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# Fault Tree

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• Pages 127 - 143 7.1 Fault Tree Symbols and Logic 7.2 Finding Cut Sets 7.3 Fault Tree Quantification 7.4 Example of a Fault Tree Construction of a Motor-pump Pressure System 7.5 Common Mistakes in Fault Trees
• ### Fault Tree

1. 1. Fault Tree Analysis
2. 2. Fault Tree Analysis <ul><li>Used in both reliability engineering and system safety engineering </li></ul><ul><li>Developed in 1961 for US ICBM program </li></ul><ul><li>Guide published in 1981 </li></ul><ul><li>Used in almost every engineering discipline </li></ul><ul><li>Not a model of all system or component failures </li></ul>
3. 3. Applying Fault Tree Analysis <ul><li>Postulate top event (fault) </li></ul><ul><li>Branch down listing faults in the system that must occur for the top event to occur </li></ul><ul><li>Consider sequential and parallel or combinations of faults </li></ul><ul><li>Use Boolean algebra to quantify fault tree with event probabilities </li></ul><ul><li>Determine probability of top event </li></ul>
4. 4. Fault Tree Logic <ul><li>Use logic gates to show how top event occurs </li></ul><ul><li>Higher gates are the outputs from lower gates in the tree </li></ul><ul><li>Top event is output of all the input faults or events that occur </li></ul>
5. 5. Terms <ul><li>Faults and failures </li></ul><ul><li>System and subsystem faults </li></ul><ul><li>Primary and secondary failure </li></ul><ul><li>Command fault </li></ul>
6. 6. Fault Tree Symbols Intermediate Event Symbol Gate Symbols AND OR Exclusive OR Priority AND Inhibit Transfer Symbols Transfer IN Transfer OUT Basic Event Conditioning Event Undeveloped Event External Event Primary Event Symbols
7. 7. Fault Tree Symbols Basic Event Conditioning Event Undeveloped Event External Event Primary Event Symbols
8. 8. Fault Tree Symbols Gate Symbols AND OR Exclusive OR Priority AND Inhibit
9. 9. Fault Tree Symbols Intermediate Event Symbol Transfer Symbols Transfer IN Transfer OUT
10. 10. Union A=B + C A=B Union C B OR C must occur for event A to occur No Current Switch A Open Battery B 0 Volts A B C
11. 11. Intersection D=E * F D= E Intersection F E AND F must occur for D to occur Over-heated Wire 5mA Current in System Power Applied t >1ms D E F
12. 12. Fault Tree Quantification <ul><li>Fault tree analysis - is not a quantitative analysis but can be quantified </li></ul><ul><li>How to </li></ul><ul><ul><li>Draw fault tree and derive Boolean equations </li></ul></ul><ul><ul><li>Generate probability estimates </li></ul></ul><ul><ul><li>Assign estimates to events </li></ul></ul><ul><ul><li>Combine probabilities to determine top event </li></ul></ul>
13. 13. Fault Tree Example Pressure Switch S Pressure Tank Switch S1 Motor Timer Relay Pump Relay K1 Relay K2 Outlet Valve
14. 14. Common Mistakes in Fault Trees <ul><li>Inputs with small probabilities </li></ul><ul><li>Passive components </li></ul><ul><li>Does quantified tree make sense </li></ul><ul><li>Don’t fault tree everything </li></ul><ul><li>Careful with Boolean expressions </li></ul><ul><li>Independent Vs dependent failure modes </li></ul><ul><li>Ensure top event is high priority </li></ul>
15. 15. FMECA, Human Factors, and Software Safety
16. 16. Non-Safety Tools <ul><li>Failure Modes, Effects, and Criticality Analysis </li></ul><ul><li>Human Factors Analysis </li></ul><ul><li>Software Safety Analysis </li></ul>
17. 17. FMEA <ul><li>Reliability engineering tool </li></ul><ul><li>Originated in 1960s </li></ul><ul><li>OSHA recognized </li></ul><ul><li>Limitation - failure does not have to occur for a hazard to be present in system </li></ul><ul><li>Used to investigate how a particular failure can come about </li></ul>
