A few reminders about IEC 61508 and IEC 61511 standards
Probability of failure of the safety function

The IEC 61508 and IEC 61511 standards are essential references for implementing functional safety. They provide a framework for all stages of the SIS lifecycle, from design to dismantling.

IEC 61508-4 defines functional safety as a subset of overall safety that relates to the controlled Equipment Under Control (EUC) system and depends on the correct operation of the safety-related E/E/EP system, safety-related systems based on other technology and external risk reduction devices.

IEC 61511-1 defines functional safety as a subset of overall safety that relates to the process and process control system and the BPCS, which depends on the correct operation of the safety instrumented system (SIS) and other layers...