Lightning Protection according to IEC 62305
Diogo Filipe da Silva Santos
Thesis to obtain the Master of Science Degree in
Electrical and Computer Engineering
Supervisor: Profª. Maria Teresa Nunes Padilha de Castro Correia de Barros
Examination Committee
Chairperson: Prof. Rui Manuel Gameiro de Castro
Supervisor: Profª. Maria Teresa Nunes Padilha de Castro Correia de Barros
Members of the Committee: Profª. Maria Eduarda de Sampaio Pinto de Almeida
Pedro
November 2015
ii
Abstract
Protection of structures against lightning has been an area of study for a very long time. Since the
middle of the 18th century it began with the work of Benjamin Franklin by protecting structures against
the effects of direct lightning discharges with his famous invention, the lightning rod, which it is still
used today as an air-termination system.
More recently, with the increasing in technology and maintenance of electrical and electronic
equipment, the low operating voltage of that equipment and the increasing height of structures, the
necessity for better lightning protection risk evaluation and measures became a very urgent matter.
That is where the IEC 62305 enters.
The objectives of this thesis are to understand the Risk Management methodology created by the
international committee of IEC, translated in the standard IEC 62305 – Lightning Protection, and apply
it to a computer program.
In order to accomplish that, it is crucial to dominate two subjects: (1) the characterization of the
lightning discharge, including the explanation of the phenomenon itself, which was very difficult to
describe until the 1950s, and the definition of the several current parameters; and (2) the lightning
incidence models adopted by the standard, including the description of the Electrogeometric Model
(EGM) and the Electrical Shadow Model.
Only when these subjects are fully understood, it is possible to take the standard’s risk management
methodology and create a user-friendly computer program, capable of supporting risk assessment and
elaborate effective solutions, according to IEC 62305.
Keywords
Lightning discharge, risk assessment, lightning protection system, IEC 62305
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Resumo
A proteção de estruturas contra descargas atmosféricas tem sido uma área de estudo há já muito
tempo. Desde meados do século 18 que começou com o trabalho de Benjamin Franklin na proteção
de estruturas contra os efeitos de descargas atmosféricas diretas, usando a sua famosa invenção, o
para-raios, que ainda hoje é utilizado como sistema de proteção aéreo.
Mais recentemente, com o aumento da tecnologia e manutenção de equipamentos elétricos e
eletrónicos, a baixa tensão a que estes operam e o aumento da altura média das estruturas, tornou a
necessidade de melhores avaliações de risco e de medidas de proteção contra descargas
atmosféricas um assunto urgente.
Os objectivos desta dissertação são perceber a metodologia de Gestão do Risco criada pelo comité
internacional CEI, traduzida na norma CEI 62305 – Proteção contra descargas atmosféricas, e aplicála a um programa computacional.
Para atingir isso, é crucial dominar dois assuntos: (1) a caracterização da descarga atmosférica,
incluindo a descrição do fenómeno em si, o que foi em si bastante difícil de descrever até à década
de 1950, e a definição dos vários parâmetros que compõem a corrente da descarga; e (2) os modelos
de incidência adotados pela norma, incluindo a explicação do Modelo Eletrogeométrico e o Modelo de
Sombra Elétrica.
Apenas quando estas matérias estão completamente compreendidas é possível pegar na
metodologia de gestão do risco presente na norma e criar um programa computacional de utilização
amigável, capaz de servir de avaliação do risco e elaborar soluções eficazes, segundo a norma CEI
62305.
Palavras Chave
Descarga atmosférica, avaliação do risco, proteção contra descargas atmosféricas, CEI 62305
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Index
Abstract.................................................................................................................................................... iii
Resumo ....................................................................................................................................................v
List of Figures .......................................................................................................................................... ix
List of Tables ........................................................................................................................................... xi
Terms, definitions, symbols and abbreviations ..................................................................................... xiii
1. Introduction .......................................................................................................................................... 1
1.1. Overview ....................................................................................................................................... 1
1.2. Objectives ..................................................................................................................................... 3
1.3. Outline ........................................................................................................................................... 3
2. Overview of international lightning protection standards..................................................................... 5
2.1. IEC standard 62305 ...................................................................................................................... 5
2.1.1. Before IEC 62305 ................................................................................................................... 5
2.1.2. New approach with IEC 62305............................................................................................... 7
2.2. Different approaches by national lightning protection standards .................................................. 8
2.2.1. Differences between national lightning protection standards and the IEC 62305 ................. 8
2.2.2. Conventional and un-conventional lightning air-termination systems .................................... 9
3. General characterization of lightning discharges .............................................................................. 11
3.1. The lightning discharge phenomenon......................................................................................... 11
3.2. Current parameters ..................................................................................................................... 13
3.2.1. Peak current, 𝐼 ..................................................................................................................... 13
3.2.2. Maximum current steepness ................................................................................................ 14
3.2.3. Charge, 𝑄 ............................................................................................................................. 14
3.2.4. Specific energy, 𝑊/𝑅 ........................................................................................................... 14
3.3. Cumulative statistical distributions of lightning peak currents .................................................... 16
3.4. Recent work in direct peak current measurements .................................................................... 18
3.5. Other parameters derived from current measurements ............................................................. 19
4. Lightning incidence models adopted by IEC 62305 .......................................................................... 23
4.1. Historical overview ...................................................................................................................... 23
4.2. The Electrogeometric Model (EGM) ........................................................................................... 24
4.2.1. Model description ................................................................................................................. 24
4.2.2. The Rolling sphere method (RSM)....................................................................................... 26
4.3. The Electric Shadow Model ........................................................................................................ 28
4.4. Lightning protection level (LPL) according to IEC 62305 ........................................................... 29
4.5. Final considerations and comparison between EGM and other lightning incidence models ..... 30
5. Risk Management.............................................................................................................................. 35
5.1. Basic Concepts and Methodology .............................................................................................. 35
5.2. Risk Evaluation ........................................................................................................................... 39
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5.2.1. Number of dangerous events, 𝑁𝑁𝑋 ......................................................................................... 40
5.2.2. Probability of damage to a structure, 𝑃𝑋 ............................................................................... 43
5.2.3. Consequent loss from damage into the structure and/or its contents, 𝐿𝑋 ............................ 48
5.3. Risk mitigation measures ............................................................................................................ 52
5.3.1. M1: Installing an LPS of an appropriate class...................................................................... 53
5.3.2. M2: Installing SPDs of an appropriate LPL at the line entrance point ................................. 55
5.3.3. M3: Protection measures against the consequences of fire ................................................ 56
5.3.4. M4: Providing zone(s) with a coordinated SPD system for the internal power and telecom
systems .......................................................................................................................................... 56
5.3.5. M5: Providing zone(s) with an adequate spatial grid-like shield. ......................................... 56
6. Application of the computer program – L.R.A. .................................................................................. 61
6.1. L.R.A. structure ........................................................................................................................... 61
6.2. Example 1: Hospital .................................................................................................................... 63
6.2.1. Structure: geometric and environmental characteristics ...................................................... 64
6.2.2. Connected lines: geometric and environmental characteristics ........................................... 65
6.2.3. Type of loss .......................................................................................................................... 68
6.2.4. Lightning protection zones ................................................................................................... 69
6.2.5. Results ................................................................................................................................. 72
6.2.6. Lightning protection measures ............................................................................................. 74
6.3. Example 2: South Tower in Instituto Superior Técnico (IST), Alameda Campus ....................... 80
6.3.1. Evaluation 1, assuming a 𝐿𝑂 = 0 − no risk of explosion ...................................................... 85
6.3.2. Evaluation 2: 𝐿𝑂 = 0.1 − assuming risk of explosion ........................................................... 88
7. Conclusions ....................................................................................................................................... 91
7.1. Summary ..................................................................................................................................... 91
7.2. Achievements ............................................................................................................................. 91
7.3. Future work ................................................................................................................................. 92
Appendices ............................................................................................................................................ 97
Appendix A: Informative tables .......................................................................................................... 97
Bibliography ........................................................................................................................................... 93
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List of Figures
Figure 1: Organization of the four parts of IEC 62305 – Protection against lightning (from IEC 62305-1
[1]) ........................................................................................................................................................... 2
Figure 2: (a) Types of cloud flashes: (i) intracloud; (ii) air discharges; (iii) intercloud; (b) Types of
ground flashes: (i) downward negative ground flashes; (ii) downward positive ground flashes; (iii)
upward positive ground flashes; (iv) upward negative ground flashes, from [19] ................................. 12
Figure 3: Impulse lightning current parameters (typically 𝑇2 < 2 𝑚𝑚𝑠) from IEC 62305-1 [1] ................. 15
Figure 4: Cumulative statistical distributions of lightning peak currents, giving percent of cases
exceeding abscissa value (from [22]) ................................................................................................... 17
Figure 5: Cumulative frequency of the current peak of the negative first stroke according to CIGRE.. 18
Figure 6: Design of an air-termination system according to the rolling sphere method (from [3]) ........ 27
Figure 8: Cumulative frequency of the current peak of the negative first stroke according to CIGRE.. 31
Figure 9: Lightning attractive radii as predicted by EGM (crosses), SLIM (solid line) and CVM (dashed
line) as a function of return stroke peak current for several structure heights (from [41]) .................... 33
Figure 10: Types of damage and types of loss according to the striking point – Source of damage.... 36
Figure 11: Types of loss and corresponding risks resulting from different types of damage ................ 37
Figure 12: Collection areas introduced in IEC 62305 [47] .................................................................... 42
Figure 13: Probabilities that a flash to or near a structure cause damage in the structure and/or its
contents ................................................................................................................................................. 44
Figure 14: Probabilities that a flash to or near a connected line cause damage in the structure and/or
its contents ............................................................................................................................................ 45
Figure 15: M1 measure organization chart of the probabilities and parameters influenced by upgrading
the LPL of the LPS, 𝑃𝐵 ........................................................................................................................... 54
Figure 16: M2 measure organization chart of the probabilities and parameters influenced by 𝑃𝐸𝐵 ...... 55
Figure 17: M4 measure organization chart of the probabilities and parameters influenced by 𝑃𝑆𝑃𝐷 ..... 57
Figure 18: M5 measure organization chart of the probabilities and parameters influenced by the mesh
widths, 𝑤𝑚 .............................................................................................................................................. 58
Figure 19: Flowchart of the L.R.A. structure.......................................................................................... 63
Figure 20: Inputs related to the geometric and environmental characteristics of the structure ............ 65
Figure 22: Number of lines (power + telecommunications) connected to the structure to be protected65
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Figure 21: Lightning ground flash density computation by the keraunic number 𝑇𝑑 and the 𝑁𝑁𝐺 world
map distribution [49] .............................................................................................................................. 66
Figure 23: Inputs related to characteristics of the connected lines to the structure .............................. 67
Figure 24: Type of loss in analysis ........................................................................................................ 69
Figure 25: Definition of the lightning protection zones .......................................................................... 70
Figure 26: Unlocking the buttons to enable the characterization of each zone .................................... 70
Figure 27: Parameters that describe the zone “”Rooms Block” for the Hospital example .................... 72
Figure 28: Risk 𝑅1 result – risk of loss of human life, for each zone, by risk component...................... 73
Figure 29: Selection of appropriate lightning protection measures according with the three most
influential risk components for the overall risk....................................................................................... 74
Figure 30: Re-calculated risk after taking the lightning protection measures of Solution 1 .................. 75
Figure 31: Re-calculated risk after taking the lightning protection measures of Solution 2 .................. 76
Figure 32: Calculated risk after taking the lightning protection measures of Solution 3 ....................... 77
Figure 33: Parameters that describe the zone “”Intensive Care Unit” from the Hospital example........ 78
Figure 34: Calculated risk, 𝑅4 – risk of loss of economic value, for each zone, by type of risk ............ 78
Figure 35: Simplified economic evaluation based on risk analysis ....................................................... 79
Figure 36: Annual savings, 𝑆𝑀 , for the three solutions presented ......................................................... 80
Figure 37: South tower in Instituto Superior Técnico ............................................................................ 80
Figure 38: Parameters that describe the laboratory zones of the South Tower in IST ......................... 83
Figure 39: Calculated risk for evaluation 1: 𝐿𝑂 = 0................................................................................ 84
Figure 40: Calculated risk for evaluation 2: 𝐿𝑂 = 0.1 ............................................................................. 84
Figure 41: Data given by the “Information” button ................................................................................. 86
Figure 42: Approximately collection area of the structure to be protected: South Tower in IST ........... 86
Figure 43: Calculated risk when protection measure against fire is applied: 𝑟𝑝 = 0.5 → 𝑟𝑝 = 0.2....... 87
Figure 44: Calculated risk when a lightning protection system of LPL III is installed in the structure to
be protected ........................................................................................................................................... 87
Figure 45: Calculated risk when a coordinated SPD system is installed in zone 3, “Laboratories” ...... 89
Figure 46: Calculated risk of a structure with a lightning protection system LPL II and the preventing
fire measures implemented ................................................................................................................... 89
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List of Tables
Table 1: Definition of the different lightning current parameters ........................................................... 15
Table 2: Comparison of return peak currents for first negative downward leaders flashes, from [22] . 20
Table 3: Tabulated values of lightning current parameters taken from CIGRE [27], [28] and adopted
by IEC 62305-1 [1] ................................................................................................................................ 21
Table 4: Coefficients of the striking distance according to Equation (3) ............................................... 26
Table 5: Minimum and maximum values of lightning parameters according with the different class of
the LPS .................................................................................................................................................. 30
Table 6: Typical values of tolerable risk, 𝑅𝑇 .......................................................................................... 38
Table 7: Risk components to be considered for each type of loss in a structure .................................. 38
Table 8: Types of number of dangerous events and collection area according with the source of
lightning ................................................................................................................................................. 41
Table 9: Loss components relation with the type of damage and the factors 𝐿 𝑇 , 𝐿𝐹 , 𝐿𝑂 ..................... 51
Table 10: Protection measures vs risk components ............................................................................. 59
Table 11: Characteristics of the structure or of the internal systems that influence the risk components
............................................................................................................................................................... 60
Table 12: Input parameters that describe the geometric and environmental characteristics of the
structure ................................................................................................................................................. 64
Table 13: Input parameters that describe the geometric and environmental characteristics of the
connected lines ...................................................................................................................................... 68
Table 14: Input parameters that describe the zone “”Rooms Block” for the Hospital example............. 71
Table 15: The input parameters that describe the geometric and environmental characteristics of the
structure ................................................................................................................................................. 81
Table 16: Parameters of power and telecommunication lines .............................................................. 81
Table 17: Distribution of people inside each protection zone................................................................ 82
Table A. 1: Values of relative location factor of the structure, 𝐶𝐷 .......................................................... 97
Table A. 2: Values of installation factor,𝐶𝐼 ............................................................................................. 97
Table A. 3: Values of line type factor, 𝐶𝑇 ............................................................................................... 97
Table A. 4: Values of environment factor, 𝐶𝐸 ........................................................................................ 98
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Table A. 5: Values of the parameter 𝑃𝑇𝐴 by implementation of measures against touch and step
voltage ................................................................................................................................................... 98
Table A. 6: Values of probability 𝑃𝐵 according to the class of the LPS installed................................... 98
Table A. 7: Values of the parameter 𝑃𝐸𝐵 in function of the LPL for which SPDs are designed ............ 99
Table A. 8: Values of the parameter 𝑃𝑆𝑃𝐷 in function of the LPL for which SPDs are designed ........... 99
Table A. 9: Values of the factors 𝐶𝐿𝐷 and 𝐶𝐿𝐼 ...................................................................................... 100
Table A. 10: Values of probability 𝑃𝐿𝐷 ................................................................................................. 100
Table A. 11: Values of probability 𝑃𝐿𝐼 .................................................................................................. 100
Table A. 12: Values of probability 𝑃𝑇𝑈 ................................................................................................. 101
Table A. 13: Values of the factor 𝑟𝑡 in function of the contact resistance of the surface ..................... 101
Table A. 14: Values of the factor 𝑟𝑝 ..................................................................................................... 101
Table A. 15: Value of the factor 𝑟𝑓 ....................................................................................................... 102
Table A. 16: Value of the factor ℎ𝑧 ...................................................................................................... 102
Table A. 17: Values of the factor 𝐾𝑆3 ................................................................................................... 103
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Terms, definitions, symbols and abbreviations
For the purposes of this document, the following terms, definitions, symbols and abbreviations are
applied.
Terms and definitions
Structure to be protected – Structure for which protection is required against the effects of lightning
in accordance with the standard IEC 62305;
Lightning protection level – LPL: number related to a set of lightning current parameters values
relevant to the probability that the associated maximum and minimum design values will not be
exceeded in naturally occurring lightning;
Lightning protection system – LPS: complete system used to reduce physical damage due to
lightning flashes to a structure;
Lightning electromagnetic impulse – LEMP: all electromagnetic effects of lightning current via
resistive, inductive and capacitive coupling, which create surges and electromagnetic fields.
LEMP protection measures – SPM: measures taken to protect internal systems against the effects of
LEMP;
Surge protective device – SPD: device intended to limit transient overvoltages and divert surge
currents;
Lightning equipotential bonding – EB: bonding to LPS of separated metallic parts, by direct
conductive connections or via surge protective devices, to reduce potential differences caused by
lightning current;
Dangerous event – lightning flash to or near the structure to be protected, or to or near a line
connected to the structure to be protected that may cause damage;
Rated impulse withstand voltage level – 𝑈𝑊 : impulse withstand voltage assigned by the
manufacturer to the equipment or to a part of it, characterizing the specified withstand capability of its
insulation against (transient) overvoltages.
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Symbols and abbreviations
𝑎 – Amortization rate;
𝐴𝐷 – Collection area for flashes to an isolated structure;
𝐴𝐷𝐽 – Collection area for flashes to an adjacent structure;
𝐴𝐷 ′ – Collection area attributed to an elevated roof protrusion;
𝐴𝐼 – Collection area for flashes near a line;
𝐴𝐿 – Collection area for flashes to a line;
𝐴𝑀 – Collection area for flashes striking near the structure;
𝐶𝐷 – Location factor;
𝐶𝐷𝐽 – Location factor of an adjacent structure;
𝐶𝐸 – Environmental factor;
𝐶𝐼 – Installation factor of the line;
𝐶𝐿 – Annual cost of total loss in absence of protection measures;
𝐶𝐿𝐷 – Factor depending on shielding, grounding and isolation conditions of the line for flashes to a line;
𝐶𝐿𝐼 – Factor depending on shielding, grounding and isolation conditions of the line for flashes near a
line;
𝐶𝐿𝑍 – Cost of loss in a zone;
𝐶𝑃 – Cost of protection measures;
𝐶𝑃𝑀 – Annual cost of selected protection measures;
𝐶𝑅𝐿𝑍 – Cost of residual loss in a zone;
𝐶𝑇 – Line type factor for a HV/LV transformer on the line;
𝑐𝑎 – Value of the animals in the zone, in currency;
𝑐𝑏 – Value of the building relevant to the zone, in currency;
𝑐𝑐 – Value of the content in the zone, in currency;
𝑐𝑠 – Value of the internal systems (including their activities) in the zone, in currency;
𝑐𝑡 – Total value of the structure, in currency;
𝑐𝑧 – Value of the cultural heritage in the zone, in currency;
𝐷1 – Injury to living beings by electric shock;
𝐷2 – Physical damage;
𝐷3 – Failure of electrical and electronic systems;
ℎ𝑧 – Factor increasing the loss when a special hazard is present;
𝐻 – Height of the structure;
𝐻𝐽 – Height of the adjacent structure;
𝑖 – Interest rate;
𝐾𝑆1 – Factor relevant to the screening effectiveness of the structure;
𝐾𝑆2 – Factor relevant to the screening effectiveness of shields internal to the structure;
𝐾𝑆3 – Factor relevant to the characteristics of internal wiring;
𝐾𝑆4 – Factor relevant to the impulse withstand voltage of a system;
𝐿 – Length of structure;
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𝐿𝑋 – Loss consequent to damages relevant to structure;
𝐿𝐽 – Length of the adjacent structure;
𝐿𝐴 – Loss due to injury to living beings by electric shock (flashes to structure);
𝐿𝐵 – Loss in a structure related to physical damage (flashes to structure);
𝐿𝐿 – Length of line section;
𝐿𝐶 – Loss related to failure of internal systems (flashes to structure);
𝐿𝑀 – Loss related to failure of internal systems (flashes near structure);
𝐿𝑂 – Loss in a structure due to failure of internal systems;
𝐿 𝑇 – Loss due to injury by electric shock;
𝐿𝑈 – Loss due to injury of living beings by electric shock (flashes to line);
𝐿𝑉 – Loss in a structure due to physical damage (flashes to line);
𝐿𝑊 – Loss related to failure of internal systems (flashes to line);
𝐿𝑍 – Loss related to failure of internal systems (flashes near a line);
𝐿1 – Loss of human life;
𝐿2 – Loss of service to the public;
𝐿3 – Loss of cultural heritage;
𝐿4 – Loss of economic value;
𝑚𝑚 – Maintenance rate;
𝑁𝑁𝑋 – Number of dangerous events per annum;
𝑁𝑁𝐷 – Number of dangerous events due to flashes to structure;
𝑁𝑁𝐷𝐽 – Number of dangerous events due to flashes to adjacent structure;
𝑁𝑁𝐺 – Lightning ground flash density;
𝑁𝑁𝐼 – Number of dangerous events due to flashes near a line;
𝑁𝑁𝐿 – Number of dangerous events due to flashes to a line;
𝑁𝑁𝑀 – Number of dangerous events due to flashes near a structure;
𝑛𝑧 – Number of possible endangered persons (victims or users not served);
𝑛𝑡 – Expected total number of persons (or users served);
𝑃 – Probability of damage;
𝑃𝐴 – Probability of injury to living beings by electric shock (flashes to a structure);
𝑃𝐵 – Probability of physical damage to a structure (flashes to a structure);
𝑃𝐶 – Probability of failure of internal systems (flashes to a structure);
𝑃𝐸𝐵 – Probability reducing 𝑃𝑈 and 𝑃𝑉 depending on line characteristics and withstand voltage of
equipment when EB is installed;
𝑃𝐿𝐷 – Probability reducing 𝑃𝑈 , 𝑃𝑉 and 𝑃𝑊 depending on line characteristics and withstand voltage of
equipment (flashes to connected line);
𝑃𝐿𝐼 – Probability reducing 𝑃𝑍 depending on line characteristics and withstand voltage of equipment
(flashes near a connected line);
𝑃𝑀 – Probability of failure of internal systems (flashes near a structure);
𝑃𝑀𝑆 – Probability reducing 𝑃𝑀 depending on shielding, wiring and withstand voltage of equipment;
𝑃𝑆𝑃𝐷 – Probability reducing 𝑃𝐶 , 𝑃𝑀 , 𝑃𝑊 and 𝑃𝑍 when a coordinated SPD system is installed;
xv
𝑃𝑇𝐴 – Probability reducing 𝑃𝐴 depending on protection measures against touch and step voltages;
𝑃𝑈 – Probability of injury to living beings by electric shock (flashes to a connected line);
𝑃𝑉 – Probability of physical damage to a structure (flashes to a connected line);
𝑃𝑊 – Probability of failure of internal systems (flashes to connected line);
𝑃𝑋 – Probability of damage relevant to a structure;
𝑃𝑍 – Probability of failure of internal systems (flashes near a connected line);
𝑟𝑡 – Reduction factor associated with the type of surface;
𝑟𝑓 – Factor reducing loss depending on risk of fire;
𝑟𝑝 – Factor reducing the loss due to provisions against fire;
𝑅 – Risk;
𝑅𝐴 – Risk component (injury to living beings – flashes to structure);
𝑅𝐵 – Risk component (physical damage to a structure – flashes to a structure);
𝑅𝐶 – Risk component (failure of internal systems – flashes to structure);
𝑅𝑀 – Risk component (failure of internal systems – flashes near structure);
𝑅𝑆 – Shield resistance per unit length of a cable;
𝑅𝑇 – Tolerable risk;
𝑅𝑈 – Risk component (injury to living being – flashes to connected line);
𝑅𝑉 – Risk component (physical damage to structure – flashes to connected line);
𝑅𝑊 – Risk component (failure of internal systems – flashes to connected line);
𝑅𝑋 – Risk component for a structure;
𝑅𝑍 – Risk component (failure of internal systems – flashes near a line);
𝑅1 – Risk of loss of human life in a structure;
𝑅2 – Risk of loss of service to the public in a structure;
𝑅3 – Risk of loss of cultural heritage in a structure;
𝑅4 – Risk of loss of economic value in a structure;
𝑅4′ – Risk 𝑅4 when protection measures are adopted;
𝑆 – Structure;
𝑆𝑀 – Annual saving of money;
𝑆1 – Source of damage: Flashes to a structure;
𝑆2 – Source of damage: Flashes near a structure;
𝑆3 – Source of damage: Flashes to a line;
𝑆4 – Source of damage: Flashes near a line;
𝑡𝑧 – Time in hours per year that persons are present in a dangerous place;
𝑇𝐷 – Thunderstorm days per year;
𝑈𝑊 – Rated impulse withstand voltage of a system;
𝑤𝑚 – Mesh width;
𝑊 – Width of structure;
𝑊𝐽 – Width of the adjacent structure.
