Fault Tree Analysis: the Best Tool for Equipment Failures

Author: Rob De La EspriellaBD3, CEO and Founder, BlueDragon IPS

Fault Tree Analysis (FTA): A Comprehensive Guide

This comprehensive blog will probe into the advantages of FTA, how it is performed, the symbols used, and best practices.

Origins: Fault Tree Analysis (FTA) was originally developed in 1962 at Bell Laboratories by H.A. Watson, under a U.S. Air Force Ballistics Systems Division contract to evaluate the Minuteman I Intercontinental Ballistic Missile (ICBM) Launch Control System.

The origins of fault tree analysis (FTA) is the Minuteman Missile Program in the 1960s.

Most industries utilize fault tree analysis (FTA) as a powerful method to identify the causes of equipment failures. FTA is a vital tool for engineers, project managers, and maintenance teams to determine the possible failure modes for equipment. A process of elimination is then used to rule out which failure modes are not viable options. The remaining failure modes must be addressed through corrective actions or compensatory measures.

Key Takeaways:

  • Fault Tree Analysis (FTA): FTA is a method used to identify the possible causes of equipment failures, helping maintenance teams prioritize corrective actions.
  • How it Works: Once we create the Fault Tree, the analysis consists of a process of elimination (i.e., testing, troubleshooting, inspections, etc.) to rule out non-viable failure modes.
  • Symbols and Gates: Understanding the symbols and gates in fault tree analysis diagrams is crucial to analyzing and determining the failure modes and the root causes of failures.
  • Advantages of FTA: FTA can be used as a design tool as well as a diagnostic tool.

Fundamentals of Fault Tree Analysis

Uses of FTA

  • FTA is best used to analyze equipment issues, because they have a finite set of components that can fail.
  • The FTA chart can be used to develop troubleshooting plans and diagnostic manuals for equipment.
  • The FTA chart can be used as a design tool. By assigning a failure probability for each component, the overall probability of triggering the top event can be calculated using Boolean Algebra.
  • Although human error can be one of the causes of the equipment failure, FTA should NOT be used to analyze human failures vs. equipment failures.  That is because when analyzing human failures, humans can add infinite variability to any system and a Fault Tree cannot capture all the possibilities.

One word of caution: developing the Fault Tree requires input from the subject matter experts and/or the vendors for the equipment being analyzed.

Most Common Symbols Used in FTA

Fault tree analysis begins with the construction of a fault tree diagram. This diagram is a visual representation of events using logic symbols and event symbols. The logic symbols, often called gates, allow you to link events together in the fault tree and are represented by Boolean logic gates.

Here are some of the most common logic symbols used in Fault Trees.

Top Event

The symbol for the Top Event is:

Top Events describe the equipment that failed.  It is the same symbol as normal events except that it is at the top of the chart.  Top Events are the starting point for the FTA.

AND Gates

An AND gate is represented by this symbol:

AND Gate

An AND gate is used when the output occurs only if all the inputs are true. The logic statement of the AND gate is: All events that input to this gate must be TRUE in order for the output to be TRUE.

OR Gate

An OR gate is represented by this symbol:

OR Gate

An OR gate is used when the output occurs if any of the inputs are true.  The logic statement of the OR gate is: The output is TRUE if at least one input is TRUE.


The symbol for an Event is:

Events are the building block for the Fault Tree.  We use this symbol for all events except for the Basic Events as described below.  Events are the higher order failure modes for which there are still even lower (basic) failures.

Basic Events

The symbol for a Basic event is:

A Basic Event is the lowest level in a fault tree branch, indicating there are no lower causes. For example, Basic events can be hardware failures, human errors, process failures, software failure, or any type of system failure.

Image of a Simple Fault Tree

A simple Fault Tree only requires AND and OR gates for troubleshooting equipment failures. In this example, the Top Event cannot happen because two of the Basic Events are not TRUE, and the logic gates above them cannot be triggered because they are AND gates.

Basic construct of the fault tree used in fault tree analysis (FTA).

Image of a Simple Fault Tree But More Involved

This is an example of a simple Fault Tree with only AND and OR gates, but more more intricate. In this example, the Top Event cannot happen because one of the four Events below its AND gate is not TRUE, as all of the Basic Events in that tree branch are not TRUE.

Basic construct of the fault tree used in fault tree analysis (FTA).

The Process of Performing Fault Tree Analysis

An integral part of FTA is how the causes of the Top Event are systematically identified.  The Fault Tree is created by identifying the high level events that must be true to cause the failure of the Top Event.  AND and OR gates are used to establish how events above those gates are triggered (i.e. the event takes place).  We continue the process by working our way through the equipment subcomponents, all the way down until the most Basic Events are identified.  Basic Events are the failure modes that start the failure sequence and trigger the gates above it.

