Fault Tree Analysis (FTA)
Fault Tree Analysis is a graphical, deductive method for identifying the combination of factors
such as equipment failure and human errors that canresult in the occurrence of the hazardous
scenarios (top event) (IEC, 2008; Sutton, 2010). Additionally, the probability of the top event
can be estimated by failure rate calculation of each individual component. FTA is an efficient
method to analysis the complex systems. This methodis often used in combination with other
CHALMERS, Product and Production Development, Master’s thesis 2011 29
hazard analysis method such as HAZOP, when a hazardous scenario is identified and required
further investigation.
The Fault Tree is built up of gates and events. Or Gate gives a positive outcome if one or more of
the inputs are positive. In contrast for And Gate, all the inputs to an And Gate need to be positive
for the output to be positive. The Voting Gate has at least three inputs; two or more of which
need to be positive for the outcome to be positive (IEC, 2008; Sutton, 2010).
There are three events in Fault Tree Analysis, Top Event, Intermediate vents, and Basic events.
As it is mentioned before, Fault Tree Analysis is used to identify how a hazardous scenario is
caused; this undesirable event is referred to the Top Event. To definition of top event the
following questions should be included: how much? How long? What is the safety impact? What
is the environmental impact? What is the productionimpact? What is the regulatory impact? The
intermediate event is defined to further development of the analysis of top event. Intermediate
events are developed through the OR and AND Gates to finally become the base event. The
Base Event cannot be developed further. Once the tree has been constructed, it can be quantified
(Sutton, 2010)