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Q8. What is Fault Tree Analysis? Where do you think it is most effective and in what circumstances is it likely to be least useful? 

 

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Fault Tree Analysis (FTA) is a graphical technique for Reliability and Safety Analysis of Systems. It is used:

  • to investigate potential faults
  • its mode and causes and
  • quantify their contribution to system unreliability in the course of product design.

The basic constructs in a fault tree diagram are gates (conditions) and events (causes leading to failure).

 

Fault tree diagrams are logic block diagrams that display the state of a system (top event) in terms of the states of its components (basic events).

An FTD is built top-down in term of events. It begins with the foreseeable, undesirable loss event (or a fault condition). Subsequently, it attempts to determine the specific causes (events) by constructing a logic diagram using a graphic model of the pathways within a system that can lead to the failure. Each cause is further broken down till a basic fault: human, hardware or software is reached. The pathways connect contributory events and conditions, using standard logic symbols (AND, OR, etc.).

 

Example of an FTD – The Root Causes of Hazard to Patients during surgery [1]

image.png.9bc9655cbfc861602f0cc6a5623b4850.png

 

The two most commonly used gates in a fault tree are the AND and OR gates.

OR Gate represent Logical Addition.

  • Even if one of the Inputs to an OR gate is “1” or “TRUE”, then the Output is “1” or “TRUE”.
  • If all the inputs are “0” or “FALSE”, then the Output is “0” or “FALSE”

AND Gate represents Logical Multiplication.

  • Even if one of the Inputs to an AND gate is “0” or “FALSE”, then the Output is “0” or “FALSE”.
  • If all the inputs are “1” or “TRUE”, then the Output is “1” or “TRUE”

 

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, efforts can be focused on improving system safety and reliability.

 

Situations where FTD is most effective:

 

  1. It works well to identify possible causal relationships in cases where Output has a Boolean (True/False) relation with inputs, especially in small and medium sized systems where all causes /events can be conceived.
  2. It can be used in situations where specific data regarding known failure rates of components is known.
  3. It is used to supplement Root Cause analysis on engineered systems, by reviewing assumptions and design decisions made during initial system design.

 

Situations where FTD is least useful:

  1. It is not effective in large complex systems as it is difficult to conceive all possible scenarios leading to the top event. The construction of fault trees can become very tedious and are prone to have errors.
  2. It does not function well as a Root Cause Analysis tool because FTD does not work well when some of the causes could be Human actions. This is because wide variance of possible human failure rates prevents FTD from providing accurate results.
  3. FTD is not very effective when there is event dependency or load sharing i.e. the occurrence of each event (cause) affects the probability of occurrence of the other events.

 

[1] http://asq.org/quality-progress/2002/03/problem-solving/what-is-a-fault-tree-analysis.html

 

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What is FTA??

Fault tree analysis (FTA) is a top down, deductive failure analysis in which an undesired state of a system is analyzed using Boolean logic to combine a series of lower-level events.

 

Purpose??

Like Failure Mode and Effect Analysis (FMEA), FTA is a method used to predict the failure modes / events of failures of functional / Process Parts and assessing the impact of those modes/ events on a system/ product/process. Though FMEA and FTA have marked differences in approach, the purpose is same holistically.

Fault tree analysis can be used to:

-       Understand the logic leading to the top event / undesired state.Basically, understand the cause of failure

-       To identify the effects of human errors

-       Show compliance with the (input) system safety / reliability requirements.

-       Prioritize the contributors leading to the top event - Creating the Critical Equipment/Parts/Events lists for different importance measures.

-       Monitor and control the safety performance of the complex system (e.g., is a particular aircraft safe to fly when fuel valve x malfunctions? For how long is it allowed to fly with the valve malfunction?).

-       Minimize and Optimize resources.

-       Assist in designing a system. The FTA can be used as a design tool that helps to create (output / lower level) requirements.

-       Function as a diagnostic tool to identify and correct causes of the top event. It can help with the creation of diagnostic manuals / processes.

  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 high hazard industries like aerospace, Pharmaceutical, petrochemical  and  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.

 

Structure:

Whereas FMEA is a “Bottom-Up” technique which examines the failure modes of the components within a system (i.e. the failure symptoms), and traces forward the potential effects of each component failure mode on system performance. As such it is a CAUSE => EFFECT model. FTA is the reverse of FMEA in that it is concerned with the identification and analysis of conditions (including component failures) that lead to the occurrence of a defined effect. In contrast with FMEA,  FTA is therefore a “Top-Down” technique, and so is an EFFECT(Undesired Event) => CAUSE model.

 

Both the above designs are, interchangeably, required for fault identification, correction and tolerance to improve design of process.

FTA is the logical model of a relationship of the undesired event to more basic events. Obviously, the top event of the fault tree is the undesired event, the middle events are the intermediate events and the events at the bottom are the basic events.

