Review of claimed enhanced lightning protection of buildings by early streamer emission air terminals

D.Mackerras M.Darveniza A.C.Liew

Indexing terms: Lightning protection of buildings, Early streamer emission air terminals

IEE Proc.-Sci. Meas. Technol., Vol. 144, No. 1, January 1997

Abstract: Simple air terminals are earthedconducting objects able to launch a connectingstreamer or leader discharge that intercepts adownward lightning leader discharge and divertsto itself the lightning strike. Early streameremission (ESE) air terminals are claimed toinitiate the connecting streamer earlier in timethan would a simple air terminal in the sameposition, and are therefore claimed to be able toattract the lightning discharge from a largerdistance than would a simple air terminal. Thechief objection raised is that this implies that astreamer or leader discharge from an ESE airterminal is able to continue propagating when theelectric field ahead of the advancing tip of thestreamer or leader is below the minimum valuethat would apply to a streamer or leader from asimple air terminal. Once a streamer or leaderdischarge has propagated into the space remotefrom the air terminal, its further progress dependsupon the supply of energy from the electric fieldin the space near the tip of the discharge andupon the dielectric properties of the airundergoing breakdown. As neither of thesefactors can be influenced by the air terminal, it isconcluded that it is not possible to gain asignificant improvement in lightning interceptionperformance by causing the early emission of astreamer from an air terminal.

1 Introduction

Our objective is to review critically the claims forenhanced lightning protection characteristics of earlystreamer emission (ESE) air terminals for lightninginterception. This review is intended to be a contribu-tion to the discussion of the possibility of improving

© IEE, 1996

IEE Proceedings online no. 19960649

Paper received 15th March 1996

D. Mackerras and M. Darveniza are with the Department of Electricaland Computer Engineering, University of Queensland, St. Lucia, 4072,Australia

A.C. Liew is with the Department of Electrical Engineering, NationalUniversity of Singapore, Kent Ridge, Singapore, 0511

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the lightning interception performance of an air termi-nal by causing the early emission of the connectingstreamer or leader discharge from the air terminal.

A recent proposal in the USA to produce a NationalFire Protection Association (NFPA) Standard on earlystreamer emission air terminals for lightning protection(Draft Standard NFPA 781 - F93TCR) has made itnecessary to examine the basis for the claims ofenhanced lightning protective properties. In examiningthese claims we have studied an extensive literature[1, 41] relevant to lightning phenomena and to bothstandard and nonstandard lightning protection meth-ods. The situation at the end of 1995 was that theNFPA had decided not to issue NFPA 781. However,there are proposals to consider recognition of ESE airterminals in the IEC Standards on lightning protectionthat are likely to be based on material similar to thatcontained in Draft Standard NFPA 781 - F93TCR. Itis our view that there are at least three incorrectassumptions in the procedures that lead to claims ofimproved lightning interception performance, for ESEair terminals.

2 Review of the physics of the lightning attachment process for simple air terminals

Descriptions of the physics of the lightning attachmentprocess have been given [1, 14, 15, 22, 40]. The term‘air terminal’ refers to any metallic object with an elec-trical connection to earth to which a lightning strikemay attach. The attachment process starts when thedownward first leader of the ground flash has reacheda point about 100 – 200m above the ground or thebuilding that is about to be struck. The first leader typ-ically takes between about 10 and 40ms to progressfrom cloud to ground, and the attachment process

Fig.1 Stages in lightning attachment process: commencement of con-necting streamer At time t1 a connecting streamer commences from building

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occupies the last 1ms or less of that time. The situationin the last stage of the downward progression of theleader is shown in Figs. 1–3.

The leader channel current results in the transfer ofcharge (usually negative) from the charged regions ofthe thundercloud towards earth. The lower end of theleader channel is at a negative potential somewhatbelow the potential of those regions, probably in therange 10 – 100MV. The attachment process terminateswhen an upward connecting leader meets the down-ward leader and the return stroke propagates up theleader channel. The peak current I in the return strokeis mainly derived from the charge on the leader chan-nel, so these two quantities are related. Berger [5] hasshown that if Q is the charge transferred to ground inthe first 1 ms or so of the return stroke, presumablymainly derived from the leader channel, then the bestfit to the available data for 89 negative first strokes is

where I is in kA and Q is in coulombs. Returning to the situation just before the connection

is made, the electric field at parts of the building risesas the leader approaches, and reaches a value at time t1when streamers arise from points on the building, asshown in Fig. 1. Shortly after, one of the streamersleads to the development of an upward leader, denotedthe ‘upward’ or ‘connecting’ leader which propagatestowards and finally reaches the tip of the downwardleader, shown as occurring at time t3 in Fig. 3. Theexistence of other ‘unsuccessful’ upward leaders issometimes seen in photographs of lightning striking theground [14, 40]. The striking distance ds shown inFig. 3 is described [14, 40] as the distance between theobject to be struck and the tip of the downward leaderat the instant (time t1 in Figs. 1–3) when the connectingleader is initiated from the object. Since the electricfield between the leader and the building just before theconnection is made depends on the leader charge, andthe leader charge is related to the peak current in thereturn stroke, there is a relationship between ds and I

