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www.dehn.de14 LIGHTNING PROTECTION GUIDE

2. Characteristics of lightning current

2.1 Lightning discharge and sequenceof lightning current

Every year, an average of around 1.5 million light-ning strikes discharge over Germany. For an areaof 357,042 km2 this corresponds to an averageflash density of 4.2 lightning discharges per squarekilometre per year. The actual lightning density,however, depends to a large extent on geographicconditions. An initial overview can be obtainedfrom the lightning density map contained in Fig-ure 3.2.3.1. The higher the resolution of the light-ning density map, the more accurate the informa-tion it provides about the actual lightning fre-quency in the area under consideration.Using the BLIDS Blitzinformationsdienst vonSiemens (lightning information service bySiemens), it is now possible to locate lightning towithin 200 m in Germany. For this purpose, eight-een measuring outposts are spread throughoutthe country. They are synchronised by means ofthe highly accurate time signal of the global posi-tioning system (GPS). The measuring posts recordthe time the electromagnetic wave produced bythe lightning discharge arrives at the receiver.From the differences in the times of arrival of theelectromagnetic wave recorded by the variousreceivers, and the corresponding differences in thetimes it takes the electromagnetic wave to travelfrom the location of the lightning discharge to thereceivers, the point of strike is calculated. The datadetermined in this way are filed centrally andmade available to the user in form of various pack-ages. Further information about this service can beobtained from www.blids.de.Thunderstorms come into existence when warmair masses containing sufficient moisture are trans-ported to great altitudes. This transport can occurin a number of ways. In the case of heat thunder-storms, the ground is heated up locally by intenseinsolation. The layers of air near the ground heatup and rise. For frontal thunderstorms, the inva-sion of a cold air front causes cooler air to bepushed below the warm air, forcing it to rise. Oro-graphic thunderstorms are caused when warm airnear the ground is lifted up as it crosses risingground. Additional physical effects furtherincrease the vertical upsurge of the air masses. Thisforms updraught channels with vertical speeds ofup to 100 km/h, which create towering cumu-lonimbus clouds with typical heights of 5 – 12 kmand diameters of 5 – 10 km.

Electrostatic charge separation processes, e.g. fric-tion and sputtering, are responsible for chargingwater droplets and particles of ice in the cloud. Positively charged particles accumulate in theupper part, and negatively charged particles in thelower part of the thundercloud. In addition, thereis again a small positive charge centre at the bot-tom of the cloud. This originates from the coronadischarge which emanates from sharp-pointedobjects on the ground underneath the thunder-cloud (e.g. plants), and is transported upwards bythe wind.

If the space charge densities, which happen to bepresent in a thundercloud, produce local fieldstrengths of several 100 kV/m, leader discharges(leaders) are formed which initiate a lightning dis-charge. Cloud-to-cloud flashes result in chargeneutralisation between positive and negativecloud charge centres, and do not directly strikeobjects on the ground in the process. The lightningelectromagnetic impulses (LEMP) they radiatemust be taken into consideration, however,because they endanger electrical and electronicsystems.

Fig. 2.1.1 Downward flash (cloud-to-earth flash)

www.dehn.de LIGHTNING PROTECTION GUIDE 15

Lightning flashes to earth lead to a neutralisationof charge between the cloud charges and the elec-trostatic charges on the ground. We distinguishbetween two types of lightning flashes to earth:

⇒ Downward flash (cloud-to-earth flash)

⇒ Upward flash (earth-to-cloud flash)

