Fault Tree Analysis

Updated on April 4, 2024
Article byRutan Bhattacharyya
Edited byRutan Bhattacharyya
Reviewed byDheeraj Vaidya, CFA, FRM

What Is Fault Tree Analysis?

A fault tree analysis or FTA refers to a systematic approach that involves determining an event’s primary cause using a diagram. Typically, the event is a failure of a process, system, product, or some other issue. It makes assessing risk easier for commercial, public service, and medical organizations.

Fault Tree Analysis

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An FTA serves as a framework that guides an organization to systematically transform the available information into a well-defined plan of action. When entities utilize this approach with failure mode and effects analysis (FEMA) or any other analysis method, it can offer an enhanced picture that can help organizations take well-informed maintenance decisions.

Key Takeaways

  • The fault tree analysis definition refers to a graphical tool that businesses and professionals can use to explore the reasons behind failures at the system level.
  • It is based on a top-down structure; typically, it starts at a particular point (the top-level event). Then, it moves downwards like a tree. On the other hand, FMEA involves a bottom-up approach.
  • There are various advantages of fault tree analysis. For example, the approach highlights the essential system elements contributing to system failure. In addition, this systematic approach considers human error.
  • FTA has three components — the diagram, the events, and the gates.

Fault Tree Analysis Explained

The fault tree analysis definition refers to a systematic approach to troubleshooting, problem-solving, and spotting an undesired result or failure’s root causes with the help of a diagram. Companies or professionals can conduct this analysis to investigate a single failure. Alternatively, they can use it to look into several components grouped systematically. Thus, it is a versatile tool that can help one conduct root cause analysis.

This approach involves using Boolean logic to join several lower-level events. FTA utilizes a top-down approach to spot the basic event that led to the system-level failure. This approach has three main components. They are as follows:

  • FTA diagram;
  • The events that occurred and to which entities apply the FTA diagram;
  • Lastly, the gates connect the events.

Let us look at some uses of FTA:

  • First, it optimizes the resources being used by an organization.
  • Second, FTA demonstrates compliance with rules and regulations concerning safety.
  • Third, it assists in redesigning, overhauling, and reviewing systems.
  • Finally, FTA helps understand the sequence of events that caused the machine or system to fail.

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The fault tree analysis procedure involves the following steps:

#1 – Defining The Top Event  

The first step involves defining what went wrong; it is the FTA diagram’s starting point. The more precisely a company defines the starting event, the better this process will follow. A few examples of clearly-defined top events are as follows:

  • A crucial component of a machine keeps failing from time to time.
  • The government has updated certain regulations, and the company must comply with the changes within a particular date.  

The main objective of defining the top event is clearly stating the issue. If an organization does not define the top event precisely, the analysis will not be as effective.    

#2 – Understanding The System

This step involves accumulating maximum details concerning the system. This step aims to determine how the system managed to work before the top event defined by the organization became a serious issue.

#3 – Listing The Top Event’s Potential Causes

Organizations must list down the top event’s potential causes. They can do it in the following way:

  • Thinking of five potential causes;
  • Projecting the likelihood of each resulting in the top event;
  • Arranging all causes in order of probability.

Another way of doing this involves predicting the likelihood of the system’s most vulnerable areas failing. Professionals and organizations familiar with FTA can develop their techniques over time. As one can observe, this part of the fault tree analysis procedure is flexible.

#4 – Drawing The FTA Diagram

When creating or drawing the FTA diagram, one must begin with the top event and map out the top event’s various causes. Afterward, they must connect every step using the ‘OR’ and ‘AND’ gates. The diagram will resemble a flow chart. Since this step heavily relies on the previous three steps, one must draw it based on a solid foundation.

#5 – Gauge The Risk

The fifth step involves assigning a probability and base level to every base event. It can be very complicated. Let us look at a few things one can do to evaluate the risk better.

  • Depend on as much data as possible
  • Transform the available data into future insights

Lastly, one must consult with individuals or organizations with extensive knowledge of the system.

#6 – Reducing The Risk

Organizations or professionals must take the necessary steps to mitigate the risk and reduce the probability of the event.


Let us look at a few fault tree analysis examples to understand the concept better.

Example #1

FTA is crucial in the automotive industry as it enables automobile manufacturers to assess the risk of probable life-threatening events and look for ways to eradicate them.

Let us consider some popular features of vehicles:

  • Traction control;
  • Rearview camera;
  • Lane warning systems.

Each of the features mentioned above can enhance the safety of the vehicle owner and improve the driving experience. Nevertheless, various problems may arise with any of the functions. Some issues could result from mechanical problems, while others could arise due to road conditions, human error, etc. FTA can help companies uncover the problem of a certain automotive function and the events that contribute to failure.

Example #2

Companies in the oil and gas sector also use FTA software to spot and eliminate issues that can cause excess emissions and impure fuel supply. Without this analysis, issues like inadequate pipeline flow and subpar fuel grades could become uncontrollable.

Let us consider some issues.

