Abstract — The primary objective of a conventional

lightning protection system is to maximize the electric field

above the structure on which it is installed when it is subject to

the electrostatic meteorological environment of a

thunderstorm. This results in the promotion of successful

streamer to leader propagation which ultimately improves the

likelihood of the controlled earth termination of a cloud to

ground (CG) lightning strike. A conventional lightning

protection system is designed to prevent damage to a building

by providing a number of preferential strike receptors (air

terminals) with low impedance paths to conduct the lightning

discharge safely to ground. In this study, via the utilization of

Finite Element Analysis (FEA) software and solid modelling

Computer Aided Design software, several air terminal

topologies comprising conventional and non-conventional

geometrical surface characteristics were modelled in detail and

then subject to an electrostatic simulation environment

(thunderstorm replica) for analysis of their respective electric

field distributions. Results indicate discrepancies of up to

100KV in electric field distribution between that of the

conventional and non-conventional air terminal topologies.

This suggests that significant research opportunities lie within

optimizing the geometrical design of a conventional air

terminal with respect to its corona emission capabilities and

could ultimately lead to a review of LPS protocols embodied in

many national and international Lightning Protection

Standards.

Keywords – Lightning, Lightning Protection System, LPS, Air

Terminal Design, Surface Characteristics, Electric Field

Distribution, finite element modelling.

I. INTRODUCTION

HAT happens on the earth’s surface during a

thunderstorm is extremely relative to what is

happening in the clouds above. The negative charge in a

thundercloud induces a positive charge on the earth’s surface

beneath it. The natural space charge produced by pointed

corona emitting objects such as trees, power lines, antennas

buildings and the like, limits the electric field at ground level

to typically around 10kV/m [3]. In the presence of a

thunderstorm, and on closer approach of the negatively

charged descending stepped leader, the fields at the sharp

conducting points of exposed objects are intensified to such

Manuscript received October 14, 2015; Revised October 14, 2015. Paper

no. RMIT-3332395-2015. James T. Fenwick is an undergraduate student with the school of

Electrical and Computer Engineering, Royal Melbourne Institute of

Technology, Melbourne, Victoria, Australia; (e-mail: s3332395@student.rmit.edu.au)

an extent that they can reach the voltage breakdown level of

air. [3][6][7][8] When this happens, the pointed object emit

corona current which induces a positive spacial charge

above it that is strong enough to produce a positive electrical

discharge (streamer) that propagates towards the descending

leader. If the leader is close enough to ground such that the

electric field between them is strong enough to sustain a

positive streamer, successful streamer to leader propagation

occurs and the provision of a successful lightning discharge

path is established. [2]

The purpose in which a conventional lightning protection

system is intended to operate, is via the meticulous

positioning of lightning strike receptors (air terminals) with

low impedance paths (down conductors) to conduct the

lightning discharge safely to ground (earth termination

network). Preliminary insight into the relation of lightning to

grounded objects was first explored by Benjamin Franklin in

the 18th

century. Franklin later went on to conceptualize the

architecture of what is now considered to be a conventional

lightning protection system by identifying three key

elements; metallic rods (Air Terminals), horizontal roof

conductor networks, and vertical down conductors

connected to earth termination networks. The first ever air

terminals used by Franklin were thin, sharp tipped needles

mounted on to the top of iron rods, since then various

geometrical topologies have been investigated in an effort to

improve their lightning interception capabilities.[11]

Generally speaking, air terminals are considered to offer a

zone of protection to a structure relative to their electric field

distribution capabilities and other risk factors according to

many national and international lightning protection

standards. The basic principles of LPS design have been

embodied in such standards as the Australian and New

Zealand Lightning Protection Standard. This standard, like

many others, does not endorse or imply the endorsement of

non-conventional LPSs that claim enhanced performance. [1]

Irrespective of limitations imposed by such standards, a vast

array of literature exists investigating the claimed enhanced

performance of non-conventional LPSs but on a consistent

basis there is an absence of relevant conclusive results and

this suggests that they do not function as they claim.[2][3][4][5]

In this study, several air terminals comprising

conventional and non-conventional surface characteristics

are modelled in detail using Solid Works Computer Aided

Optimization of Conventional Air Terminal Design for Lightning

Protection Systems with Respect to Geometrical Surface

Characteristics

James T. Fenwick, Student, RMIT

W

Design (CAD) software. To allow for electrostatic analysis

of these designs, a 3-dimensional electrostatic simulation

model, replicating the meteorological conditions of a

thunderstorm, was built using ANSYS Maxwell Finite

Element Analysis (FEA) software. Each air terminal

topology was subject to the same simulation conditions for

comparative analysis of their respective electric field

distribution. Results outline the impact that geometry has on

an air terminal’s electrostatic performance capabilities and

provides preliminary insight into the optimization of

conventional air terminal design.

