There Is No Magic To Lightning Protection: Charge …

1 There IsNoMagicTo LightningProtection:ChargeTransferSystem sDoNotPreventLightningStrikesWilliamRiso nProfessorofElectricalEngineeringNew MexicoInstituteofMiningandTechnologySoco rro,New Mexico87801 InitsNovember2001issue,PowerQualitypubli shedanarticlebyDonaldZipse, PreventLightningStrikeswithChargeTransfe rSystems (referredtoasZipse s articleinthisletter),whichdescribesa ,therehave beenmany magical lightningprotectionsystemsmarketedwhichf ailtolive verify theirclaimswithtestimonialsfromsatisfied customers,butwhohave s shortstory,TheLightningRodMan, andthereputationofthesesalesmenandtheirs ystemsoftenputslightningprotectioninthes amecategoryassnake oilinthepublic s beentwo highlymarketed magical systems theChargeTransferSystem(CTS)describedin Zipse s article,andEarlyStreamerEmission(ESE) s claimto preventlightningstrikestoa protectedarea,whileESEsystemsclaimthata singleairterminal(anothernamefora lightningrod)willcollectalllightningwith inanexceptionallylargeradius(typically10 0meters).

2 Whensubjectedtohardscrutiny, bothsystemshave failedtolive s NASA waslookingfora waytoprotecttheSpaceShuttleandothermanne dvehicleswhilethey studyofCTS s (thencalledDissipationArraySystems,thena meofthesystemsmarketedbyLightningElimina torsandConsultants,Inc.).They studieda varietyoftowers,someequippedwitha oflightningtothetowerswithCTS s wasnotsignificantlydifferentthantothosew ithoutCTS s. Inshort,thestudiesdemonstratedthata CTSdidnotpreventorsignificantlyreducethe probabilityoflightningstrikestoa tower. (NASA decidedtoprotecttheSpaceShuttlewitha wireabove theShuttletoactasa preferentialstrike documentedincidencesoflightningstrikingt heoverheadwire,protectingtheShuttleasdes igned).

3 SincetheNASA studiestherehave , laboratoryandfieldstudiesofESEairtermina ls,donebyseveralindependentresearchorgan izations,show thatESEairterminalsrespondtolightningint hesameway, andprovidethesamezoneofprotection,astrad itional,inexpensive s describedinZipse s articleis ( ,raindropscarrynegligibleamountsofcharge togroundduringa thunderstorm),asis theunderlyingtheoryclaimedforthebasisofC TS s. Thearticleclaimsthata CTSwillinjectseveralcoulombsofpositive chargeintotheairabove theprotectedstruc-ture,andthatthischarge willneutralizeanapproachinglightninglead er. Figure3 ofthearticlepresentstheequationfordeterm iningthenumberofpoints, , neededtoprovidethischarge: "! #%$(1)1where is theamountofchargeneededtoneutralizethele ader, is theamountofcoronacurrent( s fire)emittedbya singlepointundera thunderstorm,and is theamountoftimeneededtoaccumulatecharge.

4 Thenumbersintheformulaimplythat, leader, thesystemneeds1,500pointswhicheachemit17 0&A physicallyimpossible, pointundera thunderstormistheelectricalforcefromthe( commonly)negative chargeinthebaseofa chargeis typically2,000to 5,000V/meter. (Fieldsasstrongas30,000V/meterhave beenmeasured,butfieldsthisstrongarerare. )Whentheelectricfieldundera thunderstormreachesabout1,000V/meter, coronacurrentarenotfreeelectronswhichcan move rapidlyawayfromtheCTS,butareions,typical lyionizedoxygenandnitrogenmolecules, witha speedofabout10 about100meters,sothespacechargecreatedby thecoronacurrentisconfinedtoa regionofabout100metersabove reducedbelowthe1,000 CTScanemitis theamountneededtoreducethefieldto1,000V/ m, chargeis givenbytheformula'( )+*.

5 /102 (2)To producea fieldof4,000V/meter(enoughtoreducethe5,0 00V/meterfieldtypicalunderthunderstormst othecoronaonsetstrengthof1,000V/meter)at a distanceof100m(themaximumdistancethechar gecouldtravel in10seconds)wouldrequirea chargeof) ) 3 4coulombs,almosta CTScannotproduceanywhereneartheamountofc hargeclaimedinZipse s , thesimpleformulain Equation(1)wouldindicatethatit wrongwithEquation(1)?Therearetwo thingswrong oneminorandonemajor. Theminorerroris thatEq.(1)indicatesthata singlepointwillemit170&A ofcurrentundera measurementshave beenmadeoncoronacurrentsbeneaththunderst orms,andtheseshow typicalcurrentsofa few toperhaps10& to10&A inEq.(1)wouldchangethenumberofpointsto25 ,000insteadof1,500,soonewouldneedtobuild anarraywithatleast25,000points,if Eq.

