医疗器械漏电流测试示意图.docx
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医疗器械漏电流测试示意图.docx
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医疗器械漏电流测试示意图
Revisedasof23November2020
医疗器械漏电流测试示意图
SafetyTestingofMedicalElectricalEquipment
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1HazardsofMedicalElectricalEquipment
Medicalelectricalequipmentcanpresentarangeofhazardstothepatient,theuser,ortoservicepersonnel.Manysuchhazardsarecommontomanyoralltypesofmedicalelectricalequipment,whilstothersarepeculiartoparticularcategoriesofequipment.Listedbelowarevarioustypesofcommonhazards.
MechanicalHazards
Alltypesofmedicalelectricalequipmentcanpresentmechanicalhazards.Thesecanrangefrominsecurefittingsofcontrolstoloosefixingsofwheelsonequipmenttrolleys.Theformermaypreventapieceoflifesupportingequipmentfrombeingoperatedproperly,whilstthelattercouldcauseseriousaccidentsintheclinicalenvironment.
Suchhazardsmayseemtooobvioustowarrantmentioning,butitisunfortunatelyalltoocommonforsuchmundaneproblemstobeoverlookedwhilemoreexoticproblemsareaddressed.
Riskoffireorexplosion
Allmainspoweredelectricalequipmentcanpresenttheriskoffireintheeventofcertainfaultsoccurringsuchasinternalorexternalshortcircuits.Incertainenvironmentssuchfiresmaycauseexplosions.Althoughtheuseofexplosiveanaestheticgasesisnotcommontoday,itshouldberecognisedthatmanyofthemedicalgasesinusevigorouslysupportcombustion.
AbsenceofFunction
Sincemanypiecesofmedicalelectricalequipmentarelifesupportingormonitorvitalfunctions,theabsenceoffunctionofsuchapieceofequipmentwouldnotbemerelyinconvenient,butcouldthreatenlife.
Excessiveorinsufficientoutput
Inordertoperformitsdesiredfunctionequipmentmustdeliveritsspecifiedoutput.Toohighanoutput,forexample,inthecaseofsurgicaldiathermyunits,wouldclearlybehazardous.Equally,toolowanoutputwouldresultininadequatetherapy,whichinturnmaydelaypatientrecovery,causepatientinjuryorevendeath.Thishighlightstheimportanceofcorrectcalibrationprocedures.
Infection
Medicalequipmentthathasbeeninadequatelydecontaminatedafterusemaycauseinfectionthroughthetransmissionofmicroorganismstoanypersonwhosubsequentlycomesintocontactwithit.Clearly,patients,nursingstaffandservicepersonnelarepotentiallyatriskhere.
Misuse
Misuseofequipmentisoneofthemostcommoncausesofadverseincidentsinvolvingmedicaldevices.Suchmisusemaybearesultofinadequateusertrainingorofpooruserinstructions.
Riskofexposuretospuriouselectriccurrents
Allelectricalequipmenthasthepotentialtoexposepeopletotheriskofspuriouselectriccurrents.Inthecaseofmedicalelectricalequipment,theriskispotentiallygreatersincepatientsareintentionallyconnectedtosuchequipmentandmaynotbenefitfromthesamenaturalprotectionfactorsthatapplytopeopleinothercircumstances.Whilstallofthehazardslistedareimportant,thepreventionofmanyofthemrequiremethodspeculiartotheparticulartypeofequipmentunderconsideration.Forexample,inordertoavoidtheriskofexcessiveoutputofsurgicaldiathermyunits,knowledgeofradiofrequencypowermeasurementtechniquesisrequired.However,theelectricalhazardsarecommontoalltypesofmedicalelectricalequipmentandcanminimisedbytheuseofsafetytestingregimeswhichcanbeappliedtoalltypesofmedicalelectricalequipment.Forthesereasons,itistheelectricalhazardsthatarethemaintopicofthissession.
2Physiologicaleffectsofelectricity
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Electrolysis
ThemovementofionsofoppositepolaritiesinoppositedirectionsthroughamediumiscalledelectrolysisandcanbemadetooccurbypassingDCcurrentthroughbodytissuesorfluids.IfaDCcurrentispassedthroughbodytissuesforaperiodofminutes,ulcerationbeginstooccur.Suchulcers,whilenotnormallyfatal,canbepainfulandtakelongperiodstoheal.
Burns
Whenanelectriccurrentpassesthroughanysubstancehavingelectricalresistance,heatisproduced.Theamountofheatdependsonthepowerdissipated(I2R).Whetherornottheheatproducesaburndependsonthecurrentdensity.
Humantissueiscapableofcarryingelectriccurrentquitesuccessfully.Skinnormallyhasafairlyhighelectricalresistancewhilethemoisttissueunderneaththeskinhasamuchlowerresistance.Electricalburnsoftenproducetheirmostmarkedeffectsneartotheskin,althoughitisfairlycommonforinternalelectricalburnstobeproduced,which,ifnotfatal,cancauselonglastingandpainfulinjury.
Musclecramps
Whenanelectricalstimulusisappliedtoamotornerveoramuscle,themuscledoesexactlywhatitisdesignedtodointhepresenceofsuchastimulus.itcontracts.Theprolongedinvoluntarycontractionofmuscles(tetanus)causedbyanexternalelectricalstimulusisresponsibleforthephenomenonwhereapersonwhoisholdinganelectricallyliveobjectcanbeunabletoletgo.
