Creating a Fault Tree Analysis is a fairly simple process but does require some practice in creating the tree and assigning the probabilities. The process in general goes as follows:
Define the top event: The first step in completing a Fault Tree Analysis (FTA) is to clearly define the failure or incident that is being analysed. This is known as the “top event,” and it serves as the starting point for the analysis. It is critical to clearly define the top event so that the analysis is focused and relevant.
Top events could include:
- A system failure that causes a shutdown or loss of production
- A safety incident that causes injury or damage to equipment
- A financial loss caused by an error or fraud
It is critical to consider the scope of the analysis when defining the top event. If the goal is to identify the causes of a system failure, for example, it may be more useful to define the top event as the failure itself rather than an indirect consequence such as the resulting downtime. Furthermore, ensure that the top event is specific and measurable, and that it is clearly linked to the underlying causes that will be investigated in the fault tree.
Identify contributing events and conditions: The next step in completing a Fault Tree Analysis (FTA) is to work backwards from the top event to identify all of the immediate causes and their respective contributing factors. This step contributes to a thorough understanding of the events and conditions that led up to the top event.
The analyst will typically begin by reviewing any available data, such as incident reports, maintenance records, and process logs, to identify the contributing events and conditions. This data can provide valuable insights into the events and conditions that led up to the main event.
In addition, the analyst will use their knowledge of the system, process, or industry to identify potentially contributing events and conditions. This can be accomplished through brainstorming sessions, interviews with subject matter experts, or a review of industry standards and best practises.
Once the contributing events and conditions have been identified, they are represented as “gates” (symbols) in the fault tree to show the logical relationship between the events. Gates are used to demonstrate how the contributing events and conditions are related and how they lead to the outcome.
The following are the most common FTA gates:
- AND gate: Represents a condition in which all of the events connected to it must occur in order for the top event to occur.
- OR gate: Represents a condition in which at least one event connected to it must occur in order for the top event to occur.
- NOT gate: Denotes a condition in which the event connected to it must not occur in order for the top event to occur.
This step is critical because it allows the analyst to see how various events and conditions can interact to lead to the top event, providing a better understanding of the complex interactions that led to the failure.
Build the tree: After identifying and representing the contributing events and conditions as gates, the next step is to connect the gates and events together to show all of the possible combinations of events and conditions that could lead to the top event.
The fault tree diagram, which represents the logical relationship between the events and conditions, is created during this step. Starting with the top event at the top of the diagram, the tree is built by adding gates and events as needed to show all of the possible events and conditions that could lead to the top event.
It is critical to note that the tree should be as detailed and comprehensive as possible while avoiding irrelevant information or unnecessary complexity. The key is to strike the proper balance of detail and simplicity.
It is also important to note that the tree should be constructed in a logical and consistent manner so that the analysis can be easily understood, followed, and reviewed.
Once the tree is complete, the analyst can review it to ensure that it includes all of the possible combinations of events and conditions that could lead to the top event and that it is logically consistent.
The Fault Tree Analysis (FTA) diagram can be a complex and detailed representation of the problem, but it provides a clear visual representation of how various events and conditions can combine to lead to the top event, assisting in the identification of the underlying causes of the failure.
Assign probabilities: After completing the fault tree, the next step is to assign probabilities to each event or condition. This is a critical step because it allows the analyst to assess the likelihood of each event or condition occurring as well as identify the most likely causes of the failure.
There are several methods for assigning probabilities to events and conditions, including:
- Historical data: If available, historical data can be used to assign probabilities based on previous incidents or failures.
- Expert judgement: In the absence of historical data, probabilities can be assigned based on the expertise of experts with knowledge of the system, process, or industry.
- Combination of both: Probabilities can also be assigned using a combination of historical data and expert judgement.
It is critical to use the most appropriate and accurate method when assigning probabilities, taking into account the available data and the level of uncertainty involved.
It’s also worth noting that the assigned probabilities should be consistent and realistic, and that the accuracy of the analysis is highly dependent on their accuracy.
Once the probabilities are assigned, the analyst can multiply the probabilities of the contributing events and conditions to calculate the overall probability of the top event occurring. This enables the analyst to determine the most likely causes of the failure and assess the risk associated with each contributing event or condition.
Determine the likelihood of the top event: After assigning probabilities to each event or condition, the next step is to compute the overall probability of the top event occurring. The probabilities of the contributing events and conditions are multiplied.
For example, if event A has a probability of 0.2, event B has a probability of 0.3, and event C has a probability of 0.4, the probability of all three events happening at the same time can be calculated by multiplying the individual probabilities: 0.2 x 0.3 x 0.4 = 0.024.
Depending on the type of gate used in the fault tree, the process of calculating the overall probability of the top event differs.
- The probability of the top event in an AND gate is the product of the probabilities of the events connected to it.
- The probability of the top event for an OR gate is the sum of the probabilities of the events connected to it less the probability of all events occurring at the same time.
- The probability of the top event is the complement of the probability of the event connected to it for the NOT gate.
The probability of the top event shows the overall probability of the failure or incident occurring, and it enables the analyst to determine the most likely reasons of the failure and assess the risk associated with each contributing event or condition.
It is crucial to remember that the chance of the top event might change when new information is gathered and probabilities are re-evaluated, therefore the analysis should be reviewed and updated as needed on a regular basis.
Identify and assess risk: The final stage in completing a Fault Tree Analysis (FTA) is to identify and assess the risk associated with the contributing events or conditions that have been discovered. This is accomplished by determining the possibility and potential impact of each incident and then taking necessary action to reduce or eliminate the risks.
The analyst can utilise the overall probability of the top event estimated in the previous stage, as well as the probability of each contributing event or condition, to identify the risk. This enables the analyst to determine which events or conditions are most likely to cause the failure and which must be addressed immediately.
The analyst might also analyse the probable impact of each event or situation while assessing risk. This includes appraising the event’s or condition’s probable implications, such as harm, damage, or financial loss.
After identifying and evaluating the risks, the analyst can take necessary measures to mitigate or eliminate them. Implementing new procedures, changing equipment, or offering more training are all examples of this.
It is crucial to highlight that the risk management process should be iterative, as new information or events may surface and affect the risk profile, therefore the risk management plan should be reviewed and updated on a regular basis.
The overall purpose of the Fault Tree Analysis (FTA) is to discover the underlying reasons of a failure or incident and to give a methodical strategy to mitigating risks, improving the system, and preventing similar accidents in the future.
It’s worth noting that, depending on the complexity of the problem, the process may require multiple iterations and reviews to ensure that the analysis is complete and accurate.