Fault Tree Analysis (FTA) is a powerful analytical tool used to consider a "top event", which is often an unwanted hazard, and work out the causes that could lead to it. FTA is used extensively in various industries and contexts to ensure safety, reliability, and quality of products and services.
Top-down approach of Fault Tree Analysis - FTA
Unlike a natural tree which builds from the roots, a fault tree is constructed from the top downwards (top-down). The Fault Tree Analysis developer starts from the undesired event and looks to describe possible immediate causes that can lead to it. The tree is developed with more intermediate events until so-called "basic events" are reached at the roots of the tree.
These basic events are typically failures of equipment, components or human error that can be assigned with a frequency of occurrence or a probability of failure on demand (PFD). By visually representing these relationships, fault tree analysis can help in communicating where there are weaknesses in a design that might lead to safety or availability issues.
Gates and Events
To make sense, Fault Trees must have gates between the events that describe the relationship between them. For example, the relationship between a top event and two intermediate events below it in the tree can be assigned with an OR or an AND gate. The OR gate gets used if any one of the intermediate events could cause the top event, or alternatively, the AND gate gets used when all the intermediate events must occur simultaneously. Gates can have two, three or even more inputs provided they represent the structure of what is being modeled.
Fault Tree Analysis Symbols and their Meanings
There are several symbols used in FTA, each with a specific meaning. Here are some of the most commonly used symbols in fault tree analysis:
- Event - Represented by a rectangle, events start from the top event and progress down the tree with intermediate events until basic events are reached.
- Basic Event - Represents a component or process that is considered a failure when it occurs.
- Gate - Represents a logical operator (AND, OR, or NOT) used to combine the events or conditions leading to the failure mode.
- Undeveloped Event - Represents a failure mode that has not been fully analyzed.
FTA in contrast with other methods
LOPA:
Layer of Protection Analysis (LOPA) is a simplified risk assessment technique used in the process industry and the mining sector. LOPA scenarios are similar to paths through a fault tree - starting with causes (LOPA calls these initiating event frequencies) and developing to consequences via independent protection layers (IPL). LOPA can be used for single cause/consequence pairs, or multiple causes (paths) can be assessed provided they lead to the same consequence. If LOPA principles are strictly followed (which they must be to be valid), the technique may overestimate risk if a system is complex or has dependencies.
FMEA:
Failure Modes and Effects Analysis (FMEA) is a method that can be used to approach a system product from the opposite direction to FTA. FMEA and FMEDA ("D" adds Diagnostics analysis to a traditional FMEA) are bottom-up approaches that look at component level failure. FMEA is useful for component level analysis of electronic circuits, and can result in tables of data that can be quantified to produce a product failure rate with the distribution of failures into different modes.
RBD:
Reliability Block Diagrams (RBD) are another way of representing a system that can be useful when looking at system success. Blocks (rectangles) represent component successful operation, with series, parallel, or mix of series and parallel connections describing the paths to overall success. The boolean logic relationships of RBDs are equivalent to fault tree analysis, although RBD can be confusing to follow when trying to represent system failure.