Overview
IEC 61025:2006 - Fault tree analysis (FTA) is the International Electrotechnical Commission standard that defines the principles, symbols and procedures for performing Fault Tree Analysis. The second edition (2006) explains both qualitative and quantitative FTA approaches, provides mathematical modelling guidance, and specifies rules for identifying assumptions, events, failure modes, and labelling. The standard is published by IEC TC 56 (Dependability) and includes informative annexes with commonly used symbols and a detailed disjointing procedure.
Key topics
- Scope and objectives: Definition of the top event, scope of analysis, and objectives for safety, reliability, availability or maintainability studies.
- Terms and definitions: Clear definitions for top event, final event, basic/primary events, intermediate events, cut sets and minimal cut sets.
- Graphical structure and symbols: Standard gate and event symbols, gate types (AND, OR, PAND, etc.), and recommended labelling practices (see Annex A).
- Qualitative FTA: Traditional FTA techniques for identifying causal relationships without assigning probabilities - used to find fault combinations and minimal cut sets.
- Quantitative FTA: Methods to assign probabilities or failure rates to basic events and compute the probability of the top event; includes guidance on rare-event approximations and disjointing (Annex B).
- Development and evaluation: Stepwise procedures for system familiarization, fault tree construction, assumptions, failure-rate use, and report content.
- Relationships to other methods: Guidance on combining FTA with other dependability techniques (e.g., Markov analysis - IEC 61165) and when to use mixed approaches.
Applications
IEC 61025 is applicable to:
- Safety analysis of transportation systems, power plants, nuclear facilities and industrial plants.
- Reliability and availability studies for product development, operation and maintenance planning.
- Risk assessment and root-cause analysis where structured identification of event combinations is required.
Fault Tree Analysis under this standard supports both forensic (why did a failure occur?) and predictive (what is the probability of a system-level failure?) use cases.
Who should use this standard
- Reliability, safety and systems engineers
- Risk analysts and certification bodies
- Product designers and maintenance planners
- Regulators and auditors involved in dependability and safety assurance
Related standards
- IEC 60050(191) - IEV chapter on dependability terms
- IEC 61165 - Application of Markov techniques (referenced for combined modelling)
IEC 61025 provides a standardized, practical framework for documenting, analysing and quantifying fault combinations and system-level failure modes using Fault Tree Analysis (FTA).