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from the terminal in an attempt to increase theprobability of inception o f a c on ne ct in g leader fro m th eterminal during the approach of a downward lightningleader [2 ,3). According to the proponents of ESE thetime advantage realized by the early inception of theconnecting leader from ,\ ESE terminal illcomparison.to ~ n orm al F ran klin rod would provide a p os sib ility f orrhe connecting leader generated by all ESE terminal totravel a longer distance til comparison to that from aFranklin mel. Consequently, it is c la im ed that nudersimilar circumstances an ESE terminal will have a largerprotection area than ~ Franklin rod of similardimensions. However, recent experirnental andtheoretical investigations f1ndresults that are in conflictwith t he c la imed performance of E SE devices [41.

Lightning Intercept ionNon C onventio na l LightningP ro tec tio n S ystem sVERNON COORAY ON BEHALF OF ClGRE WORKING GROUP C4.405(This is the second of hi'O r ep o rt s, t he firsf o f w hic h a pp ea re d ill th e A ug ust e dit io n a fE le etT CI)

M. Bernardi (IT). C. Bouquegneau (BE), V. Cooray (5E).G. Diendorfer (AT). M. fernando (SE), M. Ishii (JP),T. Kawamura (JP), C. Mazetti (IT). C. A. Nucd (IT).

A. Piantini (BR), F. Rac:hidl (CH), V. Rakov (US), T. Shindo (JP),H. Torres (CO), 5. Visacro (BR). 5. Yokoyama (JP)

Non conventional lightningprotection systemsThe external lightning protection systems used by

e ng in ee rs in different countries can b e divided into twocategories, namely, conventional and non-conventionallightning protection sy stems . T he c on ve ntio na l s ys tem su se F rank li n rods..Man')' decades of experience showsthat b y c om b in in g Franklin rods located at criticalpoints on a structure with ~1proper down conductorand grounding system the damage due to lightningcould be reduced siguificantly [1]. The Early StreamerEmission rods and Dissipation Arrays (sometimescalled Charge Transfer Systems) belong to the cate.goryof non conventional lightning protection systems. Thelatter' systems have been introduced into severallightning protection standards without testing themover the same long period of time in the field as donefor conventional 0nes to assess and valid ate thei rpe rfo rmances, In th i s uo te we will 51.1]]11113 r ize theresults of studies pertinent to these systems as reportedin the scientific literature,

The Early Streamer Emission(ESE) conceptTI"e ESE term inals used in practice are eq IIipped

with a discharge triggering.device toiniriate streamers

Experimental data that are in conflict with theconcept of ESE

Case studies conducted by Hartono et a i, [S] inMalaysia, provide clear evidence that lightning dobypass the ESE terminals and strike the protectedstructures wellwithin the claimed protective: region ofthe ESE devices. The same study showed that nodamages were observed at the corners of structuresequipped with Franklin rods installed according to theinternarlonalligh \11 ing protection standard to cover thevulnerable pain Is such !lS edges or comers of thestmet-me. However, in structure" where. Franklin rods

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were installed wirhout.ccnsideraucn of these high riskinterception points, lightning strikes have beenobserved at these poi n r s.

In another study conducted in New Mexico [6],ESE lightning rods were allowed to compete withsymmetrically spaced Franklin rods to validate tileenhanced at tractive zone of ESE devices claimed by itsproponents. If, as claimed, ESE rods can initiate anupward leader before the Franklin rods and if the)' havea larger attractive zone, one would. expect ESE reds tob e the preferential point of arrach men r of the lightningstrikes. However, according to the observations all thelightning strikes got attached to Franklin conductorsand not a single one terminated on the ESE devices, Itis worth mentioning that among Pranklin conductorsonly those with blunt rods were struckby lightning,while those witb sharp rods were 1301 struck. Thisexperiment represent an additional indication that theESE terminals do not have an advantage over theFranldin rods and the claimed enhanced protective['l;mge does no t exist.

Proponents of .ESE sometimes refer to an expe-riment conducted in France using triggered lightning[7 ] to support the action of ESE terminals. [1 1 thisexperiment an ESE terminal. was put in competitionwith a Franklin rod to get attached to a down comingleader created ill an altitude triggered lightningexperiment. The downward moving leader got attachedto the ESE terminal and the proponents of ESE claimthat this proves the superior action o f li.,)E terminals .incornparison to Franklin rods. However, it is importantto note that iJ1 the experiment the ESE terminal waslocated closer to the rocket launcher than theconventional one. The reason fO T the attachment of theJ i.ghtn ing flash to the ESE rod could simply be due tothe spatial adva IJ tagc it had with respect ro theconventional rod, Unfortunately the positions of therods were not interchanged to validate the claimeden hanced a ttractive raJlge of the ESE terminal, Thus,one has to conclude that this experiment does netprovide evidence for the Claimed superiority of the ESEterminals against the convent ional ones.

