ANSI/ISA-61511-3-2018 / IEC 61511-3:2016

ANSI/ISA-61511-3-2018/IEC 61511-3:2016, Functional Safety—Safety Instrumented Systems for the Process Industry Sector – Part 3: Guidance for the Determination of the Required Safety Integrity Levels (IEC 61511-3:2016, IDT), offers guidance on determining the required safety integrity levels (SIL) of safety instrumented functions (SIF) within safety instrumented systems (SIS) used in the process industry sector.This standard outlines various qualitative, semi-quantitative and quantitative approaches, including event tree analysis, safety layer matrix, risk graphs, Layer of Protection Analysis (LOPA) and the “as low as reasonably practicable” (ALARP) principles, to assess risks and allocate safety requirements. It emphasizes the importance of considering multiple protection layers, common causes of failures and systemic dependencies when calculating risk reduction. This document also includes informative annexes that provide detailed examples, methodologies and best practices for applying these techniques, supporting consistent and effective safety lifecycle management from concept through decommissioning. This standard also highlights the need for professional judgment, regulatory compliance and documentation throughout the SIL determination process.