Learning objective
Read AND/OR branches as different causal paths to a defined top event.
Why it matters
A team lists hazards but cannot see which common causes or combinations defeat several safeguards at once.
Core concept
Define one top event, decompose necessary and sufficient causal paths with AND and OR gates, identify shared causes and minimum combinations, then verify against system evidence. Probabilities require justified data and independence assumptions.
Scenario
An orbital habitat risks losing breathable air. Two nominally independent scrubbers share a power bus, while manual fallback depends on the same alarm.
Worked decision
Map air-loss as the top event, expose the shared bus and alarm as common causes, then test independent power and detection paths.
Decision rule
Choose controls and validate the tree against incident and test evidence.
Ethical boundary
Drawing only component failures while ignoring shared power, human response and unknown modes; inventing precise probabilities.
AI lens
Use AI to organize evidence, then verify its source and keep decision authority with people.
Quick check
What would invalidate the first proposed action? Drawing only component failures while ignoring shared power, human response and unknown modes; inventing precise probabilities.
Next step
Complete the Fault Tree Analysis case.