Considerations for the assessment of early streamer emission lightning protection

I.D.Chalmers FIEE, J.C.Evans and W.H.Siew MlEE

Abstract: The procedure used for assessing the zone of protection of early streamer emission (ESE) systems is outlined, according to standards whch have recently been adopted in the national codes of some countries. Much importance has been placed on the upward leader velocity by previous investigators, when assessing the zone of protection offered by an ESE device. However, by applying a simple model to the attachment process, it is shown that it is the velocity ratio, between the upward and downward leaders, that determines the theoretical zone of protection offered by any air terminal. Various assumed values of the velocity ratio are examined, with reference to existing attachment process models and measurements on natural lightning discharges. The influence of discharge intensity on ESE zone of protection is examined. A question is raised on the ability of the upward connecting discharge from an ESE device to propagate as effectively as that from a Franklin rod, in view of the fact that a leader launched ‘early’ will be subjected to a lower electric field. Evidence in the literature suggests that this consideration could prove detrimental to discharge propagation.

1 Introduction

7.7 The lightning attachment process The attachment process is discussed thoroughly in [I], but a brief synopsis will be given here. During the attachment phase of a lightning discharge the strike point is deter- mined. A downward leader, usually negative, propagates downwards from a thundercloud. As it approaches the earth, the electric field level at ground level rises. Eventu- ally, this leads to the formation of positive upward leaders, usually from grounded objects which protrude from the earth’s surface, which propagate towards the downward leader. If, as frequently happens, several upward leaders are initiated from different protrusions on the ground, then these upward leaders must compete to attach to the down- ward leader. The strike point of the lightning discharge is determined by whichever of these upward leaders is first to intercept the downward leader.

An important parameter when analysing the attachment process is the striking distance, d,. This is defined as the dis- tance between the downward leader tip and the object eventually struck, at the instant when the aforementioned object launches its upward leader. The importance of the striking distance will become apparent in the following par- agraphs.

1.2 Lightning protection - conventional and nonconventional Conventional lightning protection is based upon the use of the Faraday cage, which is usually constructed by placing metallic conductors in a grid pattern across a building’s

~ ~

0 IEE, 1999 IEE Proceedings online no. 19990025 DO1 lO. l049/ ipsmt:195 Paper fmt r&vd 28th July and in r e d form 3rd November 1998 The authors are with the Department of Electronic and Electrical Engineering, University of Strathclyde, Royal College Building, 204 George Street, Glasgow GI IXW,UK

roof and down the sides. These conductors are all intercon- nected and well earthed. Particular attention is paid to all likely strike attachment points (i.e. places from which an upward leader could originate). The lightning protection system is designed such that each of these vulnerable points is fitted with an air termination - a grounded metal conduc- tor, either vertical or horizontal.

Early streamer emission (ESE) lightning protection is regarded as nonconventional and generally comprises a special unit mounted on a vertical rod. This arrangement is designed, in theory, to protect a larger radius than a Fran- klin rod (or vertical air termination) of the same height. This is supposedly achieved by causing the positive upward leader to be launched earlier in the attachment process than would be the case for a passive Franklin rod subject to the same conditions. The ESE unit may have one of several features (radioactive elements, a special shape, the facility for generating small sparks, voltage generation at the rod tip [2]), each of which is purported to expedite upward leader inception. The authors wish to exclude radioactive air terminals from the following discussion since their effi- cacy has been discussed elsewhere [2-51, besides which radi- oactive lightning protection is banned in some countries and the European scene, at least, is dominated by nonradi- oactive devices.

Following on from the claimed enhanced lightning pro- tection capabilities of ESE devices, lightning protection sys- tems have been proposed, and installed, which comprise a few ESE terminals (and in some instances only one), where previously a multiple-air termination Faraday cage arrangement would have been employed.