 

image.png.179aa653b8520ffe3c6a12e7275cf94f.png

 

The logic relationships of events are shown by logic symbols or gates

 

fault-tree.png

 

5 Step Procedure

1) Define top event

  • Define the undesired event

2) Define overall Structure

  • Define the intermediate events and combination of failure that will lead to the top event
  • Arrange them accordingly, using logical relationship

3) Explore each branch in successive level of detail

  • Continue the Top down process until the root cause for each branch is identified and/ or until further decomposition is not required
  • Thus, each branch will end with a basic event or an undeveloped event.
  • Consider common cause failure and systematic failures in the process of decomposition.
  • A good tip to stop decomposing is to go no further than physical or functional bounds set by the top event.

4) Solve the fault tree

  • Assign probabilities of failure to the lowest level event in each branch of the tree
  • From this data the intermediate event frequency and the top level event frequency can be determined using Boolean Algebra and minimal Cut Set Methods.

5) Perform Corrections if required and take decisions

  • Application of Boolean algebra and minimal cut set method will help to identify the BASIC EVENTS (A) and combination of Basic Events (B.C.D) that have major influence on the top effect.
  • This will give clear indication on what needs to be attended to and where resources need to be allocated for problem solving.

Advantages of FTA

  • Deals well with parallel, redundant or alternative fault paths
  • Searches for possible causes of an end effect, which may not have been seen before.
  • The cut sets derived in the FTA can give worthwhile insights into various ways top event occurs.
  • Very useful tool for focused analysis where analysis is required for one or two major outcomes.

Faults/ Disadvantages of FTA

  • Time-consuming in constructing the graphs
  • Requires a separate fault tree for each top event, hence making it difficult to analyse complex systems
  • Many Systems may involve many dependencies.
  • Difficult to detect inconsistencies
  • Difficult to focus on the most critical parts of the design on high coupling systems
  • Not always easy to spot when there many systems involved.
  • Fault trees developed by different individuals are different in structure, producing different cut set elements and results.
  • Sometimes the same event may appear in different parts of the tree , leading to confusion.

Circumstances when FTA is not used

  • When you suspect that a large number of distinct system conditions exist with a range of unacceptable consequences. FMEA will be more appropriate than FTA
  • When there is a need to analyse systems that contain little or no redundancy and does not examine the effects of multiple failures at the system level (apart from common cause failures). FMEA is more suited for this
  • When the system contains novel technology and the effects of failure of the components contained within the system need to be explored with insightful judgement, instead of FTA, consider using FMEA For any system FMEA is generally good for exhaustively identifying and recording the local effects within it that arise from component failures and then inferring the effects of those failures at system level.
  • When there is a need (a) to establish appropriate levels of redundancy within the design of a system, (b) to ensure “fail safe” outputs, (c) to increase the derating of components, or (d) to otherwise enhance the design generally- we must consider using FMEA
  • When you want to record the system effects of each failure cause in a tabular format as normally FTA enables the fault/failure logic within a system of a particular effect of interest to be represented in diagrammatic form

 

In summary

  • FTA will identify combinations of conditions and component failures which will lead to a single defined adverse effect
  • FMEA, on the other hand, considers all single component failures in turn and identifies the range of their effects on the system.

Either of the two methods of analyzing faults is required to enhance any process.

 

 

 

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Fault Tree Analysis was first introduced by Bell Laboratories and a popular technique for reliability, maintainability and safety analysis. It is a top-down deductive failure analysis technique used to determine the various combinations of events / causes that could cause undesired events. In simple words it is a technique to decompose a problem and identify situation that may lead to failure.

 

Usage Scenarios:

-       Understand the logic leading to top event or undesired state

-       Show compliance with input system safety / reliability requirements

-       Prioritize contributors to top event

-       Minimize and optimize resources

 

Steps for performing FTA:

-       Step 1: Define scope of analysis

-       Step 2: Define the fault condition and top level failure

-       Step 3: Identify possible causes for top level failure to occur

-       Step 4: Consider the relationship between elements to decide logic gates

o   or (one event can cause failure alone)

o   and (requires multiple events to result in failure)

o   exclusive or (output occurs if exactly one event occurs)

o   priority and (output occurs if events occur in a specific sequence specified by a conditioning event)

o   inhibit (output occurs if event occurs under an enabling condition specified by a conditioning event)

-       Step 5: Continue to breakdown each level by repeating Step 3 and 4

-       Step 6: Identify probability of the causes

-       Step 7: Analysis the fault tree

 

Illustration

image.png.c63bffbb49d594d384e00022d4fadd88.png

Practical applications

-       Hospital uses FTA to identify how incorrect prescriptions may be given through combination of events

-       Airplane parts manufacturer uses FTA to identify critical faults that could lead to hazardous failure

-       NASA uses FTA for risk and reliability analysis post Challenger accident

-       Used for process hazard analysis in high hazard process industry

-       Software industry uses FTA for debugging and software testing

 

Situations wherein FTA is not effective

-       Complex man-made systems with too many causes for failure

-       Scarce or insufficient data

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Fault tree analysis is logical analysis of root cause of a fault that has occurred.

