Power Electronics Applications Center

Knoxville TN 37932 USA

Amansoor@epri-peac.com

ç

National Institute of Standards and Technology

Gaithersburg MD 20899 USA

f.martzloff@ieee.org

© 1996 IEEE

Reprinted, with permission, from IEEE Transactions on Power Delivery, Vol.12, No.3, July 1997

Significance:

Part 2 – Development of standards – Reality checks

Part 4 – Propagation and coupling of surges

In the case of a direct lightning stroke to a building, the earth-seeking current is dispersed among all

available paths to earthing electrodes, including intentional made electrodes and opportunistic electrodes. 

A substantial part of that current will exit the building via its connection to the power distribution system.

The initial part of the stroke current dispersion is dominated by the relative inductance values of the

multiple paths and the tail of the stroke current dispersion is dominated by the relative resistance values of

the multiple paths.

The configuration of this power distribution system (daisy chain from the transformer or radial from the

transformer) influences the sharing of the current among these possible paths.

From simulations performed with a 10/350 waveform, the paper provides quantitative information on these

effects.

Filename: Dispersion
IEEE Transactions on Power Delivery, Vol. 13, No. 3, July 1998 

The Effect of Neutral Earthing Practices on 

Lightning Current Dispersion in a Low-Voltage Installation 

Arshad Mansoor, Member, IEEE 

Power Electronics Applications Center 

Knoxville TJX 37932 USA 

Abstract - Computer modeling with the EMTP code has been 

applied to several configurations and earthing practices in use in 

various countries to show the effect of any differences in the 

dispersion (sharing) of a lightning stroke current among the available 

paths for the earth-seeking lightning current. Simplifying assumptions 

have been made to some details of the configurations to focus on the 

main difference -- earthing practices. Identifying such differences 

provides the necessary perspective on their significance and the strong 

need to take them into consideration when developing international 

standards on surge-protective device applications. 

I. INTRODUCTION 

When designing a lightning protection scheme for a lowvoltage power system within a building, several scenarios must 

be considered for the point of termination of the lightning 

stroke. Common wisdom classifies these by decreasing order 

of severity: directly to the building, directly to overhead lowvoltage distribution lines (or other utilities) outside of the 

building, to other objects near the building, distant cloud-toearth strokes, and finally perhaps cloud-to-cloud discharges. 

Several standards-writing projects are underway, at the IEEE 

and at the IEC, based on present knowledge of the lightning 

flash characteristics and on assumptions about the way the 

lightning current divides among the many paths available for 

distributing (dispersing) this current to the ill-defined "earth" 

which is the termination of the cloud-to-earth strike. 

The purpose of our paper is to show the effect that different practices for neutral earthing in the low-voltage distribution 

system can have on the relative dispersion of the lightning 

current which is seeking the path of least impedance to earth. 

* Electricity Division, Electronics and Electrical Engineering Laboratory, Technology Administration, US. Department of Commerce. 

PE-968-PWRD-0-05-1997 A paper recommended and approved by the 

IEEE Surge Protective Devices Committee of the IEEE Power 

Engineering Society for publication in the IEEE Transactions on Power 

Delivery. Manuscript submitted January 2, 1997; made available for 

printing May 23, 1997. 

Franqois Martzloff, Life Fellow, IEEE 

National Institute of Standards and Technology* 

Gaithersburg MD 20899 USA 

To accomplish this purpose in an eight-page paper, and to 

concentrate on the essential difference, the models we present 

are simplified from the detailed reality, so that one of our first 

tasks will be to explain and justify the simplification. To avoid 

confusion in the meaning of the word "distribution" which can 

relate to the distribution of electric power by the utility or to the 

distribution of the lightning current among the available paths, 

we will use the term "dispersion" for the second meaning, 

lighting current dispersion. Another term used by some authors 

to convey the concept is "sharing" (among available paths). 

Note that the actual return stroke actually goes from earth to 

cloud in the majority of cases, but the scenario is generally 

described as if the stroke "t~erminated" on earthbound objects. 

In the case of a low-voltage power distribution system, 

different countries have adopted different practices on earthing 

the neutral conductor, and writing a history of why that is so 

would give an interesting insight into the development of power 

systems. The fact is thai today, two approaches are well 

entrenched in their respective territories, the so-called TN 

system and TT system where the difference lies in the mode of 

earthing the neutral. We will give a brief overview of the 

differences in ,a following section. Our purpose is to show how 

the difference in these practices affect the sharing, or 

dispersion, of the lightning current among the available paths to 

earth, and coiisequently affect the rating of surge-protective 

devices which may be included in these paths. We used the 

EMTP simulation code [I] to model several scenarios in each 

of the TN and TT systems, with small but possibly significant 

differences in the configuration. By postulating a direct stroke 

to one building, and requesting EMTP to compute currents in 

the (simplified) complete power system, we obtained results for 

the two most severe cases of lightning termination: the case of 

a direct stroke to one building, and the case of a nearby stroke 

which propag,ates and impinges at the service entrance of many 

buildings on that part of the low-voltage distribution system. 

The literature and draf standards contain many examples 

of such scenarios, but it seems that each is confined to a 

specific appr~oach or power system configuration with fairly 

detailed arrangements of load connections. The result is that 

from this plurality of examples, it is difficult to extract a clear 

perception of the significant parameters in the dispersion of the 

lightning current resulting from different earthing practices. In 

this paper, we: will simplify the scenarios to concentrate on the 

fundamental difference between the neutral earthing practices. 