Fig.2 Stages in lightning attachment process: connecting streamerapproaching leader At time t2 connecting streamer and leader approach each other

Fig.3 Stages in lightning attachment process: connecting streamer meet-ing leader At time t3 connecting streamer and downward leader meet at junction point.Striking distance ds is defined as shown

IEE Proc.-Sci. Meas. Technol., Vol. 144, No. 1, January 1997

[12–15, 31]. The following relationship between ds inmetres and I in kA is attributed in the IEEE WorkingGroup Report [24] to Love [31]:

This and other relationships between ds and I andexperimental points from Eriksson [13] are shown inFig. 6.1 of [40]. The significance of the striking distancefor the attachment process is that if a downward leadertip goes within a distance ds from a point on a building,and that point is capable of launching a connectingupward leader, then that point will be the one to whichthe lightning channel connects.

Since the point of attachment of the lightning chan-nel on the building has served as the ‘collection point’for the strike, the term ‘collection distance’ couldappropriately be used for the distance denoted ds inFig. 3 when discussing building lightning protection.The basic assumption in conventional lightning protec-tion is that the collection distance is equal to the strik-ing distance, whereas the essential claim made for ESEair terminals is that their collection distance can becaused to be greater than the collection distance forsimple air terminals.

The term ‘streamer’ is used here to denote a spatiallydiffuse discharge process with relatively low currentdensity and high resistivity. Details of positive streamerphenomena, which are extremely complex, have beeninvestigated by the Les Renardières Group [29]. Therewill usually be a transition from a streamer process to aleader process with a thermally ionised core of highcurrent density and low resistivity.

There is considerable uncertainty concerning theextent to which the discharge processes in a positivelaboratory streamer/leader a few metres long corre-spond to those in a positive upward streamer/leadertens of metres long in natural lightning. We assumethat the processes are similar, at least in the initialparts of the discharge, provided that the initial valueand rate of change of electric field for the laboratorytest are similar to those in natural lightning.

3 Electric field aspects of attachment process

The direction of progression of the downward leaderand the initiation of streamers leading to the develop-ment of an upward connecting leader are probablyguided mainly by the electric field in their vicinity.Thus, if the electric field in the space between thedownward leader and the building could be calculatedfor successive positions of the leader as it progressedtowards earth, then the path of the leader could be pre-dicted. The launching of the connecting leader could bepredicted from the electric field around the building.Such a procedure is generally considered impracticalfor routine application to the lightning protection ofbuildings. However, a consideration of the electric fieldbetween a building and a downward leader in geometri-cally simple cases can provide valuable insight into theattachment process. Dellera and Garbagnati [11] havedescribed a method of simulating the progression of thedownward leader and the upward connecting streamer/leader for natural lightning to tall objects such as mastsand electric power lines using electric field calculationsbased on the charge simulation method. Rizk [36] hasmodelled the process in even more detail.

Fig. 4 shows the approximate form of the equipoten-tial lines in the situation when the tip of the downward

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leader is 300m above ground and the leader is descend-ing vertically along the axis of a cylindrical building100m high and 50m in diameter. This is the situation 1to 2ms before the return stroke commences. Theleader, represented as a charged object 10m in diameterand 100m long, is shown projecting from a horizontalsheet of charge at a height of 400m. Both the leaderand the upper charge sheet are at a potential of 20MV.The equipotential lines are 0.5MV apart. The plotshows how the electric field is enhanced at the upperouter edges of the building, and indicates an electricfield enhancement factor of about two near the edge ofthe roof, compared with the average field (20/200 =0.1MV m–1) between the leader and the roof of thebuilding.

It is to be expected that the electric field at a point inthe space close to the upper outer corner of the build-ing will increase as the point approaches the corner.This could be visualised in terms of the electric fieldlines (normal to the equipotential lines) crowding closertogether as the corner is approached. Electric fieldenhancement at exposed upper parts of a building willgenerally result in dielectric breakdown of the airnearby. This type of dielectric breakdown is known asa corona, and will occur anywhere that sharp edges arepresent at exposed upper parts of a building or on theground under the thundercloud. Consequently, the airnear the building protuberances and edges will bepartly ionised, and the air surrounding the building willcarry a net positive charge. Soula [38] estimated thatthe air up to about 600m height under a thundercloudhad a charge density of about 1nC m–3. A net positivecharge density of this magnitude will cause the electricfield to increase with height at the rate of about340Vm–1 per metre increase in height and will thereforehave only a second-order effect on the electric field inthe situation shown in Fig. 4.