1.3 Current situation regarding ESE lightning protection The NFPA (US standard issuing body) has rejected pro- posals to introduce a standard (NFPA 781, documented in [6]) on ESE lightning protection, after conducting an exten- sive survey on all existing ESE literature at the time [5]. However, to date, and as far as the authors are aware, two

51 IEE Proc-Sci. Meas. Technol., Vol. 146, No. 2, March 1999

countries have recognised the use of ESE lightning protec- tion. namely, France [7] and Spain [SI. In an attempt to understand the likely advantages of an ESE air terminal, over a Franklin rod, the authors have examined several of the assumptions underlying the assessment procedures for ESE devices (as described in [7, SI), with particular refer- ence to discharge velocity. It is intended that the discussion contained herein will enhance and broaden the arguments raised by such authors as Mackerras et ul. [2].

2 Assessing the performance of an ESE device

2. I Introduction A problem arises when assessing lightning protection in that field testing, under natural lightning conditions, is not always practicable, and, even in regions where there is abundant lightning activity, the probability of a single air terminal (say) being struck is very small. Thus, to collect a statistically meaningful amount of data, a test would have to run for years, perhaps even decades. The time necessary to collect data could be reduced by setting up multiple tests, but the duration of experimentation would still be measured in terms of months and years. Another option would be to use rocket-triggered lightning to enhance the number of strikes to ground around the desired test site.

The draft standard NFPA 781 [6] proposed using both laboratory tests and natural lightning tests to determine the ultimate protection capabilities of an ESE device. However, the French [7] and Spanish [SI standards rely solely upon laboratory testing. The aforementioned laboratory test is described in the following paragraphs.

-HV planar electrode

a i r terminal L t o be tested

ground plane Fig. 1 of m ESE device

kih(!rutory electrode configuutwn, to drtemiine the rime crdvantuge

2.2 Test procedure Electrodes are arranged as shown in Fig. 1. The air ter-

minal under test (be it Franklin rod or ESE device) is earthed to form the anode.

The HV plane electrode is subjected to a negative volt- age which comprises a DC component and an impulse component. The DC voltage is intended to cause a field which simulates the relatively static field generated at ground level by charges in the base of a thundercloud. The impulse voltage is superimposed on top of the DC voltage and is intended to represent the time-varying field gener- ated by an approaching downward leader. The applied impulse must have a time-to-crest of between 100 and 1000 p and the composite voltage must be sufficient to cause lOOU/o breakdown probability [7].

The composite voltage is applied 100 times, per air ter- minal.

For every application of the composite voltage, the, ‘. . . upward leader triggering time . .’ [7]. The authors find the above definition of the relevant test

’ is measured .

parameter ambiguous, since it could be interpreted in more than one way. By considering the way in which the ‘upward leader triggering time’ is subsequently manipulated in [A, we believe the parameter of interest to be the time of continuous leader inception. In this context, continuous leader inception refers to the formation of a freely propa- gating leader that is relatively uninfluenced by the presence of the HV plane during the early stages of propagation. By referring; to [9], it can be seen that the HV plane can inftu- ence discharge development, particularly if the test is con- ducted in a small gap (i.e. small parameter d in Fig. 1). The experiments reported in [9] employed a gap geometry like that shown in Fig. 1, with H = 2.5m and h = lm. The gap geometry was in compliance with [7], where various values of H and h are permissible. It was found that although the discharge mechanism involved leader activity, the leader growth was intermittent, and no continuous leader phase was derected. Breakdown was brought about by the streamer zone boundary, ahead of the discontinuous leader tip, reaching the HV plane and therefore breakdown was initiated before the conditions required for continuous leader growth were fulfilled. Obviously, then, the time of continuous leader inception could not be determined. Therefore, based on our interpretation of the ‘upward leader triggering time’, tests conducted in small gaps are invalid in terms of determining the ‘upward leader trigger- ing time’.

The a.verage times of continuous leader inception for the Franklin rod and ESE air terminal, T: FR and T. E respec- tively, are calculated for each air terminal.

