Lighting Protection Theory

THE LIGHTNING PHENOMENON

Lightning is the phenomenon which accompanies the discharge of atmospheric charges from cloud to cloud or from cloud to earth. As lightning seeks the

path of the least resistance, it naturally tends to follow the shortest way between cloud and earth, such as buildings or towering projections. As illustrated

positive electrical charges gather in the clouds and negative charges gather in the ground. When the attraction between these two charges are strong

enough they come together in the form of lightning.

Lightning arrester equipment, properly manufactured and installed, dissipates the charges. In temperate climates a large majority of lightning is negative downwards lightning, as the negatively-charged cloud-base discharges to the ground.

The most important parameters are the following :

Amplitude

Rise time

Decay time

Current variation rate (di/dt)

Polarity

Charge

Specific energy

Number of strikes per discharge.

The first three parameters are independent in terms of statistics .

The expected effects of the characteristic lightning parameters are as follows:

Optical effects

Acoustical effects

Electro-chemical effects

Thermal effects

Electro-dynamic effects

Electro-magnetic radiation

 

LIGHTNING PROTECTION

Lightning Protection system are seperated two parts ;

External Lightning Protection System (LPS) and Internal Lightning Protection System.

External Lightning Protection System - LPS

External LPS is designed for protection of structures opposite to the direct lightning strokes.

For external LPS the following three different methods are used,

Air Rod

Mesh Method

Early Streamer Emission Lightning Rods

The protection level must be calculated before one of the above mentioned methods can be chosen. (IEC 62305- 1 - 1 and standarts of Europe)

 

LIGHTNING PROTECTION THEORY

Requirements for the the safety of people staying at work, home etc., made it necessary that special tasks were assigned to design engineers to take care

about the quality of lightning protection system on every higher building. The main function of the lightning protection system installed on the existing

building is to capture a lightning strike and then conduct the discharge current safely to the ground. Taking the fact into consideration that there are up to

100 lightning discharges to the ground every second throughout the world, it is always possible that a lightning strike hits the place which is particulary

close and important to you. The lightning protection system exists to intercept an atmospheric discharge in order to safely convey its current to the ground.

Lightning is formed as a result of processes occurring in the storm clouds. When air masses, ice crystals, water vapour drift and interact, then electrical

charges are generated. There are two types of storms (depending on the way how they are formed):

heat storms-formed as a result of strong heating and drifting upwards of the bottom air masses.

frontal storms-caused by the impact of a front of cold air on a warm moist air mass which is lifted above the advancing cold front.

In a typical storm cloud, the positive charges are concentrated in the upper part, whereas the negative charges build up at the base of the cloud. A further

charge growth causes an escalation of the electric field intensity until it exceeds the critical value. A cloud-to-ground discharge proceeds towards the

ground (small upward discharge can also be initiated from elevated ground points and this kind of discharge is called a ground-to-cloud discharge) or

towards the neighbouring cloud, which is called cloud-to-cloud discharge. Sound and visual effects of a lightning discharge are preceded by an invisible

initiated process. A high negative cloud potential (of the order of 108 V) is conveyed towards the ground by the downward leader, with the relatively small

decline of potential in its channel. When one of the upward leaders comes into contact with the downward leader, a conductive path of ionized air is created

allowing a powerful current to flow equalizing the potential difference between the cloud and the ground. In general, lightning installations are divided into:

conventional and active ones.

Conventional lightning protection system

Conventional lightning systems are based on the protection of a structure by making an installation of horizontal or vertical air terminals which are

connected to the earth with the help of downconductors. By means of the procedure described below, it is possible to decide according to the chosen

protection level whether the lightning protection is required or not. On the ground of our observations and experiences gained in this field, we recommend to

install the lightning protection on the structure regardless of the existance of a strike hazard level.

The selection of the protection level allows to minimize the risk of damage to people as well as complex and sophisticated equipment and structures. The

higher the efficiency of a lightning conductor is, the lower the risk of damages caused by lightning strikes will become. A protection level selection depends

on the kind of building, its structure and value.

A List of the effectiveness of lightning conductor with corresponding protection levels follows below.

Protectio

n Level

Effectiveness of lightning conductor E

I 0,98

II 0,95

III 0,90

IV 0,80

The Zones of protection can be determined by the "Cone of Protection" and "Rolling Sphere" methods.