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1. Introduction
1.1. Overview
The first concept of a lightning protection system (LPS) takes back to the middle of the 18th century
with Benjamin Franklin. His electrical experiences led to the invention of the lightning rod. He assumed
that this could help protect buildings from lightning by attaching "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 of the
building into the ground” [5].
Since then, the lightning protection system has evolved and today consists in 2 components – an
internal and an external protective system.
The external protection is composed by 3 interconnected systems:
(1) Air-termination system;
(2) Down-conductor system;
(3) Earth-termination system.
So, over 250 years later, Franklin’s concept of an LPS is fundamentally the same as the todays
external LPS, which has the function to protect structures from direct lightning strikes.
The introduction and constant improving of the internal lightning protective system is mainly due to the
advance of technology that includes equipment operating at very low voltages which can be easily
damaged with an overvoltage. As these equipment are internally connected and ultimately connected
with exterior line(s), the internal LPS has the goal to prevent dangerous sparking from happening
within the structure, protecting both equipment and more importantly, human beings.
Taking these concepts (internal and external LPS), the International Electrotechnical Commission
(IEC) Standard 62305 ed. 2.0 (2010) – Protection against lightning [1],[2],[3],[4] – uses the following
definitions:
- An external LPS is intended to intercept a lightning flash to the structure with an air-termination
system (1); conduct the lightning current safely towards earth using a down-conductor system (2); and
disperse the lightning current into the earth using an earth-termination system (3).
- An internal LPS prevents dangerous sparking within the structure using either equipotential bonding
or a separation distance (providing electrical insulation) between the external LPS components and
other electrically conducting elements internal to the structure.
Also, the distinction between direct and indirect lightning flashes also occurred much later. An indirect
flash – near the structure to be protected – has lightning current that induces high voltage and a strong
electromagnetic field that generate powerful electric pulses that can damage sensitive electronic
devices.
1
The knowledge of the nature of lightning and understanding the physical phenomena is a crucial step
in trying to find efficient protection systems against the hazardous effects of lightning strokes. Based
on that knowledge, experience and field experiments turn lightning protection systems in a very
important measure that should be implemented in every structure.
As the aim of this work is to develop a computer program that makes the risk assessment of a
structure based on IEC 62305, it is important, on a first approach, to analyze this standard.
It is a four part standard prepared by the IEC technical committee 81: Lightning protection.
The first part provides general principles to be followed for protection of structures against lightning,
including their installations and contents, as well as persons [1].
The need for protection, the economic benefits of installing protective measures and the selection of
adequate measures should be determined in terms of risk management.
So, risk management is the subject of IEC 62305-2 [2] and the suggested protection measures
considered in the standard are proved to be effective in risk reduction.
All measures for protection against lightning form the overall lightning protection. For practical
reasons, the standard is divided in two separated parts according to the criteria for design, installation
and maintenance of lightning protection measures:
– the first part, concerning protection measures to reduce physical damage and life hazard in a
structure is given in IEC 62305-3 [3];
– the second part, concerning the system protective measures (SPM) for reduction of failures of
electrical and electronic systems in a structure is given in IEC 62305-4 [4].
Figure 1 demonstrates schematically this selection and separation of criteria over the four parts of the
standard.
Figure 1: Organization of the four parts of IEC 62305 – Protection against lightning (from IEC 62305-1 [1])
2
1.2. Objectives
The need of lightning protection is becoming an imperative requisite for the protection of structures
and its contents. A possible response for this rising of case studies projects is the development of a
practical and intuitive program for risk assessment of structures.
So, the main objective of this work is to develop a user-friendly computer program that supports the
lightning protection engineer by making a risk assessment of the structure to be protected, based on
IEC 62305. By accomplishing this, the process of protecting a structure from the effects of lightning
becomes much faster and intuitive.
It allows a simple and practical input of the various parameters that characterize the structure, its
contents, and its surroundings according with the standard IEC 62305-2 [2]. With these inputs the
program calculates, in final instance, the value of the annual average probability of occuring a loss in
the structure to be protected, also called Risk.
As it will be explained later, this value of the risk will be higher or lower than a tolerable value of risk
associated with the mentioned loss. Being higher, the program gives a series of possible protective
measures in order to lower the value of the risk to tolerable values.
All these parameters that compose the calculation of the risk value are characterized in the standard.
For that reason it is essential to understand the standard and how its application in the design of
lightning protection measures leads to safer structures against the hazardous effects of lightning
discharges.
1.3. Outline
The outline of this work follows the steps to achieve the main objective of this work – the computer
program. To accomplish that, it is imperative to comprehend the IEC standard for lightning protection,
the IEC 62305, how it evolved and what brought new and finally compare it with similar standards
applied all around the world. Then characterize the lightning discharge phenomenon, which was very
difficult for the scientific community to fully understand until the middle of the 20th century. After that,
present the methodology that the IEC 62305 uses to calculate the value of the risk and finally apply it
to the program, leading to fast and easy answers. The features of the program are shown in two
different examples.
•
Chapter 1. Introduction: introduction of the work with a brief presentation of the standard IEC
62305. It is explained the need of lightning protection systems and two kinds of solutions according to
the type of damage: an LPS for reducing physical damage and life hazard in a structure and SPM to
reduce failures of electrical and electronic systems in a structure.
It is introduced also the objectives and this outline of the work.
3
•
Chapter 2. Overview of international lightning protection standards: before and after the
lightning protection standard IEC 62305. The new approaches with the introduction of the two parts
referring to the effects of indirect lightning discharges and mainly the new methodology for risk
assessment, are discussed. Then, it is made an overview of some international lightning protection
standards by mainly locating the differences and understand some of the choices adopted by the
standard IEC 62305 technical committee.
•
Chapter 3. General characterization of lightning discharges: Four topics are presented: the
lightning discharge phenomenon itself; the description of the lightning current parameters; the used
cumulative statistical distributions of the lightning peak current; the recent work developed in direct
current measurements.
•
Chapter 4. Lightning incidence models adopted by IEC 62305: in this chapter the main subject
is the characterization of the lightning incidence models adopted by the standard – the
Electrogeometric Model (EGM) and the Electrical Shadow Model. The motivation, the choice and the
implementation of these models in lightning protection are the main points of this chapter.
These three last chapters are preceded of a short historic overview for better understanding the
present results and conclusions related with these subjects.
•
Chapter 5. Risk Management: source, damage, loss and risk are some of the concepts that
are essential to evaluate the structure’s risk. The relation between them results in a methodology that
gives ultimately the value of risk.
This methodology is based on various risk components that composes the calculation of the risk of the
structure. So, in this chapter is performed a risk assessment where these components are extensively
explained.
Finally, a series of risk mitigation measures are introduced based on their effectiveness in the
reduction of the risk value.
•
Chapter 6. Application of the computer program – : using the developed program L.R.A. –
Lightning Risk Assessment – two examples, a Hospital and the South Tower of Instituto Superior
Técnico, are shown to verify the results presented in the standard IEC 62305. They were chosen due
to its complexity which will illustrate most of the features of the program.
4
2. Overview of international lightning protection standards
In this chapter the goal is to understand the worldwide role of the IEC standard and what it brought
new to lightning protection of structures.
Then, in order to understand a few of the different choices implemented by some countries, examples
of national lightning protection standards are given.
Finally, a few insights and possible explanations are discussed about the differences between IEC
62305 and some nationals lightning protection standards.
2.1. IEC standard 62305
The IEC (International Electrotechnical Commission) is a worldwide organization for standardization,
comprising all national electrotechnical committees with the objective of trying to harmonize the
different national standards.
So, first of all, it is to accept that the guidelines in an international standard cannot be perfectly
accurate for all the regions around the world and for that reason there is space for national committees
to change some parameters for their benefit and, in that way, obtain better and more efficient lightning
protection in their countries.
The IEC 62305, firstly elaborated in 2006 and then revised in 2010, is the standard intended to
address the subject of Protection against lightning.
2.1.1. Before IEC 62305
Before IEC 62305, lightning protection was made based on the standard IEC 61024 – “Protection of
structures against lightning”. In this standard the lighting protection measures were intended to protect
structures and its contents against direct lightning flashes. So, only injuries to living beings by electric
shock and physical damages to the structure were taken in account IEC 61024 [6],[7],[8].
Moreover, the only real implemented measure against the hazardous effects of lightning that the
standard IEC 61024 predicted was the design and construction of a lightning protection system (LPS)
and it was ruled by a set of construction rules according to a desired lightning protection level (LPL)
𝐼 − 𝐼𝑉.
So, according to IEC 61024 [6], the goal of the LPS consisted on the protection of the structure and its
contents against direct lightning flashes, which are the requirements of the, nowadays known, external
LPS. Also, the effects of indirect flashes were not taken into account.
The challenge then was to determine an appropriate lightning protection level (LPL) for the structure to
be protected. So, a lightning risk assessment was needed to be created. According to IEC 61024 [7],
5
the method for determining the LPL of the LPS was based on an efficiency factor, 𝐸, firstly introduced
by this standard. It was defined to show the percentage of possible flashes, which could be controlled
by the LPS without resulting in damage. For example, an LPS with an LPL I had an efficiency of 98%
of controlling direct lightning to the structure without resulting in damage (table 3 from [7]).
The procedure for obtaining the efficiency, 𝐸, was based in two concepts: 𝑁𝑁𝑑 - expected direct
lightning strikes at the structure; and 𝑁𝑁𝑐 - acceptable damage frequency. The first is related with the
local flash density (𝑁𝑁𝐺 ) and geometric measures of the structure and its surroundings; and the second
in five coefficients that ultimately describes the structure’s vulnerabilities, according to IEC 61024: the
revetment material of the facade and roof; the content of the structure; the number of people inside the
structure; the consequences of lightning in the service and the environment; and the emergency
service response.
If 𝑁𝑁𝑑 < 𝑁𝑁𝑐 then no protection is needed or the protection is optional. Otherwise, if 𝑁𝑁𝑑 ≥ 𝑁𝑁𝑐 , protection
is required. The protection levels will be determined by the ratio between 𝑁𝑁𝑐 and 𝑁𝑁𝑑 (i.e., calculation of
effectiveness). Equation (1) shows how to obtain the efficiency 𝐸:
𝐸 = 1 − 𝑁𝑁𝑐 /𝑁𝑁𝑑
(1)
So, at the time, the big achievement was the creation of LPS classes – the lightning protection levels
(LPL) through the new concept, the efficiency factor. This helped in the standardization of the LPSs,
leading to cheaper solutions. In the end, the lightning risk assessment made in IEC 61024 was only
the determination of the lightning protection level of the lightning protection system, which is today just
a part of the risk assessment presented in IEC 62305, the standard that followed IEC 61024.
For the particular case of Portugal, the ‘Direção Geral de Geologia e Energia’ (DGE) made a guide for
the design, construction and maintenance of LPSs installed in structures up to 60 𝑚𝑚, with the same
bases as the standard IEC 61024 – the ‘Guia Técnico de Pára-Raios’ [9]. However, instead of using
the lightning risk assessment present in the IEC 61024, this guide bases the need of a lightning
protection system in a structure on a table (table 1 from [9]). According to this table, with the height
and the type of construction of the structure in one hand, and the structure’s susceptibility for damage
on the other hand, it is possible to determine the necessity for the installation of an LPS. It is only
applicable to the so-called “conventional” systems: Faraday cages and Franklin rods.
So in the end, before IEC 62305, if the structure had an appropriate LPS installed, it was considered
sufficient to secure the lightning protection of a structure. In most of the cases, this premise was valid
because the “simplified risk calculations” shown above gave overestimated solutions.
6
2.1.2. New approach with IEC 62305
As it was said, the standard IEC 62305 is formed by 4 parts for protection against lightning, covering
the external and internal structure and direct and indirect effects of lightning – structure and its content
and electrical and electronic systems.
The introduction of the third and fourth parts related to “Physical damage to structures and life hazard”
– IEC 62305-3 [3] and “Electrical and electronic systems within structures” – IEC 62305-4 [4],
respectively, permit to appear a wide range of lightning protection measures, each one very focused to
a particular gap in lightning protection of the structure.
With those measures, the engineers had the tools to protect any structure. Then, the problem was to
know which measures were the most adequate to implement in a specific structure.
To determine the right lightning protection measures to use, the IEC 62305 created a new
management method presented in “Risk Management” – IEC 62305-2 [2]. It is used to find the value
of the risk of a structure against lightning; compare that value of risk to a risk tolerable value; and to
select precise protection measures in the cases that the calculated risk is bigger than the tolerable
value.
In the process, the concept of lightning protection zones, LPZs, turned the risk evaluation much more
efficient, because they permit to divide the structure to be protected into a number of zones according
to the level of threat posed by Lightning Electromagnetic Pulses (LEMP). The general idea is to
identify or create zones within the structure where there is less exposure to some or all of the effects
of lightning and to coordinate them with the immunity characteristics of the electrical or electronic
equipment installed within the zone.
So, the IEC 62305 permits to analyze the structure as a whole or by zones, which can turn the
lightning protection process more efficient, and ultimately, cheaper to implement.
This methodology is going to be extensively debated in chapter 5, Risk Management.
Recapping the upsides of the newer standard, it can be said that the great advantages of the IEC
62305 can be summarized in two big upgrades.
(1) Unlike the previous standard, that only assesses the structure’s overall lightning risk, by installing
an external LPS of an appropriate LPL, with IEC 62305, in addition to that, the internal part of the
structure is also analyzed. So, it permits to separate the LPS design in two different approaches:
external and internal LPS.
The main responsible for this achievement is the introduction of the two separated parts 3 and 4 of
IEC 62305 with a wide set of measures for lightning protection; and the new concept of lightning
protection zones, LPZs.
(2) The second big upgrade is the risk management methodology described in the second part of the
standard. As it going to be seen on the next sections, the weighting factors and probabilities that
translate the characteristics of the structure and its contents were open to a very subjective reasoning
7
by the different national committees. So, the IEC 62305 standardized these values and a methodology
which “always” gives a safe solution for lightning protection.
It is to remind again that the guidelines on IEC 62305 are general guidelines which still give a margin
to national’s lightning protection committees to alter some of the factors and probabilities values in
order to better translate its national requirements.
2.2. Different approaches by national lightning protection standards
The lightning is a non-predictable phenomenon and for that reason there is no possibility to develop a
model in laboratory and apply that model to structures everywhere. Therefore, the national standards
from country to country are not the same due to the changing requirements of the different regions.
There are two things that must be in mind when looking to worldwide lightning protection standards.
First the differences between the many national standards that adopted the methodology presented in
IEC 62305; and second the discussion regarding conventional and un-conventional lightning airterminal systems.
2.2.1. Differences between national lightning protection standards and the IEC 62305
The acceptance of most of the scientific community of the new methodologies introduced by the
standard IEC 62305 was not immediate.
For some years, some national standards maintained its methods. Some standards didn’t even
include the approach of the concept so-called as “collection area”. This area can be defined as the
ground area (around and including the structure) that have the same yearly direct lightning flash
probability as the structure. Also, the analysis of failure of electrical and electronic systems was still
non-existent. Therefore, the effects of indirect flashes were not taken into account.
With this risk analysis, the resultant values of the overall risk were many times inaccurate and over
dimensioned.
An example of those standards was the American standard – NFPA 780 [10], that consisted its risk
analysis in weighting factors as the ones discussed in section 2.1.1. for IEC 61024. The American
standard only adopted the detailed risk assessment implemented by IEC 62305 on the upgrade
edition of 2011 [11], five years after the first edition of the IEC 62305 in 2006.
The big differences that can be found are present in the weighting factors that characterize the
structure and its contents. Depending on the country needs and priorities the factors can assume
different weights in the risk calculation.
8
For example, if some standards are compared, like the British and American standards, they give
special importance to public health, children and historic structures while in the Spanish and Turkish
standards, priority is given to the environmental effects of lightning such as explosions [10].
2.2.2. Conventional and un-conventional lightning air-termination systems
Besides the differences in the weighting factors values between some national lightning standards and
the IEC 62305, there is a big controversy around the lightning air-termination systems used for
lightning protection.
First of all, as it was said in the introductory section, an air-termination system is an integrated part of
an LPS responsible to intercept lightning flashes using metallic elements such as rods, mesh
conductors or catenary wires that considerably decrease the probability of the structure being directly
hit by lightning.
This is the “conventional” lightning protection air-termination system approved by IEC 62305-3 [3].
In the 1970s, two types of un-conventional air terminals had been commercially reinvented and
introduced in the world market. They are the lightning prevention air terminal and the lightning
attracting air terminal.
As their names imply, the lightning prevention systems are claimed to be able to prevent lightning from
occurring, hence protect the building and the lightning attracting air terminals which claimed to be able
to attract the lightning to it, hence away from the structure and so, protect the structure where it was
installed on.
The following two examples represent a lightning attracting and lightning prevention air-termination
systems, respectively:
Early Streamer Emission (ESE): More similar to the conventional lightning rod, ESE systems are
lightning attractors. According to their manufacturers, they are designed to trigger the early initiation of
upward streamers, which increases the efficiency of lightning attraction as a way to extend the
effective range of protection far beyond that of lightning rods. ESE air terminals can typically be
distinguished from ordinary lightning rods due to the presence of a small object near the top, a
discharge trigger, and they also can be more geometrically complex. This discharge trigger increases
the probability for initiating an upward streamer discharge at or near the tip of the rod when an
downward leader approaches. Increasing the probability of streamers and leaders meeting is how
ESE systems serve as improved lightning attractors.
However, according to the National Institute of Standards and Technology, it is difficult to judge ESE
performance: “It is nearly impossible to make quantitatively meaningful statements on the relative
performance of ESE devices and conventional Franklin rods.” [12].
Charge Transfer System (CTS): Unlike lightning “attractors”, CTS is specifically designed to prevent a
lightning strike from terminating where it is not wanted – in a designated area of protection. This is the
9
only system in which lightning strikes are actively discouraged, rather than encouraged. CTS
technology is based on existing physics and mathematical principles.
In order to prevent lightning from striking within a specified zone, a CTS collects the induced charge
from thunderstorm clouds within this area and transfers it through an ionizer into the surrounding air,
thus reducing the electric field strength in the protected zone. The resulting reduced electrical potential
difference between the site and the cloud suppresses the formation of an upward streamer. With no
leader-streamer connection, the strike is prevented.
Even NASA conducted a series of studies using this concept in a variety of towers in the 1970s
decade to ultimately protect space shuttles. These studies found that the frequency of lightning to the
towers with CTSs was not significantly different than to those without CTSs. In short, the studies
demonstrated that a CTS did not prevent or significantly reduce the probability of lightning strikes to a
tower [13].
So in the end, the inventors of these un-conventional air terminals, ESE and CTS, have never been
capable to provide any sustainable scientific basis for their inventions. For this reason, these inventors
and manufacturers have not been able to get their un-conventional air terminals approved by the
general scientific community. So, a standard that uses LPS with these air terminals have been
classified as “non-standard” and the air-termination systems have the tag of “un-conventional” by
academics, scientists and the various standards bodies around the world.
However, despite of the rejection of these systems, there are several national standards [14], as
France [15], Spain [16], Portugal [17], Argentina, Macedonia, Romania, Slovakia and Serbia that
adopt one of the mentioned un-conventional lightning air-termination system – the Early Streamer
Emission air-termination system – ESE system.
The result is that the experience is dictating that the ESE concept does not give additional protection
than the conventional lightning rod and there are cases where its installation resulted in damage to the
structure under lightning conditions. For example, in Malaysia, the use of the un-conventional air
terminals has led to many buildings being damaged by direct lightning strikes [18]. Some of these
reported damages were found to be life threatening since they occur in buildings used to mass
occupation eg. a school building.
10
3. General characterization of lightning discharges
Since lightning has been seen by virtually everyone, one might think that lightning is very well
understood. That is not the case. Lightning’s seemingly random occurrence in space and time and the
wide range of its significant time variation, from tens of nanoseconds for many individual processes to
almost a second for the total discharge, and its obscuration by the thundercloud producing it makes
lightning particularly difficult to study. Nevertheless, more than a century of measurement has
produced a relatively complete picture of the phenomenology of lightning.
This chapter is related with the characterization of lightning discharges. There are 2 subjects that have
to be addressed to accomplish this:
1. The lightning discharge phenomenon, addressed in section 3.1.;
2. The characterization of the lightning discharge current, which includes the definition of the lightning
current parameters by recent works, addressed in section 3.2..
3.1. The lightning discharge phenomenon
There are various lightning discharges generated in Earth’s atmosphere such as the ones originated in
cumulonimbus clouds, volcanic eruptions, dust storms and snow storms [19]. In this work, the study of
the lightning discharges is confined to the ones produced by cumulonimbus clouds.
These clouds can extend by 14 𝑘𝑚𝑚 in altitude from its base, normally at 2 𝑘𝑚𝑚 in altitude [20]. Due to its
great extension, high currents of ascending air are formed, making the water droplets turn into ice
crystals with the increasing altitude which, by colliding with each other, create positive and negative
charges. The classification of lightning discharges is based on the observed polarity of the charge it
brings to ground and the direction of propagation of the initial leader.
So, first it is important to distinguish the different types of discharges [19]: flashes that make contact
with the ground are referred as ground flashes and flashes that do not make contact with the ground
are referred as cloud flashes.
The cloud flashes are divided in three categories: intracloud flashes, air discharges and intercloud
flashes, as are illustrated in Figure 2 (a). These flashes do not have any interaction with the ground
and by that will not be discussed any further.
The ground flashes are divided in four categories according with the polarity of the charge it brings to
the ground (from the cloud) and its point of initiation. So, there are downward negative ground flashes,
downward positive ground flashes, upward positive ground flashes and upward negative ground
flashes, as illustrated in Figure 2 (b).
The form of lightning discharge most frequently examined is the negative cloud-to-ground
lightning (negative downward lightning), as more than 90 % of all flashes meet this type of discharge
[21], [22].
11
Figure 2: (a) Types of cloud flashes: (i) intracloud; (ii) air discharges; (iii) intercloud; (b) Types of ground flashes:
(i) downward negative ground flashes; (ii) downward positive ground flashes; (iii) upward positive ground flashes;
(iv) upward negative ground flashes, from [19]
Therefore it is of fundamental importance for the design of lightning protection systems and the only
type of lightning discharge studied in this work.
So, it is important to explain this type of lightning discharge - negative downward lightning:
1. A downward negative lightning flash is initiated by a negative charged leader that travels from cloud
to ground in a stepped manner. As the distance between the ground and the downward stepped
leader becomes smaller, the opposite of charges enhances the strength of the electric field at ground
level leading its value to a steady increase. Due to the ground field enhancement, the electric field at
the pointed tips of a grounded structure may reach values which are several times the magnitude of
the electric field produced by the stepped leader [23];
2. When the stepped leader reaches a height of about a few hundred of meters or less above ground,
an upward positive leader discharge is incepted from the structure. This leader, created by the action
of the electric field generated by the stepped leader is called a connecting leader [24]. According with
[25], Golde assumed that upward streamers from the ground are produced when the electric field due
to the descending leader space charge reaches 300 𝑘𝑉/𝑚𝑚, assuming an average peak current of
25 𝑘𝐴. Later, with Berger’s measurements, the average value of the peak current was defined in
30 𝑘𝐴, and so the needed induced electric field reached the value of 500 𝑘𝑉/𝑚𝑚.
Once incepted, the connecting leader starts to grow toward the down coming stepped leader.
3. The moment a connection is made between the stepped leader and ground (via connecting leader),
a wave of near ground potential (or zero potential) travels at a very high speed along the channel
towards the cloud. The current associated with this wave heats the channel to several tens of
12
thousands of degrees Kelvin, making the channel luminous. This event is called the return stroke. The
point of attachment of the downward negative lightning flash on the structure is the point of initiation of
the connecting leader that made the final connection with the stepped leader.
4. Subsequent discharges are likely to occur as well. About 80% of the lightning discharges have
subsequent discharges (two or more stokes), where 90% does not overcome 8 discharges and the
mean value is 3 to 5 [22].
