Purpose
For a critical top event with several possible triggers, the method logically models failure paths. It makes visible which combinations of conditions can lead to damage.
Operations Mapping tool
Fault Tree Analysis
Fault tree analysis models a top event with AND and OR branches to make critical paths and causes visible.
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.
Which individual or combined basic events are logically sufficient to produce the defined top event?
- Participants
- 3-8
- Time
- One to three working days for a bounded tree; add expert and evidence review.
Evidence building, analysis, and effectiveness review form a multi-stage initiative.
Top event
Defined top event
State the undesired event precisely and without a solution, including system state and boundary.
Gates
First logic level
Arrange immediate causes beneath the top event and choose AND or OR from actual logic.
Decompose
Evidence-backed tree
Expand intermediate events to controllable basic events or justified development boundaries.
Cut sets
Minimal cut sets
Derive minimal event combinations whose occurrence logically produces the top event; identify shared events.
Controls
Control plan
Prioritise controls against single events and combinations; test effect and remaining paths.
Add new evidence at review and version decisions with the prior state.
A good fit when
Bei einem kritischen Top-Event mit mehreren möglichen Auslösern modelliert die Methode Versagenspfade logisch. Sie macht sichtbar, welche Kombinationen aus Bedingungen zu einem Schaden führen können.
Ideal for
- Technical disturbances
- Safety analyses
- Complex failure patterns
Not good for
- Unclear problem definitions
- Pure ideation
- Very simple cause chains
Outputs
Control plan
Reviewed fault tree with AND/OR logic, basic events, minimal cut sets, evidence, and prioritised controls.
Fault Tree Analysis · working structure
Fictional example
Fictional values show the actual method fields.
Defined top event
State the undesired event precisely and without a solution, including system state and boundary.
Top event: reserved pickup cannot be completed Saturday between 09:00 and 12:00.
First logic level
Arrange immediate causes beneath the top event and choose AND or OR from actual logic.
Top = OR [payment unavailable, unit not handoff-ready]. Handoff-ready failure = OR [not found, not released].
Evidence-backed tree
Expand intermediate events to controllable basic events or justified development boundaries.
Payment unavailable = AND [terminal offline, no manual fallback]. Terminal offline = OR [network outage, device fault].
Minimal cut sets
Derive minimal event combinations whose occurrence logically produces the top event; identify shared events.
Cut sets: {unit not found}; {not released}; {terminal offline + no fallback}.
Control plan
Prioritise controls against single events and combinations; test effect and remaining paths.
Fallback payment breaks the payment combination; scan requirement addresses not found; owner and test date recorded.
All values are fictional.
First step
Define the top event as an observable failure with system boundary, operating state, and time horizon.
Deep dive
How it works
Starting from the end event, logical links are broken down so that single and combined failures become testable. This shows where redundant safeguards really hold and where a small gap becomes critical. The structure supports risk assessment where gut feeling would be too coarse.
In detail
Fault Tree Analysis starts with a clearly described failure event and works backward to possible causes. Logical links show whether one cause is enough or whether several conditions must combine. That turns a vague disturbance into a testable causal tree. The tree helps identify especially critical paths and place controls more precisely.
Facilitation
Start with a precise top event and keep the logic between causes explicit. Work with people who know the system behavior, the process, and the failure patterns. Check every branch against evidence and avoid writing assumptions into the tree as if they were confirmed causes.
The method guide walks execution through 5 phases, timing, 5 pitfalls, and clear stop criteria.
Open method guideYour perspective
Does this method help with your work?
Your feedback helps shape the next improvements to methodatlas.
Sources and topics
Topics
Artifact templates
Fault Tree Analysis: WorksheetSet subject, evidence, roles, flow, and review.markdown
Fault Tree Analysis: WorksheetSet subject, evidence, roles, flow, and review.
markdownfault-tree-analysis-working-template.md
Set subject, evidence, roles, flow, and review.
