医疗器械漏电流测试示意图.docx

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.Manysuchhazardsarecommontomanyoralltypesofmedicalelectricalequipment,whilstothersarepeculiartoparticularcategoriesofequipment.Listedbelowarevarioustypesofcommonhazards.

1.1MechanicalHazards

Alltypesofmedicalelectricalequipmentcanpresentmechanicalhazards.Thesecanrangefrominsecurefittingsofcontrolstoloosefixingsofwheelsonequipmenttrolleys.Theformermaypreventapieceoflifesupportingequipmentfrombeingoperatedproperly,whilstthelattercouldcauseseriousaccidentsintheclinicalenvironment.

Suchhazardsmayseemtooobvioustowarrantmentioning,butitisunfortunatelyalltoocommonforsuchmundaneproblemstobeoverlookedwhilemoreexoticproblemsareaddressed.

1.2Riskoffireorexplosion

Allmainspoweredelectricalequipmentcanpresenttheriskoffireintheeventofcertainfaultsoccurringsuchasinternalorexternalshortcircuits.Incertainenvironmentssuchfiresmaycauseexplosions.

Althoughtheuseofexplosiveanaestheticgasesisnotcommontoday,itshouldberecognisedthatmanyofthemedicalgasesinusevigorouslysupportcombustion.

1.3AbsenceofFunction

Sincemanypiecesofmedicalelectricalequipmentarelifesupportingormonitorvitalfunctions,theabsenceoffunctionofsuchapieceofequipmentwouldnotbemerelyinconvenient,butcouldthreatenlife.

1.4Excessiveorinsufficientoutput

Inordertoperformitsdesiredfunctionequipmentmustdeliveritsspecifiedoutput.Toohighanoutput,forexample,inthecaseofsurgicaldiathermyunits,wouldclearlybehazardous.Equally,toolow

anoutputwouldresultininadequatetherapy,whichinturnmaydelaypatientrecovery,causepatientinjuryorevendeath.Thishighlightstheimportanceofcorrectcalibrationprocedures.

1.5Infection

Medicalequipmentthathasbeeninadequatelydecontaminatedafterusemaycauseinfectionthroughthetransmissionofmicroorganismstoanypersonwhosubsequentlycomesintocontactwithit.Clearly,patients,nursingstaffandservicepersonnelarepotentiallyatriskhere.

1.6Misuse

Misuseofequipmentisoneofthemostcommoncausesofadverseincidentsinvolvingmedicaldevices.Suchmisusemaybearesultofinadequateusertrainingorofpooruserinstructions.

1.7Riskofexposuretospuriouselectriccurrents

Allelectricalequipmenthasthepotentialtoexposepeopletotheriskofspuriouselectriccurrents.Inthecaseofmedicalelectricalequipment,theriskispotentiallygreatersincepatientsareintentionallyconnectedtosuchequipmentandmaynotbenefitfromthesamenaturalprotectionfactorsthatapplytopeopleinothercircumstances.Whilstallofthehazardslistedareimportant,thepreventionofmanyofthemrequiremethodspeculiartotheparticulartypeofequipmentunderconsideration.Forexample,inordertoavoidtheriskofexcessiveoutputofsurgicaldiathermyunits,knowledgeofradiofrequencypowermeasurementtechniquesisrequired.However,theelectricalhazardsarecommontoalltypesofmedicalelectricalequipmentandcanminimisedbytheuseofsafetytestingregimeswhichcanbeappliedtoalltypesofmedicalelectricalequipment.Forthesereasons,itistheelectricalhazardsthatarethemaintopicofthissession.

2Physiologicaleffectsofelectricity

2.1Electrolysis

Themovementofionsofoppositepolaritiesinoppositedirectionsthroughamediumiscalledelectrolysisandcanbemadetooccurby

passingDCcurrentthroughbodytissuesorfluids.IfaDCcurrentispassedthroughbodytissuesforaperiodofminutes,ulcerationbeginstooccur.Suchulcers,whilenotnormallyfatal,canbepainfulandtakelongperiodstoheal.