The detailed mechanisms of downward positive ground flashes are outside the scope of this work
because their main features are qualitatively similar to those of downward negative ground flashes,
mentioned above, with differences in the finer details.
Upward lightning is typically object initiated. It primarily occurs when there is a nearby cloud to ground
flash. The electric field change caused by the preceding flash causes an upward positive leader to
initiate from a tall object such as a building, tower or wind turbine.
3.2. Current parameters
The knowledge of the current parameters is of primary importance for lightning protection for a number
of reasons. In particular, for a better understanding of the physics of the lightning discharge, for
evaluating its electromagnetic effects, for an effective protection design of electric and electronic
systems and for the risk evaluation related to lightning damages and consequential losses.
It is well known that most of the hazardous lightning effects are caused fundamentally by the return
stroke current. Therefore, the knowledge of the parameters of this current has major interest for
lightning protection. So, in accordance with various researchers and standards, the most important
properties of lightning current that cause damage on the structure to be protected and/or its contents
are: the peak current, the maximum current steepness, the charge and the specific energy, all derived
from direct current measurements (measurements realized in instrumented towers) [1], [19], [22],
[26].
The concepts of protection and damage will be extensively debated along the work.
3.2.1. Peak current, 𝑰
As it was said, the current associated with the return stroke has large values (~𝑘𝐴), that potentially
can be very dangerous to various systems both outside and inside of the structure. The name of this
current is peak return stroke currents, or simply peak current.
The value of the peak current is even more important in cases where the struck object essentially
presents a resistive load because the voltage of the object or system will be proportional to the current
(𝑉 = 𝑅. 𝐼). Examples of those objects can be the surge impedance of a long power line, a tree, ground
rods driven into earth, and so on. In a case when a lightning return stroke with a peak current of 30 𝑘𝐴
strikes a power line with a surge impedance of 400 Ω, it can produce a prospective overvoltage of
6000 𝑘𝑉, assuming division of current. This large voltage can cause flashover across insulators, from
13
line to ground, to adjacent lines and to other objects nearby, leading to possible loss of service of
these lines and ultimately of services in the structure.
As it will become clear on section 4.2., the discovery of the relation between the peak current and the
striking distance dictated a revolution in lightning protection systems effectiveness.
𝒅𝒊
3.2.2. Maximum current steepness, � �
𝒅𝒕 𝒎𝒂𝒙
The occurrence of damage of internal systems due to indirect effect of lightning discharges, has
assumed a lot of importance in recent times, essentially due to the increasing sensitive electric and
electronic equipment which are highly vulnerable to such electromagnetic effects. These electronic
devices are normally connected to different electrical services such as the mains supply and the data
link which, depending on the line routing and grounding inside the structures, can built up large openloop networks.
The maximum current steepness is responsible for the maximum of the magnetically induced voltages
in such open loops. Just need to see the following example: for objects that present an essentially
inductive impedance, such as, under some circumstances, wires in an electronic system, the peak
voltage will be proportional to the maximum rate-of-change of the lightning current (𝑉 = 𝐿 𝑑𝑖/𝑑𝑡,
where 𝐿 is the inductance of the length of wire and 𝑉 is the voltage difference between the two ends of
the wire). So, the higher the current steepness is, the greater the induced voltages are going to be –
main responsible for electric and electronic systems failure, which is one of the focuses present in the
risk management chapter.
3.2.3. Charge, 𝑸 = ∫ 𝒊. 𝒅𝒕
The charge 𝑄 is responsible for melting effects at the attachment points of the lightning channel.
Charge is the integral of the current over time, so most of the charge induced damages in structures or
lines are due to the so-called “long” continuing current that follows some of return strokes. Even a big
return stroke that lasts perhaps a few tens of microsecond may not transfer as much charge
transferred as a low-level (100 – 1000 𝐴) continuing current that lasts a few tens to hundred
milliseconds [26].
So, for example, if a lightning discharge hits a roof structure with a thin metal sheet or a combustible
surface material (e.g. a wooden roof; a thatched roof), fire hazards and melting effects are very real
possibilities.
For this parameter there are some relevant distinctions that need to be made. There is the charge
referent to the entire lightning flash duration, 𝑄𝐹𝐿𝐴𝑆𝐻 , which includes almost every time numerous
impulses; and the charge for the duration of a single impulse, 𝑄𝑆𝐻𝑂𝑅𝑇 .
3.2.4. Specific energy, 𝑾/𝑹 = ∫ 𝒊𝟐 . 𝒅𝒕
The specific energy is represented as the time integral of the square of the current. Also known as the
action integral, the specific energy is a measure of the heat generated by a lightning strike in an object
of resistance 𝑅. So, it is responsible for the heating and melting effects arising when the lightning
current flows through a metallic conductor of a certain cross-section and material. These two
14
parameters (material and cross-section of the conductor) are the ones that need more attention when
dimensioning the conductors subjected to lightning strikes effects.
For example, in a case where a conductor is poorly dimensioned (bad conducting materials and/or
with thin cross-section), the heat generated by the lightning strike can vaporize the internal material,
as aluminum or copper, resulting in gas pressure that causes damages on the conductor.
In Table 1 and Figure 3 it can be seen the definition and representation of the different lightning
current waveform parameters, respectively.
Table 1: Definition of the different lightning current parameters
𝐼 [𝑘𝐴]
𝑑𝑖
[𝑘𝐴/𝜇𝑠]
� �
𝑑𝑡 𝑚𝑎𝑥
𝑊/𝑅 [𝑘𝐽/Ω]
𝑄𝐹𝐿𝐴𝑆𝐻 [𝐶]
𝑄𝑆𝐻𝑂𝑅𝑇 [𝐶]
𝑇 [𝑚𝑚𝑠]
𝑇1 [𝜇𝑠]
𝑇2 [𝜇𝑠]
𝑂1
Current peak value: Maximum value of the lightning current
Maximum current steepness
Specific energy for the duration of the impulse
Flash charge: Value resulting from the time integral of the lightning
current for the entire lightning flash duration
Impulse charge: Value resulting from the time integral of the
lightning current in an impulse
Flash duration: Time for which the lightning current flows at the point
of strike
Front time of impulse current
Time to half value on the tail of impulse current
Virtual origin of impulse current: Point of intersection with time axis
of a straight line drawn through the 10 % and the 90 % reference
points on the stroke current front.
Figure 3: Impulse lightning current parameters (typically 𝑻𝟐 < 𝟐 𝒎𝒔) from IEC 62305-1 [1]
15
3.3. Cumulative statistical distributions of lightning peak currents
Analyzing the above four parameters allows to say that the value of the current is the common
parameter among the four and for that, the main one. So, since early time in lightning current studies,
became clear the need to characterize this current. And that was part of the legacy of K. Berger. His
direct current measurements made on the lightning measurement station tower on Monte San
Salvatore, in Switzerland (1975) [27] still remain today the primary reference for both lightning
research and lightning protection studies. Also, recommended lightning current waveform parameters
are based on Berger data, although the current rate-of-rise parameters estimated by Anderson and
Eriksson (1980) [28] from Berger’s oscillograms are likely to be significantly underestimated, due to
limitations of the used instrumentation [22].
Figure 4 introduces the cumulative statistical distributions of lightning peak currents, giving percent of
cases exceeding abscissa value, from direct measurements in Switzerland (Berger - 1975). The
distributions are assumed to be log-normal and given for (blue) negative first strokes �𝑁𝑁𝑠𝑎𝑚𝑝𝑙𝑒𝑠 = 101�,
(red) positive first strokes �𝑁𝑁𝑠𝑎𝑚𝑝𝑙𝑒𝑠 = 26�, and (green) negative subsequent strokes �𝑁𝑁𝑠𝑎𝑚𝑝𝑙𝑒𝑠 = 135�
[27]. The log-normal nature is due to the distributions being positively skewed, meaning that they
show long “tails” extending toward higher values.
In relation with the negative subsequent strokes, if additional negative charge is immediately available
in the cloud in the vicinity of the first return stroke, the additional charge may start to move toward
ground in the conductive path left by the first return stroke. These subsequent leaders are called dart
leaders. Because the channel is already established, the charge can move down the main channel in
a much smoother manner than the stepped leader, and at a much greater speed .
First stroke current peaks are typically a factor of 2 to 3 larger than subsequent stroke current peaks.
However, about one third of cloud-to-ground flashes contain at least one subsequent stroke with
electric field peak, and, by theory, current peak, greater than the first-stroke peak [22].
By observation of Figure 4 it can be seen that only a few percent of negative first strokes exceed
100 𝑘𝐴, while about 20% of positive strokes have been observed to do so. On the other hand, it is
thought that less than 10% of global cloud to ground lightning (ground flashes) has positive charge.
So, generally only downward negative first strokes flashes are taken into account in lightning
performance studies considering that [1], [28]: i) upward flashes occur mainly from very tall structures
or mountain-top installations; ii) the majority of downward flashes are of negative polarity, which in IEC
62305 assumes a polarity ratio of 10 % positive and 90 % negative flashes and iii) subsequent stroke
peak current is much less severe in terms of peak current.
16
Figure 4:
Cumulative
statistical distributions of lightning peak currents, giving percent of cases exceeding abscissa value (from [22])
About 95% of negative first strokes are expected to exceed 14 𝑘𝐴, 50% exceed 30 𝑘𝐴, and 5% exceed
80 𝑘𝐴. The corresponding values for negative subsequent strokes are 4.6, 12, and 35 𝑘𝐴, and 4.6, 35,
and 250 𝑘𝐴 for positive strokes. Subsequent strokes are typically less severe in terms of peak current
and therefore often neglected in lightning protection studies.
There are two main distributions of lightning peak currents for negative first stroke adopted by lightning
protection standards: the International Council on Large Electrical Systems (CIGRE) distribution and
Institute of Electrical and Electronics Engineers (IEEE) distribution. Figure 5 shows the comparison
between the negative first stroke lightning current distributions adopted by CIGRE and IEEE.
For the CIGRE distribution (see Figure 5) 98% of peak currents exceed 4 𝑘𝐴, 80% exceed 20 𝑘𝐴, and
5% exceed 90 𝑘𝐴.
For the IEEE distribution (see Figure 5), the probability to exceed a certain value of current between 5
and 200 𝑘𝐴 is ruled by Equation (2):
𝑃(𝐼) =
Where 𝑃(𝐼) is in per unit and 𝐼 is in 𝑘𝐴.
1
(2)
𝐼 2.6
1+� �
31
17
Figure 5: Cumulative frequency of the current peak of the negative first stroke according to CIGRE
(from CIGRE WG 33.01 [29])
In IEC 62305, the CIGRE distribution is the one adopted. Between the two distributions presented
there is not much difference in the range of 10 − 100 𝑘𝐴. For higher peak current values, IEEE Std.
1243 – 1997 recommends the use of the CIGRE distribution [22]. Outside this range, due to data
insufficiency, it is not clear which distribution is better fitted to describe this phenomenon.
3.4. Recent work in direct peak current measurements
Berger’s measurements are still the most reliable measurements ever done. However, in his study,
neither the “zones” of very low and very high peak current values are well defined mostly because the
lack of data. So, in the following years after Berger measurements, the goal was to complement his
work by adding more samples of return peak current measurements. These studies were made
throughout the world and the results helped to verify and consolidate Berger’s measurements.
The revision made by CIGRE in 1991, which permitted the realization of the two above mentioned
distributions of lightning peak current for negative downward first stroke, included the measurements
made in Switzerland (𝑁𝑁 = 125), Australia (𝑁𝑁 = 18), Czechoslovakia (𝑁𝑁 = 123), Poland (𝑁𝑁 = 3),
South Africa (𝑁𝑁 = 81), Sweden (𝑁𝑁 = 14) and USA (𝑁𝑁 = 44) , resulting in a total sample size of 408
return peak current values.
18
Since then, direct current measurements on instrumented towers were made in Russia, South Africa,
Canada, Germany, Brazil, Japan, Austria, and again in Switzerland (on a different tower).
The results from some of these studies [22] are presented and compared with oldest measurements in
Table 2.
Looking at the mean peak current value, it can be seen that most of the measurements are consistent
with the data from Berger’s study, around 30 𝑘𝐴 [27], [28].
However, there are differences that should be mentioned:
- The Brazilian study presents a 45 𝑘𝐴 mean peak current value. There are some possible reasons to
explain the differences between Berger and the Brazilian studies, including: the relative small sample
size (38 samples) and the geographical location of Brazil, which can influence certain lightning
parameters. One way to maybe lower this value is to continue the measurements. In some studies,
the mean peak current value became smaller with the increasing of the overall sample size.
- The Japanese study is presently the study with the largest sample size of negative first strokes with
𝑁𝑁 = 120. The average peak current value was 29 𝑘𝐴, which is very similar with the 30 𝑘𝐴 from
Berger’s measurements.
Also, there were return peak current values, in both studies, higher than 100 𝑘𝐴, which helped
compensate the lack of data problem in this range of current values.
3.5. Other parameters derived from current measurements
As it was already said early in the work, there are various parameters that characterize the lightning
discharge current.
Besides lightning peak current, there are other lightning parameters that can be measure from the
studies above described, including the maximum current derivative (maximum current steepness), the
charge transfer, the specific energy and the current rise time.
Similar to the case of the peak current measurements, the most reliable and complete information on
these parameters is based on direct current measurements of K. Berger and co-workers in
Switzerland.
Table 3 shows the tabulated values (from CIGRE) of lightning current parameters which comprises the
negative and positive first stroke current and the subsequent short strokes current.
So, these parameters are the ones based on Berger data (1975), reviewed by CIGRE [27], [28] and
later adopted by the standard IEC 62305.
Also, these are the adopted parameters of the CIGRE log-normal distribution for the characterization
of the lightning current waveform parameters.
19
20
Switzerland
Italy
South Africa
Japan
Brazil
Anderon and Eriksson (1980)
Dellera et al. (1985)
Geldenhuys et al. (1989)
Takami and Okabe (2007)
Visacro et al. (2012)
CIGRE Report 63 (1991)
Anderson and Eriksson (1980)
Switzerland
Berger et al. (1975)
Switzerland (𝑁𝑁 = 125),
Australia (𝑁𝑁 = 18),
Czechoslovakia (𝑁𝑁 = 123),
Poland (𝑁𝑁 = 3), South
Africa (𝑁𝑁 = 11), Sweden
(𝑁𝑁 = 14), and USA
(𝑁𝑁 = 44)
Switzerland (𝑁𝑁 = 125),
Australia (𝑁𝑁 = 18),
Czechoslovakia (𝑁𝑁 = 123),
Poland (𝑁𝑁 = 3), South
Africa (𝑁𝑁 = 81), Sweden
(𝑁𝑁 = 14), and USA
(𝑁𝑁 = 44)
Location
References
−
9
−
408
21
10
7
338
38
120
29
14
42
80
14
31 (33)
30 (34)
45
29
33 (43)
33
31
30 (~30)
−
101
94
85
162
−
69
80
Percent exceeding tabulated value
𝟗𝟗𝟗𝟗%
𝟓𝟓𝟓𝟓%
𝟓𝟓%
101
Sample
size
Table 2: Comparison of return peak currents for first negative downward leaders flashes, from [22]
Same as Anderson and
Eriksson’s (1980) sample plus
70 additional measurements
from South Africa
Combined direct and indirect
measurements
Direct measurements on 70 𝑚𝑚
towers
Direct measurements on 70 𝑚𝑚
towers
Direct measurements on 40 𝑚𝑚
towers
Direct measurements on 60 𝑚𝑚
mast
Direct measurements on
40 to 140 𝑚𝑚 transmission line
towers
Direct measurements on 60 𝑚𝑚
mast
Remarks
The values between parentheses are related to the actual data. The values outside the parentheses
are the ones calculated with the log-normal distribution.
Table 3: Tabulated values of lightning current parameters taken from CIGRE [27], [28] and adopted by IEC
62305-1 [1]
Parameter
𝐼 [𝑘𝐴]
𝑄𝐹𝐿𝐴𝑆𝐻 [𝐶]
𝑄𝐼𝑀𝑃𝑈𝐿𝑆𝐸 [𝐶]
𝑊/𝑅 [𝑘𝐽/Ω]
𝑑𝑖
[𝑘𝐴/𝜇𝑠]
� �
𝑑𝑡 𝑚𝑎𝑥
𝑇1 [𝜇𝑠]
Values
50%
5%
4.9
11.8
28.6
1.3
7.5
40
Negative flash
1.1
4.5
20
Negative first stroke
16
150
Positive first short (single)
6
52
Subsequent negative short
65
Negative first stroke 𝑏
32
Positive first short (single)
4𝑎
4.6
20
35
90
Negative first stroke 𝑏
250
Positive first short (single)
350
0.22
0.95
4
6
55
2
0.55
25
3.5
30
Time interval [𝑚𝑚𝑠]
7
Negative first stroke
39.9
161.5
Subsequent negative short 𝑏
5.5
18
Negative first stroke
22
200
Positive first short (single)
32
140
Subsequent negative short
150
Multiple negative strokes
900
Negative flash (without single)
2.4
0.22
Subsequent negative short
Positive first short (single)
0.2
1.8
Positive flash
15000
24.3
9.9
550
Subsequent negative short 𝑏
650
9.1
6.5
duration [𝑚𝑚𝑠]
20 𝑎
80
Stroke duration [𝜇𝑠]
Total flash
Type of stroke
95%
1.1
75
4.5
Subsequent negative short
200
Negative first stroke
25
230
2000
Positive first short (single)
0.15
13
1100
Negative flashes (all)
85
500
Positive flash
31
14
33
180
𝑎
The values of 𝐼 = 4 𝑘𝐴 and 𝐼 = 20 𝑘𝐴 correspond to a probability of 98% and 80%,
𝑏
Parameters and relevant values reported in Electra No. 69
respectively
21
22
4. Lightning incidence models adopted by IEC 62305
In order to start this chapter and understand the need of a lightning incidence model, a historical
overview should be taken.
4.1. Historical overview
The first concept of lightning protection system takes back to the middle of the 18th century with
Benjamin Franklin. His electrical experiences led to the first “draft” of what it is now called as airtermination system. The protective effect of a lightning vertical conductor (rod) was expressed by a
protected area around it, whose radius is related to the height of rod. If a line is traced from the tip of
the rod towards the ground in all directions, which must include the structure to be protected, with a
certain angle, the protection volume given by this vertical conductor can be viewed as a cone.
Later based on Franklin’s approach, the interception method known today as protective angle method
based on the “Electric Shadow” model was formulated. The tangent of the protection angle – the apex
angle of the cone – equals to the ratio of the base radius of the cone and the rod height. This method
considers that any object located inside a zone limited by the envelope surface generated from a
lightning rod or a shielding wire to the ground, at a fixed angle to the vertical, is protected against a
direct lightning strike.
The protected space concept became particularly interesting with the spreading of the power
transmission lines at the beginning of 20th century. The lightning protection of these lines could be
favorably handled with the protection angle, because of their simple two-dimensional arrangement and
the fairly well statistical data available. Allying that to the lack of knowledge related to the physics of
the lightning discharge at the time, the Electric Shadow model, which depends only on the geometry of
the object to be protected, was the implemented model to design the interception systems used until
the decade of 1940.
However, in the middle of the 20th century, the increase of the power grid and the appearance of the
new 345 𝑘𝑉 transmission lines, led to recurrently flashovers across insulators, from line to ground, to
adjacent lines and to other objects nearby.
This led to a rush for a better understanding of the lightning discharge phenomenon.
Also, as it was the main problem at the time, the hazardous effects of lightning strikes were mainly
recorded in power transmission lines, so the studies of lightning protection of grounded structures
were brought to the fore by the implementation of these incidence models on power transmission
lines. Therefore, it is no surprise to see that the lightning incidence models applied to grounded
structures had its origin in the protection of power transmission lines.
23
F.S. Wagner et al [30], in 1941, was one of the pioneers of these studies. He started the study of
shielding failures in transmission lines. One of the reasons was that, by this time, the first 345 𝑘𝑉
transmission line systems began to appear in North America [31] and with that the need of lightning
protection of transmission lines gained a renewed importance.
Wagner’s experiences led to a better understanding of the lightning discharge phenomenon and in
1945, Golde determined for the first time the relation between the new concept called striking distance
and the lightning current, which concluded in the proposition of an incidence model called
Electrogeometric Model (EGM) [25].
Many researchers, including Wagner (in 1961 and in 1963), have studied and applied the EGM to
power systems: Young et al. (1963), Armstrong and Whitehead (1968), Brown and Whitehead (1969)
and later the IEEE working groups (1985 and 1997), have contributed to the EGM.
After more than 50 years the EGM and the Electric Shadow model are still the adopted models by
international standards in lightning protection of power transmission lines and (after) of grounded
structures and used all around the world.
4.2. The Electrogeometric Model (EGM)
4.2.1. Model description
The EGM describes the attachment of the downward leader to the grounded object and its core is the
concept of “striking distance”. The striking distance can be defined as the distance from the tip of the
downward leader to the object to be struck at the instant the upward connecting leader is initiated from
this object.
As it was mentioned in chapter 3, as the distance between the ground and the downward stepped
leader becomes smaller, the electric field at the top of the grounded structure increases steadily and
when it reaches a critical value an upward connecting leader arises from the structure.
With that, if a critical value of electric field is defined, the striking distance increases with the electric
charge of the downward leader, which is related with the return peak current value. Therefore, it can
be found a relation between the striking distance and the return peak current.
So, based on the electrogeometric model, to each lightning current peak value 𝐼, a length of the final
jump 𝑟𝑠 can be linked. Enormous research work on this subject was performed, for instance again in
the frame of CIGRE and IEEE. Nowadays, the most popular striking distance expression and
especially used in international lightning protection standards is represented by Equation (3).
𝑟𝑠 = 𝐴 × 𝐼𝑏
(3)
where 𝐼 is the first stroke peak current, in 𝑘𝐴, and 𝑟𝑠 is in meters [1]. The constants 𝐴 and 𝑏 are
empirical constants.
24
The procedure to obtain such expression typically involves assumptions of leader geometry, total
leader charge, distribution of charge along the leader channel, velocity of propagation of the leaders
and critical average electric field between the leader tip and the strike object at the time of the initiation
of upward connecting leader from this object [26].
One of the discrepancies among the researchers for the value of the two above mentioned constants,
𝐴 and 𝑏, is the value of the electric field that leads to the inception of the upward connecting leader,
which ultimately leads to different values of the distance between the downward leader and the
grounded structure for various values of the lightning charge.
Firstly, Golde in 1945 determined that the electric field at ground level that causes the inception of the
upward leader should have a value around 1000 𝑘𝑉/𝑚𝑚 and around 300 𝑘𝑉/𝑚𝑚 close to the lightning
protection systems of the transmission lines. In this assumptions, the adopted value of the average
peak current was 20 − 25 𝑘𝐴.
Later, in 1963 (according to [25]) Wagner determined that the value of the electric field that defines the
final jump condition was 600 𝑘𝑉/𝑚𝑚. This difference was due to the improving knowledge of the
lightning phenomenon and to the better definition of the ratio between the velocity of both downward
and upward leaders and the average peak current value, which were also considered a struggle to
accurately define.
These and other constant alterations provided by several researchers of the values of velocity of
propagation of the leaders, electric field value and average peak current value, allied with the lack of
information of the lightning discharge phenomenon and insufficient data from field experiences,
resulted in different assumptions, which led to very distinct results, which not helped for a consensus
method accepted by the general scientific community.
When the results from the Berger‘s measurements [27] appeared, they cleared doubts. With his
measurements the average peak current value was finally defined around ~30𝑘𝐴 and permitted to
gather information that resulted in a standardized value of electric field in 500 𝑘𝑉/𝑚𝑚, [23], [32], [33],
[34] – critical condition for the inception of the upward leader.
Also, by evaluating the charge dissipated by the first return strokes studied by Berger [27], the charge
stored on the stepped leader channel was estimated and a relation between the return stroke peak
current and the charge was found.
Then, by knowing the distribution of the charge along the leader channel and assuming an electric
field of 500 𝑘𝑉/𝑚𝑚 in the final gap, it was possible to estimate the striking distance of the stepped
leader to flat ground as a function of the prospective return stroke peak current – Equation (4).
This permitted to speed up the process of standardizing the model and improve the lightning
protection.
In Table 4 it can be seen the values of the constants 𝐴 and 𝑏.
