IEC_61508-4.pdf

61508-4 IEC:19971Version 4.0 05/12/97COMMISSIONCEIELECTROTECHNIQUEIECINTERNATIONALE61508-4INTERNATIONALELECTROTECHNICALCOMMISSIONFunctional safety of electrical/electronic/programmable electronic safety-related systemsPart 4:Definitions and abbreviations61508-4 IEC:19972Version 4.0 05/12/97ContentsForeword.3Introduction.51Scope.72Normative references.93Definitions and abbreviations of terms.103.1Safety terms.103.2Equipment and devices.113.3Systems:general aspects.133.4Systems:safety-related aspects.163.5Safety functions and safety integrity.183.6Fault,failure and error.203.7Lifecycle activities.233.8Confirmation of safety measures.24Index.28Figures1Overall framework of this standard.82Programmable electronic system(PES):structure and terminology.153Electrical/electronic/programmable electronic system(E/E/PES):structure and terminology.164Failure model.22Tables1Abbreviations used in this standard.1061508-4 IEC:19973Version 4.0 05/12/97FUNCTIONAL SAFETY OF ELECTRICAL/ELECTRONIC/PROGRAMMABLEELECTRONIC SAFETY-RELATED SYSTEMSPart 4:Definitions and abbreviationsFOREWORD1)The IEC(International Electrotechnical Commission)is a worldwide organization for standardization comprising all nationalelectrotechnical committees(IEC national committees).The object of the IEC is to promote international cooperation on allquestions concerning standardization in the electrical and electronic fields.To this end and in addition to other activities,theIEC publishes international standards.Their preparation is entrusted to technical committees;any IEC national committeeinterested in the subject dealt with may participate in this preparatory work.International,governmental and non-governmentalorganizations liaising with the IEC also participate in this preparation.The IEC collaborates closely with the InternationalOrganization for Standardization(ISO)in accordance with conditions determined by agreement between the twoorganizations.2)The formal decisions or agreements of the IEC on technical matters,prepared by technical committees on which all thenational committees having a special interest therein are represented,express,as nearly as possible,an internationalconsensus of opinion on the subjects dealt with.3)They have the form of recommendations for international use published in the form of standards,technical reports or guidesand they are accepted by the national committees in that sense.4)In order to promote international unification,IEC national committees undertake to apply IEC international standardstransparently to the maximum extent possible in their national and regional standards.Any divergence between the IECstandard and the corresponding national or regional standard shall be clearly indicated in the latter.5)Attention is drawn to the possibility that some of the elements of IEC 61508 may be the subject of patent rights.IEC shall notbe held responsible for identifying any or all such patent rights.6)The IEC has not laid down any procedure concerning marking as an indication of approval and has no responsibility when anitem of equipment is declared to comply with one of its standards.IEC 61508-4 has been prepared by sub-committee 65A:System aspects,of IEC technical committeeFORMTEXT65:Industrial process measurement and controlFORMTEXT.The text of this part is based on the following documents:FDIS Report on voting 65A/xxx 65A/xxxFull information on the voting for the approval of this standard can be found in the voting report indicated inthe above table.IEC 61508 consists of the following parts,under the general title“Functional safety of electrical/electronic/programmable electronic safety-related systems”:Part 1:General requirements;Part 2:Requirements for electrical/electronic/programmable electronic safety-related systems;Part 3:Software requirements;Part 4:Definitions and abbreviations;Part 5:Examples of methods for the determination of safety integrity levels;Part 6:Guidelines on the application of parts 2 and 3;61508-4 IEC:19974Version 4.0 05/12/97 Part 7:Overview of techniques and measures.This part 4 is to be used in conjunction with all other parts.61508-4 IEC:19975Version 4.0 05/12/97IntroductionSystems comprised of electrical and/or electronic components have been used for many years to performsafety functions in most application sectors.Computer-based systems(generically referred to asprogrammable electronic systems(PESs)are being used in all application sectors to perform non-safetyfunctions and,increasingly,to perform safety functions.If computer system technology is to be effectivelyand safely exploited,it is essential that those responsible for making decisions have sufficient guidance onthe safety aspects on which to make those decisions.This standard sets out a generic approach for all safety lifecycle activities for systems comprised ofelectrical and/or electronic and/or programmable electronic components(electrical/electronic/programmableelectronic systems(E/E/PESs)that are used to perform safety fun