Safety Instrumented Systems (SIS) and Safety Integrity Levels (SIL)
emerged from the growing use of electronic and programmable systems for safety functions in the 1980s to 1990s, driven by major accidents and the need for probabilistic risk reduction. The foundational standard, IEC 61508 ("Functional Safety of Electrical/Electronic/Programmable Electronic Safety-Related Systems"), was published in 1998 to 2000 (Edition 1), with Edition 2 in 2010. It introduced SIL as a measure of risk reduction (SIL 1 to 4, with SIL 4 being the highest integrity), covering the full safety lifecycle. For the process industry, IEC 61511 (Edition 1 in 2003, Edition 2 in 2016) adapted IEC 61508, focusing on SIS design, operation, and maintenance in sectors like oil & gas and chemicals. These performance-based standards shifted from prescriptive rules to quantitative targets for reliability, influencing global regulations and practices.