0885-8977/98/$10.00 O 1997 IEEE 
IT. THE TT AND THE TN SYSTEMS 

The IEC has promulgated a letter code system describing 

the arrangement of the neutral earthing in single-phase and 

polyphase power systems [2]. For the purposes of our paper, 

we can summarize the TT system as being a distribution system 

where the neutral is earthed only at the distribution transformer 

secondary, and the protective earth in a building is obtained 

from a local earth electrode. This system is used in some 

countries. The TN system has its neutral earthed at any 

available opportunity outside of a building, including the 

distribution transformer secondary, some or all poles, and the 

service entrance. In the United States, an "Equipment 

Grounding Conductor" (EGC) is created at the service 

entrance, bonded to the incoming power system neutral and to 

the common local earthing point, after which the neutral 

conductor and the EGC are carefully (and by mandate from the 

National Electrical Code [3]) kept separate from one another. 

111. NECESSARY SIMPLIFICATIONS 

Another difficulty in making a detailed comparison of 

results Erom different authors is that different models are often 

used. When apparently different results are reported, a 

lingering question is that of differences attributable to the 

simplifying assumptions and possible modeling artifacts. We 

have used the well-known EMTP code [l] for which our 

previous experience in cross-validation between the computer 

model and full-scale experimental measurements [4], [5] gave 

us great confidence in the validity of the results. 

The literature offers many contributions on the system 

simulation but our purpose is not literature review -- again, our 

purpose is only to focus on the neutral practices considerations. 

However, to support some of our postulates, we will cite some 

papers to show that in the maze of assumptions, 

simplifications, and simulations, we are not alone. 

A. Down-conductor representation 

Some authors have included in their modeling a downconductor feeding the stroke current to the common bonding 

point of the building [6]. In our model, since we postulate that 

the current is delivered from a current source, the impedance of 

the down-conductor has no effect on the current being injected 

at the common bonding point which is the point at which 

dispersion (sharing) begins. Therefore, we did not include a 

down-conductor in our models. 

B. Earthing impedance as a function of time and current 

Some authors consider the fact that the exact value of the 

earthing impedance is variable as a function of time and current 

level. For instance, [6] initially proposes a model involving 

resistance, capacitance, and inductance, with some dependency 

on time or current, or both. But after studying the problem 

closer, the authors of [6] conclude that a reasonable approximation is merely a fixed 10-8 resistance. We have used this 

value in our models of the building earthing, and postulated an 

improved, lower 5-8 resistance at the earthing electrode of the 

distribution transformer. 

C. Other available current paths 

Some standard proposals include telephone, water and gas 

connections as possible paths for the earth-seeking lightning 

current. Considering that the telephone service is a balanced 

system normally isolated from earth (until a network interface 

device becomes involved), that some water and gas services can 

include a cathodic-protection isolation or be implemented with 

plastic pipes, we chose a conservative approach of not 

including these as additional paths to earth. 

D. Actual Circuit Configuration for Service Entrances 

Figure 1 shows a schematic of a single-phase 3-wire TN 

1201240-V service to a building. One surge-protective device 

(SPD) is connected between each of the two lines and the 

common earth at the service entrance, ignoring any SPDs within 

the building under the assumption that in a well-coordinated 

cascade [7] the majority of the current is carried by the service 

entrance SPD which has the lowest limiting voltage in the 

installation. The stroke current, postulated to have terminated 

on a point of the earthing system of the building, can seek a 

path to earth in two ways: directly through the earth electrode 

of the building, and by means of the three conductors back 

toward the power system. 

Figure 1. Service connections in a 3-wire TN system 

Figure 2 shows a schematic of a three-phase 4-wire TT 

2301400-V service to a building. A dedicated protective earth is 

created and connected to a local earth electrode, while the 

incoming neutral of the power distribution system is not bonded 

to this protective earth. At the service entrance, SPDs are 

connected between the local earth and each of the incoming 

lines and the neutral. 

Figure 2 - Service entrance connections in a 3-phase, 4-wire TT system 
E. Postulated lightning stroke current 

While some authors propose a 200 kA, 101350 ps surge 

[S], [9], others suggest that even a 100 kA peak might already 

be too high a value 161, [lo], [ll]. In agreement with the latter 

three references, we postulated a 100 kA peak, 1013 50 ps surge 

current. This selection also offers the convenience that when 

we report current levels in kiloamperes in the various circuits, 

the numbers also represent the percentage of the sharing, 

making it easier to follow the process. Since many standards 

for surges impinging on SPDs (at the service entrance) are still 

based on an 8/20 ps current waveform, we will also show one 

example of the energy deposition in the SPDs when such an 

8120 ps surge is postulated. 

The surge currents are modeled using the EMTP Type 60 

Slave Source. Using the "Freeform FORTRAN expression, 

any surge current waveform that can be expressed as a closedform equation can be used as signal source in the main EMTP 

program. The equations for the 101350 ys and 8120 ps waveforms with a 100 kA peak are respectively (1) and (2) below: 

10/350 ,us: I(t) = [IJq] [exp(-t/zJ - exp(-t/zJ] (1) 

where I, = 100 kA 

7 = 0.9542 

z, = 480 

2, = 4 

8/20ps: I(t)=AI,fexp(-t/z) (2) 

where I, = 100 kA 

A = 0.01243 

z =3.911 

(In both equations, t and z's are in ps; I(@ is in same units as I,) 

F. Influence of Distribution Transformer Simplzj?cation 

The presence of distribution transformers has been 

included in many models in the literature, but their characteristics are not the same among authors. Some authors have used 

a coupled inductor with parasitic capacitor to represent the 

inter-winding capacitor in the transformer model [12]. 