 

Least useful - Since it is used to analyse faults that have occurred, it may not unearth any other potent faults in the system.

 

Most useful - When used during design of system to analyse probable faults and their causes (FMEA), and build in the mitigants.

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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 Bell
Laboratories,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  logic
gates , 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 B)

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 B)

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 B)

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:

image.png.fc2a6cf906ae04cfa63c3cdfe3804bab.png

 

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

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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.

 

Advantages of Applying FTA

FTA can be advantageous to software projects in at least three ways:

Value addition: FTA has the potential to serve as a defect-prevention tool. If FTA is performed before baselining the design, it can provide valuable information on application failures and their mechanisms. This information could be utilized to improve the design by preventing the potential defects or by introducing fault-tolerating abilities. FTA is most effective for more complex functions but may not be adding much value when applied to the simple functions of a software application. FTA utilizes the potential of teamwork to bring in a variety of ideas and broaden thinking.

Simplicity: FTA is very simple and can be prepared by project teams with minimum training. Its graphical presentation improves readability and makes it easy to maintain in the event of changes.

Traceability: Some of the conventional test case tools provide a unique identification to individual test cases. Such traceability could be added to FTA by appropriately identifying the individual scenario.

 

Fault tree analysis can be used to:

·         understand the logic leading to the top event / undesired state.

·         show compliance with the (input) system safety / reliability requirements.

·      prioritize the contributors leading to the top event - Creating the Critical Equipment/Parts/Events lists for different importance measures.

·    monitor and control the safety performance of the complex system (e.g., is a particular aircraft safe to fly when fuel valve x malfunctions? For how long is it allowed to fly with the valve malfunction?).

·         minimize and optimize resources.

·         assist in designing a system. The FTA can be used as a design tool that helps to create (output / lower level) requirements.

·         function as a diagnostic tool to identify and correct causes of the top event. It can help with the creation of diagnostic manuals / processes.

Limitation: A limitation of the fault tree analysis is that the undesired event evaluated must be foreseen and all significant contributors to the failure must be anticipated. This effort may be very time-consuming and expensive. And finally, the overall success of the process depends on the skill of the analyst involved.

 

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Fault tree analysis (FTA) is a top down, deductive failure analysis in which an undesired state of a system is analyzed using Boolean logic to combine a series of lower-level events.

Usage -

 

Fault tree analysis can be used to:

  • understand the logic leading to the top event / undesired state.
  • show compliance with the (input) system safety / reliability requirements.
  • prioritize the contributors leading to the top event - Creating the Critical Equipment/Parts/Events lists for different importance measures.
  • monitor and control the safety performance of the complex system (e.g., is a particular aircraft safe to fly when fuel valve xmalfunctions? For how long is it allowed to fly with the valve malfunction?).
  • minimize and optimize resources.
  • assist in designing a system. The FTA can be used as a design tool that helps to create (output / lower level) requirements.
  • function as a diagnostic tool to identify and correct causes of the top event. It can help with the creation of diagnostic manuals / processes.

Not Useful -

It can be used to find faults in current running processes...Where as it can not be used in inception or making of the product or process. It can not be used at the initial stage of product or process inception.

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FTA is top down failure analysis tool when system or process is in undesired state, the system or process is analysed using Boolean logic. This tool used mainly where high safety measure have to be implemented like aerospace, nuclear power, chemical and process, pharma, pertro and etc.
 
Why mainly the FTA used in Safety Hazard industry?
Failing to implement all safety measures can damage environment, loss of human life, can damage economy and reputation loss of county.
It is very critical to find all failures and failures causes and classify sever ones. and perform micros level analysis for all severe ones because.
 
FTA can be used in any process as a proactive/reactive condition even in safety hazard industries. finding failures is one of the critical activity for any process and to achieve first time right.
one can perform FTA at the time of process design, or one can perform when process is failed
performing FTA increases success rate of the process / system
Edited by ksrivasu76

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Fault Tree Analysis is a systematic analysis of a faulty condition and all the possible contributing factors broke down logically. This helps to make sure that all possible factors are explored and unearthed.

 

In the areas involved in managing inventory of large components classes, this would be handy to assess the optimal method to manage the inventory by figuring out possible risks involved in the inventory process. Areas like aerospace components inventory management.

Fault Tree Analysis will not work out in areas where there are interactions between contributing factors. These interactions cannot be brought out in FTA.

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Fault Tree Analysis is a technique where drill down is done to the lowest extent to dertermine the various causes why a particular process can fail. It implies reaching to the most granular level and understand what can cause the process to fail using analytical or statistical approaches.

