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1、FireAlarmsandSecurityDetectionAfirealarmcircuit,asitsnameimplies,soundsanalarmintheeventofafire.Therecanbeoneorseveralalarmsthroughoutabuilding,andtherecanbeseveralalarmpoints,whichactivatethewarming.Thealarmpointscanbeoperatedmanuallyorautomatically;inthelattercasetheymaybesensitivetoheat,s
2、mokeorionization.Thereareclearlymanycombinationspossible,andthischaptergivessomesystematicaccountofthewaytheyarebuiltup.Severalalarmpointsareconnectedinparallel,andwheneveroneofthemisactuatedthecircuitiscompletedandthealarmsounds.Thisisdescribedasanopencircuit,anditwillbeseenthatitisnotfails
3、afe,becauseifthereisafailureofsupply,thefirealarmcannotwork.Anothercharacteristicofthiscircuitisthateveryalarmpointmustbecapableofcarryingthefullcurrenttakenbyallthebellsorhootersworkingtogether.Thealarmpointsareconnectedinserieswitheachotherandwitharelaycoil.Therelayisnormallyclosedwhende-e
4、nergized,andopenswhenthecoilisenergized.Thuswhenanalarmpointisactivatedtherelaycoilisde-energized,therelayclosesandthealarmsounds.Thissystemfailssafetotheextentthatifthecoilcircuitfailsthemaincircuitoperatesthealarm.Itisnotofcoursesafeagainsttotalfailureofthesupplybecauseinthateventthereisno
5、supplyavailabletoworkthebells.Thealarmpointsdonothavetocarrytheoperatingcurrentofthebellsorhooters.Thisarrangementiscalledaclosedcircuitincontrasttotheopencircuit.Wecannoticethatinanopencircuitthealarmpointsarewiredinparallelandarenormallyopen,whilstinaclosedcircuittheywiredinseriesandarenor
6、mallyclosed.Atypicalmanuallyoperatedfirealarmpointiscontainedinarobustredplasticcasewithaglasscover.Thematerialischosenforitsfireresistingproperties.Thecasehasknockoutforconduitentriesattopandbottombutthematerialcanbesufficientlyeasilycutforthesiteelectriciantomakehimselfanentryinthebackifhe
7、needsit.Alternativeterminalsareprovidedforcircuitsinwhichthecontactshavetoclosewhentheglassissmashedandforcircuitsinwhichthecontactshavetoopen10whentheglassissmashed.Intheformercase,thereisatestswitchwhichcanbereachedwhenthewholefrontisopenedwithan
