Answering the 5 W’s of Fault Tree Analysis
1. What is Fault Tree Analysis?
Fault Tree Analysis, sometime abbreviated as FTA, is a methodology used to determine the probability that an unwanted event will occur. The “unwanted event” is typically considered to be some type of failure of a product, system, process, or an issue of any kind. The undesired events can be major, life-threatening incidents, such as the crash of an airliner; other critical events, such as a cellphone catching fire; or even less crucial failures, such as my personal drone crashing on landing. Fault Tree Analysis enables you to model the safety and risk potential of events related to the failure of any type of system or process – from electro-mechanical systems to human errors in complex processes. Typically, an analyst or engineer employing FTA wants to assess the likelihood, or probability, of an undesirable event, and ultimately eliminate or minimize its occurrence, or mitigate its severity. In some instances, analysts may have a contractual requirement to ensure a critical event falls below a specific likelihood of occurring. FTA is a tool that can provide the metrics needed to meet this requirement.
Fault tree analysis begins with the construction of a fault tree diagram. This diagram is a visual representation of events using logic gates. You start at the top level, by defining the main undesired event you are investigating. The events that could lead to the top event are then delineated, and are connected to the top-level event using logic gates that describe the relationship of events and their outcomes. The process continues until all basic events are identified.
A fault tree diagram consists of Boolean logic gates, such as AND, OR, NOR, NOT, XOR, and Voting gates coupled with events, as shown in the example below.
Once the tree is defined and the underlying event probabilities determined, there are various mathematical approaches to obtaining various risk metrics, such as the probability of the occurrence topmost undesirable event.
Some terminology you will often hear associated with fault tree analysis:
- Logic gates: Used to define the conditions in which a set of events will lead to a given output. For example, if event A is “failure of smoke detector” and event B is “shorted connection”, then the combination of event A AND event B will lead to a fire. Or, if event A is “brake failure” and event B is “deer in road”, then event A OR event B will lead to a crash.
- Events: The low-level building blocks of the fault tree. Events are associated with probabilities, and various input models can be used to mathematically describe their occurrence.
- Risk measures: Values such as unavailability, or the probability that a system is inoperable at a given time, are used to quantitatively measure risk.
- Importance Measures: Indicate how much, or how little, a specific event contributes to the overall system reliability.
- Cut Sets – the sets of events that lead to the occurrence of the topmost event.
2. Who does Fault Tree Analysis?
Engineers working on high risk or safety critical projects often perform Fault Tree Analysis. However, FTA is a widely used tool in a broad range of industries in order to measure the likelihood of an undesirable event occurring. Engineers, analysts, or any team member can capably perform fault tree analysis when a software tool is employed.
3. When should Fault Tree Analysis be performed?
Fault Tree Analysis should be considered any time you want to assess the risk and probability of an event you want to prevent.
FTA is employed in high-risk sectors, such as the nuclear power industry, and military and aerospace industries where mission success and safety is paramount. In some cases, in industries associated with highly critical outcomes, such as those involving loss of life or mission failure, fault tree analyses may be a contractual or regulatory requirement.
FTA is used across industries, however, since all companies have their own known risk, safety, and quality issues to address. For example, in our own company, our important issues may not be severe as those in the aerospace realm, however, we want to understand, assess, and measure software failures in order to prevent them or lessen their likelihood of occurrence.
In some cases, fault tree analysis grows out of FMEA (Failure Mode and Effect Analysis). FMEAs are used to organize and assess potential failure modes of a design or process. While performing a FMEA, a potential failure mode may be identified as a particularly high-risk concern. This may be due to the fact that the results of the failure have a high severity, potentially occur frequently, and are difficult to detect. In this situation, it may be beneficial to more thoroughly evaluate this failure with a fault tree assessment. Fault tree analysis can provide more depth and specific metrics that simply are not available in a FMEA.
4. Where should Fault Tree Analysis be performed?
Fault Tree Analysis is beneficial in a wide range of industries and organizations. Some industries that rely on FTA include the nuclear power industry, medical manufacturing, and the aerospace and defense sectors. The adoption of FTA techniques in these areas has broadened its use across many market sectors. Other industries that have used fault tree analysis to aid in risk and safety assessment include telecommunications, automotive, locomotive, oil and gas, and a wide range of commercial and manufacturing businesses.
5. Why should Fault Tree Analysis be performed?
While various tools can assess risk, there are distinct advantages of using Fault Tree Analysis for this task:
- Proven technique: Fault tree analysis is an accepted, proven technique for risk assessment. Fault tree analysis has been done for major, world-known failures such as the explosion of the Columbia space shuttle and the Deepwater Horizon incident.
- Clear metrics: Unlike other qualitative analysis tools used in reliability assessments, fault tree can provide clear quantitative metrics. Having measurable outputs is a significant advantage of FTAs.
- Comprehensive analysis: Fault trees can be as comprehensive as you desire. In some cases, fault tree diagrams can become incredibly large and fully detailed. The depth of evaluation is up to the analyst, but the ability to dive deep and get a thorough understanding of the issues at hand is a significant advantage of fault trees.
- Easy to understand: While the underlying mathematics is complex, the diagrammic representations and output metrics of a fault tree are easy to comprehend and explain to anyone. This make FTAs a powerful tool for engineers for their immediate task at hand, as well as for explanations to managers, department heads, and any team members who need to understand the risk assessment.
- Continual Improvement. Fault trees can be part of continual improvement efforts. As previously stated, fault trees can be performed as an outgrowth of a FMEA. So fault tree can be employed during the design phase, during the production phase, or as a post-mortem of a catastrophic failure occurrence. Any lessons learned from fault tree analysis can be used to aid in future designs or future process improvement initiatives.
6. And the H: How do I do Fault Tree Analysis?
Once a fault tree diagram is constructed, the mathematical computations can be performed manually, but can easily become too complex for that to be realistic. In some cases, analytical solutions are not possible, and simulation techniques must be used in order to obtain probability values. This is why using a software tool specifically designed for fault tree analysis is needed. The underlying computational engine in a fault tree application, such as Relyence Fault Tree, are built to handle to complexities of the calculations, employ optimization techniques, and provide the accuracy needed for clear assessment.
For more information about Relyence Fault Tree, click here, or test out our completely free online trial, or check out some of our Relyence Fault Tree online videos. Feel free to contact us at 724.832.1900 to schedule a 1-on-1 demo of Relyence Fault Tree. We are here to help answer your questions!