 

The technique is most effective in circumstances where the slightest of mistake can spell doom eg: building a space craft. While building a space craft the slightest of error is unacceptable as there is a lot of money involved in making the space craft and human life will be in danger if something goes wrong with the space craft. Hence, this technique in most effective under such circumstances to keep an eye on all critical aspects of space craft built.

 

The Fault Tree Analysis would not be effective in circumstances where the data and information regarding the causes is insufficient or unknown.

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Fault Tree Analysis (FTA)

 

FTA is a top down, deductive- failure analysis. As a deductive approach, FTA starts with an undesired event, such as failure of a main system, and then determines its causes using a systematic, backward-stepping process. In determining the causes, a fault tree (FT) is constructed as a logical illustration of the events and their relationships that are necessary and sufficient to result in the undesired event, or top event. The logic relationship of events are shown by logic symbols or gates like AND or OR gate. Levels of a FTA are Top Undesired event which is broken down into Intermediate events which is further broken down into basic events.

 

FTA is most commonly used in system reliability, maintainability and safety analysis. FTA is useful both in designing new products/services and in dealing with identified problems in existing ones. 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 safety process improvement, it can be used to help identify root causes of undesired events such as occupational injuries and illnesses. If we have the quantitative date of failure probability, failure rate and repair rate we can also use FTA to evaluate the probability of occurrence of the top event using analytical or statistical methods. Aerospace engineering also has found applications of FTA.

 

FTA is least useful in analyzing complex systems, as it may require separate fault tree for each top event making it difficult to analyse. And mostly it relies on the tacit / subjective knowledge of the people involved in constructing the FTA and the level of detail and completeness will be limited.

 

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Fault is what a system has in it inherent. It’s a flaw in the design. One does realize it( at times not – which is bad).

Error, on the other hand is something where in an act deviates from the course. Actual needed against measured ones.

The Failure, is inability to perform to the requirements. Not conforming to what it set out to be.

Hence, when the system has a fault, it has every possibility of contributing an error which makes it a failure. All failures can be caused by faults ( since designed ), yet all faults do not result in a failure (cannot be lucky always !!!).

That’s for the background.

 

When we talk about Fault Tree Analysis, we will be using a fault in the system and try to dissect/analyze it by putting the same at top and graphically modeling all the way down. Consider it as a tree with a flow determined to arrive at a CAUSE from the EFFECT. It is deductive in approach.

FTA uses the conditions for the effect and establishes whether they are immediate, necessary and sufficient. Its more logical in approach and depicts an algorithm which is ruled by the clause - if yes, then true. It can be explained as a path of base events and intermediate events at the bottom leading to the fault to expose and subsequent failure thereafter. FTA suggests the states where the system is least reliable or otherwise.

Example : The unfortunate explosion of Challenger shuttle in 1986 and then the FTA on the mishap. The cause apparently was human error of neglecting the temperature that day near the launch pad and the punctured O rings leaking the gases.

 

Primarily used to those system conditions which are a threat and relatively lesser in number to handle.  It helps one to understand the sensitivity to the violation of assumptions from the systems standpoint to the faults inherent. Systems which are complex and with well acknowledged faults can prompt one to use FTA.

It might not serve the individual when all the undesirable events are not foreseen. Each cause should have two modes and are to be independent. The other tools at disposal are FMEA, Event tree Analysis, What-If Analysis, Checklist etc.

 

Depending on the principle of mistake proofing (Elimination, Replacement, Prevention, Detection, Facilitation and Mitigation) one deploys, the corresponding tool should assist the purpose.

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Fault tree analysis (FTA) is a top down, deductive failure analysis in which an undesired state of a system is analyzed using Boolean logic to combine a series of lower-level events. It is a logical, graphical diagram that organizes the possible element failures and combination of failures that leads to the top level fault being studied.

 

With every product, there are numerous ways it can fail. Some more likely and possible than others. The FTA permits a team to think through and organize the sequences or patterns of faults that have to occur to cause a specific top level fault. The top level fault may be a specific type of failure, say the car will not start. Or it may be focused on a serious safety related failure, such as the starter motor overheats starting a fire. A complex system may have numerous FTA that each explore a different failure mode.

 

The primary benefit of FTA is the analysis provides a unique insight into the operation and potential failure of a system. This allows the development team to explore ways to eliminate or minimize the occurrence of product failure. By exploring the ways a failure mode can occur the changes impact the root cause of the potential failures.

 

FTA is used extensively to analyze the reliability and safety of complex systems such as nuclear power plants and weapon systems.  It identifies faults in a system design that may cause potential accidents and helps to eliminate costly design changes and retrofits. Typically it can be used in any field where failure needs to be analysed. Like in a hospital where incorrect prescriptions may be given or in a company where personnel evaluation system is not effective or an airplane parts manufacturer performs FTA as a standard part of the design process to identify critical faults which could cause hazardous failure. When identifying ways in which an item may fail, one should try looking at the problem from different angles. Like Excessive stresses and strains, Potential misuse and abuse, Environmental extremes, Natural variation in the system, Failure of dependent systems, Failure of related processes.