If Events are assigned a probability of occurrence, we can use Boolean Algebra to calculate the overall probability of the Top Event taking place.  One can see how the FTA is useful in the design phase, when we need to design equipment with specific failure criteria.

Integration of Fault Tree Analysis in Asset Management

Role of FTA in Asset Maintenance Strategies

One of the key roles of Fault Tree Analysis (FTA) in asset maintenance strategies is its ability to identify the potential causes of system failures. By utilizing fault tree diagrams, maintenance teams can pinpoint the major factors contributing to failures and prioritize corrective actions. FTA helps in streamlining maintenance efforts by focusing on the Basic Events (the most likely root causes of failures), ultimately leading to improved asset reliability and performance.

Enhancing Predictive Maintenance with FTA

One way to enhance predictive maintenance strategies is by integrating Fault Tree Analysis (FTA) into asset management practices. FTA allows maintenance professionals to identify failure modes, assess their probabilities, and understand the relationships between different events leading to system failures. By incorporating FTA into predictive maintenance programs, organizations can proactively address potential failures before they occur, resulting in increased equipment uptime, reduced maintenance costs, and improved operational efficiency.

Improving Safety and Reliability of Assets

Many industries benefit from using Fault Tree Analysis (FTA) to improve the safety and reliability of their assets. With FTA, organizations can identify potential causes of system failures and address them proactively. By analyzing failure events and their probabilities, maintenance teams can prioritize corrective actions to enhance the overall safety and reliability of their assets.

Cost Reduction and Efficiency Gains

Safety and efficiency go hand in hand in asset management. To achieve cost reduction and efficiency gains, organizations utilize FTA to pinpoint the root causes of failures and implement preventive measures. FTA helps in identifying common failure modes across systems, enabling teams to focus on preventing the most likely failures first. This proactive approach leads to reduced downtime, improved asset performance, and ultimately, cost savings.

How BlueDragon IPS Integrates Fault Tree Analysis

FTA is Data Analysis That Leads to Causal Analysis

For equipment failures, BlueDragon IPS uses FTA as the primary data analysis tool in Phase 1.  Once the FTA yields the failure modes for the equipment, we use those insights to develop lines of inquiry questions that will be used in Phase 2 Cause and Effect Analysis.

In a real-world example, a motor failure was caused by the lack of greasing of the bearings for over 3 years.  Other methodologies might call this the root cause of the motor failure.  In BlueDragon, we continue with our investigation by asking why the bearings were not greased for over 3 years.

  • The cause and effect analysis that followed identified that a junior engineer sent an email to the motor vendor to stop the vendor from greasing the bearings.
  • However, the junior engineer did not establish an in-house Preventive Maintenance card to grease the bearings.
  • Further causal analysis identified that the junior engineer was assigned as a contract manager for the motor vendor with little training, and they did not recognize that his email resulted in a contract mod.
  • The root cause was therefore the program for training and qualifying contract managers was flawed, and it affected more than that one junior engineer.

BlueDragon Best Practices for Equipment Failures and FTA

  1. Do not use FTA to analyze people problems or human errors.  FTA evaluates a finite number of possible failure modes, and humans can generate an infinite number of possibilities.
  2. Use FTA as data analysis.  Understand that the failure mode is not the root cause.
  3. The failures identified by FTA (the Basic Events that led to the Top Event) are used to generate lines of inquiry questions.
  4. The lines of inquiry questions are used to determine the deepest-seated causes that led to those failure modes. Those will be the true root causes.

The Benefits of Fault Tree Analysis

The benefits of Fault Tree Analysis (FTA) in asset management cannot be overstated. Fault tree analysis (FTA) is a powerful tool for proactively identifying potential failure modes and their causes in complex systems. By incorporating FTA into reliability-centered maintenance (RCM) and asset management programs, organizations can optimize maintenance resources, reduce downtime, and extend the life of critical assets, ultimately leading to improved operational efficiency and cost savings.

When applied reactively, FTA supports the root cause analysis of equipment failures, helping maintenance teams identify the underlying causes and implement corrective actions to prevent future occurrences. Overall, Fault Tree Analysis is a powerful tool that empowers maintenance teams to make informed decisions, prevent potential failures, and optimize asset performance.


About the Author:

Rob De La EspriellaBD3, CEO and Founder, BlueDragon IPS

Deming Prize winning team member and pioneering Nuclear Quality Assurance expert Rob De La Espriella draws from four decades of experience in the commercial nuclear power sector and the nuclear weapons complex to offer deep insights into Root Cause Analysis and Total Quality Management. Rob is a leading expert in solving complex human-centric problems in our modern work environments. He has re-defined how organizations solve complex problems with the BlueDragon Integrated Problem-solving System (IPS).

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