While these models are more accurate in studying transformer failure modes due to low-side surges, for our main 

focus which is current dispersion among available paths, we 

have chosen the simple model postulated in [8] of a simple 

inductor to represent the winding. As results show, the 

presence of a transformer at the far end of a daisy-chain lowvoltage distribution system does not have considerable effect 

on the results. Therefore, we felt justified in adopting the same 

transformer model as described in [8] for all of our circuit 

configurations. 

G. Simplifying the Circuit for Modeling 

The circuit impedances have been modeled in EMTP using 

discrete components. The wiring between buildings and from 

building to transformer is modeled as a series inductance with 

the following parameters: R = 1 mQ/m and L = 1 pWm, typical 

values for aluminum conductors of 34 mm2 cross section 

(#2 AWG) [13]. The SPDs are modeled using the EMTP Type 

92 Nonlinear Element model. Because of the simplified nature 

of the model, we performed parametric variations on factors 

such as line impedance and transformer inductance, and found 

that their influence on current dispersion is not large enough to 

warrant concern on the somewhat arbitrary values we have 

postulated in the baseline wenario. 

IV. MODIELING RESULTS 

In this section, we present selected results of EMTP runs 

for each of three TN or TT system configurations with points 

of lightning 'termination next to the distribution transformer 

("first" case) or at the opposite end of the transformer ("last" 

case), for a total of seven scenarios. We postulated a separation 

of 100 m between buildings and 20 m from the transformer. 

For each scenario, a pair of figures is given. The first 

figure of each pair is a schematic showing the configuration and 

point of stroke, together vvith indications of the peak current 

values in the circuit branches. The second figure of each pair 

shows selected current waveforms, generally currents leaving 

the house by way of the earthing electrode and the service 

conductors. Note that the peaks can occur at different times 

so that the sum of peak brimch currents shown on the figures, 

Kirkhoff notwithstanding, is not always exactly zero. 

A. TN-Radial, strike on one of the buildings 

A distribution transformer supplies three buildings in a 

radial arrangement where all the service drops originate at the 

pole where the transformer is installed (Figure 3). This 

configuratior~ is a typical U.S. residential configuration. The 

lightning stroke is postullated to terminate on the earthing 

system of one of the three buildings. Figure 4 shows the 

current waveforms. 

"Peak: occurs very 

late In event - 

Figure 3 - Radial TN configuration with three buildings supplied by one 

distribution transformer, one building struck by a 101350 ps, 100 kA surge, 

showing peak values of currents shared among available paths. 
I Time (vs) I 

SPD - Current into each line of service drop, through SPDs 

GND - Current into local building earth electrode 

Nout - Current into neutral conductor of service drop 

Figure 4 - Waveforms of currents leaving Building 1, as defined in Figure 3, 

for a 100 kA, 101350 ps surge terminating on the building earthing system 

B. TN-Daisy clzain, strike on first building 

Another typical arrangement uses a distribution 

transformer which supplies several buildings along a street, 

with short service drops from the poles to each building. The 

lightning stroke is postulated to occur upon the first building, 

next to the transformer. Figure 5 shows the circuit 

configuration and the peak currents in the branches; Figure 6 

shows the waveforms of the currents leaving the building. 

Note the early peak of the current in the neutral -- directly 

connected to earth at the pole, thus a lower inductance 

compared with the inductance of the line conductors that 

include the transformer winding. 

Buildlng 1 Buildlng 2 Buildlng 3 

20 m loom - loom * 8k.A 65kA I 

Figure 5 - Daisy chain TN configuration with building next to transformer 

struck by a 101350 ps, 100 kA surge, showing peak values of currents 

C. TN-Daisy clzain, strike on last building 

This is the same configuration as B, but the building being 

struck is at the opposite end (Figure 7). The difference, if any, 

would give insight on the relative importance of modeling the 

presence of a specific transformer. In fact, the difference in the 

SPD stress for a strike on the first building (20 kA) compared 

with a strike on the last building (26 kA) is small, showing the 

small effect of transformer position. In the building earthing, 

where there are no SPDs, a strike on the last building produces 

42 kA compared with 23 kA for a strike on the first building. 

SPD -Current into each line of service drop, through SPDs 

GND - Current into local building earth electrode 

Nout - Current into neutral conductor toward the transformer earth 

Figure 6 -Waveforms of currents leaving Building 1, as defined in Figure 5, for 

a 100 kA, 101350 ps surge terminating& the building earthing system 

' Peak occurs early In event 

- 

Figure 7 - Daisy chain TN configuration with building at opposite end of 

transformer struck by a 101350 ps, 100 kA surge, showing peak currents 

SPD - Current into each line of service drop, through SPDs 

GND - Current into local building earth electrode 

Nout - Current into neutral conductor toward the transformer earth 

Figure 8 - Waveforms of currents leaving Building 3, as defined in Figure 7, 

for a 100 kA, 101350 ps surge terminating on the building earthing system 

For Figures 7-8, the greater distance (inductance) from the 

transformer earth electrode forces initially more current flow in 

the building earth than in Figures 5-6 for a closer transformer. 
D. TT Zwire, strike on first building 

A transformer (single-phase or one phase of a three-phase 

transformer) supplies several buildings along a street, with 

short service drops from the street poles to each building. The 

lightning stroke is postulated to occur upon the building next 

to the transformer (Figure 9). The waveforms of the currents 

leaving the building are shown in Figure 10. 