 

In some circumstances, it is desirable to ensure the system continues to operate even if there is an internal failure. An aircraft navigation system should be able to operate even if an internal dc-dc regulator fails, for example. Not everything within some systems benefits by being fault tolerant .For example, a failure of a cabin reading light over a passenger seat is not critical to the safe operation of the aircraft, thus is likely not created to be fault tolerant. One criterion to determine what should be fault tolerant is the criticality of the function the system provides. This also applies to specific subsystems within a system allowing some elements to be created fault tolerant and others within the system not.Evaluating each element may help determine the specific elements that may benefit with fault tolerance. How critical is the component to the essential function or how likely is the component to fail? If an element is very unlikely to fail, the effort to create a fault tolerant system may be wasting resources better used to improve other elements of the system. Cost is another factor.

 

Thus for 911 emergency services phone systems, some banking and commerce system, safety-critical systems of aircraft, public transit, or chemical plants systems fault tolerance is essential but where the decision to add some level of fault tolerance is not so clear it requires careful consideration of the costs, benefits, and criticality.

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What is FTA ?

Fault tree analysis (FTA) is a tool originally developed in 1962 by Bell Labs for use in studying failure modes in the launch control system of the Minuteman missile project. The tool now finds wide use in numerous applications from accident investigation to design prototyping and is also finding use for protection and control related applications.

Fault tree analysis is a top-down approach to identify all potential causes leading to a defect. Each cause is further broken down into least possible events or faults. The analysis begins with a major defect. All the potential events – individual or in combination – that may cause the defect are identified. Potential events are further traced down in a similar way to the lowest possible level. A fault tree diagram is used to conduct fault tree analysis (or FTA). Fault tree analysis helps determine the cause of failure or test the reliability of a system by stepping through a series of events logically.

 

Purpose of FTA:

The main purpose of the fault tree analysis is to help identify potential causes of system failures before the failures actually occur. These calculations involve system quantitative reliability and maintainability information, such as failure probability, failure rate and repair rate

 

Difference between FMEA & FTA:

 

FMEA (Failure Mode Effect Analysis)

FTA (Fault Tree Analysis)

FMEA is a “Bottom-Up” technique which examines the failure modes of the components within a system and identifies the potential effects of each component failure mode. As such, it is a CAUSE => EFFECT model.

FTA is the reverse of FMEA in that it is concerned with the identification and analysis of conditions including component failures that lead to the occurrence of a defined effect. As such, it is an EFFECT => CAUSE model.

FMEA will be more appropriate than FTA when you suspect that a large number of distinct system conditions exist with a range of unacceptable consequences.

Consider using FTA rather than FMEA when you are particularly concerned about one or just a few system conditions that pose unacceptable consequences.

FMEA is more suited to analysing systems that contain little or no redundancy and does not examine the effects of multiple failures at system level (apart from common cause failures).

FTA is very good at showing how robust a system will be to one or more initiating faults. Thus for systems with high levels of redundancy FTA will be more appropriate.

FMEA records the system effects of each failure cause in a tabular format.

FTA enables the fault/failure logic within a system of a particular effect of interest to be represented in diagrammatic form

FMEA implicitly considers all adverse effects that may occur as a result of any single failure.

FTA focuses on a defined adverse effect.

 

In summary

  • FTA will identify combinations of conditions and component failures which will lead to a single defined adverse effect.
  • FMEA on the other hand considers all single component failures in turn and identifies the range of their effects on the system.

 

Usefulness and Advantages of using FTA

The biggest advantage of using FTA is that it starts from a top event that is selected by the user for a specific interest and the tree developed will identify the root cause. The FTA has the ability to be used with computer and generate results using computer applications for improved analysis.

 

A fault tree creates a visual record of a system that shows the logical relationships between events and causes that lead to failure. It helps others quickly understand the results of your analysis and pinpoint weaknesses in the design and identify errors.

 

A fault tree diagram will help prioritize issues to fix that contribute to a failure. In many ways, the fault tree diagram creates the foundation for any further analysis and evaluation.

 

For example, when changes or upgrades are made to the system, you already have a set of steps to evaluate for possible effects and changes. You can use a fault tree diagram to help you design quality tests and maintenance procedures

 

Fault tree analysis is useful in engineering, especially in industries where failure can have huge consequences such as nuclear power or aeronautics. However, fault tree analysis can also be used during software development to debug complex systems.

