Allindividualandoverallshields(screen)frommultipaircableshallbeterminatedintorespectiveinstrumentearthbusbaratmarshallingcabinet.
Instrumentbusbarwillbeconnectedtogroundingdispatcherby25mmsqgreenyellowstrippedcable.Fromgroundingdispatcher,itwillbeconnectedtomaininstrumentearthloopby70mmsqgreenyellowstrippedcable.
3.Intrinsicallysafeearthing
IsolationandterminationofISfieldcableshields(screen)atfielddevices,junctionboxesandmarshallingcabinetsshallbedoneinthesamemannerasforinstrumentearthexplainedabove.Howevertheoverallshield(screen)ofmultipaircableforISsignalsgoestomarshallingcabinetshallbeterminatedindividuallyandconnectedtoitsISbusbar.Theindividualshield(screen)ofthiscablewillbeterminateddirectlytoagalvanicisolatorandthenconnectedtotherespectiveISbusbar.
Thefollowingmaximumresistancelimitsshallbeachievedaftertheinstrumentearthingsysteminstalled.Thisresistanceisminimizedasmuchaspossiblesotheubnormalcurrentcanbesafelygroundedatsteelstructure.
required,consequencestofacility,andtheareaofthefireevents.
Smokedetectors
Themostusedsmokedetectorsareionizationtypeandphotoelectrictype.Theionizationtypeutilizesaconstantcurrentthatproducedbyelectrodefromanionizationofsomeisotope.Anyparticlethatgoesthroughthechamberandinterrupttheionizationwillmakeareductionofconstantcurrent.Thusalarmwillbeactivated.Thephotoelectrictypeutilizesascatteredlightfromsmokeparticlestomeasurethesmokepresent.Anyparticlesthatinterruptthelightline(itwillbescattered)willcausethedetectoractivated.
Thesmokedetector(bothionizationandphotoelectrictype)areverysensitiveandshallnotbeusedatdirtyenvironments,smokyatmosphere,andopenareaswherethereisanunpredictableairflows.Forexample,smokedetectorshallnotbeusedatkitchen,mechanicalworkshop,etc.
Anothertypeofsmokedetectorsthatmostusedatoffshoreoil&gasindustriesareHighSensitiveSmokeDetectors.Thisdetectorusuallyplacedontheverycriticalareassuchascableducting,controlroom,andelectricalroom.Thisdetectortakeasampleofeachsamplelocationbyanairsamplingtubesandthendrawthesampletoacentralizeddetectorwhichanalyzeanysmallpresentofsmoke.Itwillallowthesystemtogiveveryfastandearliestalerttoanoperatoroffireevent(eventitstillonthesmokeformingstage).
FlameDetectors
Flamedetectorsarecategorizedasfastresponsedevices.Itsensesthe
flameonalineofsight.Afireconsistsofthreepart,smoke,heat,andflame.Thisdetectorisutilizingatapplicationwhenflameisthefirstpartoffirethatformed,suchasahydrocarbonleakarea.
Therearethreemajortypesofflamedetectors:
UltraViolet(UV)FlameDetectors
Infrared(IR)FlameDetectors
CombinationofUVandIRFlameDetectors
UltraViolet(UV)FlameDetectors
UVtypeflamedetectorsaredetectorsthatsensetheUVlightemittedfromaflame.ItssensitivetoasunUVlight,weldingarcs,xrays,andlightning.ThereforetheuseofUVflamedetectorislimitedontheclosedareawherethedisturbancefromsun,lightningetcisabsentsuchasaturbinecompressorclosure,turbinegeneratorclosureetc.
UVflamedetectorsarealsoshouldntusetodetectfirethathaveheavysmoke(i.e.crudeoil)orfiresthatdoesnthavevisibleflame(i.e.alcoholetc).