* Peak occurs early in the event I L 

Figure 9 - Daisy chain TT 2-wire configuration with building next to 

distribution transformer struck by a 101350 ps, 100 kA surge 

Time (ps) 

N-E SPD - Current through neutral-to-earth SPD 

L-E SPD - Current through line-to-earth SPD 

GND - Current into building earthing electrode 

Figure 10 - Waveforms of currents leaving Building 1, as defined in Figure 9, 

for a 100 kA, 101350 ps surge terminating on the building earthing system 

E. TT 2-wire, strike on last building 

The configuration is the same as in D, but the lightning 

stroke is postulated to strike the building at the opposite end of 

distribution line, away from the transformer (Figure 11). 

Figure 12 shows the waveforms of the currents leaving 

building 3. 

Where end-users are provided with three-phase service, a 

three-phase transformer supplies several buildings along a 

street, with short service drops from the street poles to each 

building. In this configuration, the difference from a 2-wire, 

single-phase service is that four conductors instead of two are 

available as exit paths for the lightning current postulated to 

have stmck the building of interest (first or last building). 

To conserve space, we do not present two pairs of figures 

for that configuration, but the summary of Table 1 includes the 

current values computed by EMTP for the two scenarios in that 

configuration. 

Bulldlng 1 Buiidlng 2 Building 3 ---+-----+< 20 rn 100 rn 100 m 

Q kA 28 W 

r--4 100 kA 

I I - "Peak occurs late In the event -t 

Figure 11 - Daisy chain TT 2-wire configuration with building at opposite end 

of distribution transformer struck by a 101350 ps, 100 kA surge 

N-E SPD -Current through neutral-to-earth SPD 

L-E SPD - Current through line-to-earth SPD 

GND - Current into building earthing electrode 

Figure 12 - Wavt:fonns of current; leaving Building 3, as defined in Figure 11, 

for a 100 kA, 101350 ps surge terminating on the building earthing system 

G. Compariwn of the seven scenarios 

Results of our model runs for the seven scenarios (Table 1) 

show that, contrary to some speculations or intuitive 

considerations on the sharing among service conductors, the 

earthing connection of the building does not carry anywhere 

near the 50% quoted in some proposed standards [9]. 

The most severe stress, for the parameters postulated, 

occcurs in the neutral SPD in Scenario D (TT 2-wire, first 

building struck) for whicZl the configuration has the lowest 

impedance to earth and thus invites the largest share. Other 

scenarios generally reflect ]primarily the number of service-drop 

wires available for the current exit. 
TABLE 1 

SUMMARY OF CURRENT SHARING AMONG CONDUCTORS FOR THREE CONFIGURATIONS IN SEVEN SCENARIOS FOR 100 kA STROKE 

Scenario: Most severe but rare - Building being directly struck Less severe but more frequent 

Configuration: Building See Currents leaving building via building earthing Currents impinging onto 

Distribution being figures and service conductors (peak kA or %) * adjacent buildings (peak kA or %) 

system struck -- 

-- Building Service SPD in SPD in SPD in SPD in 

earthing neutral the neutral the lines the neutral the lines 

TN Radial Any 3 -4 2 1 33 NIA 23x2 NIA 10 x 2 

TN Daisy First 5-6 23 27 NIA 20 x 2 NIA 7x2 

TN Daisy Last 7-8 42 26 NIA 26x2 NI A 8x2 

TT 2-wire First 9-10 26 N/A 5 1 28 x 1 10 lox 1 

TT 2-wire Last 11-12 48 NIA 3 8 38x 1 13 13x1 

TT 4-wire Fmt -- 22 NIA 32 16 x 3 5 5x3 

TT 4-wire Last -- 3 8 NI A 20 20 x 3 6 5x3 

* Peak values do not occur at the same time in the different paths so that totals of numbers shown may be more than the impmging 100 kA peak 

V. ENERGY CONSIDERATIONS 

In the model parameters, to start the iterative process, we 

have postulated that the SPD consists of a metal-oxide varistor 

(MOV) with relatively large cross-section that might be capable 

of absorbing the energy involved in diverting the 101350 ps 

surge. For the TN configurations, we selected a 150 V rms 

rating, and a 300 V rms rating for the TT configurations. For 

the cross section, we postulated an area equal to ten 20-mm 

discs in parallel because available manufacturer's data [14] 

readily gives the 20-mm disc characteristic. Such a combination 

would have a total one-shot joule rating of 800 joules for a 

1011000 ps surge in the 150 V rating. 

Because we suspect that even this array of ten discs might 

not be capable of dissipating the energy involved in a 101350 ps 

surge, the next step in this iterative process is then to compute 

the energy that would be deposited in the SPDs, under the 

current distribution patterns computed in the seven scenarios. 

As one example, Table 2 shows the energy deposited in the 

MOVs, computed for the case of the TN Radial configuration 

where one SPD is connected between each of the two lines and 

the earth point of the installation (Figure 3). Two waveforms 

are shown in the table, the 101350 ys and the 8/20 ps surges. 

TABLE 2 

ENERGY DEPOSITION IN SERVICE ENTRANCE MOVS 

FOR THE TN RADIAL CONFIGURATION AND TWO WAVEFORMS 

Rating for ten Energy deposition Waveform 

20-mm discs Direct strike Nearby strike 

101350 ps 800 J 3500 J 840 J 

8/20 !AS 800 J 200 J 805 

For the 101350 ps waveform, the rare scenario of a direct 

strike (energy deposited is 3500 J) would require a very large 

varistor at the service entrance -- four times the ten discs we 

postulated, while this ten-disc array would be sufficient in the 

less rare scenario of a nearby strike (840 J). 

