Lightning Air Terminals - Is Shape Important

7/23/2019 Lightning Air Terminals - Is Shape Important

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Lightning Air Terminals s Shape Important?

William Rison Senior Member, IEEE),

Charles

B.

Moore,

and Graydon

D.

ulich

Langmuir Laboratory for Atmospheric Research

New Mexico Institute of Mining and Technology

Socorro, New Mexico USA

[email protected]

Abstrad-Benjamin Frank lin originally proposed th e use of

sharp pointed lightning rods

as

a way to prevent lightning strikes.

It

was

soon

found that such rods did not prevent lightning

strikes, hut that they worked to prevent damage to a strncture

when it

as

struck by lightning. Conventional lightning protection

systems evolved from this finding. Conventional lightning

protection systems consist of air terminals (lightning rods) used

to intercept a lightning discharge, downcondnctors used to carr y

the current, and a grounding system used to dissipate the curre nt

away from the protected structure. However, it has long been

recognized by the scientific community that lightning protection

systems do not prevent lightning, and that the sharp points

on

lightning rods traditionally used in North America are not

needed for that purpose. To be effective air term inal s should be

designed so tbat they are much more likely to be struck by

lightning than objects

on

the structure they are protecting.

Recent field studies indicate that

a

lightning rod with a blunt tip

is

more effective than a lightning rod with a sh arp tip.

Two nonsonventiona l lightning protection systems are heavily

marketed in North America Early Streamer Emission @SE) ai r

terminals and Charge Transfe r Systems (CTS). Proponents of

ESE air terminals claim that such devices have a much larger

zone of protection than do conventional air terminals. Proponents

of

CTSs claim tbat corona current emilted from their arrays

of

sharp points can prevent lightning strikes to protected

structures.

Field studies of ESE a ir terminals show that their performance is

similar to that of conventional sharp-pointed air terminals, and

that they do

not

have the greatly enhanced zone of protection

claimed for them. Field studies of Charge Transfer Systems show

that they do not prevent lightning strikes

Kqwords-air lerminals, Charge Transfer Systems, Early

Streamer Emission air terminals, lightning,

ligltlning prorec l ion,

lightning rods

I. INTRODUCTlON

Conventional lightning protection systems (LPSs) have

developed from the pioneering work of Benjamin Franklin in

the late

1700s

[I]. Franklin was the first to conclusively

demonstrate that lightning is a n electrical discharge, similar in

phenomenology (though not magnitude)

to

the small sparks

Franklin generated in his laboratory experiments. Franklin

found that if be approached a charged object with a grounded

blunt conductor, a spark would develop between the two.

However, if be approached the cha rged object with

a

grounded

sharp conductor, a spark did not develop; rather, the charged

object was silently discharged. This led Franklin to the

hypothesis that it might be possible to p revent lightning strikes

to a structure by erecting sharp grounded conductors on the

structure the sharp conductors might be able to silently

discharge the thundercloud, preventing a lightning discharge:

houses, churches and ships [should be provided] on

the highest parts o f those edifices, upright rods of iron

made sharp as a needle, and gilt

to

prevent rusting, and

from the foot of those rods

a

wire down the outside o f the

building into the ground,

or

down round one of the

shrouds of a ship, and down her side till it reaches the

water.

Franklin and others installed lightning rods on structures

to try

to protect them by preventing lightning. While there is

no evidence th t lightning strikes were prevented, it

soon

became evident that, when lightning did strike a structure

equipped with lightning rods, the lightning would attach

to

one

of the elevated rods, the lightning current was carried to ground

through the down conductors, and the structure was protected.

This led Franklin to modify his claim for lightning

rods

to

include the ability to provide a safe path

to

ground for the

lightning current:

I have mentioned in several of my

letters,

and except

once, always in the olternutive viz, that pointed rods

erected on buildings, and communicating with the moist

earth, would either prevent a stroke, or, if not prevented,

could conduct it,

so as

hat the building should suffer no

damage.

Since Franklins invention of the lightning rod it has been

convincingly demonstrated that conventional lightning

protection systems are extremely effective in preventing

damage

to

protected structures from direct lightning strikes [2,

31.

During the 1800s most improvements in conventional

LPSs came about through trial and error analysesof system

failures

led

to improvements

in

the placement of air terminals

and down conductors, and in grounding system installation.