The average times of continuous leader inception are normalised to an applied impulse with a time-to-crest of 650p (this is presumably because the contributors to [7] believe that the 650p wave shape creates a time-varying field that is the most representative of that seen in natural lightning). The laboratory impulse voltage varies with time, and both inception times will correspond to particular val- ues of electric field, E , FR and E; E respectively, where elec- tric fields are calculated by dividing- the instantaneous applied voltage (at times r;, FR and T : E ) by the plane- plane distance, H (see Fig. 1). The normalised leader incep- tion times, r, FR and-Ti E, are the times at which the electric field of Ei F~ and Ei E are achieved on the normalising 6 5 0 ~ waveform. To perform this normalisation, the assumption is that the field at leader inception would be the same regardless of the time-to-crest of the applied wave. It is not obvious that such a normalisation procedure is valid for the case of a simple Franklin rod let alone for every type of ESE device, regardless of the ESE operating mech- anism. For example, many ESE devices appear to extract their energy from the electric field generated by an approaching downward leader (in natural lightning) or from the field created by the applied impulse voltage (in the laboratory test). If the ESE operates in a way such that the efficiency of energy extraction has any dependence on the rate of change of field (dEldt), then it seems highly unlikely that a linear normalisation procedure can be legitimately applied. It should be noted that macroscopic value of field (i.e. E ; FR and E , E ) will be lower than the corresponding localised field seen at the air terminal tip at times T: FR and T;. E .

The time advantage, AT, is calculated:

AT = T%FR - T L ~ (1)

AL = v+AT (2)

The dimensional advantage, AL, is calculated:

IEE Proc.-Sci. Mens. Techno/.. L’ol. 146. No. 2. .March I999 58

where v, represents the velocity of the upward leader in a real lightning discharge and AL. is used to represent the amount by whch the stnking distance for an ESE air ter- minal is larger than that for a Franklin rod.

3 Leader velocity

The larger the strilung distance, the larger the theoretical zone of protection of an air terminal. From eqn. 2 it can be seen that AL is proportional to the upward leader velocity v+, thus making the actual value of v, crucial. The value of v, is a matter of controversy. In [7], the assumed upward leader velocity is 106m/s. This figure has been strongly dis- puted by Mackerras et al. [2], who suggest that this value is at least an order of magmtude too large. However, it will be shown in this paper that, if several assumptions are made, the parameter AL. is governed by the ratio of the two leader velocities (upward and downward) rather than their actual magnitudes.

a b

Fig.? Thoretical conditions in a uaturul lightning dkcharge at tmie T = T . c Down%d leader tip at a distance d, + AL from lightning rod tip a ESE air termination h Franklin rod air termination

downward leader

v y..” . upword

.....,......

\ t i s j;/,/////,,,/;L, b

FigL3 T= T , FR Domward leader tip at a distance 4, from lightning rod tip a ESE air termination Upward leader length is AL b Franklin rod air termination

Theoretical conditions in a natural lightning dkcharge at tmw

3.1 The velocity ratio vJv+ Fig. 2u shows an ESE device approached by a downward leader whose tip has reached a distance ds + AL from the ESE air terminal. If the theory in [7] is valid, then the elec- tric field conditions enable the ESE air terminal to launch a continuous upward leader when the macroscopic field in its vicinity has reached a magnitude of Ei E. In Fig. 2b, a Franklin rod is subjected to the same electrical conditions as the ESE air terminal in Fig. 2a. The downward leader tip has not yet attained striking distance from the Franklin

IEE Proc.-Sci. Meus. Technol.. Vol. 146, No. 2. March 1999

rod, and therefore the conditions required for upward leader inception are not yet satisfied. In Figs. 3a and b the downward leader has propagated further downwards and the Franklin rod (Fig. 3b) is able to launch an upward leader since the macroscopic field in the vicinity of the air terminal has reached ELFR. According to [7], by this time the upward leader originating from the ESE air terminal (Fig. 3u), subject to the same conditions, would have attained a length AL.

