Fillable Printable Fault Tree Analysis Sample Template
Fillable Printable Fault Tree Analysis Sample Template
Fault Tree Analysis Sample Template
Fault Tree Analysis
HS02-015B (9-06)
Goal
This training program will introduce the basics of Fault Tree
Analysis (FTA).
Objective
The individual will demonstrate understanding of Fault Tree
Analysis principles.
Background
Engineers developed FTA to improve the safety of missile sys-
tems. They reasoned most accidents/incidents result from failures
or malfunctions within a system. A system consists of people,
equipment, material, and environmental factors. This system per-
forms specifi c tasks using prescribed methods. The components
of a system and its environment are interrelated, and a failure in
any part can affect the other parts.
A negative event can be a near miss or an incident that could
have resulted in personal injury to an employee or equipment/
property damage.
Analytical Trees
Analytical trees are graphic representations or pictures of
a project or event. They use deductive reasoning in that they
start with a general top event or output event and develop down
through the branches to specifi c input events that must occur in
order for the output to be generated. Analytical trees are called
trees because their structure resembles a tree, narrow at the top
with a single event symbol and then branching out as the tree is
developed.
Fault Trees
Negative analytical trees or fault trees are excellent trouble-
shooting tools. They can be used to prevent or identify failures
prior to their occurrence, but are more frequently used to analyze
accidents or as investigative tools to pinpoint failures. When an
accident or failure occurs, the root cause of the negative event
can be identifi ed.
Each event is analyzed by asking, “How could this happen?” In
answering this question, the primary causes and how they interact
to produce an undesired event are identifi ed. This logic process
continues until all potential causes have been identifi ed.
Throughout this process, a tree diagram is used to record the
events as they are identifi ed. Tree branches stop when all events
leading to the negative event are complete.
Symbols are used to represent various events and describe
relationships:
And gate - represents a condition in which all the
events shown below the gate (input gate) must be pres-
ent for the event shown above the gate (output event) to
occur. This means the output event will occur only if all
of the input events exist simultaneously.
Or gate - represents a situation in which any of the
events shown below the gate (input gate) will lead to
the event shown above the gate (output event). The
event will occur if only one or any combination of the
input events exists.
There are fi ve types of event symbols:
1. Rectangle - The rectangle is the main building
block for the analytical tree. It represents the negative
event and is located at the top of the tree and can be
located throughout the tree to indicate other events
capable of being broken down further. This is the only symbol
that will have a logic gate and input events below it.
2. Circle – A circle represents a base event in the
tree. These are found on the bottom tiers of the tree
and require no further development or breakdown.
There are no gates or events below the base event.
3. Diamond – The diamond identifi es an
undeveloped terminal event. Such an event is one
not fully developed because of a lack of informa-
tion or signifi cance. A fault tree branch can end
with a diamond. For example, most projects require personnel,
procedures, and hardware. The tree developer may decide to
concentrate on the personnel aspect of the procedure and not the
hardware or procedural aspects. In this case the developer would
use diamonds to show “procedures” and “hardware” as undevel-
oped terminal events.
4. Oval – An oval symbol represents a special
situation that can only happen if certain circum-
stances occur. This is spelled out in the oval
symbol. An example of this might be if switches
must be thrown in a specifi c sequence before an action
takes place.
5. Triangle – The triangle signifi es a transfer of a
fault tree branch to another location within the tree.
Where a triangle connects to the tree with an arrow, everything
shown below the connection point transfers to another area of
the tree. This area is identifi ed by a corresponding triangle that
is connected to the tree with a vertical line. Letters, numbers
or fi gures identify one set of transfer symbols from another. To
maintain the simplicity of the analytical tree, the transfer symbol
should be used sparingly.
Texas Department of Insurance
Division of Workers’ Compensation
Workplace & Medical Services, Outreach & Education
Fault Tree Analysis
FTA involves the following steps:
1. Defi ne the top event.
2. Know the system.
3. Construct the tree.
4. Validate the tree.
5. Evaluate the tree.
6. Study tradeoffs.
7. Consider alternatives and recommend action.
Defi ne the top event. To defi ne the top event the type of fail-
ure to be investigated must be identifi ed. This could be whatever
the end result of an incident may have been, such as a forklift
overturning.
