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ALARP vs SFAIRP.pdf
ABB Safety and Functional Safety a General Guide
NTM162011
D-010r1
e ida.
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Summary of Draft IEC 61511 Standard for Functional Safety: Safety Instrumented Systems for the Process Industry
Rachel Amkreutz Reliability Engineer, exida.com Sellersville, PA 18960 +215-453-1720 IEC 61511 has been developed as a Process Sector implementation of the international standard IEC 61508: “Functional safety of electrical / electronic / programmable electronic safety-related systems.” The standard has two concepts, which are fundamental to its application; the safety lifecycle and safety integrity levels (SIL). The safety lifecycle forms the central framework which links together most of the concepts in this international standard. It is a good engineering procedure for safety instrumented system (SIS) design. In the safety lifecycle, process risks are evaluated and SIS Performance requirements are established (availability and risk reduction). Layers of protection are designed and analyzed. Finally, a SIS (if needed) is optimally designed to meet the particular process risk. Safety integrity levels are order of magnitude levels of risk reduction. There are four SIL’s defined in this standard, just as in IEC 61508. SIL1 has the lowest level of risk reduction. SIL4 has the highest level of risk reduction. The standard suggests that applications which require the use of a single safety instrumented function of SIL 4 are rare in the process industry and that they shall be avoided where reasonably practicable. The standard is primarily concerned with safety-instrumented systems for the process industry sector (sensors, logic solvers and final elements are included as part of the safety instrumented system). It also deals with the interface between safety-instrumented systems and other safety systems in requiring that a process hazard and risk assessment be carried out. This draft consists of three parts: Part 1: Framework, definitions, system, hardware and software requirements (Version CDV, 26/05/00) Part 2: Guidelines in the application of IEC 61511-1 (Version 2.3, 18/05/00) Part 3: Example methods for determining safety integrity in the application of Hazard & Risk Analysis. (Version CDV, 06/07/00)
1 Part 1: Framework, definitions, system, hardware and software requirements
Part 1 specifies requirements for system architecture and hardware configuration, application software, and system integration. This includes sections on management of functional safety, safety lifecycle requirements, verification, process hazard and risk analysis, and allocation of safety functions to protection layers. These last two sections only contain general requirements and no detailed requirements. Furthermore, there are sections on SIS safety requirements specification, and SIS design and engineering, and requirements for application software (including selection criteria for utility
© exida.com Rachel Amkreutz summary iec 61511.doc, Sep. 14, 2000 Page 1 of 1
3.com excellence in dependable automation software). Finally there are sections on factory acceptance testing.com Rachel Amkreutz summary iec 61511. Parts 1. The key differences between IEC 61508 and IEC 61511 are discussed in Part 1. Annex D provides three examples of the application of IEC 61511. Part 1. Annex C provides an example of the application of IEC 61511. a typical SIS architecture development. Part 2 generally contains information and guidelines on IEC 61511.0. Annex A. and 4 of IEC 61508 have thus been combined into part 1 of IEC 61511. 2000 Page 2 of 2 . Annex E provides © exida. and information requirements. PROCESS SECTOR SAFETY SYSTEM STANDARD PROCESS SECTOR HARDWARE PROCESS SECTOR SOFTWARE DEVELOPING NEW HARDWARE DEVICES USING PROVEN IN USE HARDWARE DEVICES USING HARDWARE DEVELOPED AND VALIDATED ACCORDING TO IEC 61508 DEVELOPING EMBEDDED (SYSTEM) SOFTWARE DEVELOPING APPLICATION SOFTWARE USING FULL VARIABILITY LANGUAGES DEVELOPING APPLICATION SOFTWARE USING LIMITED VARIABILITY LANGUAGES OR FIXED PROGRAMS FOLLOW IEC 61511 FOLLOW IEC 61508 FOLLOW IEC 61511 FOLLOW IEC 61511 FOLLOW IEC 61508-3 FOLLOW IEC 61508-3 Figure 1 2 Part 2: Guidelines in the application of IEC 61511-1 Part 2 contains sections on scope. as shown in figure 1. SIS operation and maintenance. either from