Fault Tree Analysis: Worksheet
Question
To be confirmed
Desired outcome
To be confirmed
Scope
Whole initiative Time: One to three working days for a bounded tree; add expert and evidence review.
Complete this worksheet on paper or in your own document during the work.
Preparation for this scope
Method setup
- Materials: Precise top event, system boundary and operating state, architecture and failure data, AND/OR gate notation, evidence register
- Roles: FTA lead · system architecture · reliability/safety · component experts · control owners
- Advance information: Define the top event as an observable failure with system boundary, operating state, and time horizon.
- Overall time needed: One to three working days for a bounded tree; add expert and evidence review.
- Setup: Prepare a workspace with Top event · Gates · Decompose · Cut sets · Controls; separate example from live data.
Prepare for the work steps
Top event: Defined top event
- 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
Gates: First logic level
- Defined top event
Decompose: Evidence-backed tree
- First logic level
Cut sets: Minimal cut sets
- Evidence-backed tree
Controls: Control plan
- Minimal cut sets
Fault Tree Analysis · working structure
Defined top event
State the undesired event precisely and without a solution, including system state and boundary.
...
First logic level
Arrange immediate causes beneath the top event and choose AND or OR from actual logic.
...
Evidence-backed tree
Expand intermediate events to controllable basic events or justified development boundaries.
...
Minimal cut sets
Derive minimal event combinations whose occurrence logically produces the top event; identify shared events.
...
Control plan
Prioritise controls against single events and combinations; test effect and remaining paths.
...
The blank worksheet contains prompts only.
Reference · original teaching example
Top event: Defined top event
Expected artifact: Defined top event
State the undesired event precisely and without a solution, including system state and boundary.
Entry:
...
- Is the top event binary-testable and bound to an operating state?
Gates: First logic level
Expected artifact: First logic level
Arrange immediate causes beneath the top event and choose AND or OR from actual logic.
Entry:
...
- Does OR mean one input suffices and AND mean all inputs are jointly required?
Decompose: Evidence-backed tree
Expected artifact: Evidence-backed tree
Expand intermediate events to controllable basic events or justified development boundaries.
Entry:
...
- Are basic events concrete, independently understandable, and labelled by evidence or assumption?
Cut sets: Minimal cut sets
Expected artifact: Minimal cut sets
Derive minimal event combinations whose occurrence logically produces the top event; identify shared events.
Entry:
...
- Are cut sets minimal and derivable from documented gate logic?
Controls: Control plan
Expected artifact: Control plan
Prioritise controls against single events and combinations; test effect and remaining paths.
Entry:
...
- Does each priority control break a relevant cut set while residual logic stays visible?
Open questions and next steps
...
Method guide: https://methodatlas.meierhoff-systems.de/en/methods/fault-tree-analysis/run-sheet
Alternatives and neighbors
Fitting alternatives with decision guidance, followed by other related methods.
Operations
Fishbone Diagram
Instead of Fault Tree Analysis, choose Fishbone Diagram when you want to gather causes into clear categories and keep discussions structured.
Operations
5 Whys
Instead of Fault Tree Analysis, choose 5 Whys when you want to quickly open up several possible causes and get the team into an initial cause chain.
Operations
8D Problem Solving
Coordinates team, immediate actions, root cause analysis, and prevention across eight steps to a verified solution.
Operations
MORT Analysis
A structured accident tree separates management controls, barriers, and events so serious incidents become auditable.
Operations
Barrier Analysis
Hazards, targets, and failed barriers are mapped so controls can be strengthened at the point of exposure.
Operations
Family Tree Analysis
A recurring incident pattern links related cases across time so systemic causes become visible beyond one event.
Constraints and origin
HighWorkshop + async2-6 h
Profile
- Method form
- Mapping tool
- Participants
- 3-8
- Time
- 2-6 h
- Maturity
- Canonical
- Origin
- H. A. Watson and Bell Labs
Next up: Failure Mode and Effects Analysis