Referring to Figs. 2 and 3, at the time of upward leader inception the downward leader tip is at a height DE and DFR for the ESE device and Franklin rod respectively. The time taken for the downward leader to travel between these two heights is governed by the downward leader velocity, v . In fact;

where AT is equivalent to the time advantage determined from the laboratory experiment described earlier. Combin- ing eqns. 2 and 3;

(4) U+ A L = -(DE - D F R ) 2)-

The authors acknowledge that eqn. 4 is not of any great practical significance, in view of the difficulty encountered when quantifying the parameters on the r.h.s. of the equa- tion. However, an attempt will be made to quantify the r.h.s. parameters of eqn. 4 in Section 3.4, using the similar assumptions to those used in [7]. Eqn. 4 will merely be employed to illustrate that the choice of parameters used to assess the zone of protection offered by an ESE device have an extreme influence on the magnitude of the calculated zone of protection. Also, the authors believe that too much emphasis is being placed on the upward leader velocity.

From eqn. 4, AL. has no dependence upon the actual velocity of leader propagation, rather it is proportional to the ratio of the upward leader velocity to the downward leader velocity. The negative leader velocity is incorporated in the testing procedure outlined in [7] when calculating the impulse voltage to be used during testing and normalisa- tion of the laboratory leader inception times. In [7], the upward and downward leader velocities are considered to be equal, i.e. v-/v+ = 1.

3.2 Measured magnitude of the velocity ratio Yokoyama et al. [lo] performed velocity measurements on the attachment phase of lightning discharges using a photo- diode array positioned at the focal point of a camera and concluded that, ‘The progress velocity of an individual step of stepped leaders can be expressed by a log-noma1 distri- bution and its mean value is 1.6 x 106m/s for upward lead- ers and 4.0 x 106m/s for downward leaders.’ and , ‘The mean progress velocity of a total leader process is given as 0.8 - 2.7 x 105m/second for upward leaders and 2.9 x 105m/second for downward leaders, . . ’. It is important to stress that the period over which the ESE device is being assessed is not the whole attachment process, but the period between upward leader initiation by an ESE device and upward leader initiation by a Franklin rod. Whether the step speed or the mean speed is used to calculate the performance of an ESE device, v./v+ appears to be greater than 1. However, the research of Yokoyama et al. was car- ried out on Winter thunderstorms in Japan, in which posi- tive cloud-to-ground discharges are common [ 1 13, and this consideration could influence the relevance of these results to the more common negative flash.

59

3.3 Theoretical magnitude of the velocity ratio Several models of the lightning attachment process exist, attempts having been made to correlate each model with existing lightning data. Three models will be mentioned here. Eriksson [I21 and Rizk [13] both assume that the velocity ratio is unity during attachment. However, Dellera and Garbagnati [14] assume, based on observations in the field and in the laboratory, that the velocity ratio v lv , is four at upward leader inception and during attachment the velocity ratio is assumed to decrease reaching unity imme- diately prior to attachment of the two leaders. In the dis- cussion of [13], Rizk makes reference to the velocity ratio used by Dellera and Garbagnati [14], ‘If the instantaneous value varies between 4 and 1 during positive leader propa- gation, the average value, which is relevant to this model, will be considerably lower than 4.’ In other words, the velocity ratios employed by Rizk and DellerdGarbagnati do not necessarily contradict one another and differentia- tion may be necessary between the average velocity ratio and instantaneous velocity ratio during the attachment phase.

The test procedure described in [7] concerns the very early stages of upward leader propagation, whilst the downward leader is between the heights DE and D,. Therefore, it is the instantaneous velocity ratio which is of most interest here, assuming it differs from the average. If the upward leader accelerates as it propagates, then v-lv, may be considerably larger than 1 during the period in which the performance of the ESE device is assessed.

3.4 Quantifying the effect of velocity ratio on the parameter A L Aleksandrov et u1. [ 151 presented laboratory measurements of the average positive upward leader inception field deter- mined using a procedure which appears to be the same as that described in Section 2.2. The applied impulse had a time-to-crest of around 500p. The ESE air terminal in question was found to have a continuous leader inception field (macroscopic) of 127kVlm whereas a passive Franklin rod, of the same dimensions, required a field of 213kVlm to form a continuous upward leader. The associated con- tinuous leader inception times were 75 and 175ps respec- tively. This would give AT > loop since the leader inception times would have to be normalised to the 650p time-to-crest, if the procedure in [7] were followed. Conse- quently, for this set of laboratory results, AL > 100m. This value of AL is arrived at by assuming that the upward leader velocity is 106m/s - the disputed value assigned to v+ in [7].

