Compare IEC 61508, ISO 26262, and IEC 62304 — which applies when?

These three standards all address functional safety but target different industries and have distinct scopes. IEC 61508 is the "parent" standard — a generic framework for functional safety of electrical, electronic, and programmable electronic (E/E/PE) safety-related systems. It applies across all industries (industrial machinery, process control, railways, nuclear) and defines Safety Integrity Levels from SIL 1 (lowest) to SIL 4 (highest). IEC 61508 covers the entire safety lifecycle and provides general requirements for hardware reliability (random failure rates measured in PFH — Probability of Dangerous Failure per Hour) and systematic software development processes.

ISO 26262 is the automotive-specific adaptation of IEC 61508. It replaces the SIL classification with ASIL (A through D), adds automotive-specific concepts like HARA tailored to vehicle hazard scenarios, and provides more detailed guidance on automotive hardware metrics (SPFM — Single Point Fault Metric, LFM — Latent Fault Metric) and software development practices suited to automotive ECU development. ISO 26262 does not cover commercial vehicles over 3.5 tons (those use ISO 19638) or motorcycles (ISO 25119 for agricultural machinery). The key insight is that ISO 26262 is not simply a relabeling of IEC 61508 — it adds automotive domain knowledge, different hardware metric calculations, and a production-focused process model.

IEC 62304 covers medical device software lifecycle processes and is harmonized with IEC 61508 but tailored for the regulatory requirements of the medical industry. It defines three software safety classes: Class A (no injury possible), Class B (non-serious injury possible), and Class C (serious injury or death possible). IEC 62304 focuses heavily on software development processes — requirements management, architecture documentation, unit testing, integration testing, and risk management (linked to ISO 14971 for medical device risk management). In an interview, the answer to "which applies when" is: IEC 61508 for general industrial safety systems, ISO 26262 for passenger vehicles, and IEC 62304 for medical device software. Many automotive companies also reference ASPICE (Automotive SPICE) for process maturity alongside ISO 26262 for safety, and medical device companies pair IEC 62304 with FDA 21 CFR Part 820 for quality system requirements.