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
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