Infrared(IR)FlameDetectors
Infraredflamedetectorsaredetectorsthatsensethehotlight(infrared)emittedbyaCO2productofflame.ItsveryreliablethanUVflamedetectorbecauseofmoreimmunetoUVlight,weldingarcs,etc.Itcanbeusedinopenareasorclosedareas.Theinfraredflamedetectorssignalshallbefilteredfromhothumanbodyandsetittoinsensitivetohot
body.
CombinationofUV&IRFlameDetectors
UV/IRFlameDetectorsusebothUVandIRsensortosenseaflame.ItshouldbeselectabletobeUVonlyorIRonlyorbothsensorsactive.Itshouldbedetectanyflamewhetherindoororoutdoorapplications.IfbothUVandIRsensorareactive,itwillonlysendalarmsignalwhenbothofitsenseafire.ItwillreducethefalsealarmthatcausedbyusingUVonlyorIRonly.
HeatorThermalDetectors
Heatdetectorisslowerdevicecomparetosmokedetectorandflamedetector.Thisdetectorutilizeaheatsensitiveelement(usuallyarethermistor)tosensetheheat.Itwillactivateonlyafterthefirereachessomesignificantstagetoradiateitsheatenergyandsensedbytheheatsensitiveofheatdetectors.Eventhoughthisdetectorhasslowresponsetofirebutitsverysuitableforapplicationthatpronetoalarmfalseifweusesmokeorflamedetectorssuchaskitchenarea,workshop,laboratoryroom,smokingareaetc.
Therearethreetypesofheatdetectors:
Fixedtype.
Rateofrisetype.
Fusibleplugs.
Fixedheatdetectors
Afixedheatdetectorisadetectorthatsetatafixedtemperaturesetpoint.Oncetheambienttemperatures(causedbyafire)reachthesetpoint,thedetectorswillsendanalarmsignal.Asastandard,thefixedheatdetectorssetpointis77Celsius.Fixedheatdetectorsareverysuitableforapplicationthathasaswingstemperatureoniti.e.kitchen,workshop,laboratoryroometc.
Rateofriseheatdetectors
Rateofriseheatdetectorsisadetectorthatsensestherateofriseintheambienttemperature.Itsverysensitiveheatdetectorsandcancauseafalsealarmataroomhavingswingtemperature.Normallyitwillactivatewhentheheatrateofriseisbetween12Fahrenheitor15Fahrenheit.Itssuitabletouseatsmokingroom,warehouseetc.Itsalsosuitabletouseatroomthatdoesnthaveahighairflowthatcancauseasmokedetectorslowtoresponsesuchasstorageroomorwarehouse.
FusiblePlugs
Fusibleplugsareametalcylinderthathasasealedmetalwithlowmeltingpoint.Itisconnectedtoapneumatictubingloop.Whentheambienttemperaturesreachthemeltingpointofthesealmetal,thenthesealwillbrokeandcausingapneumaticairleakthroughit.Thisleakwillbedetectedbyapressureswitchanditwillinitiateanalarmthatactivatesaverylargecapacityvalvetooperate.Thislargecapacityvalves(calleddelugevalve)willspoutafirewatertoextinguishfires.Thefusibleplugisdesignedtoprotectaclosedvesselfromafire.Whenaclosedvesselexposedtoafire,itcancausingadangerousriseininternalpressureofvesselsandcausingablowup.Thereforeusuallyitputoutaroundaclosedvessel.
Toprotectbuildings,machinery&appliancesunderfaultconditions
Toprovideanalternativepathforthefaultcurrenttoflowsothatitwillnotendangertheuser
Toensurethatallexposedconductivepartsdonotreachadangerouspotential.
Toprovidestableplatformforoperationofsensitiveelectronicequipmentsi.e.
Toprovidesafepathtodissipatelightningandshortcircuitcurrents.
Tomaintainthevoltageatanypartofanelectricalsystemataknownvaluesoastopreventovercurrentorexcessivevoltageontheappliancesorequipment.
