Fault tree analysis (FTA) is a powerful tool for root cause analysis, helping to identify and analyze the origins of asset issues before equipment failures occur. It is particularly valuable in manufacturing environments, where understanding the potential causes of system failures is crucial for prevention.

FTA is a deductive, top-down approach designed to determine the cause of a specific undesired event within a complex system. This methodology involves dissecting the root cause of a failure into its contributing factors and representing this through a graphical model called a fault tree. This model aids managers and engineers in identifying potential failure modes and their probabilities, which is essential for safety and reliability analyses.

A Brief Background on FTA

Developed in the early 1960s by Bell Laboratories for the US Air Force to analyze potential flaws in the Minuteman missile system, FTA has since been widely adopted across various industries, including aerospace, nuclear power, chemical, and automotive sectors.

As manufacturing environments become increasingly complex, the need for effective risk management tools like FTA grows. Integrating fault tree analyses into an organization’s safety and reliability engineering practices can provide deeper insights into potential system failures, enhancing overall performance and reducing the likelihood of costly and potentially catastrophic incidents.

Who Employs FTA 

FTA is utilized by system designers, process designers, project managers, and engineers in manufacturing. These professionals often use FTA alongside methodologies like Kaizen and root cause analysis to prevent or resolve system failures.

How to Perform FTA

Performing an FTA involves seven key steps:

Step 1: Define the Undesired Event 

Clearly define the specific and measurable undesired event you want to analyze. This event serves as the starting point for your fault tree diagram.

Step 2: Identify the Contributing Events and Factors 

Identify the factors and events that might contribute to the undesired event, categorizing them into basic and intermediate events. Basic events are fundamental and cannot be further broken down, while intermediate events are higher-level and caused by other events.

Step 3: Construct the Fault Tree 

Create a graphical representation of the relationships between the undesired event and its contributing factors using standard gate and event symbols. This process is iterative and may require adjustments as new information becomes available.

Step 4: Gather Failure Data 

Collect failure data from historical records, industry databases, expert opinions, etc., for the basic events in the fault tree. This data is essential for quantifying the risks associated with the undesired event.

Step 5: Perform the Analysis 

Analyze the fault tree by calculating the probability of the undesired event occurring and identifying the most critical contributing factors. Use either qualitative or quantitative methods, depending on the analysis needs.

Step 6: Interpret the Results 

Interpret the results of the analysis and communicate relevant information to stakeholders. The results should be seen as a starting point for further investigation and validation.

Step 7: Implement Improvements and Monitor Progress 

Based on the findings, implement preventive measures and improvements to reduce the likelihood of the undesired event. Continuously monitor and update the fault tree to reflect changes in system design, operating conditions, or component performance.

Symbols Used in FTA

FTA uses standardized symbols and naming conventions across industries to illustrate the link and flow between different activities. Events and gateways (gates) are the two main categories.

Event Symbols:

• Top Event (TE): The initial event at the top of the fault tree, prompting an investigation.
• Intermediate Events (IE): Events caused by one or more other events, leading to further failures.
• Basic Events (BE): Fundamental events at the bottom of the fault tree, representing root causes.
• Underdeveloped Events (UE): Events lacking sufficient information, placed as a subtree.
• Transfer Events (TE): Used when the fault tree is too large to fit on paper, linking different parts of the tree.
• Conditional Events (CE): Conditions required for specific gate types.
• House Events (HE): Used to turn events on or off, allowing parts of the fault tree to be included or excluded.

Gate Symbols:

• AND Gate: Output occurs only if all input events occur.
• Priority AND Gate: Output occurs if all input events happen in a specific order.
• OR Gate: Output occurs if one or more input events occur.
• XOR Gate: Output occurs if exactly one input event occurs.
• k/N or Voting Gate: Output occurs when a specified number of input events occur.
• Inhibit Gate: Output occurs when all input and conditional events occur.

Pros Vs. Cons of FTA Methodology

Pros	Cons
Identifies the root cause of a failure event, saving time and money.	Relies heavily on the expertise of analysts and the quality of failure data.
Provides ways to mitigate failure consequences before they occur.	Best suited for smaller system analyses; large systems require complex and time-consuming fault trees.
Helps determine the likelihood of various failures, allowing focused prevention efforts.	The precision of calculated probabilities depends on the availability and quality of failure data.
Identifies common failure modes across systems or products, guiding necessary design changes.	Examines only one top event at a time, limiting the scope of analysis.