1. Defining the Undesired Event(s) (Major Fault(s))
a. The undesired event is most often the fault which, upon occurrence, results in complete failure of the system, the failure of a back-up system, degradation, or an undetected failure. This is considered catastrophic failure. The major fault is a failure which causes loss of availability through the degradation or system shut-down and/or poses a safety hazard to operators and/or maintenance personnel. The undesired event, however, may be an unusual failure at a sub-system level, the root cause of which is unknown. Any observable event may be chosen as the “undesired event”. The analyst must recognize that the FTA will not identify failures unrelated to the chosen event.
b. To define the undesired event, the normal system operation and environment must be known in order to allow the analysis to show the undesired event as a failure. When defining the undesired event, care must be taken to prevent the range of the faults from becoming too broad. For example, “Failure to complete trip”, for an automobile, is not specific enough to allow for ease of analysis. This is because failure could vary from an air conditioning fault, which caused discomfort, to loss of engine power, which caused loss of mobility. Both faults could be considered failure; however, loss of mobility is obviously a much more severe fault than losing air conditioning.