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Fault Tree Analysis Tutorial

Fault tree analysis (FTA) is a top down, in which a harmful part of a system is evaluated by applying Boolean logic to incorporate a sequence of lower-level scenarios. Such methodology is primarily applied in safety and reliability engineering to comprehend how systems can fail, to determine the best practices to mitigate risk or to identify the rates of a safety incident or a particular system level failure.

Some Background about Fault Tree Analysis (FTA)

FTA is a technique used in reliability and safety analysis. The notion of FTA was initiated by Bell Telephone Laboratories in 1962 for the US Air Force to use with the Minuteman system. Later it was adopted and broadly used by the Boeing Company. FTA is one of the logical techniques used in operations research and in system reliability. Other techniques include reliability block diagrams (RBDs).

Fault Tree Diagram (FTD)

FTDs are analytical block diagrams that demonstrate the phase of a system (top event) in regards to the phases of its elements (basic events). Similar to RBDs, FTDs are a graphical design approach, and thus offer a substitute to RBDs.

An FTD is constructed top-down and based on events instead of blocks. It applies a graphic “model” of the connections inside a system that can result in a predictable, harmful damage (or a failure). The routes link causal events and conditions, using standard logic symbols (AND, OR, etc.). The fundamental theories in a fault tree diagram are gates and events, in which the events have similar concept as a block in an RBD and the gates are the settings.

Fault Trees and Reliability Block Diagrams

The most essential difference between fault tree diagrams and RBDs is in an RBD one would work in the “success space”, whereas in a fault tree one would work in the “failure space”. In fact, the RBD considers success combinations, while the fault tree considers failure combinations. Plus, fault trees have conventionally been applied to assess fixed likelihoods, whereas RBDs can contain time-dependent distributions for the blocks’ success or failure, as well as for other characteristics such as resolution/restoration distributions.

Fault Tree Analysis Application

  • Comprehending the rational caused the top event/harmful state
  • Demonstrates compliance with the (input) system safety/ reliability specifications
  • Giving priority to the contributors caused the top event – Creating the Critical Equipment/Parts/Events lists for various importance measures
  • Monitor and control the safety performance of the complex system
  • Reducing and optimizing resources.
  • Helping in designing a system. The FTA can be applied as a design tool that helps to create (output / lower level) requirements
  • Operating as a diagnostic tool to determine and correct causes of the top event. It can assist with the generation of diagnostic manuals/processes.

Drawing Fault Trees: Gates and Events

Fault trees are made by gates and events (blocks). The two most employed gates in a fault tree are the AND and OR gates. The fault tree diagram for the system contains two input events connected to an OR gate which is the output event or the “top event”. If the top event is system failure and the two input events are component failures, then this fault tree specifies that the failure of A or B lead the system to fail.

Node Types

  • Basic events
  • Gate events
  • Condition events
  • Transfer events

Event Symbols

Event symbols are applied for primary and intermediate events. Primary events are not more developed on the fault tree. Intermediate events are found at the output of a gate. The event symbols are shown below:

Fault Tree Analysis

Basic event

Fault Tree Analysis

External event

Fault Tree Analysis

Undeveloped event

Fault Tree Analysis

Conditioning event

Fault Tree Analysis

Intermediate event

Basic event – failure or error in a system component or element

External event – usually planned to happen

Undeveloped event – an event about which inadequate information exists, or which is of no consequence

Conditioning event – conditions that limit or impact logic gates

Gate Symbols

Fault Tree Analysis

OR gate

Fault Tree Analysis

AND gate

Fault Tree Analysis

Exclusive OR gate

Fault Tree Analysis

Priority AND gate

Fault Tree Analysis

Inhibit gate

Basic Gates

Gates are the rational signs that link contributory events and conditions in a fault tree diagram. The AND and OR gates explained above, as well as a Voting OR gate, where the output event happens if a given number of the input events are the most fundamental forms of gates in standard fault tree analysis.

A fault tree diagram is always drawn in a top-down style with lowest item being a fundamental event block. Standard fault tree gates have no characteristics.

AND Gates

In an AND gate, the output event happens if all input events happen. In the system reliability language, this indicates that all elements must fail (input) in order for the system to fail (output). When applying RBDs, the equivalent is a simple parallel structure.

OR Gate

In OR gate, the output event happens if either of the input events happens. In regards to system reliability, this indicates that if any element fails (input), then the system will fail (output).

Relationships Between Fault Trees and RBDs

While the signs and frameworks of the two diagrams vary, most of the rational structures in a fault tree diagram can also be simulated with an RBD. Overall, a fault tree can be simply transformed to an RBD. It is usually more complicated to change an RBD into a fault tree, particularly if one provides highly complex configurations.


The gates in a fault tree are the analytical signs that link causal events and conditions. An event (or a circumstance) block in a fault tree is similar to a standard block in an RBD, in which it can have the possibilities of occurrence (or a distribution function). But, in contrast to conventional RBDs, in which a single graphical representation is used to characterize the block (or event), fault trees apply several graphical block representations.

FTA Procedures:

  • Defining the failure or event
  • Comprehending the system
  • Building the fault tree
  • Analyzing the fault tree
  • Controlling the hazards determined


  • Evaluating complex systems
  • Identifying events that can lead to failures or accidents
  • Studying safety, reliability, unavailability, accidents


  • Graphical
  • Demonstrating cause-effect relationships
  • Faulty events, normal events, pathways
  • Probability


  • Failures
  • Fault events
  • Normal events
  • Environmental effects
  • Systems, subsystems, components
  • System elements
  • Repair

FT Strengths

  • Visual model
  • Easy to follow
  • Probability model
  • Scientifically sound
  • Existing commercial software
  • FTs provide value even under incomplete information
  • Confirmed techniques

How Can You Learn More?

In order to know more about Fault Tree Analysis training, please click HERE

Fault Tree Analysis Tutorial

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