Theoretical evidence that are in conflict withthe concept of ESE

Tile whole concept of ESE is based on the observedfact that by artif icial t riggering of streamers from the tipof a lightning terminal (i.e, ESE rod) stressed by aswitching impulse, one can cause the terminal to

initiate ,1 leader earlier than from a lightning terminalplaced under identical circumstances but without theaction of artificial streamers (i.e, Franklin rod) [2 J. 1 0the laboratory, it was found that the time advantage (i.e.the rime interval between the initiation of leaders fromESE and Franklin rods), {J,f of an ESE terminal is about75 ~lo. Proponents of E5E terminals have taken thislaboratory observation and extended it to naturalconditions claiming that a 75 !A s advantage will g i v e riseto a lengthadvantage equal to the product v Mwhere vis [he speed of the upward moving leader. Assuming Jleader speed of 10 6 m/s 'ESE proponents claim that ar tESE terminal would have a length advantage of about75 III over a conventional rod, Thus, the following twocond rtions have to be satisfied for the ESE devices tofunction according to their specifications:

I) The early initiation of leaders from ESEterminals observed in the Iaboratory takes placealso under natural conditions.ln other words, anESE terminal can launch a connecting leaderlong before a conventional rod under naturalconditions.

2) The time advantage observed will translate to 3length advantage of If I : ! . t over a conventionalterminal.

Let us first assume that a time advantage exists inESE devices when exposed to lightning-generatedelectric f ie ld s . .This' time advantage was converted to IIlength advantage of-about 75 m over a conventional rodby assuming a leader speed of about 10' m/s, Themajority of speeds of upward. connecting leadersreported in the literature is from those in either rockettriggered lightning or from those in upward initiatedlightning flashes, In these cases the upward connect ingleader moves ill II more or less static backgroundelectric field created b y thunderclouds. These leaderspeeds are not relevant to the study underconsideration. Y O k O Y . 1 l l 1 . a et al. [8] managed to m e a s u r ethe speeds of upward connecting leaders initiated froman 80 m tall tower as a result of the electric fieldgenerated by downward moving leaders. In fourexamples analyzed in the study they found that theconnecting leader speeds just before the connection ismad e between them arid the downward moving leaderswere 1.3 x 10' m/s, 1.4 x lor, rn/s, 2.9 x 10' m/s and 0.5 x10 " rn/s, T he se s pe ed s a rc s im il ar to the one used by ESEmanu factures in calculating the striking distance.However, il is not correct to use these speeds in theanalysi of ESE terminals because what is required toc al cu la te t he l en gt h of the ccnnecting leader given thetime advantage is the average speed of the

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e o ,- - - - _ _ , . . - ~ - - . . . . . , - - . .- - . .- . - - - . . .--lp;;:5kA- -lp=10kA

50 1 - - " ' : " " __::....:~:.,:I~p=:;3~O~kA~-!t-nit---r--t---j'-I--l--++++---+-+--+-+--1__.J..;1--\,1

connecting leader. The average speed of connectingleaders measured by Yokoyama et al, [8] varied from0.8 x 10' m/s to 2,7 s : 10>m/s. This avenge speed is ailorder o f m ag nit ud e less than the one used by E S Emanufactures. Moreover, the connecting leadersphotographed til the study originated from an 80 m tallstructure. In general, the co n necting leaders issuedfrom tall structures are relatively longer than the onesissued b y short structures during lightninginterception, Long leaders have ample time tothermalize their channel and this makes them movefaster than short connecting leaders. If thisexperimentally observed value of average leader speedis used in the conversion of time advantage to distance,the resulting length advan rage would be of no use illman)' practical situations. Second, this conversion oftime advantage to a length advantage is .not correctbecause the e ve nt ua I l en g th advantage depends on theratio of the. speeds of both downward and upwindleaders. Jf this is taken into account the assumed lengthadvantage will be Jess than t he v alu e calculated by justmultiplying 61 by the speed of the leader. Third,according to the proponents of ESE the earlierinitiation of a con necring leader from an ESE deviceDCC .U r s i n a smal le r e le ct ri c f ie ld t han is req u i r ed f or th einitiation of a leader by a conventional rod. However,for a successful. propagation of a connecting leaderacertain background electric fielcl is needed. Jf thebackground electric field is not large enough theinitiated leader could be aborted {9]. The proponentsof the ESE do not consider the requirements for the

propagation of a leader and they do no t consid er thepossibility that the initiated leaders could be aborted ifthe background electric field requirements are not met.