Respiratoryarrest
Themusclesbetweentheribs(intercostalmuscles)needtorepeatedlycontractandrelaxinordertofacilitatebreathing.Prolongedtetanusofthesemusclescanthereforepreventbreathing.
Cardiacarrest
Theheartisamuscularorgan,whichneedstobeabletocontractandrelaxrepetitivelyinordertoperformitsfunctionasapumpfortheblood.Tetanusoftheheartmusculaturewillpreventthepumpingprocess.
Ventricularfibrillation
Theventriclesoftheheartarethechambersresponsibleforpumpingbloodoutoftheheart.Whentheheartisinventricularfibrillation,themusculatureoftheventriclesundergoesirregular,uncoordinatedtwitchingresultinginnonetbloodflow.Theconditionprovesfatalifnotcorrectedinaveryshortspaceoftime.
Ventricularfibrillationcanbetriggeredbyverysmallelectricalstimuli.Acurrentaslowas70mAflowingfromhandtohandacrossthechest,or20μAdirectlythroughtheheartmaybesufficient.Itisforthisreasonthatmostdeathsfromelectricshockareattributabletotheoccurrenceofventricularfibrillation.
Effectoffrequencyonneuro-muscularstimulation
Theamountofcurrentrequiredtostimulatemusclesisdependenttosomeextentonfrequency.Referringtofigure1,itcanbeseenthatthesmallestcurrentrequiredtopreventthereleaseofanelectricallyliveobjectoccursatafrequencyofaround50Hz.Above10kHztheneuro-muscularresponsetocurrentdecreasesalmostexponentially.
Figure1.Currentrequiredtopreventreleaseofaliveobject.
Naturalprotectionfactors
Manypeoplehavereceivedelectricshocksfrommainspotentialsandaboveandlivedtotellthetale.Partofthereasonforthisistheexistenceofcertainnaturalprotectionfactors.
Ordinarily,apersonsubjecttoanunexpectedelectricalstimulusisprotectedtosomeextentbyautomaticandintentionalreflexactions.Theautomaticcontractionofmusclesonreceivinganelectricalstimulusoftenactstodisconnectthepersonfromthesourceofthestimulus.Intentionalreactionsofthepersonreceivingtheshocknormallyservethesamepurpose.Itisimportanttorealisethatapatientintheclinicalenvironmentwhomayhaveelectricalequipmentintentionallyconnectedtothemandmayalsobeanaesthetisedarerelativelyunprotectedbythesemechanisms.
Normally,apersonwhoissubjecttoanelectricshockreceivestheshockthroughtheskin,whichhasahighelectricalresistancecomparedtothemoistbodytissuesbelow,andhenceservestoreducetheamountofcurrentthatwouldotherwiseflow.Again,apatientdoesnotnecessarilyenjoythesamedegreeofprotection.Theresistanceoftheskinmayintentionallyhavebeenloweredinordertoallowgoodconnectionsofmonitoringelectrodestobemadeor,inthecaseofapatientundergoingsurgery,theremaybenoskinpresentinthecurrentpath.
Theabsenceofnaturalprotectionfactorsasdescribedabovehighlightstheneedforstringentelectricalsafetyspecificationsformedicalelectricalequipmentandforroutinetestandinspectionregimesaimedatverifyingelectricalsafety.
3Leakagecurrents
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Causesofleakagecurrents
Ifanyconductorisraisedtoapotentialabovethatofearth,somecurrentisboundtoflowfromthatconductortoearth.Thisistrueevenofconductorsthatarewellinsulatedfromearth,sincethereisnosuchthingasperfectinsulationorinfiniteimpedance.Theamountofcurrentthatflowsdependson:
a.thevoltageontheconductor.
b.thecapacitivereactancebetweentheconductorandearth.
c.theresistancebetweentheconductorandearth.
Thecurrentsthatflowfromorbetweenconductorsthatareinsulatedfromearthandfromeachotherarecalledleakagecurrents,andarenormallysmall.However,sincetheamountofcurrentrequiredtoproduceadversephysiologicaleffectsisalsosmall,suchcurrentsmustbelimitedbythedesignofequipmenttosafevalues.
Formedicalelectricalequipment,severaldifferentleakagecurrentsaredefinedaccordingtothepathsthatthecurrentstake.
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Earthleakagecurrent
Earthleakagecurrentisthecurrentthatnormallyflowsintheearthconductorofaprotectivelyearthedpieceofequipment.Inmedicalelectricalequipment,veryoften,themainsisconnectedtoatransformerhavinganearthedscreen.Mostoftheearthleakagecurrentfindsitswaytoearthviatheimpedanceoftheinsulationbetweenthetransformerprimaryandtheinter-windingscreen,sincethisisthepointatwhichtheinsulationimpedanceisatitslowest(seefigure2).
Figure2.Earthleakagecurrentpath.
Undernormalconditions,apersonwhoisincontactwiththeearthedmetalenclosureoftheequipmentandwithanotherearthedobjectwouldsuffernoadverseeffectsevenifafairlyl
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