2.2Burns

Whenanelectriccurrentpassesthroughanysubstancehavingelectricalresistance,heatisproduced.Theamountofheatdependsonthepowerdissipated（I2R）.Whetherornottheheatproducesaburndependsonthecurrentdensity.

Humantissueiscapableofcarryingelectriccurrentquitesuccessfully.Skinnormallyhasafairlyhighelectricalresistancewhilethemoisttissueunderneaththeskinhasamuchlowerresistance.Electricalburnsoftenproducetheirmostmarkedeffectsneartotheskin,althoughitisfairlycommonforinternalelectricalburnstobeproduced,which,ifnotfatal,cancauselonglastingandpainfulinjury.

2.3Musclecramps

Whenanelectricalstimulusisappliedtoamotornerveoramuscle,themuscledoesexactlywhatitisdesignedtodointhepresenceofsuchastimulusi.e.itcontracts.Theprolongedinvoluntarycontractionofmuscles（tetanus）causedbyanexternalelectricalstimulusisresponsibleforthephenomenonwhereapersonwhoisholdinganelectricallyliveobjectcanbeunabletoletgo.

2.4Respiratoryarrest

Themusclesbetweentheribs（intercostalmuscles）needtorepeatedlycontractandrelaxinordertofacilitatebreathing.Prolongedtetanusofthesemusclescanthereforepreventbreathing.

2.5Cardiacarrest

Theheartisamuscularorgan,whichneedstobeabletocontractandrelaxrepetitivelyinordertoperformitsfunctionasapumpfortheblood.Tetanusoftheheartmusculaturewillpreventthepumpingprocess.

2.6Ventricularfibrillation

Theventriclesoftheheartarethechambersresponsibleforpumpingbloodoutoftheheart.Whentheheartisinventricularfibrillation,themusculatureoftheventriclesundergoesirregular,uncoordinated

twitchingresultinginnonetbloodflow.Theconditionprovesfatalifnotcorrectedinaveryshortspaceoftime.

Ventricularfibrillationcanbetriggeredbyverysmallelectricalstimuli.Acurrentaslowas70mAflowingfromhandtohandacrossthechest,or20μAdirectlythroughtheheartmaybesufficient.Itisforthisreasonthatmostdeathsfromelectricshockareattributabletotheoccurrenceofventricularfibrillation.

2.7Effectoffrequencyonneuro-muscularstimulation

Theamountofcurrentrequiredtostimulatemusclesisdependenttosomeextentonfrequency.Referringtofigure1,itcanbeseenthatthesmallestcurrentrequiredtopreventthereleaseofanelectricallyliveobjectoccursatafrequencyofaround50Hz.Above10kHztheneuro-muscularresponsetocurrentdecreasesalmostexponentially.

Figure1.Currentrequiredtopreventreleaseofaliveobject.

2.8Naturalprotectionfactors

Manypeoplehavereceivedelectricshocksfrommainspotentialsandaboveandlivedtotellthetale.Partofthereasonforthisistheexistenceofcertainnaturalprotectionfactors.

Ordinarily,apersonsubjecttoanunexpectedelectricalstimulusisprotectedtosomeextentbyautomaticandintentionalreflexactions.Theautomaticcontractionofmusclesonreceivinganelectricalstimulusoftenactstodisconnectthepersonfromthesourceofthestimulus.Intentionalreactionsofthepersonreceivingtheshocknormallyservethesamepurpose.Itisimportanttorealisethatapatientintheclinicalenvironmentwhomayhaveelectricalequipment

intentionallyconnectedtothemandmayalsobeanaesthetisedarerelativelyunprotectedbythesemechanisms.

Normally,apersonwhoissubjecttoanelectricshockreceivestheshockthroughtheskin,whichhasahighelectricalresistancecomparedtothemoistbodytissuesbelow,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,apersonwhoisincontactwiththeearthedmetalenclosureoftheequipmentandwit