The Application of IEC 61511 for Safety Instrumented Systems, and Its Role in Achieving and Maintaining the Target Risk

Over the past 30 years there have been a number of initiatives worldwide to develop guidelines and standards to enable the safe exploitation of programmable electronic systems used for safety applications. In the context of the process sector a major initiative has been focused on IEC 61508, and its implementation for the process sector IEC 61511 (Functional safety: Safety instrumented systems for the process industry sector). 

The presentation will cover some of the key features of IEC 61508 and IEC 61511 which has now been available for over ten years and which sets out the framework that encompasses the concept of a Safety Lifecycle from initial concept to final decommissioning. The presentation will illustrate the following key aspects of the standard:

• The concept of a Target Risk used for Safety Risk, Environmental Risk and Asset Risk;
• The concept of a Safety Lifecycle with specific compliance requirements for each phase of the Safety Lifecycle;
• The compliance requirements for all relevant Safety Lifecycle phases:
• The concept of a Safety Integrity Level (SIL) and associated Target Failure Measure determined from the Target Risk for the specific scenario and the necessary risk reduction required;
• The Importance of preparation prior to the SIL Determination Workshop (e.g. LOPA Workshop) and having verifiable reliability data;
• The potential benefits of having software tools to assist in the SIL determination and SIL verification of the design including the importance of verifiable reliability data;
• The importance of traceability of key parameters through the relevant Safety Lifecycle phases from initial hazard analysis (HAZOP)….SIL determination….operation and maintenance……modification to final decommissioning; 
• The importance of end user involvement in the key stages of the design of a Safety Instrumented System in order that both capital costs and operation and maintenance costs can be assessed;
• The application of the standard to new designs and also to legacy systems. 

The presentation will include a practical example to illustrate some of the concepts in the standard including Key Performance Indicators (KPIs) covering Competence/Training, Functional Safety Assessment and Proof Testing. In the context of Proof Testing, which is an Operation and Maintenance Key Performance Indicator for end-users, the presentation will indicate the safety and resource benefits of optimising Proof Testing of Safety Instrumented Functions (SIFs) for a specific application.  The presentation will also clarify the difference between a functional test and a proof test and will outline the concepts of Perfect and Imperfect Proof Testing.