Determine all the undesired events in operating a system.
Separate this list into groups having common characteristics.
Several FTAs may be necessary to study a system completely.
Finally, one event should be established representing all events
within each group. This event becomes the undesired event to
study.
Know the system. All available information about the system
and its environment should be studied. A job analysis may prove
helpful in determining the necessary information.
Construct the fault tree. This step is perhaps the simplest
because only the few symbols are involved and the actual con-
struction is pretty straightforward.
Principles of constructionPrinciples of construction. The tree must be constructed using
the event symbols listed above. It should be kept simple. Main-
tain a logical, uniform, and consistent format from tier to tier.
Use clear, concise titles when writing in the event symbols. The
logic gates used should be restricted to the and gate and or gate
with constraint symbols used only when necessary. An example
would be the use of the oval constraint symbol to illustrate a
necessary order of events that must happen to have an event
occur. The transfer triangle should be used sparingly if at all.
The more the transfer triangle is used, the more complicated the
tree becomes. The purpose of the tree is to keep the procedure as
simple as possible.
Validate the tree. This requires allowing a person knowl-
edgeable in the process to review the tree for completeness and
accuracy.
Evaluate the fault tree. The tree should then be scrutinized
for those areas where improvements in the analysis can be made
or where there may be an opportunity to utilize alternative proce-
dures or materials to decrease the hazard.
Study tradeoffs. In this step, any alternative methods that
are implemented should be further evaluated. This will allow
evaluators to see any problems that may be related with the new
procedure prior to implementation.
Consider alternatives and recommend action. This is the last
step in the process where corrective action or alternative mea-
sures are recommended.
Example
Benefi ts: The primary advantages of fault tree analyses are
the meaningful data they produce which allow evaluation and
improvement of the overall reliability of the system. It also evalu-
ates the effectiveness of and need for redundancy.
Limitation: A limitation of the fault tree analysis is that the
undesired event evaluated must be foreseen and all signifi cant
contributors to the failure must be anticipated. This effort may be
very time consuming and expensive. And fi nally, the overall suc-
cess of the process depends on the skill of the analyst involved.
Primary
Cause
Logic Gates
Accident/
Incident
Events
Example
Safety Violations Hotline
The Texas Department of Insurance,
Division of Workers’ Compensation (TDI/DWC)
Resources
The Texas Department of Insurance, Division of Workers’
Compensation (TDI/DWC) Resource Center offers a
workers’ health and safety video tape library. Call (512)
804-4620 for more information or visit our web site at
www.tdi.state.tx.us.
Disclaimer: Information contained in this training program
is considered accurate at time of publication.
Quiz Answers
1. B
2. C
3. D
4. C
5. F The purpose of the fault tree analysis is to illustrate
a procedure or event in a way that can be visualized and if it is
complicated, it will no longer be an effective tool.
Review Questions
1. What is meant by the term “negative event?”
a. A political rally for a negative candidate.
b. A near miss, or accident that results in personal injury or
property damage.
c. An automobile race where all the vehicles drive in reverse.
d. A series of events that if there are two together they will
actually make a positive event.
2. What is a benefi t of the fault tree analysis?
a. It offers shade on a sunny day.
b. You can determine the amount of money it will cost to
implement a procedure.
c. You will get meaningful data about the overall reliability of
the system.
d. It takes little time and expense to create.
3. What does the circle symbol used in the formation of the tree
represent?
a. The negative event that will be studied.
b. The transfer of a branch to another location on the tree.
c. A terminal event that doesn’t need to be developed further.
d. A base event with no events below it.
4. What was the fault tree analysis created to do?
a. To evaluate the production of baked goods.
b. To increase the safety level of the space program.
c. To improve the safety of missile systems.
d. To illustrate how to safely set up a Christmas tree.
5. A fault tree analysis should be as complicated as possible. T/F