On the other hand, if we were to stay with the 8120 ps as a 

postulated waveform, even the large 100 kA peak would be 

handled with comfortable margin by the ten-disc array. These 

results provide quantitative data which we will discuss further 

under the CostlRisk heading. 

VI. PARAMETRIC VARIATIONS 

We performed several parametric variations for the purpose 

of exploring the typical "what if' questions, and also to show 

whether or not our postulated values might be viewed as too 

arbitrary because of their influence on the results. 

A. Line impedance and building separation 

The value of 1 pHlm for conductors has long been used by 

many researchers as a typical value. To investigate the 

significance of that postulated 1 pH1m combined with the 100- 

m separation, we ran two cases, one with half the value and one 

with double the value. The first case corresponds to either half 

the separation for the same unit impedance or half the unit 

impedance with the same separation. For the second case, one 

of the parameters is doubled while the other is held constant. 

Table 3 shows a comparison of the baseline case with these two 

parametric variations. 

TABLE 3 

EFFECT OF LINE IMPEDANCE ON SHARING - TN DAISY LAST 

Baseline Half Double 

Percent of 100 kA peak 100 m, 1 uHlm baseline baseline 

Current into building earthing 42 32 53 

Current in service neutral 26 27 25 

Current in SPDs 26 26 25 

This comparison shows no significant differences in the 

current sharing for each of the three available conductors (there 

are two line conductors, each with an SPD) when the postulated 

unit impedance or building separation is varied over a 1:4 

range, so that our selection for these two parameters should not 

be a matter of concern. 

B. Transformer pole earthing resistance and building 

earthing system resistance 

By their relationship, these parameters can be expected to 

have an influence on the outcome. In the baseline case, we 

postulated a 5-Q pole earthing resistance and a 10-8 building 

earthing resistance. Table 4 shows the comparison of the 

baseline case with the reversed relationship between the pole 

earthing resistance and the building earthing resistance. 
TABLE 4 

EFFECT OF POLE EARTHINGBUILDING EARTHING - TN RADIAL 

Percent of 100 kA peak Baseline Reverse baseline 

5-Q pole, 10-Q bldg. 10-Q pole, 5-Q bldg. 

Current in building earth 2 1 3 1 - - 

Current ~n serv~ce earth 33 

Current in SPD 23 22 

Indeed, the relationship of pole versus building earthing 

resistance has a significant effect on the current carried by the 

neutral, but not on the current carried by the SPDs. This is 

particularly true, although not obvious in the table (where only 

the peak values are shown, reflecting the inductive effect on 

initial current dispersion), for the tail of the 101350 ps 

waveform where the subsequent sharing is determined by the 

resistance ratios [6], [12]. 

C. Length of circuit (more buildings along a street) 

Postulating a greater number of buildings along the daisy 

chain, while keeping the resistance of the building earthing 

constant, can be expected to offer a path of lesser impedance to 

the currents exiting the building, because of the greater number 

of available earth electrodes. Table 5 shows the effect of going 

from 3 buildings (baseline) to 9 buildings, still with the last 

building being struck. 

TABLE 5 

EFFECT OF NUMBER OF BUILDINGS IN TN CIRCUIT 

ON SHARING. DISTANT HOUSE STRUCK - 

Percent of 100 kA peak Baseline 3 buildings 9 bulldings 

Current in building earth 42 42 

Current in service earth 39 14 

Current in SPD 26 27 

Again in this case, a difference is noticeable in the neutral 

conductor current, but not in the SPD current. Thus, this parametric variation shows that the number of buildings between the 

building being struck and the distribution transformer, while 

affecting the neutral current, does not affect the stress imposed 

on the SPDs in this TN configuration. 

VII. DISCUSSION 

A. Effect of posiulaied waveform 

While we have adopted for our baseline the 101350 ps 

waveform, many SPD standards cite an 8/20 ys or a 4/10 ps 

surge waveform as an SPD capability requirement [15], [16] or 

as a surge environment description [17]. To explore the effect 

on sharing of the stroke current with different waveforms, in 

particular during the initial part of the 101350 ps surge where 

inductive effects dominate for the circuit parameters selected, 

we made one run with a 8120 ys surge instead of the 101350 ps 

used in the baseline case of the TN Radial. Predictably, given 

the small difference between a 10 ps and an 8 ys rise time, little 

effect was noted in the sharing during the first 20 ys. Of 

course, the energy involved for the total surge duration is 

another matter, already discussed in Section V. 

B. Selection of SPDs 

If the design objective is to provide protection for a direct 

stroke to the building (a topic which will be the subject of the 

costhisk analysis mentioned below), the SPDs must be selected 

with sufficient current-handling capability to survive the surges 

resulting from the postulated surge. 

Alternate proposals have been made to use a spark gap as 

service entrance SPD. Such a gap must then be capable of 

clearing the resulting follow current, which may be an issue for 

systems having a large available fault current, such as the 10 kA 

rms specified For U.S. installations [3], [la]. 

We have ]made one run with a spark gap model instead of 

a varistor model. From the sharing point of view, the difference 

is small, which can be readily explained by the fact that 

inserting in the lightning current paths a varistor with a limiting 

voltage of 1 kV or so, or a gap with an arc voltage of 100 V or 

so, should have a very small effect on the sharing because of 

the many kilovolts developed by the lightning current flowing 

in the inductances and resistances of the line conductors and 

earthing connections. 

