IEC Protection Against Lightning

Protection against lightning A UK guide to the practical application of BS EN 62305

Michael L Henshaw MBA, I.Eng, MlET

Ref: BIP 2118 Business Information

First published in the UK in 2007 by

British Standards Institution 389 Chiswick High Road London W4 4AL

(Q British Standards Institution 2007

All rights reserved. Except as permitted under the Copyright, Design.s a.nd Patents Act 1988, no part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means - electronic, photocopying, recording or otherwise - without prior permission in writing from the publisher.

Whilst every care has been taken in developing and compiling this publication, BSI accepts no liability for any loss or damage caused, arising directly or indirectly in connection with reliance on its contents except to the extent that such liability may not be excluded in law.

The right of Michael L Henshaw to be identified as the author of this Work has been asserted by him in accordance with sections 77 and 78 of the Copjright, Designs a.nd Patents Act 1988.

Typeset by YHT Ltd, London Printed in Great Britain by MPG Books, Bodmin, Cornwall

Bri t ish LiOramj Cataloguing in Publication Data A catalogue record for this book is available from the British Library

ISBN 978 0 580 50899 8

Con tents

Endorsement Not ice

Foreword

In trod uction

Section 1 : Basic considerations

Section 2: Risk assessment 2.1 2.2 2.3 2.4 2.5 2.6 2.7

2.8 2.9 2.10

General Type of damage and loss and source of damage Risk assessment stage 1 - determination of assigned values Risk assessment stage 2 - calculation of collection areas Risk assessment stage 3 - assessment of number of dangerous events Risk assessment stage 4 - Assessment of risks R,, R2, R3 and R4 Risk assessment stage 5 - comparison of calculated and tolerable risk and identifying risk by source of damage Risk assessment stage 6 - selection of protection measures Summary of protection measures Splitting structure into zones

Section 3: Protection measures

Section 4: Basic criteria for protection of structures

Section 5: Design of structural protection 5.1 General considerations 5.2 Reinforced concrete structures 5.3 External LPS 5.4 Internal lightning protection system

vi i ...

Vlll

X

1

7 7 8 9 16 17 18

23 26 31 31

40

41

44 44 45 47 71

V

Protection against lightning

Section 6: Joints, bonding and connections 6.1 6.2 Corrosion

Equipotential bonding of internal conductive parts

Section 7: Requirements for structures with risk of explosion, in addition to standard requirements 7.1 General requirements 7.2 7.3 Structures containing hazardous areas

Structures containing solid explosives materials

Section 8: Protection measures - touch and step voltages

Section 9: Components and materials

Section 10: Design of protection for electrical and electronic systems within a structure 10.1 Design and installation of lightning electromagnetic pulse protection

measures system (LPMS) 10.2 Basic protection measures 10.3 Earthing and bonding 10.4 10.5 Externally sited equipment 10.6 Coordinated SPD protection 10.7 Connections between structures

Magnetic shielding and line routing

Section 11: Inspection, testing and maintenance of LPS and LPMS 11.1 11.2

Inspection, testing and maintenance of LPMS Inspection, testing and maintenance Of LPS

78 79 80

83 83 84 84

86

87

91

91 94 96 98 99

100 102

103 103 104

vi

Section 1 Basic considerations

It is important for regular consultation to take place between the various parties involved in the contractual chain. This should result in an effective lightning protection system (LPS) and lightning electromagnetic pulse measures system (LPMS) at the lowest possible cost. The coordination of LPS andlor LPMS design work with construction work will often reduce the need for some bonding conductors and the frequency of those that are necessary. [BS EN 62305-3, E. 4.2.2.11

In practice it has been difficult for the lightning protection contractor to obtain sufficient information at the tender stage of any project in order to offer precise designs. It is also impractical in many cases, due to the contracting process in the UK, for the lightning protection contractor to be allowed access to the client or their professional team at tender stage in order to obtain all of the detailed information needed to derive a complete design.

