SANS 61024-1-1

SABS IEC 1024-1-1:l993*

UDC 621.31 6(.93+.98):699.887.2 ICS 91.120.40

'This standard references other standards First IEC edition

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Approved by the President of the SABS on 24 April 1995

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Specification

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Protection of structures against lightning

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Part 1: General principles

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Section 1: Guide A - Selection of protection levels for lightning protection systems

Published by THE SOUTH AFRICAN BUREAU OF STANDARDS

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SABS IEC 1024-1-1

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Amendments issued since publication Date Text affected Amdt No.

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Obtainable from the

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South African Bureau of Standards Private Bag X191 Pretoria Republic of South Africa

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Telegrams : Comparator, Pretoria : 321308SA Telex : (012) 344-1568 Fax COPYRIGHT RESERVED Printed in the Republic of South Africa by the South African Bureau of Standards

SOUTH AFRICAN BUREAU OF STANDARDS National amendment No. 1 : 13 January 1997

PROTECTION OF STRUCTURES AGAINST LIGHTNING PART 1 : GENERAL PRINCIPLES

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SECTION 1: GUIDE A - SELECTION OF PROTECTION LEVELS FOR LIGHTNING PROTECTION SYSTEMS

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SABS IEC 1024-1-1 :1993

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Approved by the President of the SABS on 13 January 1997.

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Scope of amendment

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This part of SABS IEC 1024-1 has been amended to draw attention to the existence of another specification that contains national deviations from this standard.

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National foreword

Insert the following paragraph between the first and second paragraphs:

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Deviations from the international requirements in this part of SABS IEC 1024-1 are given in SABS 0313. In the case of dispute, SABS 0313 will take precedence over this part of SABS IEC 1024-1.

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SABS IEC 1024-1-1

Notice

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This part of SABS IEC 1024-1 was approved by the President of the South African Bureau of Standards on 24 April 1995. NOTES

1 In terms of the Standards Act. 1993 (Act 29 of 1993), no person shall claim or declare that he or any other person complied with an SABS standard unless

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b) the identity of the person on whose authority such claim or declaration is made, is clear.

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a) such claim or declaration is true and accurate in all material respects, and

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2 It is recommended that authorities who wish to incorporate any part of this standard into any legislation in the manner intended by section 31 of the Act consult the SABS regarding the implications.

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This part of SABS IEC 1024-1 will be revised when the parent document is revised. In the meantime, however, comment will be welcome and will be considered when this part of SABS IEC 1024-1 is revised.

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National foreword

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The National Committee (SC 754.7F: Electrical installations, of TC 754.7:Electrical distribution systems and components) responsible for this part of SABS IEC 1024-1 has accepted the text of IEC 1024-1-1:l993, Protection of structures against lightning - Part l :General principles - Section l : Guide A - Selection of protection levels for lightning protection systems, as suitable for publication. without technical deviation or language editing, as a South African standard, with the addition of a cover page, notice, national foreword and committee list.

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The South African Bureau of Standards has in recent years taken over an increasing number of international standards for publication as national standards. In some of the take-over standards, the point (also referred to as a dot or full stop) is used as the decimal sign, and users of these standards are requested to read this point as a comma in view of the fact that the comma is the only legal decimal sign in South Africa. It should be mentioned that both the International Organization for Standardization (ISO) and the International ElectrotechnicalCommission (IEC) have formally adopted the comma as the decimal sign.

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NOTES

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1 The page numbers at the top of the pages are those of the original document. Therefore, where reference is made in the text to a specific page, refer to the page number at the top of the page. 2 Where reference is made in this part of SABS IEC 1024-1 to an IEC standard, the reader is encouraged to investigate

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the possibility of using an equivalent SABS IEC standard, should one exist. Information on currently valid national and international standards can be obtained from the South African Bureau of Standards.

Attention is drawn to the reference given in the introduction to this standard. This reference is indispensable for the application of this standard.

