2.2. Barrier concept
The barrier concept first appeared in the work of Haddon
[6,7], is prominently present in the MORT tree [8] but
has been elaborated lately in particular by Hollnagel [9,10].
The ‘classical’ barrier concept presumes a hazard (a dangerous
source of energy) and a target (a vulnerable object, like
humans, animals or the environment), which is protected by
the barrier. Obviously, this barrier is physical in nature; otherwise,
it cannot protect the target. Distance is also considered
a barrier in this context, and when the energy level is high,
the critical distance must be large.
However, in the context of fault and event trees and the
notion of ‘defence in depth’, the barrier concept has been
stretched comprising also barriers that in themselves cannot
protect the target from the hazard, but are part of a whole
set of measures that together represent the classical barrier
concept of the MORT tree. Hence, between the hazard and
the target many ‘barriers’ are put that together should prevent
the hazard from damaging the target, i.e. ‘defence in depth’.
In such a series, many barriers are not so much concerned
with actual hazards but with occurrences or events that might
lead to some sort of loss of control and, in the long run,
the unwanted release of a hazardous energy source. Barriers
therefore have different functions, based on their location in
the fault tree (or event tree).
This barrier concept is incorporated into the management
model described above. A barrier function is developed in
a risk analysis and then specified, based on its position in
the fault or event tree, i.e. in relation to the event it should
prevent. The main consideration then will be if the barrier
should have any behavioural l involvement, and if so, what the
nature of this involvement will be