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).