II. ELECTROSTATIC ENVIRONMENTAL PROPERTIES OF A

THUNDERSTORM

The cloud most commonly associated with a thunderstorm

is called the cumulonimbus cloud. This cloud structure is

typically characterized by a towering and dense, vertical

cloud in which strong rising air currents called updrafts are

present. The presence of these updrafts are believed to

instigate charge distribution within the cloud, smaller

particles rise to the top of the cloud via the updrafts and

acquire a positive charge whereas the larger particles

accumulate at the bottom of the cloud acquiring a negative

charge. The concentration of positive charges in the top of

the cloud and that of the negative charges in the bottom of

the cloud creates a significantly large potential difference

not only between different regions of the cloud, but also

between the bottom of the cloud and the earth. When this

potential difference becomes significant enough, the

phenomenon more formally known as lightning takes place

and bridges the two regions of charge via an ionized channel

that works to neutralize the intervening field.

A common method used to model the charge distribution

of a cumulonimbus cloud is the Simpson Cumulonimbus

Model. [10] In this model cumulonimbus cloud is broken into

three spheres of uniform distributing charges. The cloud

model below consists of three point charges suspended in air

at different heights from the ground. In the simulation, the

ground has been modelled with a boundary condition of odd

symmetry (Flux Normal) where E is normal to the boundary;

its normal components are 0. The surrounding walls of the

simulation region have been assigned a balloon boundary

condition, where the charge at the"infinity" balances the

charge in the drawing region. The net charge is 0. The three

charges of 3C, -40C and +40C are placed at heights of 2, 7

and 12km from ground level respectively. The charges are

modelled as spheres of radii 900m for the positive and

negative 40C charges, and radii 150m for the 3C charge. The

size of the spheres is picked in such a way that when the

meshing process takes place, the electric fields in the area of

interest within the model are accurate. The initial mesh

settings for this simulation are 1 meter. [9] On closer

inspection of the electrostatic simulation depicted in Fig. 1

we can see that the ground electric field rapidly runs to 0 as

the radius increases laterally from the vertical centreline. [10]

Fig. 1. Electrostatic simulation of Simpson Cumulonimbus Model [10]

III. LPS AIR TERMINAL MODELLING

Three air terminals were modelled in Solid Works CAD.

Each air terminal was designed at a height of 1 meter, a

radius of 9.5mm and assigned aluminum (Grade 1050)

material properties (as specified by [1]) with a bulk

conductivity of 38 Mega Siemens/m and a relative

permittivity of 1. One conventional air terminal was

modelled as the reference air terminal and is depicted in the

far left of Fig 2. This model featured conventional smooth

surface characteristics and ultimately generated the reference

data used to analyze its non-conventional counterparts. Two

non-conventional air terminals were modelled, both

comprising surface characteristic alterations. The air

terminal comprising surface characteristic #1 depicted in the

middle of Fig. 2 demonstrated a “knurled” rough surface in

which diamond like cut-outs and extrusions texturized the

terminal from top to bottom. The air terminal comprising

surface characteristic #2 depicted at the far right of Fig. 2

demonstrated a “pyramid swept” surface in which pyramid

like cut-outs and extrusions texturized the terminal from top

to bottom with a “zig-zag” like shape.

Fig. 2. Air Terminal Solid Works Models, (LEFT) Conventional

Reference Air Terminal, (MIDDLE) Non-Conventional Air Terminal

Surface Characteristic #1, (RIGHT) Non-Conventional Air Terminal

Surface Characteristic #2

IV. 3 DIMENSIONAL AIR TERMINAL ELECTROSTATIC

SIMULATION MODEL

The 3 dimensional electrostatic simulation model used for

the analysis of each air terminal topology consisted of a

2000mm x 2000mm x 200mm rectangular prism suspended

directly above the air terminal at a height of 2000mm,

therefore allowing a gap of 1000mm between the tip of the

air terminal and the under croft of the rectangular prism.