  • Oil of sub-grade viscosity
  • Gas adulterated with sulfur.

The problem of water oil could arise due to different root causes. The same can happen for gasoline that is not free from impurities. In any case, fuel impurities impact the engine health of aircraft, automobiles, etc. FTA helps the fuel industry minimize the occurrence of such issues.


Before looking at an FTA diagram, one must be aware of its symbols, which have two categories: gates and events.

Let us look at them in detail.

Fault Tree Analysis symbols

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1. Gates

  • OR Gate: The output event occurs if a minimum of one input occurs.
  • AND Gate: The output event will occur if every input event occurs.
  • Priority AND Gate: The output event happens only after a particular order of conditions.
  • Inhibit Gate: The output event happens if the input event takes place and the fulfillment of the condition associated with it is complete.
  • Exclusive OR Gate: The output event will occur if any of the two input events happen. However, one must note that both input events must not occur.

2. Events

  • Conditioning Event: This refers to a restriction imposed on a certain logic gate.
  • Intermediate Event: Usually, one places this gate above a basic event to show additional details regarding the event description.
  • Basic/Primary Event: It is a primary failure event.
  • Undeveloped Event: This means that a component in the system requires further investigation owing to the availability of limited information.
  • External Event: This is an event that one typically expects to take place.
  • Transfer Out And Transfer In: One uses them to mimic a sub-tree or branch of the FTA.  

Once individuals are familiar with the symbols, they can look at the following example. Let us consider the three main issues that could potentially result in business failure. An OR gate represents each of these problems. This means that any of the events can cause the business to collapse. Every OR gate is expanded to spot the lowest-level causes.

Fault Tree Analysis symbols Diagram

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As shown in the above diagram, the first gate spots the likelihood of business failure owing to insufficient goods. The next gate analyzes the situations that could lead to substandard sales. When looking at the reasons for the below-par sales, one can look into the two crucial aspects leading to the problem — consumers not buying and marketing-related issues. Next, the third gate identifies the possibility of fiscal insolvency and its probable root causes. Any of the causes could singlehandedly cause the business to collapse.

As one can see, using FTA, one can consider every potential problem, have more clarity on matters, and take measures to reduce or even eliminate the problems before they lead to an undesired result.

Advantages And Disadvantages

Let us look at the benefits and limitations of FTA.

#1 – Advantages

  • This approach accounts for human error.
  • FTA allows entities to break down an undesired result into several problems they can solve in an organized manner.
  • It highlights the crucial system elements contributing to the failures or undesired results.
  • FTA offers quantitative and qualitative analysis.

#2 – Disadvantages

  • It can look into only one top event.
  • Common cause failures are usually not obvious.
  • Professionals and organizations find it difficult to capture the time-related factors when using FTA.
  • FTA requires experienced people to understand the logic gates.

Fault Tree Analysis vs FMEA

While FTA and Failure Modes and Effects Analysis or FEMA might seem similar at first glance, they have some key differences. Let us look at a few of them.

  • FTA utilizes a top-down approach. On the other hand, FEMA uses a bottom-up approach.
  • Predicting every likely failure mode that one can separate based on their criticality is necessary for a successful FMEA. That said, FTA does not consider partial failures.
  • FMEA is a qualitative analysis method. In contrast, FTA is a quantitative and qualitative analysis tool.
  • It is easy for one to keep the FTA updated.
  • FTA considers external events. On the other hand, FMEA does not consider external events.  

Fault Tree Analysis vs Fishbone

The following are some distinct characteristics of fishbone and FTA:

  • FTA is based on Boolean algebra; one draws the FTA diagram using the ‘OR’ and ‘AND’ gates. On the other, fishbone is not based on Boolean algebra.
  • The purpose of a fishbone diagram is to get to the bottom of the list of suspects. On the other hand, the objective of the FTA diagram is to narrow down the possible causes and eliminate them.

Frequently Asked Questions (FAQs)

When to use fault tree analysis?

Companies or professionals choose to conduct FTA when they need to analyze the failures of complex assets and systems. This graphical tool helps them find out the root cause of failures and prevent such undesired results in the future.

What are the principles of fault tree analysis?

The muti-casualty principle forms the basis of the FTA concept. It involves tracing every branch of events that could result in failure or an accident. The approach utilizes labels, symbols, and identifiers.

Is fault tree analysis inductive or deductive?

The FTA is an inductive and systematic approach that uses a top-down approach to identify the root causes of an undesired result. The starting point is the failure or top event. From then on, it moves downwards, reaching a broader conclusion — like a diagnosis or investigation. For instance, if a business records poor sales, FTA will consider it the top event.

What is the difference between fault tree analysis and ETA?

FTA uses a deductive approach. On the other hand, event tree analysis, or ETA, utilizes an inductive approach.

This has been a guide to what is Fault Tree Analysis. We explain its examples, steps, diagram, advantages, disadvantages, & differences with FMEA & fishbone. You can learn more about it from the following articles –

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