 

Effectiveness of FTA

FTA is a very effective risk assessment tool but when it comes to a reasonably complex system, that includes a large number of equipment and process variables, the fault tree becomes enormous and takes quite of a time to be completed. A team of engineers works over it and even then it may take years to complete without the surety of weather all the failure possibilities are considered or not. The concept of partial failure in a fault tree does not exist. If the equipment is partially working it is considered as fully unavailable or in failure mode. This partial failure changes the reliability of a system but the FTA has no effect of such condition in its results. If a fault tree is developed by different safety professionals, it will be of different nature depending on the developer. This makes the fault tree a non-generic or inexact in nature.

 

Circumstances where it is likely to be least useful

Fault trees represent a deductive approach to determining the causes contributing to a designated failure. The approach begins with the definition of a top or undesired event, and branches backward through intermediate events until the top event is defined in terms of basic events.

 

A basic event is an event for which further development would not be useful for the purpose at hand. For example, for a quantitative fault tree, if a frequency or probability for a failure can be determined without further development of the failure logic, then there is no point to further development, and the event is regarded as basic.

 

Last but not the least ! Working in the service industry for so many years and using FMEA as a tool to identify cause & effect relationship, it was a good oppurtunity to deep dive and learn on a concept which is not widely talked about and also not in many curriculum of Black Belt or Master Black Belt Programs.

 

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Fault Tree Analysis.docx

 

Fault Tree Analysis (FTA)

 

FTA is a top down approach to problem solving.it means that you are taking a top level view of a process or a product , identifying  a potential failure or undesired event  .it uses deductive reasoning

 FTA is a methodology used to determine the probability that an unwanted event will occur .

FTA consists of a Boolean logic gates such as AND,OR,NOR,XOR ETC.

In FTA  we build a visual failure analysis tree diagram .the top of the tree is undesired result below this are potential causes leading to the undesired result representing the gates.

The technique starts with a failure and works backward toward s the root cause.

 

USES – FTA

 To predict failure events and potential causes.

To analyse a complete system containing numerous interconnected cause of failure

To identify causes of a failure after it has happened.

To identify causes of a potential system failure during the design process

 

Best Used in Industry-

Its best used in manufacturing ,electrical  risk management ,Aerospace, Pharma projects and nowadays in software industries.

Applicable to a system  when undesired event is suspected of being caused by more than one pathway.

 

Least used in industry

FTA is least used where the number of effects are high.

Not applicaple when undesired events can be causedonly by external events.

Not applicable when undesired events cannot be stopped,slowec or mitigated

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Fault tree analysis (FTA) is a top down analysis in which a failure in a system is analyzed to combine a series of smaller causal events. This analysis method is mainly used in the fields of engineering where system availability and reliability is very crucial to understand how systems can fail & to identify the best ways to reduce risk or to determine. 

FTA is most effective where fatality rate or mortality rate is high like in the Pharmaceuticals where studies are done with humans to patent a drug &  aerospace, defense etc.....

FTA is least effective in industries like Telecom, Retail etc... 

 

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FTA:- Fault tree analysis

A technique by which many events that interacts to produce other events can be related using simple logical relationship.

FTA is a technique generally used in reliability and safety analysis.

Bell Telephone Laboratories developed the concept in 1962 for the US Air Force for use with the Minuteman system. It was later adopted and extensively applied by the Boeing Company. Fault tree analysis is one of many symbolic "analytical logic techniques" found in operations research and in system reliability.

FTA is a Top down approach

Fault tree diagrams represent the logical relationship between sub-system and component failures and how they combine to cause system failures. The TOP event of a fault tree represents a system event of interest and is connected by logical gates to component failures known as basic events.

Limitation:-

            1.       Difficult to conceive all possible scenarios leading to the top of event

            2.       Making an FTA for large system can be tedious, as we will require a separate fault tree for each top event.

            3.       Fault trees developed by different users are generally different in structure. Which leads to confusion

            4.       Same event may occur in different part of tree, leading to some confusion.

           5.       Correlation between basic events are difficult to model and exact solution to correlated event does not exist

           6.       Subjective decisions regarding the level of details and completeness are often necessary.

           7.       Can be costly in time & effort

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FTA is a deductive analysis approach for resolving an undesired event into its causes through logic diagram and Bollean algebra. Same is developed by Bell laboratories.

 

A logic diagram called Fault tree is constructed to show the event relationship.
Probability of occurrence values are assigned to the lowest events in the tree in order to obtain the probability of occurrence of the top event.
 
 
 

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Fault Tree Analysis

 

Fault Tree concept was introduced by Bell Telephone Labs and later adopted and extensively used 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).

 

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. 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. The RBD equivalent for this configuration is a simple series system with two blocks, A and B, as shown next. 

 

Fault tree diagram can be used to model and analyze similar types of logical configurations required for system reliability and related analyses.

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FTA helps to know the causes of failure. It is a top down failure analysis. In this  analysis system, series of step by step events happen to test the system reliability. It also helps to identify the impact of human errors. The logical relationship of events are shown by logic symbols or gates.