With the development of modem inshumentation over the past

eighty years, our scientific understanding of lightning has

greatly improved, which has led to a better understanding of

and improvements

to

LPSs.

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Some aspects of LPSs are historical canyovers from

Franklins original work, and do not necessarily follow from

current scientific understanding of lightning. One

of

these is

the use in North America of traditional sharp pointed lightning

rods.

In 1878 several British.societies organized a conference on

lightning protection. In

1882,

they issued their report,

Report

of the Lighining Rod Conference, which laid out a code of

rules

for

those who installed lightning protection systems in

Britain [4]. This document (and its American successor, the

National Fire Protection Associations

Specijcations for

Proiecfion of Buildings

against

Lighfning

[ 5 ] , contained the

following statement:

A lightning conductor fulfills two functions; it

facilitates the progress of the electricity to the earth,

carrying it

off

harmlessly, and tends to prevent

disruptive discharge by neutralizing the conditions

which d etermine such discharge in the vicinity of the

conductor. _.. he second object is accomplished by

the conductor being surmounted by a point or

points.

Over the next few decades, however, it

was

realized that

lightning rods could not neutralize the conditions which lead to

lightning strikes. In 1933, M. G Lloyd

of

he National Bureau

of Standards w o te [6]:

Most of those who have given considerable study to

this problem [lightning protection] recognize that the

discharges from the points of lightning rods have little,

if any, value in preventing a stroke of lightning, and that

it is not important that the points should be sharp.

While it is now well known that lightning rods neither

eliminale nor reduce the probability of a lightning strike, the

authors have long been interested in the question of whether

the shape of

a

lightning rod might influence its effectiveness.

This paper discusses that question.

11.

THE

LIGHTNING TTACHME NT

ROCESS

In order to discuss the effectiveness of lightning air

terminals, it is necessq to have a hasic understanding of the

phenomenology of the lightning attachment process. More

detailed discussion can be found in standard references on

lightning (e.g., [7]). Physical processes in a thundercloud

separate electrical charge inside the cloud. In a typical

thundercloud, there is a mid-level negative charge at ahout

6

km

altitude and an upper positive charge at about

IO

km

altitude. (Thunderstorm charging

is

a complicated process,

depending on many environmental conditions, and many

storms have charge structures different than the typical

thunderstorm I describe here.) The negative charge in the mid

level of the thundercloud induces a positive charge on the

ground beneath it. The electric fields on the ground under a

thunderstorm are typically 5 to 20 kV/m. The fields at the

ground are intensified at the extremities of an exposed object

to such an extent that the fields at the ex tremities can reach the

value needed to break down air

(3

MV/m at sea level). When

this happens, the object emits corona current, which produces

a positive space charge above it. The corona current continues

to flow until the space charge reduces the field at the

extremities

of

the object to helow the air breakdown threshold.

All exposed pointed objects emit corona current ree leaves,

grass blades, antennas, power lines, etc. The space charge

produced by objects on the ground limits the fields at the

ground to the

5

to 20 kV/m value mentioned above. Without

this space charge, the fields at the ground under a

thunderstorm would often exceed 100kV/m.

As the charge separation continues in the thundercloud,

electric fields intensify inside the cloud. When the fields

become strong enough an electrical breakdown (lightning)

occurs, which discharges the thundercloud and reduces its

electric field. The majority of lightning is intracloud

discharges between the mid-level negative charge and the

upper positive charge. A significant fraction of lightning is

cloud-to-ground

(CG)

between the mid-level negative

charge in the thundercloud and the induced positive charge on

the ground below. (There are also positive cloud-to-ground

discharges, not discussed in this paper, between a positive

charge region in a thundercloud and an induced negative

charge on the ground.)

A negative CG discharge begins in the lower part of the

mid-level negative charge region of the thundercloud. The

breakdown propagates downward in a process known

as

a

stepped leader. It develops in steps

of

about

50 m

in length,

with an interval of about 50

ps

between the steps, and an

average velocity of about 1.5 x

IOs

d s . This gives lightning

its characteristic jagged app earance. The stepped leader

carries negative charge towards the ground. A charge

of

about

1

mC/m

is deposited on the leader channel, and a current of

about 100 A flows as he channel develops.

