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CommonSILMyths

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1.UsingaSIL3logicsolvermeansthatIhaveaSIL3system.No.WhenusingaSIL3logicsolver,itiscriticalthattheentiresystemisde

signedtoconformtoSIL3requirements.ThePFDfortheentiresystemisimportant.IfauserinstallsaSIL3logicsolverbutdoesnotemployappropriateredundancyordoesnotincorporatecomponentsintothesystemwithcorrectPFDcalculations,thentheentiresystemmaynotcomplywithaSIL3level.Achainisonlyasstrongasitsweakestlink.

2.SIL3suitableproductsarebetterthanSIL1orSIL2suitableproducts.Thisisnotnecessarilytrue.WhileahigherSILlevelcorrespondstoalowerprobabilityoffailureondemand,aSIL2suitableproductmaybeperfectlyacceptableforuseinaSIL3environmentif,forexample,theprooftestingintervalisincreasedorifredundancyisused.Itisveryimportantforanend-usertounderstandtheoperatingrequirementsoftheproductswithinagivenSILenvironm

enttoensurethatonceinstalled,theproductsmaintaintheirSILsuitabilitylevels.Incorrectinstallation,prooftesting,orconfigurationoftheproductscouldmaketheSILsuitabilitylevelinaccurate.

3.TherearemanyagenciesthatarecapableofissuingSILcertifications.Thereareveryfewnationallyaccreditedbodiesthatcanissuenationallyaccreditedcertifications,includingFM,TUV,andSira.Manyunaccreditedconsultingfirmsissuecertificatesthatindicatetheyhavereviewedtheproductand/orprocessforconformancetocertainpartsoftheIEC61508standard.Thestandarddoesnotmandatethatcertaincompaniesoragenciesareabletocertifyproductsandsystems.Rather,itissuggestedthatanalysisiseitherconductedorvalidatedbyanindependentthirdparty.

4.AvendorcandeterminewhetherasystemmeetstherequirementsofIEC61511.No.Onlytheendusercanensurethatthesafetysystemisimplementedtobecompliantwiththestandards.Itisuptotheusertoensurethatprocedureshavebeenfollowedproperly,theprooftestingisconductedcorrectly,andsuitabledocumentationofthedesign,process,andproceduresexists.Theequipmentorsystemmustbeusedinthemannerinwhichitwasintendedinordertosuccessfullyobtainthedesiredriskreductionlevel.JustbuyingSIL2orSIL3suitablecomponentsdoesnotensureaSIL2orSIL3system.

5.AcustomermustpurchaseacompleteSILbasedsolution,evenifsomefunctionsdonotrequireaSILlevel.FormostapplicationstherewillonlybeafewSIFfunctionsbeinghandledbyth

esystem,andthevastmajorityofthecircuitsmaynotneedtobeSILratedatall.IfthecustomerspecifiesSIL2orSIL3fortheentiresystemhemayaddconsiderablecostwithlittleornobenefitorimprovementinsafety.

6.SafetyandReliabilityarethesamething.No.Safetyandreliabilityareoftenlinkedbutarenotthesamething.SafetyisdefinedintheIEC61508standardsasfreedomfromunacceptablerisk.Asafesystemshouldprotectfromhazardswhetheritisperformingreliablyornot.Safetyengineeringassuresthatasafetysystemperformsasneeded,evenwhenpiecesfail.Infact,safetyengineersassumethatsystemswillfail,anddesignaccordingly.

Reliabilityisameasureofhowwellthesystemdoesexactlywhatitisintended

todowhenoperatedinaspecificmanner.Areliablesystemmaynotalwaysbeasafesystem.Thechallengeinfunctionalsafetyistoensurethatasystemisbothreliableandsafe.

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