消防报警和安全检测外文文献翻译.docx
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消防报警和安全检测外文文献翻译.docx
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消防报警和安全检测外文文献翻译
FireAlarms and SecurityDetection
Afirealarmcircuit,asitsname implies,sounds analarmintheeventofafire.Therecanbeoneor several alarmsthroughoutabuilding,andtherecanbeseveralalarmpoints,which activate thewarming. Thealarm pointscanbeoperatedmanuallyor automatically; inthelattercasetheymaybesensitiveto heat, smokeor ionization.Thereareclearly manycombinations possible,andthischaptergivessomesystematicaccount oftheway theyarebuiltup.
Several alarmpointsareconnectedin parallel,andwhenever oneofthem is actuated thecircuit iscompleted andthe alarmsounds.This isdescribedasanopen circuit,anditwill beseenthatit is notfail safe,becauseifthereisafailureofsupply, thefirealarmcannotwork.Another characteristicof this circuit isthateveryalarmpointmustbe capableofcarryingthefull currenttakenbyallthe bellsorhootersworkingtogether.
Thealarmpoints areconnected inserieswitheachother and witharelaycoil.Therelayis normallyclosedwhende-energized,andopenswhenthecoilisenergized.Thuswhen analarmpointisactivated the relay coil is de-energized, the relayclosesandthe alarmsounds.Thissystemfailssafe to the extentthatifthecoil circuitfailsthemain circuitoperates thealarm.Itisnotofcourse safeagainsttotal failureofthe supplybecauseinthateventthereisnosupplyavailabletoworkthebells. Thealarm pointsdonothavetocarrytheoperatingcurrentofthebellsor hooters.Thisarrangementiscalledaclosedcircuitincontrast totheopencircuit.We cannoticethatin anopencircuitthealarm pointsarewiredinparallelandarenormallyopen,whilstina closed circuit theywiredin seriesandarenormallyclosed.
Atypicalmanuallyoperatedfirealarmpointiscontained in a robust redplasticcase witha glass cover.Thematerialischosenforitsfireresistingproperties. The casehasknockout forconduit entries attopandbottom but thematerialcanbesufficientlyeasilycutforthesiteelectriciantomake himselfanentry inthebackifheneedsit.Alternativeterminalsareprovidedforcircuitsin whichthecontactshavetoclosewhentheglassissmashedandfor circuits inwhichthecontactshaveto openwhen theglass issmashed.Inthe formercase,there isatestswitchwhichcanbereachedwhen thewholefront is openedwithan AllenKey.In thelatter case, thetestpush is omittedbecausethe circuit is inany caseofthe failsafetype. Thealarmpoint illustratedissuitable forsurface mounting.Similar onesareavailableforflushfixingandin weatherproof versions. Thecurrentcarryingcapacityofthe contactsshouldalways becheckedwith themaker'scatalogue.
Athermallyoperatedalarmpointconsistsofa bi-metalstripthatdeflectswhenthetemperaturerises, andtherebytiltsatubehalffullofmercury. Whenthe tubetilts themercuryflows into the otherhalfof thetube where itcompletesthecircuitbetweentwocontactspreviouslyseparated byair.Alternatively, thearrangement within thetube can be suchthat the mercury breaksthecircuitwhenthe tubeistilted. Thecasingof thealarm is of stainless steel.Heatdetectorsof thistypeareusuallysettooperateat65℃. Theyarefrequently usedin boilerhouses.
Asmokeoperatedalarmpointwouldbeusedonlyinspecialcircumstanceswhich make it necessarytodetect smokerather thanheat.Thistypecan cause nuisanceoperation ofthealarmbyreactingtosmall quantities ofsmokewhich havenotbeen causedbyafire;they haveforexamplebeenknowntosoundthealarmas a result of cigarette smoke in anoffice.Modernoneshave adjustable sensitivitysothattheycanbesettoavoid nuisanceoperation.
Anionization detectorcontainsachamber whichhousessome lowstrength radioactivematerial andapairofelectrodes.The radioactivematerial makes theairinthechamberconductivesothat asmallcurrentflowsbetweentheelectrodes. The sizeofthecurrentvarieswith thenatureofthe gasinthechamberandassoonasanycombustion.Productsareaddedtotheairthereis asuddenchange inthe currentflowing.The detectoralsohasa secondchamberwhichispermanently sealedsothatthecurrentthroughitnever changes.Aslong asthecurrents throughthetwochambersareequalthereisnooutput,assoon astheybecomeunbalancedthere is anetoutputwhichisusedtooperatea transistorswitchinthe maincircuitthroughthedetector.