The "Cone of Protection" rule consists of placing the protected structure in the zone of protection of high vertical air terminal.

Angle α for the protection level 0.95 is a 45º "Rolling Sphere" rule, which involves rolling imaginary shperes over the structure. The outer structure contour

is described by spheres. Points between the sphere and the structure indicate the protected areas. The areas touched by the sphere are deemed to require

protection. Depending on a protection level for a structure, the different values of sphere radius have to be considered.

 

 

AIR ROD

Before a Lightning Strike hits, lonization leads to an increase of the electric field around the top of the rod and the lightning current is lead from the rod to

the ground. According to table 1, the protected area connects the level of protection according to the protective angle, the length of the rod, the height of the

rod above the surface to be protected. Air Rods are used for the mesh method and stretched technique.

This is done by connecting the Air Rods to the down conductors, while the building is covered by a mesh with down conductors. In this system the

protection stage result the distance of down conductors fixings are determined according to these distances conductors are fixed.

In the Roof system of Mesh Metod, Specially Air Terminals are used on conductors crossing points on firing roofs. On f i re resistant roofs (metal roofs

etc ...) It is not needed to use air terminals (IEC 62305)

Mesh Method Details

Protection Levels According to Height and α Angles

AIR ROD AND MESH METHOD PROCESSING CRITERIA ACCORDING TO PROTECTION LEVEL

Protection

Level

Air Rod Height (m) cage (m) Down conductor

distance (m)

20 30 45 60

I α angles 25 * * * 5x5 10

II 35 25 * * 10x10 15

III 45 35 25 * 15x15 20

IV 55 45 35 25 20x20 25

Surge Protection Theory

Overvoltage definition

Overvoltage is any voltage, whose peak value exceeds the appropriate peak value of the highest operating voltage in the LV power system. Overvoltage is

usually an accidental phenomenon, which differs in time history and the place of its occurrence. Its parameters are defined by its cause (lightning stroke,

switching in heavy-current network and so on) and also by electrical character of the circuit (wave resistance, ending  impedance, discharge ability and  so

on). In the past few years the range of current and voltage courses  for different uses has been  standardized.  These courses enable implementation of

testing on equipment and constructive elements under the same conditions. In the  following  text  the most  important parameters of  the most used

standardized courses will be defined (according to EN 61 643-11, IEC 60-1 and CSN 34 5640).

Peak value (amplitude) Umax,Imax

peak value  is the maximal value of voltage or current

which is achieved by monitored impulse course

Front of impulse

a part of  voltage or current  impulse before  the peak

value

Front time of current impulse T1

 

1,25multiple of the time interval between moments, when

actual current value rise from 10% to 90% of the peak value

Front time of voltage impulse T1

 

1,67multiple of the time interval between moments, when actual

voltage  value  rise  from  30%  to  90% of  the peak value

Tail of impulse

 

a part of voltage or current impulse after the peak value

Time to half-value T2 the  time  interval between virtual beginning of  impulse and the

moment, when observed course reduces to 50% of its peak value

Note: The virtual beginning is an intersection of time axis and bisector, which goes through points, where actual value of

the front of impulse at first time reaches partly given lower value and partly given higher  value …in detail  see  the following two figures.

Standardized testing current impulse

two basic types of testing current impulses are used during SPD tests:

Testing  impulse  of  lightning  current  Iimp(10/350)  –  it  is used  for simulation of  lightning current (so-called  test by lightning current)

Testing current impulse Imax(8/20) – it is used for simulation of indirect effect of lightning and switching overvoltages. Arrester must discharge

cca 17,5x higher charge during test by the testing impulse of lightning current Iimp(10/350), than during testing by the current  impulse  Imax(8/20) with the 

same  amplitude. Also  resulting  in  a  different  construction of  the  lightning current arresters  tested by  the lightning current  impulse  Iimp(10/350) and 

surge arresters tested by the current impulse Imax(8/20).

Course and parameters of lightning voltages and currents

In  the chart  shown below  there are  typical courses and parameters  of  lightning  impulse  voltages  and  currents, which occur  in conductive parts of 

landscape, building constructions and metal lines in consequence of lightning stroke (taking in account influences caused by galvanic, inductive or

capacitive coupling).The typical values of lightning impulse voltages and currents, which occur in conductive parts of landscape, building constructions and

metal lines.