For these reasons it is important for the LPS contractor to be clear with their direct contractual principal regarding the characteristics they have applied to the risk assessment process in order to derive a design - for example, the assumptions that have been made regarding the structure and line characteristics in the risk assessment in order to derive the level of protection, and the type of air-termination, down-conductor and earth-termination networks that have been allowed in the design. These all may have implications on the structure and services within it. Clarification should be sought regarding who is responsible for areas where there may be duplication -the equipotential bonding, for example.

The importance of human life and the advances in electrical and electronic technology, and its increased sensitivity and scope for increased consequential losses, together with the introduction of BS EN 62305, has further lifted the requirements for a professional approach to the provision of lightning protection. No longer is it acceptable for the professional team to ask in their tender specification simply for a 'lightning protection system to BS EN 62305' and leave it to the contractors at the lower levels of the contractual chain to derive what they think is appropriate with the information they have. The need for information to derive the requirements of the client and then to apply this to the new standard is a vital part of providing protection under BS EN 62305. Consulting engineers need to involve LPS contractors, and due to the greater technical involvement


necessary, to pay for their assessment and design expertise, during the early design phases of a project. Only if consulting engineers are prepared to do this will the requirements for protection against lightning be delivered in a non-confrontational and efficient manner.

The management of the process for determining the need and delivering an LPS to a structure andlor an LPMS to protect electrical or electronic systems within a structure would be most effective in the UK contracting sector if the process shown in Table 1.1 is followed.

Table ?.? - Management of the process of providing LPS and LPMS

Step 1 Objective I Actions by

Client and consultant to decide which risks they wish to consider, provide detailed inputs to risk assessment and employ lightning protection experts to carry out risk assessment and derive need for protection and lightning protection level.

Consulting engineer employs lightning protection expert to plan and undertake initial tender design. Both parties together with other appropriate members of the services design team discuss and determine requirements and most appropriate options for LPS and siting of services.

Lightning protection expert carries out initial LPS design and provides to consultant for tender purposes.

LPS

Initial risk assessment

LPS planning

LPS tender stage design

Check the need for an LPS and determine the level of protection required.

Consider options for protection.

Provide detailed design and specification for tender stage LPS, considering:

LPL design requirements; appropriate air- termination design method; inclusion of natural components for air- termination, down- conductor and earthing; protection of roof mounted fixtures and bonding needs including surge protection devices (SPDs); positioning of down- conductors; positioning of test points; soil resistivity; appropriate earthing arrangements; internal LPS, separation distances and bonding; touch and step potential outside building;

Basic considerations

Step

LPS tender

LPS installation

LPS commissioning

LPS documentation

Recurrent inspections and electrical tests

LPMS

Objective

other specific issues.

Obtain competitive market prices for compliant LPS installation from competent contractors.

Secure a good LPS installation coordinated with other services.

Ensure LPS installation complies with the initial and developed design in accordance with BS EN 62305-2.

Gather full information relating to the design parameters and as installed data for the project health and safety file and facilitate future maintenance. Project health and safety file passed to client upon project completion.

Ensure continuing adequacy of LPS and compliance with legislation.

Actions by

Consultant ensures coordination with all other services and any consequential amendments needed to tender stage design.

Consultant or main contractor, depending upon specific contract administration, requests bids from approved contractors based upon accurate requirements.

LPS contractor supervised ultimately by the consultant through the particular contractual chain.

LPS contractor supervised by the consultant.

LPS contractor.

Main contractor.

Client engages LPS contractor to undertake visual inspections, electrical tests and reports.

Initial risk assessment

Final risk analysis

Check the need for lightning electromagnetic pulse (LEMP) protection, and if needed, select suitable LPMS using the risk assessment method.

The cost/benefit ratio for the selected protection measures should be optimized using the risk assessment method again. As a result the following are defined:

lightning protection level (LPL) and the lightning parameters; lightning protection zones (LPZ) and their boundaries.

Client and consultant to provide detailed inputs to risk assessment and employ lightning protection expert to carry out risk assessment and derive need for protection and lightning protection level.

