INTRODUCTION OF FTA (FAULT TREE ANALYSIS)
(Written by Okasatria Novyanto)
(Written by Okasatria Novyanto)
In a complicated multi-component system, it is important to analyze the possible mechanisms for failure and to perform probabilistic analyses for the expected rate of such failures. Typically, each such system is unique in the sense that there are no other identical systems (same components interconnected in the same way and operating under the same conditions).
Furthermore, the initiating causes and the combination of events which can lead to a particular failure are of interest also. The natural way to attack a problem of this nature, where many events interact to produce other events, is to relate these events using simple logical relationships (intersection, union, etc.) and to methodically build a logical structure which represents the system.
In this respect, FTA is a systematic deductive technique which allows to develop the causal relations leading to a given undesired event. It is deductive in the sense that it starts from a defined system failure event and unfold backward its causes down to the primary (basic) independent faults. The method focuses on a single system failure mode and can provide qualitative information on how a particular event can occur, to what consequences it leads, while at the same time allowing the identification of those components which play a major role determining the defined system failure.
Moreover it can be solved in quantitative terms to provide the probability of events of interest starting from knowledge of the probability of occurrence of the basic events which cause them.
History of FTA
1. On 1962, FTA was first used by Bell Telephone Laboratories
2. Technique improved by Boeing Company.
3. Extensively used and extended during the Reactor safety study (WASH 1400).
About FTA
FTA is an acronym that stands for Fault Tree Analysis. FTA is a top-down approach to failure analysis, starting with a potential undesirable event (accident) called a TOP event and then determining all the ways it can happen. FTA uses a simple set of rules and logic symbols from probability theory and Boolean algebra.
The analysis proceeds by determining how the TOP event can be caused by individual or combined lower level failures or events which are referred to as basic events. They are “connected” trough logic gates. The lowest-level basic events that terminate fault tree paths are often called terminal events or primary events. They are either component-level events that cannot be further resolved or external events. It is a graphical representation of the major faults or critical failures associated with a product, the causes for the faults and potential countermeasures. The tool helps identify areas of concern for new product design or for improvement of existing products. It also helps identify corrective actions to correct or mitigate problems.
Terminology on FTA
1. The undesirable event at the system level is referred to as the TOP event - It generally represents a system failure mode or hazard for which predicted availability data is required.
2. The lower level events in each branch of a fault tree are referred to as basic events - They represent hardware, software, and human failures for which the probability of failure is given based on historical data.
Utilization of FTA
FTA is useful both in designing new products or in dealing with identified problems in existing products. In the quality planning process, the analysis can be used to optimize process features and goals and to design for critical factors and human error. As part of process improvement, it can be used to help identify root causes of trouble and to design remedies and countermeasures
How to use FTA
a. Select a component for analysis - Draw a box at the top of the diagram and list the component inside.
b. Identify critical failures or “faults” related to the component - Using FMEA is a good way to identify faults during quality planning. For quality improvement, faults may be identified through Brainstorming or as the output of Cause and Effect Analysis.
c. Identify causes for each fault - List all applicable causes for faults in ovals below the faults. Connect the ovals to the appropriate fault box.
d. Work toward a root cause - Continue identifying causes for each fault until you reach a root or controllable cause.
e. Identify countermeasures for each root cause - Use Brainstorming or a modified version of Force Field Analysis to develop actions to counteract the root cause of each critical failure. Create boxes for each countermeasure, draw the boxes below the appropriate root cause, and link the countermeasure and cause
Limitations
1. Narrow focus - FTA examines only one specific accident of interest. To analyze other types of accidents, other fault trees must be developed.
2. Art as well as science - The level of details, types of events included in a FTA, and organization of the tree vary significantly form analyst to analyst.
3. Quantification requires significant expertise - Using FTA results to make statistical predictions about future system performance is complex. Only highly skilled analysts can reliably perform such quantifications.
In addition, analysts often become so focused on equipment and systems that they forget to address human and organizational issues adequately in their models. While this is not an inherent limitation of FTA, it is worth noting.
Most common uses
Generally applicable for almost every type of risk assessment application, but used most effectively to address the fundamental causes of specific accidents dominated by relatively complex combinations of events. FTA Can be used as an effective root cause analysis tool in several applications, example :
1. To understand the causal factors of an accident.
2. To determine the actual root causes of an accident.
The Applications
a. Identifying safety critical components.
b. Verifying product requirements.
c. Certifying product reliability.
d. Assessing product risk.
e. Investigating accidents/incidents.
f. Evaluating design changes.
g. Displaying the causes and consequences of events.
h. Identifying common-cause failure.
Summary
1. FTA models the possible combinations of equipment failures, human errors, and external conditions that can lead to a specific type of accident.
2. Used most often as a system-level risk assessment technique.
3. Include human errors and common-cause failures.
4. Performed primarily by an individual working with system experts through interviews and fields inspections.
5. A risk assessment technique that generates
5.a Qualitative descriptions of potential problems and combinations of events causing specific problems of interest.
5.b Quantitative estimates of failure frequencies and likelihoods, and relative importance of various failure sequences and contributing events.
5.c Lists of recommendations for reducing risks.
5.d Quantitative evaluations of recommendation effectiveness.
Reference : Training Module of Sachbudi Abbas Ras, B.Eng.,M.Eng
(Head of INDONUSA Quality Center and INDONUSA Operations Research Center)