VOLTAGESTABILIZATION:
Therearemanysourcesofelectricity.Everytransformercanbeconsideredaseparatesource.Iftherewerenotacommonreferencepointforallthesevoltagesourcesitwouldbeextremelydifficulttocalculatetheirrelationshipstoeachother.Theearthisthemostomnipresentconductivesurface,andsoitwasadoptedintheverybeginningsofelectricaldistributionsystemsasanearlyuniversalstandardforallelectricsystems.
OVERVOLTAGEPROTECTION:
Lightning,linesurgesorunintentionalcontactwithhighervoltagelines
cancausedangerouslyhighvoltagestotheelectricaldistributionsystem.EarthingprovidesanalternativepatharoundtheelectricalsystemtominimizedamagesintheSystem.
DIFFERENTMETHODOFEARTHING
PLATETYPEEARTHING:GenerallyforplatetypeearthingnormalPracticeistouse
Castironplateofsize600mmx600mmx12mm.OR
Galvanizedironplateofsize600mmx600mmx6mm.OR
Copperplateofsize600mm*600mm*3.15mm
Plateburredatthedepthof8feetintheverticalpositionandGIstripofsize50mmx6mmboltedwiththeplateisbroughtuptothegroundlevel.
Thesetypesofearthpitaregenerallyfilledwithalternatelayerofcharcoal&saltupto4feetfromthebottomofthepit.
PIPETYPEEARTHING:ForPipetypeearthingnormalpracticeistouse
GIpipe[Cclass]of75mmdiameter,10feetlongweldedwith75mmdiameterGIflangehaving6numbersofholesfortheconnectionofearthwiresandinsertedingroundbyaugermethod.
Thesetypesofearthpitaregenerallyfilledwithalternatelayerofcharcoal&saltorearthreactivationcompound.
DESCRIBETHEMETHODOFCONSTRUCTIONOFEARTHPIT
ExcavationonearthforanormalearthPitsizeis1.5MX1.5MX3.0M.
Use500mmX500mmX10mmGIPlateorBiggerSizeformoreContactofEarthandreduceEarthResistance.
MakeamixtureofWoodCoalPowderSalt&Sandallinequalpart
WoodCoalPowderuseasgoodconductorofelectricity,anticorrosive,rustprovesforGIPlateforlonglife.
Thepurposeofcoalandsaltistokeepwetthesoilpermanently.
Thesaltpercolatesandcoalabsorbswaterkeepingthesoilwet.
Careshouldalwaysbetakenbywateringtheearthpitsinsummersothatthepitsoilwillbewet.
Coalismadeofcarbonwhichisgoodconductorminimizingtheearthresistant.
SaltuseaselectrolytetoformconductivitybetweenGIPlateCoalandEarthwithhumidity.
Sandhasusedtoformporositytocyclewater&humidityaroundthemixture.
PutGIPlate(EARTHPLATE)ofsize500mmX500mmX10mminthe
midofmixture.
UseDoubleGIStripsize30mmX10mmtoconnectGIPlatetoSystemEarthling.
ItwillbebettertouseGIPipeofsize2.5diameterwithaFlangeonthetopofGIPipetocoverGIStripfromEARTHPLATEtoTopFlange.
CoverTopofGIpipewithaTjointtoavoidjammingofpipewithdust&mudandalsousewatertimetotimethroughthispipetobottomofearthplate.
MaintainlessthanoneOhmResistancefromEARTHPITconductortoadistanceof15MetersaroundtheEARTHPITwithanotherconductordipontheEarthatleast500mmdeep.
CheckVoltagebetweenEarthPitconductorstoNeutralofMainsSupply220VAC50Hzitshouldbelessthan2.0Volts.
THEFOLLOWINGFACTORSAREEFFECTINGTHEEARTHPIT
SOILRESISTIVITY:Itistheresistanceofsoiltothepassageofelectriccurrent.Theearth
resistancevalue(ohmicvalue)ofanearthpitdependsonsoilresistivity.Itistheresistanceofthesoiltothepassageofelectriccurrent.
Itvariesfromsoiltosoil.Itdependsonthephysicalcompositionofthesoil,moisture,dissolvedsalts,grainsizeanddistribution,seasonalvariation,currentmagnitudeetc.