SABS IEC 1024-1-1

Committee SC 754.7F

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At the time of approval of this part of SABS IEC 1024-1 by National Committee SC 754.7F:Electrical installations, the composition of the subcommittee was as follows:

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Chamber of Mines of South Africa Copper Tubing Africa (F'ty) Ltd

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CSlR

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Department of Labour

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Department of Mineral and Energy Affairs

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Department of Public Works

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Electrical Engineering and Allied Industries Association

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Environmental Panelling Systems (F'ty) Ltd

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Exoweld

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Provincial Administration: Western Cape (Works Department)

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SA Housing Trust Limited

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SME Lightning Protection and Earthing CO

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South African Association of Consulting Engineers

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Spoornet

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Surge Technology

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Telkorn SA Limited

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The Association of Electric Cable Manufacturers of South Africa

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The Association of Municipal Electricity Undertakings (Southern Africa)

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The Electrical Contractors' Association

CEI

NORME INTERNATIONALE

IEC

INTERNATIONAL STANDARD

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Premiere edition First edition 1993-08

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Protection des structures contre la foudre

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Partie 1: Principes generaux Section 1: Guide A - Choix des niveaux de protection pour les installations de protection contre la foudre

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Protection of structures against lightning

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Part 1: General principles Section l: Guide A - Selection of protection levels for lightning protection systems

@ CEI 1990 Droits de reproduction reserves - Copyright - all rights reserved Aucune parlm 6, mtle pobltcatwn ne pea( etre reprcdqlte nl u t t h s w sous quelque loma que ce sol et par auwn procede ilectronque ou mecanlque y corms b photaople e4 les mrrol~kns.sans I accord ecrd de I editeur

No part of ;ha pubbcal~onmay be reproduoad a uthzed In any l a m or by any means. sktronc or mechanml. ndudwg photac=~y~ng and m a d ~ t n ,mlhoul m wrltvlg from the publ~sher

Bureau Central de h Commission Electrotechnique Internationale 3, rue de VaremM Gedve. Suisse

Commiss~onElectrotechnique Internationale CODE P RlX International Electrotechn~calCommission P R I C E CODE Memav~apoa~an ~nenrporex~~recndn ~OMHCCH~

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Pour p m , vorr catalogue en vgueur For prrce. see currenl catalogue

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. 3.

FOREWORD ..............................................................................................................................

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Clause

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.......................................................................................................

3.1

Lightning current parameters used for dimensioning Lightning Protection Systems (LPS) ....................................................................................

3.2

Lightning ground flash density

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Selection of protection levels for Lightning Protection Systems (LPS) ..................... Accepted frequency of lightning flashes (N. ) to a structure ............................. 19 Expected frequency (Nd)of direct lightning flashes to a structure ................. 21

4.3

Procedure for selection of LPS ..........................................................................

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4.1 4.2

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Lightning parameters

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Common structures .............................................................................................. Special structures .................................................................................................

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2.1 2.2

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Classification of structures ...............................................................................................

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Scope and object .............................. . ................................................................. Terms and definitions .........................................................................................

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1.1

1.2

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General ..............................................................................................................................

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INTRODUCTION.....................................................................................................................

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Figures .........................................................................................................................................

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Annex A . Basic values of lightning current parameters . Cumulative frequency distribution ................................................................................................................

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CONTENTS

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INTERNATIONAL ELECTROTECHNICAL COMMISSION

Part 1: General principles Section 1: Guide A Selection of protection levels for lightning protection systems

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PROTECTION OF STRUCTURES AGAINST LIGHTNING

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FOREWORD

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1) The IEC (International Electrotechnical Commission) is a worldwide organization for standardization comprising all national electrotechnical committees (IEC National Committees). The object of the IEC is to promote international cooperation on all questions concerning standardization in the electrical and electronic fields. To this end and in addition to other activities, the IEC publishes International Standards. Their preparation is entrusted to technical committees; any 1EC National Committee interested in the subject dealt with may participate in this preparatory work. International. governmental and non-governmental organizations liaising with the IEC also participate in this preparation. The IEC collaborates closely with the International Organization for Standardization (ISO) in accordance with conditions determined by agreement between the two organizations.

