Fault Tree Analysis

Investigating root causes by fault tree and cut set calculations

Figure 1. Bowtie effect of FTA and ETA
Figure 1. Bowtie effect of FTA and ETA

FTA and ETA are complementary (and are often used together) but focus on opposite sides of an undesired event. Figure 1 shows how they fit together. This is sometimes called a 'bowtie' model (because it looks like one) and when complementary FTAs and ETAs are used, it is called the bowtie technique.

FTA is concerned with analysing faults which might lead to an event, whereas ETA is interested in stopping it escalating. Both can be applied qualitatively and, if you have the data, quantitatively.

Event Tree Analysis

Fault tree construction

  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.

Fault tree analysis

  1. Find and interpret Cut Sets and Path Sets
  2. Test logic in SUCCESS Domain, verify
  3. Common Cause events/phenomena
  4. Find and interpret Cut Sets
  5. Importance
  6. Path sets
  7. Reducing vulnerability; sensitivity tests
Figure 2. Structure of fault tree analysis report
Figure 2. Structure of fault tree analysis report
  1. Title: Company, Author, Date, etc
  2. Executive Summary: (Abstract of complete report)
  3. Scope of the analysis …
    Brief system description
    TOP Description/Severity Bounding
    Analysis Boundaries
    …Physical Boundaries / Interfaces Treated
    …Operational Boundaries / Resolution Limit
    …Operational Phases / Exposure Interval
    …Human Operator In/Out / Others
  4. The Analysis
    Discussion of Method (Cite References)
    Software Used
    Presentation/Discussion of the Tree
    Source(s) of Probability Data (if quantified)
    Common Cause Search (of done)
    Sensitivity Test(s) (if conducted)
    Cut Sets (Structural and/or Quantitative Importancee, if analyzed)
    Path Sets (if analyzed)
    Trade Studies (if done)
  5. Findings …
    TOP Probability (give Confidence Limits)
    Comments on System Vulnerability
    Chief Contributors
    Candidate Reduction Approaches (if appropriate)
  6. Conclusions and Recomendations …
    Risk Comparisons ("Bootstrapping" data, if appropriate)
    Is further analysis needed? By what method(s)?
  7. Say what is is analyzed and what is not analyzed.
  8. Show Tree as Figure. Include Data Sources, Cut Sets, Path Sets etc as Tables.