Now, ",-Ie come back to the first assumption,Re cen tl y , Be ce rr a and C ooray [1 0) constructed a modelincorporating the physics of the attachment process tosirnulatc ligh ming at tach men t 1:0 structures. Tills modelha s been validated using data from altitude triggeredlightning [Ill. Since the current measured at the baseof the trigger wire .showed the occurrence of several.aborted streamer leader inceptions, in t h . e validation ofthe m odel. the space charge left: behind b y theseunsuccessful leader inceptions (precursors) were takeninto ,1CCOunl .It is worth mentioning that the effect ofcorona generated by the tugger wire on the inception ofleaders from its tip - disregarded in a firstapproximation in [ l - O J - was the subject of a recentinvestigation [12]. Using thei r model Becerra andCooray [9J have simulated the initiation anddevelopment of positive leaders under the influence oftime varyi ng electric Iielc isused in laboratory as well as1 1 1 C t im e v ar yi ng e le ct ri c f i e l d s generated a t ground levelb y the descent of the downward leaders. Their resultss J lO W that indeed one can obtain a time advantage inthe la bo rarory but also they show that such a tiinadvantage will be pract ically negligible when. the rodsare exposed to the background electric fields of leaders.As shown i.n F igure 1, in order to change the strikingdistance significan tly, ESE rods have to be supplied withMega-volt strong generators.

gflc;13.!!!".E.3,m 20+-----~--~-t-_+4-~++----_4--_+__l~_+4_~I__----+,--._~~~~~~~r r . _ - - I- I-1- I-- - ~ - -I-I- f- _- ..-

30+-----+-~~-~r++44+----j--_+__l_+~+4-~----~--~+_~~~~

1.001: :+04O+-__~~~-L~LLLY~ __ -L__~-L~~ ~_l~-L~~

HXJEt03 1.00E+05 100E+06Amplitude external pulse applied totip [V]

Figure 1: Distance b etw een The downward leader tip and the ESE rod at the moment of connection betweenthe connecting leader and the down-coming stepped leader as a function of the voltage impulse applied tothe ESE rod. Calculations arc given for three prospective return stroke currents

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The concept of dissipationar:ay.system~ and mountingscientific . evidence againsttheir principle of operationThe original idea of lighming eliminators or

dissipation arrays 1. > to utilize the space chargegenerated by one Or several grounded arrays of sharppoints to "dissipate" (i.e neutralize) the charge inthunderclouds and thus prevent lightning strikes to astructure to be protected. The 'proponents of thissystem claimed lila tthe space cha rge generated by thearray will silently discharge the thundercloud, Thefollowing argument shows that this indeed is not thecase. The mobility of small ions at ground level isabout (1 - 2) x 10" mZ V, s' [13J and v in thebackground electric fields of 10 - so kV/m the drifrvelocity of these ions ma y reach 1 to 10 m rs . EV eJ l if theanay can generate charge of sufficient quantities toneut ralise the cloud charge, in the t ime ofregenerationo f c ha rg e b etw e en lightning f la sh es i n th e thundercloudof about lO s the-s pace ch arg e can move only a d i sta neeof about 10 to 100 rn. Thus, the space charge would notbe able to reach the cloud in time to prevent th eoccurrence of lightning. Facing this challenging andconvincing opposition f rom . li gh tn ing researchers th eproponentsof lightning eliminators accepted th at theanal's nrc not capable of neutralizing the cloud charge[14]. In turn they suggested that the function of thedissipation array is to neutralize the charge al l the downcorning stepped leaders,

Now, a typical stepped leader may bri'IJg downabout 5 C of Charge to ground and th e dissipation arrayhas to generate this charge in about 10 s, the timeinterval between lightning flashes. The proponents ofdissipation arrays made the following argument toshow the effectiveness of the array in generatingsufficient quantity of charge to neutralize the steppedleader [141. According to Zipse [14J a 12 point array(four sets of ti l fee poin is) located OJl a 20 m,pole canproduce about 1 - - 2 mA as the storm sets in (no detailsas to 110w these measureruents were carried our aregiven in the paper). Thus, a typical array with 40.0.0.points can inject a charge comparable to that of astepped leader illabout lOs, the t ime in rerval betweenlightning strikes. Firstly, the proponents of dissipationa trays do not explain the physics behind this claimedneutralization proces . FOl example, since the chargegenerated by the "HI'ay Is distributed in space thestepped.leader has to move. into this space charge regionbefore it could be neutralized. Recall that the b ulk of

this space charge is located in the near vicinity of thedissipation ,rrr'

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- - - - U p ELECIRAN. 25B _DeWbre 20]1

169140< f 1202-

+"

~ 100::J 80otil 600. . . . 400

200 a 100 200 300 400

Electric field (kV/m500 600

Figure 2: The corona current as a function of the background electric field (rom dusters of needles. The number ofneedles in the cluster is show Il in the diagram.