As the leader nears the ground, the electric fields on the

ground intensify to such a level that the field near the tips of

objects becomes strong enough to produce positive sparks

(called streamers,

or

counter-leaders) which race upward

towards the descending negative leader. If the stepped leader

is too

iir

away from the ground, the electric fields between the

leader and the streamer are not large enough to sustain the

propagation of the streamer, and the streamer dies out.

Eventually the leader gets close enough to the ground so that

the intervening fields are strong enough to sustain a streamer,

and a streamer will propagate to attach to the descending

leader. When the streamer and leader meet, a conductive path

to ground is established, and the charge on the leader channel

flows to ground. This is the high current return stroke of a

lightning discharge. Typical return stroke currents are 5

kA

and currents can he as large as 200 kA.

The object which emits the streamer which wins the race

the one which reaches the stepped leader fust s the object

which gets struck by the lightning discharge. The distance

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from the grounded object to the tip o f the descending leader at

the time the successful streamer is initially emitted from the

object is called the striking distance. The striking distance for

a typical lightning strike is about

100m.

111.

EFFECTIVE

LIGHTNING RODS

An

effective lightning rod is one w hich is much m ore likely

to generate a successful propagating streamer than is a part of

the structure it is protecting. It has long been known that

tall,

exposed objects are more likely to get struck

th n

are shorter,

shielded objects. There are two reasons for this:

1) If a taller object and

a

shorter object generate streamers

at the same time, the streamer from the taller object will be

more likely to attach to the approaching leader because that

streamer has

a

shorter distance

to

travel. This is the basis of the

electrogeometric method for the placement of

air

terminals in

an LPS [PI If one assumes a 50 meter striking distance, then

air

terminals should be placed such that a streamer

om

an air

terminal will have a shorter distance to travel to meet an

approaching leader than will a streamer from any point on the

protected structure, regardless of the direction from which the

leader is approaching. A practical way to place lightning rods

using this method is the Rolling Ball method [9]

2)

A tall, exposed object will provide electrical shielding

for

a nearby lower object. The e lectric fields at the shielded

object will be lower than they would be if the taller object were

not present. Consequ ently the shielded object

will

generate a

streamer later than it would have if the taller object had not

been there. This is why an LPS made of a grid of overhead

wires

is so

effective.

The electrogeometric method is based on the assumption

that all objects are equally likely to generate streamers under

the same conditions. However, if an object of one shape were

more effective at generating a successful streamer th n

an

object of a different shape, then the electrogeometric method

will not hold

for

all placem ents of air terminals an object on

the protected structure with a more optimal shape for

generating streamers might produce a streamer earlier th n a

less optimally shaped lightning rod closer to the approaching

leader. Moore

[lo]

suggested that lightning rods with blunt

tips might be more effective at generating successful streamers

than rods with sharp tips. We recen tly conducted a twelve year

field study to

try to

determine if the shape

of

an air terminal

affects its ability to succ essh lly generate a streamer. Results

of the field studies are discussed in [I I] and are summarized

below.

A. Competition benveen Air Terminals of Different Shapes

In

order to determine if the shape of an air terminal affects

its ability to generate a successful streamer, we arranged

a

compe tition between air terminals of different shapes. We

conducted the experiment in an area with a high lightning strike

den sity on a ridge near the 3288-111 high sum mit of South

Baldy Peak in the Magdalena Mountains of central New

Mexic o. We used a number of setups, each of which had

two

rods of different shapes. The two rods were mounted on

6 meter tall poles, separated by a distance of 6 meters. The

Figure 1. Blunt-tippcdlightning ods which were struck by lightning

6meter separation insured that the Perturbation in the field

strengths at one of the tips produced by an adjacent rod was

about

1

of the ambient field strength. Each rod

was

equipped

with a fuse

or

a lightning counter

to

determine if it were struck

by lightning. The rods were

made

of aluminum, a

metal

which

is much more likely to melt at the attachment point, and hence

show physical evidence when struck by lightning than

materials such as nickel-plated steel

or

copper

are.

A typical

setup bad

a

traditional sharp-pointed air terminal and a blunt

air

terminal in compe tition. The diam eters of the blunt rods varied

from 9.5 mm to

51 nun.

