable designs. Our worldwide engineering and design expertise is unparalleled. We use proven methods of design and development with strict management of quality, value, schedule and cost.

Tags: analysis, fault tree, FTA, reliability

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