In the case of downward flashes, leader dischargespointing towards the ground guide the lightningdischarge from the cloud to the earth. Such dis-charges usually occur in flat terrain and near lowbuildings and structures. Downward flashes can berecognised by the branching (Figure 2.1.1) which isdirected earthwards. The most common type oflightning is negative lightning flashes to earth,where a leader filled with negative cloud chargepushes its way from the thunder cloud to earth(Figure 2.1.2). This leader propagates in a series ofjerks with a speed of around 300 km/h in steps of afew 10 m. The interval between the jerks amountsto a few 10 μs. When the leader has drawn close tothe earth, (a few 100 m to a few 10 m), it causesthe strength of the electric field of objects on thesurface of the earth in the vicinity of the leader(e.g. trees, gable ends of buildings) to increase.The increase is great enough to exceed the dielec-

tric strength of the air. These objects involvedreach out to the leader by growing positivestreamers which then meet up with the leader, ini-tiating the main discharge.Positive flashes to earth can arise out of the lower,positively charged area of a thundercloud (Figure2.1.3). The ratio of the polarities is around 90 %negative lightning to 10 % positive lightning. Thisratio depends on the geographic location.On very high, exposed objects (e.g. radio masts,telecommunication towers, steeples) or on thetops of mountains, upward flashes (earth-to-cloud

leader leader

Fig. 2.1.2 Discharge mechanism of a negative downward flash(cloud-to-earth flash)

Fig. 2.1.3 Discharge mechanism of a positive downward flash(cloud-to-earth flash)

Fig. 2.1.4 Upward flash (earth-to-cloud flash)


flashes) can occur. It can be recognised by theupwards-reaching branches of the lightning dis-charge (Figure 2.1.4). In the case of upwardflashes, the high electric field strength required totrigger a leader is not achieved in the cloud, butrather by the distortion of the electric field on theexposed object, and the associated high strengthof the electric field. From this location, the leaderand its charge channel propagate towards thecloud. Upward flashes occur with both negativepolarity (Figure 2.1.5) and also with positive pola-rity (Figure 2.1.6). Since, with upward flashes, theleaders propagate from the exposed object on thesurface of the earth to the cloud, high objects canbe struck several times by one lightning dischargeduring a thunderstorm.Objects struck by lightning are subject to higherstress by downward flashes (cloud-to-earthflashes) than by upward flashes (earth-to-cloudflashes). The parameters of downward flashes aretherefore taken as the basis when designing light-ning protection measures.Depending on the type of lightning flash, eachlightning discharge consists of one or more partialstrikes of lightning. We distinguish between shortstrikes with less than 2 ms duration and longstrikes with a duration of more than 2 ms. Further

distinctive features of partial lightning strikes aretheir polarity (negative or positive), and their tem-poral position in the lightning discharge (first, sub-sequent or superimposed partial strikes of light-ning). The possible combinations of partial light-ning strikes are shown in Figure 2.1.7 for down-ward flashes, and Figure 2.1.8 for upward flashes.The lightning currents consisting of both impulsecurrents and continuing currents are load-inde-pendent currents, i.e. the objects struck exert noeffect on the lightning currents. Four parametersimportant for lightning protection technology canbe obtained from the lightning current profilesshown in Figure 2.1.7 and 2.1.8:

⇒ The peak value of lightning current I

⇒ The charge of the lightning current Qflash, com-prising the charge of the short strike Qshort andthe charge of the long strike Qlong

⇒ The specific energy W/R of the lightning cur-rent

⇒ The steepness di/dt of the lightning current.

The following chapters show which of the individ-ual efficiency parameters are responsible for whicheffects, and how they influence the dimensioningof lightning protection systems.

leader leader

Fig. 2.1.5 Discharge mechanism of a negative upward flash (earth-to-cloud flash)

Fig. 2.1.6 Discharge mechanism of a positive upward flash (earth-to-cloud flash)


±I

long-time current

positive or negative t

−I

negative t

±I

first impulse current


−I

sequential impulse currents

negative t

±I

short stroke


−I

subsequentshort strokes

negative t

first long stroke

superimposedshort strokes

±I


single longstroke

±I

long stroke


−I

negative t

Fig. 2.1.7 Possible components of downward flashes

Fig. 2.1.8 Possible components of upward flashes

2.2 Peak value of lightning currentLightning currents are load-independent currents,i.e. a lightning discharge can be considered analmost ideal current source. If a load-independentactive electric current flows through conductivecomponents, the amplitude of the current, and theimpedance of the conductive component the cur-rent flows through, help to regulate the potentialdrop across the component flown through by thecurrent. In the simplest case, this relationship canbe described using Ohm´s Law.