The upward leader inception field is dependent upon the electrode geometry used during testing. However, in the absence of any alternative figures, the macroscopic leader

inception fields reported in [ 151 will be assumed applicable to the case of a free standing Franklin rod and ESE air ter- minal, under natural lightning conditions.

Using the leader model outlined in [16] the electric field below the downward leader can be approximated. It is assumed that 10% of the charge carried by the downward leader is, concentrated at the tip, the remaining 90% being uniformly distributed along the leader length. It is assumed [16] that the total charge carried by the downward leader is related it0 the peak return-stroke current such that every 15kA of current corresponds to 1 C of charge. The authors have assumed that the length of the downward leader at attachment, L, is equal to 2000m. It is assumed that the downward leader propagates vertically and approaches a perfectly conducting ground plane. The electric field at any point on the ground plane is given by:

1 -

1 E = - - (02 + d J ) $ (Lz + d 2 ) &

where 1) is the height of the negative leader tip above ground and q represents the charge distributed along the leader, per unit length. The parameter d is the horizontal distance from the vertical downward leader trajectory to the point on the plane where the field is being calculated. For calculation purposes, the case where the air terminal and the downward leader lie on the same vertical line has been considered. For d = 0 m, eqn. 5 simplifies to:

(6)

Thus the height of the leader when the inception field, for each device, is reached can be calculated. This calculation has been performed for several different intensities of downward leader, and the results are shown in Table 1. AL. has also been calculated for various values of v_lv+.

The values of AL given in Table 1 were calculated for the case where d = Om. For cases where d # 0, the predicted values of AL are larger than those indicated in Table 1, but not to an extent that affects the observations that can be made udng Table 1. The predicted increase in AL occurs because an increase in d causes the dEldt experienced by the air terminal to become lower, for a given set of down- ward leader parameters. For example, for a lOkA leader, a unity velocity ratio and d = 40m, eqn. 5 gives AL = 34.78m, as opposed to 29.92m when d = Om.

The striking distance for the Franklin rod, as calculated in Table 1 (2nd column), is somewhat large, especially when compared with the output of other formulae, which predict striking distance given a particular prospective

Table 1: Mathematical determination of AL based on EiFR = 213kVlm and EiE 127kVlm and various values of v-/v+: effect of velocity ratio on the parameter AL

( E = 213kV/m) ( E = 217kV/m) DE- DFR vJv+ = 1 vJv+ = 2 vJv+ = 4 DFR (m) DE (m) (m) AL(m) AL(m) AL(m)

5 58.80 78.36 19.56 19.56 9.76 4.89

10 86.54 116.46 29.92 29.92 14.96 7.48

20 129.09 175.69 46.60 46.60 23.30 11.65

50 224.14 309.01 84.87 84.87 42.44 21.22

70 276.09 381.29 105.20 105.20 52.60 26.30

100 344.95 475.55 130.60 130.60 65.30 32.65

120 386.53 531.37 144.84 144.84 72.42 36.21

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return-stroke current. For example, eqn. 7 is widely quoted. Th~s equation predicts striking distance in metres when given the return-stroke current in kA.

d, = 101°.65 (7) However, it can be argued that there is no definitive quan- titative relationship between d, and I, although a trend can be observed in the sparse available data. Uman [l] men- tions several different theoretical relationships between striking distance and return-stroke current, one of which (curve 4 in Fig. 6.1 of [I]) predicts striking distances close to those shown in Table 1. Also, the majority of the photo- graplucally obtained data presented in Fig. 6.1 of [I], wluch relates d, to I, lies above the curve described by eqn. 7. This suggests that striking distances may be larger than those indicated by eqn. 7.