Testing  current  impulse  in  the  waveform  of  10/350μs is most often used  for  simulation of currents  infiltrating  into power  lines and electric equipment 

in consequence of galvanic coupling. In case of  inductive and capacitive coupling the voltage and current impulses are considera-bly shorter. The

examination of interfering lightning effects in relation to  inductive surges (currents)  in consequence of  inductive  coupling  is most  often  carried  out  by 

the testing  current  impulses  in  the waveform  of  8/20μs.  The examination of  lightning effects  in  relation  to  interfering surges (currents) in

consequence of capacitive coupling is  similarly carried out by  the  testing voltage  impulses  in the waveform of 1,2/50μs.

 

Kinds of surge couplings

Generally

Disturbing  energies  (e.g.  voltages,  currents,  fields)  can infiltrate  into  the building by ways of different couplings whereas  cabling  and  its  layout 

represent  an  important part  here.

 

Galvanic coupling

During near and direct lightning strokes into the lightning conductors of buildings, the overvoltage shows in consequences of a galvanic coupling. The

galvanic coupling is  given  by  a  different  size  of  ground  potentials  along the building (earth  electrodes, protective  connection  etc.)

Capacitive coupling

There  is always a capacitive coupling  (parasitic capac-ity) between the source of interference and the receiver. The higher the front rate of  rise of the

disturbing voltage impulse (du/dt) is, the stronger its interference effect is.

Inductive coupling

There  is always an  inductive coupling  (magnetic  field) between  the  source of interference and  the  receiver. The higher  the  front  rate of  rise of  the

disturbing current  impulse (di/dt) activating the magnetic field is, the higher the interference effect is.

Types of overvoltage

Direct Lightning Stroke

A  lightning stroke is an electric discharge between an electrically charged cloud and earth surface (earth lightning), between two or more clouds and each

other or between parts of one cloud (cloud lightning). Just a small percentage of  strokes happens be  tween  the  surface and  the clouds. The lightning

strokes originate in the „storm cells“, which stretch average out up to few kilometers. Every storm cell is active for up to 30 minutes and generates from two

to three lightning strokes per minute. The storm cell often reaches  the height of over 10  kilometres, whereas  the bottom visible part of the clouds is

usually at the height of one to two kilometres. In the centre of the storm cell there exists a strong  rising air  flow, which causes separation of positive and

negative charges. The positive charge is usu-ally binded on the frazils at the top of the storm cell,  while negative charge is usually binded on water drops at

the bottom of the cell. Nearby the earth the cell  is charged with positive   charge which is usually caused by discharge especially from forest. Beyond the

storm cells originating from  the  summer heat  there are  storm cells originating from  the  frontal cloudiness as a  result of big air masses movement. The

storm frequency depends on the season. In  summer months  (July–August)  there are on average 5  times more  storms  than  in winter months 

(December–February). The environmental heating up supports the storm creation. In autumn warm water near the seacoast  gives the necessary energy 

for the storm creation.  According to IEC 1312-1:1995 and IEC 62305 it  is possible to describe lightning charges by five basic parameters.

Total impoulse lightning charge Qf max.300 C

THe first stroke charge Qs max.100 C

The first stroke peak current Iimp max.200 kA

Specific energy the first stroke current W/R max.10 MJ/Ω

Rate of rise of the current di/dt max.200 kA/μs

General distribution of lightning current when an object is thunderstruck

Protection  system of  LV power  system  composited of  lightning current arresters and  surge arresters  SPD must be able to discharge lightning currents

or their substantial parts without their damage. It is generally recommended to come out  from  the ohmic  resistance of  the building earthing, pipeline,

power distribution  system and  so on for the purposes of establishing current distribution  going through SPD in case of direct lightning stroke into a

building protected by  the outside  lightning  system. The  following figure  shows  a  typical  example  of  lightning  current distribution in an object hit by

direct lightning stroke.