Independsonthecompositionofsoil,Moisturecontent,Dissolvedsalts,grainsizeanditsdistribution,seasonalvariation,currentmagnitude.
MOISTURE:Moisturehasagreatinfluenceonresistivityvalueofsoil.The
resistivityofasoilcanbedeterminedbythequantityofwaterheldbythesoilandresistivityofthewateritself.Conductionofelectricityinsoilisthroughwater.
Theresistancedropsquicklytoamoreorlesssteadyminimumvalueofabout15%moisture.Andfurtherincreaseofmoisturelevelinsoilwillhavelittleeffectonsoilresistivity.Inmanylocationswatertablegoesdownindryweatherconditions.Therefore,itisessentialtopourwaterinandaroundtheearthpittomaintainmoistureindryweatherconditions.Moisturesignificantlyinfluencessoilresistivity
SOILCONDITION:Differentsoilconditionsgivedifferentsoilresistivity.Mostofthesoils
areverypoorconductorsofelectricitywhentheyarecompletelydry.Soilresistivityismeasuredinohmmetersorohmcm.
SoilplaysasignificantroleindeterminingtheperformanceofElectrode.
Soilwithlowresistivityishighlycorrosive.Ifsoilisdrythensoilresistivityvaluewillbeveryhigh.
Ifsoilresistivityishigh,earthresistanceofelectrodewillalsobehigh.
DISSOLVEDSALT:Purewaterispoorconductorofelectricity.
Resistivityofsoildependsonresistivityofwaterwhichinturndependsontheamountandnatureofsaltsdissolvedinit.
Smallquantityofsaltsinwaterreducessoilresistivityby80%.commonsaltismosteffectiveinimprovingconductivityofsoil.Butitcorrodesmetalandhencediscouraged.
PHYSICALCOMPOSITION:
Differentsoilcompositiongivesdifferentaverageresistivity.Basedonthetypeofsoil,theresistivityofclaysoilmaybeintherangeof4150ohmmeter,whereasforrockyorgravelsoils,thesamemaybewellabove1000ohmmeter.
WEATHERCONDITION:Increaseordecreaseofmoisturecontentdeterminestheincreaseor
decreaseofsoilresistivity.
Thusindrywhetherresistivitywillbeveryhighandinmonsoonmonthstheresistivitywillbelow.
LOCATIONOFEARTHPIT:Thelocationalsocontributestoresistivitytoagreatextent.Ina
slopinglandscape,orinalandwithmadeupofsoil,orareaswhicharehilly,rockyorsandy,waterrunsoffandindryweatherconditionswatertablegoesdownveryfast.InsuchsituationBackfillCompoundwillnotbeabletoattractmoisture,asthesoilaroundthepitwould
bedry.Theearthpitslocatedinsuchareasmustbewateredatfrequentintervals,particularlyduringdryweatherconditions.
Thoughbackfillcompoundretainsmoistureundernormalconditions,itgivesoffmoistureduringdryweathertothedrysoilaroundtheelectrode,andintheprocesslosesmoistureoveraperiodoftime.Therefore,chooseasitethatisnaturallynotwelldrained.
EFFECTOFCURRENTMAGNITUDE:Soilresistivityinthevicinityofgroundelectrodemaybeaffectedby
currentflowingfromtheelectrodeintothesurroundingsoil.
Thethermalcharacteristicsandthemoisturecontentofthesoilwilldetermineifacurrentofagivenmagnitudeanddurationwillcausesignificantdryingandthusincreasetheeffectofsoilresistivity
EFFECTOFGRAINSIZEANDDISTRIBUTION:
Grainsize,itsdistributionandclosenessofpackingarealsocontributoryfactors,sincetheycontrolthemannerinwhichthemoistureisheldinthesoil.
Effectofseasonalvariationonsoilresistivity:Increaseordecreaseofmoisturecontentinsoildeterminesdecreaseorincreaseofsoilresistivity.Thusindryweatherresistivitywillbeveryhighandduringrainyseasontheresistivitywillbelow.
