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
SafetyTestingofMedicalElectricalEquipment
1HazardsofMedicalElectricalEquipment
Medicalelectricalequipmentcanpresentarangeofhazardstothepatient,theuser,ortoservicepersonnel.Manysuchhazardsaremontomanyoralltypesofmedicalelectricalequipment,whilstothersarepeculiartoparticularcategoriesofequipment.Listedbelowarevarioustypesofmonhazards.
1.1MechanicalHazards
Alltypesofmedicalelectricalequipmentcanpresentmechanicalhazards.Thesecanrangefrominsecurefittingsofcontrolstoloosefi*ingsofwheelsonequipmenttrolleys.Theformermaypreventapieceoflifesupportingequipmentfrombeingoperatedproperly,whilstthelattercouldcauseseriousaccidentsintheclinicalenvironment.
Suchhazardsmayseemtooobvioustowarrantmentioning,butitisunfortunatelyalltoomonforsuchmundaneproblemstobeoverlookedwhilemoree*oticproblemsareaddressed.
1.2Riskoffireore*plosion
Allmainspoweredelectricalequipmentcanpresenttheriskoffireintheeventofcertainfaultsoccurringsuchasinternalore*ternalshortcircuits.Incertainenvironmentssuchfiresmaycausee*plosions.Althoughtheuseofe*plosiveanaestheticgasesisnotmontoday,itshouldberecognisedthatmanyofthemedicalgasesinusevigorouslysupportbustion.
1.3AbsenceofFunction
Sincemanypiecesofmedicalelectricalequipmentarelifesupportingormonitorvitalfunctions,theabsenceoffunctionofsuchapieceofequipmentwouldnotbemerelyinconvenient,butcouldthreatenlife.
1.4E*cessiveorinsufficientoutput
Inordertoperformitsdesiredfunctionequipmentmustdeliveritsspecifiedoutput.Toohighanoutput,fore*ample,inthecaseofsurgicaldiathermyunits,wouldclearlybehazardous.Equally,toolowanoutputwouldresultininadequatetherapy,whichinturnmaydelaypatientrecovery,causepatientinjuryorevendeath.Thishighlightstheimportanceofcorrectcalibrationprocedures.
1.5Infection
Medicalequipmentthathasbeeninadequatelydecontaminatedafterusemaycauseinfectionthroughthetransmissionofmicroorganismstoanypersonwhosubsequentlyesintocontactwithit.Clearly,patients,nursingstaffandservicepersonnelarepotentiallyatriskhere.
1.6Misuse
Misuseofequipmentisoneofthemostmoncausesofadverseincidentsinvolvingmedicaldevices.Suchmisusemaybearesultofinadequateusertrainingorofpooruserinstructions.
1.7Riskofe*posuretospuriouselectriccurrents
Allelectricalequipmenthasthepotentialtoe*posepeopletotheriskofspuriouselectriccurrents.Inthecaseofmedicalelectricalequipment,theriskispotentiallygreatersincepatientsareintentionallyconnectedtosuchequipmentandmaynotbenefitfromthesamenaturalprotectionfactorsthatapplytopeopleinothercircumstances.Whilstallofthehazardslistedareimportant,thepreventionofmanyofthemrequiremethodspeculiartotheparticulartypeofequipmentunderconsideration.Fore*ample,inordertoavoidtheriskofe*cessiveoutputofsurgicaldiathermyunits,knowledgeofradiofrequencypowermeasurementtechniquesisrequired.However,theelectricalhazardsaremontoalltypesofmedicalelectricalequipmentandcanminimisedbytheuseofsafetytestingregimeswhichcanbeappliedtoalltypesofmedicalelectricalequipment.Forthesereasons,itistheelectricalhazardsthatarethemaintopicofthissession.
2Physiologicaleffectsofelectricity
2.1Electrolysis
ThemovementofionsofoppositepolaritiesinoppositedirectionsthroughamediumiscalledelectrolysisandcanbemadetooccurbypassingDCcurrentthroughbodytissuesorfluids.IfaDCcurrentispassedthroughbodytissuesforaperiodofminutes,ulcerationbeginstooccur.Suchulcers,whilenotnormallyfatal,canbepainfulandtakelongperiodstoheal.
2.2Burns
Whenanelectriccurrentpassesthroughanysubstancehavingelectricalresistance,heatisproduced.Theamountofheatdependsonthepowerdissipated(I2R).Whetherornottheheatproducesaburndependsonthecurrentdensity.
Humantissueiscapableofcarryingelectriccurrentquitesuccessfully.Skinnormallyhasafairlyhighelectricalresistancewhilethemoisttissueunderneaththeskinhasamuchlowerresistance.Electricalburnsoftenproducetheirmostmarkedeffectsneartotheskin,althoughitisfairlymonforinternalelectricalburnstobeproduced,which,ifnotfatal,cancauselonglastingandpainfulinjury.
2.3Musclecramps
Whenanelectricalstimulusisappliedtoamotornerveoramuscle,themuscledoese*actlywhatitisdesignedtodointhepresenceofsuchastimulusi.e.itcontracts.Theprolongedinvoluntarycontractionofmuscles(tetanus)causedbyane*ternalelectricalstimulusisresponsibleforthephenomenonwhereapersonwhoisholdinganelectricallyliveobjectcanbeunabletoletgo.