Lightning protection expert and client/ consultant


Step

LPMS planning

LPMS tender design

LPMS tender

Installation of the LPMS, including supervision

Approval of the LPMS

Recurrent inspections

Notes BS EN 62305-4 calls for a lightning protection expert to undertake initial designs and independent final commissioning. This additional contracting member is not currently custom and practice in the UK and, in practice, the consulting engineer would perform this service with some input from a lightning protection contractor or surge protection specialist. It is unlikely that a client would wish to incur additional expense employing a lightning protection expert, however it would be prudent to ensure that any party undertaking detailed design and commissioning works, either directly or indirectly, are covered by professional indemnity or other appropriate forms of insurance.

For direct contracts between client and the LPS expert, the LPS contractor may replace the consultant on the basis that the contractor will have direct contact with the client in this case for the purposes of establishing exact requirements.

It is customary in the UK for the lightning protection contractor simply to be asked to 'provide a design and quotation for a system to BS 66511, with little input data to assist. A simple request through the conventional contractual chain, typically from the electrical contractor, for the lightning protection contractor to 'provide a design and quotation for a system to BS EN 62305' is likely to lead to a system that does not totally concur with the requirements of the new standard and one that is poorly coordinated with other services. These circumstances, or others where insufficient information is available, are covered in the risk assessment by applying a default to the calculation.

Objective

Definition of the LPMS considering:

spatial shielding measu res; bonding networks; earth-termination systems; line shielding and routing; shielding of incoming services; coordinated SPD protection.

Provide detailed design and specification for tender stage LPMS.

Obtain competitive market prices for LPMS installation from competent contractors.

Ensure quality of installation and provision of documentation and, where necessary, revision of the construction drawings.

Checking and documenting the state of the system.

Ensuring the adequacy of the LPMS.

Actions by

Consulting engineer employs lightning protection expert to plan and undertake initial tender design. Both parties, together with other appropriate members of the design team and electrical or electronic equipment suppliers, discuss and determine requirements and most appropriate options for LPMS.

Lightning protection expert

Consultant ensures coordination with all other services.

Consultant or main contractor, depending upon specific contract administration, requests bids from approved contractors based upon accurate requirements.

LPMS contractor supervised ultimately by the consultant through the particular contractual chain.

LPMS contractor supervised by the consultant.

Client engages LPMS contractor to undertake visual inspection, tests and documentation.

Basic considerations

Table 1.1 suggests a route that would lead to a fully coordinated optimum engineering solution. There will be circumstances in which the client does not wish to follow this route and prefers the more conventional request to a lightning protection specialist for 'a system to BS EN 62305'. In this case, the client will as a minimum need to provide the lightning protection contractor with the details shown in Table 1.2 if they wish any proposed risk assessment or solution to be remotely appropriate.

In Table 1.2, the risks to be considered are defined as:

R1 = risk of loss of human life;

R, = risk of loss of service to the public;

R3 = risk of loss of cultural heritage;

R4 = risk of loss of economic value.

Table 1.2 - Information required to enable accurate risk assessment by type of risk

Information required

Type of structure or service (for example, school, offices, hospital or warehouse).

Does the structure contain explosives?

Is the structure in an area where it is higher than or the same height as other structures or is it isolated or located on the top of a hill or knoll?

What is the postal address of the structure?

Dimensions of structure, length, width and heights. Provision of scaled roof plans and elevation drawings detailing the structural and external make up of the structure and showing details and locations of all services and equipment, especially that equipment located externally to the structure.

What are any rooms internal to the structure housing electronic equipment constructed of and what are the dimensions of the internal structures?

For service lines the following is required:

Number of service lines feeding the structure?

Is the power line single or three phase, overhead or underground, does it have armouring or other mechanical protection and if so what is the resistance of this in O/km?

For telecommunications lines, how many, how many pairs within the line, is it overhead or underground, does it have screening and if so what is the resistance of the screening in O/km? If the lines are overhead, what are their heights from the ground?

What are the lengths of all the lines between the structure to be assessed for protection and the telephone exchange or substation feeding the lines? What are the lengths, widths and heights of these structures?