OBSTRUCTIONS:Thesoilmaylookgoodonthesurfacebuttheremaybeobstructions
belowafewfeetlikevirginrock.Inthateventresistivitywillbe
affected.Obstructionslikeconcretestructurenearaboutthepitswillaffectresistivity.Iftheearthpitsarecloseby,theresistancevaluewillbehigh.
AREAAVAILABLE:Singleelectroderodorstriporplatewillnotachievethedesired
resistancealone.
Ifanumberofelectrodescouldbeinstalledandinterconnectedthedesiredresistancecouldbeachieved.Thedistancebetweentheelectrodesmustbeequaltothedrivendepthtoavoidoverlappingofareaofinfluence.Eachelectrode,therefore,mustbeoutsidetheresistanceareaoftheother.
CURRENTMAGNITUDE:Acurrentofsignificantmagnitudeanddurationwillcausesignificant
dryingconditioninsoilandthusincreasethesoilresistivity.
MEASUREMENTOFEARTHRESISTANCEBYUSINGEARTHTESTEROREARTHMEGGER:
FormeasuringsoilresistivityEarthTesterisused.ItisalsocalledtheMEGGER.
Ithasavoltagesource,ametertomeasureResistanceinohms,switchestochangeinstrumentrange,WirestoconnectterminaltoEarthElectrodeandSpikes.
ItismeasuredbyusingFourTerminalEarthTesterInstrument.Theterminalsareconnectedbywiresasinillustration.
P=PotentialSpikeandC=CurrentSpike.Thedistancebetweenthespikesmaybe1M,2M,5M,10M,35M,and50M.
Allspikesareequidistantandinstraightlinetomaintainelectricalcontinuity.Takemeasurementindifferentdirections.
Soilresistivity=2LR.
R=ValueofEarthresistanceinohm.
Distancebetweenthespikesincm.
=3.14
P=Earthresistivityohmcm.
EarthresistancevalueisdirectlyproportionaltoSoilresistivityvalue
THREEPOINTMETHODOFMEASURINGEARTHRESISTANCE:
InthismethodearthtesterterminalC1&P1areshortedtoeachotherandconnectedtotheearthelectrode(pipe)undertest.
TerminalsP2&C2areconnectedtothetwoseparatespikesdriveninearth.Thesetwospikesarekeptinsamelineatthedistanceof25metersand50metersduetowhichtherewillnotbemutualinterferenceinthefieldofindividualspikes.
Ifwerotategeneratorhandlewithspecificspeedwegetdirectlyearthresistanceonscale.
Spikelengthintheearthshouldnotbemorethan1/20thdistance
betweentwospikes.
Resistancemustbeverifiedbyincreasingordecreasingthedistancebetweenthetesterelectrodeandthespikesby5meter.Normally,thelengthofwiresshouldbe10and15Meterorinproportionof62%ofD.
Suppose,thedistanceofCurrentSpikefromEarthElectrodeD=60ft,Then,distanceofPotentialSpikewouldbe62%ofD=0.62Di.e.0.62x60ft=37ft.
FOURPOINTMETHODOFMEASURINGEARTHRESISTANCE:
Inthismethod4spikesaredriveninearthinsamelineattheequaldistance.OutertwospikesareconnectedtoC1&C2terminalsofearthtester.SimilarlyinnertwospikesareconnectedtoP1&P2terminals.Nowifwerotategeneratorhandlewithspecificspeed,wegetearthresistancevalueofthatplace.
InthismethoderrorduetopolarizationeffectiseliminatedandearthtestercanbeoperateddirectlyonA.C.
PIPEEARTHINGVSPLATEEARTHING:SupposeCopperPlatehavingofsize1.2mx1.2mx3.15mmthick.soil
resistivityof100ohmm,
TheresistanceofPlateelectrodetoearth(R)=(r/A)Xunderroot(/A)=(100/2.88)X(3.14/2.88)=36.27ohm
Now,consideraGIPipeElectrodeof50mmDiameterand3mLong.
soilresistivityof100Ohmm,
TheresistanceofPipeelectrodetoearth(R)=(100r/2L)Xloge(4L/d)=(100X100/2X3.14X300)Xloge(4X300/5)=29.09Ohm.