25
Table 4: Coefficients of the striking distance according to Equation (3)
Source
Wagner
Armstrong and Whitehead
Brown and Whitehead
Whitehead CIGRE survey
IEEE Working Group (1985)
IEEE Working Group (1997)
𝑨
𝒃
11.25
0.49
7.1
0.75
8
0.65
6.72
9.4
10
0.8
0.67
0.65
The values adopted by the IEC 62305 standard are the values referenced from the IEEE Working
Group (1997), 𝐴 = 10 and 𝑏 = 0.65, which contains the most recent information about the lightning
discharge stages (referred in section 3.1., page 11) and the lightning discharge current parameters
(referred in section 3.2., page 13). Equation (4) is the result:
𝑟𝑠 = 10 × 𝐼0.65
(4)
It is important to remember that the EGM was firstly introduced for the protection of power
transmission lines. With the need for protection of structures against lightning discharges becoming
more and more a necessity it was fundamental to create a response for this problem. So, as EGM was
so successful for power transmission lines protection, the obvious solution was to use its concept of
striking distance and apply it to structures. The rolling sphere method is the resulting solution.
4.2.2. The Rolling sphere method (RSM)
In the early 1960s, based on the concept of protected spaces boarded by circular arcs it was proposed
the use of a fictitious sphere for the location of lightning conductors on structures. The Hungarian
standard was the first to apply this theory.
The concept of the rolling sphere is directly related to the electrogeometric model in that it is based on
the assumption that a stepped leader has to approach to a critical distance, i.e. striking distance,
before it will be attracted to the structure. In other words, this concept assumes that there is a
spherical region with radius equal to the striking distance and located around the tip of the stepped
leader, with the property such that the first point of a grounded structure that enters into this spherical
volume will be the point of attachment of the stepped leader.
It was born the rolling sphere method (RSM).
So, the main application of the RSM is the positioning of air terminals on an ordinary structure, so that
one of the terminals, rather than a roof edge or other part of the structure, initiates the upward leader
that intercepts the downward leader and, hence, becomes the lightning attachment point.
26
For that reason, the rolling sphere method is suggested in IEC 62305-3 [3] to be used for the
positioning of air-termination systems. This is a method suitable in any case regarding the structure to
be protected. Figure 6 shows an example how to apply this method.
Figure 6: Design of an air-termination system according to the rolling sphere method (from [3])
So, in the IEC standard it is assumed that the critical distance of approach of the tip of the stepped
leader to a given structure so that the attachment becomes imminent is independent of the height of
the structure. This assumption is only valid in the case when a connecting leader is absent. In reality,
there will be a connecting leader issued from the structure. So unfortunately, the adopted values of the
sphere radius are only result of compromises in standardization committees, and cannot be used as
basis to perform theoretical studies or to get scientific conclusions [35] due to the fact that most of the
physics associated with the attachment process of lightning flashes with structures are neglected, as it
was firstly introduced.
The reference, in Figure 6, to the height 𝐻 = 60 𝑚𝑚 (the highest value that the rolling sphere radius can
assume) is due to the fact that until this height, the standard predicts that the occurrence of side
flashes is negligible. Above 60 𝑚𝑚 consideration should be given to install a lateral air-termination
system on the upper part of tall structures (typically the top 20 % of the height of the structure).
27
4.3. The Electric Shadow Model
Since the early years of lightning protection studies that the Electric Shadow Model is used in the
lightning protection of grounded structures. Its first application was the protection of transmission and
distribution lines and its supporting structures.
In order to protect them, two main parameters were considered to influence the incidence of direct
lightning strikes on these lines: the annual average ground flash density 𝑁𝑁𝐺 [𝑘𝑚𝑚−2 . 𝑦𝑒𝑎𝑟 −1 ] (the
number of flashes per square kilometer and per year), and the collection area of the line.
This area refers to the ground area (including the structure or line) that has the same yearly direct
lightning flash probability as the structure or line and so, it is sometimes called as the “attractiveness”
of the line and/or its supporting structures.
The common practice dictated that an “electrical shadow” is traced from the line to the earth and the
lightning strikes that normally would hit this “shadowed zone” are instead intercepted by the line.
According to Wagner [30], the width of this attractiveness distance was considered to be height
dependent and so, the width distance from the line was usually a multiple of the height of the line or
the supporting structure. Typically, it was considered twice the line height.
Since then, many researchers have developed estimations of this attractive width.
As the consensus in the general scientific community was not reached, the Institute of Electrical and
Electronics Engineers (IEEE) Working Group (1991) [29] realized a study that related the
attractiveness distance with the height of the structure (not just for lines). With this study the attractive
width from the structure to be protected became to be typically the triple of the structure height.
So, when the standard IEC 62305 was made in 2006 and revised in 2010 the collection area concept
was maintained and it is now used to calculate the average annual number of dangerous events due
to lightning flashes influencing a structure.
Following the study conducted by IEEE, the IEC 62305 states that the collection area for isolated
structures on flat ground, is the area defined by the intersection between the ground surface and a
straight line with 1/3 slope which passes from the upper parts of the structure (touching it there) and
rotating around it.
If the structure has a complex shape such as elevated roof protrusions, a graphical method should be
used to evaluate the collection area. An acceptable approximate value of the collection area for these
kind of structures (complex) is the greater between the collection area evaluated above (using the
height 1:3 ratio) and the collection area attributed to the elevated roof protrusion, where this area is a
circle with its center in the protrusion with a radius of 3 times the height of the protrusion of the
structure.
These calculations (and more) will be later detailed in the section 5.2.1., dedicated to the computation
of the number of dangerous events.
28
Still, it is important to remember that the Electric Shadow Model is also used, not only to calculate the
number of dangerous events that can affect a structure, but also in the positioning of air-termination
systems for simple-shaped structures and protrusions in more complex structures.
It uses the concept of striking distance to positioning vertical rods in order to place the structure
and/protrusions to be protected inside the circular cone shape – the vertical rod “attracts” the lightning
strikes to itself, protecting the structure and/or protrusions from direct lightning strikes.
4.4. Lightning protection level (LPL) according to IEC 62305
With the general equation established (see Equation (4)), the IEC standard assumes that for different
requirements for lightning protection systems (LPS), four lightning protection levels (LPL) are defined,
resulting in four classes of LPS �𝐼– 𝐼𝐼– 𝐼𝐼𝐼– 𝐼𝑉� [1], [3].
For each one, a set of maximum and minimum lightning current parameters is fixed (see Table 5):
-
The maximum values of lightning current parameters (section 3.2., page 13) relevant to 𝐿𝑃𝐿 𝐼
will not be exceeded with a probability of 99 % ; they are reduced to 75 % for 𝐿𝑃𝐿 𝐼𝐼 and to 50 % for
𝐿𝑃𝐿 𝐼𝐼𝐼 and 𝐼𝑉. With these values it can be made a correct design of various lightning components,
like the cross-section of conductors, thickness of metal sheets, current capability of SPDs and
separation distance for preventing dangerous sparking.
-
The minimum values of lightning current amplitude for the different 𝐿𝑃𝐿 are used to derive the
rolling sphere radius 𝑟𝑠 , which is fixed between 20 𝑚𝑚 and 60 𝑚𝑚.
Hence, planning with the rolling sphere leads to the possible point of strikes – where the air terminals
have to be placed.
This strategy to standardize levels of protection turn the process of lightning protection of structures
much more simple, faster and cheaper in most of cases. Even more, with IEC 62305 the protection of
internal systems can be made independently of the external protection, giving more accuracy in the
safety of structures.
For better understanding, consider the following:
Consider a lightning protection system design using a rolling sphere with a given radius. This radius,
being the striking distance, is associated with a certain peak return stroke current. Any stepped leader
associated with a prospective peak return stroke current larger than the “critical current” (mentioned
above) will be associated with a rolling sphere of larger radius, such that a stepped leader will not be
able to penetrate the lightning protection system. On the other hand, a stepped leader associated with
a current smaller than the designated “critical current” is going to have a smaller rolling sphere radius
and such strokes may be able to penetrate through the lightning protection system and strike the
structure. Thus, for a more sensitive structure to the effects of lightning, a smaller sphere radius
should be used when creating the lightning protection system.
29
Table 5: Minimum and maximum values of lightning parameters according with the different class of the LPS
Maximum values – Dimensioning
Minimum values – Interception criteria
criteria
Lightning
protection
level
𝐼
𝐼𝐼
𝐼𝐼𝐼
𝐼𝑉
Critical
Probabilities for
Critical
Probabilities for
maximum
the limits of the
minimum
the limits of the
Rolling
prospective
lightning current
prospective
lightning current
sphere
return stroke
are smaller than
return stroke
are greater than
radius
peak current
the maximum
peak current
the minimum
[𝒌𝑨]
values
[𝒌𝑨]
values
[𝒎]
150
98%
3
5
99%
10
97%
20
91%
45
200
99%
100
97%
100
97%
16
84%
30
60
That is why, as higher the level of protection of the LPS, the lower is the value of rolling sphere radius
and hence the lower will be the return peak current that can penetrate the LPS. So, it can be said that,
according with the definition of the level of protection of the LPS, the flashes with a return peak current
lower than the minimum values established in Table 5 should no longer be considered to be
significantly dangerous to the structure and/or its contents.
Also, recalling from previous section 3.2.1., it is from the cumulative statistical distribution of lightning
peak currents (Figure 4) that these probabilities have meaning. For example, for 𝑳𝑷𝑳 𝑰, the rolling
sphere radius equals 𝒓𝒔 = 𝟐𝟓𝟓 𝒎, which corresponds to the lightning peak current of 𝟑 𝒌𝑨. According
with the distribution of lightning peak currents, there is a 𝟗𝟗𝟗𝟗 percent probability that a flash has a
return peak current higher than 𝟑 𝒌𝑨, hence, a radius with higher value. So, that flash will not be able
to penetrate the lightning protection system. The same reasoning is valid for the other LPS levels of
protection and it can be seen in Figure 7.
The marked points are the fixed values proposed by IEC 62305-1 [1] for 𝑳𝑷𝑳 𝑰 − 𝑰𝑽, presented above
in Table 5.
4.5. Final considerations and comparison between EGM and other lightning incidence models
There are some considerations regarding the use of the electrogeometric model (EGM) as the
lightning incidence model adopted by the IEC 62305.
30
Figure 7: Cumulative frequency of the current peak of the negative first stroke according to CIGRE
(based on CIGRE WG 33.01 [29])
First of all, in order to understand the choice of the EGM, it is important to know that the physics
behind the lightning attachment to a grounded object is a very complex one and that until the middle of
the 20th century was a very grey area for the general scientific community. Now, using the latest data
from experimental field measurements and the newest simulation tools, it is possible to reasonably
model the attachment process including its physic considerations.
The first to attempt to create a solution that included the lightning attachment process was Eriksson. In
his paper [36], Eriksson presented an expression, derived from empirical data and an analytical
model, and compared with transmission line observations. Nevertheless, the observed data showed
dispersion
because
of
recorded
uncertainties.
Later,
Eriksson
introduced
an
improved
electrogeometric model [39]. Still, firstly applied to power transmission lines, it was more recently
adapted for protection of structures with the designation of Collection Volume Method (CVM).
This method, unlike the “classic” EGM explained above, suggested that the successful attachment of a
downward stepped leader depends not only on the initiation of the upward leader but also on the
relative velocity of the both leaders. The first condition, defined by the striking distance, determines the
starting point of the propagation of the connecting leader. The later determines the upward connecting
leader propagation length and defines a parabolic interception locus. Hence, Eriksson considered that
the connection of the downward leader with the newly initiated upward leader is only possible within a
volume defined by the surface generated by the striking distance and the parabolic locus. The lateral
extension of this volume, called the collection volume, defines the lateral attractive distance. Eriksson
31
used the critical radius concept to evaluate the striking distance and assumed a unitary value for the
velocity ratio between the downward and the upward leader.
With these assumptions, the calculations of Eriksson permitted him to found that the lightning
attractive distance does not only depend upon the prospective return stroke current (as the “classical”
electrogeometric model suggests) but also upon the height of the analyzed object.
Then, with the appearance of new technology, computerized solutions became an option to determine
lightning incidence. Rizk introduced a new model for assessing the exposure of horizontal conductors
to lightning strokes [34]. His model started from a developed criterion for positive leaders initiated from
a conductor under a negative descending lightning. Dellera and Garbagnati also introduced a
simulation of lightning strokes to transmission lines by means of the leader progression model based
on the physics of discharge on long air gaps [37],[38].
More recently, in 2012, Becerra and Cooray [40], following the previous work of Rizk, Dellera and
Garbagnati, updated the Self Consistent Leader Inception and Propagation Model (SLIM).
As the CVM, this method uses the concept of attractive radius but in this case the attractive radius is
defined as the first lateral distance from the structure for which the downward leader strikes the
ground instead of the grounded structure. so it is not attracted to the structure to be protected.
The big difference between this method and the previous leader propagation models is that the
parameters such as the downward leader average velocity, the prospective return stroke peak current,
the lateral distance of the downward leader channel to the structure and the electric field are
considered to influence the velocity of upward connecting leaders. This clearly shows that the velocity
of connecting leaders changes from one flash to another due to the variations of these parameters.
Thus, it is not appropriate to use generalized ratios between the velocity of the downward and upward
leaders, as assumed by the existing leader propagation models. Instead, the upward leader velocity
has to be self-consistently computed for each case.
Before proceeding further, the EGM should be remembered. As mentioned previously, according to
the EGM, a stepped leader will terminate on a point on a structure if the potential gradient between the
tip of the stepped leader and the point of the structure reaches a critical value. In general it is assumed
that this critical potential gradient is 500 𝑘𝑉/𝑚𝑚.
Also with SLIM the final attachment of the stepped leader to the structure takes place when the
potential gradient between the tip of the connecting leader and the tip of the stepped leader reaches
500 𝑘𝑉/𝑚𝑚.
So, since EGM neglects the presence of a connecting leader, for a given leader potential, EGM
provides a lower limit to the attractive radius. As the length of the connecting leader increases the
attractive radius increases. Therefore, the attractive radii obtained by SLIM will be longer than the
attractive radii predicted by EGM and for that reason, the SLIM solutions require more protection and
so, safer solutions than the EGM solutions.
32
In [41], Cooray compares the three models, EGM, CVM and SLIM. The goal is to understand how
much the lightning attachment process affects the attractive radii as a function of the return peak
current, for several structure heights. The results are shown in Figure 8.
Figure 8: Lightning attractive radii as predicted by EGM (crosses), SLIM (solid line) and CVM (dashed line) as a
function of return stroke peak current for several structure heights (from [41])
The main conclusion that can be extracted from Figure 8 is that the connecting leader does not play a
significant role in the case of lightning attachment to “normal” (average height) structures. Cooray
stated that, for structures with height less than 30 𝑚𝑚, the lightning attachment process predicted by
SLIM does not deviates significantly from the results given by EGM.
However, the importance of the connecting leader increases with increasing structure height.
Comparing the attractive radii predicted by CVM with the ones predicted by EGM and SLIM shows
that CVM, as applied in engineering practice of today, exaggerates the effect of the connecting leader
leading to unreasonably large attractive radii.
33
In 2006 the first edition of the IEC 62305 was created and the lightning incidence model adopted was
the EGM that although neglects the physics of the lightning attachment process, allows the engineer
to work with a friendly and easy model designed to give safe solutions to protection against lightning.
For the lightning interception system, planning with the rolling sphere method leads to the possible
striking points where air-terminations have to be placed. However, the RSM doesn’t give any
information related with the probability of a lightning discharge strikes these individual different points.
To complement that lack in RSM, in [42] a numerical method is implemented using existing and
internationally accepted data, relations and investigations and based on that gives the probabilities of
the lightning strikes to different points on the surface of a structure as a result.
As expected, the edges and corners of the structures are more exposed than flat surfaces [42], [43].
The tips of slim roof protrusions are even more endangered.
It is also shown in [42] that, with a comparatively small number of rods, a highly efficient airtermination system can be installed. Compared with the standardized procedure of placing rods on
roofs (and walls) described in IEC 62305-3 [3], the number of rods can be smaller, hence the lightning
protective system becomes cheaper.
The reason for this to happen is that the rolling sphere method is very conservative, giving the LPS
designer all possible striking points without providing direct information about the probability of such a
strike. This means in the end that planning air-termination rods with the standardized rolling sphere
method maintains the designer on the safe side.
Concluding this chapter of incidence models, it can be said that the lightning protection technique
introduced by Benjamin Franklin has proven its effectiveness, as evidenced by the comparative
statistics of damage caused by lightning to protected and unprotected structures.
The rolling sphere method, based on the EGM concept of striking distance, commonly used in the
design of LPS is relatively crude, in part because of its neglecting nature regarding the lightning
attachment process and its overestimated dimension of the air-termination systems, but in the end it
does represent a useful engineering tool for determining the number and positions of air terminals. For
that reason, since it was first introduced by the Hungarians in the 1960s it has been a worldwide used
method for designing lightning interception systems.
Still, mainly due to more recent studies (CVM, but particularly SLIM), there are evidences that there is
still room for improvement.
34
5. Risk Management
5.1. Basic Concepts and Methodology
There are a few concepts that should be mentioned in order to understand the following parameters
that compose the value of the risk present in a structure to be protected.
The beginning of this study is the flash striking point. Depending on the striking point, the lightning
current can cause different type of damage to a structure. Then, this damage can result in
consequential loss in the structure. This loss is translated in a relative value of probable annual loss,
which it calls risk, 𝑅. Finally, the risk is compared with a tolerable value depending on the type of loss
and either the risk value is higher or lower than the tolerable risk value, so protection measures should
be implemented or not, respectively.
Since the flash striking point is the entry point of the lightning current in the structure it can be called
the source (𝑆) of possible damage:
𝑆1 : flashes to a structure;
𝑆2 : flashes near a structure;
𝑆3 : flashes to a line;
𝑆4 : flashes near a line.
Then, as it was said, the lightning current may cause damage to the structure. The types of damage
(𝐷) can be:
𝐷1 : injury to living beings by electric shock;
𝐷2 : physical damage;
𝐷3 : failure of electrical and electronic systems.
As these different types of damage occur, they can result in consequential loss (𝐿) in the structure to
be protected which may be on the structure itself or its contents as it can be seen:
𝐿1 : loss of human life (including permanent injury);
𝐿2 : loss of service to the public;
𝐿3 : loss of cultural heritage;
𝐿4 : loss of economic value (structure, content, and loss of activity).
At this point, it is already important to establish the relations between the source of damage and
consequent damage and loss. Figure 9 helps to understand these relations.
35
𝑎
𝑏
Only for properties where animals may be lost.
Only for structures with risk of explosion and for hospitals or other structures where failures of internal systems
immediately endangers human life.
Figure 9: Types of damage and types of loss according to the striking point – Source of damage
Two things can be observed in Figure 9:
- Direct flashes, either to a structure (𝑆1 ) or to a line (𝑆3 ), can originate the three types of damage in
the structure, 𝐷1 , 𝐷2 , 𝐷3 .
As these flashes are related very often with their destructive effects due to the high value of the
lightning current, it seems appropriate that they are associated with all three types of damage;
- Indirect flashes, either near a structure (𝑆2 ) or near a line (𝑆4 ), originate only the type of damage
related with the failure of internal systems, 𝐷3 .
36
Induced overvoltages are a danger to the electronic equipment. More and more often electronic
equipment are a crucial and needed part of the society and because of that the need of its protection
has now, more than ever, a very high importance.
With the source and type of damage characterized and as well its consequential loss it remains to
conceptualize the risk.
The risk is evaluated by the type of loss which may appear in the structure, consequent from damage.
It is the annual average value of the probability to occur the considered loss. As there are four types of
loss, so the risk has also four types.
𝑅1 : risk of loss of a human life (including permanent injury);
𝑅2 : risk of loss of service to the public;
𝑅3 : risk of loss of cultural heritage;
𝑅4 : risk of loss of economic value.
For better understanding, Figure 10 outlines these relations.
1)
2)
Only for hospitals or other structures where failure of internal systems immediately endanger human life.
Only for properties where animals may be lost.
Figure 10: Types of loss and corresponding risks resulting from different types of damage
With the risk calculated it is time to compare its value with a tolerable risk value, 𝑅𝑇 .
− If 𝑅 ≤ 𝑅𝑇 , lightning protection is not necessary;
− If the opposite happens, 𝑅 > 𝑅𝑇 , protection measures should be implemented in order to reduce 𝑅
to tolerable values.
Typical values of tolerable risk, 𝑅𝑇 , can be observed in Table 6. As the type of risk, these values are
associated with the type of loss.
37
Table 6: Typical values of tolerable risk, 𝑹𝑻
Types of Loss
𝐿1
Loss of human life or permanent injuries
𝐿3
Loss of cultural heritage
𝐿2
𝑹𝑻 (𝒚𝒆𝒂𝒓−𝟏 )
10−5
10−3
Loss of service to the public
10−4
By principle, for loss of economic value (𝐿4 ), the route to be followed is an evaluation of cost/benefit
comparison. If the data for this analysis are not available the representative value of tolerable risk
𝑅𝑇 = 10−3 may be used.
So, it can be drawn a sequence of steps which results in the final value of risk:
Source (flash striking point) → Damage (caused by lightning current) → Loss (consequential loss
derived by the types of damage) → Risk (value of probable annual loss) → Comparison with a
tolerable risk value, 𝑅𝑇 → Implementation of protective measures if 𝑅 > 𝑅𝑇 → Re-calculate risk.
After this conceptualization, it is time to describe the components that compose each type of risk.
Each type of risk has its components (partial risks) and the sum of them gives the total value of risk, 𝑅.
It can be seen in Table 7 that these risk components result from the source and type of damage of
each type of risk.
Table 7: Risk components to be considered for each type of loss in a structure
(according to IEC 62305-2 [2], table 2, page 22)
Source of damage
Risk Component
Risk for each type of loss
𝑅1
38
𝑅4
Flash near a
structure
structure
𝑺𝟏
𝑺𝟐
Flash to a line
Flash near a line
connected to
connected to the
the structure
structure
𝑺𝟑
𝑺𝟒
𝑅𝐴
𝑅𝐵
𝑅𝐶
𝑅𝑀
𝑅𝑈
𝑅𝑉
𝑅𝑊
𝑅𝑍
*
*
*𝑎
*𝑎
*
*
*𝑎
*𝑎
*
*
𝑅2
𝑅3
Flash to a
*
*
*
𝑏
*
*
*
*
*
*
*
*
*
𝑏
*
𝑎
Only for structures with risk of explosion, and for hospitals or other structures where failure of internal
𝑏
Only for properties where animals may be lost.
systems immediately endangers human life.
Considering that direct flashes to a structure (𝑆1 ) can originate the three types of damage in the
structure (𝐷1 , 𝐷2 , 𝐷3 ), in terms of risk components, it translates in the three risk components:
𝑅𝐴 , 𝑅𝐵 , 𝑅𝐶 , (respectively).
Still for direct flashes, the ones striking a line (𝑆3 ) can originate as well the three types of damage in
the structure, 𝐷1 , 𝐷2 , 𝐷3 , which result, in this case, in the three risk components: 𝑅𝑈 , 𝑅𝑉 , 𝑅𝑊 ,
respectively.
For indirect flashes only the type of damage related with the failure of internal systems, 𝐷3 is
originated. So, if the flash striking point is near a structure (𝑆2 ), the risk component resultant is 𝑅𝑀 and
if the flash striking point is near a line (𝑆4 ) the risk component resultant is 𝑅𝑍 .
So, here are the eight risk components derived by the relation of the source and type of damage for
each type of risk:
𝑅𝐴 – Risk component related to injury to living beings – flashes to a structure;
𝑅𝐵 – Risk component related to physical damage to a structure – flashes to a structure;
𝑅𝐶 – Risk component related to failure of internal systems – flashes to a structure;
𝑅𝑀 – Risk component related to failure of internal systems – flashes near a structure;
𝑅𝑈 – Risk component related to injury to living being – flashes to a connected line;
𝑅𝑉 – Risk component related to physical damage to a structure – flashes to a connected line;
𝑅𝑊 – Risk component related to failure of internal systems – flashes to a connected line;
𝑅𝑍 – Risk component related to failure of internal systems – flashes near a line.
With this introduction of Basic Concepts and Methodology, it can begin the characterization of each
component of the risk.
Each component has a series of parameters, factors and probabilities, which condition it respective
value. This section has the objective of explaining them.
5.2. Risk Evaluation
Each risk component 𝑅𝐴 , 𝑅𝑈 , 𝑅𝐵 , 𝑅𝑉 , 𝑅𝐶 , 𝑅𝑀 , 𝑅𝑊 and 𝑅𝑍 may be expressed by Equation (5):
𝑅𝑋 = 𝑁𝑁𝑋 × 𝑃𝑋 × 𝐿𝑋
(5)
Where
𝑁𝑁𝑋 is the number of dangerous events per year;
𝑃𝑋 is the probability of damage;
𝐿𝑋 is the consequent loss.
