Apportionment of safety integrity levels in complex electronically controlled systems

摘要

Currently, technical systems become more and more complex. A main contribution is coming from electronic control systems that allow for a larger number of functions. The concept of Safety Integrity Levels (SILS) has been developed within different systems of standards (IEC 61508, EN 50129/EN 50128 and DEF-STAN 00-56). These standards are applied in different areas: control technology (IEC 61508), railway technology (EN 50129 and EN 50128) and Defence technology (DEF-STAN 00-56). SILs are a tool for assigning safely targets to systems. The concept of a Safety Integrity Level (SIL) within different standards is explained. The problem of assigning safety integrity levels to functional units within a system is discussed, especially how a safety integrity level is realised by a certain safety architecture of a system. Some standards give rules for system architecture and how to achieve a higher SIL for a system designed from constituents that have a lower SIL. Examples for system architectures and SIL assignment are given. For all systems, a choice of SILs for hardware and software is discussed and the problems arising are described.