Fig.4 Approximate form of equipotentials at instant when a downwardleader tip is 300 m from the earth and is 200 m from the top of a buildingrepresented by a conducting circular cylinder 100 m high and 50 m diame-ter The leader is represented by conducting circular cylinder 100m long and 10mdiameter, is at a potential of 20MV and the equipotentials are 0.5MV apart.(Field map provided by W. Zaengl)

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As the leader approaches the building, say to within100 m, the electric field at the upper parts will increase,and streamers will be launched upward from points ofelectric field enhancement on the building roof. Thiswill involve a transition from a corona type of dis-charge to a streamer. One or more of these streamerswill undergo transition into one or more upward lead-ers. These leaders will advance into regions of dimin-ishing field, and will be unable to obtain sufficientenergy from the field to continue their progress. Thereexists a minimum electric field required for the contin-ued successful propagation of a leader. The criterionhas been stated by Golde [14] in the form that the min-imum average field is about 0.5MV m–1. However, Del-lera and Garbagnati have restated the criterion (privatecommunication, 1995) in the form that a field of0.5MV m–1 or more is required in the region ahead ofthe advancing leader to ensure continued propagation.The field remote from the advancing tip of the dis-charge is not considered relevant to its progression.

Thus, in the example shown in Fig. 4, the tip of thedownward leader will have to advance to within about40m of the building top before the average fieldreaches 0.5MV m–1 (using the Golde criterion) and aconnecting leader can be launched. When this happens,the leader will probably originate from the edge of thebuilding roof, in a region of electric field enhancement,despite the fact that the midpoint of the roof of thebuilding is the closest point to the leader tip. Thisshould be kept in mind when determining strike attach-ment points and priority should be given to placing airterminals at points of electric field enhancement suchas the upper outer edges and corners of buildings.Selection of a critical average electric field lower than0.5MV m–1 can be accommodated by reducing theassumed potential of the leader and the electric field.Otherwise the discussion is unchanged.

4 National codes of practice for lightning protection

Most countries have a national standard or code ofpractice on lightning protection. Examples include:AS1768-1991 for Australia [1], BS6651-1992 for theUK [6], the NFPA (National Fire Protection Associa-tion) code used in the USA [33], the Norm Francaise inFrance [34] and CP33:1985 for Singapore [37]. TheInternational Electrotechnical Commission (IEC) hasproduced codes IEC 1024.1-1990 and IEC 1024.1.1-1993 [25] for general use in countries that do not havetheir own code, or as a guide to revision for thosecountries that do have their own code. There are majoraspects of interception lightning protection in whichthe recommendations of the various codes are in agree-ment. There is general agreement that the major com-ponents of a lightning protection system are: airterminals, downconductors, earth electrodes, equipo-tential bonding and overvoltage protection. Most codesare confined to describing only conventional or stand-ard methods of protection, avoiding reference to non-standard methods.

5 Rolling sphere method of predicting lightning strike attachment points and the collection surface concept

Many recent codes have adopted the rolling spheremethod of determining probable strike attachment

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points. This method has been described by Lee [1, 27]where a sphere radius of 45m is used, corresponding tothe striking distance and collection distance (for simpleair terminals) for a peak lightning current of about10kA. The rolling sphere method is derived directlyfrom the electrogeometric method of predicting light-ning strike attachment to phase wires and shield wiresof electric power transmission lines. The validity of therolling sphere method for predicting lightning strikeattachment to buildings is supported by the success ofthe electrogeometric method in explaining shieldingfailure rates in transmission lines.

The essence of the argument in favour of the rollingsphere method is as follows. We consider a particularpeak lightning current, for example 10kA, and the cor-responding striking distance and sphere radius of 45m.We allow the sphere to roll over and around the build-ing in any direction with the sphere surface touchingthe building. Each radial line from the point of spherecontact with the building to the centre of the sphererepresents a possible path of the lowest portion of theleader channel. The surface traced out by the centre ofthe sphere defines all possible points in space fromwhich a leader can attach to the building. This surface,shown in Fig. 5, could be regarded as the lightning col-lection surface for the building.

Once a leader tip has touched the collection surface,the electric field between the tip of the leader and thebuilding will be dominated by the charges on the leaderchannel and the charges induced on the building.Charges elsewhere will have relatively little effect onthe electric field in this space. Once the leader tip hasreached this surface, the average electric field will havereached about 0.5MV m–1, ensuring that a connectingleader will occur. Thus, the rolling sphere method doesaccount for electric field requirements in an indirectmanner. The procedure can be repeated for any otherpeak lightning current and corresponding sphereradius. The main deficiency of the method is that ittends to lead designers to expect strikes to flat buildingsurfaces that have, in actuality, a very low probabilityof being struck. This deficiency is partly compensatedfor in [1] by recommending that priority be given toplacing air terminals at exposed corners and edges ofbuildings, usually in the form of a conducting strip. Analternative approach would be to assign a collection

Fig.5 Lightning leader collection surface for flashes with a particularstriking distance and collection distance, ds Lightning downward leaders reaching portion of collection surface marked aaawill terminate at point A etc.