Where an  individual evaluation  is not possible,  it can be assumed that:

50% of  the  total  lightning current  Iimp=200kA  (10/350)….  IS1=  100kA  (10/350) enters  the earth  termination  system of  the  LPS 

(lightning protection  system) of  the  structure considered

50% of  Iimp=200kA  (10/350)….    IS2=  100kA  (10/350)  is distributed among  the  services  entering  the  structure (external  conductive

parts, el.power,  communication lines, etc.) The value of the current flowing in each service Ii is given by  IS/n, where n  is  the number of  the above

mentioned services (see the above figure). For evaluating  the current Iv in  individual conductors  in unscreened cables,  the cable current  Iiis divided by

m, the number of conductors, i.e. Iv = Ii /m.

For shielded cables, the current will flow along the shield. Requirement on dimensioning of protective system SPD in the most usual connection of the

building and LV power system (TNC  -  system  230/400V/50Hz)  results  from  this reasoning: For maximum lightning current size Iimp = 200kA (10/350) it

is enough to dimension the protective cascade of each phase conductor entering the object on cca 4% Iimp, that is on cca 8kA (10/350) in most cases.

Distribution of protected area into the lightning protection zones

The standard IEC 13 12-1 and IEC 62 305 defines the lightning protection zones LPZ  from  the  respect of  the direct even indirect lightning

effect. These zones are characteristic thanks to fundamental breaks of the electromagnetic conditions in their limited zones.  

LPZ OA

Zone where  items are  subject  to direct  lightning  strokes, and  therefore may have  to carry up  to  the  full  lightning current;  the

unattenuated electromagnetic  field occurs here.

LPZ OB Zone where items are not subject to direct lightning strokes, but the unattenuated electromagnetic field occurs

LPZ OCZone  where  items  are  not  subject  to  direct  lightning strokes and where currents on all conductive parts within this zone are further

reduced compared with zones 0B. In this zone the electromagnetic field may also be attenuated depending on the screening measures

LPZ 1Zone  where  items  are  not  subject  to  direct  lightning strokes and where currents on all conductive parts within this zone are further

reduced compared with zones 0B. In this zone the electromagnetic field may also be attenuated depending on the screening measures

LPZ 2

Zone  where  items  are  not  subject  to  direct  lightning strokes and where currents on all conductive parts within this zone are further

reduced compared with zones 0B. In this zone the electromagnetic field may also be attenuated depending on the screening measures

If a further reduction of conducted currents and/or elec-tromagnetic  field  is  required,  subsequent  zones  shall be introduced. The  requirement  for  those

zones shall be selected  according  to  the  required  environmental zones of the system to be protected. In general, the higher the number of the zones, the

lower the electromagnetic environment parameters. At the boundary of the individual zones, bonding of all metal penetrations shall be provided and

screening measures might by installed.

Note:  Bonding at  the boundary between  LPZ  0A,  LPZ  0B and  LPZ  1  is defined  in  IEC  13  12-1 and  IEC  62  305.  The electromagnetic  fields 

inside a  structure are  influenced by opening windows, by currents on metal conductors (e.g. bonding bars, cable shields and tubes), and by cable routing.

The following figure shows an example for dividing a structure into several zones.

There all electric power and signal lines enter the protected volume (LPZ 1) at one point,

and are bonded to bonding bar 1 at the boundary of LPZ 0A, LPZ 0B and LPZ 1. In

addition, the lines are bonded to the internal bonding bar 2 at the boundary of LPZ 1 and

LPZ 2.

Furthermore, the outer shield 1 of the structure is bonded to bonding bar 1 and the inner

shield 2 to bonding bar 2. Where cables pass from one LPZ to another, the bonding must

be executed at each boundary. LPZ 2 is constructed in such a way that partial lightning

currents are not transferred into this volume and cannot pass through it.