An essential aspect of designing an effective surge 

protection system is to perform a costlrisk analysis involving 

the probability of a building being struck by a large surge, such 

as 100 kA, versus the cost of ensuring survival of the service 

entrance SPDs to be installed. This analysis introduces factors 

such as the flash density in the locale, the randomness of the 

distribution of the flashes over the area of attraction of the 

building which depends in part on the height of the building, 

and the distribution of peak amplitudes of lightning strokes. 

For instance, [l I] reports statistics [19] whereby an 80 kA 

amplitude is exceeded for less than 5% of the strokes. Such an 

analysis is beyond the scope of our paper, but it must be 

mentioned here to keep the situation in perspective and remind 

developers of SPD application guides or standards to include it 

in their recommendations. 

D. Applying-field experience lo standards 

The ultimate test of the usefulness of a standard is that 

equipment manufactured according to that standard has 

satisfactory fi~ald experience, while being produced at a cost that 

users are willing to accept. Very low field failure rates can be 

seen as overdiesign, high failure rates obviously as underdesign. 

It is the dream of one of the authors to establish a clearinghouse 

where field experience of manufacturers could be collected and 

applied to optimize the definition of the environmental stress 

[20]. Given the competitive nature of the industry, this is likely 

to remain only a dream. However, many U.S. utilities are now 

offering to their customers the installation of a meter-base 

adapter SPD. The field experience for these SPDs might be 

collected from utilities ..- with safeguards on proprietary 

information -- and become an input to the process of 

moderating slome proposals for high-stress requirements, on the 

basis of the successful field experience of SPDs with 

capabilities below those irnplied in proposed standards. 
VIII. CONCLUSlONS [4] Martzloff, F.D., Mansoor, A., Phipps, K.O., and Grady, W.M., "Surging 

the Upside-Down House: Measurements and Modeling Results," 

Modeling several typical TN and TT configurations of Proceedings, EPRI PQA'95 Conference, 1995. 

neutral earthing practices and scenarios of lightning strike point [5] Mansoor, A. and Martzloff, F.D., "Driving High Surge Currents into 

provides insights on significant effects, which should lead to Long Cables: More Begets Less," Paper 96 SM399-6 PWRD, IEEE PES 

more effective application of surge-protective devices (SPDs). Summer Meeting, Denver CO, 1996. 

161 Rakotomalala. A,. Auriol. Ph.. and Rousseau. A,. "Lirhtnine Distribution 

A direct lightning stroke to a building can produce high 

stresses on the service entrance SPDs as the earth-seeking 

current will exit in part by way of the utility service drop. 

SPDs in that building will be strongly affected, while nearby 

buildings will be impacted by much lower surge currents. 

The major difference among the scenarios we have modeled 

appears in the current carried away from the building by the 

neutral conductor. 

In a TN system where the neutral is bonded to earth at the 

service entrance, there is no SPD in that path, and thus no 

concern about neutral SPD integrity. In typical residential 

single-phase U.S. systems, the line SPDs can carry about 

25% of the stroke current. 

In a TT system where there is an SPD in the neutral path, 

a single-phase two-wire configuration can have 50% of 

the stroke current being carried by the neutral SPD. In a 

three-phase TT system where there are four conductors to 

carry away the stroke current, the neutral SPD can carry 

up to 30% of the stroke current. 

L * , , , , u-~- ~--

.-. 

Through ~arthini System." Symposium Record, IEEE International 

Symposium on EMC, Chicago IL, USA, August 1994, pp 419-423. 

[7] Lai, J.S. and Martzloff, F.D., "Coordinating Cascaded Surge Protection 

Devices: High-Low versus Low-High," IEEE Transactions IA-29 No.4, 

JulyIAugust 1993, pp 680-687. 

[8] Birkl, J., Hasse, P., and Zahlmann, P., "Systemgerechter Einsatz von 

Ableitern in Niederspannungsnetzen," Elektrotechnische Zeitschrift 

N0.17, 1994, pp 964-971 

[9] Flisowski, Z., and Mazzetti, C., "Efficiency of Lightning Protection 

System as Screening Measure Against LEMP," Proceedings, EMC'96 

ROMA, September 1996. 

[lo] Darveniza, M., Sargent, Limbourn, Liew, Caldwell, Currie, Holcombe, 

Stillman & Frowd, "Modelling for Lightning Performance Calculations" 

IEEE Transactions PAS-98, No.6, NovIDec 1979, pp 1900-1908. 

[l 11 Kirder, E.P. and Uman, M.A., "Cloud-to-Ground Lightning Mechanisms 

of Damage and Methods of Protection," Seminars in Neurology, Vol. 15, 

No.3, September 1995. 

[12] Dugan, R.C. and Smith, S.D., "Low-Voltage-Side Current-Surge 

Phenomena in a Single-Phase Distribution Transformer System," IEEE 

Transactions PWRD-13, NO. 2, April 1988. 

[13] Steigenvald, R.L., Ferraro, A,, and Tompkins, R.E, "Investigation of a Family of Power Conditioners Integrated into the Utility Grid," Sandia 

ReportSAND81-7031, 1981, page 20. 

1141 

. . Transient Volta,qe - Suppression Devices, Harris Corp., 1991. 

