what is IEC 61508?

What is IEC 61508?

Managing safety functions from design through operation

IEC 61508 provides a functional safety framework for safety functions performed by electrical, electronic and programmable electronic systems. Its purpose is to identify hazards and maintain the required risk reduction through design, software development, installation, operation and maintenance. Among industrial standards, it is a basic safety publication and provides a foundation for sector applications within the IEC standards family. Compliance does not mean that a system can never fail.

Where does a safety function begin and end?

A function includes the sensor that detects the hazardous condition, the logic solver that makes the decision and the final element that acts on the process or machine. The logic solver may be a suitable safety controller; using an ordinary PLC does not automatically establish safety capability. Depending on the application, a contactor, valve, actuator or intermediate relay may also be part of the assessment. Connections, power supplies and responses to faults belong within the function's boundary.

For example, where high pressure presents a hazard in a tank, a function might close an inlet valve in response to a pressure sensor. The specification should define the initiating condition, safe state, maximum response time and conditions for restarting. This is an application example: removing power from every process does not always produce a safe outcome. Some systems need cooling or another utility to remain available.

What does SIL describe?

SIL means Safety Integrity Level. IEC 61508 defines four levels, SIL 1 through SIL 4; a higher level imposes more demanding safety integrity requirements on the function. The target follows from hazard analysis, tolerable risk and the contribution of other protective measures. Choosing the highest SIL for every project cannot replace risk assessment.

In low demand mode, PFDavg is the average probability that the safety function will fail dangerously when demanded. In high demand or continuous mode, PFH describes the average frequency of dangerous failures per hour. These are different quantities. Meeting a numerical target also requires compliance with architectural constraints and measures against systematic failure. SFF and HFT refer to safe failure fraction and hardware fault tolerance respectively; these values alone do not demonstrate SIL compliance.

Editor's note: A device described as “suitable for SIL 2” does not make the entire connected machine or plant SIL 2. Review the scope of the certificate, the conditions in the safety manual, the connection architecture and the testing assumptions together. The assessment concerns the complete defined safety function.

Why does the safety lifecycle matter?

The work starts with hazard and risk analysis. Risk reduction is then allocated to appropriate measures and a safety requirements specification (SRS) is prepared. Hardware and software design, installation, commissioning, operation, modifications and decommissioning follow within the lifecycle. Random hardware failures require different measures from systematic failures caused by incorrect requirements, software defects or maintenance errors.

Verification checks whether the outputs of a lifecycle phase meet their applicable requirements. Validation demonstrates that the completed system performs the specified safety functions in its intended application. Functional safety assessment considers the adequacy of the relevant activities and evidence. Competent personnel, the required degree of independence and traceable records must be addressed throughout the work, not left until the final test.

The seven parts of the 2010 edition

The following summary covers the second edition, published in 2010. When the IEC catalogue was checked on 17 October 2026, Part 1 of the third edition was offered as a PRV/FDIS pre-release. Distinguish a pre-release from the final publication and identify the edition and any amendments used for the project.

Part 1: General requirements. Covers the overall safety lifecycle and the management of functional safety.

Part 2: Requirements for E/E/PE safety-related systems. Addresses hardware and system realisation requirements.

Part 3: Software requirements. Covers the lifecycle, development and verification of safety-related software.

Part 4: Definitions and abbreviations. Establishes the terminology used throughout the series.

Part 5: Examples of methods for determining safety integrity levels. Presents approaches that support determination of the required SIL.

Part 6: Guidelines on the application of IEC 61508-2 and IEC 61508-3. Provides guidance on hardware calculations and applying software requirements.

Part 7: Overview of techniques and measures. Describes techniques available for preventing and controlling failures.

Practical distinctions in installation and maintenance

Two channels sharing a power supply may both be affected by its failure. Assess common cause failures and the independence of protection layers. The electromagnetic environment, cable routing, suitable EMC filters and grounding also form part of the design; these measures do not establish safety integrity on their own.

In motor applications, an ordinary stop command on a variable frequency drive should not be treated as equivalent to a documented safety function. Assess the load on the electric motor, the time needed for motion to stop and any required mechanical measures together. Define the safe state for the hazards of the application.

A periodic proof test aims to reveal dangerous failures that normal diagnostics have not detected. The interval and coverage must match the assumptions used in the calculations; there is no universal maintenance interval. Verify restoration of the function after testing and control temporary bypasses. Software or equipment changes also require impact analysis and appropriate reverification and revalidation.

Choosing the standard for the application

IEC 61508 provides the general framework. IEC 61511 addresses safety instrumented systems in the process industries, while IEC 62061 and, where applicable, ISO 13849 address machinery safety control systems. ISO 13849 uses Performance Levels (PL). Although relationships exist between PL and SIL, they cannot be substituted simply by relabelling a rating. Select the applicable standard by its scope and the characteristics of the safety function.