The present paper deals with safety barriers,
describing their characteristics and presenting a new
classification for them based on their type and operating mode.
While the assessment of safety barriers performance is often
achieved by tests or inspections in order to determine the
probability of failure on demand (PFD) and detect the so-called
hidden failures, a new methodology to evaluate the adequacy of a
safety barrier is proposed by linking the safety integrity level of
an assessed safety barrier with the probability of occurrence of
the hazardous event that it should protect. Based on the study of
all possible safety function failures there is a selective approach
to determine the ones classified as dangerous undetected in a way
to use them on the determination of the PFD. Applying the
methodology proposed it is also possible to estimate the
probability of occurrence of a hazardous situation, once it
depends on the simultaneity of happening the initiating event and
the safety barrier fault, when a demand occurs. The impact of a
specific safety barrier assessment on risk can thus be evaluated.