Over the twelve years of the

study

none of the sharp rods

were struck by lightning, whereas 13 of the blunt rods were. A

photograph

of

six blunt rods which were struck is shown in

Figure 1. None of the blunt rods with diameters of

9.5

nun or

51

mm were struck. Most of the strikes have been to 19

mm

diameter rods. These results indicate

that a

moderately blunt

tipped object is more likely to generate a successful streamer

than is a

sharp

tipped object

or

an object with an extremely

blunt tip. Reasons for this are discussed in detail in [12,13].

B Streamers fr om Lighfning Rods

In

an

attempt to learn more about the generation of

streamers from lightning rods, we measured the streamer

currents generated from several rods of different shapes in

response

to

approaching leaders. We used

three

air terminals

installed on

6

m tall masts separated by 6 m, in the

form

on an

equilateral triangle. We measured the currents from the masts

using 12-bit 5 MHz digitizers. We also measured the electric

field change induced by an approaching stepped leader.

Figure 2shows the results for on e lightning strike. The black

trace in Figure

2

shows the increase in the electric field

as

a

stepped leader approaches. Note that the ambie nt electric field

increased from about 30 kV/m to about

60

kV/m. While the

electric field tends to increase with time, there are instances

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where the field decreases for a short period of time. These

decreases are due to the shielding effect of space charge

produced by objects in the vicinity. As a negatively charged

leader approaches, the fields at the tips of nearby objects

become strong enough to induce the objects to emit positive

corona current. That positive charge provided some shielding

from the approaching negative charge, and temp orarily reduced

the field at the ground.

The colored traces in Figure 2 show the currents emitted

from three air terminals. The blue trace is the current 6 om a

19 nun diameter blunt rod, the green trace is the current f7om a

traditional

sha rp

pointed rod, and the red trace is the current

6om a so-called Early Streamer Emission air terminal

(discussed more fully in Section IV. The figure shows the

responses from about 500 ps to about

50

p before the high

current return stroke of the lightning strike. Starting at about

370 p before the return stroke, all three rods s m e d emitting

attempted streamers streame n which died out because the

fields between the tip of the air terminal and the approaching

leader were

too

small to

sustain

the streamer's propagation.

The currents in the early attempted streamers were about 1 A.

s the fields increased with the approaching leader, the

attempted streamer currents became larger. At about

150 ps

before the return stroke, the blunt rod (blue trace) generated a

streamer which did not die out. M e r he current initially rose

to about 4 A, it fell to about A, but the streamer continued to

propagate. At about 100 ps before the return stroke, the current

began to increase rapidly as the streamer developed in

intensity. When the current reached a value of 8 A it saturated

the instrumentation; a short while later, a fuse used to protect

the instrumentation blew. A subsequent inspection of the blunt

rod showed fused spots where the lightning had attached to

the

rod.

E L E C ~ C r l E L D C H * N O E A N D C U R R E N T T O L I G ~ N C i R O O S . W ~ 1 7 .l f f l . r n 1 7 u T

Black DBlm

E

( lor kVlm multiply by

10 :

Blue: 14mm

I

Red.

RBdimcCv~

ESP

devi-:

Green:

FranWin md

L s

U0 a3

250 2m ~1 5 0

-,m 50

TIME RELA TIVE TO TRIGGER SIGNAL

microremnds)

Figure 2. Lightning

rod responses

(colored t r a c e s

to increasing

electric field

(black

trace from an approaching lightning stepped

leader.

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The data indicate that all three rods generated attempted

streamers in response to an approaching leader. However, only

the blunt rod generated a successful streamer a streamer

which propagated to meet the approaching leader.

Our esults show that a blunt tipped object is more likely to

generate a successful streamer than is a sharp tipped one. If a

lightning protection system using sharp tipped lightning rods

protects a stlucture, then a blunt object on the structure may

generate a propagating streamer earlier than one of the

lightning rods.

A

more effective LPS would be one w hich used

blunt tipped lightning rods, so that an object on the protected

building is less likely to generate a propagating streamer earlier

than i s one of the ligh tning rods.

N N O N C O T I 0 N A L LIGHTNINGROTECTION

SYSTEMS

Currently there are two widely promoted non-conventional

lightning systems which are claimed to have a scientific basis.

