火灾报警和安全检测毕业论文外文翻译.docx

火灾报警和安全检测毕业论文外文翻译.docx

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火灾报警和安全检测毕业论文外文翻译

FireAlarmsandSecurityDetection

Afirealarmcircuit,asitsnameimplies,soundsanalarmintheeventofafire.Therecanbeoneorseveralalarmsthroughoutabuilding,andtherecanbeseveralalarmpoints,whichactivatethewarming.Thealarmpointscanbeoperatedmanuallyorautomatically;inthelattercasetheymaybesensitivetoheat,smokeorionization.Thereareclearlymanycombinationspossible,andthischaptergivessomesystematicaccountofthewaytheyarebuiltup.

Severalalarmpointsareconnectedinparallel,andwheneveroneofthemisactuatedthecircuitiscompletedandthealarmsounds.Thisisdescribedasanopencircuit,anditwillbeseenthatitisnotfailsafe,becauseifthereisafailureofsupply,thefirealarmcannotwork.Anothercharacteristicofthiscircuitisthateveryalarmpointmustbecapableofcarryingthefullcurrenttakenbyallthebellsorhootersworkingtogether.

Thealarmpointsareconnectedinserieswitheachotherandwitharelaycoil.Therelayisnormallyclosedwhende-energized,andopenswhenthecoilisenergized.Thuswhenanalarmpointisactivatedtherelaycoilisde-energized,therelayclosesandthealarmsounds.Thissystemfailssafetotheextentthatifthecoilcircuitfailsthemaincircuitoperatesthealarm.Itisnotofcoursesafeagainsttotalfailureofthesupplybecauseinthateventthereisnosupplyavailabletoworkthebells.Thealarmpointsdonothavetocarrytheoperatingcurrentofthebellsorhooters.Thisarrangementiscalledaclosedcircuitincontrasttotheopencircuit.Wecannoticethatinanopencircuitthealarmpointsarewiredinparallelandarenormallyopen,whilstinaclosedcircuittheywiredinseriesandarenormallyclosed.

Atypicalmanuallyoperatedfirealarmpointiscontainedinarobustredplasticcasewithaglasscover.Thematerialischosenforitsfireresistingproperties.Thecasehasknockoutforconduitentriesattopandbottombutthematerialcanbesufficientlyeasilycutforthesiteelectriciantomakehimselfanentryinthebackifheneedsit.Alternativeterminalsareprovidedforcircuitsinwhichthecontactshavetoclosewhentheglassissmashedandforcircuitsinwhichthecontactshavetoopenwhentheglassissmashed.Intheformercase,thereisatestswitchwhichcanbereachedwhenthewholefrontisopenedwithanAllenKey.Inthelattercase,thetestpushisomittedbecausethecircuitisinanycaseofthefailsafetype.Thealarmpointillustratedissuitableforsurfacemounting.Similaronesareavailableforflushfixingandinweatherproofversions.Thecurrentcarryingcapacityofthecontactsshouldalwaysbecheckedwiththemaker'scatalogue.

Athermallyoperatedalarmpointconsistsofabi-metalstripthatdeflectswhenthetemperaturerises,andtherebytiltsatubehalffullofmercury.Whenthetubetiltsthemercuryflowsintotheotherhalfofthetubewhereitcompletesthecircuitbetweentwocontactspreviouslyseparatedbyair.Alternatively,thearrangementwithinthetubecanbesuchthatthemercurybreaksthecircuitwhenthetubeistilted.Thecasingofthealarmisofstainlesssteel.Heatdetectorsofthistypeareusuallysettooperateat65℃.Theyarefrequentlyusedinboilerhouses.

Asmokeoperatedalarmpointwouldbeusedonlyinspecialcircumstanceswhichmakeitnecessarytodetectsmokeratherthanheat.Thistypecancausenuisanceoperationofthealarmbyreactingtosmallquantitiesofsmokewhichhavenotbeencausedbyafire;theyhaveforexamplebeenknowntosoundthealarmasaresultofcigarettesmokeinanoffice.Modernoneshaveadjustablesensitivitysothattheycanbesettoavoidnuisanceoperation.

Anionizationdetectorcontainsachamberwhichhousessomelowstrengthradioactivematerialandapairofelectrodes.Theradioactivematerialmakestheairinthechamberconductivesothatasmallcurrentflowsbetweentheelectrodes.Thesizeofthecurrentvarieswiththenatureofthegasinthechamberandassoonasanycombustion.Productsareaddedtotheairthereisasuddenchangeinthecurrentflowing.Thedetectoralsohasasecondchamberwhichispermanentlysealedsothatthecurrentthroughitneverchanges.Aslongasthecurrentsthroughthetwochambersareequalthereisnooutput,assoonastheybecomeunbalancedthereisanetoutputwhichisusedtooperateatransistorswitchinthemaincircuitthroughthedetector.

Thehumanbeingisthemostefficientfiredetector,providedthatheisactuallythereatthetime.Thecombinationofhiseyes,earsandnoseallowshimtodifferentiatebetweenvariousphenomena.Hecansmellthedifferencebetweencigarettesmoke,burningtoastandsomethingmoresinistersuchasburningpaper.Hecanseesmokeandflamesandwilloftenhearafirebeforehehasnoticedeithe