18. 18. FMEA Process <ul><li>Define system & analysis scope </li></ul><ul><li>Construct block diagrams </li></ul><ul><li>Assess each block for effect on system </li></ul><ul><li>List ways that components can fail </li></ul><ul><li>Assess failure effects for each failure mode </li></ul><ul><li>Identify single point failures </li></ul><ul><li>Determine corrective actions </li></ul><ul><li>Document results on worksheet </li></ul>
19. 19. System Breakdown Subsystem 1 Subsystem 2 Subsystem 3 Assembly 1 a Assembly 1 b Assembly 1 c Subassembly 1c.1 Subassembly 1c.2 Subassembly 1c.3 Component 1c.3.1 Component 1c.3.2 Component 1c.3.3 Part 1c.3.3.a Part 1c.3.3.b Part 1c.3.3.c Total System
20. 20. FMEA Worksheet <ul><li>Component #, name, function </li></ul><ul><li>Failure modes </li></ul><ul><li>Mission phase </li></ul><ul><li>Failure effects locally </li></ul><ul><li>Failure propagation to the next level </li></ul><ul><li>Single point failure </li></ul><ul><li>Risk failure class </li></ul><ul><li>Controls, recommendations </li></ul>
21. 21. Failure Modes <ul><li>Premature operation </li></ul><ul><li>Failure to operate on time </li></ul><ul><li>Intermittent operation </li></ul><ul><li>Failure to cease operation on time </li></ul><ul><li>Loss of output or failure during operation </li></ul><ul><li>Degraded output or operational capability </li></ul><ul><li>Unique failure conditions </li></ul>
22. 22. Failure Modes, Effects, & Criticality Analysis <ul><li>Virtually same as FMEA </li></ul><ul><li>Identifies criticality of components </li></ul><ul><li>Emphasizes probability of failure </li></ul><ul><li>Criticality components </li></ul><ul><ul><li>Failure effect probability </li></ul></ul><ul><ul><li>Failure mode ratio </li></ul></ul><ul><ul><li>Part failure rate </li></ul></ul><ul><ul><li>Operating time </li></ul></ul>
23. 23. Human Factors Safety Analysis <ul><li>Many different techniques </li></ul><ul><li>Human element must be considered in engineering design </li></ul><ul><li>The merging of three fields: </li></ul><ul><ul><li>Human factors </li></ul></ul><ul><ul><li>Ergonomics </li></ul></ul><ul><ul><li>Human reliability </li></ul></ul>
24. 24. Performance & Human Error <ul><li>Why do people make mistakes? </li></ul><ul><li>Combination of causes - internal/external </li></ul><ul><li>Performance shaping factors (factors that influence how people act) </li></ul><ul><ul><li>External PSF </li></ul></ul><ul><ul><li>Internal PSF </li></ul></ul><ul><ul><li>Stressor PSF </li></ul></ul>
25. 25. Human Error <ul><li>Out of tolerance action within human/machine system </li></ul><ul><li>Mismatch of task and person </li></ul><ul><li>Significant contributor to many accidents </li></ul><ul><li>False assumptions </li></ul><ul><ul><li>Human error is inevitable </li></ul></ul><ul><ul><li>People are careless </li></ul></ul><ul><li>More complex systems must be less dependent on how well people operate them </li></ul>
26. 26. Human Error Categories <ul><li>Omission - leaving out a task </li></ul><ul><li>Commission </li></ul><ul><ul><li>Selection error </li></ul></ul><ul><ul><li>Error of sequence </li></ul></ul><ul><ul><li>Time error </li></ul></ul><ul><ul><li>Qualitative error </li></ul></ul>
27. 27. HF Safety Analysis The Process <ul><li>Describe system goals and functions </li></ul><ul><li>List & analyze related human operations </li></ul><ul><li>Analyze human errors </li></ul><ul><li>Screen errors & select </li></ul><ul><li>Quantify errors & affect on system </li></ul><ul><li>Recommend changes to reduce impact of human error </li></ul>