R1

X

X

X

X

X

X*

X

X

X

X

R2

X

X

X

X

X

X

X

X

X

X

R3

X

X

X

X

X

X

X

X

%

X

X

X

X

X

X

X

X

X

X


Information required

Do the lines run through areas where they are higher than or the same height as other structures or are they isolated or located on the top of hills or knolls?

Do the lines run through urban areas with tall buildings or an ordinary urban environment or do they run through suburban or rural areas?

For power lines, is there a transformer provided to the structure or is there a service line only?

Where telecommunication/data systems are installed within the structure, will routing precautions be taken to avoid induction loops, for example by routing cables away from external walls and running together power and data cables feeding the same equipment? Will the cables be shielded or run in mechanical protection offering shielding against LEMP effects? If so what will the resistance of the shielding be in O/km?

How many floors does the structure have and how many people will be in it? Are there likely to be any difficulties of evacuation, for example aged or infirm people in hospitals? Are there likely to be any hazards or contamination for the surroundings or environment in the event of a strike?

What is the value of the soil resistivity in O/m?

What are the design voltage withstand levels for the power and electronic systems within the structure?

What are the floor finishes inside the structure and outside in the zone up to 3 m away from the structure?

What provisions are fitted to protect against fire risk?

What is the fire loading of the structure?

Separate costs of the structure, contents, systems within the structure and any animals on site.

The interest and amortization rates applicable to the total costs of structure, contents and systems.

X = Required information. Only required for this risk if the structure is a hospital or contains explosives.

** Only required where the services feeding the structure are underground or where risk component RA (relating to the zone three metres outside the structure - see Table 2.9 of this guide for further detail) is to be considered. *** Only required for this risk where there is a risk of loss of animals.

R1

X

X

X

X*

X

X**

X

X

X

X

R2

X

X

X

X

X

X**

X

X

X

R3

X

X

X

X**

X

X

X

%

X

X

X

X

X

X**

X

X***

X

X

X

X

Section 2 Risk assessment

2.1 General

The risk assessment is the vital first part in the application of BS EN 62305 and is required to determine the need for protection measures so as to reduce the risk of loss due to lightning below a tolerable level.

BS EN 62305-2 states: 'The values assigned for certain parameters used as part of the risk evaluation process in this British standard, are merely values proposed by the IEC (specifically in Annexes B, C and the case studies in Annex H). It is recognized by IEC that these identified values may not be appropriate for application in all the countries that utilize this standard. Different values may be assigned by each national committee based upon each country's perception and importance they attribute to the relevant risk category. The UK committee GEU81 has reviewed the relevant parts of this standard and have provided appropriate UK interpretations which can be found in national annexes at the end of this standard'.

For completeness of understanding, BSI has issued the IEC version of 62305, adopted by CENELEC, in its entirety, including those parts that it has reconsidered in light of the British interpretation. It is important to the accurate outcome of the risk assessment for application in the UK that Annexes B, C and H be disregarded and replaced with their appropriate nationally determined appendices NB, NC and NH.

The risk assessment is a complicated process requiring much information, some of which under the current customs and practices within the UK construction industry is not likely to be available to the lightning protection designer at the initial design and tender stage. It is vital therefore that the lightning protection contractor undertakes a calculation using the information they are provided with, together with any reasonable assumptions made and the inputs and outcomes presented in tabular form identifying all the characteristics used, so that the client andlor their professional representatives can see that all areas have been appropriately considered and included in the final assessment.


Where there is a desire that there be no avoidable risk, the decision to provide lightning protection may be taken regardless of the outcome of any risk assessment. [BS EN 62035- 2, Introduction]

2.2 Type of damage and loss and source of damage

The lightning current is the primary source of damage. The following sources of damage are represented by the strike attachment point shown in Table 2.1:

S1: flashes to a structure;

S2: flashes near a structure;

S3: flashes to a service;

S4: flashes near a service. [BS EN 62035-2, 4.1.11

Three basic types of damage can occur as a result of lightning flashes:

Dl: injury to living beings;

D2: physical damage;

D3: failure of electrical and electronic systems [BS EN 62035-2, 4.1.21

Each type of damage, singularly or a combination, could produce a different consequential loss in the object to be protected. This depends on the characteristics of the object and its content. Losses to be taken into account are as follows:

[BS EN 62305-2, 4.1.31

L1: loss of human life

I%: loss of service to the public

L3: loss of cultural heritage

L4: loss of economic value (structure and its content, service and loss of activity)

Note that L1 to L4 are all types of loss associated with a structure.

