What is IEC 61508?
IEC 61508 is the international foundation standard for functional safety of electrical, electronic and programmable electronic systems (E/E/PE). It describes the complete safety lifecycle process: from risk assessment, design, realisation and use through to decommissioning.
IEC 61508 forms the foundation for sector-specific standards such as IEC 61511 (process industry), IEC 62061 (machinery) and ISO 26262 (automotive).
🎯 Purpose of the standard
The standard aims to enable the design of systems that:
- continue to function safely, even during faults
- meet a predefined risk level
- have a demonstrable safety level: the SIL (Safety Integrity Level)
🧱 Structure of the standard
| Part | Topic |
|---|---|
| IEC 61508-1 | General requirements |
| IEC 61508-2 | Requirements for E/E/PE systems |
| IEC 61508-3 | Software-related requirements |
| IEC 61508-4 | Definitions of terms |
| IEC 61508-5 | Substantiation of risk reduction |
| IEC 61508-6 | Testing and validation |
| IEC 61508-7 | Recommended techniques and methodologies (RMTs) |
🧠 What is a Safety Integrity Level (SIL)?
SIL is a measure of the reliability and safety of a function. The higher the SIL, the lower the probability of a dangerous failure (PFH or PFD):
| SIL | Continuous (PFH) | Demand-mode (PFDavg) |
|---|---|---|
| SIL 1 | ≥10⁻⁶ to <10⁻⁵ 1/h | ≥10⁻² to <10⁻¹ per demand |
| SIL 2 | ≥10⁻⁷ to <10⁻⁶ 1/h | ≥10⁻³ to <10⁻² |
| SIL 3 | ≥10⁻⁸ to <10⁻⁷ 1/h | ≥10⁻⁴ to <10⁻³ |
| SIL 4 | ≥10⁻⁹ to <10⁻⁸ 1/h (rarely required) | ≥10⁻⁵ to <10⁻⁴ |
In practice, SIL 4 is rare and SIL 1–3 are the levels typically applied in industry.
🧩 Relationship with other standards
| Standard | Relation to IEC 61508 |
|---|---|
| IEC 61511 | Sector-specific application for the process industry |
| IEC 62061 | Machinery safety; derived from 61508 |
| ISO 13849-1 | Alternative for machinery; without formal SIL, but PL-oriented |
| ISO 26262 | Functional safety in the automotive industry |
| IEC 61496 | Protection using ESPE (light curtains) based on 61508 |
🔧 When do you apply IEC 61508?
| Application | Explanation |
|---|---|
| Design of Safety PLCs | SIL certification required (e.g. SIL 2 or 3) |
| Sensors/actuators with a safety function | SIL-conformant development required |
| Development of safety relays | Components are validated against 61508 |
| Integration of complete safety functions | Systems are validated to the required SIL |
✅ Best practices
- Follow the full safety lifecycle (hazard analysis → design → verification → use)
- Choose components with demonstrable SIL capability (certificates, PFH/PFD values)
- Document your measures in a safety dossier
- Carry out validation and FMEA in line with the recommendations in parts 6 & 7
- Pay attention to software development practices in line with part 3
📌 In summary
IEC 61508 is the international standard for functional safety of electronic systems. It provides the basis for SIL classifications, design principles and validation of safety-related control systems.