FromtheabovecalculationtheGIPipeelectrodeoffersamuchlesserresistancethanevenacopperplateelectrode.
AsperIS3043Pipe,rodorstriphasamuchlowerresistancethanaplateofequalsurfacearea.
GIEARTHINGVSCOPPEREARTHING:AsperIS3043,theresistanceofPlateelectrodetoearth(R)=(r/A)X
underroot(P/A).
Wherer=ResistivityofSoilOhmmeter.
A=AreaofEarthingPlatem3.
TheresistanceofPipeelectrodetoearth(R)=(100r/2L)Xloge(4L/d).
WhereL=LengthofPipe/Rodincm
d=DiameterofPipe/Rodincm.
TheresistivityofthesoilandthephysicaldimensionsoftheelectrodeplayimportantroleofresistanceofRodwithearth.
Thematerialresistivityisnotconsideredimportantroleinearthresistivity.
Anymaterialofgivendimensionswouldofferthesameresistancetoearth.Exceptthesizingandnumberoftheearthingconductorortheprotectiveconductor.
LENGTHOFPIPEELECTRODEANDEARTHPIT:Theresistancetoearthofapipeorplateelectrodereducesrapidly
withinthefirstfewfeetfromground(mostly2to3meter)butafterthatsoilresistivityismostlyuniform.
Afterabout4meterdepth,thereisnoappreciablechangeinresistancetoearthoftheelectrode.Exceptanumberofrodsinparallelaretobepreferredtoasinglelongrod.
AMOUNTOFSALTANDCHARCOAL(MORETHAN8KG):Toreducesoilresistivity,itisnecessarytodissolveinthemoisture
particleintheSoil.
SomesubstancelikeSalt/Charcoalishighlyconductiveinwatersolutionbuttheadditivesubstancewouldreducetheresistivityofthesoil,onlywhenitisdissolvedinthemoistureinthesoilafterthatadditionalquantitydoesnotservethePurpose.
5%moistureinSaltreducesearthresistivityrapidlyandfurtherincreaseinsaltcontentwillgiveaverylittledecreaseinsoilresistivity.
Thesaltcontentisexpressedinpercentbyweightofthemoisturecontentinthesoil.Considering1M3ofSoil,themoisturecontentat10percentwillbeabout144kg.(10percentof1440kg).Thesaltcontentshallbe5%ofthis(i.e.)5%of144kg,thatis,about7.2kg.
AMOUNTOFWATERPOURING:Moisturecontentisoneofthecontrollingfactorsofearthresistivity.
Above20%ofmoisturecontent,theresistivityisverylittleaffected.Butbelow20%theresistivityincreasesrapidlywiththedecreaseinmoisturecontent.
Ifthemoisturecontentisalreadyabove20%thereisnopointinaddingquantityofwaterintotheearthpit,exceptperhapswastinganimportantandscarcenationalresourcelikewater.
LENGTHVSDIAMETEROFEARTHELECTRODE:Apartfromconsiderationsofmechanicalstrength,thereislittle
advantagetobegainedfromincreasingtheearthelectrodediameterwiththeobjectinmindofincreasingsurfaceareaincontactwiththesoil.
Theusualpracticeistoselectadiameterofearthelectrode,whichwillhaveenoughstrengthtoenableittobedrivenintotheparticularsoilconditionswithoutbendingorsplitting.Largediameterelectrodemaybemoredifficulttodrivethansmallerdiameterelectrode.
Thedepthtowhichanearthelectrodeisdrivenhasmuchmoreinfluenceonitselectricalresistancecharacteristicsthanhasitsdiameter.
MAXIMUMALLOWABLEEARTHRESISTANCE:Majorpowerstation=0.5Ohm.
MajorSubstations=1.0Ohm