Nowadays, different industries such as nuclear, automotive, process, chemical, and oil gas industry, must meet specific requirements in order to keep and reduce safety risks to as low as reasonably practicable (ALARP) level. As a result, the number of electrical/electronic/programmable electronic safety-related systems (E/E/PES) to control, prevent and mitigate hazardous events has increased. Moreover, new guidelines and procedures have been developed to guarantee the availability and function of safety systems over their service life. Based on Markov processes, this paper proposes a reliability model to assess the integrity and verify the design of E/E/PES safety-related systems. The average probability of failure on demand (PFDavg) and Safety Integrity Level (SIL), are used to determine the reliability performance of an E/E/PES in a low demand mode of operation in accordance with the functional safety principles of the IEC 61508 and IEC 61511 standards.