These are the so-called Early Streamer Emission @SE) air

terminals, and Charge Transfer Systems

(CrS's) .

A

Early Streamer Emission Air Terminals

ESE air terminals have either a unique shape or active

elements at the air terminal tip. Proponents of ESE air

terminals claimed that these features cause the terminals to

generate streamers significantly earlier than conventional air

terminals [14]. The earlier streamer generation is claimed to

effectively increase the height of the air terminal by AL =

v

AT,

where v is the velocity of the streamer and

AT

is the time

advantage of the ESE air terminal. Manufacturers of ESE air

terminals measure AT in laboratory studies. Typical reported

values of AT are 50 to 300

p

and the value

used

for v is

lO m/s. This purportedly gives an ESE air terminal an

advantage

AL

of about

100

m over a conventional air terminal.

There are

three

significant problems with

this

claim:

1)

Proponents of ESE d evices claim a streamer propagation

speed of about IO6

s .

However both laboratory and field

measurements of streamer speeds show that the speeds are on

the order

of

IO4

m/s

tn

10' d s ,

one

to

two orders of magnitude

lower than claimed [I5 ,161. Using the actual speed of leaders,

the adv antage of ESE rods is decreased by one to two orders of

magnitude.

2) Even if

an

ESE device were to emit a streamer

significantly earlier than a conventional air terminal did, it

would be e ffective only if the streamer were able to propagate

from the ESE device to the approaching leader. The

propagation of a streamer does not depend on the conditions at

the tip of the air terminal, but on the conditions between the tip

and the approaching leader, which are not affected by ESE

devices. In an an alysis of ESE d evices, Mackerras et al. [17]

conclude that it is not possible to

gain

a significant

improvement in lightning interception performance by causing

the early emission of a streamer kom an

air

terminal.

3)

Our field studies

[ I l l

(e.g., Figure

2)

show that ESE

devices do not generate streamers any earlier th nconventional

air terminals do. We tested the two most popular ESE air

terminals marketed in North America, and found that their


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responses to approaching leaders were similar to that of a

conventio nal sharp painted lightning rod. During ow twelve

year field study we found no evidence that any of the ESE

device were struck by lightning, although we have many

documented cases in which lightning struck with the claimed

zones of protection of the ESE devices.

B.

Charge Transfer Systems

A

Charge Transfer System typically consists of an array of

many sharp conducting points erected over a facility to be

protected. Corona current 6om the points on the array

supposedly transfers a significant amount of charge 60m the

array into a region above the array. Condu ctors betwee n the

array of points and a grounding system provide a path for the

current 6om ground to the points in the array. The primary

claim made for Charge Transfer Systems is that this space

charge above the array prevents lightning discharges to the

protected facility. When originally introduced , the claim was

similar to the original hypothesis

of Franklin:

that the charge

generated by the

CTS

would silently discharge a thundercloud

and prevent lightning [18]. When it becam e obvious that the

additional charge from a few thousand paints would not add

significantly to the charge generated 6 0m the large number of

natural points (tree leaves, grass blades, etc.), the proponents

changed their arguments to say that a CTS would inject

sufficient charge (about 5 C ) above it to neutralize an

approaching leader [19]. It is easy to demonstrate that it is

impossible to inject the necessary charge into a small volume

above such an array [16]. The current claim is that a small

amount of space charge above the array will inhibit the

formation of steamers from the array. causing the lightning to

strike a nearby object rather than the array and the protected

structure below [20]. Howe ver, all of these mech anisms

require that an array of closely spaced points generate

significantly more corona current than a single point. Field

studies of arrays of

paints

show that this is not the case

[21,22,23].

Because the mechanism by which

a

Charge Transfer

System is supposed to work in not clear, the

best

way to

investigate the effectiveness of

a

CT S is to determine whether

or

not such systems get struck by lightning. Anec dotal

accounts of arrays being struck do not necessarily invalidate

the systems an array could have been imprope rly installed or

maintained, which could reduce its effectiveness. The best way

to assess the effectiveness of CTS s is to compare two similar

structures, one of which is equipped with a CTS and

a

similar

control structure without a CTS. There have been several such

studies done [22,24,25,26]. All of the studies have shown that

the Charge Transfer Systems were struck by lightnihg (as were

the control structures), and that there was no significant

reduction in the frequency of lightning strikes to a structure

equipped with a CTS.