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distance for strikes to flat surfaces smaller than the col-lection distance for corners and edges.

6 Comparison of standard and nonstandard methods of lightning protection

In all national standards or codes on lightning protec-tion, there is a convergence of views on the function ofair terminals, namely, that any simple metallic objectwill serve the purpose provided it is adequately earthedor bonded to an earthed part, and has adequate metal-lic cross-sectional area to carry the full lightning cur-rent. That is, the effectiveness of the air terminal inlaunching streamers and a connecting leader and actingas the connection point for the lightning channel, isattributed entirely to its position on the building, and isindependent of the fine details of its shape or structureon a scale of a few tens of centimetres.

Thus the basic philosophy in designing the intercep-tion system is to determine all probable strike attach-ment points on the building using a method such as therolling sphere method. Then, simple air terminals arerequired at all such possible attachment points, unlessexisting metallic structures will serve adequately as anair terminal. Inconspicuous air terminals are usuallypreferred and rods protruding above the building areavoided except where needed to protect an adjacentsurface or volume. Such systems will have a largenumber of points on the upper parts of the buildingthat will serve as launching points for upward stream-ers and connecting leaders and therefore for the attach-ment of the lightning channel to the building. Theytherefore cater for the arrival near the building of light-ning leaders from all possible directions. The air termi-nals connect directly to downconductors that lead thelightning current to the earth electrodes.

In contrast, special-purpose or proprietary intercep-tion lightning protection methods are generally basedon the claim that it is possible to enhance the protec-tive properties of an air terminal by various design fea-tures. These protection systems are denotednonstandard because they are not included in majorstandards for lightning protection. Reports favourableto the use of nonstandard systems include [2–4, 17–20,23, 35]. Reports arguing against the effectiveness ofnonstandard systems include [7–10, 14, 26, 32, 39, 41].Depending on the origin of the particular device, vari-ous means of enhancing the protective property havebeen used including radioactive ionisation devices, non-radioactive ionisation devices, special shapes of the airterminal, creating small sparks between two parts of adevice and causing voltage pulses on some part of thedevice.

All special purpose proprietary air terminals areclaimed to provide enhanced lightning protection bycausing the emission of an upward streamer/leader thatwill propagate towards the tip of the downward leaderat an earlier stage in the attachment process thanwould occur for a simple air terminal in the same posi-tion. The enhanced protective property may beexpressed in the form that the special purpose device isequivalent to a simple air terminal of greater heightthan its physical height. Alternatively, the advantagemay be stated in terms of an increased collection dis-tance compared with a simple device in the same posi-tion. Thus, the special purpose device is claimed toattract the lightning channel to itself from a greater dis-tance than would a simple air terminal in the same

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position; thus the special purpose device will competesuccessfully with the simple device even though it isfurther from the tip of the downward leader than thesimple device.

A characteristic feature of a complete lightning inter-ception system using special purpose devices is thatonly one, or at most, a very small number, of promi-nent air terminals will be used. These systems willsometimes employ one insulated downconductor perair terminal, with earthing systems separate from thebuilding, or insulated downconductors for part of theheight of the building connecting to reinforcing steel atan intermediate height [35]. It may be claimed thatthere is value in keeping the lightning current awayfrom the fabric of the building. Darveniza [8] hasexpressed major reservations about the claim that aninsulated downconductor can isolate the lightning cur-rent from the metalwork of the building.

7 Types of early streamer emission air terminals

7.1 Radioactive air terminals Heary et al. [23] have presented the results of HV labo-ratory tests of air terminals with and without radioac-tive sources which show that the radioactive deviceshave a height advantage of the order of 10cm wheredischarge path lengths are in the order of a metre. It isargued that this effect can be extrapolated to a heightadvantage substantially greater than 10cm in effectiveheight of the air terminal under natural lightning con-ditions. However, Wu et al. [41] were unable to detectany height advantage for radioactive terminals in HVlaboratory tests of a proprietary radioactive air termi-nal using discharge lengths of about 5m. Possibly asmall effect would have been observed if a constantelectric field with superimposed impulse had been usedas in [23].

Assuming that a height advantage of a few centime-tres can be achieved by a radioactive device, it is thennecessary to question whether this effect can be extrap-olated to a larger height advantage under natural light-ning conditions. The mechanism for the heightadvantage appears to be the emission of ionising (suchas alpha) particles from the radioactive source causingionisation and increased conductivity of the air for adistance of a few centimetres (the range in air of theemitted particles) from the air terminal. This mecha-nism is the same for laboratory and natural lightningconditions, so there is a sound reason to believe thatthe height advantage will be the same for the two situa-tions, and that the height advantage will be of theorder of centimetres.