28. 28. Software Safety <ul><li>Newest member of system safety field </li></ul><ul><li>Software controls millions of systems </li></ul><ul><li>Treat software like any system component </li></ul><ul><ul><li>Determine the hazards </li></ul></ul><ul><ul><li>If software is involved in hazard - deal with it </li></ul></ul><ul><li>Common tools </li></ul><ul><ul><li>Software Hazard Analysis </li></ul></ul><ul><ul><li>Software Fault Tree Analysis </li></ul></ul><ul><ul><li>Software Failure Modes & Effects </li></ul></ul>
29. 29. Software Facts <ul><li>Software is not a hazard </li></ul><ul><li>Software doesn’t fail </li></ul><ul><li>Health monitoring of software only assures it performs as intended </li></ul><ul><li>Every line of code cannot be reviewed </li></ul><ul><li>Fault tolerant is not the same as safe </li></ul><ul><li>Shutting down a computer may aggravate a an already dangerous situation </li></ul>
30. 30. Software Safety Analysis (SSA) Flow Process <ul><li>Software Requirements Development </li></ul><ul><li>Top-level System Hazards Analysis </li></ul><ul><li>Detailed Design Hazard Analysis </li></ul><ul><li>Code Hazard Analysis </li></ul><ul><li>Software Safety Testing </li></ul><ul><li>Software User Interface Analysis </li></ul><ul><li>Software Change Analysis </li></ul>
31. 31. SSA <ul><li>Required when software is used to: </li></ul><ul><li>Identify a hazard </li></ul><ul><li>Control a hazard </li></ul><ul><li>Verify a control is in place </li></ul><ul><li>Provide safety-critical information or safety related system status </li></ul><ul><li>Recovery from a hazardous condition </li></ul>
32. 32. Safety Tool Categories <ul><li>Software safety requirements analysis </li></ul><ul><ul><li>Flowdown analysis </li></ul></ul><ul><ul><li>Criticality analysis </li></ul></ul><ul><li>Architectural design analysis </li></ul><ul><li>Detailed design analysis </li></ul><ul><ul><li>Soft tree analysis </li></ul></ul><ul><ul><li>Petri-Net </li></ul></ul><ul><li>Code analysis </li></ul>
33. 33. Software Testing <ul><li>Software testing </li></ul><ul><li>System safety testing </li></ul><ul><li>Software changes </li></ul><ul><li>IV &V organization </li></ul>
34. 34. Other Techniques
35. 35. MORT <ul><li>Qualitative tool used in 1970s </li></ul><ul><li>Merges safety mgt & safety engineering </li></ul><ul><li>Analyses mgt policy in relation to RA and hazard analysis process </li></ul><ul><li>Uses a predefined graphical tree </li></ul><ul><li>Analyze from top event down </li></ul><ul><li>Too large and doesn’t tailor well to smaller problem </li></ul>
36. 36. Energy Trace Barrier Analysis (ETBA) <ul><li>Qualitative tool for hazard analysis </li></ul><ul><li>Developed as part of MORT </li></ul><ul><li>Traces energy flow into, through, & out of system </li></ul><ul><li>Four typical energy sources </li></ul><ul><li>Energy transfer points & barriers analyzed </li></ul><ul><li>Advantages </li></ul>
37. 37. ETBA Procedure <ul><li>Examine system / identify energy sources </li></ul><ul><li>Trace each energy source through system </li></ul><ul><li>Identify vulnerable targets to energy </li></ul><ul><li>Identify all barriers in energy path </li></ul><ul><li>Determine if controls are adequate </li></ul>
38. 38. Sneak Circuit Analysis <ul><li>Standardized by Boeing in 1967 </li></ul><ul><li>Formal analysis of all paths that a process could take </li></ul><ul><li>Find sneak paths, timing, or procedures that could yield an undesired effect </li></ul><ul><li>Review engineer drawings, translate, & identify patterns </li></ul><ul><li>Disadvantages </li></ul>