Table 2.1 summarizes these sources of damage.

Table 2.1 - Sources of damage

Point of strike

To the structure

Near the structure

To the service

Near the service

') Only for structures with risk of explosion, and for hospitals or other structures where failure of internal systems immediately endangers human life. ') Only for properties where animals may be lost.

Source of damage

S1

S2

S3

S4

Type of damage

D l D2 D3

D3

D l D2 D3

D3

Type of loss

L1, ~ 4 ~ ) L1, L2, L3, L4 LI", L2, L4

LI", L2, L4

L1, ~ 4 ~ ) L1, L2, L3, L4 LI'), L2, L4

LI'), L2, L4

Risk assessment

Over the last 20 years, BS 6651 has been used in the UK and many other parts of the world and is recognized as one of the leading standards for the protection of structures against lightning. Over these years, many thousands of risk assessments have been carried out to this standard resulting in a requirement for a system of protection. BS EN 62305 also produces a risk assessment and proposals for protection systems, but is much more complicated and requires the input of much more detailed design data in order to determine the level of protection required. This is a significant shift from BS 6651.

One added complication in calculating the risk assessment is the introduction of the ability to calculate a risk for separate zones within the same structure. As BS EN 62305 has many new practices it is expedient in the early days of its introduction to consider all structures as single zones until and unless practical experience of the new methodologies demonstrates a need otherwise, or where, in the case of particularly vulnerable structures, R < RT cannot be satisfied (R and RT represent calculated risk and the tolerable risk, respectively). In these occasional cases, reference should be made to Annex NH of BS EN 62305-2. For completeness however, the zoning principle and sample calculations appear at the end of this risk assessment section.

On the above basis, the procedure for selection of protection measures follows the flow chart in Figure 2.1.

[Source: BS EN 62305-2, Figure 31

2.3 Risk assessment stage 1 - determination of assigned values

Assume we have an office and storage plant distributing goods to the public, constructed of part reinforced (noncontinuous) concrete columns, conventional block and brick walls and a flat roof 100 m long, 20 m wide and 16 m high located in Hertfordshire. All services are underground and no details of their lengths or the 'a' end structures are known. No details of the types of internal cabling or their routes are available. People are present only inside the structure, there is an automatic fire alarm system fitted and there are no spatial shields at the boundary or internal to the structure. The structure will be classed as a single zone for the purposes of this exercise.

The first stage of the risk assessment process is to identify the risks to be evaluated. In the past, it has been customary to consider mainly those risks associated with human life. However, the new assessment method provides the client with the ability to derive the value for several risks:

R1: risk of loss of human life (this is the primary consideration in the application of any assessment within the UK);

R2: risk of loss of service to the public (which could relate to loss of human life in hospital situations for example);

R3: risk of loss of cultural heritage (applicable only in areas where loss of cultural heritage is possible);

R4: risk of loss of economic value (only likely to be applicable by strong and informed request from the client).


ldentify the structure to be protected - Stage 1 data and characteristics

1 I Determine from client which risk(s) islare to I

beassessed - R,, R , & R ,

1 Calculate collection areas - Stage 2

Calculate number of dangerous events - Stage 3 ldentify and calculate appropriate risk components - Stage 4

and calculate R , , R , 8 R , as reauired.

Compare calculated and tolerable values of the risk being assessed and identify risk by source of damage - Stage 5

R>R ,? Structure

Figure 2.1 - Procedure for selecting protection measures in structures

installed?

