Chapter58-SafetyApplicationsSYSTEMSANALYSISManhTrungHoAsystemcanbedefinedasasetofinterdependentcomponentscombinedinsuchawayastoperformagivenfunctionunderspecifiedconditions.Amachineisatangibleandparticularlyclear-cutexampleofasysteminthissense,butthereareothersystems,involvingmenandwomenonateamorinaworkshoporfactory,whicharefarmorecomplexandnotsoeasytodefine.Safetysuggeststheabsenceofdangerorriskofaccidentorharm.Inordertoavoidambiguity,thegeneralconceptofanunwantedoccurrencewillbeemployed.Absolutesafety,inthesenseoftheimpossibilityofamoreorlessunfortunateincidentoccurring,isnotattainable;realisticallyonemustaimforaverylow,ratherthanazeroprobabilityofunwantedoccurrences.Agivensystemmaybelookeduponassafeorunsafeonlywithrespecttotheperformancethatisactuallyexpectedfromit.Withthisinmind,thesafetylevelofasystemcanbedefinedasfollows:“Foranygivensetofunwantedoccurrences,thelevelofsafety(orunsafeness)ofasystemisdeterminedbytheprobabilityoftheseoccurrencestakingplaceoveragivenperiodoftime”.Examplesofunwantedoccurrencesthatwouldbeofinterestinthepresentconnectioninclude:multiplefatalities,deathofoneorseveralpersons,seriousinjury,slightinjury,damagetotheenvironment,harmfuleffectsonlivingbeings,destructionofplantsorbuildings,andmajororlimitedmaterialorequipmentdamage.PurposeoftheSafetySystemAnalysisTheobjectofasystemsafetyanalysisistoascertainthefactorswhichhaveabearingontheprobabilityoftheunwantedoccurrences,tostudythewayinwhichtheseoccurrencestakeplaceand,ultimately,todeveloppreventivemeasurestoreducetheirprobability.Theanalyticphaseoftheproblemcanbedividedintotwomainaspects:1.identificationanddescriptionofthetypesofdysfunctionormaladjustment2.identificationofthesequencesofdysfunctionsthatcombineonewithanother(orwithmore“normal”occurrences)toleadultimatelytotheunwantedoccurrenceitself,andtheassessmentoftheirlikelihood.Oncethevariousdysfunctionsandtheirconsequenceshavebeenstudied,thesystemsafetyanalystscandirecttheirattentiontopreventivemeasures.Researchinthisareawillbebaseddirectlyonearlierfindings.Thisinvestigationofpreventivemeansfollowsthetwomainaspectsofthesystemsafetyanalysis.MethodsofAnalysisSystemsafetyanalysismaybeconductedbeforeoraftertheevent(apriorioraposteriori);inbothinstances,themethodusedmaybeeitherdirectorreverse.Anapriorianalysistakesplacebeforetheunwantedoccurrence.Theanalysttakesacertainnumberofsuchoccurrencesandsetsouttodiscoverthevariousstagesthatmayleaduptothem.Bycontrast,anaposteriorianalysisiscarriedoutaftertheunwantedoccurrencehastakenplace.Itspurposeistoprovideguidanceforthefutureand,specifically,todrawanyconclusionsthatmaybeusefulforanysubsequentapriorianalyses.Althoughitmayseemthatanapriorianalysiswouldbeverymuchmorevaluablethananaposteriorianalysis,sinceitprecedestheincident,thetwoareinfactcomplementary.Whichmethodisuseddependsonthecomplexityofthesysteminvolvedandonwhatisalreadyknownaboutthesubject.Inthecaseoftangiblesystemssuchasmachinesorindustrialfacilities,previousexperiencecanusuallyserveinpreparingafairlydetailedapriorianalysis.However,eventhentheanalysisisnotnecessarilyinfallibleandissuretobenefitfromasubsequentaposteriorianalysisbasedessentiallyonastudyoftheincidentsthatoccurinthecourseofoperation.Astomorecomplexsystemsinvolvingpersons,suchasworkshifts,workshopsorfactories,aposteriorianalysisisevenmoreimportant.Insuchcases,pastexperienceisn...
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