Comparison of Programmable Electronic Safety
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www.safetyusersgroup.com Page 1 / 1 Comparison of Programmable Electronic Safety-Related System Architectures Anton A. Frederickson, Mr., Dr. Independent Consultant – member of Safety Users Group Network 10 January, 2003 Abstract This paper discusses the concepts of risk, safety lifecycle, and safety integrity for safety-related electrical/ electronic/ programmable electronic systems ( E/E/PES) contained in the International Electrotechnical Commission (IEC) 61508 Standard: Functional safety of electrical/electronic/programmable electronic safety-related systems, Parts 1 through 7. This paper utilizes information from various parts of the IEC 61508 so the concepts and methodologies can be presented in an abridged form. This paper also shows a number of PES architectures used in safety-related applications. Markov Models are used to calculate the PFDavg so the suitability of using the architectures in applications requiring different safety integrity levels can be determined. Markov Models are also used to compute MTTFspurious for all the PES architectures so the impact of spurious trips can be taken into account when selecting a PES architecture. 1. INTRODUCTION The emphasis in this paper is on computer-based systems (referred to as programmable electronic systems (PESs)) that are increasingly being used to p erform safety fun ctions. While the PESs provide a flexible way of implementing safety functions and providing extensive diagnostics, great care must be taken to ensure the resulting sy stems meet the required lev el of safety int egrity. The IEC 61508 addresses design and assessment methodologies that must be used to ensure the PESs are safe. The IEC 61508 Standard consists of seven parts. The first three parts are normative; Part 1 provides the general requirements, Part 2 provides the hardware requirements, and Part 3 provides the software requirements. The remaining four informative part s provide definitions, bibliog raphies, and guidelines for applying Parts 1, 2 and 3. The seven-part standard is generic and applies to all safety-related systems irrespective of the application. Examples of the application sectors coming with in the scope include but are not limited t o: • Process industries (emergency shutdown systems, fire and gas detection systems, burner controls); • Manufacturing industries (industrial robots, machine tools); • Transportation (railway signaling, braking systems, lifts); • Medical (miscellaneous electro-medical apparatus, radiography ); The IEC 61508 introduces the concept of Safety Integrity Levels that relate to the safety integrity require d for the hardware and software used in the safety -related syste m. The IEC 61508 also introduces th e concept of an Overall Safety Lifecycle to ensure that all activities necessary to achieve the required Safety Integrity Le vel are performed. Figure 1 shows the Overall Safe ty Lifecycle and for each phase of the lifecycle the standard specifies: • The objectives to be achieved • The requirements to meet the objective • The scope of each phase • The required inputs to the phase • The deliverables required to meet the requirements of each phase
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