Revision of IEC 61508 - Learning from our experience
May 2008

Barrie Reynolds, Chair of BSI committee GEL/65/1^* urged members of The 61508 Association at their meeting in March 2008 to get involved in the revision of IEC 61508. He emphasised the importance of learning from the experience of functional safety practitioners for whom the Association provides a unique focal point.

Last chance for substantial changes
He said that the revision of IEC 61508 had been under way for four years. The Association had been involved in commenting on the committee drafts of Parts 1, 2, 3 and 4 in 2006. The committee drafts for voting (CDVs) should be available in August 2008 with comments to be received by the UK committee by November 2008. This would be the last chance to make substantial changes to the technical content of the standard. He urged members to review the drafts and submit their comments via Ian Cleare on the IEC commenting template which would be provided.

Key changes proposed
The key changes from the current edition of IEC 61508 being proposed in the draft second edition are:

Safety requirements specification - has been given its own lifecycle phase, necessitating the renumbering of subsequent phases from Box 9 onwards. The new section will specify what information

Element - a new term to cover components and sub-systems. A sub-system is defined as an element, the failure of which will cause the failure of the safety function to perform its task.

Systematic capability - the concept is developed for systematic safety integrity compliance for elements and sub-systems. Three routes are proposed:

• Compliance with techniques and measures tables
• Proven in use concepts
• For software that does not satisfy the first two routes

Element safety manual - to be provided by element supplier and providing all data required to integrate the element into the system.

Element safety function and synthesis - the concept of the element safety function independent of the application context is being developed and the architectural systematic capability of the combination of elements, e.g. does 1+1 = 2 is addressed.

Continuous control - to be better addressed.

Competence - to be a normative requirement.

Digital communications - are addressed to recognise the requirements for Fieldbus and wireless networks.

ASICS (Application Specific Integrated Circuits) - some requirements have been included.

Security - is recognised as being important with a general duty for security risks to be considered. Detailed requirements will be catered for in another IEC publication, a reference to which will be included in IEC 61508.

Qualitative SIL requirement assessment - will have to be treated as requiring the highest level of risk reduction for the given SIL, e.g. SIL 1 provides 10 to 100 times risk reduction but only the 100 times could be specified where a qualitative SIL requirement assessment has been carried out.

Diagnostic functions - will require FMEA and failure rate analysis, supported by evidence.

Safe Failure Fraction - requirements are being reconsidered and may be drastically changed or even omitted.

Members of The 61508 Association will be given the opportunity to review the proposed changes and to submit their comments to the UK national committee for onward transmission to the IEC.

Work continues on Software Assessment and Legacy Systems
The 61508 Association is continuing its work on two current issues. The need for a consistent and transparent method for assessing software for compliance with IEC 61508 Part 3 is recognised and the first phase of a project to develop assessment templates is making progress. Trials with software developers and instrument manufacturers are planned, with the resultant assessment templates to be published towards the end of 2008.

The Association has teamed up with other organizations to prepare guidelines for the review and management of safety-related systems designed prior to the introduction of IEC 61508 - referred to as legacy systems. It is planned for the first edition of the guidelines to be available by July 2008, with the intention to revise them in the light of experience in their use.

If you are interested in either of these projects, or in becoming involved more generally as a member of The 61508 Association, contact Ian Cleare at ian.cleare@61508.org.

^* The UK mirror committee to IEC SC65A, responsible for IEC 61508. see http://www.61508.org/standards.htm

To view previous issues of the IEC Association Forum click on the links below:
December 2007 - Progress on key issues