 

It is very useful in software development field, safety analysis etc.

 

It is not very useful for multiple top events. Because for each top event, seperate FTA required. This increases the complexity. 

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To Design the System that work correctly we often need to understand and correct how they can go wrong”

 

The Fault Tree Analysis, briefly FTA, was developed originally in the sixties in the area of the American telecommunication industry and aircraft industry. Only in the beginning of the eighties a formalization of these methods occurred at the instigation of American authorities. The Fault Tree Analysis is used for reliability- and safety security analyses. The proceeding is very similar to the Reliability Block Diagram RBD. The aim is to determine possible combinations of causes which can lead to certain undesirable events (event), the so-called top level events.

Fault tree diagrams represent the logical relationship between sub-system and component failures and how they combine to cause system failures. The TOP event of a fault tree represents a system event of interest and is connected by logical gates to component failures known as basic events.

After creating the diagram, failure and repair data is assigned to the system components. The analysis is then performed, to calculate reliability and availability parameters for the system and identify critical components.

 

Methodology (Rules)

1.   The “Immediate, Necessary & Sufficient”

2.    Rule 2. The “Clear Statement”

3.   Rule 3. The “No Miracles”

4.   Rule 4. The “Complete-the-Gate”

5.   Rule 5. The “No Gate-to-Gate”

6.   Rule 6. The “Component or System Fault?” Rule M

 

Benefits and limitations

 Prepared in early stages of a design and further developed in detail concurrently with design development.

• Identifies and records systematically the logical fault paths from a specific effect, to the prime causes

• Allows easy conversion to probability measures

 • But may lead to very large trees if the analysis is extended in depth.

• Depends on skill of analyst

• Difficult to apply to systems with partial success

 • Can be costly in time & effort

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Fault Tree analysis is a technique used for analysis of System Reliability, Maintainability and safety analysis. It is a deductive procedure for determining the various combinations of hardware and software failures and human errors that could potentially cause an undesired event or events. Such events are called as Top events.

 

It is also used to evaluate of a occurrence of a top event using analytical and statistical methods

 

Advantages:

 

Deals well with parallel, redundant or alternative fault paths

Searches for possible causes of an end effect which may not have been foreseen

The cut sets derived in FTA can give enormous insight into various ways a top event occurs

Useful for focused analysis of one or two major outcomes

 

Disadvantages:

Requires a separate fault tree for each top event and makes it difficult to analyse complex systems

Fault trees developed by different individuals are usually different in structure producing different cut set elements and results

Same event may appear in different parts of the tree leading to confusion

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Fault tree analysis is a type of Tree diagram.As known tree diagrams are useful in situations where we want to discover or define a hierarchical relationship between events-desirable or undesirable.A FTA is constructed to relate an undesirable "top event" or failure to a sequence of events that led to the top event.In other words, the FTA depicts logical pathways from set of basic causal events to a single undesirable result or top event.We typically use logical operators, such as AND or OR gates,to connect lower level events with higher events.Hence,once the logic has been described, quantification can take place and risk level can be assessed.The steps involved are:.    

 

1.Identify the top event.                        

2. Identify the next level events.        

3.Develop logical relationships between the top and next level events.                                    

4.Identify and link lower level events.                                    

5.Quantify the FT.                                          

 

A fault tree does not contain all possible failure modes or all possible fault events that could cause system failure.Howevee ,an FT is capable of considering human error,hardware and software failures and act of nature.It has got widespread usage in the field of reliability,safety and risk analysis.The FT is a more focused tool than the FMEA.FTA works well for independent events.

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What is Fault Tree Analysis?
    Represents the logical relationship between sub-system and components failures and how they combine to cause system failures. 
The TOP event of a fault tree represents a system event of interest and is connected by logical gates to component failure know as basic events


FTA analysis involves five steps:


*Define the undesired event to study 
*Obtain an understanding of the system 
*Construct the fault tree 
*Evaluate the fault tree 
*Control the hazards identified 

 

Where do you think it is most effective?


FTA functions as a diagnostic tool to identify and correct causes of the top event. It can help with the creation of diagnostic manuals / processes. FTA goes from product failure to Part failure & FMEA goes from part failure to Product failure. FTA examines system from TOP to DOWN, 

FTA is used to Investigates potential faults, Its modes & causes and quantify their contribution to system unreliability in the course of product design.


Methodology (Rules)
1. The “Immediate, Necessary & Sufficient” Rule
2. The “Clear Statement” Rule
3. The “No Miracles” Rule
4. The “Complete-the-Gate” Rule
5. The “No Gate-to-Gate” Rule
6. The “Component or System Fault?” Rule

 

Methodology (Rules - 1) – Immediate, necessary and sufficient cause
Immediate - Closest in space, time and derivation of the event
Necessary - There is no redundancy in the statement or gate linkage
The event could not result from a sub set of the causal.
Sufficient - The events will, in all circumstances and at all times, cause
the event 

 

Methodology (Rules - 2) – The clear statement rule
Write event box statements clearly, stating precisely what the event is and when it occurs.