The above described segmentation of the protected ob-ject into protection zones gives

possibilities of active protection of the LV power system thanks to insertion of the

protective SPDs (usually at the zone boundary LPZ 0→1and LPZ 1→2) and other

protective SPDs at the zone boundary LPZ 2→3. Standardly it is recommended to insert

so-called 1ststage protection – surge arrester class I tested by lightning current

Iimp(10/350) at  the  zone boundary  LPZ 0→1. It  is  recommended  to  insert 2nd  stage

protection - surge arrester class II tested by testing impulse Imax(8/20) at the boundary

zone LPZ 1→2. At the boundary of LPZ 2→3 and subsequently along the consequential

circuit there is also recommended  to  shoulder after  every  cca  10m by  socalled 3rd

stage protection class  III also  tested by  testing impulse  Imax(8/20) or UOC.  For extra 

important protected equipment  it  is  recommended  to  secure  it by a quality continuous

surge protection class  III with high-frequency filter at  the boundary of  LPZ 2→3.  If  there

are adjacent structures  between  which  power  and  communication cables pass, the

earthing system shall be interconnected, and it is beneficial to have many parallel paths to

reduce current in the cables. A meshed earthing system fulfills this requirement. The 

lightning currents are  further  reduced, e.g. by enclosing all the cables in metal conduits or

gridlike reinforced concrete ducts, which must be integrated into the meshed earthing

system.

E.S.E Lightning Conductor

SCHIRTEC E.S.E Lightning Conductor

The main function of the lightning protection system installed on the existing building is to capture a lightning stroke and then

conduct discharge current safety to the ground.

In some conditions, however the active lightning system is the only possible method to protect from direct lightning strokes. Due

to the arguments mentioned above, we recommended to use the active lightning protection whenever the conventional solution

solution is inconvenient or when the former is more preferable to the latter as in the case of the efficient protection of architect.

The lightning discharge is initiated by so-called down conductor which creates an ionised air path (downwards or upwards)

between the cloud and the ground for the necessary flow of any lightning currents.

Operating Principle:

Schirtec-A lightning conductor is formed by two armatures. One of them is connected to ground, while the other remains at

atmospheric potential. The great magnitude of the electric field during the thunderstorm produces that, although armatures are

separated by a very short distance, the difference of potential between them during lightning approaching becomes

considerable.

This difference of potential is the power supply of the lightning conductor internal device. The internal device is located in the

body of Schirtec-A and is called Variable Impedance Unit . Therefore, the device working is regulated by the atmospheric field.

The advantages of these characteristics are, on one side, that in normal conditions the device is not working, avoiding then

unnecessary stress to the components. On another side, during thunderstorms the device detects when a proper electric field

exists, and when the downward leader is approaching, because it provokes a strong and rapid increase of the atmospheric

electric field.

During normal atmospheric conditions, the charge is also neutral in every area (also at the air), and the internal device is not

working.

The first difference with a simple lightning rod starts already when storm clouds appear. Inside the components of the internal

device, equi-potential lines become very close together, causing the necessity of a strong concentration of charges at the

armature surface. The device is designed in such a way that the transitory current does not get lost, but remains as

electromagnetic fields in the components of the electrical device. The electric field value, able to ionise the air around the tip, is

reached earlier than with a simple rod, because the internal device makes the voltage increase over ground level. Then, air

charges become also a part of the internal current. Therefore, the ionised area is growing much faster than with a simple rod.

The previous phase to the formation of the upward leader is the formation of corona discharges (streamers) that propagate

towards the downward leader. One of these streamers will become the upward leader, which will propagate continuously to the

downward leader, forming then the lightning discharge path.

Inside the lightning conductor, the downward leader approaching and the strong increase of the electric field caused by it are the

factors that activate the mean function of the internal device .When the voltage between the armatures exceeds a certain value

which the circuit is designed for, then the internal trigger works, using the accumulated energy for pumping to designed for, then

the internal trigger works, using the accumulated energy for pumping to inside the ionised area. The strong and sudden

concentration of positive charges cause repellent forces in the ionised area, which break the existing border. The device has

provoked then a streamer effect, avoiding the ‘'glow regime'' that was lowering the effectiveness of a simple lightning rod.

The streamer emission under these conditions favours the upward leader formation, which will progress continuously till

reaching the downward leader, forming then the discharge path. Then, as the Schirtec-A is the point where the upward leader

was formed. It will be the receiver of the lightning strike.

Details of  SCHIRTEC-A (S-A)

1. Air Terminal2. Ion Generator3. Accelerator and Atmospheric Electrodes4. Grounding Connection Terminal

Explanation Proteciton Radius Table

Here you will find an explanation on how to read the protection radius table correctly.

Protectionlevel

The protection level of the object depends on several factors such as:

if the object is inhabited or not

if the object is expolosiv or not,

material of the roof and the roof construction (wood, metall,...)

