Fault Tree Analysis Basics

A Fault Tree Analysis (FTA) is a systematic deductive, top-down method of analyzing system design and performance. It involves specifying a top event to analyze (such as catastrophic system behavior), followed by identifying all of the associated elements in the system that could cause that top event to occur. It clearly identifies fault relationships and provides a methodical approach to predict the probability that some top-level events will or will not occur.

Fault Trees provide a convenient symbolic representation of the combination of events resulting in the occurrence of the top event. Fault tree analyses are generally performed graphically using a logical structure of AND and OR gates.



This analysis is often applied to identify critical design and/or safety weaknesses early in the development process. It is also useful for the development of diagnostic approaches.

Fault Trees are one of the most widely used methods in system reliability and failure probability analysis. A Fault Tree is a graphical representation of events in a hierarchical, tree-like structure. It is used to determine various combinations of hardware, software, and human error failures that could result in a specified risk or system failure. System failures are often referred to as top events. A deductive analysis using a Fault Tree begins with a general conclusion or hazard, which is displayed at the top of a hierarchical tree. This deductive analysis is the final event in a sequence of events for which the Fault Tree is used to determine if a failure will occur or, alternatively, can be used to stop the failure from occurring. The remainder of the Fault Tree represents parallel and sequential events that potentially could cause the conclusion or hazard to occur and the probability of this conclusion.

A fault tree is a graphical representation of a logical structure representing undesired events ("failures") and their causes. You create the logical structure by using gates and represent undesired events by using basic events. Reliability parameters are assigned to the basic events. Widely used in system reliability studies, fault tree analysis offers the ability to focus on an event of importance, such as a highly critical safety issue, and work to minimize its occurrence or consequence. The probability of the top-level event can then be determined by using mathematical techniques. The resulting fault tree diagram is a graphical representation of the chain of events in your system or process, built using events and logical gate configurations.

Fault Trees are composed of events and logical event connectors (OR-gates, AND-gates, etc.). Each event node's sub-events (or children) are the necessary pre-conditions that could cause this event to occur. These conditions can be combined in any number of ways using logical gates. Events in a Fault Tree are continually expanded until sub-events are created for which you can assign a probability. Events in a Fault Tree describe the different levels of system abstraction. Higher nodes represent a high level of abstraction while lower nodes represent a low level of abstraction.

List of different FTA events and gates supported by ALD FTA Software provided below:

OR gate	OR gate - output event occurs if any of the input events occurs
AND gate	AND gate - output event occurs only when all the input events occurs simultaneously.
NAND gate	NAND gate - NOT AND operation
NOR gate	NOR gate - NOT OR operation
XOR gate	XOR gate - Exclusive OR operation
K-out-of-N gate	K-out-of-N gate - output event occurs if K or more of the input events occurs
Transfer gate	Transfer gate - transfer to another tree (sub-tree)
Basic Event	Basic event - represents a basic equipment fault or failure that requires no further development into more basic faults or failures.
Undeveloped event	Undeveloped event - represents a fault event that is not examined further because information is unavailable or because its consequence is insignificant.
House Event	House event - represents a condition or an event which is TRUE (ON) or FALSE (OFF) (false).

The main purpose of Fault Tree Analysis is to evaluate the probability of the top event using state-of-the-art analytical and/or statistical methods. These calculations involve system quantitative reliability and maintainability data, such as failure probability, failure rate, expected failure, down time, repair rate, etc.

Several analysis types can be conducted:
• Qualitative Analysis: performed by means of Minimal Cut Sets (MCS) building
• Quantitative Analysis: calculating the Absolute probabilities, i.e. the probabilities of system failures
• Importance/Sensitivity Analysis: Importance analysis results help to select those fault tree events, which contribute most to the system's unavailability. Sensitivity analysis helps to choose those events, where a relatively small change will lead to a relatively large system unavailability changes.

Recommended reading on Fault Tree Analysis

Fault Tree Analysis Tutorial by Marvin Rausand, Norwegian University of Science and Technology
Introduction to Fault Tree Analysis by Jorn Vatn
Fault Tree Handbook with Aerospace Applications by NASA Office of Safety and Mission Assurance


See also:
Fault Tree Analysis Software by ALD
Download Fault Tree Analysis Software Free Demo.