In the field of functional safety, the aim of this work is to model and give a probabilistic assessment of safety related systems. This systems, called Safety Instrumented Systems (SIS), are automated systems responsible for the industrial facilities operating safety and ensuring a safe fallback state or even the shutdown of such industrial facilities in order to avoid dangerous accidents or injuries. The proposed approach shows how these systems must be analyzed by the means of the Probability of Failure on Demand (PFD) and what are the different parameters that make it possible to improve this probability and therefore the safety of the industrial facilities that need to be protected.