7.2 Nonradioactive ionising devices, sparking devices and special shapes of air terminal There is a class of special purpose air terminals basedon the creation of ionised air around the device bysparking or otherwise, or on the special shape of thedevice. However, during the close approach of thedownward leader, all prominent conducting earthedobjects on the top of a building will be in a high ambi-ent electric field and there will be local electric fieldenhancement. Consequently, many objects will be emit-ting ions in corona discharges in sufficient quantities toprevent the local field adjacent to the objects from ris-ing above the dielectric breakdown field for air, about3MV m–1. Order of magnitude calculations in typical

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situations show that the thickness of the corona sheatharound prominent elevated objects will be of the orderof centimetres. The fine details of the shapes of deviceswill be surrounded by a blanket of ionised air and anyions emitted by the special purpose device will be incompetition with a much greater quantity of ions fromall nearby prominent objects. Any possible externaleffect of sparking on or within the device will beblocked off by the shielding effect of the corona sheathof ionised and weakly conducting air. It is thereforephysically unrealistic to expect that the lightning pro-tection properties of the device will be enhanced.

7.3 Early streamer emission devices In the class of devices specifically labelled ESE, voltagepulses are produced on one part of the device by meansthat are a commercial secret. Air terminals that areclaimed to operate by producing small sparks also mustpresumably produce voltage pulses on some part of thedevice. From the limited information available in theliterature on one type of ESE device, it appears that theenergy to produce the voltage pulses and sparks mustbe derived from the increasing electric field during theapproach of the downward leader.

8 Methods of testing ESE devices in a HV laboratory to determine the collection distance

In HV laboratory tests comparing a particular type ofESE air terminal with a simple air terminal (‘Franklinrod’), Berger [2] found that the ESE air terminal emit-ted a positive streamer between 10µs and 50µs earlierin the front time of the impulse voltage than the simpleair terminal. A simplified illustration of one form oftest is shown in Fig. 6 where a time difference, ∆T, ismeasured between the times of initiation of the currentsin the ESE air terminal and the simple air terminal. Inanother form of the comparative test, the two types of

Fig.6 HV laboratory test used to compare ESE air terminal with simpleair terminal (‘Franklin rod’) Negative switching impulse is superimposed on a steady negative field. Timeadvantage ∆T is the time difference between the times of initiation of currentfrom the two terminals. See Berger [2] for details of the procedure

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air terminal were tested alternately, with a total of 100impulses applied to each type. The value of ∆T was thedifference between the averages of the times of dis-charge initiation over all the tests of each type of airterminal.

Negative switching impulse voltages were used with asuperimposed steady electric field. The rate of increaseof electric field during the switching impulse front wasabout 1kV/m per microsecond. The tests were carriedout using a very large upper electrode, area about300m2, with a steady applied negative voltage on whichwas superimposed the negative switching impulse volt-age. It was stated that voltage pulses were producedand applied to an undisclosed part of the ESE termi-nal, but no information was given about the voltagepulses in terms of their timing, polarity, waveshape,repetition frequency or point of application, making itvery difficult to assess the significance of the resultobtained. In the tests carried out at Les RenardièresHigh Voltage Laboratory, the speed of propagation ofstreamers and leaders could be measured. Measuredinitial speeds were about 2cm/µs. The procedures notedabove were also specified in Draft Standard NFPA-781- F93 TCR.

Berger [3] has argued that the observed time advan-tage ∆T would result in the streamer from an ESE ter-minal, under natural lightning conditions, propagating(presumably after transition to a leader) between 10mand 50m further than a streamer/leader from a simpleair terminal. This implied an assumed constantstreamer/leader propagation speed of 106m s–1. Thisassumed propagation speed is stated explicitly withoutsupporting evidence in Draft Standard NFPA 781-F93TCR, where the distance advantage, ∆L, for anESE air terminal is calculated in metres as

where ∆T is in seconds. Berger [3] argued from theabove that the electrogeometrical model would be mod-ified by increasing the striking distance by ∆L above thenormal striking distance for a given peak current whenusing an ESE terminal, implying that an ESE air termi-nal could attract lightning from a greater distance thana simple air terminal, the difference between the twodistances being between about 10m and 50m.

Fig.7 Building equipped with ESE air terminal at A and simple air ter-minal at B Tip of downward lightning leader at C is within normal striking distance andcollection distance (ds) for B (earthed metallic object at edge of building) andwithin claimed collection distance (ds + ∆L) of A

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9 Examination of claimed collection distances for ESE air terminals

Consider the physical situation of an ESE air terminalon the roof of a building during the last stages of theapproach of a downward leader towards the building.If the claims noted above are correct, then the ESE airterminal will be able to launch streamers and a con-necting leader from point A in Fig. 7 at an earlier timethan point B on the building can launch a connectingleader. It is implied that the tip of the connectingleader from point A will continue to advance at theassumed speed and will maintain a distance advantageover the tip of the connecting leader from point B. Weuse here the Golde criterion, although it will be shownthat our conclusions are not dependent on the particu-lar criterion adopted.

