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A fault tree analysis (FTA) is a deductive failure analysis method that models the pathways within a system that can lead to failures or undesired results. It is a top-down method which starts at a single point and then branches out downwards to display different states of the system using logic symbols. The starting point in this case is a fault or undesired event, and then it is resolved downward to show the causes of the undesired event and then show the causes of those events.
A fault tree analysis is a systematic and stylized deductive process in which an undesired event, such as a catastrophic failure of a system, is defined and then broken down to its individual causes. The very top of the diagram is the event and branching from it are its immediate causes. Each immediate cause is then also resolved into its own immediate causes, and this continues until the most basic causes are identified. This results in a tree with numerous branches highlighting the possible chain of events that caused the failure at the top of the diagram.
The resulting fault tree explicitly shows all of the different relationships that are necessary in order to result in the event at the very top. By constructing the tree, a thorough understanding of the basic and logic causes of the event on top can be obtained. This then provides a tangible record and a framework for a thorough quantitative and qualitative evaluation of the event in question.
To construct a tree, a final event is placed at the very top, and then connected to logic symbols which represent the conditions for the event to occur, which then connects to the intermediate events that caused the higher event. For example, the OR symbol means that at least one intermediate event needs to happen for the higher event to occur, while the AND symbol means that at least two or more intermediate events need to happen for the higher event to occur.
As an example, a computer becoming fried due to a lightning strike could be the top event. In this case, two things need to happen in order for the top event to occur, one being that a lightning bolt hit the main electricity line of the house, and the other intermediate event or state is that the computer is plugged into the main socket. Both of these intermediate events or states must happen first before the top event, the computer getting fried, can occur.