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2) The formal decisions or agreements of the IEC on technical matters, prepared by technical committees on which all the National Committees having a special interest therein are represented. express. as nearly as possible, an international consensus of opinion on the subjects dealt with. 3) They have the form of recommendations for international use published in the form of standards, technical reports or guides and they are accepted by the National Committees i n that sense. 4) In order to promote international unification. IEC National Committees undertake to apply IEC International

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Standards transparently to the maximum extent possible in their national and regional standards. Any divergence between the IEC Standard and the corresponding national or regional standard shall be clearly indicated in the latter.

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International Standard IEC 1024-1-1 has been prepared by IEC technical committee 81: Lightning protection.

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The text of this standard is based on the following documents: Amendment to DIS

Report on Voting

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81(C0)18

81(CO)20

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8l(C0)14

Report on Voting

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Full information on the voting for the approval of this standard can be found in the report on voting indicated in the above table.

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IEC 1024 consists of the following parts, under the general title: Protection of structures against lightning.

- Part 1: General principles.

Annex A forms an integral part of this standard.

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The Part 1, General principles, establishes the fundamental definitions and general principles of lightning protection, as well as providing the necessary information concerning design, construction and materials to facilitate the basic installation of external and internal lightning protection systems (LPS) of common structures. The Part 1, also gives the basic requirements for good maintenance and inspection practice.

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INTRODUCTION

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Guide A contains information on assignment of protection levels to structures to be protected. It gives guide-lines for the selection of LPS and represents the consensus view of many countries' experts as to the best general practice based on the present state of the art.

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However it should be kept in mind that the matter is so complicated, due to the involved parameters, that only a thorough risk analysis can give the correct evaluation of the required protection level.

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Where the selection of protection levels for structures is based on the assessment of the risk of damage due to lightning, a Technical Report (future IEC 1024-1-2) will assist the national authorities concerned.

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This guide is used in conjunction with Part 1 , when the particular aspects of protection assessment and physical design and construction of an LPS are considered.

PROTECTION OF STRUCTURES AGAINST LlGHThlNG

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General

1.1

Scope and object

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Part 1: General principles Section 1 : Guide A - Selection of protection levels for lightning protection systems

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This guide is applicable to the selection of protection levels for LPS covered by IEC 1024-1.

Terms and definitions

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It provides information on the classification of structures according to the consequential effects of a lightning stroke and on procedures for selection of an LPS giving an adequate level of protection.

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For the purpose of this guide, the following definitions apply:

(4: The.current flowing at the point of strike.

lightning current

1.2.2

peak value (I): The maximum value of the lightning current in a flash.

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1.2.3 average steepness of lightning current (dildt): The difference between the lightning current values at the start and at the end of a specified time interval [i(t2) - i(f,)] divided by the specified time interval [t2 - $1.

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1.2.4 flash duration (7'): Time for which the lightning current flows at the point of strike.

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1.2.5 total charge (QtotaI): The time integral of the lightning current for entire lightning flash duration.

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1.2.6 impulse charge (Q,p,)l: The time integral of the lightning current for the impulse part of the lightning flash duratcon.

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1.2.7 specific energy (WIR): The energy dissipated by the lightning current in a unit resistance. It is the time integral of the square of the lightning current for the duration of the lightning flash. 1.2.8 probability of damage (p): Probability of a lightning flash causing damage to the structure. 1.2.9 risk of damage: Probable average annual losses (humans and goods) in a structure due to lightning flashes.

1.2.10 direct lightning flash frequency to a structure (NJ: Expected average annual number of direct lightning flashes to the structure.

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1.2.12 accepted lightning flash frequency (N,): The maximum accepted average annual frequency of lightning flashes which can cause damage to the structure.