In order to mimic the excitation values of a thunderstorm

and its associated charge distribution, the rectangular prism

was assigned an excitation value of -10MV to emulate the

potential difference associated with the tip of the negative

downward leader and ground as stated in [10]. The air

terminals were then assigned an excitation value of +10KV

which is believed to be the electric field potential at ground

level under the presence of a thundercloud. Once all

excitations had successfully been assigned, a virtual force

was assigned to the rectangular prism and a region was

created in which the simulations would populate with

results. The floor of the region was assigned a boundary

condition of odd symmetry (normal flux). Finally a polyline

was drawn on the simulation as a non-model object. The

poly line was drawn horizontally 500mm directly above the

tip of the air terminal spanning 1.5 meters each way, this line

was ultimately used as a reference for the rectangular plots

that were utilized in the analysis of each air terminal’s

electric field distribution. Initial mesh settings for each

simulation were set at 0.001mm so to allow for recognition

of surface characteristic alterations.

V. PRELIMINARY INDIVIDUAL SIMULATION RESULTS

The following simulations depicted in Figures 3, 4, and 5

embody the preliminary stages of the electrostatic analysis

performed on each respective air terminal topology.

Fig. 3. 3 Dimensional Conventional Air Terminal Electrostatic Simulation

Figure 3 depicts the electric field distribution of a

conventional air terminal when a field overlay displaying

electric field magnitude is applied. From these results, a

rectangular plot was generated based on the values generated

at each point along the polyline, and the data was exported

and tabulated in excel.

Fig. 4. 3 Dimensional Non-Conventional Air Terminal (Surface Characteristic #1) Electrostatic Simulation

Figure 4 depicts the electric field distribution of the non-

conventional air terminal comprising surface characteristics

#1 when a field overlay displaying electric field magnitude

is applied. From these results, a rectangular plot was

generated based on the values generated at each point along

the polyline, and the data was exported and tabulated in

excel.

Fig. 5. 3 Dimensional Non-Conventional Air Terminal (Surface

Characteristic #2) Electrostatic Simulation

Figure 5 depicts the electric field distribution of the non-

conventional air terminal comprising surface characteristics

#2 when a field overlay displaying electric field magnitude

is applied. From these results, a rectangular plot was

generated based on the values generated at each point along

the polyline, and the data was exported and tabulated in

excel.

Upon finer inspection of the preliminary non-conventional

simulations, it can be seen that the Electric Field at the

lateral most parts of the Poly Line reference line is of a

higher magnitude than that of the conventional. When

tabulated (Table 1), this discrepancy is quantified and it can

be seen that although electric fields are of a higher

magnitudes at the most lateral parts of the distribution,

magnitudes are significantly smaller directly above the non-

conventional air terminals when compared with that of the

conventional.

Table 1. Poly Line Electric Field Data Extracted From Preliminary

Electrostatic Simulations

VI. SECONDARY SIMULATION RESULTS

To further explore the preliminary electrostatic simulation

results of Part V., a secondary set of simulations were run to

analyze whether or not the conventional and non-

conventional air terminals would complement each other if

laid out in an array like manor. For this simulation, each air

terminal topology was laid out in the same configuration and

put under the same analysis for electric field distribution

capabilities. A simulation was also run utilizing both air

terminal topologies and results were analyzed.

The layout of air terminals for this configuration consisted

of a star like formation utilizing a total of five air terminals.

The electrostatic simulation environment they were subject

to was equivalent to that of Part V. Air terminals were offset

600mm in +x, -x, +z, and –z directions from the reference

air terminal located at 0,0,0,0 (+x, -x, +z, -z) at ground level.

Each air terminal was assigned a positive voltage of 10KV

similar to that of the preliminary simulation environment

and all results were exported to a single rectangular plot

which is analyzed in Part VII. of this report. For these

simulations the non-conventional air terminal model

comprising surface characteristics #1 was omitted from

simulations due to meshing problems.

Fig. 6. 3 Dimensional Conventional Air Terminal Electrostatic

Simulation (Secondary Array)

Figure 6 depicts the electric field distribution of the

conventional air terminal laid out in an array like manor

when a field overlay displaying electric field magnitude is

applied. From these results, a rectangular plot was generated

based on the values generated at each point along the

polyline, the data was exported and tabulated in excel.

Fig. 7. 3 Dimensional Non-Conventional (Surface Characteristics #2)

Air Terminal Electrostatic Simulation (Secondary Array)

Figure 7 depicts the electric field distribution of the

conventional air terminal laid out in an array like manor

when a field overlay displaying electric field magnitude is

applied. From these results, a rectangular plot was generated

based on the values generated at each point along the

polyline, the data was exported and tabulated in excel.