39
These are the three pillars that characterize the value of risk. If a dangerous event occurs, there is a
probability that it causes damage to the structure or its contents and consequently originates a loss.
The risk is the annual average value of the probability of occurring the loss considered.
5.2.1. Number of dangerous events, 𝑵𝑿
The concept of dangerous events is associated with the source of damage – the striking point.
And the number of dangerous events per year is evaluated by Equation (6):
𝑛
𝑁𝑁𝑋 = 𝑁𝑁𝐺 × 𝐴𝑋 × � 𝑋𝑖 × 10−6
𝑖=1
(6)
Where
𝑁𝑁𝐺 is the lightning ground flash density. It is the number of lightning flashes per square quilometer per
year [𝑘𝑚𝑚−2 × 𝑦𝑒𝑎𝑟 −1 ];
𝐴𝑋 is the collection area according to the source of damage [𝑚𝑚2 ];
𝑋𝑖 are the parameters (structure and lines characteristics) that turn the total events 𝑁𝑁𝐺 × 𝐴𝑋 into
dangerous events.
The value of the ground flash density is available from lightning location networks in many areas of the
world. According with the standard [2], if a map of 𝑁𝑁𝐺 is not available, in temperate regions it may be
estimated by Equation (7):
𝑁𝑁𝐺 ≈ 0.1 × 𝑇𝐷
(7)
Where 𝑇𝐷 is the thunderstorm days per year (which can be obtained from isokeraunic maps).
The factor 10−6 corresponds to converting 𝑚𝑚2 into 𝑘𝑚𝑚2 of the collection area.
Multiplying the lightning ground flash density by the relevant collection area gives the total number of
events that can occur in that area.
Remind the following: there are 4 sources of lightning – flashes to a structure (𝑆1 ), flashes near a
structure (𝑆2 ), flashes to a line (𝑆3 ) and flashes near a line (𝑆4 ). So, each of these sources requires a
different collection area and, by consequence, results in a different number of events occurring in the
corresponding area.
On Table 8 and Figure 11 it can be seen the different types of collection areas where can occur the
corresponding number of dangerous events, according with the source of the striking point.
40
Table 8: Types of number of dangerous events and collection area according with the source of lightning
Source of
Type of number of
lightning
dangerous events
Structure
𝑁𝑁𝐷
Adjacent structure
Near a structure
𝑁𝑁𝐷𝐽
𝑁𝑁𝑀
Near a line
𝑁𝑁𝐿
Source of
Type of collection
lightning
area
Line
𝑁𝑁𝐼
N. of
Equation
𝑁𝑁𝐷 = 𝑁𝑁𝐺 × 𝐴𝐷 × (𝐶𝐷 ) × 10−6
Equation
(8)
−6
𝑁𝑁𝐷𝐽 = 𝑁𝑁𝐺 × 𝐴𝐷𝐽 × �𝐶𝐷𝐽 × 𝐶𝑇 � × 10
𝑁𝑁𝑀 = 𝑁𝑁𝐺 × 𝐴𝑀 × 10−6
𝑁𝑁𝐿 = 𝑁𝑁𝐺 × 𝐴𝐿 × (𝐶𝐼 × 𝐶𝐸 × 𝐶𝑇 ) × 10−6
−6
𝑁𝑁𝐼 = 𝑁𝑁𝐺 × 𝐴𝐼 × (𝐶𝐼 × 𝐶𝐸 × 𝐶𝑇 ) × 10
Equation
(9)
(10)
(11)
(12)
N. of
Equation
For rectangular structures:
Structure and
Adjacent structure
Near a structure
Line
Near a line
𝐴𝐷
𝐴𝐷 = 𝐿 × 𝑊 + 2 × (3 × 𝐻) × (𝐿 + 𝑊) + 𝜋 × (3 × 𝐻)2
For complex structures (e.g. elevated protrusions):
𝐴′𝐷 = 𝜋 × (3 × 𝐻𝑃 )2
(13)
𝐴𝑀
𝐴𝑀 = 2 × 500 × (𝐿 + 𝑊) + 𝜋 × 5002
(14)
𝐴𝐼
𝐴𝐼 = 4000 × 𝐿𝐿
(16)
𝐴𝐿
𝐴𝐿 = 40 × 𝐿𝐿
(15)
The collection area of a structure (or line), as it was said, refers to the ground area having the same
yearly direct lightning flash probability as the structure (or line).
Depending on the source of the striking point, the standard IEC 62305-2 [2], established a set of
general equations for calculating the collection areas:
-
For isolated structures (on flat ground) the IEC standard determines the collection area of the
structure as an area defined by the intersection between the ground surface and a straight line with
1/3 slope which passes from the upper parts of the structure (touching it there) and rotating around it.
In effect the collection area reaches out from the perimeter of the roofline three times the height of the
structure.
-
For flashes striking near the structure, the collection area 𝐴𝑀 extends to a line at a distance of
500 𝑚𝑚 from the perimeter of the structure.
Again, the value of 500 𝑚𝑚 is a compromise made by the committee in order to result in a much safer
solution.
-
For direct and indirect flashes to a connected line the important parameters are the average
earth resistivity (𝜌), the threshold voltage level (𝑈𝑊 ) and the length of the line (𝐿). Using the reference
values given in the standard, the resultant collection areas are (approximately) translated in Equation
(15) and Equation (16), according respectively with [45], [46].
41
As it was said in section 2.2.1. (in page 8) there are some differences between parameters during the
risk assessment. The computation of the different collections is an example: the British version of the
IEC standard [44] – BS IEC 62305 – for flashes striking near the structure, the collection area 𝐴𝑀
instead of extending to a line at a distance of 500 𝑚𝑚 from the perimeter of the structure, it only extends
to a line at a distance of 250 𝑚𝑚. The same happens in the Portuguese standard – NP 4426 [17].
Also, before the standard IEC 62305, some countries including England, considered that an
appropriate collection area for direct strikes to a structure is an area defined by the intersection
between the ground surface and a straight line with 1/1 slope which passes from the upper parts of the
structure (touching it there) and rotating around it, instead of the later standardized 1/3 slope, resulting
in a collection area much smaller.
Figure 11: Collection areas introduced in IEC 62305 (from [47])
The parameters 𝑋𝑖 are the ones who restrict the total number of events to the number of dangerous
events according to structure and connected lines geometric characteristics. The concept of
“dangerous” is important because it is the one who indicates what type of event contributes to the
value of risk that the structure is in.
The above mentioned parameters 𝑋𝑖 are factors which characterize the structure,𝐶𝐷 , adjacent
structures, 𝐶𝐷𝐽 , and connected lines to the structure, 𝐶𝑇 , 𝐶𝐼 and 𝐶𝐸 .
The relative location of the structure, compensating for surrounding structures or an exposed location
will be taken into account by a location factor 𝐶𝐷 . Its value range from 0.25 (Structure surrounded by
42
higher objects) to 2 (Isolated structure on a hilltop or a knoll). For detail analysis consult Table A.1 in
the Appendix A section.
From its analysis, it can be seen that the value of the factor 𝐶𝐷 is higher as the structure to be
protected is more isolated from surrounding structures. As the factor 𝐶𝐷 gets higher so risk
components, 𝑅𝐴 , 𝑅𝐵 , 𝑅𝐶 (related to flashes to the structure to be protected, as expected), get higher,
meaning in this case that an isolated structure (e.g. rural areas) is likely to be more at risk than a
surrounded structure (e.g. urban areas).
(𝐶𝐷𝐽 as the same concept as 𝐶𝐷 but the 𝐶𝐷𝐽 is related to an adjacent structure of the structure to be
protected)
This factor is the 𝑋𝑋 which multiplied by the collection area of the structure (or the adjacent structure)
and the lightning ground flash density give the number of dangerous events due to flashes to a
structure, 𝑁𝑁𝐷 .
Note: for flashes to an adjacent structure, 𝑁𝑁𝐷𝐽 , besides the 𝐶𝐷𝐽 parameter, as the adjacent structure it is assumed
to be connected to the structure to be protected by a power or telecommunication line, the parameter 𝐶𝑇 – line
factor – must be taken into account as well, as expected.
In order to characterize the dangerous events cause by flashes to or near a line, three factors must be
taken into evaluation:
The installation factor takes into account the reduction in the coupling between the lightning flash and
the line due to the installation of the line underground, when compared with an aerial installation.
So, 𝐶𝐼 gives that an aerial line is more at risk than a buried line, as expected.
The line type factor, 𝐶𝑇 , gives that a LV power, telecom or data line is more at risk than a HV power
line with a HV/LV transformer.
And environment factor of a line, 𝐶𝐸 , gives that, just as the factor 𝐶𝐷 is influenced by objects that
surrounds the structure to be protected, objects that surrounds a line connected to the structure to be
protected influence the value of 𝐶𝐸 .
For more information see Table A.2, Table A.3 and Table A.4 in the Appendix A section.
These three factors are directly proportional to the value of risk components, 𝑅𝑈 , 𝑅𝑉 , 𝑅𝑊 , 𝑅𝑍 - related to
flashes to or near a line connected to the structure to be protected, as it was expected.
So, 𝐶𝐼 , 𝐶𝑇 , 𝐶𝐸 are the factors 𝑋𝑋 which multiplied by the collection area of the structure (or the adjacent
structure) and the lightning ground flash density give the number of dangerous events due to flashes
to a line, 𝑁𝑁𝐿 , and the number of dangerous events due to flashes near a line, 𝑁𝑁𝐼 .
5.2.2. Probability of damage to a structure, 𝑷𝑿
Each value of 𝑃𝑋 is a combination of various parameters. It can almost be said that 𝑃𝑋 is a factorization
43
composed by these parameters. They translate the characteristics of the structure and connected
lines that are relevant for the risk calculation.
Following the same reasoning as the risk components (𝑅𝐴 , 𝑅𝐵 , 𝑅𝐶 , 𝑅𝑀 , 𝑅𝑈 , 𝑅𝑉 , 𝑅𝑊 , 𝑅𝑍 ), there are
eight components of probabilities, 𝑃𝑋 , and they are also grouped according to the source and type of
damage. These are the probabilities of a dangerous event occurs causing a type of damage:
𝑃𝐴 − probability that a flash to a structure will cause injury to living beings by electric shock;
𝑃𝐵 − probability that a flash to a structure will cause physical damage;
𝑃𝐶 − probability that a flash to a structure will cause failure of internal systems;
𝑃𝑀 − probability that a flash near a structure will cause failure of internal systems;
𝑃𝑈 − probability that a flash to a line will cause injury to living beings by electric shock;
𝑃𝑉 − probability that a flash to a line will cause physical damage;
𝑃𝑊 − probability that a flash to a line will cause failure of internal systems;
𝑃𝑍 − probability that a flash near a line will cause failure of internal systems.
One more time it can be seen that direct flashes to a structure or to a line can lead to the three types
of damage and indirect flashes (near a structure or near a line) can lead to damage of internal
systems (induced overvoltages).
On the following Figure 12 and Figure 13 it can be observed the relations and dependencies between
the probabilities and the parameters that compose them.
Figure 12: Probabilities that a flash to or near a structure cause damage in the structure and/or its contents
44
Figure 13: Probabilities that a flash to or near a connected line cause damage in the structure and/or its contents
With that said, it can begin the characterization of each parameter that composes these probabilities.
•
𝑃𝐴 is the probability that a flash to a structure will cause electric shock to living beings due to
dangerous touch and step voltages. 𝑃𝑇𝐴 is one of the parameters that reduces this probability by
implementing additional protection measures against step and touch voltages.
When a fault occurs in the structure, in this case, caused by a lightning, the current will pass through
any metallic object and enter the earth.
Step voltage is the voltage between the feet of a person standing near an energized grounded object.
So, a person could be at risk of injury during a fault simply by standing near the grounding point.
Touch voltage is the voltage between the energized object and the feet of a person in contact with the
object.
So, by the definition of step and touch voltages, there are two things that should be improved:
-
Grounding resistance;
-
Safe distance between points of danger and public areas in the structure.
𝑃𝑇𝐴 ranges from 1 (No protection measures) to 0 (Physical restrictions or building framework used as a
down-conductor system). Table A.5 shows that the protection measures mentioned in it are indeed
very suitable in reducing the risk of dangerous events due to touch and step voltages to occur.
•
One important parameter is the probability that a flash hits a structure and causes physical
damage, 𝑃𝐵 .
The value of the probability 𝑃𝐵 is one of the most difficult to lower because implies the
construction/upgrade of a lightning protection system, an LPS. These alterations by lowering 𝑃𝐵 , in
some cases, results in changes in the structure at structural level. So, the construction of a lightning
45
protection system, LPS, can be very expensive, even more, as the lightning protection level, LPL, gets
higher.
In light of that, the LPS designer should be aware that other measures can reduce the risk to tolerable
values without the construction or improvement of a LPS in the structure.
𝑃𝐵 ranges from 1 to 0.02 according with the class of the installed LPS. For better consultation see
Table A.6, in the Appendix A section.
To understand the influence of an LPS in the calculation of the risk it is important to know that an LPS
can be of two kinds: External and Internal.
- An external LPS is intended to intercept direct lightning flashes to the structure, including flashes to
the side of the structure, and conduct the lightning current from the striking point to the ground. The
external LPS is also intended to disperse this current into the earth without causing thermal or
mechanical damage, or dangerous sparking which may trigger fire or explosions.
- An internal LPS shall prevent the occurrence of dangerous sparking within the structure to be
protected due to lightning current flowing in the external LPS or in other conductive parts of the
structure. These dangerous sparking may occur between the external LPS and other components
such as:
– metal installations: requires Lightning equipotential bonding for metal installations;
– internal systems: requires Lightning equipotential bonding for internal systems;
– external conductive parts and lines connected to the structure: requires Lightning equipotential
bonding for external conductive parts and lines connected to the structure to be protected.
So, changing the class of the LPS (𝑃𝐵 ) acts on both external and internal LPS. Knowing this, there are
two parameters that are influenced by altering the internal LPS class: 𝑃𝐸𝐵 and 𝑃𝑆𝑃𝐷 .
•
𝑃𝐸𝐵
depends on the lightning equipotential bonding for connected electrical and
telecommunication lines. It ranges from 1 to 0.01, depending on the lightning protection level, LPL, for
which its SPDs are designed (see Table A.7 in the Appendix A section for more detail in 𝑃𝐸𝐵 values).
Analyzing Figure 13, it can be observed that the parameter 𝑃𝐸𝐵 influences the probabilities 𝑃𝑈 and 𝑃𝑉 :
probability that a direct flash to a line will cause injury to living beings by electric shock and probability
that a flash to a line will cause physical damage, respectively.
An interesting fact that already can be observed is that the probability 𝑃𝐵 influences the corresponding
probabilities but for the case of direct flashes to a structure: 𝑃𝐴 and 𝑃𝐵 itself.
So, it is no surprise when the standard says that altering the LPS class, in order to reduce the
probability that a flash to a structure causes physical damage, 𝑃𝐵 , includes a mandatory lightning
equipotential bonding of the connected power and telecommunication lines by installation of SPDs of
equal or higher class.
46
In resume, lowering the value of the probability 𝑃𝐵 imposes a lowering on the parameter 𝑃𝐸𝐵 . Later on
it will be seen how important these parameters are to the calculation of the final value of the risk and
how to reduce its potential hazardous effects if protective measures are not implemented.
•
The other parameter influenced by altering the internal LPS is 𝑃𝑆𝑃𝐷 . It depends on the
coordinated SPD system installed and, as 𝑃𝐸𝐵 , it ranges from 1 to 0.01, depending on the lightning
protection level, LPL, for which its SPDs are designed (see Table A.8 in the Appendix A section for
more detail in 𝑃𝑆𝑃𝐷 values).
In this case, unlike the probability 𝑃𝐸𝐵 , the probability 𝑃𝑆𝑃𝐷 does not depend on the value of 𝑃𝐵
(provided that an LPS is installed, regardless of its class). This is valid because 𝑃𝑆𝑃𝐷 value changes by
installing a coordinated SPD protection system in order to reduce the risk components directly related
to the protection of internal power and telecommunication systems from its failure, 𝑅𝐶 , 𝑅𝑀 , 𝑅𝑊 and 𝑅𝑍 ,
through the reduction of the corresponding probabilities, 𝑃𝐶 , 𝑃𝑀 , 𝑃𝑊 and 𝑃𝑍 .
In section 5.3. (risk mitigation measures), this parameter will be better explained.
The previous parameter is related to specific characteristics of the structure that influences the
probabilities that a failure of internal systems may occur when a direct or indirect flash hits the
structure. The following two factors are the ones related with specific characteristics of lines connected
to the structure to be protected: shielding, grounding and isolation.
•
𝐶𝐿𝐷 is a factor depending on shielding, grounding and isolation conditions of the line for
flashes to a line.
•
𝐶𝐿𝐼 is a factor depending on shielding, grounding and isolation conditions of the line for flashes
near a line.
For these factors it is important to notice the differences between the two of them.
𝐶𝐿𝐷 is always equal or higher than 𝐶𝐿𝐼 . This is explained by looking at the definitions of the factors.
One depends on flashes in a line and the other on flashes near a line. So, as direct flashes are
significantly more likely to be hazardous to the structure and its contents, 𝐶𝐿𝐷 is the factor that will
contribute more in increasing the value of the risk.
Another interesting fact that these two parameters give is what probabilities they influence: the
probabilities that a direct flash to a structure and direct or indirect flashes to a line cause failure to
internal systems − 𝑃𝐶 , 𝑃𝑈 , 𝑃𝑊 for direct flashes and 𝑃𝑍 for indirect flashes.
To see in detail the relations between the two factors see the Table A.9 in the Appendix A section.
Still in the factors that influence the probabilities of damage to the structure derived by flashes to or
near a connected line, the factors 𝑃𝐿𝐷 and 𝑃𝐿𝐼 are introduced.
47
•
The probabilities 𝑃𝐿𝐷 and 𝑃𝐿𝐼 (Table A.10 and Table A.11 respectively, present in the Appendix
A section) are the components of the probabilities that takes into account the effects of failure of
internal systems within the structure due to a flash to (𝑃𝑈 , 𝑃𝑉 , 𝑃𝑊 ) or near (𝑃𝑍 ) a line.
Modern equipment is particularly vulnerable and may be damaged by milijoules of energy. Where
there are connections using long cables, any discharge into the area around these cables is likely to
induce currents in the cables. Against a direct strike of lightning, there is very little chance that any
protection measure results in avoiding partial or even total destruction of the equipment. But if the
strike of lightning is near the cable, the likely induced currents that will occur in the cables can be
managed and reduced to safer values if some measures have been applied. This effect of induced
currents can be greatly reduced if the impedance through which flows is small, that is, if the resistance
of the circuits is small. This keeps the voltages low.
In this case, the resistance of the cable screen is the parameter under revision. The probability of the
impulse withstand voltage of an equipment is reached increases as the resistance of the cable screen
increases.
For the withstand voltage of the equipment, it is observed for both factors that increasing its value, the
probability of a flash to or near a line damage the structure or its contents decreases, as expected.
Furthermore, the different geometrical characteristics between the two lines, power and
telecommunication results in different conduction of induced currents and so on different values of the
probability of failure of internal systems due to a flash near a line, 𝑃𝐿𝐼 .
Finally, in order to fully characterize 𝑃𝑈 , the probability that a flash to a line will cause injury to living
beings by electric shock, as it was done for 𝑃𝐴 , protective measures against touch voltages must be
taken care of. So, the parameter 𝑃𝑇𝑈 is presented.
•
𝑃𝑇𝑈 depends on protection measures against touch voltages, such as physical restrictions or
warning notices. In this case, if it wants to reduce the probability 𝑃𝑈 , the value of 𝑃𝑇𝑈 must be
reduced. Table A.12, present in the Appendix A section, shows the different values that 𝑃𝑇𝑈 can
assume depending on protective measures against the dangerous of touch voltages.
So, 𝑃𝑇𝐴 , 𝑃𝐵 , 𝑃𝐸𝐵 , 𝑃𝑆𝑃𝐷 , 𝐶𝐿𝐷 , 𝐶𝐿𝐼 , 𝑃𝑇𝑈 , 𝑃𝐿𝐷 , 𝑃𝐿𝐼 are the parameters that characterize the probabilities of
occurring damage in the structure. There is another parameter (𝑃𝑀𝑆 ) that it will be discussed later on
this chapter. It relates to the probability that a flash near a structure will cause failure of internal
systems, 𝑃𝑀 .
5.2.3. Consequent loss from damage into the structure and/or its contents, 𝑳𝑿
The loss 𝐿𝑋 refers to the mean relative amount of a particular type of damage for one dangerous event
caused by a lightning flash, considering both its extent and effects.
48
The loss value 𝐿𝑋 varies with the type of loss considered, as it was already said:
– 𝐿1 (Loss of human life, including permanent injury): the endangered number of persons (victims);
– 𝐿2 (Loss of public service): the number of users not served;
– 𝐿3 (Loss of cultural heritage): the endangered economic value of structure and content;
– 𝐿4 (Loss of economic values): the endangered economic value of animals, the structure (including its
activities), content and internal systems,
and, for each type of loss, it varies with the type of damage (𝐷1 , 𝐷2 and 𝐷3 ) causing the loss.
So, as the risk components, the losses are grouped according to the source and type of damage.
Moreover, the loss 𝐿𝑋 should be determined for each zone of the structure into which it is divided, in
order to make the process more effective and keep the cost at lower values.
The factors that influence the consequent loss are:
•
Factor 𝑟𝑡 can reduce the loss of human life, 𝐿1 , and the economic loss, 𝐿4 , depending on the
type of soil or floor.
The resistance of a conductor depends on the atomic structure of the material or its resistivity
(measured in Ω. 𝑚𝑚), which is the material property that measures its ability to conduct electricity. A
material with a low resistivity will behave as a “good conductor” and one with a high resistivity will
behave as a “bad conductor”.
That is why the value of the factor 𝑟𝑡 decreases when the contact resistance of the surface is higher.
For more detail, see Table A.13 in the Appendix A section.
•
The factor 𝑟𝑝 gives the information about the measures applied in the structure in order to
reduce the consequences of fire. Applying any of these measures reduces, by consequence, physical
damage in the structure and its contents.
Once again, it should be done an economic viability study when reducing the value of the factor 𝑟𝑝 . For
example, installing manual alarms and extinguishers maybe has a significant reduction in the value of
the risk (reduction by 50 % of 𝑟𝑡 ) and it is relatively cheap instead of a reduction of the factor to 0.2 by
installing automatic equipment with a much higher investment.
As the consequence of fire can be very hazardous for the structure and its contents, 𝑟𝑝 , influences all
four types of loss and so it is a very important factor for the value of the risk (see Table A.14).
•
The next factor is related with the risk of fire or the risk of explosion in the structure (as a
whole or by zones) − 𝑟𝑓 .
49
According to the amount of risk present in a zone, 𝑟𝑓 assumes a value between 1 and 0, where 1 is for
zones with continuously explosive atmosphere and 0 is for zones with no risk of explosion or fire.
These values are better presented in Table A.15 in the Appendix A section.
Usually this factor is associated with 𝑟𝑝 because with the structure at more risk of fire or explosion
(higher 𝑟𝑓 ) comes a higher need of protection measures (smaller 𝑟𝑓 ). So, as factor 𝑟𝑝 , 𝑟𝑓 influences all
four types of loss.
•
ℎ𝑧 , by definition, is the parameter that translates the possible presence of special hazards in
the structure (as a whole or by zone).
Its value is translated in the level of panic and the evacuation difficulties present in the zone (or in the
entire structure) in study. It depends on the characteristics of the zone and on the number of persons.
ℎ𝑧 ranges from 1 (No special hazard ) to 10 (High level of panic – number of people greater than 1000
persons). For more detail see Table A.16 in the Appendix A section.
This parameter is directly related to risk components that characterize the risk of physical damage, 𝑅𝐵
and 𝑅𝑉 , through the components of loss of human life, 𝐿𝐵 and 𝐿𝑉 .
So, as higher is the level of panic or the difficulties of evacuation in the structure or in a zone in the
structure, higher is the value of ℎ𝑧 and consequently the value of the loss of human life, 𝐿1 .
Finally, as it was said, the components of loss are related with the source and type of damage that can
occur in a structure. These are the factors that restrict its effects in each loss component:
-
For 𝐿1 , loss of human life:
ℎ𝑧 : factor increasing factor due to physical damage, 𝐷2 , depending on the presence of a special
hazard in the zone;
𝐿 𝑇 : typical mean relative numbers of victims injured by electric shock (𝐷1 ) due to one dangerous event;
𝐿𝐹 : typical mean relative numbers of victims by physical damage (𝐷2 ) due to one dangerous event;
𝐿𝑂 : typical mean relative numbers of victims by failure of internal systems (𝐷3 ) due to one dangerous
event.