Calculate new values of risk components

NO -

v Install an

adequate level of LPS

I 1 I

Install an adequate LPMS

Install other protection methods

Risk assessment

For demonstration purposes all risks R1, R2, R3 and Rq are to be considered, however R3 would be disregarded in this case as there is no risk of loss of cultural heritage associated with this structure. Rq will usually only be undertaken after an informed request by the client, as the safety of people within the structure is of paramount importance and the derivation of any theoretical loss or gain of economic value is a secondary consideration.

In order to start the process, various parameters relative to the structure, power and telecommunication lines feeding it need to be established. These will form inputs to the risk calculation. The parameters, symbols and values attributed to the structure in question, together with comments on the approach to each, are shown in Table 2.2.

Table 2.2 - Parameters associated with the structure and its environment

Parameter

Dimensions (m) of the structure to be protected, referred to as the 'b' end

Dimensions (m) of the structure at the source 'a' end of the service cables

Location factor

Touch and step voltage protection

LPS

Symbol and value

Lb = 100 Wb = 20 Hb = 16

La = 20 Wa = 20 Ha = 8

Cd = 0.5 'a' and 'b' ends

PA = 0

Ps = 1

BS EN 62305-2 reference

Actual dimensions

Actual dimensions

Table A.2

Table NB.l

Table NB.2

Comments

The subscript Ib' identifies the structure to be protected. The length, width and height should be the largest dimension of each of the planes (see collection area for further detail).

The lines coming into the structure will originate from a structure at the 'a' end but more often than not the information regarding the 'a' end building characteristics will not be available. This 'a' parameter fits into the risk calculation and affects the calculated values of R, and R, but only to a small degree and may be disregarded for practical reasons if the information is not readily available, without adversely affecting the outcome of the assessment. However for consistency and completeness, see the calculations below. Should the practitioner wish to include values for the 'a' end telecommunication and power structures and definitive information is not available, it is reasonable to assume dimensions of 20 m, 20 m and 8 m for L, Wand H respectively, which is typical of an average telephone exchange or substation.

Choice between four factors depending upon relative location to other buildings.

Only used when assessing R1 for the zone 3 m outside the structure; in this example this is not applicable so we will disregard it. However, it will be covered at the end of this section on zoning. PA considers the danger to living beings from touch and step potentials developed as a result of a flash to the structure. Where physical restrictions are provided or no one is in the area, PA is negligible so insert a 'zero' value.

Choice between seven factors. When undertaking the first risk assessment, the factor 1 should be applied to indicate no protection is currently installed.


Parameter

Shield extemal to structure at boundary

Shield internal to structure

Flash density

Soil resistivity (Om)

Type of ground outside the structure

Failure of internal systems

Loss of service to the public

Failure of intemal services due to a flash near a service

Symbol and value

Ksl = 1

Ks2 = 1

Ng = 1

p = 500

r, = 0

For this example Lo = 0 for R, as it is not applicable in this case

For R2, Lo = 0.01 For R4, Lo = 0.01

PLI(P) = 0.4 PLl (n = I


Equation NB.3

Equation NB.3

Annex NK Figure 1 and Figure 2.3 of this guide

Measured and provided by the client's engineers

Table NC.2

Table NC.l

Table NC.6

Table NC.7

Table NB.7

Comments

Ksl is calculated from the equation 0.12 x w where w is the widest width of the shield at the structure external periphery (see Section 5.4.4 for details) and 0.12 is a constant. Ksl to KS4 are used to calculate PM for use when calculating R1 in explosive and hospital structures only and R2 in all cases. The maximum value attributed to Ksl is 1.

Ks2 is calculated from the equation 0.12 x w where w is the widest width of the shield intemal to the structure (see Section 5.4.4 for details) and 0.12 is a constant. The maximum value attributed to KS2 is 1.

Ng is derived from the number of strikes per year per square kilometre of ground and is shown in Figure 2.3

Soil resistivity varies greatly, even across the same site in some cases. Should the resistivity not be made available with the tender documentation, a value of 500 should be assumed for the purposes of the initial risk evaluation.

This is a factor with a choice of four values that takes account of the touch and step potentials in the zone to 3 m from the structure. It is not applicable in this example, as we are not considering the zone 3 m outside the building.