From eqn. 4 it was apparent that AL is proportional to vh,. It is the ratio of these two velocities that ultimately determines any superior performance of the ESE lightning air terminal, as opposed to the upward leader velocity alone. Table 1 illustrates the significant effect that the velocity ratio has on U, and subsequently on the theoreti- cal radius of protection offered by the ESE device.

A

I I I I

I

I I I I - %+ AT

small large Fig.4 time udvuntuge

Graph to illuvtrute the effect of rule of c h g e ofjeld on iwmred

4 Strike intensity and its influence on AT

In Section 2.2 the normalisation of the leader inception times, to a 6 5 0 ~ time-to-crest impulse, was briefly men- tioned. The implicit assumption used is that the macro- scopic continuous leader inception field will have the same magnitude, regardless of the applied impulse wavefront time. The normalisation procedure itself suggests that the greater dEldt during the region of leader inception, the smaller the difference between Franklin rod and ESE leader inception times. This is demonstrated in Fig. 4, where a constant dEldt has been employed for illustrative purposes. This in turn suggests that AL will be smaller, the greater dE1dt during the region of leader inception. During the region of leader inception dEldt is greater the weaker the discharge and this is highlighted in Table 1 where, regardless of velocity ratio, the parameter AL. decreases as the prospective return-stroke current decreases. The values of U, for v h , = I , in column 5 of Table 1 should be com- pared to the figure of AL > loom, as discussed in Section 3.4. The question arises, how suitable is the 6 . 5 0 ~ normalis- ing waveform used in the laboratory? The theory presented in Table 1 suggests that the dEldt during the critical part of

IEE Proc.-Sci. Meas. Technol.. Vol. 146, No. 2, March 1999

the attachment phase has an appreciable probabhty of being greater than that associated with the 650p wave. We can draw this conclusion since a 50kA discharge produces a smaller U (84.87m) than that suggested by the labora- tory experiment, for which AL > 100m. In lightning dis- charge, there is a substantial (> 50%) probability that the return-stroke current will be smaller than 50kA. This does suggest then, that the 6 5 0 ~ normalising wave rises too slowly, and this leads to AL being overestimated, consider- ing the intensity of the majority of lightning discharges.

5

The previous argument, used to stress the importance of the velocity ratio, assumed that ESE leaders propagate at the same velocity as upward leaders from a Franklin rod. In the following paragraphs it will be shown that this assumption may give an optimistic result for AT and AL. To successfully intercept the downward leader, the upward leader must be able to propagate after it has formed. How- ever, since an ESE device initiates its upward leader ‘early’, the electric field into which the leader has to propagate will initially be lower than would be the case for a Franklin rod which must ‘wait’ until the downward leader gets closer to the ground. The argument that the field magnitude will affect the propagation capabilities of ‘early’ leaders has been raised already in [2].

It has been shown that the upward connecting discharge velocity increases as the upward leader channel elongates [ 171. Dellera and Garbagnati attributed this phenomenon to the increase in electric stress at the leader tip, as it prop- agates. If this is the case, then the ESE leader tip, which is subjected to a lower field than the Franklin rod discharge (at initiation), can be expected to propagate more slowly than would be the case if it were launched at later instant in time, when the downward leader has had an opportunity to propagate closer to earth.

The length of the corona region ahead of the leader tip is also dependent upon the magnitude of the local electric field into which the upward leader propagates [IS]. This behaviour can be seen in long spark studies. Carrara and Thione [19] commented on the slight lengthening of the corona region during leader propagation. This lengthening coincided with an increasing stress ahead of the leader tip.

In long spark studies, it would be expected that, if the leader corona ahead of the leader tip is longer (as would be expected in the case of the Franklin rod discharge, when compared with the ESE discharge), then the time between continuous leader inception and breakdown would be shorter. Although it is unknown the exact degree to which long spark studies can be compared with natural lightning [2], this observation would suggest that it is remiss to calcu- late the zone of protection from the time difference at leader inception alone. Evidence from the laboratory sup- ports this.