We assume that the tip of the downward leader,point C, is at a potential of 20MV and has arrived at aparticular instant of time at a point 40m from point Band 60m from point A. The average field between Band C will be 0.5MV m–1, so the required condition forthe launching of a connecting leader from B has beenmet. The leader from B will therefore be able to obtainenough energy from the electric field to maintain itsprogress even though the ambient electric field isdiminishing as the distance from B increases. Thus, thestriking distance and collection distance for simple airterminals for this stroke is ds = 40m.

Let us assume that the ESE air terminal at A isclaimed to have a 20m advantage in collection distanceover a simple air terminal, so ∆L = 20m in Fig. 7.Then, at this moment, the ESE air terminal should beable to launch a connecting leader that has the sameprobability of completing the connection to the down-ward leader as the leader from B. So its collection dis-tance should be about 60m.

Once the leader from A has been launched, it will beprogressing into a region of diminishing ambient elec-tric field strength. The leader will be subject to thesame laws governing progress as any other leader. Theaverage field between A and C will be 0.33MV m–1

which is less than the 0.5MV m–1 required to ensurethe progress of the leader on the basis of the Golde cri-terion (see Section 3). The ESE air terminal will beunable to modify the dielectric characteristics of the airbetween A and C. The leader from A will therefore beunable to obtain sufficient energy from the electric fieldto maintain its progress towards C. It will therefore beunable to compete successfully with the leader from B.

The above discussion is independent of the actualnumerical values chosen. Stated in more general terms,it is physically unreasonable to expect an upwardleader to continue its progress towards the tip of thedownward leader if it is unable to obtain enoughenergy from the electric field to do so. The ability toobtain this energy is related to the average fieldbetween the downward leader tip and the point launch-ing the upward leader. On the basis of the criterionstated by Dellera and Garbagnati (see Section 3), theability to obtain the energy is related to the electricfield ahead of the advancing leader. The actual numeri-cal value of the field and the criterion selected do notaffect the conclusion. All leaders, once they have pro-gressed into air beyond their launch point, are subjectto the same laws governing their progress. It followsthat the striking distance and collection distance is adirect consequence of these laws and the properties of

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the air and is independent of any special features of theair terminal launching the leader.

10 Comments on selected streamer/leader propagation speed

Berger [4] in his Section 4 stated that: ‘... EarlyStreamer Emission (ESE) air terminals ... have beendesigned to trigger an upward leader earlier than thatissued from a Franklin rod ...’ and further states that:‘ESE conductors ... use an electrical triggering devicewhich reduces strongly the duration of the transitionfrom streamer to leader (this dead time limits the effi-ciency of any Franklin rod)’. We therefore considerthat early streamer emission air terminals are actuallyintended to operate by early emission of a leader andthat the relevant propagation speed is that of the leadersince the streamer-to-leader transition has taken placeearly in the process.

We consider that the selection of 106m s–1 (100cm/µs)as a constant streamer/leader speed is not supported bythe available evidence. The speed of 106m s–1 is aboutthe average speed of propagation of certain types ofdownward stepped leader in natural lightning (seereviews in Chaps. 5 and 12 of [40]) but is larger thanpropagation speeds in the early stages of laboratorypositive streamer/leader discharges. Measurements atLes Renardières High Voltage Laboratory [28–30] andby Gorin and Shkilev [16] indicate leader speeds in therange 1 – 3cm/µs in the early stage of leader propaga-tion during long gap breakdown under switchingimpulse conditions. Fig. 4.1.11 of [29] shows ‘realleader velocity’ to be in the range 1 – 2cm/µs for gapspacings in the range 2 – 10m. Fig. 4.2.1 of [29] showsthe ‘real velocity of the leader’ to be in the range 1.5 –2.5cm/µs for the time interval 200 – 400µs during thedischarge. Fig. 3.4.7 of [28] shows the leader lengthincreasing with time at a rate of about 1.5cm/µs over aperiod of about 400µs. Fig. 3.6.2 of [28] shows theaxial leader velocity above 20cm/µs only during the last4µs before breakdown, and exceeding 100cm/µs onlyduring the last 2µs before breakdown. This is in gen-eral agreement with Fig. 6 of [16] where the progressrate of the streamer zone boundary is plotted againstthe steepness of the front of the voltage impulse wave-shape. Progress rates reach about 100cm/µs only for afront steepness of about 1000kV/µs. These HV labora-tory tests are considered to simulate conditions at anair terminal under natural lightning conditions, as therate of rise of electric field is reasonably close to thatoccurring under natural lightning conditions. The simu-lation is improved if the negative switching impulse issuperimposed on a steady negative field [4]. For thesereasons we consider that the laboratory measurementsindicate at least the correct order of magnitude of theinitial speed of the upward connecting leader undernatural lightning conditions.