This factor takes account of the loss due to the failure of internal systems and is only applicable to assessments of R, for hospitals and structures where a risk of explosion is present. Where this criterion is not applied, a value of zero should be applied to the risk calculations.

This factor takes account of internal system failures due to flashes to or nearby the service and is only applicable to assessments of R2 and R4.

There is a PLl factor for each of the telecommunications and power lines and this factor takes into account that a flash near an incoming service could cause failure of intemal systems. When no coordinated SPDs are fitted, PLl = PZ and where coordinated SPDs are fitted, the value of Pz = Psp, or PLl whichever is the lower.

Risk assessment

Table 2.3 - Parameters associated with the incoming power line and internal connected equipment

Parameter

Length (m)

Height (m)

Transformer factor

Location factor

Line environment factor

Line shielding

Internal wiring precautions

Internal system withstand values

SPD protection measures

Symbol and value

LC = 1000

Hc = 0

C, = 1

C, = 0.25

C, = 0.1

PLD = 1

KS3 = 1

KS4 = 0.6

PspD = 1


Measured or default

Measured or assumed to default to 6

Table A.4

Table A.2

Table A.5

Table NB.6

Table NB.5

Equation NB.4

Table NB.3

Comments

LC is the distance from the structure to be protected to the substation or other structure that provides a splitting of the service. If no dimensions are provided, 1000 m should be assumed.

Hc is the height from the ground to the highest point of the overhead line. If no height is provided a reasonable assumption is 6 m.

This factor has two choices and relates to where a HV/LV transformer is located between the point of any strike and the structure to be protected. It seems logical that as any external line can intercept a strike, unless the transformer is located either within or directly adjacent to the structure to be protected, the factor 1 should be applied for a service only.

Choice between four factors depending upon relative location of the line to other buildings.

Choice between four factors depending upon the nature of the surrounding area.

This factor takes account of the probability of failure of intemal systems due to a flash to the connected service depending upon the cable screen resistance and the impulse withstand voltage of the equipment. For unshielded services or where screen or withstand details are not known, a value of PLD = 1 should be applied. PLD = Pu where no SPDs are provided for equipotential bonding purposes. Where surge protection devices (SPDs) provide equipotential bonding, Pu is the lower value of PspD (Table NB.3) or PLD (Table NB.6).

This factor takes into account the characteristics of the internal wiring. Where cable screening or details of cable routing are unknown or unclear a value of KS3 = 1 should be applied.

This factor takes into account the rated impulse withstand voltage of the system to be protected. K, = 1 .5/Uw In general electrical systems are designed at Uw = 2.5kV and this value may be used unless more detailed data is available.

This factor takes into account the failure of internal systems due to a flash to a structure. When undertaking the first risk assessment, the factor ' I ' should be applied to indicate no protection is currently installed. The application of coordinated SPDs and the consequential reduction in the value of PspD is only applicable for structures fitted with an external LPS.


Table 2.4 - Parameters associated with the incoming telecommunications line and internal connected equipment

Table 2.5 - Characteristics associated with the zone inside the structure

Parameter

Length (m)

Height (m)

Location factor

Line environment factor

Transformer factor

Line shielding

Internal wiring precautions

Internal system withstand values

SPD protection measures

Symbol and value

LC = 1000

Hc = 0

Cd = 0.25

C, = 0.1

C, = 1

PLD = 1

KS3 = 1

Ks4 = 1

PspD = 1

Parameter

Type of floor surface

Risk of fire

Fire protection


Measured

Measured

Table A.2

Table A.5

Table A.4

Table NB.6

Table NB.5

Equation NB.4

Table NB.3

Symbol and value

r, = 0.01

r, = 0

rf = 0.01

r, = 0.2

Comments

As for power characteristics.




This factor does not appear in any further calculations for the telecommunications line m in this guide. Although in the vast majority of cases a transformer will not be in the telecommunications line, there may be instances where one will be present and this factor should then be incorporated.



This factor takes into account the rated impulse withstand voltage of the system to be protected. Ks4 = 1 .5/Uw In general electronic systems are designed at Uw = 1.5kV and this value may be used unless more detailed data is available.



