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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:

The five basic steps to perform a Fault Tree Analysis are as follows:

1 Identify the Hazard

2 Obtain Understanding of the System Being Analyzed

3 Create the Fault Tree

4 Identify the Cut Sets

5 Mitigate the Risk

Top‐level event is called a Cut Set. There are many cut sets within the FTA. Each has an individual probability assigned to it. The paths related to the highest severity / highest probability combinations are identified and will require mitigation.