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1.2.11 frequency of damage by direct lightning flash: Average annual number of direct lightning flashes which cause damage to the structure.

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1.2.13 efficiency of an LPS (E): The ratio of the average annual number of direct lightning flashes which cannot cause damage to the structure to the direct lightning flash number to the structure.

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Classification of structures

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The classification of structures can be made according to consequential effects of lightning strokes which may be dangerous to structures, their contents or their surroundings.

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The direct effects of lightning which may be dangerous, are: fires, mechanical damage. injuries to people and animals, and damage to electric and electronic equipment. The effects of lightning may be responsible for panic and moreover cause explosions, and emissions of dangerous substances such as radioactive materials, chemical agents, toxic substances, biochemical contaminators, bacteria and viruses.

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The effects of lightning may be particularly hazardous to computer systems, control systems, regulation systems, and power supplies thereby causing loss of service to the public, loss of data production and business. Sensitive electronic equipment is installed in all types of structures and may require special protection.

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Examples of four classifications of different types of structures are given in table 1, but only common structures are considered in Part 1 and in this Guide.

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Common structures are structures used for ordinary purposes, whether commercial, industrial, farming, institutional or residential. Structures higher than 60 m are not considered in Part 1.

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2.2

Special structures

Descriptions of four types of special structures are given below.

Structures with confined danger

2.2.1

Structures whose construction materials, contents or occupants make the whole volume of the structure vulnerable to the consequential effects of lightning. Structures dangerous to their surroundings

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2.2.2

Structures whose contents can be dangerous to the surroundings if struck by lightning.

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Structures dangerous to social and physical environments

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Structures which may cause biological, chemical and radioactive emissions as a consequence of being struck by lightning. 2.2.4

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Miscellaneous structures

Structures for which an LPS of special design might be considered.

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temporary installations; structures under construction.

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tents, camping sites and sports fields:

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tall structures (above 60 m in height);

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Typical cases are as follows:

Table 1 - Examples of structure classification l

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Type of structure

Effects of lightning

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Puncture of electrical installations, fire and material damage

Damage normally limited to objects exposed to the point of strike or to the lightning path Farm

Primary risk of fire and hazardous step voltages

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Secondary risk due to loss of electric power, and life'hazard to livestock due to failure of electronic control of ventilation and food supply systems. etc

Damage to the electrical installations (e.g. electric lighting) likely to cause panic

Bank Insurance Company Commercial zompany. etc.

As above, plus problems resulting from loss of communication, failure of computers and loss of data

Hospital Nursing home %son

As above, plus problems of people in intensive care, and the difficulties of rescuing immobile people.

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Theatre School Department store Sports area

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Failure of fire alarms resulting in delayed fire fighting measures

Additional effects depending on the contents of factories, ranging from minor to unacceptable damage and loss of production

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Loss of irreplaceable cultural heritage

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Museums and archaeological sites

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Telecommunications Power plants Industries with fire hazards

Consequential hazards to the immediate surroundings caused by fire, etc. Consequences of fire and explosion to the surroundings

plant and its

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Refinery Service station Firework factory Munition works

Unacceptable loss of services to the public

Chemical plant Nuclear plant Biochemical laboratories ancl plants

NOTES Sensitive electronic equipment might be installed in all kinds of structures. includirlg all kinds of common 1 structures, which can be easily damaged by overvoltages due to lightning. The loss of service is the product of the time for which a single user cannot make use of the service by 2 the number of users involved, in one year.

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Lightning parameters

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Lightning parameters are usually obtained from measurements taken on high objects. The data given in this guide relates to both downward and upward flashes.

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The statistical distribution of the recorded lightning parameters can be assumed to have a logarithmic normal distribution. On this basis, the probability of occurrence of any value of each parameter can be calculated from the values given in annex A.

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The polarity ratio of lightning strokes depends on the nature of the territory. If no local information is available, 10 % positive and 90 OO/ negative should be assumed.