The main selling point used by manufacturers of

CTSs

is

that their customers report significant decreases in lightning

damage after a structure has been equipped with a

C T S .

It is

important to note that a decre ase in (or eliminationo f lightning

damage does not necessarily indicate the elimination of

lightning strikes. In fact, a well designed convention al LPS

will greatly reduce

or

eliminate damage 6o m a direct lightning

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strike without preventing strikes.

A

CTS has all the elements

of a conventional LPS, and probably reduces lightning damage

for the same reasons a conventional

LPS

does. A C T S contains

a grid of overhead wires, which functions both

as

a preferred

strike point for a lightning discharge, and

to

reduce the fields at

the protected structure below, reducing the probability of a

streamer from the structure itself A CTS has a low impedance

path to a good grounding system, which gives

a

path to ground

for the large lightning currents when the CTS is struck.

V. CONCLUSIONS

A conventional lightning protection system consist of air

terminals which act

as

preferential receptors of lightning

strikes, downconductors to cany the large lightning currents,

and

a

grounding system to dissipate the lightning current away

6 0 m

the protected structure. The r terminals in

an

LPS

should be higher than objects on the protected structure, and

should be designed to be more effective at the generation of

successfully propagating streamers. A twelve year field study

has shown that lightning rods with moderately blunt tips

(19 mm diameter) are more effective

th n

rods with sharp tips

or extremely blunt tips

(>

50m m) . Lightning protection

systems using sharp rods have a long history of effectiveness,

but systems which use blunt rods should be even more

effective.

Field studies of two non-conventional lightning protection

systems (Early Streamer Emission air terminals and Charge

Transfer Systems) show that these systems do not function as

claimed by their proponents. ESE air terminals have been

shown to be no more effective th n sharp pointed lightning

rods.

While Charge Transfer Systems may reduce damage

60m

lightning strikes, they do

so

in the same way that much

less expensive conventional lightning protection systems do.

They do not eliminate lightning strikes

as

claimed.

REFERENCES

[I] I .

B. Cohen, Benjamin Franklins Experiments, A New Edition of

Franklins Experiments and Observations on Electricity, b a r d

University

Press,

Cambridge, MA 1941

[2] Comm inee on Atmospheric and Spac e Electricity

of

the American

Geophysical Union, The scientific basis

for

mditional lightning

protection systems,

h n o : l l a s e . a r m . o r ~ ~ ~ o ~ ~ . v d f ,

001

131 Federal Interagency Lightn ing Protection Users Group, The basis of

conventional lightning protection technology M I

- ~ . l i ~ h m i n e s a f e t . ~ ~ ~ l ~ ihmiwnventionalLPT.odf.2001..

[4] G. I Symons,,

editor,

R e p R of the Lighming Rod Conference, E. B F.

N. Spohn, 16 Charing Cross Road, London, 1882..

[SI W.

Lemmon,

B.

H.

Loomis and R. P. Barbour, Specifications for

Protection

of

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MA

1904.

161 M. G . Lloyd, iigh tning protection for Irees Science,

vol. 70,

p. 603,

1933.

[q

M.

A

U-. The Lightning Discharge, Aca dem ic

Fress,

Orlando, 1987.

[SI Mousa, A.M . and R D . Srivastava, A Revised Elecbogeom ehic Model

for

the Termination of Lightning Smkes on Ground Objects, Proc.

Inter. Aerospace omi Ground Conference

on

Lightning and S t d c

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City, OK, 342-352, 1988.

[9] R. H.

Lee.

Protect your plant against lightning, IEEE

Trans.

On

Industry Applications.vol. L4-15, pp. 236-240, 1978.

[IO] C.

B.

Moore, Improved configurations of lightning rads and ir

t m n i ~ l s ,. Franklin

Inst.,

vol. 315, pp. 61-85, 1983.


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[I 11 C. B. Moore. G. D. Aulich and W. Rison,

Measuremcntr

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rerpanses

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nearby strikes, Oeophys. Res. Len ., vol. 21, pp. 1487-1490,

2000.

[I21

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