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We are interested in analysing the
different sequences of events an external grid blackout could cause.
| The thumbnail on the right (click to open) depicts an event three with
four columns for the event tree itself and four more for result and
sequence information. |
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Sequences loss of
the external.
From the left, the first column represents the initiating event,
which in this case is loss of the external grid. The following three
columns represent function events. The function events represent events
that you have decided will be of importance for the development of
sequences of events and their consequences.
In this example, input to the function events are the basic events
representing failure of the diesel generator, the isolator switch and the
connection switch.
The isolator switch isolates the diesel and the 415V bus in case of a
blackout.
You are able to split (branch) the line beneath the columns underneath
each function event. This illustrates success (straightforward or up) or failure
(down) of the function events.
In this example the function events represent failures that, given the
initiating event, are enough to cause the consequence "No power on the
415V bus". Therefore, in this case, the sequences branch only once.
For example, if the diesel has failed it doesn't matter if the switches
fail or not. You could of course insert branches underneath every function
event. This would, however not give you any more information.
You are able to calculate the frequency for each sequence as well as a set
of sequences by assigning them the same consequence.
A sequence and consequence
analysis of this event tree model will produce the following results.
Described above are the very basics of event tree modeling. You can
integrate your fault tree model in an event tree by assigning fault tree
gates as input to your function events. Using this possibility you can
assign whole fault tree models of your safety systems as input to your
function events. You are then free to set any initiating event you can
think of, model what sequences this event could generate, and analyze the
consequences they lead to.
You also have the
possibility to assign a gate in your fault trees or a consequence in
another event tree as initiating event. Some applications, e.g.
RiskSpectrum PSA Professional, also offer the possibility to branch more
than two sequences in one branch-point (multiple input for function
events).
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