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DOE-HDBK-1100-2004
The FTA method was originally developed to supplement a FMEA. Fault trees, in their original
usage, were diagrams indicating how the data developed by FMEAs interact to cause a specific
event. The FTA method is most effective in analyzing complex systems with a limited number
of well-identified hazards. In most cases, FTAs are used to perform in-depth analyses of
hazardous events identified by another hazard evaluation method.
FTA is a deductive method that uses Boolean logic symbols (i.e., AND gates, OR gates) to
break down the causes of the top event into basic equipment failures and human errors. The
analysts begin with the top event and identify the causes and the logical relationships between
the causes and the top event. Each of the causes, called intermediate events, is examined in the
same manner until the basic causes for every intermediate event have been identified. The fault
tree is a graphic representation of the relationships between basic events and the selected top
event. Table 4.24 presents the standard symbols used in fault tree construction to show these
relationships.
A fault tree is, itself, a Boolean equation relating basic events to the top event. The equation can
be analyzed quantitatively or qualitatively by hand or by using computer code(s). If it is
analyzed quantitatively, the probabilities or frequencies of the intermediate events and the top
event are calculated. If it is analyzed qualitatively, a list of the failure combinations that can
cause the top event is generated. These combinations are known as cut sets. A minimal cut set
(MCS) is the smallest combination of basic events that, if they occur or exist simultaneously,
cause the top event. These combinations are termed "minimal" because all of the basic events in
a MCS must occur if the top event is to occur. Thus, a list of MCSs represents the known ways
the top event can occur, stated in terms of equipment failures, human errors, and associated
circumstances.
4.6.2 Analysis Procedure
A FTA has four steps: (1) defining the system or process, (2) constructing the fault trees,
(3) analyzing the fault trees, and (4) documenting the results. To meet PSM Rule requirements,
defining the process for study, performing the analysis, and documenting the results can be
performed by a single person. The construction of the fault trees must be performed by a team.
DEFINING THE PROCESS.
This step identifies the specific top event or events to be evaluated and
the boundary conditions under which they are analyzed. Boundary conditions include the
following.
System Physical Boundaries
Unallowed Events
Level of Resolution
Existing Conditions
Initial Equipment Configuration
Other Assumptions
Initial Operating Condition
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