from an object in a thunderstorm electric field. It isimportant: to recognize, however, that the corona chargeissued from the terminal W O L 1 l d not.screen the sides ofthe terminal O( the tower. Thusyas tilt: stepped leaderapproaches the dissipation -army a connecting leadercould be issued from. [he sides of the terminal which is[Jot screened by the space charge.The main q uestio n iswhether the space charge from the needles can counterbalance the increase in.the electric f ie ld c au se d by th edown corning stepped leader.at thetipofthe structure10 su.. h all ex ten t that the formation of a connectingleader is inhibited. Calculations done in [15] show thata tower without the space charge produced by theneedles will launch a connecting leader before < I towerwith similar geometry but with space charge, generatedduring tile descent of the leader, at the tower top.However, th e space charge controlled field does no t lagfar behind tile field that would be present in the absenceof the space charge .. For example, the difference in thestepped leader tip height from the tower top when theelectric field at the tower top is large enough to launcha connecting leader i n t he p re se ll ce and in the absenceof space charge is no more than two meters 116J. Thisstudy indicates that the reduction ill the strikingdistance caused by the space charge may not be morethan a few meters.

J n addition to the above points, there are severalwell documented cases in which lightning has beenobserved to strike dissipation arrays, The bestprocedure to cond uct such a study is to compare twosimilar structures, one with a CTS and the otherwithout. Several such studies have been conducted [17,

18,19,20]. All the studies show that CTS systems werestruck by lightning as well as the control structure. Noreduction in the frequency of Iightning strikes 10structures has been observed.

The proponents of dissipation arrays claim thataccording to the anecdotal evidence ofthe users there isa reduction in the cases of lightning damage after theinstallation of arrays. However , til is does not necessarilymean that the a (ray bas prevented any lightningstrikes.First, since the array is well grounded, it provides -apreferential path for the lightning current to go toground. This it self will reduce the damage due tolightning strikeseven if it does not prevent a lightningstrike. Second, as suggested by Golde [21], theconnection of an umbrellashaped array at the top ofatower willincrease the radius of curvatureofits tip andinhibit the upwar I initiated lightning flashes byreducing the field enhancing effect of the tip. This maylead to a reduction in the number of upward initiatedtlashes from the tower. But, as noted b y Mousa [.22]'upward initiated flashes are of interest in the case oftowers of effective heights larger than about JOO m ormore. The dissipation arrays will not have any effect allthe number of lightning strikes to smaller structures,

ConclusionsBoth theory and experiments 51:1OW tbat (i) ESE

principle, namely that the ESE rods have longer st rikingdistances than conventional Franklin rods, does notwork under natural field conditions and there is no

l iII

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justificationat present to assumethat the ESE rodsperform better than Franklin rods and (ii) tbedissipa ri on arrays cann 0 t d issi pate 1111 imminentlightning flash either to the protectedsrrocture or latheterminal itself.

References[I I Tobias, ]. M., C. L.Wakefield, L.W. Strother,

v.Mazur, J. Covino, J . R. Fredlund, H. J . Christ ian, Jr.,M. Bateman, W. K. Jordan, G. Hales, The Basis ofConven tional Lightning Protectin n Tech nology, Reportof the "Federal Interagency lightning protection usergroup, US Army, Communications- ElectronicsCommand, Directorate for safety, FOIt Monmouth,New Jersey, June 2001.

12] G . BtrgH. "The early streamer emissionlightning rod conductor", Proceedings 15th Int. Conf.Aerospace an d ground, rCO lSE . paper 38, 1992.

[3 1 R . J . Van Brunt, T . L Nelson, K. L. Stricklen,E i U ' l y streamer emission lightning protection systems:An overview, IEEE Electrical Insulation Magazine, VolL6,No.1, pp . 5-24,2000.

l4J M.A. Urnan, V. Rakov, A critical review of non-conventional approaches to I.ighUliiJg protection,Bulletin of the American Meteorological Society,pp.1S09 - \820, December 2002.

l5 1 A. Hartono, I. Robiah, A study of non-conventional air terminals and striken points illa highthunderstorm region, Proceedings 25th InternationalConference on Lightning Protect ion, ICLP Rhodes,Greece, pp, 357-361, 2000.

[6J C. B.Moore, G. D. Aulkh,W. Rison,Measurements of lightning rod responses to nearbystrikes, Geophvs. Research Lett , Vol . 27, No. 10, pp 1487-14.90,'2000.

[7] A. Eybert-Berard, A. Lefort.B, Thirion, Onsiretests. Proc. 24th Int. Conference on LightningProtection, Birmingham, England, StaffordshireUnivers i ty , ~p. 425-435. 1998.

ls I S. Yokoyii,ma, K . Miyake, '1 : Suzuki , S. Kanao,Winter lightning on Japan sea coast =developmenr ofmeasuring system on progressi ng feature of lightningdischarge-, TEEE Trans. Power Delivery, Vol, 5, No.3,pp. 141S}