What is IEC 61508?

What is IEC 61508? IEC 61508 is the international standard for functional safety for electrical, electronic and programmable electronic safety related systems.

IEC 61508:2010 was published as Edition 2 in 2010. Edition 3 is already under consideration.

Functional safety is the part of the overall safety relating to the equipment under control and its associated control system that depends on the correct functioning of the safety-related system. Functional safety is achieved when all the specified safety functions, of the safety-related system, can satisfy their required safety performance. Functional safety is undertaken by active systems.

We recognise IEC 61508 as the benchmark for achieving functional safety and manageing risks in a proportionate way.

IEC 61508 sets out the requirements for ensuring that systems are designed, implemented, operated and maintained to provide the required safety integrity level (SIL). Four SILs are defined according to the risks involved in the system application, with SIL 4 being used to protect against the highest risks. The standard specifies a process that can be followed by all links in the supply chain so that information about the system can be communicated using common terminology and system parameters.

Some organisations or people can find functional safety and IEC 61508 compliance as a daunting prospect. One of the key principles of the Association is to promote and support the best routes for compliance to IEC 61508 and related standards.

The CASS Scheme Association has published several templates for the assessment of functional safety according to IEC 61508 (examples are CASS-508-FSM and CASS-508-SW). These are free to use and available via our downloads page.

Due to the development of sector standards, the main roles of IEC 61508 are now the development of products (e.g., PLC’s, process instruments, variable speed drives) for use in relevant sectors / sector standards and the support of niche applications such as non-power-generation nuclear applications.

The IEC 61508 standard family is in seven main parts (plus a technical report):

  • IEC TR 61508-0, Functional safety and IEC 61508
  • IEC 61508-1, General requirements
  • IEC 61508-2, Requirements for E/E/PE safety-related systems
  • IEC 61508-3, Software requirements
  • IEC 61508-4, Definitions and abbreviations
  • IEC 61508-5, Examples and methods for the determination of safety integrity levels
  • IEC 61508-6, Guidelines on the application of IEC 61508-2 and IEC 61508-3
  • IEC 61508-7, Overview of techniques and measures


IEC 61508 has been adopted in the UK as BS EN 61508 and in the EU by EN 61508, with the “EN” indicating adoption by the European electrotechnical standardisation organisation CENELEC. Other standards have been produced for the application of the IEC 61508 principles to particular sectors. One of the key roles of IEC 61508 continues to be the support of further sector standards and the development of new technologies.

Sector specific standards related to IEC 61508 include:

  • IEC 61511 Process industries
  • IEC 61513 Nuclear power plants
  • IEC 62061 Machinery sector
  • ISO 13849 Machinery sector
  • IEC 61800-5-2 Power drive systems
  • ISO 26262 Automotive sector

Please review the CASS webpages and consider joining the CASS Scheme Association if you are involved in functional safety

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