5.7 Laboratory observations Berger [20] presented two streak photographs of discharges, one discharge emanating from an ESE air termination, the other from a Franklin rod. The test gap had parameter H = 13.5m (Refer to Fig. I). The rod height, h, is not quoted in [20] but we believe it to have been 3Sm, based on the assumption that references [15, 201 refer to the same experi- mental programme.

These ‘typical’ streak photographs have been roughly sketched and are presented in Fig. 5. For the ESE dis- charge, the leader channel is unshaded and lies below the shaded leader corona. The leader velocity is represented by

Comparative velocity of ‘early’ and ‘late‘ leaders

61

the gradient of the line between the leader zone and the leader corona. The leader velocity is seen to increase with time, in the case of the ESE device. At inception of the ESE leader (A) the velocity of the leader is visibly slower (lower gradient) than at the end of the image converter record (B).

1

A B

a

D E b

Fig. 5 Rough sketch of two streuk photograph Discharge emanating from: U an ESE air termination b a Franklin rod adapted from [20]

During the initial stages of discharge development, the Franklin rod is unable to launch a continuous leader. Instead there are intermittent bursts of discharge growth, between (C) and (D), and eventually the continuous leader channel forms. At leader inception (D), the leader velocity appears to be greater than the leader velocity at ESE leader inception (A). The Franklin rod leader appears to have a speed at (E) comparable to the ESE leader at (B). How- ever, this observation is only approximate. Berger draws attention to the comparatively slow initial ESE leader velocity and allowance is made for this phenomenon in [20], although no such correction is required for compliance with [7].

A further point to note is that the leader corona extends further in the case of the Franklin rod leader. For a given leader length, the Franklin leader corona has attained a longer length when compared with the ESE leader corona for the same leader length.

In [7] the performance of an ESE device is assessed purely on the progression of the ESE leader from its incep- tion (A) until a time at which a Franklin rod, subject to the same electrical conditions, would be able to launch a con- tinuous upward leader (D). It is the initial stages of the ESE leader which are important, and it is this portion of the discharge which is expected to be the slowest. Ths gives rise to the suspicion that the advantage of the ESE device over the Franklin rod is larger at inception than at break- down.

6 Suggested modifications to assessment procedure

It is debatable whether a laboratory test can be used to determine the zone of protection of any air terminal. How-

62

ever, the alternative of field testing under natural lightning conditions is inevitably costly, time-consuming and may not, at the end of the allotted test duration, yield results. Therefore, it is perhaps inevitable that some reliance be placed on laboratory tests to determine the effectiveness of ESE lightning protection systems.

The authors have reservations about the simple method used to determine AL using the laboratory test and accom- panying theory of [7]. Nonetheless, the test procedure described in [7] at least provides a starting point for assess- ing the differences in discharge behaviour exhibited by dif- ferent air terminal types. In the first instance, we suggest that the following issues need addressing.

The minimum permissible gap length should be increased, such that a continuous leader formation occurs, as it would in a real lightning discharge, prior to the onset of breakdown. The respective discharge velocities of ESE and Franklin rod discharges should be investigated. This issue could be partially addressed by analysing breakdown times, in conjunction with leader inception times in labora- tory experiments (where propagation is assured due to the over-voltage of the gap). A longer minimum gap length is desirable here also, since any differences between the time advantag,e at leader inception and breakdown might not be apparent in a short gap. Analysis of leader inception times and times-to-breakdown would not, however, ultimately answer questions raised by other authors [2] on the ability of the ESE discharge to propagate in a real lightning situa- tion (where the propagation problem is signlficantly more complex in view of downward leader dynamics, longer propagation distances and competing upward leaders).

7 Conclusions

Using a simple model of the downward leader, it has been shown that the parameter AL is strongly dependent upon the intensity (peak-return stroke current) of the discharge to which the Franklin and ESE air terminals are subjected. This model indicates that, based on the assumption that v h , = 1, the natural lightning dEldt is likely to be greater than that achieved in laboratory tests of the type described in [7]. In view of this consideration, AL could be overesti- mated. For example, assuming v /v+ = 1, the model pre- sented in this paper predicts hL = 29.92m for a lOkA discharge. This should be compared with the value of AL > lOOm mentioned in Section 3.4.