J. For line conductors, the difference reflects primarily the 11 51 ANSILEEE C62.11-199 1, IEEE Standard for Metal-Oxide Surge 

total number of conductors in the system, which can be two, 

three or four. The earth-seeking lightning current will divide 

(but not always equally) among these conductors. While the 

initial dispersion (during the first 20 ps) is controlled by the 

inductances, the later dispersion is controlled by the relative 

values of the earthing resistances. 

If the postulated stroke is as high as some of the proposed 

standards suggest, modeling the behavior of service entrance 

SPDs of the type installed in increasing numbers by U.S. 

utilities shows that some failures could be expected. As 

field experience seems to indicate an acceptable failure (if 

any) rate, one can question the need fo; imposing such 

severe requirements, unless the mission of the,facility is 

such that even a rare failure would be unacceptable. 

IX. ACKNOWLEDGMENTS 

Support and encouragement for this work was provided by 

the parent organization of each author. Additional support was 

received from Delmarva Power Company, from Pacific Gas & 

Electric Company, and from PECO Energy Company. Gerald 

FitzPatrick and Roger Witt contributed comments on the draft. 

X. REFERENCES 

[I] EPRI, "Electromagnetic Transient Program (EMTP), Version 2.0; Volume 

1: Main Program: Volume 2: Auxiliary Routines, EPRIReport EL-6421-L, 

July 1989. 

[2] IEC International Standard 364-1, Electrical Installations ofBuildings - 

Part I: Scope, Object and Fundamental Principles, 1992. 

[3] NFPA 70, Natzonal Electrical Code, 1996 

- - Arresters for AC Power circuits. 

- 

[16] ANSIIIEEE C62.34-1996, IEEE Standard for Performance of LowVoltage Surge-Protective Devices (Secondary Arresters). 

[17] ANSILEEE C62.4 1 - 1991, IEEE Recommended Practice on Surge 

Voltages in Low-Voltage AC Power Circuits. 

[18] UL Std. 1449, Transient Voltage Surge Suppressors, 1996. 

[19] Berger, K., Anderson, R.B., and Kroninger, H., "Parameters of Lightning 

Flashes," Electra, 1975-80, pp 1548-1565 

[20] Martzloff, F.D., "Keeping up with the Reality of Today's Surge 

Environment," Power Quality Solutions, September 1995 Proceedings, 

pp 243-249. 

Arshad Mansoor (M' 1995) is an 

Electrical Systems Engineer at the EPRI 

Power Electronics Applications Center 

(PEAC). He received his MS and Ph.D. in 

electrical engineering from the University of 

Texas, Austin in 1992 and 1994 

respectively. His areas of interest include 

Power Quality, power systems transients 

analysis, harmonics, surge propagation and 

protection, and EMTP model development. 

Franqois Martzloff (M' 1956, F' 1983) 

Born and educated in France, with 

additional MS degrees from Georgia Tech 

and Union College, worked at General 

Electric for 29 years and now 12 years at the 

National Institute of Standards and 

Technology. He is contributing to several 

committees for the development of standards 

on EMC, surge protection and Power 

Quality in the IEEE and the IEC. 
Discussion 

P. Hasse and J. Birkl (Dehn + Sohne, 92318 Neumarkt, 

Germany): The problem of lightning current distribution 

depending on the different kinds of current distribution 

systems and under the influence of possible variables has been 

very clearly represented with this contribution. 

The curve development gained by the EMTP program 

matches very well with the simulation calculations conducted 

in Germany with the PSPICE program. 

In respect to the described results, however, a few additions 

are necessary from our point of view: 

1. Section 111. G. and VI. A.: 

The induction of a conductor system always results out of 

the geometric system of the slip-knot consisting of coming 

and going conductor. A separation in coming conductor 

impedance and going conductor impedance is not realistic. 

In particular, in case of multiple conductor cables it is to be 

observed that in case of the same flow direction of the 

lightning current, the inductivity of the total system 

differentiates to a single conductor system. 

2. Section VI. Schedules 3 - 5: 

In particular, in case of longer connecting cables between 

buildings and between building and transformer a change 

in waveform of the surge flowing through these cables. 

Only the observance of the amplitude factor of the flowing 

lightning currents is not sufficient. In this situation, it 

would be more meaningful to consider also the energy 

distribution. 

Section VII. B.: 

For decades now, in Europe, spark gap arresters, with a 

mains follow current quenching capability, are being 

installed successfully as lightning current arresters at the 

building entrance. In particular, the high down-lead ability 

and impulse-time shortening of the rest impulse make a 

favourable co-ordination with connecting MOV's possible. 

Section VII. 4: 

The lightning protection necessity for a system, as well as 

the deduced lightning protection class resulting from this, 

is described in IEC 1662. At the same time the lightning 

protection class is determined, the layout of the lightning 

protection system necessary lightning parameters are 

defined (IEC 13 12-1). A deviating layout of protection 

measures on the basis of test currents 8/20 ps is therefore 

not permissible. 

Manuscript received November 3, 1997 

Frangois D. Martzloff (National Institute of Standards and 

Technology, Gaithersburg MD) : 

We thank the two authors of the discussion for taking the time 

to review our ;paper and provide comments aimed at broadening 

the consensus on the subject. In particular, we are delighted to 

hear that our computations based on EMTP matches very well 

with the simulation calculations conducted in Germany with the 

PSPICE program. With respect to their specific four comments, 

we offer the following responses, preceded by the general 

remark that the purpose of our computations was to reveal the 

differences among various postulates for the circuit configurations, as influenced by the grounding practices for the neutral 

in effect in different countries, rather than the precise values for 

a particular set of parameters. We emphasize the concept of 

postulate, lest we fall into the trap of taking electromagnetic 

environment standards as an exact duplication of reality, while 

they are in fact only the documentation of an industry consensus 

on how reality might be represented '. 

