Specifications

Appendix-c Grounding Considerations, Surge Protection
November 11, 2013
c‐2
Copyright©2013,HarrisBroadcast
WARNING:Disconnectprimarypowerpriortoservicing. 888‐2628‐300
Theprimaryelectrostaticshieldoftheisolationtransformer,ifused,connectstotheACneutralwire(white)sothat
intheeventofatransformerprimaryfault,faultcurrentisreturneddirectlytotheACsourceratherthanthrougha
commongroundsystem.TheACneutralisconnectedtoearthground
attheserviceentry.
Useofseparategroundspreventscross‐couplingofpowerandsignal currentsasaresultofanyimpedancethatmay
becommontotheseparatesystems.Itisespeciallyimportantinlow‐levelsystemsthatnoise‐producingandnoise‐
sensitivecircuitsbeisolatedfromeachother;separatingthegroundingpathsis
onestep.
NoiseGroundingPlate.Whereexcessivehigh‐frequencynoiseontheACgroundisaproblem,ametalplatehaving
anareaofatleast10squarefeetembeddedinconcreteandconnectedtotheACgroundwillassistinnoise
suppression.TheconnectiontoACgroundshouldbeshorterthan5
feet,asdirectaspossible,andwithoutsplices.
Localwiringcodeswilldictatetheminimumwiresizetobeused.
PeripheralEquipmentGrounds.Allperipheralsaresuppliedwithaseparategroundingwireorstrap.Allbranch
circuitreceptaclesmustpermitconnectiontothisground.Thisservicegroundmustbeconnectedthroughthe
branchcircuittoacommongroundingelectrodebytheshortestandmostdirectpathpossible.Thisisasafety

groundconnection,notaneutral.
Often,circuitcommonintestequipmentisconnectedtopowergroundandchassis.Inthesecases,isolatedAC
powermustbeprovidedfromaseparateisolationtransformertoavoidagroundloop.
c.2.3 DC Ground
DCgroundsinthetransmitterareconnectedtoagroundbus,whichinturnisroutedtoacommoncabinetground
andthenconnectedtoanearthground.Theuseofseparategroundbussesis asuggestedmethodofisolationused
topreventcross‐couplingofsignals.Thesegroundbusesare
thenroutedtothecabinetgroundandtoearthground.
c.2.4 Earth Ground
Thetransmittermustbeconnectedtoearthground.Theconnectionmusthaveanimpedanceof5ohmsorless.For
example,aone‐inchmetalroddriven20feetintomoistearthwillhavearesistanceofapproximately20ohms,and
alargegroundcounterpoiseburiedinmoistearthwillexhibit
aresistanceontheorderof1to5 ohms.
Theresistanceofanelectrodetogroundisafunctionofsoilresistivity,soilchemistryandmoisturecontent.Typical
resistivityofunpreparedsoilcanvaryfromapproximately500ohmsto50kohmspersquarecentimeter.
Theresistanceoftheearthgroundshouldbeperiodicallymeasuredtoensurethattheresistanceremainswithin
installationrequirements.
c.2.5 RF Ground
Electricalandelectronicequipmentmustbeeff ectivelygrounded,andshieldedtoachievereliableequipment
operation.Thefacilitygroundsystemformsadirectpathoflowimpedanceofapproximately10ohmsbetween
earthandvariouspowerandcommunicationsequipment.Thiseffectivelyminimizesvoltagedifferentialsonthe
groundplanetobelowlevelswhich
willproducenoiseorinterferencetocommunicationcircuits.
Thebasicearthelectrodesubsystemconsistofdrivengroundrodsuniformlyspacedaroundthefacility ,
interconnectedwith2or4inchcopperstrap.Thestrapandrodsshouldbeplacedapproximately40inches(1meter)
outsidetheroofdriplineofthestructure,andthestrapburiedatleast20inches
(0.5meters).Thegroundrods
shouldbecopper‐cladsteel,aminimumofeightfeet(2.5meters)inlengthandspacedapartnotmorethantwice
therodlength.Brazingorweldingshouldbeusedforpermanentconnectionsbetweentheseitems.
Wherearesistanceof10ohmscannotbeobtainedwiththeaboveconfiguration,alternatemethodsmustbe
considered.
Ideally,thebestbuildinggroundplaneisanequipotentialgroundsystem.Suchaplaneexistsinabuildingwitha
concretefloorifagroundgrid,connectedtothefacilitygroundsystematmultiplepoints,isembeddedinthefloor.
Theplanemaybeeitherasolidsheetorwiremesh.Ameshwillactelectricallyasasolidsheetaslongasthemesh
openingsarelessthan1/8wavelengthatthehighestfrequenciesofconcern.Whenitisnotfeasibletoinstallafine
mesh,copper‐cladsteelmeshes
andwiresareavailable.Eachcrossoverpointmustbebrazedtoensuregood
electricalcontinuity.Equipotentialplanesforexistingfacilitiesmaybeinstalledatorneartheceilingabovethe
equipment.