39. 39. Cause-Consequence Analysis <ul><li>Uses symbolic logic trees </li></ul><ul><li>Determine accident or failure scenario that challenges the system </li></ul><ul><li>Develop a bottom-up analysis </li></ul><ul><li>Failure probabilities calculated </li></ul><ul><li>Consequences identified from top event </li></ul><ul><li>Consequence may have variety of outcomes </li></ul>
40. 40. Dispersion Modeling <ul><li>Quantitative tool for environmental and system safety engineering </li></ul><ul><li>Used in chemical process plants, can determine seriousness of chemical release </li></ul><ul><li>Internationally recognized model - CAMEO </li></ul><ul><li>Features of the system </li></ul><ul><li>Advantages </li></ul>
41. 41. Test Safety <ul><li>Not an analysis technique </li></ul><ul><li>Assures safe environment during testing </li></ul><ul><li>Must integrate system safety process into test process </li></ul><ul><li>Three layers of test environment </li></ul><ul><li>Safety analysis needed at each level </li></ul><ul><li>Test readiness review </li></ul>
42. 42. Comparing Techniques <ul><li>Complex Vs simple </li></ul><ul><li>Apply to different phases of system life cycle </li></ul><ul><li>Quantitative Vs qualitative </li></ul><ul><li>Expense </li></ul><ul><li>Time and personnel requirements </li></ul><ul><li>Some are more accepted in certain industries </li></ul>
43. 43. Selecting A Technique <ul><li>All techniques are good analyses </li></ul><ul><li>Consider advantages and disadvantages </li></ul><ul><li>Select technique most suited to the problem, industry, or desired outcome </li></ul><ul><li>Ask yourself a few questions </li></ul><ul><ul><li>What’s the purpose? </li></ul></ul><ul><ul><li>What is the desired result? </li></ul></ul><ul><ul><li>Does it fit your company and achieve goals? </li></ul></ul><ul><ul><li>What are your resources and time available? </li></ul></ul>
44. 44. Data Sources and Training
45. 45. Data Reliability <ul><li>Start with company historical data </li></ul><ul><li>Analyses only as good as the data that is used </li></ul><ul><li>Caution about misunderstanding data </li></ul><ul><li>Quantifiable data is not always the best </li></ul><ul><li>Always cite sources and assumptions </li></ul>
46. 46. Data Limits <ul><li>Most failure data is generic </li></ul><ul><li>Break large items into smaller parts </li></ul><ul><li>Data may not consider environmental changes </li></ul><ul><li>Use expert judgement to convert generic data into realistic values </li></ul>
47. 47. Government Data Banks <ul><li>Government Printing Office </li></ul><ul><ul><li>Books from DoD, NASA, EPA, & OSHA </li></ul></ul><ul><li>Government-Industry Data Exchange Program </li></ul><ul><ul><li>Army, Navy, FAA, Dept of Labor, Dept of Energy, National institute of Standards and Technology </li></ul></ul><ul><li>Databases of other countries </li></ul>
48. 48. Industry Data Banks <ul><li>Corporations </li></ul><ul><li>Insurance companies </li></ul><ul><li>Electronics Industries Associations </li></ul><ul><li>Consumer Product Commission </li></ul><ul><li>System Safety Society </li></ul><ul><li>Material Safety Data Sheets </li></ul>
49. 49. Creating Your Own Databank <ul><li>Collect data on system </li></ul><ul><ul><li>Design </li></ul></ul><ul><ul><li>Assessments </li></ul></ul><ul><ul><li>Hazard identification </li></ul></ul><ul><ul><li>Compliance verification </li></ul></ul><ul><li>Make the data easily accessible and consolidated in one place </li></ul><ul><li>Computers and new software make collection easier </li></ul>