-
For 𝐿2 , loss of public service:
𝐿𝐹 : typical mean relative number of users not served, resulting from physical damage (𝐷2 ) due to one
dangerous event;
𝐿𝑂 : typical mean relative numbers of users not served resulting from failure of internal systems (𝐷3 )
due to one dangerous event.
-
For 𝐿3 , loss of cultural heritage:
𝐿𝐹 : typical mean relative value of all goods damaged by physical damage (𝐷2 ) due to one dangerous
event.
-
For 𝐿4 , loss of cultural heritage:
𝐿 𝑇 : typical mean relative value of all goods damaged by electric shock (𝐷1 ) due to one dangerous
event;
𝐿𝐹 : typical mean relative value of all goods damaged by physical damage (𝐷2 ) due to one dangerous
event;
50
𝐿𝑂 : typical mean relative value of all goods damaged by failure of internal systems (𝐷3 ) due to one
dangerous event;
To reinforce that, Table 9 shows the relation between the loss components with the type of damage
𝐷1 , 𝐷2 , 𝐷3 , and with that associate the loss components with the factors 𝐿 𝑇 , 𝐿𝐹 , 𝐿𝑂 , respectively.
Loss Components
Table 9: Loss components relation with the type of damage and the factors 𝑳𝑻 , 𝑳𝑭 , 𝑳𝑶
𝑳𝑨
Type of Damage 𝑫𝟏
X
Type of Damage 𝑫𝟐
𝑳𝑩
X
𝑳𝑪
𝑳𝑴
𝑳𝑼
X
X
X
𝑳𝑽
X
𝑳𝑾
𝑳𝒁
Type of Damage 𝑫𝟑
X
X
𝑳𝑻
𝑳𝑭
𝑳𝑶
Typical mean relative number of victims according to the Type of Damage
There are still some parameters that can restrict the value of loss.
-
Time of permanency of people in a zone:
𝑛𝑧 is the number of persons in the zone;
𝑛𝑡 is the total number of persons in the structure;
𝑡𝑧 is the time in hours per year for which the persons are present in the zone.
If the structure is considered as a whole zone, 𝑛𝑧 = 𝑛𝑡 .
-
Cost of the loss:
𝑐𝑎 : is the value of animals in the zone;
𝑐𝑏 : is the value of building relevant to the zone;
𝑐𝑐 : is the value of content in the zone;
𝑐𝑠 : is the value of internal systems including their activities in the zone.
The introduction of these parameters in the calculation of the loss can lower the value of the risk, in
some cases, very significantly.
With this revision of the parameters that comprise the calculation of the components of the risk, it is
interesting to see how which and each one of these factors and probabilities influence the value of
risk.
51
By analysis of the data in [48], an article that discusses the sensitivity of the parameters in risk
calculations, it shows for two examples (present in the standard) the order of importance of these
parameters in respect to the risk of loss of human life.
The examples are for a Country House and for an Office Building.
For these studied cases the results demonstrate that 𝑁𝑁𝐺 , the lightning ground flash density, is the most
sensitive parameter. That can be explained by the influence of this parameter in the calculation of
every risk component. Next in the list in the most sensitive parameters, come the ones regarding
physical damage, 𝑟𝑝 , ℎ𝑧 , 𝑟𝑓 , 𝐿𝐹 and 𝑃𝐸𝐵 . Beyond these, factors that give information about the location
of the structure and the lines connected to it are relevant in the risk calculation.
For results more accurate these parameters should be investigated in more detail. But even though
they are not 100% precise, they already give an idea in how and where it should be done the
interventions so the risk reduces to tolerable values. This evaluation it will be taken into consideration
in the next section of risk mitigation measures.
5.3. Risk mitigation measures
This section assumes great importance because, after the characterization of the structure to be
protected, it must be implemented measures in order to reduce the risk to tolerable values and confirm
that the structure is finally protected against direct and indirect lightning strikes and its effects.
First of all, lets remind the risk components:
𝑅𝐴 – Risk component related to injury to living beings – flashes to structure;
𝑅𝐵 – Risk component related to physical damage to a structure – flashes to a structure;
𝑅𝐶 – Risk component related to failure of internal systems – flashes to structure;
𝑅𝑀 – Risk component related to failure of internal systems – flashes near structure;
𝑅𝑈 – Risk component related to injury to living being – flashes to connected line;
𝑅𝑉 – Risk component related to physical damage to structure – flashes to connected line;
𝑅𝑊 – Risk component related to failure of internal systems – flashes to connected line;
𝑅𝑍 – Risk component related to failure of internal systems – flashes near a line.
As it was already said, the risk components are associated with the source and types of damage that
can occur in the structure and its contents. Therefore, possible measures to reduce these risk
components should be such that these damages are less probable to occur and, in the case of they
occur, decrease their consequences on the structure and its contents.
So, in order to accomplish this, there is a consideration to be made.
Not all of the factors listed and tabled in the previous section can be changed, meaning, not all
parameters are suitable or economically viable to modify.
For example, the dimensions of the building; the lightning ground flash density 𝑁𝑁𝐺 (number of lightning
flashes per 𝑘𝑚𝑚2 per year, a parameter that depends on the region where the structure is in); the
52
structure location factor, 𝐶𝐷 ; the factor that increases with the level of panic and the difficulty of
evacuation present in a zone of the structure, according to the number of people present in the zone in
study, ℎ𝑧 . These are some parameters that can’t be change.
And there are some examples of parameters that may not be economically viable to alter in an already
built structure. Like the value of the probability of a lightning strike to the structure results in physical
damage, 𝑃𝐵 ; the parameter the depends on lightning equipotential bonding (EB) conforming on the
lightning protection level (LPL) for which its SPDs are designed, 𝑃𝐸𝐵 ; the value of the rated impulse
withstand voltage, 𝑈𝑊 , of some equipment present in the structure, among others. These parameters
are rated like this mainly because changing their values may implicate structural modifications in the
structure to be protected, or very expensive modifications.
With this in mind and with all the information that the standard gives mentioned in the previous
section, a list of measures can be done in order to reduce these risk components values:
5.3.1. M1: Installing an LPS of an appropriate class
This measure is directly related in the reduction of the probability of a flash to a structure to cause
physical damage to the structure, 𝑃𝐵 .
This probability is determined by the class of the lightning protection system, LPS, installed.
Upgrading the LPS implicates sometimes structural changes in order to protect both external and
internal structure, as it was said before. So, 𝑃𝐵 acts directly in both changes introduced by the upgrade
of the LPS.
With the alteration of the class of the LPS (both external and internal - 𝑃𝐵 ) it is time to look more
carefully at the internal LPS. This one is influenced by altering the value of the parameters 𝑃𝐸𝐵 , related
to lightning equipotential bonding.
There are three ways to achieve lightning equipotential bonding according to the IEC 62305-3 [3]. By
interconnecting the LPS with:
– metal installations;
– internal systems;
– external conductive parts and lines connected to the structure.
The parameter, 𝑃𝐸𝐵 , relates to the third way to achieve lightning equipotential bonding, more specific,
the equipotentialization of the lines connected to the structure.
With this in mind, two probabilities should be reminded: 𝑃𝑈 is the probability that a flash to a line
entering the structure will cause injury to living beings by electric shock due to touch voltage and 𝑃𝑉 is
the probability that a flash to a line entering the structure will cause physical damage to the structure.
𝑃𝐸𝐵 is the parameter common to these two probabilities which sometimes can significantly influence
the value of the risk. As higher as this level is, less probable is a flash to a line to cause injury to living
53
beings by electric shock (𝑃𝑈 ) and less probable is a flash to a line to cause physical damage (𝑃𝑉 ), by
the same factor.
And as it was already said, the probability 𝑃𝐵 influences the corresponding probabilities but for the
case of direct flashes to a structure: 𝑃𝐴 and 𝑃𝐵 itself.
As a result, upgrading the class of the LPS (M1 measure), the class of the surge protective devices,
SPDs, installed at the point of entrance of the lines connected to the structure, should be upgraded as
well with the same (or higher) LPL.
So, it can be said that the value of 𝑃𝐸𝐵 depends on 𝑃𝐵 when measures are taken to upgrade the LPS.
But, in the case of the surge protective devices should be of a higher class than the class of the LPS
installed, there is no mandatory requirement to upgrade the LPS in question. Therefore, it can be
implemented a measure only for upgrading the SPDs class (M2 measure). This can happen when, for
example, the function of the structure takes very serious the protection of the equipment inside the
structure, or the information flowing in the telecommunication lines are of greatly importance − Banks,
data centers, server storages, hospitals, etc.
Figure 14 shows an organization chart can make these relations more obvious.
Figure 14: M1 measure organization chart of the probabilities and parameters influenced by upgrading the LPL of
the LPS, 𝑷𝑩
To sum up, a decrease in the value of the probability of a flash to a structure to cause physical
damage to the structure, 𝑃𝐵 , implicates a direct reduction in the probability of a flash to a structure to
cause injury to living beings by electric shock, 𝑃𝐴 .
54
Also, changing the value of 𝑃𝐵 , implicates a mandatory change in the value of 𝑃𝐸𝐵 . By reducing 𝑃𝐸𝐵 ,
the probability of a flash to a line to cause injury to living beings by electric shock (𝑃𝑈 ) and the
probability of a flash to a line to cause physical damage (𝑃𝑉 ) decreases by the same factor, as it was
said before.
So, in terms of the reduction on the value of the risk, 𝑃𝐵 has a significant impact.
5.3.2. M2: Installing SPDs of an appropriate LPL at the line entrance point
As it was said, there are a number of situations when the surge protective devices should be of a
higher class than the class of the LPS installed.
Some rules for SPDs installation must be respected:
For external conductive parts, lightning equipotential bonding shall be established as near as possible
to the point of entry into the structure to be protected and all the conductors of each line should be
bonded directly or with an SPD. This will increase the level of effectiveness of these devices.
More over SPDs shall have the following characteristics:
– tested with 𝐼𝑖𝑚𝑝 ≥ 𝐼𝐹 where 𝐼𝑖𝑚𝑝 is the withstand current of the SPD, and 𝐼𝐹 is the lightning current
flowing along the lines;
– the protection level 𝑈𝑃 lower than the impulse withstand level of insulation between parts.
If these conditions are fulfilled the value of 𝑃𝐸𝐵 decreases the values of the probability of a flash to a
line to cause injury to living beings by electric shock (𝑃𝑈 ) and the probability of a flash to a line to
cause physical damage (𝑃𝑉 ) decreases by the same factor. Using the same reasoning as before for
measure M1 the figure (Figure 15) below shows better these relations.
Figure 15: M2 measure organization chart of the probabilities and parameters influenced by 𝑷𝑬𝑩
55
5.3.3. M3: Protection measures against the consequences of fire
Providing zone(s) with protection measures against the consequences of fire (such as extinguishers,
automatic fire detection system etc.). This will reduce ultimately the components 𝑅𝐵 and 𝑅𝑉 via the
reduction factor 𝑟𝑝 .
In this case, 𝑟𝑝 , the factor that reduces the loss due to provisions against fire influences the
components of loss in a structure related to physical damage, 𝐿𝐵 (flashes to structure) and 𝐿𝑉 (flashes
to line). As 𝐿𝐵 and 𝐿𝑉 decreases so the value of the risk components related to physical damage, 𝑅𝐵
and 𝑅𝑉 decreases (remind Equation (5)).
5.3.4. M4: Providing zone(s) with a coordinated SPD system for the internal power and telecom
systems
Lightning flashes to a structure (source of damage 𝑆1 ), near the structure (𝑆2 ), to a service connected
to the structure (𝑆3 ) and near a service connected to the structure (𝑆4 ) can cause failures or
malfunction of internal systems.
An equipment is protected if its rated impulse withstand voltage 𝑈𝑊 at its terminals is greater than the
surge overvoltage between the live conductors and earth. If not, an SPD must be installed.
SPDs properly selected, coordinated and installed result in a system intended to reduce failures of
electrical and electronic systems − a coordinated SPD system. It should be designed to protect the
cables crossing borders of the different lightning protection zones, LPZs, and possibly at the
equipment to be protected in order to limit conducted surges due to lightning on electrical lines.
The probability that an SPD does not adequately protect the equipment for which it is intended, is
equal to the probability that the discharge current at the point of installation of this SPD exceeds the
current at which the protection level was determined. That is where the probability 𝑃𝑆𝑃𝐷 takes its value.
As higher as the level of protection provided by the coordinated SPD system, less probable is the
discharge current exceeds the current at which the protection level was determined.
So, changing the value of 𝑃𝑆𝑃𝐷 implicates changes in the risk components influenced by failure of
internal systems 𝑅𝐶 , 𝑅𝑀 (through 𝑃𝐶 and 𝑃𝑀 , respectively) and 𝑅𝑊 , 𝑅𝑍 (through 𝑃𝑊 and 𝑃𝑍 ,
respectively), as shown in Figure 16 .
5.3.5. M5: Providing zone(s) with an adequate spatial grid-like shield.
This measure was chosen to reinforce the protection against negative consequences of current surges
in electrical equipment.
56
Figure 16: M4 measure organization chart of the probabilities and parameters influenced by 𝑷𝑺𝑷𝑫
Spatial shields define protected zones, which may cover the whole structure, a part of it, a single room
or the equipment enclosure only. These may be grid-like, or continuous metal shields, or comprise the
"natural components" of the structure itself.
So, there are three ways to ensure effectiveness in attenuation of the magnetic field effects in internal
systems.
The parameters that characterize spatial shields are 𝑤𝑚1 and 𝑤𝑚2 : mesh widths of grid-like spatial
shields, or of mesh type LPS down-conductors or the spacing between the structure metal columns, or
the spacing between a reinforced concrete framework acting as a natural LPS.
The indexes 1 and 2 in 𝑤𝑚 corresponds to the factor relevant to the screening effectiveness of the
structure, 𝐾𝑆1 , and the factor relevant to the screening effectiveness of shields internal to the structure,
𝐾𝑆2 , respectively.
Inside a lightning protection zone, LPZ, at a safety distance from the boundary screen at least equal to
the mesh width, 𝑤𝑚 , factors 𝐾𝑆1 and 𝐾𝑆2 for LPS or spatial grid-like shields may be evaluated as:
𝐾𝑆1 = 0.12 × 𝑤𝑚1
𝐾𝑆2 = 0.12 × 𝑤𝑚2
(17)
(18)
For continuous metal shields with thicknesses not lower than 0.1 𝑚𝑚𝑚𝑚, 𝐾𝑆1 = 𝐾𝑆2 = 10–4 .
Ultimately, in order to reduce the probability that a flash near a structure cause failure of internal
systems, 𝑃𝑀 , a grid-like LPS, screening, routing precautions, increased withstand voltage, isolating
interfaces and coordinated SPD systems are suitable as protection measures.
So, the mesh widths 𝑤𝑚 , affect the value of the factors 𝐾𝑆 that changes the value of 𝑃𝑀𝑆 that alters the
value of the probability 𝑃𝑀 by the same factor (see Figure 17).
57
At last, the reduction of the probability 𝑃𝑀 results, as well, in the reduction of the value of the risk
component 𝑅𝑀 (related to failure of internal systems – flashes near structure).
In top of all this, there are yet two other parameter that can be help on the reduction of the risk
component 𝑅𝑀 .
− 𝐾𝑆3 takes into account the characteristics of internal wiring: shielded or unshielded cables; cables
with or without routing precautions to avoid large loops; cables protected inside metal conduits or not.
Reducing this parameter can decrease the value of the risk component related to failure of internal
systems, 𝑅𝑀 , by the same or even higher factor than 𝐾𝑆1 and 𝐾𝑆2 .
Its value ranges from 1 (Unshielded cable – no routing precaution in order to avoid loops) to 0.0001
(Shielded cables and cables running in metal conduits). Consult Table A.17 in the Appendix A section
for more information.
− 𝐾𝑆4 takes into account the impulse withstand voltage of the system to be protected.
𝐾𝑆4 =
1
𝑈𝑊
(19)
So, there are two sub-measures that can also be taken into account:
M5.1: Check the internal wiring and upgrading the level of protection of the cables by shielding them
and take routing precautions;
M5.2: Upgrade the electrical and the telecommunication equipment with higher values of withstand
voltage.
These two sub-measures will decrease the probability that a flash near a structure cause failure of
internal systems, 𝑃𝑀 , and consequently, the equivalent risk component, 𝑅𝑀 .
In Figure 17, the relations between these parameters can be better understood.
Figure 17: M5 measure organization chart of the probabilities and parameters influenced by the mesh widths, 𝒘𝒎
58
Table 10 reminds which protection measures influences each risk components.
Table 10: Protection measures vs risk components
Protection Measures
M1
M2
M3
M4
M5
M5.1
M5.2
X
X
X
Risk Components
𝑹𝑨
X
𝑹𝑩
X
𝑹𝑼
X
𝑹𝑽
X
X
X
X
X
𝑹𝑪
X
𝑹𝑾
X
𝑹𝑴
X
𝑹𝒁
X
By analyzing Table 10 it can be found some relations:
− M1 is an important measure because it reduces the values of risk components related to injury
(caused by electric shock) and physical damage (caused by fire or explosion) to living beings.
Applying M1 it is on the best interests of everyone in the structure;
− M4 is the measure that must be applied if failure of internal systems is the number one priority by
designing a coordinated SPD system. In association with M5 measure that accounts the effects of
LEMP, the risk component 𝑅𝑀 becomes of great relevancy. As 𝑅𝑀 is a risk component important to the
good functioning of the structure, reducing it implicates a significantly reduction in the value of risk.
As physical damage and failure of internal systems are usually the mainly consequences that
influenced the value of the risk, these measures are adequate and effective in the reduction of the risk.
Moreover, Table 11, present in the standard, relates various characteristics with the respective risk
component.
59
Table 11: Characteristics of the structure or of the internal systems that influence the risk components
(according to IEC 62305-2 [2], table 3, page 23)
Characteristics of structure
𝑅𝐴
𝑅𝐵
𝑅𝐶
𝑅𝑀
𝑅𝑈
𝑅𝑉
𝑅𝑊
𝑅𝑍
Collection area
X
X
X
X
X
X
X
X
Surface soil resistivity
X
Floor resistivity
X
X
X
X
X
X
X
X
X
X
X
X
or of internal systems
Protection measures
Physical
restrictions,
insulation, warning notices,
soil equipotentialization
LPS
X
X
Bonding SPD
X
X
Isolating interfaces
Coordinated SPD system
X𝑎
X𝑏
X𝑏
X𝑐
X𝑐
X
X
X
X
X
X
Spatial shied
X
X
X
Shielding external lines
X
Shielding internal lines
X
X
Routing precaution
X
X
Bonding network
X
X
Fire precaution
X
X
Fire sensitivity
X
X
Special hazard
X
X
Impulse withstand voltage
𝑎
𝑏
𝑐
Only for grid-like external LPS
Due to equipotential bonding
Only if they belong to equipment
60
X
X
X
X
6. Application of the computer program – L.R.A.
As it was stated in the beginning of the work, the objective of this work is to develop a computer
program that supports lightning protection projects for structures based on the standard IEC 62305.
The program has the goal to make project analysis of risk management of lightning protection systems
a process much faster and intuitive.
The aim of this chapter is to show how the program works.
6.1. L.R.A. structure
Take Equation (5), in 5.2. (Risk Evaluation, page 39) section – 𝑅𝑋 = 𝑁𝑁𝑋 × 𝑃𝑋 × 𝐿𝑋 – as a reference to
describe the methodology adopted in the program to calculate the risk components and consequently,
the overall risk (sum of the risk components).
In order to accomplish that, the program is divided in 6 tabs.
The first step is to determine the number of dangerous events per year (section 5.2.1. Number of
dangerous events, page 40), given by Equation (6). According to this equation, 𝑁𝑁𝑋 depends on the
lightning ground flash density, the collection area and on parameters that depend on the structure and
connected lines environmental and geometric characteristics. Conclusion, 𝑁𝑁𝑋 number of dangerous
events, is directly related with the source of the lightning (see Table 8).
So, in the program, the first two tabs are related to, (1) the structure and (2) the connected lines
characteristics and its environmental surroundings. These tabs allow to calculate directly all the
collection areas and all the number of dangerous events per year, just as Table 8 describes.
Then, the type of loss is analyzed (section 5.2.2., page 43), meaning, the mean relative amount of a
particular type of damage for one dangerous event caused by a lightning discharge, considering both
its extent and effects.
Taking this definition, it is obvious that the structure internal characteristics are dependent on the type
of loss. So, only after choosing the type of loss it can begin the study of the structure internal
characteristics. This defines the third and fourth tab in the program, respectively.
For this evaluation, as it was already said, the structure can be considered as a whole or divided by
zones. Either way, for each zone, a set of parameters must be associated with it in order to
characterize the internal composition of the structure to be protected. In terms of lightning protection
system, this evaluation describes the internal component of the LPS, as the first two tabs generally
describe the conditions of the structure regarding to the external LPS.
61
To calculate the risk components still remain to describe the probability of damage to the structure to
be protected. According to section 5.2.3. (page 48), as there are eight risk components, there are
eight probability components, 𝑃𝑋 , and they are grouped according to the source and type of damage.
With that said and reminding that the previous parameters are also dependent on the source and type
of damage of lightning, it is safe to say that these probabilities components are either present in the
external and in the internal components of the LPS. So, for example, the 𝑃𝐵 and 𝑃𝐸𝐵 probabilities are
defined in the first tab (related to the structure external characteristics) and 𝑃𝑆𝑃𝐷 , 𝑃𝑇𝑈 and 𝑃𝑇𝐴
probabilities are defined in the fourth tab (related to the structure internal characteristics).
With all the dangerous events, probabilities and losses characterized, it only remains to calculate the
risk. The fifth tab is where the risk calculations are presented. In the form of a table, the eight risk
components and the overall risk are shown and finally the overall risk is compared with the value of
tolerable risk (according with the type of loss chosen) in order to conclude that the structure is at risk
or not.
Also, if the risk evaluated is 𝑅4 (risk of loss of economic value of a structure), the program permits an
economic evaluation, in which compares the cost of loss based on the risk of the structure without
taking any protective measures and the sum between the cost of loss based on the risk of the
structure calculated when protective measures are taken and the cost of construction and
maintenance of those measures. All these are calculations made in ‘per year’.
This comparison gives the annual savings which can be a very useful indicator for the engineer when
designing the lightning protection measures.
For last, with the value of the risk calculated, it is required to determine the need of protection
measures against lightning: if the value of the overall risk, 𝑅, is higher than the tolerable value defined
by the standard, 𝑅𝑇 , by definition, and more importantly, for security reasons, protection measures
against lightning must be taken. The program provides the user with a set of protective measures and
they are given according with the three higher risk components, for each zone.
For example, if the risk components related to physical damage (𝑅𝐵 , 𝑅𝑉 ) are the ones that most
influence the overall risk value, suitable protective measures are the construction/upgrade of an LPS
and equipotential bonding at the entrance of the connected lines to the structure.
The following flowchart shows the structure of the computer program L.R.A.. As it can be seen, it
follows the 6 tabs of the program (see Figure 18):
(1) Structure: geometric and environmental characteristics;
(2) Connected lines: geometric and environmental characteristics;
(3) Type of loss;
(4) Lightning protection zones characteristics;
(5) Results;
(6) Lightning protection measures.
62
Figure 18: Flowchart of the L.R.A. structure
6.2. Example 1: Hospital
The case studies examples presented in IEC 62305-2 [2] are a good reference for demonstrating its
risk management approach. For that reason, the goal of this section is to validate the results that the
developed program gives by comparing the resultant risk values with the ones presented in the
standard 62305. The chosen example of application is the standard’s case study of an “Hospital”. This
case study is very complex because:
-
The structure is divided in 4 different zones;
-
Takes into account the effect of an adjacent structure connected by a line;
-
Calculates the risk 𝑅1 – risk of loss of human life in a structure – and the risk 𝑅4 – risk of loss of
economic value in a structure, which includes an economic evaluation;
-
Presents 3 solutions using different protective measures in order to reduce the value of the risk.
Implementing this example will permit to describe almost all the features of the program, which are:
-
Calculate the risk and determine the need for protection – Primary objective;
-
Show the contribution of the different risk components to the overall risk, 𝑅;
63
-
According with these risk components suggest appropriate lightning protective measures, and recalculate the risk including them;
-
Provide additional information data regarding the collection areas and the number of dangerous
events, which can help in the design of the lightning protection system;
-
Make a (basic) economic evaluation when requested.