Thehumanbeing isthemost efficientfiredetector,providedthatheisactuallythereatthe time. Thecombinationofhiseyes,ears andnoseallowshim to differentiate between variousphenomena.He can smellthedifference betweencigarettesmoke, burningtoast andsomethingmoresinistersuchasburningpaper. Hecanseesmoke andflamesandwilloftenhear a fire beforehe hasnoticedeither theflamesorthesmoke.Sometimes,thefirst thinghenoticesisheat,forexample,ahotpartitionwall. Itis thesephenomena thatfiredetectordesignersmeasuretodetecttheonset of fire. Itis theonsetoffire that iscrucial; verysmallfirescanbeputoutquickly andsafelywith portablefireextinguishers,orevenacupof coffeeif detectedsoon enough.Obviously,thesmallerthefire,thelessitmaybeobserved. Firedetectors must,therefore,bedesignedwithouttheriskof falsealarms.So whattechnology isused todaytodetect thesephenomena?
Two techniquesare used for thedetection of smoke:
photoelectric smoke detectorsandionization smoke detectors.Photoelectric detectorsarearrangedso that,althoughvisiblelightcannotenter,smokecanenterfreely.Thistype ofdetector isbasedonamattblackchamberaboutthesizeofacoffeecup.Adividingwall is placedin the middle of the chamberoneithersideofwhichislocated a light transmitterandalightreceiver.Undernormalcircumstances, lightfromthetransmitterisnotreceived bythereceiver,asthechamberisnot reflective. Whensmokeparticles enter thechamber,light bouncesofftheseparticleswhich resultsinasignalbeing received. A similareffectcanbeobservedwhensunshine entersa smokyroom.Thesephoto-electricdetectorsarehighly sensitive andreliable,solong asthe smokeparticlesarevisible.
Certainclassesoffire,however,emitinvisiblesmokeparticleslongbeforethefireis actuallyalightandin thesecasesan ionization smoke detectormustbepresent.Althoughthedesign ofthese detectorsisslightlymorecomplicatedthaninthecaseof photo-electric detectors,thebasic principleisverysimple.Twometalplatesareseparated byseveralmillimetersandavoltageisapplied acrossthem.Sinceairdoesnotconductelectricity, nocurrentflows.Ifanionizingradioactivesourceis broughtclose totheplates then theair itselfisionized,thatis, electrically chargesparticles are presentbetween the platesandtheseallow atinycurrentto flow.Anyparticlesenteringthe chamber,eithervisibleor invisible,tendtoneutralizetheions,thusdecreasingthe current flow.Itisthisreductionincurrentwhichismonitored inmeasuringtheamountofsmoke thatis present.
Firedetectionanditscorresponding safetysystems arecrucialpartsofan intelligent building.Thispaperreviews thecurrentstateofdevelopment offiredetectionand alarm systems intheintelligentbuilding.New technologiesandconceptsdeveloped in intelligent buildings,suchasadvancedmulti-functionsensors,computer visionsystems andwirelesssensors,real-time controlviatheInternet,and integratedbuildingservice systems,have alsobeen reviewed and discussed. Thesenewtechnologiesandconceptswillimprove thecapabilityoffiredetectionsystems todiscriminatebetween fire and non-firethreatsandwillincreasethe time availablefor propertyand lifeprotection.However,mucheffortisstill neededtoremove barriersto thefurtherdevelopmentofthesenew technologies.
Newsensor technologieswillbekeycomponentsinthe nextgenerationofintelligent buildings.Current intelligentbuildingsoftenhaveembedded processorsanddedicatedinformationnetworks. Thenewgenerationis expectedtoaddthecapabilitytolearnaboutthebuildings circumstancesanditsoccupants needsand changethe behavior ofitscontrolsystemsaccordingly. Theemploymentof alargenumber ofsensorswithin the buildingwillallowit tooperateina responsivemanner,rather thanusingpre-programmedcontrolmodels asareemployedinthe first twogenerationsof intelligentbuilding. Theinformationprovided bysensorsincludeschangesinboth internalandexternal environmentsofabuilding,suchas smoke,temperatureandhumidity, airquality,air movement,andthenumberofbuildingoccupantsaswell asahostofother properties. Thesystemwillusesensorstoidentifyhowaparticularpersontendstoreacttoparticularcircumstancesandtolearndifferentbehaviorsfor differentpeople. Thenumber ofsensors required toobtainthistypeoffunctionality isquitehigh,especiallysinceone of themajorgoalsofintelligentbuildingsisto allowindividualizedcontrolofanenvironment.Thisneedwillincreasethe costof intelligent buildingsand makeitdifficulttomanage theresultinglarge amountofdata.Developmentofcost-effectivesensors hasconsequentlybeen identifiedasa keyneedforintelligentbuildings. Fortunately,manyof thepropertiesthatneedtobemonitored can beusedformultiplepurposes. Securitysystemsthatcantracktheentryand exit ofoccupantsfrom anofficebuilding canalsobeused toensurecompleteevacuationofabuilding duringafireor even,in moreadvancedforms,determinewhereoccupantsmaybetrappedandunable to escape.Similarly,para
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