The fault tree analysis (FTA) was first introduced by Bell Laboratories and is one of the most widely used methods in system reliability, maintainability and safety analysis. It is a deductive procedure used to determine the various combinations of hardware and software failures and human errors that could cause undesired events (referred to as top events) at the system level.

 

The deductive analysis begins with a general conclusion, then attempts to determine the specific causes of the conclusion by constructing a logic diagram called a fault tree. This is also known as taking a top-down approach.

 

The main purpose of the fault tree analysis is to help identify potential causes of system failures before the failures actually occur. It can also be used to evaluate the probability of the top event using analytical or statistical methods. These calculations involve system quantitative reliability and maintainability information, such as failure probability, failure rate and repair rate. After completing an FTA, you can focus your efforts on improving system safety and reliability. It is most effective in the manufacturing industry... 

The fault tree analysis (FTA) was first introduced by Bell Laboratories and is one of the most widely used methods in system reliability, maintainability and safety analysis. It is a deductive procedure used to determine the various combinations of hardware and software failures and human errors that could cause undesired events (referred to as top events) at the system level.

The deductive analysis begins with a general conclusion, then attempts to determine the specific causes of the conclusion by constructing a logic diagram called a fault tree. This is also known as taking a top-down approach.

The main purpose of the fault tree analysis is to help identify potential causes of system failures before the failures actually occur. It can also be used to evaluate the probability of the top event using analytical or statistical methods. These calculations involve system quantitative reliability and maintainability information, such as failure probability, failure rate and repair rate. After completing an FTA, you can focus your efforts on improving system safety and reliability.

This analysis method is mainly used in the fields of safety engineering and reliability engineering to understand how systems can fail, to identify the best ways to reduce risk or to determine (or get a feeling for) event rates of a safety accident or a particular system level (functional) failure. FTA is used in the aerospace,nuclear power, chemical and process,pharmaceutical, petrochemical and other high-hazard industries; but is also used in fields as diverse as risk factor identification relating to social service system failure. FTA is also used in software engineering for debugging purposes and is closely related to cause-elimination technique used to detect bugs.

I don't think that FTA is less likely to be used in any process or projects. The unlikely use will only if the project is small and there is very minimum error/failures.

Fault Tree Analysis

 

 

What is it:

Fault Tree Analysis (FTA) is a part of the Root Cause Analysis ( RCA) toolkit used during the Analyze and Improve phases. It is a tool/approach that helps to identify underlying reasons for a specific failure or event. Unlike a Fishbone diagram where all possible causes are listed without their inter-relation, FTA also helps to determine the relationships between causes and failures. It may highlight both 'OR' and 'AND' conditions for multiple causes to a failure mode. A single fault tree is used to analyze only one top event which can then be a basic cause of another higher level event.

 

FTA is commonly used to search for the causes of an already observed or potential failure so that problems relating to processes, products, service, or quality can be either controlled or eliminated. Therefore, this is useful for both RCA and also to find solutions for root causes as even solutions can be verified through inter-relationships between factors that constitute the optimum solution.

 

FTA is essentially a top-down approach whereas FMEA ( Failure Mode & Effects Analysis) is a bottom-up approach.

 

Approach:

FTA starts with identifying the top event that is in question. It's then followed by identifying probable high level causes along with their relations ( 'OR'  gate or 'AND' gate) towards the effect. Naturally, causes with 'AND' relation need to happen together for the failure to happen and any of the causes with 'OR' relation may result in the failure. All these causes then are drilled down further until low level actionable causes are identified. 'Minimal cut set' approach is followed that identify the smallest combination of basic events which, if occur, will cause the top event to occur. With the help of subject matter experts, probabilities are assigned to each basic cause and they are prioritized. The last logical step is to evaluating the tree and brainstorm to find solutions to root causes so in effect the failure can be either controlled or eliminated.

 

Most effective at: 

In manufacturing industry, automotive or health care where large and complex systems of mechanical and structural components exist FTA is highly effective to analyze system/process/ structure reliability. For software bug analysis or risk assessment , FTA is proven to be immensely effective.