U I R= ⋅

If a current is formed at a single point on a homo-geneously conducting surface, the well-knownpotential gradient area arises. This effect alsooccurs when lightning strikes homogeneousground (Figure 2.2.1). If living beings (people oranimals) are inside this potential gradient area, astep voltage is formed which can cause a shock cur-rent to flow through the body (Figure 2.2.2). Thehigher the conductivity of the ground, the flatterthe shape of the potential gradient area. The riskof dangerous step voltages is thus also reduced.If lightning strikes a building which is alreadyequipped with a lightning protection system, thelightning current flowing away via the earth-ter-mination system of the building gives rise to apotential drop across the earthing resistance RE ofthe earth-termination system of the building (Fig-ure 2.2.3). As long as all conductive objects in thebuilding, which persons can come into contactwith, are raised to the same high potential, per-sons in the building cannot be exposed to danger.This is why it is necessary for all conductive parts inthe building with which persons can come intocontact, and all external conductive parts entering


ϕ potential

r distance frompoint of strike

ϕ

r

Î

Û

Î

air-terminationsystem

down-conductorsystem

earth-termination systemwith earth resistance RE

remote earth

lightning current

time

curr

ent

Fig. 2.2.1 Potential distribution of a lightning strike into homoge-nous soil

Fig. 2.2.2 Animals killed by shock current due to hazardous stepvoltage

Fig. 2.2.3 Potential rise of the earth-termination system of a build-ing compared to the remote earth due to the peak valueof the lightning current


the building, to have equipotential bonding. If thisis disregarded, there is a risk of dangerous shockhazard voltages if lightning strikes.The rise in potential of the earth-termination sys-tem as a result of the lightning current also createsa hazard for electrical installations (Figure 2.2.4).In the example shown, the operational earth ofthe low-voltage supply network is located outsidethe potential gradient area caused by the light-ning current. If lightning strikes the building, thepotential of the operational earth RB is thereforenot identical to the earth potential of the con-sumer system within the building. In the presentexample, there is a difference of 1000 kV. Thisendangers the insulation of the electrical systemand the equipment connected to it.

2.3 Steepness of lightning currentThe steepness of lightning current Δi/Δt, which iseffective during the interval Δt, determines theheight of the electromagnetically induced volt-ages. These voltages are induced in all open or

closed conductor loops located in the vicinity ofconductors through which lightning current isflowing. Figure 2.3.1 shows possible configura-tions of conductor loops in which lightning cur-rents could induce voltages. The square wave volt-age U induced in a conductor loop during theinterval Δt is:

M Mutual inductance of the loop

Δi/Δt Steepness of lightning current

As already described, lightning discharges com-prise a number of partial strikes of lightning. As faras the temporal position is concerned, a distinctionis made between first and subsequent short strikeswithin a lightning discharge. The main differencebetween the two types of short strikes consists inthe fact that, because the lightning channel has tobe built, the gradient of the lightning current ofthe first short strike is not as steep as that of thesubsequent short strike, which can use an existing,

U M i t= ⋅ / Δ Δ

distance r

1000 kV

UE

L1 L2 L3 PEN

RE = 10 Ω UE

I = 100 kAsubstation

RB 100% lightning current90%

10%

front time T1

U

T1

induced square-wave voltage

time

time

curr

ent

volta

ge

Î

s 3

s2

s 1

1

2

3

1 Loop in the down con-ductor with potentialflashover distance s1

2 Loop in the down con-ductor and installationcable with potentialflashover distance s2

3 Installation loop withpotential flashoverdistance s3

Î / T1building down conductor

Fig. 2.2.4 Threat to electrical installations by potential rise at theearth-termination system

Fig. 2.3.1 Induced square-wave voltage in loops via the currentsteepness Δi/Δt of the lightning current


fully conductive lightning channel. The steepnessof lightning current of the subsequent lightningstrike is therefore used to assess the highestinduced voltage in the conductor loops. An example of how to assess the induced voltagein a conductor loop is shown in Figure 2.3.2.