Even although the velocity ratio is assumed to be unity in [7], evidence in the literature suggests that the velocity ratio could be considerably higher during the period of the attachment process when ESE performance is ascertained. This consideration could cause the parameter AL to be fur- ther overestimated. Considering a lOkA discharge, the pre- viously described model predicts that AL. would be reduced to 7.48m if the velocity ratio were as high as 4 during the early stages of attachment.

The ESE leader is launched earlier in time, but at a lower ambient field than the Franklin rod leader. If this consider- ation causes the ESE leader to propagate at a velocity which is initially slower than the Franklin rod discharge, the advantage of the ESE device at leader inception would be partially or totally offset by the faster propagation veloc- ity of the Franklin rod discharge.

All three of the above considerations could cause assess- ment based on the model outlined in [7], to over-predict the zone of protection of the ESE device under test.

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8 References

1 UMAN, M.A.: ‘The lightning discharge’ (Academic Press Inc., Orlando, FL, 1987)

2 MACKERRAS, D., DARVENIZA, M., and LIEW, A.C.: ‘Review of claimed enhanced lightning protection of bddings by early streamer emission air terminals’, IEE Proc. Sci. Meas. Technol., 1997, 144, (l), pp. 1-10

3 GOLDE, R.H.: ‘Lightning protection’ (Edward Arnold Ltd., London, 1973)

4 ZIPSE, D.W.: ‘Lightning protection systems: advantages and disad- vantages’, IEEE Trans. Ind. Appl., 1994,30, (S ) , pp. 1351-1361

5 VAN BRUNT, R.J., NEILSON, T.L., and FIREBAUGH, S.L.: ‘Early streamer emission air terminals lightning protection systems’. National Fire Protection Association Publication, Quincy, MA, 1995

6 Draft Standard NFPA 781 - F93 TCD: Technical Committee Docu- mentation, 1993 Fall Meeting, Pheonix, AZ, National Fire Protection Association Publication, Quincy, MA, 1993

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9 CHALMERS, I.D., EVANS, J.C., SIEW, W.H., ALLEN, N.L., GREAVES, D.A., and COTTON, I.: ‘Laboratory testing of early streamer emission air terminals’. Proceedings of international confer- ence on Lightning protection, Birmingham, UK, September 1998, pp. 412417

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13 RIZK, F.A.M.: ‘Modeling of transmission-line exposure to direct lightning strokes’, IEEE Trans. Power Deliv., 1990, 5, (4), pp. 1,983- 1997

14 DELLERA, L., and GARBAGNATI, E.: ‘Part I: Description of the model and evaluation of exposure to free-standing structures’, IEEE Truns. Power Deliv., 1990, 5, pp. 2,009-2,017

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16 ABDEL-SALAM, M., EL-MOHANDES, M.T., BERGER, G., and SENOUCI, B.: ‘Onset criterion of upward streamers from a Franklin rod’, J. Electrost.. 1989, 24, pp. 4S59

17 DELLERA, L., and GARBAGNATI, E.: ‘Shielding failure evalua- tion: application to the leader progression model’. Presented at inter- national conference on Lightning and power systems, London, UK, June 1984, Paper 66,

18 GARCIA, H., and HUTZLER, B.: ‘Electrical breakdown in long air gaps - the final jump’. Proceedings of 3rd international conference on Gas discharges, London, September 1976, pp. 206-210

19 CARRARA, G., and THIONE, L.: ‘Switching surge strength of large air gaps: a physical approach, IEEE Tram. Power Appar. Syst., 1976, PAS-95, (2), pp. 512-524

20 BERGER, G.: ‘The early streamer emission lightning rod conductor’. Presented at International Aerospace & Ground conference on Light- ning & .stutic electricity, Atlantic City, NJ, October 1992, Paper 38

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