 

Methodology (Rules - 3) – The ‘component or systems fault’ rule
If the answer to the question:  “Can this fault consist of a component failure?” is Yes,
– Classify the event as a “State of component fault”
If the answer is No, – Classify the event as a “state of system fault”

 

Methodology (Rules - 4) – No miracles rule
If the normal functioning of a component propagates a fault sequence, then it is assumed that the component functions normally.

 

Methodology (Rules - 5) – the complete gate rule
All inputs to a particular gate should be completely defined before further
analysis of any one of them is undertaken

 

Methodology (Rules - 6) no gate to gate rule
Gate inputs should be properly defined fault events, and gates should not be directly connected to other gates.

 

Benefits and limitations


*Prepared in early stages of a design and further developed in detail concurrently with design development.
*Identifies and records systematically the logical fault paths from a specific effect, to the prime causes
*Allows easy conversion to probability measures.
*But may lead to very large trees if the analysis is extended in depth.
*Depends on skill of analyst.
*Difficult to apply to systems with partial success.
*Can be costly in time & effort.
"FTA is normally only performed for more serious undesired events.

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Fault tree analysis is a method used to analyse any defects or accidents or undesired event that happens in an industry. This method looks like an inverted tree. That is why it is named as fault tree.

In many cases there are multiple causes for an accident or other loss-making event. Fault tree analysis is one analytical technique for tracing events which could contribute for an accident or other loss making event.

It can be used

i.                     In an accident investigation

ii.                   In a detailed hazard assessment

The fault tree is a logic diagram based on the principle of multi-causality which traces all branches of event which could contribute to an accident or failure.

A fault tree creates a visual record of a system that shows the logical relationships between events and causes lead that lead to failure. It helps others quickly understand the results of your analysis and pinpoint weaknesses in the design and identify errors.

A fault tree diagram will help prioritize issues to fix that contribute to a failure.

In many ways, the fault tree diagram creates the foundation for any further analysis and evaluation.

For example, when changes or upgrades are made to the system, you already have a set of steps to evaluate for possible effects and changes.

You can use a fault tree diagram to help you design quality tests and maintenance procedures.

 

It uses sets of symbols, label and identifiers displayed and described as below :

image.png.2c82f86f1732cc84b8b27feaa7d92920.png

 

There are two basic types of fault tree diagram notations: events and logic gates. The primary or basic failure event is usually denoted with a circle. An external event is usually depicted with a symbol that looks like a house. It's an event that is normal and guaranteed or expected to occur. Undeveloped event usually denotes something that needs no further breakdown or investigation or an event for which no further analysis is possible because of a lack of information. A conditioning event is a restriction on a logic gate in the diagram. These gate symbols describe the Boolean relationship between outcomes.

Ø  Gate symbols can be the following:

Ø  OR gate - An event occurs as long as at least one of the input events takes place

Ø  AND gate - An event occurs only if all input conditions are met

Ø  Exclusive OR gate - An event occurs only if one of the input conditions is met, not if all conditions are met

Ø  Priority AND gate - This is probably the most restrictive scenario when an event occurs only after a specific sequence of conditions

Ø  Inhibit gate - An event will only occur if all input events take place as well as whatever is described in a conditional event

 

Approach :

Ø  A fault tree diagram is drawn from the top down

Ø  The starting point is the undesired event

Ø  The immediate contributory fault conditions leading to that event is then logically worked out and drawn.

Ø  These may each in turn be caused by other faults and so on.

Ø  The trickiest part of the whole thing is actually getting the sequence of failure dependencies worked out in the first place.

 image.png.c50f085d94db6eceab6bc689681f6c13.png

 

In the above Fault tree analysis, the undesired event is “Bridge construction getting delayed”. This event is found to be caused by either of “unavailability of construction material”, “unfavourable weather conditions” or “equipment problems”. Hence these three events are connected to the main event through an “OR gate”. That is any of the three events, individually, can cause delay in the bridge construction.

Further

“Unavailability of construction material” is drilled down and found to be caused by basic events  “Scarcity of raw material” or “shortage of funds”.

Unfavourable weather conditions cannot be drilled down further and hence have no basis events leading to it.

“Equipment problems” was found to be related to other two events (equipment damage or unavailability of equipment) through an OR gate.

“Equipment damage” was then found to be caused by either “substandard equipment” OR “Operating technicians not available”.

“Unavailability of equipment” was found to be caused by either “equipment not procured” OR “equipment not received from the supplier”.

Thus from this analysis, it was possible to get to the root cause/causes that was/were leading to intermediary undesired events which in turn were leading to the main undesired event.

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