2.4 Charge of lightning currentThe charge Qflash of the lightning current is madeup of the charge Qshort of the short strike and thecharge Qlong of the long strike. The charge

of the lightning current determines the energydeposited at the precise striking point, and at allpoints where the lightning current continues in

Q idt= ∫

the shape of an electric arc along an insulatedpath. The energy W deposited at the base of theelectric arc is given by the product of the charge Qand the anode-/cathode voltage drop with valuesin the micrometer range UA,K (Figure 2.4.1).

The average value of UA,K is a few 10 V anddepends on influences such as the height andshape of the current:

Q Charge of lightning current

UA,K Anode/cathode voltage drop

Hence, the charge of the lightning current causesthe components of the lightning protection systemstruck by lightning to melt down. The charge isalso relevant for the stresses on isolating sparkgaps and protective spark gaps and by spark-gapbased surge protective devices.

Recent examinations have shown that, as the elec-tric arc acts for a longer time, it is mainly the con-tinuing charge Qlong of the continuing currentwhich is able to melt or vaporise large volumes ofmaterials. Figure 2.4.2 and 2.4.3 show a compari-son of the effects of the short strike charge Qshortand the long strike charge Qlong.

W Q UA K= ⋅ ,

10

1

0.1

0.01

0.001

0.1 · 10-3

0.01 · 10-3

0.1 0.3 1 3 10 30

Δ iΔ t

1

1

a

a

U

s

s (m)

a = 10 m

a = 3 m

a = 1 m

a = 0.1 m

a = 0.3 ma = 0.03 ma = 0.01 m

Example of calculationbased on an installation loop (e.g. alarm system)

From the above diagram results: M2 ≈ 4.8 μH

U = 4.8 · 150 = 720 kV

M2 (μH)

a

s

10 m

ΔiΔt

kAμs

3 m

150

(high requirement)

smelt metal

tip of the down conductor

Q

UA,K

time

lightningcurrent

Qshort = ∫idt

Qlong = ∫idt

long stroke current

time

curr

ent

curr

ent

Fig. 2.3.2 Example for calculation of induced square-wave voltagesin squared loops

Fig. 2.4.1 Energy deposited at the point of strike by the load of thelightning current


2.5 Specific energyThe specific energy W/R of an impulse current isthe energy deposited by the impulse current in aresistance of 1 Ω. This energy deposition is theintegral of the square of the impulse current overthe time for the duration of the impulse current:

The specific energy is therefore often called thecurrent square impulse. It is relevant for the tem-perature rise in conductors through which a light-ning impulse current is flowing, as well as for theforce exerted between conductors flown throughby a lightning impulse current (Figure 2.5.1).

For the energy W deposited in a conductor withresistance R we have:

R (Temperature dependent) d.c. resistance ofthe conductor

W/R Specific energy

The calculation of the temperature rise of conduc-tors through which a lightning impulse current isflowing, can become necessary if the risks to per-sons, and the risks from fire and explosion, have tobe taken into account during the design andinstallation of lightning protection systems. Thecalculation assumes that all the thermal energy isgenerated by the ohmic resistance of the compo-

W R i dt R W R= ⋅ ∫ = ⋅ / 2

W R i dt/ = ∫ 2

0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100

Galvanised steel100 kA (10/350 μs)

Copper100 kA (10/350 μs)

