A system consists of people,equipment, material, and environmental factors. This system performs specific 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. FTA reasoned most accidents/incidents result from failures
or malfunctions within a system.
Method.
1. Define 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.
Example:
1. And gate - represents a condition in which all the
events shown below the gate (input gate) must be present
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.
2. 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.
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 information
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 undeveloped
terminal events.
4. Oval – An oval symbol represents a special
situation that can only happen if certain circumstances
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. 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.