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The values reported in this guide are based on polarity ratio 10 % positive and 90 % negative. 3.1

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Lightning current parameters used for dimensioning Lightning Protection Systems (L PS)

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The mechanical and thermal effects of lightning are related to the peak value of the current (I), the total charge (Qtot,,). the impulse charge (Qimpulse ) and specific energy (WIR). The highest values of these parameters occur in positive flashes.

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The damaging effects caused by induced voltage are related to the steepness of the lightning current front. In this guide the average steepness between 30 Oh and 90 % values of the peak current is used for design purposes. The highest value of this parameter occurs in subsequent negative strokes. Such negative strokes occur in almost all negative flashes to a structure.

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Provided that 10 % of positive strokes and 90 % of negative flashes is assumed, the values of lightning parameters related to the protection levels are given in table 2.

Lightning ground flash density

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The lightning ground flash density expressed in terms of ground strokes per square kilometre per year should be determined by measurement.

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If lightning ground stroke density (Ng) is not available, it may be estimated by using the following relationship:

N =0 . 0 4 - ~ ~ per ' -km2 ~ ~ per year 9

where Td

is the number of thunderstorm days per year obtained from isoceraunic maps

NOTE - This relationship varies with changes in climatic conditions.

Table 2 - Relationship of lightning current parameters to protection levels (see 3.1) Protection level Lightning parameter

Total charge

'total ( C ) 'impulse (1'

Impulse charge Specific energy

W I R (kJIQ)

Average steepness

150

300

225

150

100

75

50

10 000

5 600

2 500

200

150

100

100

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dild t301g0 C& k A 1 ~ s

200

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Current peak value

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Selection of protection levels for Lightning Protection Systems (LPS)

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The purpose of selecting a protection level is to reduce, below the maximum tolerable level, the risk of damage by a direct lightning flash to a structure, or to a volume to be protected.

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For each structure the risk of damage can be estimated taking into account the annual frequency of direct lightning flashes to the structure ( N d ) , the probability with which lightning causes damage and the average possible loss amount which may appear as a consequence of lightning to the structure.

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NOTE There are cases in which the indirect strokes should be taken into account in the risk assessment.

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The damage depends on several parameters, among which are: the use and the content (humans and goods) of the volume to be protected; construction materials and measures taken to reduce the consequential effects of lightning.

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The structure is classified according to the consequential effects of lightning as indicated in clause 2.

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Once the maximum tolerable level of the risk of damage to the structure considered has been selected. the maximum accepted value N, of annual frequency of lightning flashes which can cause damage to the structure can be evaluated.

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Therefore the selection of the adequate level of protection for the LPS to be provided can be based on the expected frequency Nd of direct lightning flashes to the structure to be protected and on the accepted annual frequency N, of lightning flashes.

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Accepted frequency of lightning flashes (N,) to a structure

The values of Nc are the responsibility of National Committees where human, cultural and social losses are involved.

The values of Ncmay be established by the owner of the structure or by the designer of LPS where losses are relevant to private property only.

- type of construction;

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presence of flammable and explosive substances; number of people concerned by the damage;

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type and importance of the public service concerned;

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value of goods having suffered damage;

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- other factors (see table 1). NOTE - Local regulations may impose values of Nc in particular cases.

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- measures provided to reduce the consequential effects of lightning;

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The values of N, may be estimated through the analysis of the risk of damage taking into account appropriate factors such as:

Expected frequency Nd of direct lightning flashes to a structure

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The average annual frequency Nd of direct lightning flashes to a structure can be assessed from:

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Nd = Ng . A, . 1 0 - ~per year

in which,

is the average annual ground flash density. in lightning flashes per square kilometre per year, concerning the region where the structure is located (see 3.2);

A,

is the equivalent collection area of a structure (m2).

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Ng

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The equivalent collection area of a structure is defined as an area of ground surface which has the same annual frequency of direct lightning flashes as the structure.

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For isolated structures the equivalent collection area A, is the area enclosed within the border line bl obtained from 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 (see figure 1 for flat country and figures 2A and 2B for hilly country).