The speeds noted above agree with those reported byBerger [4]. He reported a leader speed of 20km s–1

(2cm/µs) for leaders from both ESE air terminals and‘Franklin rods’ (simple air terminals) in HV laboratorytests. These tests were carried out using switchingimpulses with a superimposed steady electric field, asnoted in Section 8.

Thus, the available laboratory measurements indicatethat leader speeds are typically a few cm/µs initially anddo not reach 100cm/µs until the final stages of theattachment process, when the electric field across the

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unbridged part of the gap is rising rapidly. For thesereasons we conclude that an increased collection dis-tance for an ESE air terminal under natural lightningconditions cannot validly be calculated by measuring atime advantage as described by Berger [2] and in DraftStandard NFPA 781-F93TCR, under HV laboratoryconditions, and multiplying this time by 106m s–1. If allother assumptions were valid, the distance advantage∆L in metres would be more correctly calculated with∆T in seconds as

11 Proposed alternative HV laboratory and field tests for ESE air terminals

We consider that a more appropriate way to establish acollection distance advantage for an ESE air terminalwould be to place an ESE air terminal and a simple airterminal a few metres apart in a HV laboratory test asdescribed in Section 8. The ESE air terminal should ini-tially be at the same height as the simple air terminaland be lowered in stages until the probability of initia-tion of discharges from each terminal became equal.The proposed test is illustrated in Fig. 8. The differ-ence in height of the two terminals would then be aphysically realistic measure of the increase in collectiondistance for the ESE terminal.

A similar requirement should be placed on compara-tive field tests of simple and ESE air terminals undernatural lightning conditions. Tests of this type havebeen described by Gumley [19] and are proposed in theDraft Standard NFPA 781-F93TCR. We propose atype of field test shown in Fig. 9 where an ESE air ter-minal is required to ‘protect’ a surrounding elevated

Fig.8 Proposed HV laboratory test to compare performance of an ESEair terminal and a simple air terminal (‘Franklin rod’) Height difference (∆H) is adjusted until there is equal probability of a dis-charge to each terminal

Fig.9 Proposed ESE terminal field test Test aims to determine whether or not terminal can ‘protect’ a surroundingobject (represented by suspended wire) within claimed distance (∆L) by whichESE collection distance exceeds striking distance for simple air terminals

8

conductor in the form of a suspended wire insulatedfrom ground and placed at a radial distance of about0.9 ∆L from the ESE terminal. Currents from the ESEair terminal and the suspended wire would be recorded.The objective would be to record the streamer andleader currents for close lightning that does not strikethe ESE air terminal or the suspended wire, and torecord return stroke current for strikes to either theESE air terminal or the suspended wire.

If the ESE claims are correct, the ESE air terminalwill have larger (and earlier) emission currents than thesuspended wire for all nearby lightning strikes that donot strike either the wire or the ESE terminal, and onlydirect strikes to the ESE air terminal should berecorded.

12 Comments on the protective radii claimed for the ESE air terminals

Draft Standard NFPA 781-F93TCR specifies a methodof calculating the protective range of an ESE air termi-nal based on the protective radius concept. This con-cept is a corollary of the collection volume concept fortall slender objects illustrated in Fig. 10. According tothis concept, for a simple air terminal as shown inFig. 10a, a spherical surface is drawn with centre A,the tip of the slender object, and radius equal to thestriking distance, ds, for a selected peak current. The

Fig.10 Geometrical construction used to establish the protective radiusfor a tall slender object a Simple air terminal at A b ESE air terminal at C

Fig.11 Reproduction of Fig. 4-2.3(b) from Draft Standard NFPA 781-F93TCR giving, for protection level II, values of Rpe as functions of heighth for various values of ∆L – – – ∆L = 0, this curve has been added to original

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point B on the surface is equidistant from A and theground. The radial distance from the centre line to B isRpn, the protective radius for a simple air terminal at A.Vertically descending leaders within a radial distanceRpn of the centre line will strike point A, leaders outsidethis distance will strike the ground.

With ESE terminals having a claimed collection dis-tance ds + ∆L, the situation shown in Fig. 10b applies,and the protective radius is increased to Rpe. Values ofRpe as a function of height h of the ESE air terminaland the value of ∆L are shown in Fig. 11, taken fromDraft Standard NFPA 781-F93TCR. For example, foran ESE air terminal 10m high, having ∆L = 30m, Rpeis about 66m. This concept is then transferred to build-ings of arbitrary shape as shown in Fig. 12. However,a concept developed for tall slender objects cannot val-idly be transferred to objects of arbitrary shape,because the boundary conditions for the electric field(or for electrogeometrical modelling) have beenchanged.

