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Updated on 9/16/2019
Relyence User Guide
Introduction to Fault Tree Analysis
Direct link to topic in this publication:

Performing a Fault Tree Analysis


Fault tree analysis (FTA) is a top down, deductive failure analysis methodology in which conditions and factors that can contribute to a specified undesired event are identified and organized in a logical manner and represented pictorially. It is the most commonly used technique for causal analysis in risk and reliability studies. It has been widely used in safety-critical applications and high risk and high hazard industries to understand how systems can fail, to determine the contributing factors and pathways of failures, to identify the best ways to reduce risk, and to quantify event probabilities of a safety accident or a particular system level failure. FTA methodology is described in several industry and government standards, including NRC NUREG–0492 for the nuclear power industry, Fault Tree Handbook with Aerospace Applications for use by NASA, SAE ARP4761 for civil aerospace, MIL–HDBK–338 for military systems, and IEC 61025 for cross-industry.

Performing a fault tree analysis (FTA) is a continual process. You begin with a risk you want to evaluate, and then build out your analysis based on information about events that can lead to a potentially hazardous condition. As information is gathered over time, you can update your fault tree analysis to become more comprehensive and more exact. 

Fault trees can be as simple or as complex as needed for your particular situation. 

We recommend going through Getting Started with Relyence Fault Tree as a starting point for learning Relyence Fault Tree. From there, you can proceed to building your own analyses. The following process is intended to be a starting point; you can adapt it as required for your needs.

1. Define the risk you want to evaluate

The object of fault tree analysis is to either quantitatively or qualitatively assess the risk of occurrence of an undesirable event. That event can be anything you choose. Oftentimes, fault trees are used in industries where safety is a topmost concern; however, fault tree analysis can be used in any industry where the prevention of an undesired event is critical. For example, you may want to evaluate the potential of a particular product failure if you feel that its impact on your company reputation or warranty claims would be substantial.

2. Build your fault tree diagram

You begin your FTA by developing a diagrammatic representation of events using logic gates. Start by defining the top event, which represents the failure you are analyzing. The events that could lead to the top event are then determined and are connected to the top event using a logic gate depending on the logical relationship between the events and its causes. The process continues until basic causes are identified. A variety of logic gates can be utilized in defining your fault tree depending on which one accurately models the relationships between events. You can use AND, OR, NOR, NOT, XOR, Priority AND, and Voting Gates to build out your fault tree. 

3. Add in the data associated with your events

Once your fault tree is built, you then need to define the probabilities of the events in your diagram in order to perform a quantitative analysis. Relyence Fault Tree supports several ways to model the probability of occurrence values of your events. Data parameters used to model event probabilities include unavailability, probability, frequency, and failure rate values.

4. Calculate your fault tree

Once you have your fault tree diagram built and event probabilities defined, you perform computations to calculate the overall probability of the Top Gate, as well as the probabilities of all intermediate gates if you want. Fault trees results are computed based on the application of Boolean logic and other mathematical techniques. The techniques utilized vary in complexity, accuracy, and computational time. To do this part of FTA by hand would be difficult and error prone, which is why using a package designed specifically to perform FTA is important. Relyence Fault Tree has a powerful calculation engine which enables you to quickly and accurately perform fault tree calculations.

5. Analyze the results

Once you have the results of your FTA, you then review the probabilities of the undesired event or events. Depending upon your risk level requirements, you will have to assess if design changes are required, or if you must perform other necessary tasks to mitigate or prevent risk. You cannot completely eliminate risk, but the goal of a fault tree is to help determine how to ensure the probability of risk is as low as tolerable.

As you make changes or perform other risk-lowering activities, you can then go back and adjust your fault tree data parameters accordingly and recalculate risk results based on your modifications.