FAULT
TREE
ANALYSIS
4th Edition

provided as a free service by www.fault-tree.net -- Pat L. Clemens and Jacobs Sverdrup

P. L. Clemens
May 1993

Sverdrup

1

TOPICS COVERED . . .
• Fault Tree Definition
• Developing the Fault Tree
• Structural Significance of the Analysis
• Quantitative Significance of the Analysis
• Diagnostic Aids and Shortcuts
• Finding and Interpreting Cut Sets and Path Sets
• Success–Domain Counterpart Analysis
• Assembling the Fault Tree Analysis Report
• Fault Tree Analysis versus Alternatives
• Fault Tree Shortcomings / Pitfalls / Abuses
All fault trees appearing in this training module have been drawn, analyzed, and printed using FaultrEASE ™, a computer application available from: Arthur D. Little, Inc. / Acorn
Park / Cambridge, Massachusetts, 02140-2390 — Phone (617) 864-5770.

provided as a free service by www.fault-tree.net -- Pat L. Clemens and Jacobs Sverdrup

2

FIRST —
A BIT OF
BACKGROUND . . .

• Origins of the technique
• Fault Tree Analysis defined
• Where best to apply the technique
• What the analysis produces
• Symbols and conventions

provided as a free service by www.fault-tree.net -- Pat L. Clemens and Jacobs Sverdrup

3

ORIGINS . . .
• Fault tree analysis was developed in
1962 for the U.S. Air Force by Bell
Telephone Laboratories for use with the Minuteman system…was later adopted and extensively applied by the Boeing Company…is one of many symbolic logic analytical techniques found in the operations research discipline. provided as a free service by www.fault-tree.net -- Pat L. Clemens and Jacobs Sverdrup

4

THE FAULT TREE IS . . .
• …a graphic “model” of the pathways within a system that can lead to a foreseeable, undesirable loss event. The pathways interconnect contributory events and conditions, using standard logic symbols. Numerical probabilities of occurrence can be entered and propagated through the model to evaluate probability of the foreseeable, undesirable event.

• …only one of many System Safety analytical tools and techniques.

provided as a free service by www.fault-tree.net -- Pat L. Clemens and Jacobs Sverdrup

5

FAULT TREE ANALYSIS IS BEST
APPLIED TO CASES WITH . . .
• Large, perceived threats of loss…i.e., high risk.
• Numerous potential contributors to a mishap.
• Complex or multi-element systems/processes.
• Already-identified undesirable events. (A must!)
• Indiscernible mishap causes (i.e., autopsies).

CAVEAT: Large fault trees are resource-hungry and should not be undertaken without reasonable assurance of need.

provided as a free service by www.fault-tree.net -- Pat L. Clemens and Jacobs Sverdrup

6

FAULT TREE ANALYSIS PRODUCES . . .
• Graphic display of chains of events/conditions

leading to the loss event.
• Identification of those potential contributors to failure that are “critical.”
• Improved understanding of system characteristics. • Qualitative/quantitative insight into probability of the loss event selected for analysis.
• Identification of resources committed to preventing failure.
• Guidance for redeploying resources to optimize control of risk.
• Documentation of analytical results.

provided as a free service by www.fault-tree.net -- Pat L. Clemens and Jacobs Sverdrup

7

SOME DEFINITIONS . . .
• FAULT: An abnormal, undesirable state of a system or a system element* induced (1) by presence of an improper command or absence of a proper one, or (2) by a failure (see below). All failures cause faults; not all faults are caused by failures. A system which has been shut down by safety features has not faulted.

• FAILURE: Loss, by a system or system element*, of functional integrity to perform as

intended. E.g., relay contacts corrode and will not pass rated current when closed, or the relay coil has burned out and will not close the contacts when commanded — the relay has failed ; a pressure vessel bursts —