 

Least Useful at:

• It does not work well when human actions are inserted as a cause. This is because human failure rates have wide variance and this prevents accurate results.
• Construction of a FTA for a large complex system is often tedious. Basic shortcomings of FTA are often identified as (i) correlation between all basic events are difficult to model, (ii) an exact solution to correlated events does not always exist. Therefore, FTA is prone to subjective decisions.
• Because this technique is highly reliant on judgment and insight that are based on subjective opinions, there is a risk of erroneous information, which may cause erroneous result . If wrong failure sources are identified in an FTA, the subsequent results may face a ripple effect of this error. Results may, therefore, not be valid or accurate.
• FTAs may fail if the technique is not implemented in a disciplined fashion or if the system problem is so complex that multiple levels of potential causes exist for each problem type. When the system of focus for the FTA is very large, quantitative analysis software may be required to analyze the results.

 [Sources used: https://wesharescience.com , https://www.slideshare.net/ ,http://reliawiki.com ]

Fault Tree Analysis (FTA) is a logical, structured process that can help identify potential causes of system failure before the failures actually occur.

FTA has the same basic approach as the Root Cause Analysis. FTA allows a backward approach to systematically identify potential causes of failures, and it provides an overview of interrelationships between causes and effects and breaks down failures into more detail.

 

When to Apply FTA:

Applied any time during the life of a plant, system, subsystem, or equipment item

Primarily used to examine incidents or accidents whose consequences would be classified as catastrophic

Often initiated after a major hazard has been recognized for the first time

 

When not to apply FTA:

FTA can be least used when the definition of the undesired event can be very hard to catch.

Getting exact numbers for the probabilities leading to the event is usually impossible for the reason that it may be very costly and time consuming to do so

 

How FTA is Used:

Fault trees are used to determine the frequency of top events for which historic data is not available or applicable

Once the top event is defined, the fault tree is constructed by working downwards through the system, one level at a time, to determine what failures could occur which could lead to the top event

 

FTA and FMEA Compared:

FTA – focus on failure outcome – Examines the applicable components, processes and conditions retroactively to identify all possible contributing factors that could have worked alone or in combination to cause the outcome

FMEA – focus on each system component, and examines before-the-fact all things that could possible go wrong with that component

FTA is very good at showing how resistant a system is to single or multiple initiating faults. It is not good at finding all possible initiating faults. FMEA is good at exhaustively cataloging initiating faults, and identifying their local effects. It is not good at examining multiple failures or their effects at a system level.  FTA considers external events, FMEA does not.

FTA
What is Fault Tree Analysis? Where do you think it is most effective and in what circumstances is it likely to be least useful?

 

Background:
Fault Tree Analysis (FTA) introduced by BellLaboratories,are prevalently used in the fields of Reliability and Safety Engineering

Fault Tree Analysis:

As the name itself implies, this technique talks about the analysis of a failure or fault

 

Why its called Fault Tree:

The technique uses a top-down approach to analyze the potential reasons (causes) for the failure by stepping through a serious of steps logically.

 

To explain more on this, a general conclusion /an undesirable state called an event (top event), is made at the beginning (at the top) of the analysis, followed by specific causes that lead to the conclusion/undesirable state, by developing a logic diagram.  In other words, a visual representation of a system showing the logical relationship between the events and the causes that leads to the failure is created

 

Purpose of FTA:
1.    To help identify potential causes of system failure before the failures actually happen.
2.    Additionally, probability (of the occurrence) of the top event or the undesired state can be measured either statistically or analytically  

 

How does it work?
1.    Two types of basic diagrammatic notations: Events and logic gates
2.    Primary or basic event(s) is/are depicted with a circle.
3.    The symbols used in the logic diagram are called  logicgates , much akin to the symbols used in Digital Electronics
4.    AND Gate, OR gate, Exclusive OR gate(XOR) are the primary logic gates used
5.    The logic gates describe the Boolean relationships between the various event outcomes

 

Logic Gates:

1.     1. ‘OR’ Gate:  Given two or multiple causes (events) that could lead to a failure (top event), the failure can happen even if any one of the causes (events) occurs.

