Getting Started with Windchill Risk and Reliability > Windchill FTA > Understanding FTA Calculations
Understanding FTA Calculations
Windchill FTA includes both qualitative and quantitative calculations. Qualitative calculations analyze the unwanted event (the top gate in the fault tree) and determine the minimum combination of events that cause this event to occur. These calculations are useful for identifying unexpected root cause combinations, designing for weak points in a system, and finding common causes for an issue. Quantitative calculations measure the unreliability and unavailability of the system based on the fault tree, and provide a more precise and less subjective evaluation of a fault tree.
A sampling of supported calculations is outlined below and brief descriptions are provided. For more information, see the help or guide for Windchill Risk and Reliability.
Calculation
Description
Minimal cut sets
A cut set identifies which unique combination of component failures and/or events can cause an undesired event to occur. A minimal cut set is the smallest set of events, which, if they all occur, cause the top event to occur.
Reliability importance measures
A method of ranking the basic events in the fault tree to show their relative culpability in causing the top event to occur. Importance measures are used to detect critical design weaknesses and component failures. They can assist you in identifying the fault tree event that, if improved, is most likely to produce the greatest improvement in system performance. They can also help you determine if allocating additional resources or adding redundancy to the system would improve the overall system reliability.
Unavailability
The probability that, at a given time, the system does not function, due to either a failure or a repair.
Unreliability
The probability that, during a given time period, the top event occurs, and the system is unable to function.
Frequency
The frequency of failures over a time period.
Number of failures
The number of failures in a period of time.
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