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For complexe topography (see figures 2C and 2D) the construction can be simplified taking into account some characteristic parts of the outline replacing them with straight lines or circle sections. Surrounding objects significantly influence the equivalent area if their distances from the structure are less than 3 ( h + hs) where: h is the height of the structure under consideration; h, is the height of the surrounding object.

In this case, the equivalent areas of structure and proximate object overlap each other and the equivalent area Ae is reduced to a distance:

xs =

d + 3 (h, - h) 2

where: d is the horizontal distance between the structure and the object (see figure 3).

Only those objects which have permanent durability and adequate resistance against lightning stresses shall be taken into account.

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NOTE - Other more sophisticated methods are available and can be used for a more precise evaluation of the equivalent collection area.

Procedure for selection of LPS

4.3

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For every structure considered, the designer of the LPS protection shall decide whether or not an LPS is needed. If it is, he should select a proper level of protection.

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The first step in LPS selection procedure requires an adequate assessment of the structure under consideration according to its features. Structure dimensions and localization, thunderstorm activity (annual lightning flash density) in the considered region as well as the structure classification shall be determined. These data give the background for the assessment of:

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- average annual frequency of lightning flashes Nd as a product of the local ground density Ng and the equivalent collection area A, of the structure;

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average annual frequency of flashes Nc which can be accepted for the considered structure (see 4.1).

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The value of the accepted frequency of flashes Nc shall be compared with the actual value of frequency of lightning flashes Nd to the structure.

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This comparison allows a decision to be made as to whether and LPS is necessary. and if so, of what type.

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If Nd I Nc the LPS is not needed.

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If Nd > N c , an LPS of efficiency E r 1 - Nc/Ndshould be installed and the proper level of protection selected according to table 3.

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The design of an LPS shall meet the requirements given in the standard for the selected protection level.

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If an LPS of efficiency P lower than E is installed, additional measures of protection shall be provided.

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Additional protective measures are for instance:

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In any case a minimum value of the equivalent collection area is to be assumed equal to the horizontal projection of the structure itself.

measures limiting touch and step voltages; measures limiting fire propagation;

measures to mitigate the effects of lightning-induced overvoltages on the sensitive equiprnents.

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0.98 0.95 0.90 0.80

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LPS efficiency

Protection level

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- LPS efficiencies corresponding with protection levels

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Table 3

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Critical values of required efficiency E, of the LPS, as a function of the direct lightning frequency Nd to the structure, and accepted lightning frequency of flashes Ncare shown in figure 5.

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More detailed explanation of LPS selection procedure is given in the flow diagram of figure 4.

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Input data:

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Structure dimension and position Ground flash density (N ) 9 Class of the structure

Assess equivalent area A , and calculate frequency of flashes to the structure: Nd = N x A e g

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Establish from National Standard number of critical events Nc. according to the class of the structure

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NO .

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Provide LPS of efficiency E 2 Ec

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Calculate

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Protection is not needed

Figure 4

Establish protection level adequate to the E value. and LPS dimensions according to this level

Establish protection level adequate to E value and LPS dimensions according to this level. Design additional measures of protection

- Flow diagram for LPS selection procedure

dr a

W ith

n

w

on 07

20

ia

ar t

;p

01

6-

-0

lly re pl ed

ac

by

S

SA N

62

51: 20 07

30

.

Annex A (normative)

-

Peak current (kA) (min 2 kA) Cumulative frequency Lightning stroke

4

4.6

Positive flashes

4.6

5% 90

12

30

35

250

by

Subsequent negative strokes

SA N

20

50 %

30

80 %

62

First negative strokes

95 %

S

98 %

51: 20 07

.

