【導讀】Lice. nse. dC. op. y:. Inst. itu. te. Of. Te. ch. no. log. yT. all. ag. ht,In. sti. tut. eo. fT. ech. no. log. y,Mo. nJu. n1. 11. 6:1. 6:3. 3G. MT. +0. 0:0. 02. 00. 7,Un. co. ntr. oll. ed. C. op. y,B. SI. Incorporating. Amendmentto. BSIEC61508-5:1998. (renumbersthe. BSIECas. BSEN61508-5:2020)Functionalsafetyof. programmable. electronic. safety-related. systems—。BritishStandard. ICS;L. ice. nse. dC. op. y:. Inst. itu. te. Of. Te. ch. no. log. yT. all. ag. ht,In. sti. tut. eo. fT. ech. no. log. y,Mo. nJu. n1. 11. 6:1. 6:3. 3G. MT. +0. 0:0. 02. 00. 7,Un. co. ntr. oll. ed. C. op. y,B. SI. BSEN61508-5:2020. Nationalforeword. April1999.Cross-references. Catalogue.application.directionofthe. Committee,waspublished. esintoeffect. on15July1999. ©BSI15March2020. ISBN0580327280. Summaryofpages. Lice. nse. dC. op. y:. Inst. itu. te. Of. Te. ch. no. log. yT. all. ag. ht,In. sti. tut. eo. fT. ech. no. log. y,Mo. nJu. n1. 11. 6:1. 6:3. 3G. MT. +0. 0:0. 02. 00. 7,Un. co. ntr. oll. ed. C. op. y,B. SI

　　

【正文】 tware requirements IEC 615084:1998, Functional safety of electrical/electronical/programmable electronic safety related systems – Part 4: Definitions and abbreviations of terms IEC 615086,— Functional safety of electrical/electronical/programmable electronic safety related systems – Part 6: Guidelines on the application of parts 2 and 3 1) IEC 615087,— Functional safety of electrical/electronical/programmable electronic safety related systems – Part 7: Overview of techniques and measures 1) ISO/IEC Guide 51:1990, Guidelines for the inclusion of safety aspects in standards IEC Guide 104:1997, Guide to the drafting of safety standards, and the role of Committees with safety pilot functions and safety group functions 3 Definitions and abbreviations For the purposes of this standard, the definitions and abbreviations given in part 4 apply. 1 ) To be published. Page 9 EN 61508?5:2020 Licensed Copy: Institute Of Technology Tallaght, Institute of Technology, Mon Jun 11 16:16:33 GMT+00:00 2007, Uncontrolled Copy, (c) BSI Annex A (informative) Risk and safety integrity – General concepts General This annex provides information on the underlying concepts of risk and the relationship of risk to safety integrity. Necessary risk reduction The necessary risk reduction (see of IEC 615084) is the reduction in risk that has to be achieved to meet the tolerable risk for a specific situation (which may be stated either qualitatively1) or quantitatively2)). The concept of necessary risk reduction is of fundamental importance in the development of the safety requirements specification for the E/E/PE safety related systems (in particular, the safety integrity requirements part of the safety requirements specification). The purpose of determining the tolerable risk for a specific hazardous event is to state what is deemed reasonable with respect to both the frequency (or probability) of the hazardous event and its specific consequences. Safetyrelated systems are designed to reduce the frequency (or probability) of the hazardous event and/or the consequences of the hazardous event. The tolerable risk will depend on many factors (for example, severity of injury, the number of people exposed to danger, the frequency at which a person or people are exposed to danger and the duration of the exposure). Important factors will be the perception and views of those exposed to the hazardous event. In arriving at what constitutes a tolerable risk for a specific application, a number of inputs are considered. These include: – guidelines from the appropriate safety regulatory authority。 – discussions and agreements with the different parties involved in the application。 – industry standards and guidelines。 – international discussions and agreements。 the role of national and international standards are being increasingly important in arriving at tolerable risk criteria for specific applications。 – the best independent industrial, expert and scientific advice from advisory bodies。 – legal requirements, both general and those directly relevant to the specific application. 1 ) In achieving the tolerable risk, the necessary risk reduction will need to be established. Annexes D and E of IEC 615085 outline qualitative methods, although in the examples quoted the necessary risk reduction is incorporated implicitly rather than stated explicitly. 2 ) For example, that the hazardous event, leading to a specific consequence, shall not occur with a frequency greater than one in 108 h. Page 10 EN 61508?5:2020 Licensed Copy: Institute Of Technology Tallaght, Institute of Technology, Mon Jun 11 16:16:33 GMT+00:00 2007, Uncontrolled Copy, (c) BSI Role of E/E/PE safetyrelated systems E/E/PE safetyrelated systems contribute towards meeting the necessary risk reduction in order to meet the tolerable risk. A safetyrelated system both – implements the required safety functions necessary to achieve a safe state for the equipment under control or to maintain a safe state for the equipment under control, and – is intended to achieve, on its own or with other E/E/PE safetyrelated systems, other technology safetyrelated systems or external risk reduction facilities, the necessary safety integrity for the required safety functions ( of IEC 615084). NOTE 1 – The first part of the definition specifies that the safetyrelated system must perform the safety functions which would be specified in the safety functions requirements specification. For example, the safety functions requirements specification may state that when the temperature reaches x, valve y shall open to allow water to enter the vessel. NOTE 2 – The second part of the definition specifies that the safety functions must be performed by the safety related systems with the degree of confidence appropriate to the application, in order that the tolerable risk will be achieved. A person could be an integral part of an E/E/PE safetyrelated system. For example, a person could receive information, on the state of the EUC, from a display screen and perform a safety action based on this information. E/E/PE safetyrelated systems can operate in a low demand mode of operation or high demand or continuous mode of operation (see of IEC 615084). Safety integrity Safety integrity is defined as the probability of a