Safety Instrumented Systems

IEC 61508 is European Norm throughout the EU and thus in the UK it is BS EN 61508. IEC 61508 is the master standard. A number of standards existed to provide the appropriate guidance for different industry sectors. For example the UKOOA guidelines cover the Offshore Oil & Gas industry, IEC 61511 (also known as ISA S84.01-2004 in the USA) covers the process industries and so on

Rowan House Ltd are members of the 61508 Associationn. In 2003 Clive de Salis, MD of Rowan House, became the Chair of the Association's working group developing Competence and Accreditation guidance for people working on IEC61508 SIL rated systems. In 2005 Clive de Salis became the first Chair of the 61508 Association.

Rowan House Ltd are certified by SIRA, using the UKAS accreditted CASS system, for the assessment of your Safety Integrity Level ("SIL") requirements for your process plant under both BS EN 61508 and IEC 61511 standards. The CASS accreditation is the approval of the organisation's delivery of auditted studies and SIL determination that meets the requirement of safety authorities such as the HSE.

HSE policy on CASS

1. It is recognised that relevant good practice such as contained in HSE Approved Codes of Practice and Guidance together with relevant standards properly applied within their scope can play an important role in the achievement of good health and safety performance. In the context of functional safety of electrical/ electronic/programmable electronic (E/E/PE) safety-related systems BS EN 61508 is such a reference standard for determining whether a reasonably practicable level of safety has been achieved. Sector standards and specific industry standards or guidelines may also form relevant good practice.
2. HSE believes that, in general, accredited conformity assessment schemes when properly applied, as part of a systematic approach to the management of health and safety risks, can assist duty holders and can be helpful in targeting regulatory activity.
3. Accredited certification schemes such as CASS (Conformity Assessment for Safety-related Systems) when properly applied and as part of a systematic approach can help duty holders in making a demonstration that the functional safety of E/E/PE safety-related systems has been properly addressed. In the context of regulatory regimes where safety cases are required, the adoption of schemes such as CASS could provide useful evidence as part of the demonstration that risks have been reduced as low as reasonably practicable (ALARP).

HSE information can be accessed at http://www.hse.gov.uk/

CASS is the scheme for assessing the compliance of safety related systems with the requirements of IEC 61508 and associated standards. It provides a systematic approach to be used by certification bodies and others when assessing compliance at all stages from the specification of safety requirements through the design, development and manufacture of system components to integration, commissioning, operation and maintenance. At each stage CASS takes the conformity assessor through the logical steps of defining the scope of the assessment, the target of evaluation, the requirements to be met and the process of demonstrating and recording conformity.

BS EN 61508 - Functional safety of electrical/electronic/programmable electronic safety-related systems, includes far more than the design of a Safety Instrumented System (SIS). It is a life cycle safety concept concerned with the operated unit from inception to decommissioning.

The standard includes for: assessment of risk to people and the environment, determination of acceptable risk and the methods to achieve and maintain this level of acceptable risk.

In addition, the operator needs to have an Operating, Maintenance and Design Change Control policy in place as part of the life cycle maintenance of the designed and install Safety Integrity Level (SIL). The establishment by the equipment operator of appropriate SIS Standards, Documentation, Proof Testing, Maintenance, Audits and a formal procedure for Change Management are required to achieve this objective.

BS EN 61508 is a generic standard on which sector specific safety standards are to be based. For the process sector IEC 61511 is available and is to be the bases for the next issue of S84 the USA equivalent.

BS EN 61508 is a European Norm; however it is not a directive or part of UK legislation.

[The author has been made aware from different sources that the HSE are using the standard as a 'bench mark' of good practice.]