Safety, Risk, and Failure Analysis
Risk concepts
Risk combines severity and likelihood. Know the hierarchy: identify hazards, assess risk, reduce risk, and verify. Risk reduction follows a preferred order: design out the hazard, add safeguards, use warnings and procedures, then personal protective equipment. Traps: accepting warnings as equivalent to design elimination, and treating detection controls as prevention.
FMEA and FMECA
FMEA examines failure modes, effects, and causes. RPN = severity x occurrence x detection; higher RPN means higher priority. FMECA adds criticality analysis. Candidates should know that RPN has limitations (ordinal scales, ties) and that severity of 10 should trigger action regardless of RPN. Watch for questions that treat detection as reducing severity; it does not.
Fault tree and event analysis
Fault tree analysis is deductive (top-down) and uses AND/OR gates; FMEA is inductive (bottom-up). Minimal cut sets identify combinations causing the top event. Event tree analysis is inductive and forward-looking, used for consequence modeling.
Safety standards and integrity
Know concepts of safety integrity levels (SIL), layers of protection, and common-cause failure. Redundancy without diversity may not reduce risk if common causes dominate.
Worked example
A failure mode has S = 9, O = 4, D = 3: RPN = 108. Reducing occurrence to 2 by design change gives RPN = 54. Reducing detection to 1 instead gives 36 but leaves the same severity and occurrence; the better fix addresses occurrence, not inspection.