1. Section I1 G and IV A 

Indeed, the concept of inductance is based on a conductive 

loop that carries the current in a closed circuit. However, in the 

circuits we postulated for our computations, the conductors in 

question - phase and neutral - may be considered as one part 

of the closed circuit and might be called "coming," according to 

the tenninolcgy used by our colleagues, while the path consisting of the earth, the distant return to the cloud, the lightning 

channel, and [even the down-conductor (see Section I11 A) may 

be considered as the other part of the loop and might be called 

"going" conductors. 

For this reason, we represented in our figures the phase and 

neutral as if they were separate, while in reality they can be at 

some finite distance from each other (the so-called "open 

wiring" used jn overhead lines) or in close vicinity, as in the case 

of an underground cable or an overhead "triplex." Aware of the 

differences, we started our computations for a given, postulated 

configuration - always the same for the variations in the neutral 

grounding -. but performed a parametric variation in the line 

inductance (taking twice or half the value used in the baseline), 

as stated in the subject paragraph, to convince ourselves that the 

influence on dispersion is not large enough to cause concern. 

Space limitations for the paper prevented us from providing 

detailed numerical results - as they also do here - and we 

were hoping that our simple statement that we did consider the 

issue and found little effect on the differences among neutral 

grounding scenarios might be acceptable. 

2. Section VI 3 to 5 

One of the results of our computations based on a postulated 

101350 ps waveform was to show that, for the distances we 

selected, the impedance of the cable between buildings - and 

therefore their length - has only a small influence on the longterm current waveform and dispersion among conductors, which 

is primarily influenced by the postulated values for respective 

earthing resistances. With the values selected for inductances, 
the current dispersion is substantially affected by the respective 

inductances only for the first 20 or 30 ps. 

We agree that additional information might be conveyed by 

reporting the energy distribution along the complex path of the 

lightning current, but here again space limitations intervene. We 

can offer the response, however, that in view of the large values 

of the earthing resistance compared to the other resistances in 

the circuit - cable resistances and dynamic "resistances" of the 

varistor or gap SPDs - the latter are not a priority in reporting 

results. The EMTP model of course has the capability of 

reporting any set of parameters if "asked" to do so. 

For specific applications of one type or another of SPD 

technology, the EMTP model can provide detailed information 

on the energy that will be deposited in these SPDs for the 

various scenarios to be considered. 

Section VII B 

We are aware that in some countries, the installation of a 

service-entrance arrester is a common practice, and that gapped 

arresters may be used for that purpose. The issue is one of cost 

vs. benefits for an arrester designed for the large lightning 

currents associated with a rare direct strike to the building. We 

have observed, during our interactions with several international 

or IEEE technical committees, that consensus has not been 

reached on what current waveform and peak amplitudes should 

be considered when making the cost vs. benefits analysis. 

Depending on the nature of the installation, the cost vs. benefits 

equations are different. Several proposals for "risk analysis" are 

currently under consideration in several standards-developing 

bodies, and consensus is clearly not achieved at this point. This 

lingering question is addressed in our response to the fourth and 

last comment after the present one. 

Our intention in making the remark on available fault 

current in the second paragraph of this section was not to contest 

the successful European experience cited by our colleagues, but 

to alert our readers at large to the importance of considering that 

requirement. The point that mains follow-current quenching 

capability is not trivial was confirmed in a comment by one of 

the reviewers of our forthcoming paper, "Gapped Arresters 

Revisited" (scheduled for presentation at the IEEE-PES Winter 

1998 Meeting and later publication in IEEE Transactions). 

Section VII 4 

We are aware of the work conducted in the IEC Technical 

Committee 8 1, the responsible body for development of the IEC 

61662 and IEC 61312 publications. We are also aware of some 

discomfort among other parties concerning the stipulations from 

that body which might result in less than fully cost-effective 

solutions to the question of real necessity for protection against 

worst-case scenarios. The footnote offered in support of our 

introductory remark applies here also. There is a long and 

successful history of application of surge-protective devices 

based on a postulated 8/20 ps surge current waveform, using the 

appropriate values of amplitudes. For that reason, we included 

in our paper as alternate postulate the 8/20 ps waveform. From 

the point of view of EC TC 81, their recommendations might be 

considered normative and thus non-negotiable, but protection 

measures in the various countries are typically determined - if 

at all - by bodies that promulgate codes based on a consensus 

drawn from experience based not exclusively on TC 81 recommendations. Therefore, the use of the term "not permissible" 

appears somewhat strong in the context of voluntary or even 

regulatory practices. 

In conclusion, we appreciate the opportunity to present more 

detailed background information on our computations and 

underlying postulates, thanks to the discussion contributed by 

our colleagues. 

1. Long ago, my mentor, Frank Fisher, taught me this concept 

which I recite in the following terms, well worth repeating in the 

present context: "The criterion of validity of an environment 

standard is not so much how closely it duplicates reality but 

rather how well equipment designed in accordance with this 

standard pe~orm in the field. If equipment designed in 

accordance with the standard pedorm well in the field, while 

equipment ignoring the standard do not perform well, the 

chances that the standard be a good standard are pretty good." 

Manuscript received January 7, 1998.