2. A few reminders about IEC 61508 and IEC 61511 standards

2.1 Functional safety

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 of protection.