Furthermore, assuming for the moment that theclaimed collection distance for the ESE air terminal iscorrect, the ESE air terminal in Fig. 13 could only pro-tect the building edge to a distance of about ∆L sincethe edge will have its own normal collection distance.Applying the rolling sphere method to this situation asshown in Fig. 13 shows that a 10-m high ESE air ter-minal with a claimed collection distance of 75m (45m+ 30m) cannot protect the edge of the building with itsown collection distance of 45m when the edge is 66mfrom the ESE terminal. The lightning collection surfacefor point A (the ESE terminal) is shown as aaa. Thelightning collection surface for the building edge (B) isshown as bbb. All leaders reaching surface bbb, fromwhatever angle of approach, will connect with thebuilding edge rather than the ESE terminal.

Support for the view that nonstandard air terminalsin general and ESE air terminals in particular cannotprotect buildings in accordance with the claimed

Fig.12 Reproduction of Fig. B-2.2 from Draft Standard NFPA 781-F93TCR comparing protective zones of a ‘Franklin rod’ with those of anESE air terminal whose effective height is claimed to be increased by ∆L

Fig.13 Building with ESE air terminal which fails to protect edge ofbuilding at B Claimed collection distance 75m (∆L = 30m) Protective radius 66m Edge at B has normal striking and collection distance, 45m

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enhanced zones of protection (as defined by theclaimed protective radii noted above) is provided bythe observations of Hartono and Robiah [21] who haveobserved lightning strike attachment points on build-ings in Malaya well within the claimed zones of protec-tion. Their findings are as follows.

‘It was found that the majority of buildings whichwere installed with the non-standard lightning protec-tion systems have been repeatedly struck by lightningwith a significant number of these strikes falling wellwithin their claimed enhanced zone of protection. Thefailure of the non-standard lightning protection systemto prevent leader attachments to buildings is wellknown and has been documented. Some countries haveeven banned the use of the non-standard lightning pro-tection systems for safety reasons.’

The observed protection failures support our viewthat the collection distances of nonstandard (includingESE) air terminals is not significantly different fromthe collection distances of simple air terminals. Theobservations of Hartono and Robiah [21], mainlybased on tall buildings in Kuala Lumpur, show thatneither standard nor nonstandard air terminalsmounted away from the building edge are able to pro-tect the building edge. A conductive strip mountedalong the upper outer edge, with bonding to buildingreinforcing, appears to be the essential minimumrequirement for protecting tall buildings.

13 Conclusions

The arguments presented above lead us to the follow-ing conclusions regarding the claimed collection dis-tances for ESE air terminals.

There is no sound physical foundation for claims ofenhanced lightning protective properties for ESE airterminals. Even if a streamer from an ESE air terminalcan be launched at an earlier time than a streamer froma simple air terminal, it will be subject to the same lawsgoverning streamer to leader transition and upwardpropagation as any other leader, and will not completeits path to the downward leader tip unless the electricfield is sufficiently high. Thus it is not physically rea-sonable to assign a collection distance to a special pur-pose terminal that is larger than the collection distanceof a simple terminal in the same position.

The choice of 106m s–1 (100cm/µs) as an assumedconstant upward positive leader propagation speed isnot in agreement with the available HV laboratoryobservations. It would be more reasonable to assumean initial leader propagation speed of about 2cm/µs,increasing as the gap between the leader tip and thedownward leader diminished, and reaching 100cm/µs inthe later stages of the attachment process.

There appears to have been a misconception regard-ing the application of the protective radius concept tobuilding lightning protection. Even if it is assumed thatthe claimed collection distances for ESE air terminalsare correct, a simple analysis shows that the edge of abuilding will not be protected by an ESE air terminalwhen the edge is at a distance equal to the protectiveradius from the ESE air terminal as stated in DraftStandard NFPA 781-F93TCR.

There is so much doubt about the validity of theclaims for ESE air terminals that we consider that theclaims should be subjected to independent scrutiny andindependently conducted field tests before they areaccepted and embodied in any standard.

9

Our conclusion that nonstandard air terminals can-not have collection distances up to 50m greater thanthe collection distances of simple air terminals is sup-ported by observations in Malaya by Hartono andRobiah [21] of lightning strikes to unprotected parts(usually upper outer edges) of buildings well within theclaimed zones of protection of the nonstandard air ter-minals.

14 Acknowledgments

An earlier draft version of this paper was sent to sev-eral experts worldwide. We gratefully acknowledgetheir comments and contributions, many of which havehelped us to improve the paper. In particular, weacknowledge the assistance and information providedby the following persons: L. Dellera, A.J. Eriksson, E.Garbagnati, Z.A. Hartono, A.E. Pedersen, H. Stein-bigler, M.A. Uman, J. Wiesinger and W. Zaengl whoprovided the equipotential plot in Fig. 4. C.B. Mooresupplied evidence supporting our conclusions fromfield tests of ESE air terminals in New Mexico. J.J.Lowke and R. Morrow supplied evidence from theoret-ical analyses of electrical discharges supporting ourstatements concerning the propagation of streamersand leaders. We acknowledge the helpful comments ofthree referees, one of whom suggested the use of theterm ‘collection distance’ as distinct from striking dis-tance.

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