The following sections are related with the computation of the value of the risk of loss of human life in
a structure – 𝑅1 and the value of risk of loss of economic value in the structure, 𝑅4 .
6.2.1. Structure: geometric and environmental characteristics
This is the tab where the structure’s geometric and environmental characteristics are defined. The
parameters that translate those characteristics are presented in Table 12:
Table 12: Input parameters that describe the geometric and environmental characteristics of the structure
Input Parameter
Symbol
Lightning ground flash density
𝑁𝑁𝐺
Length, Width, Height
Location factor
Probability that a flash to a structure will cause
physical damage
Parameter that relates the lightning equipotential
bonding with the lightning protection level for
which SPDs are designed
Parameter that takes into account the screening
effectiveness of the structure, of the LPS
[𝑘𝑚𝑚−2 . 𝑦𝑒𝑎𝑟 −1 ]
𝐿, 𝑊, 𝐻
[𝑚𝑚]
𝐶𝐷
Value
Description
4
-
50, 150, 10
-
1
Isolated structure
1
No LPS installed
𝑃𝐸𝐵
1
No SPD installed
𝐾𝑆1
1
𝑃𝐵
No spatial shielding
considered
Figure 19 shows how the input of these parameters can be made. It can be made manually by tipping
a valid number 1, or automatically by pressing the corresponding button with the tag “Hint”. These
buttons open a dialog box where it is explained what the respective parameter translates, which
values can assume and choose the desired value.
1
NOTE: By valid number it means a value according to the standard 62305-2 [2]. Besides the standard predicts
that some of the values here presented can be modify by national committees to better translate the national
conditions. At the present form, the program does not allow the introduction of new values.
64
Figure 19: Inputs related to the geometric and environmental characteristics of the structure
For example, in the case of the “Hint” button for the 𝑁𝑁𝐺 parameter it permits the calculation of the
lightning flash density by the keraunic number (number of thunderstorm days per year) (see Equation
(7)) or by the visualization of a world map of lightning flash density [𝑘𝑚𝑚−2 . 𝑦𝑒𝑎𝑟 −1 ], as it can be seen
next in Figure 21.
6.2.2. Connected lines: geometric and environmental characteristics
The first question to be made is:
Figure 20: Number of lines (power + telecommunications) connected to the structure to be protected
65
Figure 21: Lightning ground flash density computation by the keraunic number 𝑻𝒅 and the 𝑵𝑮 world map
distribution [49]
66
Although the number of lines could be any one, there are going to be just two lines of interest: one
power line and one telecommunication line. The program processes as many lines as the user wants
but to the calculation of the risk it only goes the values that correspond to the lines (one for power and
one for telecommunication) that translate the higher risk to the structure, meaning, the lines that result
in a higher number of dangerous events, 𝑁𝑁𝐿 (number of dangerous events due to flashes to a line) and
𝑁𝑁𝐼 (number of dangerous events due to flashes near a line).
The parameters under scrutiny are presented in Table 13.
In this example two lines are considered, one of power and one of telecommunication. As it can be
seen from Figure 22 the line of telecommunication is connected to another structure, the so-called
adjacent structure.
Figure 22: Inputs related to characteristics of the connected lines to the structure
It is important to remember that by including the characterization of the lines connected to the
structure to be protected, it permits to quantify the effects of lightning strikes to and near a line, which
influences 4 of the 8 risk components.
67
Table 13: Input parameters that describe the geometric and environmental characteristics of the connected lines
Input Parameter
Power line
Telecommunication line
Length of line section [𝑚𝑚]
𝐿𝐿 = 500 𝑚𝑚
𝐿𝐿 = 300 𝑚𝑚
Installation factor
Line type factor
Environment factor
Factor depending on shielding,
grounding and isolation conditions of
the line for flashes to or near a line
Length, width and height of an
adjacent structure [𝑚𝑚]
Location factor of an adjacent
𝐶𝐼 = 0.5 : Buried line
a system [𝑘𝑉]
Shield resistance per unit length of a
cable [Ω/𝑘𝑚𝑚]
Probability of failure of internal
systems due to a flash to or near the
connected line depending on the
line and equipment characteristics
𝐶𝑇 = 1 : LV
𝐶𝑇 = 0.2 : HV power line
telecommunication line
environment
environment
𝐶𝐸 = 0.5 : Suburban
𝐶𝐸 = 0.5 : Suburban
𝐶𝐿𝐷 = 1 : Shielded buried line
𝐶𝐿𝐷 = 1 : Shielded buried line
𝐶𝐿𝐼 = 0 : Shielded buried line
𝐿𝐽 = − ; 𝑊𝐽 = − ; 𝐻𝐽 = −
𝐶𝐿𝐼 = 0 : Shielded buried line
𝐿𝐽 = 20 𝑚𝑚;
𝑊𝐽 = 30 𝑚𝑚; 𝐻𝐽 = 5 𝑚𝑚
𝐶𝐷𝐽 = −
𝐶𝐷𝐽 = 1 : Isolated structure
𝑈𝑊 = 2.5 𝑘𝑉
𝑈𝑊 = 1.5 𝑘𝑉
𝑅𝑆 ≤ 1 Ω/𝑘𝑚𝑚
1 Ω/𝑘𝑚𝑚 < 𝑅𝑆 ≤ 5 Ω/𝑘𝑚𝑚
structure
Rated impulse withstand voltage of
𝐶𝐼 = 0.5 : Buried line
𝑃𝐿𝐷 = 0.2
𝑃𝐿𝐼 = 0.3 : Power line
𝑃𝐿𝐷 = 0.8
𝑃𝐿𝐼 = 0.5 :
Telecommunication line
6.2.3. Type of loss
In this tab the goal is to choose the type of loss – see Figure 23.
Each type of damage, alone or in combination with others, may produce a different consequential loss
in the structure to be protected and in this tab the user can choice which type of loss it wants to be
analyzed. As it was said earlier in the work, there are 4 types of loss that the standard considers to be
relevant (see section 5.1., page 35):
L1: loss of human life (including permanent injury);
L2: loss of service to the public;
L3: loss of cultural heritage;
L4: loss of economic value (structure, content, and loss of activity).
68
For each type of loss, a set of zone parameters are assigned. For the “Hospital” example it will be
made the evaluation for the loss of human life, 𝐿1 , and for the loss of economic value, 𝐿4 .
Figure 23: Type of loss in analysis
6.2.4. Lightning protection zones
This tab permits the selection of several zones (up to ten), which can be considered in detail. Normally
these zones are distinguished because there is some kind on particularity that can significantly
increase or decrease the value of the risk.
The program gives some example of zones that usually fulfill those particularities.
Also, the user must input the total number of people that are regularly present in the structure – 𝑛𝑡 .
Figure 24 shows that for the “Hospital” example the chosen zones are the “Operating room; “Rooms
block”; “Outside of the building” and “Other” (in this case is the “Intensive Care unit) and that there are
regularly 1000 persons inside the structure.
Then, each zone is described by a set of parameters that, according with the chosen type of loss, will
contribute more or less to the value of the risk. It can be accessed by defining the number of people
that are regularly in each chosen zone, 𝑛𝑧 , and pressing the button “Ok” – see Figure 25.
69
Figure 24: Definition of the lightning protection zones
Figure 25: Unlocking the buttons to enable the characterization of each zone
70
The parameter 𝑛𝑧 is important for the loss of human life, 𝐿1 , because it translates the “weight of the
zone”, meaning that the percentage given by the relation 𝑛𝑧/𝑛𝑡 represents the contribution of the zone
for the loss of human life in the structure and by consequence, the overall risk 𝑅1 .
Pressing the button “Ok” unlocks the zone and the parameters can now be inserted – Figure 26. The
button “Hint” shows a table with a summary description of these parameters (see section 5.2.3., page
48). In Table 14 the input parameters of the zone “Rooms Block” are presented.
Table 14: Input parameters that describe the zone “”Rooms Block” for the Hospital example
Input Parameter
Symbol
Value
Comment
Type of floor
𝑟𝑡
𝑟𝑡 = 1 × 10−5
Linoleum floor
𝑃𝑇𝐴 = 1
None
𝑃𝑇𝑈
𝑃𝑇𝑈 = 1
None
Protection against shock (flash to
a structure)
Protection against shock (flash to
a line)
Risk of fire
Fire protection
Internal spatial shield
Power line
Internal wiring
Coordinated SPDs
Telecom
Internal wiring
line
Coordinated SPDs
𝑃𝑇𝐴
𝑟𝑓
𝑟𝑓 = 1 × 10−2
Ordinary
𝐾𝑆2
𝐾𝑆2 = 1
None
𝐾𝑆3
𝐾𝑆3 = 0.2
conductors in the same
𝑃𝑆𝑃𝐷
𝑃𝑆𝑃𝐷 = 1
None
𝐾𝑆3
𝐾𝑆3 = 0.01
conductors in the same
𝑃𝑆𝑃𝐷
𝑃𝑆𝑃𝐷 = 1
None
𝐿𝑇
𝐿 𝑇 = 1 × 10−2
𝐿𝐹
𝐿𝐹 = 1 × 10−1
𝐿𝑂
𝐿𝑂 = 1 × 10−3
𝑟𝑝
ℎ𝑧
𝐿1 : Loss of human life
Factor for persons in the zone
(𝑛𝑧 /𝑛𝑡 ) ×
(𝑡𝑧 /8760)
𝑟𝑝 = 1
ℎ𝑧 = 5
0.95
None
Unshielded (loop
conduit)
Unshielded (loop
cable)
Difficult to evacuation
𝐷1 : due to touch and
step voltage
𝐷2 : due to physical
damage
𝐷3 : due to failure of
internal systems
“Weight of the zone”
71
Figure 26 show the input of above parameters made for the zone “”Rooms Block”, in the case of the
type of loss 𝐿1 is chosen. The tolerable risk value associated with this type of loss is 𝑅𝑇 = 1 × 10−5
(see Table 6).
Figure 26: Parameters that describe the zone “”Rooms Block” for the Hospital example
6.2.5. Results
Finally, the results tab, where the values of all the components of the risk are shown. With this, it is
possible to reach to the value of the overall risk, 𝑅, which by comparison with the value of the tolerable
risk, 𝑅𝑇 , enables the user to respond to the most important question: “Is the structure in study at risk
from lightning?”
-
If the answer is negative, the program reaches its final step. This happens when the overall
risk is smaller than the value of the tolerable risk imposed by the type of loss in analyzes.
However, if the user wants to pursuit a smaller value than 𝑅, it can go to the final tab of the program
and select appropriate protective measures.
-
If the answer is affirmative, the program gives two choices: go to the final tab and select
protective measures; or go to the previous tabs and alter values of some parameters and re-calculate
the risk.
Also, in this tab, it is possible to visualize the resultant values of the different collection areas and
annual dangerous events, by pressing the button “Additional Information”.
72
So, for the “Hospital” example, the value of the risk is shown in Figure 27. Observing this figure, it can
be seen that the overall risk has a very high value, in relation with the associated tolerable risk value –
𝑅𝑇 = 1 × 10−5 . So, lightning protection is needed for this structure.
The overall risk is 𝑅1 = 69.9585 × 10−5 (≫ 𝑅𝑇 ). Analyzing the results in Figure 27, it can be seen that
the zone denominated by 𝑍2 - the “Room Block” zone, has a substantially contribution to 𝑅1 . Still in 𝑍2 ,
the risk component with the higher value is 𝑅𝐵 – related to physical damage to the structure caused by
direct lightning discharges to the structure. The reason can be explained by remembering Table 11
that dictates that fire precautions, fire sensitivity and special hazards are the factors that more
influence this risk component, which in this case, assume the values 𝑟𝑝 = 1, 𝑟𝑓 = 10−2 (high value) and
ℎ𝑧 = 5 (high value), respectively. Finally, in 𝑍2 , 𝑛𝑧 = 950 (number of persons in the zone 𝑍1 ) of
𝑛𝑡 = 1000 (total number of persons in the structure): the zone has the higher “weight” in the overall
value of the risk.
Figure 27: Risk 𝑹𝟏 result – risk of loss of human life, for each zone, by risk component
73
6.2.6. Lightning protection measures
Further to evaluating the need of lightning protection, the program suggests adequate protective
measures and allows to anticipate the decrease on the risk value that can be achieved by each of the
selected ones.
So, the program gives to the user the set of protective measures that are more likely to decrease the
value of the three higher risk components of each zone, as it can be seen in Figure 28.
The user selects the desired protective measures and by pressing the button “Apply measures”, the
value of the risk is re-calculated and the tab of “Results” re-appear with the new values provided by
the introduction of protective measures.
Figure 28: Selection of appropriate lightning protection measures according with the three most influential risk
components for the overall risk
In the example of the “Hospital”, the standard gives 3 solutions:
74
Solution 1)
– Protect the building with a Class I LPS (𝑃𝐵 = 0.02, including also 𝑃𝐸𝐵 = 0.01);
– Install coordinated SPD protection on internal power and telecom systems for (1.5 ×) better than
𝐿𝑃𝐿 𝐼 (𝑃𝑆𝑃𝐷 = 0.005) in zones 𝑍1 , 𝑍2 , 𝑍4 ;
– Provide zone 𝑍2 with an automatic fire protection system (𝑟𝑝 = 0.2, for zone 𝑍2 only);
– Provide zone 𝑍1 and 𝑍4 with a meshed shield with 𝑤𝑚 = 0.5 𝑚𝑚.
Figure 29 shows the results for Solution 1: 𝑅1 = 0.3377 × 10−5 .
As the risk component 𝑅𝐵 (physical damage to the structure) in the zone 𝑍2 (Rooms Block) has the
higher value, the construction of a LPS in the structure and the installation of an automatic fire
protection system in zone 𝑍2 are the measures that theoretically more contribute to its reduction
(remember Table 11) and by consequence, the value of the overall risk.
It is now when the computer program L.R.A. can be very useful. It permits to analyze measure by
measure and let the user conclude what lightning protection measure is more effective in risk
reduction.
So, analyzing the effectiveness of the first two measures, if only one is selected the decrease in the
risk 𝑅1 is around ~73% and 59%, respectively. But when both measures are implemented the
reduction does not increase substantially (~74%). This happens because both measures actuated in
the same risk component and so, the effectiveness of the reduction decreases. This information is
very important because, in this case, instead of implementing both measures, the substantial
reduction of the risk can be achieve by only using one of the measures – cheaper solution.
Figure 29: Re-calculated risk after taking the lightning protection measures of Solution 1
75
In the end, Solution 1 gives a reduction of 99.52% in relation with the value of the risk when the
structure is not protected. The increase of the reduction of the risk is due to the introduction of the
measures that actuates in preventing the failures of internal systems 𝑅𝐶 , 𝑅𝑀 , 𝑅𝑊 and 𝑅𝑍 , which in this
case is achieved by installing a coordinated SPD protection on internal power and telecom systems
and by installing a meshed shield with 𝑤𝑚 = 0.5 𝑚𝑚. The same effectiveness analysis can be made for
these two measures and the conclusion is that implementing the coordinated system is much more
effective than constructing a mesh shield in terms of reducing the risk: 98% and 5.5% decrease in the
overall risk value 𝑅1 , respectively (with the LPS of class I and the fire protection system also installed).
Solution 2)
– Protect the building with a Class I LPS (𝑃𝐵 = 0.02, including also 𝑃𝐸𝐵 = 0.01);
– Install coordinated SPD protection on internal power and telecom systems for (3 ×) better than 𝐿𝑃𝐿 𝐼
(𝑃𝑆𝑃𝐷 = 0.001) in zones 𝑍1 , 𝑍2 , 𝑍4 ;
– Provide zone 𝑍2 with an automatic fire protection system (𝑟𝑝 = 0.2, for zone 𝑍2 only).
In this case, as in Solution 1, the construction of the LPS and the improved fire protection system give
the same reduction of the value of the risk.
As, in this case of the Hospital example, constructing the mesh shield is not very effective in reducing
the risk, to have better protection, meaning, lower even more the risk 𝑅1 , the coordinated SPD
systems can be improved. Figure 30 shows the results for Solution 2: 𝑅1 = 0.2224 × 10−5 .
Figure 30: Re-calculated risk after taking the lightning protection measures of Solution 2
76
Solution 3)
– Protect the building with a Class I LPS (𝑃𝐵 = 0.02, including also 𝑃𝐸𝐵 = 0.01);
– Install coordinated SPD protection on internal power and telecom systems for (2 ×) better than 𝐿𝑃𝐿 𝐼
(𝑃𝑆𝑃𝐷 = 0.002) in zones 𝑍1 , 𝑍2 , 𝑍4 ;
– Provide zone 𝑍2 with an automatic fire protection system (𝑟𝑝 = 0.2, for zone 𝑍2 only);
– Provide zone 𝑍1 and 𝑍4 with a meshed shield with 𝑤𝑚 = 0.1 𝑚𝑚.
This solution reinforces the conclusion made in Solution 2 as the installation of the coordinated SPD
protection on internal power and telecom systems reduces much more the value of the overall risk
than the construction of a mesh shield.
The results for Solution 3 can be seen in Figure 31: 𝑅1 = 0.2505 × 10−5 .
Figure 31: Calculated risk after taking the lightning protection measures of Solution 3
Looking at the new values of the overall risk 𝑅1 , the differences between them are not very big. So,
how to choose between the three solutions? This is when the assessment of the risk 𝑅4 can be very
helpful.
The process is identical to the previous presented. The only differences occurs when choosing the
type of loss, in this case, 𝐿4 and when the input of the zone parameters associated with the loss 𝐿4 𝐿 𝑇 , 𝐿𝐹 , 𝐿𝑂 and the costs 𝑐𝑎 , 𝑐𝑏 , 𝑐𝑐 , 𝑐𝑠 (remember from 5.2.3., page 48).
Figure 32 illustrates that better, using for example the “Intensive care unit” zone.
77
Figure 32: Parameters that describe the zone “”Intensive Care Unit” from the Hospital example
With the risk 𝑅4 calculated for Solution 1: 𝑅4 = 0.3012 × 10−5 (see Figure 33), an economic evaluation
can be made. As this subject is outside of the scope of the work, the program only allows the user to
better choose from the 3 solutions by giving the annual savings for each of the solutions presented.
78
Figure 33: Calculated risk, 𝑅4 – risk of loss of economic value, for each zone, by type of risk
Figure 34 shows the process for Solution 1 – the program requires the input of the (approximate)
overall cost of the protective measures for each solution in study and the relevant rates that will
applied: 𝑖 – interest rate; 𝑎 – amortization rate; 𝑚𝑚 – maintenance rate.
Figure 34: Simplified economic evaluation based on risk analysis
Finally, the program takes the previous inputs and calculates four parameters: the cost of loss when
the structure is unprotected, 𝐶𝐿 ; the cost of loss when the structure is protected, 𝐶𝑅𝐿 ; the annual cost of
the measures including the relevant rates, 𝐶𝑃𝑀 ; and finally the value of annual savings 𝑆𝑀 , which
provides the value for comparison between the different solutions. Remember the 𝑐𝑡 designation:
overall cost.
In the case of this practical example of the “Hospital” the best solution is Solution 2, because it has the
set of lightning protection measures that provides the highest value of annual savings with the lowest
annual investment of the three solutions, as it can be seen in Figure 35.
As the measure for the construction of a mesh shield is the common element in Solutions 1 and 3, the
conclusion is that it must be more expensive in relation with the installation of a coordinated SPD
system.
It is important to remind that this example has the goal to validate the developed program, which it
has. All the reached values using the L.R.A. program are exactly the same as the ones presented in
the examples from standard IEC 62305 [2]. That is why the values in Figure 34 and Figure 35 are in
dollars.
Solution 1)
79
Solution 2)
Solution 3)
Figure 35: Annual savings, 𝑺𝑴 , for the three solutions presented
6.3. Example 2: South Tower in Instituto Superior Técnico (IST), Alameda Campus
In the following example, the south tower of IST is going to be evaluated. Figure 36 shows the
structure.
Figure 36: South tower in Instituto Superior Técnico
80
The input parameters that describe the geometric and environmental characteristics of the structure
and assuming that the tower does not have installed any lightning protection system, surge protective
devices or special screening, the following parameters are presented in Table 15.
Table 15: The input parameters that describe the geometric and environmental characteristics of the structure
Input Parameter
Symbol
Lightning ground flash density
𝑁𝑁𝐺
Length, Width, Height
Location factor
Probability that a flash to a structure will cause
physical damage
Parameter that relates the lightning equipotential
bonding with the lightning protection level for
which SPDs are designed
Parameter that takes into account the screening
effectiveness of the structure, of the LPS
[𝑘𝑚𝑚−2 . 𝑦𝑒𝑎𝑟 −1 ]
𝐿, 𝑊, 𝐻
[𝑚𝑚]
𝐶𝐷
Value
Description
1.2
-
30, 30, 50
-
1
Isolated structure
1
No LPS installed
𝑃𝐸𝐵
1
No SPD installed
𝐾𝑆1
1
𝑃𝐵
No spatial shielding
considered
Then, the input parameters that characterize the connected lines: one for power and other for
telecommunication that lead to the higher values of dangerous events are presented in Table 16.
Table 16: Parameters of power and telecommunication lines
Power line
Telecommunication line
𝐿𝐿 = 250 𝑚𝑚
𝐿𝐿 = 150 𝑚𝑚
𝐶𝐼 = 0.5 : Buried line
𝐶𝐼 = 0.5 : Buried line
𝐶𝑇 = 1 : LV power line
𝐶𝑇 = 1: LV telecommunication line
𝐶𝐿𝐷 = 1 : Shielded buried line
𝐶𝐿𝐷 = 1 : Shielded buried line
𝐶𝐸 = 0.1 : Urban environment
𝐶𝐸 = 0.1 : Urban environment
𝐶𝐿𝐼 = 0 : Shielded buried line
𝐶𝐿𝐼 = 0 : Shielded buried line
𝑊𝐽 = −
𝑊𝐽 = −
𝐶𝐷𝐽 = −
𝐶𝐷𝐽 = −
𝐿𝐽 = −
𝐻𝐽 = −
𝑈𝑊 = 2.5 𝑘𝑉
𝐿𝐽 = −
𝐻𝐽 = −
𝑈𝑊 = 1.5 𝑘𝑉
𝑅𝑆 ≤ 1Ω/𝑘𝑚𝑚
1Ω/𝑘𝑚𝑚 < 𝑅𝑆 ≤ 5Ω/𝑘𝑚𝑚
𝑃𝐿𝐼 = 0.3 : Power line
𝑃𝐿𝐼 = 0.5 : Telecommunication line
𝑃𝐿𝐷 = 0.2
𝑃𝐿𝐷 = 0.8
81
With the structure and lines geometric and environmental characteristics taken care of, it is possible to
calculate the collections areas and the number of dangerous events, following the equations in Table
8.
As the type of loss under study is the loss of human life (𝐿1 ) the distinctive zone parameters are
ℎ𝑧 , 𝐿 𝑇 , 𝐿𝐹 and 𝐿𝑂 . Also, the tolerable risk value associated with this type of loss is once again
𝑅𝑇 = 1 × 10−5 .
In this case, there are several zones that deserve special attention due to some features that define
them:
-
Offices: due to the its high number (80!); it can turn the evacuation of these zones specially
difficult;
-
Library: zone always associated with a high level of risk of fire;
-
Chemical laboratory: due to the high level of electric and electronic equipment that operate at very
low voltages (possible damage due to overvoltages) and the chemical components that may
cause explosions;
-
Multimedia room: again, the high level of electric and electronic equipment is a serious risk to
failure of internal systems;
-
Classroom / Auditorium / Study room: due to the regularly high number of people present in these
zones.
