What are Fault Trees?
Reliability engineering is a sub-discipline of engineering that applies scientific knowledge to components, products, plants, or processes to ensure that they perform their intended functions without failure [1]. Reliability engineering ensures the system, product, or process is dependable and operates consistently without failure or breakdown. It is particularly important in industries where failures of a system or product can have severe consequences and in industries where reliability is critical to ensure customer satisfaction [1]. Reliability engineering commonly uses fault tree analysis, a graphical technique to identify the potential causes of system failures and determine if a technology is safe and reliable. This method systematically breaks down high-level system failures into their causal factors.
Fault trees indicate how individual component failures propagate through the system and lead to system failure. [2], [3]Fault trees have a hierarchical organisation. Starting with a top-level event, this event is then iteratively refined via gates that indicate how the event may occur. If there are no further refinements, possible one arrives at the basic events (the leaves of the tree). The basic events model the basic failure causes (often component failures) [2], [3]. An example of a static fault tree can be seen in Figure 1.
A static fault tree only exists of boolean gates, for example, the OR-gate and AND-gate that can be seen in the images on the right. The OR-gate outputs failure when at least one of its children fails, and the AND-gate outputs failure when all its children fail. [2]
There are two types of analyses, qualitative and quantitative. Qualitative fault trees illustrate a system's components and causal failure paths, and quantitative fault trees focus on dependability metrics.
Fault trees can aid in multiple activities [2]:
Understanding failure propagation and root causes. Fault trees systematically break down complex system failure scenarios into smaller pieces until the root causes have been identified. This helps with understanding system failures.
Improving system design. Fault trees help make systems more reliable by preventing the top event from occurring or reducing its probability.
Diagnosing a fault that has happened. Using an existing fault tree or constructing a new one can help pinpoint where the fault originated.
Monitoring risks during operations.
Complying with requirements.
Fault tree analysis is a powerful tool commonly employed in the aerospace and defence industries to assess the safety and reliability of complex systems, such as missiles, spacecraft, and aeroplanes. However, its usefulness extends beyond these fields. For example, it is utilized to evaluate the safety of nuclear power plants, where identifying potential failures is crucial. Similarly, fault tree analysis is used to assess the safety and reliability of transportation systems, including railways, highways, and air traffic control systems. Furthermore, fault tree analysis is used to analyse the reliability of manufacturing systems and identify potential failures that could lead to production downtime or quality issues. Another example is applying fault tree analysis on dike safety by identifying and analysing possible causes of dike failures [2].