Fault tree analysis
A top-down method that starts from one undesired event and maps every credible combination of failures that could cause it.
Definition, in the source's words
NUREG-0492, Fault Tree Handbook (U.S. NRC, 1981), PDF p. 34(opens nrc.gov in a new tab)“A fault tree analysis can be simply described as an analytical technique, whereby an undesired state of the system is specified (usually a state that is critical from a safety standpoint), and the system is then analyzed in the context of its environment and operation to find all credible ways in which the undesired event can occur. The fault tree itself is a graphic model of the various parallel and sequential combinations of faults that will result in the occurrence of the predefined undesired event.”
What it means
Events combine through gates. For independent events, an AND gate multiplies probabilities (every input must happen); an OR gate is 1 minus the product of the complements (any input is enough).
Example
A tank overflows if the level switch fails (P = 0.1) OR the operator misses the alarm (P = 0.2). P(top) = 1 − (0.9 × 0.8) = 0.28. If both had to fail (AND), P = 0.1 × 0.2 = 0.02.
Where it appears on the exam
ASP: ASP11 Domain 2, Safety Programs and Concepts
Practice: System Safety Analysis: Fault Trees and FMEA, Statistics and Probability
CSP: CSP11 Domain 3, Risk Management
Practice: System Safety: Fault Trees, FMEA and MIL-STD-882E, Risk Matrix and Fault Tree Probability
Questions
- Is fault tree analysis top-down or bottom-up?
- Top-down. It starts with the undesired event and works back to its causes. FMEA is the usual bottom-up comparison.
Related
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