Skip to main content

Fault Tree Analysis(FTA)

Risk & Failure Analysis
mechanicalelectricalinstrumentationprocess

Definition

A top-down, deductive analysis technique that uses Boolean logic (AND/OR gates) to model the combinations of events that can lead to an undesired top event (typically a system failure or hazardous condition). FTA starts with the top event and systematically decomposes it into contributing causes at progressively lower levels until basic events (component failures, human errors, external events) are reached. The fault tree can be evaluated qualitatively (identifying minimal cut sets) or quantitatively (calculating top event probability).

Context

FTA complements FMEA — while FMEA is bottom-up (component failure to system effect), FTA is top-down (system failure to component causes). FTA is particularly useful for analyzing safety systems, identifying common-cause failures, and evaluating the adequacy of redundancy. It is required for SIL verification of safety instrumented systems.

Examples

  1. A fault tree for 'Loss of Cooling Water' identifies 4 minimal cut sets: dual pump failure (AND gate), single common-cause failure of power supply, control system common-mode failure, and cooling tower structural failure.
  2. Quantitative FTA calculates the top event probability for 'uncontrolled hydrocarbon release' at 2.3 x 10^-5 per year, which is within the facility's risk tolerance criteria.

Applicable Standards

  • IEC 61025:2006
  • NUREG-0492
  • ISO 31010:2019

Related Terms

Root Cause Analysis(RCA)

A systematic investigation methodology used to identify the fundamental underlying cause(s) of a failure, incident, or undesired outcome. RCA goes beyond the immediate or proximate cause to uncover the latent organizational, procedural, or design factors that created the conditions for the event. Common RCA techniques include 5-Why analysis, fishbone (Ishikawa) diagrams, fault tree analysis, and barrier analysis.

Failure Mode and Effects Analysis(FMEA)

A systematic, proactive methodology for evaluating a process, product, or design to identify all possible failure modes, determine their effects on system performance, rank them by risk priority, and recommend actions to reduce or eliminate the highest risks. FMEA examines each component's potential failure modes, their causes, effects at local/system/end levels, current controls, and risk priority numbers.

Reliability Block Diagram(RBD)

A graphical method for modeling system reliability by representing components as blocks connected in series, parallel, or complex configurations that reflect the functional relationship between components. Series connections mean all components must work for the system to function; parallel connections provide redundancy where only one of multiple components needs to work. RBDs enable calculation of system reliability from component-level reliability data.

Safety Integrity Level(SIL)

A discrete level (SIL 1 through SIL 4) for specifying the safety integrity requirements of safety instrumented functions (SIFs). Each SIL level corresponds to a target probability of failure on demand (PFD): SIL 1 = 0.1-0.01, SIL 2 = 0.01-0.001, SIL 3 = 0.001-0.0001, SIL 4 = 0.0001-0.00001. Higher SIL levels require more rigorous design, testing, and maintenance.

System Reliability

The reliability of a system composed of multiple interconnected components, calculated from the individual component reliabilities and their functional configuration (series, parallel, complex). System reliability accounts for the way component failures combine to cause system failure, considering dependencies, common-cause failures, and load-sharing effects that may not be captured by simple series-parallel models.

Where this appears in Reliatic

This concept is used in the FMEA & Risk module.