2. Build the fault tree

In hazard studies, the part of the bow-tie located upstream of the dreaded event is generally made up of a tree structure on which :

• all causes and combinations of causes are represented;

• prevention barriers are represented symbolically (by a vertical bar that opposes the accident sequence).

As a result, this tree structure does not strictly speaking correspond to a fault tree, since barrier failures are not explicitly represented. An example of a simplified representation is shown in the figure "Simplified fault tree for the dreaded event: Chlorine leakage during wagon unloading".