50. 50. Data Bank <ul><li>Hazardous materials </li></ul><ul><li>MSDS </li></ul><ul><li>System design info </li></ul><ul><li>Safety critical systems </li></ul><ul><li>Best design practices </li></ul><ul><li>Testing history </li></ul><ul><li>Failure history </li></ul><ul><li>Safety analyses </li></ul><ul><li>Accident histories </li></ul><ul><li>Safety Standards </li></ul><ul><li>Identified hazards </li></ul><ul><li>Causes of hazards </li></ul><ul><li>Proven hazard controls </li></ul><ul><li>Hazard consequences </li></ul><ul><li>Hazard tracking system </li></ul>Systems Info System Safety Data
51. 51. Safety Training <ul><li>Twofold approach </li></ul><ul><ul><li>Employee training </li></ul></ul><ul><ul><li>Emergency response </li></ul></ul><ul><li>Types of training </li></ul><ul><ul><li>Initial training </li></ul></ul><ul><ul><li>Refresher training </li></ul></ul><ul><ul><li>New training for changes </li></ul></ul>
52. 52. Employee Training <ul><li>Training needs assessment </li></ul><ul><li>Purpose of training </li></ul><ul><li>Assess current operations </li></ul><ul><li>Review hazard analysis data </li></ul><ul><li>Develop and implement training </li></ul><ul><li>Record training </li></ul>
53. 53. Emergency Preparedness and Response Training <ul><li>Train all personnel affected by possible emergency </li></ul><ul><li>Training subjects </li></ul><ul><ul><li>Evacuation procedures </li></ul></ul><ul><ul><li>Shutdown of equipment </li></ul></ul><ul><ul><li>Firefighting and first aid </li></ul></ul><ul><ul><li>Crowd control and panic prevention </li></ul></ul><ul><li>Conduct exercises </li></ul>
54. 54. Certification for Hazardous Operations <ul><li>Determine personnel that require training </li></ul><ul><li>Certification program elements </li></ul><ul><ul><li>Certification examination </li></ul></ul><ul><ul><li>Physical examination </li></ul></ul><ul><ul><li>Classroom and hands-on training </li></ul></ul><ul><ul><li>Test of safe working practices </li></ul></ul><ul><ul><li>Recertification schedule </li></ul></ul>
55. 55. Safety Awareness <ul><li>Highlight safety in organization </li></ul><ul><li>Positive incentives </li></ul><ul><li>Establish safety representatives in each area </li></ul><ul><li>Conduct meetings to discuss safety program </li></ul><ul><li>Safety reps should be trained in workplace safety inspections and program monitoring </li></ul>
56. 56. Accident Reporting, Investigation, and Documentation
57. 57. Reporting the Accident <ul><li>Accident reporting without retribution </li></ul><ul><li>Posting of reportable accidents </li></ul><ul><li>New-employee briefing </li></ul><ul><li>Management involvement </li></ul>
58. 58. Setting Up a Closed-Loop Reporting System <ul><li>Pre-accident plan </li></ul><ul><li>Report within 24 hours </li></ul><ul><ul><li>Pass data up the chain </li></ul></ul><ul><ul><li>Initiate board </li></ul></ul><ul><ul><li>Capture perishable information </li></ul></ul><ul><li>Investigate all accidents </li></ul>
59. 59. Forming a Board <ul><li>Company policy </li></ul><ul><ul><li>Accident classification </li></ul></ul><ul><ul><li>Standing list of board candidates </li></ul></ul><ul><li>Selecting the Board members </li></ul><ul><ul><li>Various backgrounds </li></ul></ul><ul><ul><li>Voting members and advisors </li></ul></ul><ul><li>Board responsibilities </li></ul>
60. 60. Conducting the Investigation <ul><li>Preparing for investigation </li></ul><ul><li>Gathering evidence and information </li></ul><ul><li>Analyzing the data </li></ul><ul><li>Discussion of analysis and conclusions </li></ul><ul><li>Recommendations </li></ul>
61. 61. Investigation Report <ul><li>Abstract of report </li></ul><ul><li>Summary of F & R </li></ul><ul><li>Procedure used </li></ul><ul><li>Background </li></ul><ul><li>Sequence of events </li></ul><ul><li>Analysis methodology </li></ul><ul><li>Analysis results </li></ul><ul><li>Conclusions </li></ul><ul><li>Detailed F & R </li></ul><ul><li>Minority reports </li></ul><ul><li>Appendixes </li></ul>