According to the IST website [50], Table 17 is created:
Table 17: Distribution of people inside each protection zone
Zone
Description
Number of
people
𝑍1
Offices
73 offices
80
Library
2 library
20
𝑍3
Chemical laboratory
7 chemical laboratory
35
𝑍4
Multimedia room
6 multimedia room
15
Classroom /
4 auditoriums /
𝑍5
Auditorium /
4 classroom /
Study room
2 study room
𝑍2
TOTAL
200
350
So, 5 zones are defined and for each one, the following parameters are the ones that describe them:
-
The floor is made of wood or linoleum – 𝑟𝑡 = 1 × 10−5 ;
No special measures are taken against shock – 𝑃𝑇𝐴 = 𝑃𝑇𝑈 = 1 ;
Zones with low risk of fire and with no provisions for fire protection – 𝑟𝑓 = 0.001 and 𝑟𝑝 = 1, except
in the library and chemical laboratories zones where the risk of fire is considered high – 𝑟𝑓 = 0.1,
and with the last with fire protection system – 𝑟𝑝 = 0.5;
82
-
It is assumed no internal spatial shielding and no SPD system installed for internal systems –
𝐾𝑆2 = 1, 𝑃𝑆𝑃𝐷 = 1, respectively. For the internal wiring, 𝐾𝑆3 = 0.2 for power wiring (with routing
precautions) and 𝐾𝑆3 = 1 for telecommunication wiring (with no routing precautions), except in the
zone of laboratories and multimedia room where routing precautions are taken for both wiring,
-
𝐾𝑆3 = 0.01;
Although the average number of people in some zones and the number of floors makes the overall
structure to be considered difficult to evacuate (ℎ𝑧 = 5), the structure in question provides its
occupants double escape routes by stairs, double average high speed elevators and well
distributed, in every floors, security illumination and evacuation maps providing efficient escape
-
routes. So, in a case of an evacuation situation it is considered a low level of panic – ℎ𝑧 = 2;
Damage 𝐷1 : considered injuries due to shock – 𝐿 𝑇 = 0.01 ;
Damage 𝐷2 : accounts physical damage considering the structure as a school – 𝐿𝐹 = 0.1 ;
Damage 𝐷3 : failure of internal systems – this type of damage it only makes sense to be different
from 0 on the laboratory zones, where the high number of electric and electronic systems can be
affect by an overvoltage or spark that can result in an explosion and/or initiate a fire. So, it is
interesting to make two evaluations, one considering a risk of explosion when lightning affects the
internal systems of the zone of the laboratories and another considering no additional risk to
failure of these internal systems:
− 𝐿𝑂 = 0, assuming no risk of explosion (1);
− 𝐿𝑂 = 0.1, assuming risk of explosion (2).
Figure 37 show the input of the parameters for the laboratory zones:
Figure 37: Parameters that describe the laboratory zones of the South Tower in IST
83
The results for the two evaluations are shown in Figure 38, for 𝐿𝑂 = 0 and Figure 39, for 𝐿𝑂 = 0.1.
Figure 38: Calculated risk for evaluation (𝟏): 𝑳𝑶 = 𝟓𝟓
84
Figure 39: Calculated risk for evaluation (2): 𝐿𝑂 = 0.1
Clearly, the value of the risk components are significantly higher when the risk of explosion is
accounted for (2), than the evaluation where the risk of explosion is not assumed (1). In the end, both
cases need lightning protection. Remember that 𝐿𝑂 is the loss in the structure due to failure of internal
systems. Therefore 𝐿𝑂 directly influence the risk components related to failure of internal systems,
𝑅𝐶 , 𝑅𝑀 , 𝑅𝑊 , 𝑅𝑍 . That explained why the these risk components are equal to 0 and different from 0 in the
evaluation (1): 𝐿𝑂 = 0 and evaluation (2): 𝐿𝑂 = 0.1, respectively.
Before evaluate the best set of lightning protection measures there is some information that can be
useful to analyze. For example, the button “Additional Information” gives the value of the number of
dangerous events that can happen in the respective collection area. This area is calculated through
the geometric measures of the structure and connected lines. These collection areas are also given in
the table generated by the mentioned button, as Figure 40 shows.
In Figure 41, it can be seen (approximately) the collection area of the South Tower in IST. As the
structure to be protected has 𝐻 = 50 𝑚𝑚, 𝑊 = 30 𝑚𝑚 and 𝐿 = 30 𝑚𝑚, its the collection area is
approximately a circle which covers an area of 92095 𝑚𝑚2 (it is not an exact circle, but the difference
can be neglected, less than 3%).
As it can be seen from the figure, the area can be considerate as overestimated with the 1:3 ratio
adopted by the standard.
6.3.1. Evaluation (𝟏), assuming a 𝑳𝑶 = 𝟓𝟓 − no risk of explosion
First of all, the risk components related to failure of internal systems, 𝑅𝐶 , 𝑅𝑀 , 𝑅𝑊 , 𝑅𝑍 are equal to 0.
Then, as it was said, the risk of fire in the library and the laboratories is considered high. So, the
obvious protective measure is to upgrade the fire protection system, by implementing fixed
automatically operated extinguishing installations and/or automatic alarm installations: 𝑟𝑝 = 0.5 → 𝑟𝑝 =
0.2 in zones 𝑍2 and 𝑍3 .
Applying this measure, the overall risk is reduced significantly but the value of the risk, 𝑅 = 8.6030 ×
10−5 as it can be seen in Figure 42, still remains higher than the tolerable risk value, 𝑅𝑇 : 𝑅1 > 𝑅𝑇 .
So, more protective measures are needed. For this example, where the “critical” risk components are
related to physical damage, 𝑅𝐵 and 𝑅𝑉 (see Figure 38), the measure that most affects these risk
components, is the construction of a lightning protection system, an LPS.
Starting with an LPS of class III: 𝑃𝐵 = 1 → 𝑃𝐵 = 0.1 and 𝑃𝐸𝐵 = 1 → 𝑃𝐸𝐵 = 0.05 , the results are
significant in the reduction of the value of the overall risk, 𝑅1 . The down side of this measure is the
costly investment that can be around a few thousands of euros.
The re-calculated values of the risk are presented in Figure 43. For this evaluation (𝟏), the overall risk
𝑹𝟏 becomes smaller than the tolerable risk 𝑹𝑻 – objective achieved: 𝑹𝟏 = 𝟓𝟓. 𝟖𝟗𝟗𝟖𝟕 × 𝟏𝟓𝟓−𝟗𝟗 < 𝑹𝑻 = 𝟏 ×
𝟏𝟓𝟓−𝟗𝟗 .
85
Figure 40: Data given by the “Information” button
Figure 41: Approximately collection area of the structure to be protected: South Tower in IST
86
Figure 42: Calculated risk when protection measure against fire is applied: 𝑟𝑝 = 0.5 → 𝑟𝑝 = 0.2
Figure 43: Calculated risk when a lightning protection system of LPL III is installed in the structure to be protected
87
Recapping the lightning protective measures adopted:
-
Protect the building with a Class III LPS (𝑃𝐵 = 0.1, including also 𝑃𝐸𝐵 = 0.05);
Provide zone 𝑍2 (library) and zone 𝑍3 (laboratories) with an automatic fire protection system
(𝑟𝑝 = 0.2 for zone 𝑍2 only).
Obviously, the user may not be still satisfied and desire more protection against lightning or even alter
his choices and select different measures to reach other risk values.
The economic evaluation can be helpful in this matter. Different set of lightning protective measures
can give similar risk values, so the necessary investment on these measures must be revised.
6.3.2. Evaluation (𝟐): 𝑳𝑶 = 𝟓𝟓. 𝟏 − assuming risk of explosion
In this case, the “critical” risk components of the laboratory zones are the ones related with failure of
internal systems, 𝑅𝐶 , 𝑅𝑀 , 𝑅𝑊 , 𝑅𝑍 (≠ 0). So, the most effective lightning protection measure to
implement in this zone is the reinforcement of the security of the internal wiring.
The measure that better translates that is:
-
Install a coordinated SPD protection system on internal power and telecom systems for (3 ×)
better than LPL I (𝑃𝑆𝑃𝐷 = 0.001) in zone 𝑍3 (laboratories).
This measure consists in a coordinated system where a surge protective device, SPD, shall be located
at the line entrance (both power and telecommunication lines) in each protection zone. This device will
protect the equipment (which operates at very low voltages) inside the zone from overvoltages created
by lightning surges.
With this measure the overall risk decreases significantly (see Figure 44).
However, the risk is still higher than the tolerable risk value: 𝑅1 = 18.9930 × 10−5 < 𝑅𝑇 = 1 × 10−5 .
The “critical” zone is still the one referent to the laboratories, but the higher risk components are the
ones related to physical damage, 𝑅𝐵 and 𝑅𝑉 , with the lead on the zones with high risk of fire: 𝑍2
(library) and 𝑍3 (laboratories).
So, constructing an LPS is again the most suitable lightning protection measure along with the fire
protection measure providing zone 𝑍2 and 𝑍3 with an automatic fire protection system (𝑟𝑝 = 0.2).
With some tries in the program, the lightning protection level (LPL) that the LPS should be so that
𝑅 < 𝑅𝑇 is LPL of II: 𝑃𝐵 = 0.05 and 𝑃𝐸𝐵 = 0.02.
The construction of an LPS of class II together with the automatic fire protection system lead to an
overall risk of:
𝑅1 = 0.6445 × 10−5 < 𝑅𝑇 = 1 × 10−5 . Confirmed in Figure 45.
88
Figure 44: Calculated risk when a coordinated SPD system is installed in zone 3, “Laboratories”
Figure 45: Calculated risk of a structure with a lightning protection system LPL II and the preventing fire measures
implemented
As before, the user has the chance to continue to reduce the value of the risk or choose to adopt
another approach with a different set of combination of measures. It all depends on the budget and the
practicality of applying those measures.
89
90
7. Conclusions
7.1. Summary
The International Electrotechnical Committee (IEC) has created a very good and reliable lightning
protection standard – the IEC 62305. Mostly thanks to the introduction of the analysis of the
hazardous effects in electrical and electronic failure due to lightning, the analysis of the effects of
indirect lightning and to the detailed risk management methodology.
It is important to realize that the results obtained with this methodology are purposely overestimated,
which results in a higher level of lightning protection and so, in a safer solution. The down side is that,
with a higher level of lightning protection, the higher can be the cost of the needed set of lightning
protective measures.
That is why IEC allows that the factors and probabilities values that compose the risk analysis can be
altered by national committees for better suit their countries characteristics and result in an overall
more cost effective lightning protection. Therefore, the goal of every national committee is to take the
IEC 62305 risk management methodology [2] (which is debated in chapter 5, Risk Management), and
apply it accordingly to each country requirements.
Regarding the un-conventional technology products, the discussion has dragged itself for over 30
years, which according with the scientific community, was considered a sufficient amount of time for
the manufactures of these systems to deliver acceptable data results that definitely proven the
efficiency and effectiveness of these systems. As the data is not conclusive in favor of the
manufactures, the standards that use LPS with this technology have been classified as “non-standard”
and the air-termination systems have been tagged as “un-conventional”, accordingly with several
international standardization committees and researchers [51], [52], [53].
On the other hand, it is surprising that the air-termination system used in modern days is essentially
the same system invented by Franklin in middle of the 18th century, when the average structures were
much smaller and had much less electrical and electronic requirements from today’s structures to
protect. Nevertheless, with the increasing height of structures and higher levels of lightning protection
needed, it is crucial to constantly analyze if the risk assessment and lightning protection measures
adopted by IEC 62305 are the most suitable to carry on the lightning protection all around the world.
That is the main role of the IEC, to guarantee a safe and reliable solution that can be adopted
internationally.
7.2. Achievements
Regarding the computer program, L.R.A., it was created to assess the value of the risk of a structure
being affected by lightning discharges. As it was demonstrated, it follows the IEC 62305 risk
computation methodology.
91
The L.R.A. gives three big advantages to its user:
(1) It is easy to navigate between the tabs that compose the program and has an intuitive way of
input the parameters;
-
Just by analyzing the table of results, where every risk component is discriminated with its value
and by zone, it is possible to see what are the types of damage (𝐷1 : injury to living beings by
electric shock, 𝐷2 : physical damage, 𝐷3 : failure of electrical and electronic systems) that more
contribute to increase the overall risk value. With that information, the user narrows down where
he should invest more to reduce the risk. Even more, being these risk components calculated by
zone, the user has that additional information and can direct its efforts to a particular zone
resulting in a less costly and effective investment;
(2) The above advantage is translated on the program when it gives a set of lightning protection
measures for each zone according with the 3 higher risk components, resulting in effective
solutions and with that becoming the process much faster – the user doesn’t need to make any
analysis or calculations;
(3) It allows an economic evaluation for all four types of risk: for 𝑅1 , 𝑅2 and 𝑅3 allows comparing the
different solutions based on the cost of the lightning protective measures; for 𝑅4 allows to
calculate the cost of loss when the structure is unprotected and when the structure is protected.
With those and with the annual cost of the lightning protective measures, the program calculates
the annual savings by applying the measures. Being able to know the annual savings, the user
can compare between the different solutions and see which one has the higher annual savings,
as it was seen in the “South Tower in IST” example.
7.3. Future work
For the developed program, the L.R.A., the next step in its improvement is the introduction of new
features: a tab dedicated to the design of the lightning protection system, LPS: the air-termination
system; the down-conductor system; and the earth-termination system, using the IEC 62305 concepts
and methodologies.
As it was seen in the application of the examples in chapter 6, an economic evaluation can be an
important tool to the design engineer as the cost of the selected solution is always a crucial parameter.
So, improve this evaluation could be, for example, by elaborating a data base with the most recent
prices of the protective measures and the user could choose from a list; or even associate the L.R.A.
program with a company or companies that make budgets for lightning protection.
This tool could help in the decision making between different solutions and complement the risk
management methodology.
92
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95
96
Appendices
Appendix A: Informative tables
The next tables are related to parameters and factors that characterize the structure to be protected
as well the lines connected to it.
A.1. The relative location of the structure, 𝐶𝐷 :
Table A.1: Values of relative location factor of the structure, 𝑪𝑫
Relative Location
Structure surrounded by higher objects
Structure surrounded by objects of the same height or smaller
𝑪𝑫
0.25
0.5
1
Isolated structure: no other objects in the vicinity
2
Isolated structure on a hilltop or a knoll
A.2. Installation factor of a line, 𝐶𝐼 : power or telecom line
Table A.2: Values of installation factor,𝑪𝑰
𝑪𝑰
Routing
1
Aerial
Buried
Buried cables running entirely within a meshed earth termination
0.5
0.01
A.3. Line type factor, 𝐶𝑇 :
Table A.3: Values of line type factor, 𝑪𝑻
Installation
LV power, telecommunication or data line
HV power (with HV/LV transformer)
𝑪𝑻
1
0.2
97
A.4. Environment factor of a line, 𝐶𝐸 : power or telecom line
Table A.4: Values of environment factor, 𝑪𝑬
𝑪𝑬
Environment
1
Rural
0.5
Suburban
Urban
Urban with tall buildings 𝑎
𝑎
Buildings higher than 20 𝑚𝑚
0.1
0.01
A.5. 𝑃𝑇𝐴 : parameter that reduces the probability that a flash to a structure will cause injury to living
beings by electric shock, 𝑃𝐴 , through the implementation of the measures tabled below against touch
and step voltage.
Table A.5: Values of the parameter 𝑷𝑻𝑨 by implementation of measures against touch and step voltage
𝑷𝑻𝑨
Additional protection measure
1
No protection measures
Warning notices
Electrical insulation (e.g. at least 3 mm cross-linked polyethylene) of exposed parts
(e.g. down-conductors)
Effective soil equipotentialization
Physical restrictions or building framework used as a down-conductor system
0.1
0.01
0.01
0
A.6. Probability that a flash to a structure will cause physical damage, 𝑃𝐵 .
Table A.6: Values of probability 𝑷𝑩 according to the class of the LPS installed
Characteristics of structure
Structure not protected by LPS
Structure protected by LPS
-
𝑷𝑩
𝐼𝑉
0.2
𝐼𝐼
0.05
Class of LPS
𝐼𝐼𝐼
𝐼
Structure with an air-termination system conforming to LPS I and a continuous
metal or reinforced concrete framework acting as a natural down-conductor system
Structure with a metal roof and an air-termination system, possibly including natural
components, with complete protection of any roof installations against direct
lightning strikes and a continuous metal or reinforced concrete framework acting as
a natural down-conductor system
98
1
0.1
0.02
0.01
0.001
A.7.
𝑃𝐸𝐵 :
depends
on
the
lightning
equipotential
bonding
for
connected
electrical
and
telecommunication lines and on the lightning protection level, LPL, for which its SPDs are designed
Table A.7: Values of the parameter 𝑷𝑬𝑩 in function of the LPL for which SPDs are designed
𝑳𝑷𝑳
No SPD
𝑷𝑬𝑩
𝐼𝐼𝐼 − 𝐼𝑉
0.05
𝐼
0.01
1
𝐼𝐼
0.02
The values of 𝑃𝐸𝐵 may be reduced for SPDs having better protection characteristics
(higher nominal current 𝐼𝑁 , lower protective level 𝑈𝑃 , etc.) compared with the
0.005 − 0.001
requirements defined for LPL I at the relevant installation locations.
A.8. 𝑃𝑆𝑃𝐷 : depends on the coordinated SPD system and to the lightning protection level (LPL) for
which its SPDs are designed.
Table A.8: Values of the parameter 𝑷𝑺𝑷𝑫 in function of the LPL for which SPDs are designed
No coordinated SPD system
𝑷𝑺𝑷𝑫
𝐼𝐼𝐼 − 𝐼𝑉
0.05
𝐼
0.01
LPL
𝐼𝐼
The values of 𝑃𝑆𝑃𝐷 may be reduced for SPDs having better protection
characteristics (higher nominal current 𝐼𝑁 , lower protective level 𝑈𝑃 , etc.)
1
0.02
0.005 − 0.001
compared with the requirements defined for LPL I at the relevant installation
locations.
A.9. 𝐶𝐿𝐷 and 𝐶𝐿𝐼 : Factors related with specific characteristics of lines connected to the structure to be
protected: shielding, grounding and isolation, when a flash to or near a connected line occurs,
respectively.
99
External line type
Table A.9: Values of the factors 𝑪𝑳𝑫 and 𝑪𝑳𝑰
Connection at entrance
𝑪𝑳𝑫
𝑪𝑳𝑰
1
1
1
Aerial line unshielded
Undefined
Buried line unshielded
Undefined
Multi grounded neutral power line
None
Shielded buried line (power or TLC)
Shield not bonded to the same bonding bar as equipment
Shielded aerial line (power or TLC)
Shield not bonded to the same bonding bar as equipment
Shielded buried line(power or TLC)
Shield bonded to the same bonding bar as equipment
Shielded aerial line (power or TLC)
Shield bonded to the same bonding bar as equipment
1
1
0.2
1
0.1
1
0.3
1
0
1
Lightning protective cable or wiring
0
Shield bonded to the same bonding bar as equipment
0
0
(No external line)
No connection to external lines (stand-alone systems)
Any type
Isolating interface according to IEC 62305-4
0
0
in lightning protective cable ducts,
metallic conduit, or metallic tubes
0
0
A.10. and A.11. The probabilities 𝑃𝐿𝐷 and 𝑃𝐿𝐼 are the components of the probabilities that takes into
account the effects of failure of internal systems within the structure due to a flash to (𝑃𝑈 , 𝑃𝑉 , 𝑃𝑊 ) or
near (𝑃𝑍 ) a line.
Line
type
Table A.10: Values of probability 𝑷𝑳𝑫
Routing, shielding and bonding conditions
Power
Aerial or buried line, unshielded or shielded whose
Lines
shield is not bonded to the same bonding bar as
equipment
or
5 Ω/𝑘𝑚𝑚 < 𝑅𝑠 ≤ 20 Ω/𝑘𝑚𝑚
Shielded aerial or buried
whose shield bonded to
Telecom
lines
the same bonding bar
𝑅𝑠 ≤ 1 Ω/𝑘𝑚𝑚
as equipment
Line type
Power lines
TLC lines
100
1 Ω/𝑘𝑚𝑚 < 𝑅𝑠 ≤ 5 Ω/𝑘𝑚𝑚
Withstand voltage 𝑼𝑾 in 𝒌𝑽
𝟏
𝟏. 𝟗𝟗
𝟐. 𝟗𝟗
𝟒
𝟔
1
1
1
1
1
1
1
0.95
0.9
0.8
0.4
0.2
0.04
0.02
0.9
0.6
0.8
0.6
Table A.11: Values of probability 𝑷𝑳𝑰
𝟏
1
1
Withstand voltage 𝑼𝑾 in 𝒌𝑽
𝟏. 𝟗𝟗
𝟐. 𝟗𝟗
0.5
0.2
0.6
0.3
𝟒
0.16
0.08
𝟔
0.1
0.04
0.3
0.1
A.12. 𝑃𝑇𝑈 : parameter that reduces the probability that that a flash to a line will cause injury to living
beings by electric shock, 𝑃𝑈 , through the implementation of the measures tabled below against touch
voltage.
Table A.12: Values of probability 𝑷𝑻𝑼
Protection measure
𝑷𝑻𝑼
1
No protection measures
0.1
Warning notices
0.01
Electrical insulation
0.005 − 0.001
Physical restrictions
A.13. 𝑟𝑡 : Factor that reduces the loss of human life, 𝐿1 , and the economic loss, 𝐿4 , depending on the
type of soil or floor.
Table A.13: Values of the factor 𝒓𝒕 in function of the contact resistance of the surface
Type of surface 𝒃
Agricultural, concrete
Marble, ceramic
Gravel, moquette, carpets
Asphalt, linoleum, wood
Contact resistance 𝒌𝛀
≤1
𝒂
1 − 10
10 − 100
≥ 100
𝒓𝒕
10−2
10−3
10−4
10−5
𝑎
Values measured between a 400 𝑐𝑚𝑚2 electrode compressed with a uniform force of 500 𝑁𝑁 and
𝑏
A layer of insulating material, e.g. asphalt, of 5 𝑐𝑚𝑚 thickness (or a layer of gravel 15 𝑐𝑚𝑚 thick)
a point of infinity.
generally reduces the hazard to a tolerable level.
A.14. 𝑟𝑝 : Factor translated in measures, applied in the structure, in order to reduce the consequences
of fire.
Table A.14: Values of the factor 𝒓𝒑
Provisions
No provisions
One of the following provisions: extinguishers; fixed manually operated extinguishing
installations; manual alarm installations; hydrants; fire compartments; escape routes
One of the following provisions: fixed automatically operated extinguishing installations;
automatic alarm installations
𝑎
𝑎
𝒓𝒑
1
0.5
0.2
Only if protected against overvoltages and other damages and if firemen can arrive in less than 10 min.
101
A.15. 𝑟𝑓 : Factor related with the risk of fire or the risk of explosion in the structure (as a whole or by
zones).
Risk
Table A.15: Value of the factor 𝒓𝒇
Amount of risk
Zones 0, 20 and solid explosive
Zones 1, 21
Explosion
Zones 2, 22
High
Ordinary
Fire
Low
Explosion or fire
None
𝒓𝒇
1
10−1
10−3
10−1
10−2
10−3
0
Reminding the zone definition: place in which an explosive atmosphere consisting of a mixture of air
and flammable substances in the form of gas, vapour or mist:
-
Zone 0: is present continuously or for long periods or frequently;
-
Zone 1: is likely to occur in normal operation occasionally;
-
Zone 2: is not likely to occur in normal operation but, if it does occur, will persist for a short period
only;
And: place in which an explosive atmosphere, in the form of a cloud of combustible dust in air:
-
Zone 20: is present continuously, or for long periods, or frequently;
-
Zone 21: is likely to occur in normal operation occasionally;
-
Zone 22: is not likely to occur in normal operation but, if it does occur, will persist for a short period
only.
A.16. ℎ𝑧 : Factor that translates the level of panic and the evacuation difficulties present in the zone (or
in the entire structure) in study. It depends on the characteristics of the zone and on the number of
persons.
Table A.16: Value of the factor 𝒉𝒛
Kind of special hazard
No special hazard
Low level of panic (e.g. a structure limited to two floors and the number of persons
not greater than 100)
Average level of panic (e.g. structures designed for cultural or sport events with a
number of participants between 100 and 1 000 persons)
Difficulty of evacuation (e.g. structures with immobile persons, hospitals)
High level of panic (e.g. structures designed for cultural or sport events with a
number of participants – greater than 1 000 persons)
102
𝒉𝒛
1
2
5
5
10
A.17. 𝐾𝑆3 takes into account the characteristics of internal wiring.
Table A.17: Values of the factor 𝑲𝑺𝟑
𝑲𝑺𝟑
Type of internal wiring
Unshielded cable – no routing precaution in order to avoid loops
𝑎
Unshielded cable – routing precaution in order to avoid large loops
Unshielded cable – routing precaution in order to avoid loops
Shielded cables and cables running in metal conduits
𝑎
𝑑
𝑐
𝑏
1
0.2
0.01
0.0001
Loop conductors with different routing in large buildings (loop area in the order of 50 𝑚𝑚2 ).
𝑏
Loop conductors routed in the same conduit or loop conductors with different routing in
𝑐
Loop conductors routed in the same cable (loop area in the order of 0.5 𝑚𝑚2 ).
𝑑
small buildings (loop area in the order of 10 𝑚𝑚2 ).
Shields and the metal conduits bonded to an equipotential bonding bar at both ends and
equipment is connected to the same bonding bar.
103