IEC 61508

IEC 61508 is the international standard for functional safety of electrical, electronic, and programmable electronic safety-related systems, published by the International Electrotechnical Commission. It provides a framework for managing risk throughout the lifecycle of safety-critical systems, from concept and design through operation and decommissioning. The standard is 'generic' — it applies across industries — and has been adapted into sector-specific standards including ISO 26262 for automotive, IEC 61511 for process industries, and EN 50128 for railway applications.

The standard's core contribution is the Safety Integrity Level (SIL), a graded scale from SIL 1 (low risk reduction) to SIL 4 (catastrophic consequence prevention) that quantifies the required reliability of a safety function. Each SIL imposes specific targets for probability of dangerous failure, hardware fault tolerance, and systematic capability — the last being a measure of how well the development process prevents design errors. IEC 61508 was heavily influenced by DO-178B, adopting its emphasis on lifecycle documentation, traceability, and structured verification.

IEC 61508's generic design was intended to make it universally applicable, but this universality is also its weakness. The standard abstracts away domain-specific failure modes — a chemical plant's runaway reaction is not a railway signal's wrong-side failure — and leaves the mapping from generic requirements to domain hazards as an exercise for each industry. The result is that sector-specific adaptations often add more than they clarify, and the 'generic' standard becomes a bloated requirements document that certifiers interpret inconsistently across borders.