the statistical probabilities from the bottom up.
Drawing Fault Trees: Gates and Events
Gate symbols represent results of interactions among contributing failure events and can vary among tools. Basic gates used to construct the Fault Tree can be seen below:
| Gate Symbol | Name | Causal Relation |
|
|
OR | Output event occurs if any one of the input events occurs |
|
|
AND | Output event occurs if all input events occur |
|
|
BASIC | Basic event for which failure data is available. |
|
|
INTERMEDIATE EVENT | System or component event description |
|
|
TRANSFER | Indicates that this part of the fault tree is developed in a different part of the diagram or on a different page. |
FTA Gate Notes
Fault Tree probabilities can be computed by simple arithmetic only if basic events (component failures without lower level contributors) are independent. Independence is determined by ensuring the failure of one basic event has no effect on any other and groups of basic events cannot fail from common causes such as shock. For independent basic events with very small failure rates, typically found in electronic components, an AND gate output probability can be computed as the product of its input failure probabilities, and an OR gate output probability can be computed as the sum of its input failure probabilities.
Below is a Basic Fault Tree Analysis Example Structure:
The five basic steps to perform a Fault Tree Analysis are as follows:
1 Identify the Hazard
2 Obtain Understanding of the System Being Analyzed
3 Create the Fault Tree
4 Identify the Cut Sets
5 Mitigate the Risk
Top‐level event is called a Cut Set. There are many cut sets within the FTA. Each has an individual probability assigned to it. The paths related to the highest severity / highest probability combinations are identified and will require mitigation.
How to Undertake a Fault Tree Analysis?
Although the nature of the undesired event may be quite different, fault tree analysis has the same procedure for any types of undesired event. To do a comprehensive fault tree analysis, simply follow the process below:
1 Define and identify the fault condition (hazard) as precisely as possible based on the aspects such as the amount, duration, and related impacts.
2 Using technical skills and existing facility details to list and decide all the possible reasons for the failure occurrence.
3 Break down the tree from the top level according to the relationship between different components until you work down to the potential root cause. The structure of your fault tree analysis diagram should be based on the top, middle (subsystems), and the bottom (basic events, component failures) levels.
4 If your analysis involves the quantitative part, evaluate the probability of occurrence for each of the components and calculate the statistical probabilities for the whole tree.
5 Double‐check your overall fault tree analysis diagram and implement modifications to the process if necessary.
6 Collect data, evaluate your results in full details by using risk management, qualitative, and quantitative analysis to improve your system.
Benefits of Fault Trees
A fault tree creates a visual record of a system that shows the logical relationships between events and causes that lead to failure. It helps others quickly understand the results of your analysis and pinpoint weaknesses in the design and identify errors. A fault tree diagram will help prioritize issues to fix that contribute to a failure. In many ways, the fault tree diagram creates the foundation for any further analysis and evaluation. For example, when changes or upgrades are made to the system, you already have a set of steps to evaluate for possible effects and changes. You can use a fault tree diagram to help design quality tests and maintenance procedures.
Identify the Root Cause
Look over your list of potential causal factors and determine the real reason this problem or issue occurred in the first place. These data should have provided enough insight into the failure for the investigator to develop a list of potential or probable reasons for the failure. Dig deep to examine each level of cause and effect and the events that led to the unfavorable outcomes. The problem is that in the real world it is never possible to prove a single event that solely initiates a whole chain of other events. That is because there are always other events before the so‐called “root cause event.” This may seem like semantics, but for problem‐solvers, it is important to keep in mind that there never is a silver‐bullet answer.
Analyzing the short list of potential root causes is to verify each of the suspect causes is essential. In almost all cases, a relatively simple, inexpensive test series can be developed to confirm or eliminate the suspected cause of equipment failure.
Most equipment problems can be traced to misapplication, operating or maintenance practices and procedures. Some of the other causes that are discussed include training, supervision, communications, human engineering, management systems, and quality control. These causes are the most common reasons for poor plant performance and equipment reliability. However, human error may contribute to, or be the sole reason for, the problem.
Recommend and Implement Solution
When working on solutions, keep your Root Cause Analysis aim in view. You don’t just want to solve the immediate problem. You want to prevent the same problem from recurring.
Ask the following