2.4Respiratoryarrest
Themusclesbetweentheribs(intercostalmuscles)needtorepeatedlycontractandrela*inordertofacilitatebreathing.Prolongedtetanusofthesemusclescanthereforepreventbreathing.
2.5Cardiacarrest
Theheartisamuscularorgan,whichneedstobeabletocontractandrela*repetitivelyinordertoperformitsfunctionasapumpfortheblood.Tetanusoftheheartmusculaturewillpreventthepumpingprocess.
2.6Ventricularfibrillation
Theventriclesoftheheartarethechambersresponsibleforpumpingbloodoutoftheheart.Whentheheartisinventricularfibrillation,themusculatureoftheventriclesundergoesirregular,uncoordinatedtwitchingresultinginnonetbloodflow.Theconditionprovesfatalifnotcorrectedinaveryshortspaceoftime.
Ventricularfibrillationcanbetriggeredbyverysmallelectricalstimuli.Acurrentaslowas70mAflowingfromhandtohandacrossthechest,or20µAdirectlythroughtheheartmaybesufficient.Itisforthisreasonthatmostdeathsfromelectricshockareattributabletotheoccurrenceofventricularfibrillation.
2.7Effectoffrequencyonneuro-muscularstimulation
Theamountofcurrentrequiredtostimulatemusclesisdependenttosomee*tentonfrequency.Referringtofigure1,itcanbeseenthatthesmallestcurrentrequiredtopreventthereleaseofanelectricallyliveobjectoccursatafrequencyofaround50Hz.Above10kHztheneuro-muscularresponsetocurrentdecreasesalmoste*ponentially.
Figure1.Currentrequiredtopreventreleaseofaliveobject.
2.8Naturalprotectionfactors
Manypeoplehavereceivedelectricshocksfrommainspotentialsandaboveandlivedtotellthetale.Partofthereasonforthisisthee*istenceofcertainnaturalprotectionfactors.
Ordinarily,apersonsubjecttoanune*pectedelectricalstimulusisprotectedtosomee*tentbyautomaticandintentionalrefle*actions.Theautomaticcontractionofmusclesonreceivinganelectricalstimulusoftenactstodisconnectthepersonfromthesourceofthestimulus.Intentionalreactionsofthepersonreceivingtheshocknormallyservethesamepurpose.Itisimportanttorealisethatapatientintheclinicalenvironmentwhomayhaveelectricalequipmentintentionallyconnectedtothemandmayalsobeanaesthetisedarerelativelyunprotectedbythesemechanisms.
Normally,apersonwhoissubjecttoanelectricshockreceivestheshockthroughtheskin,whichhasahighelectricalresistanceparedtothemoistbodytissuesbelow,andhenceservestoreducetheamountofcurrentthatwouldotherwiseflow.Again,apatientdoesnotnecessarilyenjoythesamedegreeofprotection.Theresistanceoftheskinmayintentionallyhavebeenloweredinordertoallowgoodconnectionsofmonitoringelectrodestobemadeor,inthecaseofapatientundergoingsurgery,theremaybenoskinpresentinthecurrentpath.
Theabsenceofnaturalprotectionfactorsasdescribedabovehighlightstheneedforstringentelectricalsafetyspecificationsformedicalelectricalequipmentandforroutinetestandinspectionregimesaimedatverifyingelectricalsafety.
3Leakagecurrents
3.1Causesofleakagecurrents
Ifanyconductorisraisedtoapotentialabovethatofearth,somecurrentisboundtoflowfromthatconductortoearth.Thisistrueevenofconductorsthatarewellinsulatedfromearth,sincethereisnosuchthingasperfectinsulationorinfiniteimpedance.Theamountofcurrentthatflowsdependson:
a.thevoltageontheconductor.
b.thecapacitivereactancebetweentheconductorandearth.
c.theresistancebetweentheconductorandearth.
Thecurrentsthatflowfromorbetweenconductorsthatareinsulatedfromearthandfromeachotherarecalledleakagecurrents,andarenormallysmall.However,sincetheamountofcurrentrequiredtoproduceadversephysiologicaleffectsisalsosmall,suchcurrentsmustbelimitedbythedesignofequipmenttosafevalues.
Formedicalelectricalequipment,severaldifferentleakagecurrentsaredefinedaccordingtothepathsthatthecurrentstake.
3.2Earthleakagecurrent
Earthleakagecurrentisthecurrentthatnormallyflowsintheearthconductorofaprotectivelyearthedpieceofequipment.Inmedicalelectricalequipment,veryoften,themainsisconnectedtoatransformerhavinganearthedscreen.Mostoftheearthleakagecurrentfindsitswaytoearthviatheimpedanceoftheinsulationbetweenthetransformerprimaryandtheinter-windingscreen,sincethisisthepointatwhichtheinsulationimpedanceisatitslowest(seefigure2).
Figure2.Earthleakagecurrentpath.
Undernormalconditions,apersonwhoisincontactwiththeearthedmetalenclosureoftheequipmentandwithanotherearthedobjectwouldsuffernoadverseeffectsevenifafairlylargeearthleakagecurrentweretoflow.T
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