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.