6 (1) (1)is thatit assumesthatthecurrentfromanarrayofpoints is simplythecurrentfroma singleisolatedpointmultipliedbythenumber ofpointsinthearray. Thisis , considerthewaterdeliveredfroma fire,oneora fewhydrantscanproducea prodigiousflow , if many firehydrantsareopen(perhapsopenedbykidsc oolingoff ona hotsummerday)theflowofwateroutofa singlehydrantis considerablylessthantheflow whenonlya few becausetherearephysicalconstraints(water pressureandsizeofpipes)whichlimitthetota lamountofwaterwhichcanbedeliveredthrough a thearray, thefieldfromthispositive chargeoffsetsthedrivingfieldfromthecharg einthethundercloud, ,ina 10secondtimeperiod,anarrayofpointswitha dimensionofabout100meterscanemita maximumof) ) 3 4coulombsofcharge,fora totalmaximumcurrentof) )

7 5 % ,500pointsonanarray, thiswouldbea ,000points,2thesametotalcurrentwouldbeem itted,fora have donedonestudiesoncurrentemissionsfrommul tipointarraysattheLangmuirLaboratoryforA tmosphericResearch,New MexicoTech s mountain-topthunderstormresearchlaborato ry. Inourexperi-mentswehave found,forexample,thatanarrayof80pointsem itsa coronacurrentabouttwicethevalueofthatfro ma (1) (1)is thereareno Magic lightningprotectionsystemsavailable,is it possibletocompletelyprotecta facilityfromlightningdamage?Withproperap plicationoftraditionallightningprotectio nprinciples,theanswerisanunqualified Yes .AtLangmuirLaboratorywehave beenconductingresearchina have a specially-builtinstrumentationshelter(ca lledKiva II)whichwasbuiltto take dataduringa directstrike to theshelter.

8 Becausetheprobabilityofnaturallightnings trikinga specificpointis low, eveninthehighlightningdensityofourmounta in-toplaboratory, webringlightningtoourinstrumentationbytr iggeringit weshoota smallrockettrailinga occasionsI, aswellasotherresearchersandobservers,hav e beeninsideKiva II whenit hasbeenstruckbylightning,withthelightnin gterminatingona current-measuringshuntonthetopofKiva II,abouttwo feetabove II wasproperlydesignedtowithstandtheeffects ofa directlightningstrike, personnelandinstrumentationinsideit 1)providepreferentialstrikespointforligh t-ning(anarrayofconductorshigherthantheo bjectsbeingprotected),a goodgroundingsystem,andconduc-torsbetwee nthetwo toconductthedamagingcurrentfroma lightningdischargeawayfromthestructureto beprotected;and2) taketoprovidecompleteprotectionfromlight ning?

9 It requiresa meanstoensurethatthecurrentsfroma directstrike cannotenterthestructure, preventthecurrentsfroma directlightningstrike fromenteringKiva II,it wascon-structedof1/4 steel,withonlya fewopeningsforsignalandpowerwirestoenter . To preventtransientsfromenteringKiva II,everysignalandpowerwireenteringintoit steelis usuallyprohibitive,anda structurewithoutwindowsis ,it is oftenverydifficulttoidentifyandprovidead equatetransientprotectiontoeveryconducto renteringintoa practicallightningprotectionsystem(LPS)u suallycomesdowntoa (bothmonetarycostsforprovidingmoreprefer entialstrike pointsandtransientprotection,andaestheti ccostsintheappearanceofthebuilding) (withappropriatemodificationstothedesign ofsystemsfromstudiesofsystemfailures), ,theNationalFireProtectionAssociation sStandard fortheInstallationofLightingProtectionSy s-tems(NFPA780)

10 Istheprimarystandardgoverningtheinstalla tionofLPS ,thepredecessortothecurrentNFPA780, ,therehave beennumerousupdatestoNFPA s s article,thestatusofNFPA 780wasupintheairfora ,theNFPA Standard s Councilhaddecidedtodelaypublicationofa revisiontoNFPA 780untilit reports,onefroma groupoffederalgovernmentusersofLPS s andonefroma hearingonthematteratitsOctober2001meetin g,theCouncilvotedtoissuethenewestrevisio nofNFPA780, 780containsnumerouschangesfromthe1997edi tionreflectingtoday s 780providesa standardforinstallationofLPS s toprotecta structurefroma directlightningstrike,it sayslittleabouthowtoprotectsensitive ,suchasIEEES tandard1100, s donotpreventlightningthey dofunctionasprettygoodtraditionalLPS providea systemofoverheadwireswhichfunctionaspref erentialstrike points(similartotheoverheadwireusedtopro tecttheSpaceShuttle),a goodgroundingsystem, , LPS s basedonNFPA 780areequallyeffective inprotectingstructures,at a ,ZipseaccusesthoseofusopposedtoCTS s of relyingonobsoleteinformationorrefusingto considera viabletechnology.