Basic values of lightning current parameters Cumulative frequency distribution

ed

Total charge (C)

ac

Cumulative frequency

Lightning stroke

50 %

5%

1.1

5.2

24

0.2

1.4

11

1.3

7.5

40

re pl

95 Vo

lly

First negative strokes

ia

Subsequent negative strokes

;p

ar t

Negative flashes

20

80

350

Impulse charge (C)

-0

6-

01

Positive flashes

07

Cumulative frequency

Lightning stroke

1.1

43

Subsequent negative strokes

0.22

0,951

n

20

20

on

5%

First negative strokes

w dr a W ith

50 %

95 %

Positive flashes

1

16

4 150

l

Specific energy (JIR) Cumulative frequency Lightning stroke 50 4 '0

95 %

5%

-

5.5 X

1oS

lo3

5.2 X

lo4

lo5

1.5 X 10'

6.0 X

lo3

5.5 X 10'

Subsequent negative strokes

5.5

X

102

6.0

Positive flashes

2.5

X

10'

6.5 X

62

30

51: 20 07

.

First negative strokes

SA N

S

Rate of rise (kA/ps) Cumulative frequency First negative strokes

50 %

9.1

Average steepness between: 30 % and 90 % of peak current 10 % and 90 % of peak currant

2.6 1.7

24

5% 65

7.2

5

20 14

lly

re pl

ac

ed

Maximum rate of rise

by

95 %

50 %

5%

10

40

162

4.1

20

3.3

15

99 72

20

07

-0

10 % and 90 O/O of peak current

Cumulative frequency

95 O h

ar t

and 90 % of peak current

6-

O/O

01

Average steepness between:

;p

Maximum rate of rise

30

ia

Subsequent negative strokes

Cumulative frequency

95 %

Maximum rate of rise

0.2

W ith

dr a

w

n

on

Positive flashes 50 % 2.4

5% 32

Front time duration (PS) Cumulative frequency

50 %

5 Yo

1 .8

5.5

18

1.5 2.2

3.8 5.6

51: 20 07

Total rise time

95 %

Rise time between:

10 14

62

30

30 % and 90 % of peak current 10 % and 90 % of peak current

I

50 %

SA N

95 %

I

Total rise time

1 .l

4.5

3.0

0.2

0.6 0.8

3.5

re pl

ac

0.1 0.2

ed

Rise time between:

30 % and 90 % of peak current 10 % and 90 % of peak current

5%

1

by

I

S

Cumulative frequency Subsequent negative strokes

50 %

5%

3.5

22

200

Stroke duration (vs)

-0 6

-0

1;

pa rt

ia

Total rise time

Cumulative frequency

95 %

lly

Positive flashes

Cumulative frequency

20

07

Lightning stroke

on

First negative strokes

30 6.5

50 O/o

5%

75

200

32

140

200

2 000

w

n

Subsequent negative strokes

95 %

W ith

dr a

I

Positive flashes

25

.

First negative strokes

Total flash duration (PS) Cumulative frequency Lightning stroke

0,15

13

1 l00

31

180

900

positive flashes

14

85

500

62

30

Subsequent negative strokes

.

All negative flashes

5%

50 %

51: 20 07

95 %

SA N

S

Time intervals between lightning strokes (ms) Cumulative frequency Lightning stroke

95 %

by

Multiple negative strokes

50 %

7

5%

150

re pl

ac

ed

33

l

W ith

dr a

w

n

on

20

07

-0 6

-0

1;

pa rt

ia

lly

The cumulative frequency distribution of lightning parameters is reported in figure A.1.

. 51: 20 07 30 62 S SA N by ed ac re pl lly ia pa rt 1; -0 -0 6 on

I

First negative stroke

kA

0

w W ith

dr a

Qirnpu~se

Subsequent negative strokes

0.

Positive stroke

C

W/R

kJ111

d i d tmX

kAlp S

diM '30/90 Sb

kAl~s

Negative flash

8 8 c9 0

Positive flash

0.

C

n

~tota~

Scale of abscissa

07

20

Parameter

O @

8. 0

0 0.

@

0.

Figure A . l - Cumulative frequency distribution of lightning parameters

sabs pta

19