Fault Tree Analysis

Used in both reliability engineering and system safety engineering

Used in almost every engineering discipline

Bell Telephone Laboratories developed the concept of fault tree analysis in 1962 for the U.S. Air Force for use with the Minuteman system. It was later adopted and extensively applied by the Boeing Company. A fault tree diagram follows a top-down structure and represents a graphical model of the pathways within a system that can lead to a foreseeable, undesirable loss event (or a failure). The pathways interconnect contributory events and conditions using standard logic symbols (AND, OR, etc.).

Fault tree diagrams consist of gates and events connected with lines. The AND and OR gates are the two most commonly used gates in a fault tree. To illustrate the use of these gates, consider two events (called "input events") that can lead to another event (called the "output event"). If the occurrence of either input event causes the output event to occur, then these input events are connected using an OR gate. Alternatively, if both input events must occur in order for the output event to occur, then they are connected by an AND gate. The following figure shows a simple fault tree diagram in which either A or B must occur in order for the output event to occur. In this diagram, the two events are connected to an OR gate. If the output event is system failure and the two input events are component failures, then this fault tree indicates that the failure of A or B causes the system to fail.

Fault tree analysis is also a tool for discovering product failure, engineering failure, ranking the effects of item failures and human error.

•Fault Tree Analysis (FTA) is a deductive reasoning technique that focuses on one particular accident event.

The fault tree itself is a graphic model that displays the various combinations of equipment faults and failures that can result in the accident event
•The solution of the fault tree is a list of the sets of equipment failures and human/operator errors that are sufficient to result in the accident event of interest.
The strength of FTA as a qualitative tool is its ability to break down an accident into basic equipment failures and human errors. This allows the safety analyst to focus preventive measures on these basic causes to reduce the probability of an accident

In many cases there are multiple causes for an accident or other loss-making event. Fault tree analysis is one analytical technique for tracing the events which could contribute. It can be used in accident investigation and in a detailed hazard assessment. The fault tree is a logic diagram based on the principle of multi-causality, which traces all branches of events which could contribute to an accident or failure. It uses sets of symbols, labels and identifiers. But for our purposes, you’ll really only use a handful of these, shown below:

Fault Tree Analysis: a Systematic and Stylized Deductive Process

An undesired event is defined

The event is resolved into its immediate causes

This resolution of events continues until basic causes are identified

A logical diagram called a fault tree is constructed in the process of carrying out the analysis

Why Fault Tree Analysis (FTA) is carried out

To gain an understanding of the system

To document the failure relationships of the system

To exhaustively identify the causes of a failure

To assure compliance with requirements or a goal

To identify any weaknesses in a system

To prioritize contributors to failure

To identify effective upgrades to a system

To optimize operations and processing

To quantify the failure probability and contributors

The Fault Tree

FTA produces a Fault Tree.

The fault tree is the logical model of the relationship of the undesired event to more basic events.

The top event of the fault tree is the undesired event.

The middle events are intermediate events.

The bottom of the fault tree is the causal basic events or primary events.

The logical relationships of the events are shown by logical symbols or gates.

Basic Fault Tree Importance Measures

FV Importance = Relative contribution to the system failure probability from a component failure

RAW = Factor increase in the system failure probability when a component is assumed to be failed

RRW = Factor decrease in the system failure probability when a component is assumed to succeed

FV Importance = “Fussell-Vesely Importance”

RAW = “Risk Achievement Worth”

RRW = “Risk Reduction Worth”

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