The more valuable and vulnerable the object is the higher the level of protection needed. e.g. an inhabited house, with a metal

roof could be attributed to protection class 1. A telephone pole of low value could attributed to the protection class 4. The safety

class can be calculated with our lightning risk program.

h

Installation height, the higher the lightning conductor is installed the greater the protective radius Rp (m)

Rp(m)

Radius protection, we recommend  to install the lightning conductor in a height of 5 to 6 meters because you can then achieve a

high level of protection.

Protection Level Calculator

 

FORMULAS VALUES RESULTS

The Equivalent Collection Area of Structure

L=50  

Ae=LW+6H(L+W)+9πH2

(For rectangular fields)W=30 Ae=15061.7251

H=15  

 

Expended Lightning Frequency

Td=30

Ng=0,04.Td1.25

Nd=Ng.Ae.C1.10-6

Ng=2.8084

Ae=15061.7251 Nd=0.0423

C1=1  

Accepted Lightning Frequency

 

C=C2.C3.C4.C5Nc=5,5.10-3/C

C1=1

C2=0.5

C3=1 Nc=0.0022

C4=1  

C5=5

C=2.5

If Nd ≤ Nc...Optional Protection

If Nd>Nc...Protection Required Efficiency , E=1-(Nc/Nd)

EfficiencyE= 0.95

Active Lightning Protection Levels

E > 0,98 LEVEL I + Add

0,95 <E ≤ 0,98 LEVEL I

0,90 <E ≤ 0,95 LEVEL II

0,80 < E≤ 0,90 LEVEL III

0<E≤0,80 LEVEL IV

Example 1 for S-A:

Single-familiy-home; Protection Level 1

Example  2 for S-A:

Factoryhall; 150 m x 50m, Protection Level 1

The lightning conductor  S-A has been installed at a

height of 4m.

The protection radius has to be specified according

to NF C 17-102 standard with 63m, the effective

protection is bigger, according to our test reports.

There were two lightning conductors S-A installed at a height

of 6m.

The radii overlap and provide for the entire production facility

sufficient protection.

The protection radius must be specified according to NF C

17-102 with 79m, the effective protection is bigger, according

to our test reports

Protection Radius Table

Rp(m) S-A (��L: 60m) S-AS (��L: 30m) S-DA (��L: 60m) S-DAS (��L: 45m)

h I II III IV I II III IV I II III IV I II III IV

2 31 35 39 43 19 22 25 28 31 35 39 43 25 28 32 36

The Protection Radius For SCHIRTEC E.S.E. Lightning Conductors(According to NFC 17 102)

Rp SCHIRTEC-AS(ΔL=15m)

SCHIRTEC-DAS(ΔL=45m)

SCHIRTEC-A/DA(ΔL=60m)

H(m) I II III IV I II III IV I II III IV

2 13 15 18 20 25 28 32 36 31 35 39 43

4 25 30 36 41 51 57 64 72 63 69 78 85

5 32 37 45 51 63 71 81 89 79 86 97 107

6 32 38 46 52 63 71 81 90 79 87 97 107

8 33 39 47 54 64 72 82 91 79 87 98 108

10 34 40 49 56 64 72 83 92 79 88 99 109

20 35 44 55 63 65 74 86 97 80 89 102 113

30 35 45 58 69 65 75 89 101 80 90 104 116

60 35 45 60 75 65 75 90 105 80 90 105 120

4 63 69 78 85 38 44 51 57 63 69 78 85 51 57 64 72

5 79 86 97 107 48 55 63 71 79 86 97 107 63 71 81 89

6 79 87 97 107 48 55 64 72 79 87 97 1007 63 71 81 90

8 79 87 98 108 49 56 65 73 79 87 98 108 64 72 82 91

10 79 88 99 109 49 57 66 75 79 88 99 109 64 72 83 92

20 80 89 102 113 50 59 71 81 80 89 102 113 65 74 86 97

30 80 90 104 116 50 60 73 85 80 90 104 116 65 75 89 101

60 80 90 105 120 50 60 75 90 80 90 105 120 65 75 90 105

Key:

h: Installation height

I/II/III/IV: Protectionlevel

Rp(m):Protectionradius

in m

5

6

Recommended

installation height