医疗器械漏电流测试示意图Word文档下载推荐.docx

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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,or20µ

Adirectlythroughtheheartmaybesufficient.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