Fault tree analysis decomposes a top event into logical combinations of individual and combined failures. The structure shows where redundant protections hold and where a small gap becomes critical.

1. 1

   Top event

   Defined top event

   

   Goal

   Defined top event: State the undesired event precisely and without a solution, including system state and boundary.

   Inputs for this step

   • Define the top event as an observable failure with system boundary, operating state, and time horizon.
   • Precise top event, system boundary and operating state, architecture and failure data, AND/OR gate notation, evidence register

   Actions

   • State the undesired event precisely and without a solution, including system state and boundary.

   Output

   Defined top event

   State the undesired event precisely and without a solution, including system state and boundary.

   Quality check

   Is the top event binary-testable and bound to an operating state?

   Pitfall

   Bad pickup system is neither binary nor bounded. Add failure state, system, period, and success criterion.

   Timing: 30–45 minutes

2. 2

   Gates

   First logic level

   

   Goal

   First logic level: Arrange immediate causes beneath the top event and choose AND or OR from actual logic.

   Inputs for this step

   • Defined top event

   Actions

   • Arrange immediate causes beneath the top event and choose AND or OR from actual logic.

   Output

   First logic level

   Arrange immediate causes beneath the top event and choose AND or OR from actual logic.

   Quality check

   Does OR mean one input suffices and AND mean all inputs are jointly required?

   Pitfall

   AND is chosen because factors belong together although either alone suffices. Write a truth sentence for every gate and test counterexamples.

   Timing: 45–75 minutes

3. 3

   Decompose

   Evidence-backed tree

   

   Goal

   Evidence-backed tree: Expand intermediate events to controllable basic events or justified development boundaries.

   Inputs for this step

   • First logic level

   Actions

   • Expand intermediate events to controllable basic events or justified development boundaries.

   Output

   Evidence-backed tree

   Expand intermediate events to controllable basic events or justified development boundaries.

   Quality check

   Are basic events concrete, independently understandable, and labelled by evidence or assumption?

   Pitfall

   A branch ends at human error without an observable state. Phrase a concrete state and state its control and evidence boundary.

   Timing: 2–6 hours

4. 4

   Cut sets

   Minimal cut sets

   

   Goal

   Minimal cut sets: Derive minimal event combinations whose occurrence logically produces the top event; identify shared events.

   Inputs for this step

   • Evidence-backed tree

   Actions

   • Derive minimal event combinations whose occurrence logically produces the top event; identify shared events.

   Output

   Minimal cut sets

   Derive minimal event combinations whose occurrence logically produces the top event; identify shared events.

   Quality check

   Are cut sets minimal and derivable from documented gate logic?

   Pitfall

   Frequent events are called critical paths without logical derivation. Evaluate Boolean paths and remove every non-minimal superset.

   Timing: 45–90 minutes

5. 5

   Controls

   Control plan

   

   Goal

   Control plan: Prioritise controls against single events and combinations; test effect and remaining paths.

   Inputs for this step

   • Minimal cut sets

   Actions

   • Prioritise controls against single events and combinations; test effect and remaining paths.

   Output

   Control plan

   Prioritise controls against single events and combinations; test effect and remaining paths.

   Quality check

   Does each priority control break a relevant cut set while residual logic stays visible?

   Pitfall

   Only the most frequent event is treated while a single-event path remains open. Check controls against every cut set and record remaining single and combined paths.

   Timing: 45–75 minutes