The Fault tree analysis was first introduced at Bell laboratories by H.A. Watson 1962. It is one of the most widely used methods in system reliability,maintainability and safety analysis. It is a deductive procedure used to determine the various combinations of hardware, software failures and human errors that could cause  undesirable event.It is carried out to minimize and optimize resources.
     The deductive analysis begins with a general conclusion, then attempts to determine the specific causes of the conclusion by constructing a logic diagram called Fault Tree.
      So, Fault Tree is a logical model of the relationship of undesirable events to more basic events.The top level of the fault tree is the undesirable event. The middle events are intermediate events and the basic events are at the bottom.

The logic relationship of the events are shown by logic symbols or Gates.
The main purpose of the FTA is to help identify potential causes of system failures before the failures actually occur. It can also be used to evaluate the probability of the top event using analytical or statistical methods. These calculations involve system quantitative reliability and maintainability information such as failure probability, failure rate and repair rate. After completing FTA one can focus his efforts on improving system safety and reliability.

Fault tree construction

To do a comprehensive FTA, follow these steps:
  1. Define the fault condition, and write down the top level failure.
  2. Using technical information and professional judgments, determine the possible reasons for the failure to occur. Remember, these are level two elements because they fall just below the top level failure in the tree.
  3. Continue to break down each element with additional gates to lower levels. Consider the relationships between the elements to help you decide whether to use an "and" or an "or" logic gate.
  4. Finalize and review the complete diagram. The chain can only be terminated in a basic fault: human, hardware or software.
  5. If possible, evaluate the probability of occurrence for each of the lowest level elements and calculate the statistical probabilities from the bottom up.


Skindleberg said…
Thanks for bringing up this fault analysis method. It would help if there was an associated example and a comparison to other fault analysis methods.
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