62. 62. Accident Documentation <ul><li>Investigation Report </li></ul><ul><ul><li>Retained with supporting documents </li></ul></ul><ul><ul><li>Corrective action implemented </li></ul></ul><ul><ul><li>Available for future safety analysis </li></ul></ul><ul><li>Retain the records </li></ul><ul><li>Public release of information </li></ul>
63. 63. Risk Assessment
64. 64. What is Risk? <ul><li>Severity of consequences of an accident times the probability of occurrence </li></ul><ul><li>Risk perception may vary from actual risk </li></ul><ul><li>Risk: realization of unwanted, negative consequences of an event (Rowe) </li></ul><ul><li>Risk: summation of three elements </li></ul><ul><ul><li>Event scenario </li></ul></ul><ul><ul><li>Probability of occurrence </li></ul></ul><ul><ul><li>Consequence </li></ul></ul>
65. 65. Risk Perception <ul><li>Factors concerning perception of risk </li></ul><ul><li>Voluntary Vs nonvoluntary </li></ul><ul><li>Chronic Vs catastrophic </li></ul><ul><li>Dreaded Vs common </li></ul><ul><li>Fatal Vs nonfatal </li></ul><ul><li>Known Vs unknown risk </li></ul><ul><li>Immediate or delayed danger </li></ul><ul><li>Control over technology </li></ul>
66. 66. Risk Assessment Methodology <ul><li>1 Define objectives </li></ul><ul><li>2 Define system </li></ul><ul><li>3 Develop scenarios </li></ul><ul><li>4 Develop event trees </li></ul><ul><li>5 Quantify scenarios </li></ul><ul><li>6 Consequences </li></ul><ul><li>7 Risk evaluation </li></ul><ul><li>8 Risk management </li></ul>Formal process of calculating risk and making a decision on how to react
67. 67. Risk Assessment Methodology Define Objectives Define System Develop Event Trees Develop Scenarios Quantify Scenarios Consequences Determination Risk Management Risk Evaluation Step 1 Step 2 Step 3 Step 4 Step 5 Step 6 Step 7 Step 8
68. 68. Identifying Risk in a System <ul><li>Risk identified through analysis techniques </li></ul><ul><li>Use several techniques </li></ul><ul><li>Construct fault tree </li></ul><ul><li>Use analysis tools to focus on which component is the trigger </li></ul>
69. 69. Risk Communication <ul><li>Communicating with public </li></ul><ul><ul><li>Acknowledge the community </li></ul></ul><ul><ul><li>Do not imply irrationality or ignorance </li></ul></ul><ul><li>Methods to promote communication </li></ul><ul><ul><li>Community participation </li></ul></ul><ul><ul><li>Approach “group” appropriately </li></ul></ul><ul><ul><li>Consultation with community </li></ul></ul><ul><ul><li>Involve community in negotiations </li></ul></ul><ul><ul><li>Be open with information </li></ul></ul>
70. 70. Risk Evaluation
71. 71. A Probabilistic Approach <ul><li>Quantifying risk through probability of failure </li></ul><ul><li>Hard to quantify probability of some events </li></ul><ul><li>Understand the data, the sources, & the limitations </li></ul><ul><li>Follow rules of probability </li></ul>
72. 72. Risk Analysis Model <ul><li>Developing accident scenarios & initiating event </li></ul><ul><li>Event Trees </li></ul><ul><li>Consequences determination </li></ul><ul><li>Uncertainty </li></ul><ul><li>Risk evaluation - Risk profiles </li></ul>
73. 73. Calculating Safety Costs <ul><li>Tracking data costs </li></ul><ul><ul><li>System downtime (lost productivity) </li></ul></ul><ul><ul><li>Equipment damage and replacement </li></ul></ul><ul><ul><li>Accident clean-up </li></ul></ul><ul><ul><li>Personnel injuries and death </li></ul></ul><ul><li>Expected value </li></ul><ul><li>Cost-benefit analysis </li></ul>