Analysis methods
FTA · IEC 61025
A top-down analysis of the combinations of failures that can lead to an undesired event.
Fault tree analysis (FTA)
- Category
- Analysis methods
- Scope
- International
- Edition / refs
- IEC 61025:2006
MSA output
- FTA · FMEA · Bowtie analysis
- SIL verification
- Full traceability
What is FTA · IEC 61025?
FTA is a top-down (deductive) method. Starting from an undesired top event — for example, hazardous motion continuing while a guard is open — it uses logic gates such as AND and OR to model the combinations of lower-level events and basic failures that can cause it. IEC 61025:2006 defines the principles, symbols and procedures for qualitative and quantitative FTA.
Qualitative analysis identifies minimal cut sets: the smallest combinations of basic events that cause the top event, exposing single points of failure and common cause dependencies. Quantitative analysis combines basic event probabilities or failure rates to estimate the probability or frequency of the top event. A bowtie diagram, widely used in process safety, places a fault tree of causes and an event tree of consequences either side of a single hazardous event.
Who it applies to
- Safety and reliability engineers
- Functional safety engineers verifying non-standard architectures
- Process safety teams using bowtie analysis
- Machine builders with complex safety functions
Key requirements
- 01
Define the top event and boundaries
A precise undesired event, system boundary, operating mode and assumptions.
- 02
Construct the tree
Develop intermediate events down to basic events using standard gates and symbols.
- 03
Find minimal cut sets
Identify the combinations that lead to the top event, and single points of failure.
- 04
Quantify
Apply failure data to estimate top event probability or frequency where required.
- 05
Address dependencies
Model common cause failures and shared dependencies explicitly.
- 06
Document and review
Record data sources and assumptions, and review the tree with the design team.
Summary for orientation only. Always refer to the current official text of FTA · IEC 61025 and any applicable legislation.
MSA for FTA · IEC 61025
What MSA generates
Evidence produced from your engineering inputs, traceable end to end and exported to Word, PDF or Excel.
FTA · FMEA · Bowtie analysis
Fault trees and bowtie views created alongside your other safety analyses.
SIL verification
Analysis results feeding the verification of safety functions.
Full traceability
Each tree linked to the hazards and functions it analyses, updated when inputs change.
Frequently asked questions
When should I use FTA instead of a simple PL or SIL calculation?
When the architecture does not match standard designated architectures, when several failures must combine to cause the hazard, or when common cause and shared dependencies need to be made explicit.
What is a minimal cut set?
The smallest set of basic events that, if they all occur, cause the top event. A minimal cut set with a single event is a single point of failure.
How does a bowtie relate to a fault tree?
The left side of a bowtie is essentially a simplified fault tree of threats and preventive barriers; the right side shows consequences and mitigating barriers.
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