Truth Table:        Determines the outcome of the various input combinations either as true or false represented by ‘1’ and ‘0’ respectively

Cause 1 (Input A)	Cause 2 (Input	Outcome Formula =A+B
0	0	0
0	1	1
1	0	1
1	1	1

So the table implies that for the failure /event at higher level   to happen, one cause (input in the above table) is suffice 

 

2.       2. ‘AND’ Gate:  Given multiple causes (events) that could lead to a failure (top event), the failure can happen if and only if all the causes (events) occurs.

 

Cause 1 (Input A)	Cause 2 (Input	Outcome (Formula =A.B)
0	0	0
0	1	0
1	0	0
1	1	1

 

As the table shows, the failure/event at higher level happens , only if the 2 causes occur. In case of multiple inputs, then all have to occur.

 

3 3.       ‘Exclusive OR’ (XOR) Gate:  Given multiple causes (events) that could lead to a failure, the failure can happen if and if only one cause (event) occurs.

Cause 1 (Input A)	Cause 2 (Input	Outcome Formula = A.(~B)+ (~A). B
0	0	0
0	1	1
1	0	1
1	1	0

 

How does the formula work: ~ àgives the inverse of the condition. If ‘A’ =0, then ~A=1

So the table implies that for the failure /event at a higher level to happen,  only one can be a cause at a given time.

 

How is it different from other techniques such as Fishbone or 5-Whys:

While those techniques do a failure analysis on the damage or problem has occurred, FTA does identify potential causes for a failure to happen

Essential Steps of FTA:
1.    Define the undesirable state (top event) – the failure, for which the analysis is being done
2.    Drill down the immediate causes (which will become events if further  break-down is possible) for the undesirable state(top event)
3.    Repeat  # 2 until no further drill-down is not possible which means the most basic causes are found out
4.    Develop the fault tree diagram based on the above steps
5.    Do the evaluation of the developed FTA as what needs to be done
6.    Take necessary actions post the evaluation to avoid the failure and potential risks and make your system fool-proof

 

 Benefits of FTA:
1.    It prioritizes issues, the fixes of which can address the failure
2.    It acts as a base for future analysis
3.    It highlights the existing design flaws cum errors.
4.    Provides a simple and self-explanatory view of the causes for the failure (in a tree structure)

Industries where FTA is very much useful
FTA is useful in industries where failure can have severe impacts/consequences. Some of the industries where FTA can be very useful are:
a).  Nuclear Power
b).  Aeronautics
c).   Healthcare
d).  Mission Critical Systems across industries
e).  Debugging Complex systems in Software Engineering,
f).   Space Programmes

 

Eg:

 

Circumstances where FTA is least useful:
1.    Where probability of failures happening is known to be very less
2.     Where the impact/severity of failure is minimal or not relevant

Fault tree analysis: A technique by which many events that interact to produce other events can be related using simple logical relationship.

It is effective in Qualitative description of potential problems, Quantitative estimates of failures frequencies and helps to prioritize issues, 

 

Limitations:  Fault tree diagram construction is very tedious for large systems and also it is a back ward method which is used to think about consequences which may occur.

Fault tree analysis introduced by Bell Laboratories is a deductive failure analysis where we try to find the cause of any undesirable state of the system using Boolean logic.

 

It is mainly used in Safety and Reliability engineering to identify the reasons causing a system to fail, Risk analysis of system, and finding event rate of the system.

 

FTA is used in complex and hazardous processes of Aerospace sector, Pharma sector, chemical engineering, nuclear power sector and software engineering for debugging 

Fault tree analysis introduced by Bell Laboratories is a deductive failure analysis where we try to find the cause of any undesirable state of the system using Boolean logic.

 

It is mainly used in Safety and Reliability engineering to identify the reasons causing a system to fail, Risk analysis of the system, and finding event rate of the system.

 

FTA is used in complex and hazardous processes of Aerospace sector, Pharma sector, chemical engineering, nuclear power sector and software engineering for debugging 

 

Where do you think it is more effective

It is more effective for analysing faults of complex / Hazardous systems using boolean logic

 

Where do you think its least useful

Its least useful when you have to analyse the effect of a single component, equipment and subsystem. It is good at showing how a system is resistant to single/ multiple initiating faults but it cannot help in the exhausting analysis that is in analysing all the faults.