10.00 mm

Aluminiumd = 0.5 mm; 200 A, 350 ms

10.00 mm

Copperd = 0.5 mm; 200 A, 180 ms

10.00 mm

Stainless steeld = 0.5 mm; 200 A, 90 ms

10.00 mm

Steeld = 0.5 mm; 200 A, 100 ms

10.00 mm

Galvanised steeld = 0.5 mm; 200 A, 100 ms

specific energyW/R

force onparallel

conductors

heatinglightningcurrent

time

specificenergy

force

Fig. 2.4.2 Effect of an impulse current arc on a metal surface

Fig. 2.4.3 Plates perforated by the effects of long-time arcs

Fig. 2.5.1 Heating and force effects by the specific energy of light-ning current


nents of the lightning protection system. Further-more, it is assumed that, because of the brevity ofthe process, there is no perceptible heat exchangewith the surrounding. Table 2.5.1 lists the tempera-ture rises of different materials used in lightningprotection, and their cross sections, as a functionof the specific energy.

The electrodynamic forces F generated by a cur-rent i in a wire with a long, parallel section oflength I and a distance d (Figure 2.5.2) can be cal-culated as an approximation using the followingequation:

F(t) Electrodynamic force

i Current

μ0 Magnetic constant in air (4π ⋅ 10-7 H/m)

l Length of conductor

d Distance between the parallel conductors

The force between the conductors is attractive ifthe two currents flow in the same direction, andrepulsive if the currents flow in opposite direc-tions. It is proportional to the product of the cur-rents in the conductors, and inversely proportionalto the distance of the conductors. Even in the caseof a single, bent conductor, a force is exerted onthe conductor. Here, the force is proportional tothe square of the current in the bent conductor.The specific energy of the impulse current thusdetermines the load which causes a reversible orirreversible deformation of components and arraysof a lightning protection system. These effects aretaken into consideration in the test arrangementsof the product standards concerning the require-ments made on connecting components for light-ning protection systems.

2.6 Assignment of lightning currentparameters to lightning protec-tion levels

In order to define lightning as a source of interfer-ence, lightning protection levels I to IV are laiddown. Each lightning protection level requires aset of

⇒ maximum values (dimensioning criteria usedto design lightning protection components tomeet the demands expected to be made ofthem) and

⇒ minimum values (interception criteria neces-sary to be able to determine the areas withsufficient protection against direct lightningstrikes (radius of rolling sphere)).

F t μ i t l d( ) ( )= / ⋅ ⋅ / 022π

I

d

F

i i

F

i i

4 10 16 25 50 100Cross section[mm2]

AluminiumW/R [MJ/Ω]

IronW/R [MJ/Ω]

CopperW/R [MJ/Ω]

Stainlesssteel

W/R [MJ/Ω]

Mat

eria

l

2.5 – 564 146 52 12 3

5.6 – – 454 132 28 7

10 – – – 283 52 12

2.5 – – 1120 211 37 9

5.6 – – – 913 96 20

10 – – – – 211 37

2.5 – 169 56 22 5 1

5.6 – 542 143 51 12 3

10 – – 309 98 22 5

2.5 – – – 940 190 45

5.6 – – – – 460 100

10 – – – – 940 190

Table 2.5.1 Temperature rise ΔT in K of different conductor mate-rials

Fig. 2.5.2 Electrodynamic effect between parallel conductors

The Tables 2.6.1 and 2.6.2 show the assignment ofthe lightning protection levels to maximum andminimum values of the lightning current parame-ters.


Lightningprotection

level

Max.lightningcurrent

peak value

Probability ofthe actually

upcoming light-ning current tobe less than themax. lightningcurrent peak

value

I

II

III

IV

200 kA

150 kA

100 kA

100 kA

99 %

98 %

97 %

97 %

Maximum values(Dimensioning criteria)

Lightningprotection

level

Min.lightning

current peakvalue

Probability ofthe actually

upcoming light-ning current tobe higher thanthe min. light-ning currentpeak value

Radiusof therollingsphere

I

II

III

IV

3 kA

5 kA

10 kA

16 kA

99 %

97 %

91 %

84 %

20 m

30 m

45 m

60 m

Minimum values(Interception criteria)

Table 2.6.1 Maximum values of lightning current parameters andtheir probabilities

Table 2.6.2 Minimum values of lightning current parameters andtheir probabilities

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