A Brief Guide to Fault Tree Analysis (FTA)

Fault tree analysis is primarily used in safety and reliability engineering. Though FTA is essential and known in the aerospace industry, any business that aims to assess the vulnerabilities of their system to failure-causing events can use fault tree analysis.

What is a Fault Tree Diagram?

A fault tree diagram is a key feature of FTA and is best used to explain the relationship between events. Though the FTA diagram is not the intended end result or purpose of fault tree analysis, it helps safety and reliability engineers spot and address issues that could lead to multiple undesired events.

How to Do a Fault Tree Analysis

Easily conduct a fault tree analysis with the following steps:

1. Define the undesired event.
2. Identify the requirements for the undesired event to occur.
   What are the essential components of this undesired event? Without these components, the undesired event cannot occur at all.
3. Decide the relationship between the requirements and the undesired event and use the corresponding fault tree analysis symbols.
   Guide Questions:
   • Do all of them need to be present for the undesired event to occur?
     • If yes, is their order relevant to the undesired event occurring?
       • If yes, use the Priority AND gate symbol.
       • If no, use the AND gate symbol.
         
     • If no, is a specific number of them needed for the undesired event to occur?
       • If no, use the OR gate symbol.
       • If yes, is the specific number more than 1?
         • If no, use the Exclusive OR gate symbol.
         • If yes, use the VOTING gate symbol.
           
     • If a requirement only leads to the undesired event under a certain condition, use the INHIBIT gate symbol.
       
4. Identify the causes of each requirement.
5. Decide the relationship between the causes and the requirement.
   Follow the same procedure as Step 3, only replace requirements with causes and undesired event with requirement.
   Repeat until you reach the root causes or basic events.
   

   

Fault Tree Analysis Example in Detail

Below is an example of a fault tree analysis in an electric power system:

Top event: short-circuit fault
[OR gate connecting top event to 1A and 1B]

• Intermediate event 1A: breakdown of transmission lines
  [OR gate connecting 1A to 1A-2A and 1A-2B]
  • Intermediate event 1A-2A: conductor resistance
    • Basic event 1A-2A-3A: rise in conductor temperature
  • Intermediate event 1A-2B: corona loss
    • Basic event 1A-2B-3A: ionization of air molecules near
      the transmission line conductors
      
      
• Intermediate event 1B: transformer failure
  • Intermediate event 1B-2A: insulation failure
    [INHIBIT gate connecting 1B-2A to 1B-2A-3A and conditioning event]
    Conditioning event: fault current passes through transformer
    • Intermediate event 1B-2A-3A: sufficient insulation deterioration
      [OR gate connecting 1B-2A-3A to 1B-2A-3A-4A and 1B-2A-3A-4B]
      • Intermediate event 1B-2A-3A-4A: thermal aging
        • Intermediate event 1B-2A-3A-4A-5A: excessive temperature
          • Basic event 1B-2A-3A-4A-5A-6A: transformer overloads
      • Intermediate event 1B-2A-3A-4B: mechanical stress
        • Basic event 1B-2A-3A-4B-5A: transient power frequency currents

The above fault tree analysis example in a visual format:

Disclaimer: Since the focus of this example is to show a fault tree analysis, other technical elements relating to a short-circuit fault may be incorrect.

Application of Fault Tree Analysis

The most critical part of fault tree analysis is the evaluation of the fault tree diagram. Using the diagram as a visual representation of failure paths, safety and reliability engineers can better identify which elements need to be removed or modified to prevent failure. 

Beyond simple identification of failure hazards, the gate and event symbols also help safety and reliability engineers strategically plan for how best to attack failure. They are able to know when and where they should add failure control measures and prioritize and allocate resources accordingly.

Another benefit of fault tree analysis is the ability to get Common Cause Failures (CCFs) when comparing the events of multiple fault trees within a single system. CCFs can be described as the highest priority risks to a system’s safety and reliability. 

Since these elements are the most likely to cause failure within a system, CCFs should be closely monitored to help prevent them from going further up failure paths and causing overall system failure.

Building Up a Database of Information: SafetyCulture

How do you fix a system if you don’t know the exact problems that need fixing? While most teams already have an idea of the problems they need to solve, some teams may still have difficulty in identifying the problems that are harming their system.

For those who want to ensure that they start their fault trees with the right problems and not waste their time making fault tree diagrams for random issues, consider using SafetyCulture (formerly iAuditor). 

Using SafetyCulture, you can see which undesired events have the largest negative impact on your system and spot trends in system failures. You can do the following and more with SafetyCulture:

• Help those on-the-ground gather information with photos and annotations
• Incorporate inspections into your failure information database
• View timelines of failure resolutions using the Actions feature

Book a demo to see how SafetyCulture works, get started for free, or take a look at some of our featured templates for root cause analysis below.

Root Cause Analysis Templates