In addition to the simulations conducted in Figures 6 and

7, a third simulation was run that utilized both conventional

and non-conventional (Surface Characteristic #2) air

terminal topologies. To achieve the best possible layout for

this simulation, the conventional air terminal was used as the

reference air terminal located at 0,0,0,0 (+x, -x, +z, -z), the

non-conventional air terminals were offset in the same

fashion as the previous simulations and the results were also

included in the rectangular field plot in Part VII. for

analysis.

0.00 0.50 1.00 1.50 2.00 2.50 3.00Distance [meter]

4.00E+006

4.50E+006

5.00E+006

5.50E+006

6.00E+006

6.50E+006

7.00E+006

Y1

[V

_p

er_

me

ter]

Maxwell3DDesign1Electric Field Distribution Plot ANSOFT

Curve Info

MixedImported

ConventionalSetup1 : LastAdaptive

Non-Conventional SC2Imported

Fig. 8. 3 Dimensional Non-Conventional & Conventional(Surface

Characteristics #2) Air Terminal Electrostatic Simulation (Secondary

Array)

Figure 8 depicts the electric field distribution of the mixed

conventional/non-conventional air terminal layout when a

field overlay displaying electric field magnitude is applied.

From these results, a rectangular plot was generated based

on the values generated at each point along the polyline, the

data was exported and tabulated in excel.

VII. ANALYSIS OF RESULTS

Upon the successful completion of all secondary

simulations the electric field distribution data for each layout

was plotted on one rectangular plot and is depicted below in

Graph. 1.

Graph 1. Electric Field Distribution Rectangular Plot

When analyzing discrepancies between electric field

distributions it is evident when looking at the plot that the

conventional system is still the most effective system for

overhead electric field maximization. The air terminals

comprising surface characteristic alterations have little

beneficial effect on electric field distribution other than

producing higher electric field magnitude at the most lateral

parts of the field distribution, this includes when they are

laid in conjunction with conventional air terminals. These

results have remained consistent throughout both

preliminary and secondary simulations. For a more

quantifiable analysis of the secondary simulations, data was

exported from the distribution plot and tabulated in excel

where discrepancies between electric field magnitude were

noted. This data can be seen below in Table. 2.

Table 2. Poly Line Electric Field Data Extracted From Secondary Electrostatic Simulations

It can be deduced from Table 2 that utilizing the non-

conventional air terminals increases the electric field

magnitude at the most lateral parts of the electric field

distribution. This was expected as these results were

consistent in preliminary simulations. When utilizing both;

conventional and non-conventional systems in the same

array, whilst discrepancies between the electric field

associated with a fully conventional system are minimized;

there is no significant advantage of arranging a system in

this way in the context of this simulation environment.

To fully utilize the advantages of lateral electric field

maximization provided by non-conventional air terminals

comprising geometrical surface characteristics we must

compensate for the deficiency in overhead electric field

magnitudes when compared with fully conventional systems.

VIII. CONCLUSIONS

Determining the electric field distribution capabilities of

air terminals utilized in lightning protection systems is

critical in identifying the zone of protection in which the

system provides a structure against the damaging effects of

lightning. In this paper, via the utilization of SolidWorks

CAD software and ANSYS Maxwell FEA software, several

conventional and non-conventional air terminals were

modelled and subject to an electrostatic simulation

environment for analysis of their electric field distribution

capabilities when in the presence of a thunderstorm.

It can be deduced from both preliminary and secondary

results, that when altering the geometrical surface

characteristics of grounded structures in a thunderstorm

environment, their electric field distribution capabilities

change considerably. In the context of this analysis, although

electric field magnitudes were maximized at the most lateral

parts of electric field distribution for air terminals

comprising geometrical surface characteristic alterations,

this comes at a somewhat proportional depreciation in

overhead electric field intensity. Ultimately, to gain full

advantage of lateral electric field maximization provided by

such non-conventional air terminals we must compensate for

the co-ordinate reduction in overhead electric field intensity.

This paper has analyzed the affects that minor alterations

made to a conventional air terminal’s geometry can have on

its respective electric field distribution capabilities. Both

industry standard conventional, and non-conventional air

terminals have been analyzed, and the effects of air terminal

surface geometry have been identified as worthy of

consideration when designing a lightning protection system.

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