merged files interview q & a.pdf

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