IEC 61508. This section contains a detailed safety lifecycle overview for application software. abbreviations and definitions (process sector specific).e. SIS installation and commissioning. definitions and abbreviations (same apply as for part 1). IEC 61511-1 furthermore has sections on: scope.e ida. and conformance. references. It gives information on attributes of a programming language for SIS. related to various aspects of application programming. Part 1. i. The relationship between IEC 61508 and IEC 61511 is also defined in Part 1. Sep. an example of the development of application code for a process sector programmable electronic SIS.doc. this part of IEC 61511 corresponds to part 6 of IEC 61508. and 6 informative annexes. 6 (safety lifecycle requirements) and 7 (software requirements) of IEC 61511 Part 1 and sets out the functional steps in their application. Annex A gives a brief overview of the requirements of clause 5 (functional safety management). Part 6 Annex B or ISA TR84. Annex B refers to example techniques for calculating the probabilities of failure on demand. Part 1 in a chemical company.02. and an example that illustrates how a major SIS logic solver manufacturer/integrator develops safety application software for customers. In this way. SIS decommissioning. 2. 14.
Part 7. 3 Part 3 Guidelines in the Application of Hazard & Risk Analysis This part of IEC 61511 contains guidelines in the area of selecting safety integrity level (SIL) in hazards and risk analysis. see figure 2. shortly stating. of which Annex A covers the ALARP principle (As Low As Reasonably Practicable) and tolerable risk concepts. 2000 Page 3 of 3 . It provides information on the underlying concepts of risk and the relationship of risk to safety integrity and a number of methods that should enable the safety integrity levels for the Safety Instrumented Functions to be determined. and 3 of this standard. description and reference of the specific technique. and SIS matched to the necessary risk reduction Figure 2 Risk and safety integrity concepts Part 3. a risk graph (qualitative). and in this way corresponds to Part 5 of IEC 61508. For other techniques it refers to IEC 61508. © exida. other protection layers. The information provided is not of sufficient detail to implement any of these approaches. Part 3 consists of a clause on the underlying concepts of risk and the relationship of risk to safety integrity (general guidance). Annex F contains an overview of relevant safety techniques and measures relevant to Parts 1. Consequence of hazardous event Non-SIS prevention/ mitigation protection layers Other protection layers Tolerable risk target Process risk Frequency of hazardous event SIS Necessary risk reduction Process and the process control system Safety integrity of non-SIS prevention/mitigation protection layers. IEC 61511. aim. The information is intended to provide a broad overview of the wide range of global methods used to do hazards and risk analyses. 14. a Calibrated risk graph (semiqualitative). All methods have been simplified in order to illustrate the underlying principles.e ida. Annex B through F cover both quantitative and qualitative approaches to SIL selection. Furthermore there are several informative annexes. Sep. It only gives an overview of additional process sector references.doc. These include a qualitative method (the Safety Matrix Method).com excellence in dependable automation an example of a safety PLC manufacturer’s approach in developing a programmable logic solver certified to IEC 61508 for the process sector.com Rachel Amkreutz summary iec 61511. and Layer Of Protection Analysis (semi-quantitative) are described. 2.
com is an internet based knowledge company focusing on automation safety and reliability. Coaching and consulting services to cost effectively implement IEC 61511 are proudly offered. Figure 1 even shows that. Training courses on all aspects of IEC 61511 are available. books and selfstudy guides are available on our website. Manuals. 14. Sep.doc. exida.com.exida. www. It is expected that engineering companies and instrumentation users will find the most value from this document. © exida.com Rachel Amkreutz summary iec 61511.e ida. 2000 Page 4 of 4 . IEC 61511 is considered a standard for users.com excellence in dependable automation Overall.
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