Table NC.2

Table NC.4

Table NC.3

Comments

This is a factor with a choice of four values that takes account of the touch and step potentials inside the structure. This is a factor with a choice of four values that takes account of the touch and step potentials outside the structure, but is not applicable in this example as we are only considering the inside of the structure.

This is a factor with a choice of four values that takes account of the specific fire load of a structure. Other than structures with a risk of explosion or paper mills or industrial warehouses with flammable stock, the risk will usually be 'ordinary', thus attracting a value of f' = 0.01.

This is a factor with a choice of three values that takes account of the provisions taken to reduce the consequences of fire within a structure.

Risk assessment

Table 2.6 - Characteristics required to calculate R4 and evaluate costs of loss

Parameter

Special hazards

Injury by touch and step voltages

Loss due to physical damage

Symbol and value

For R1, h, = 5

For R4, h, = 1

For R,, L, = 0.0001 For R4, L, = 0

For R1, Lf = 0.42 For R2, Lf = 0.1 For R3, Lf = 0.1 For R4, Lf = 0.5

Parameter

Cost of animals on the site

Cost of the structure (£1 Cost of the contents (£1

Cost of the systems in the structure (£)

Interest rate (%)

Amortization rate (%)

Maintenance rate


Table NC.5

Table NC.l applies to R1 and R2 Table NC.7 applies to R4 Table NC.l

Table NC.6

Annex NC.4 Table NC.7

Symbol and value

CA = 0

CB = 20M

Cc = 2.5M

C, = 3M

i = 7.25

a = 4

m= 5

Comments

This is a factor with a choice of seven values which increases the relative amount of loss due to the presence of a special hazard and is applicable to calculations for R1 and R4 only. The only three values relevant to an R4 risk are 1. 20 or 50.

This factor takes account of people inside and/ or outside the building only and is used to assess R,, R2 and R4 where there is a risk of loss of animals.

This factor takes account of people inside the structure and can be derived in most cases from the typical values given in the table. For structures not listed, a calculation of Lf can be undertaken. Note that Lf is derived from different tables for the calculation of R1, R2, R3 and R4.


Annex G

Annex G

Annex G

Annex G

Annex G

Annex G

Annex G

Comments

This factor is only applicable if animals are present. If no animals are present then the value of CA is zero.

This is the total cost of initially building the structure.

This is the cost of all contents including stock but excluding the cost of supplying and installing the electrical and electronic systems and wiring.

This is the cost of supplying and installing the electrical and electronic systems and wiring.

This is the rate of interest applying to any loan to purchase the LPS and LPMS systems. If there is no loan or information is not readily available then the higher of the opportunity cost of not having the money to invest or the Bank of England base rate plus 2.5 % should apply.

This is the length of time that the owner of the structure, systems and contents writes off the costs of the LPS and LPMS. If the information is not readily available a default rate of 4 % (relating to 25 years) should be applied.

This is the cost of maintenance of the LPS and LPMS systems as a percentage of the initial installation cost. Allow 5 % of the cost of the protection measures if maintenance details are unknown at this stage.


2.4 Risk assessment stage 2 - calculation of collection areas

The next stage in the assessment process is to derive the collection areas.

To calculate the number of dangerous events in later stages of the process, there is a requirement to derive collection areas for the structure, near the structure, to the power and telecommunications lines (which could be aerial or buried) and near the power and telecommunication lines (which again could be aerial or buried).

I I

H a

overhead Hc service I I

I LC I (1000m max.)

Figure 2.2 - Collection areas

BS EN 62305-2, Annex A defines several different methods for deriving collection areas. However for consistency of approach and to ensure collection areas are not understated, unless the practitioner has sufficient detail or the benefit of bespoke software, it is suggested that the formulae and methods in Table 2.7 are applied. The characteristics, LC, Ha etc can be obtained from the tables, calculations or measurements referred to in Tables 2.2 to 2.6.

Table 2.7 - Collection areas

Collection area (m2)

Structure to be protected

Near the structure to be protected

Structure at 'a' end

Equation and value

Adp = L x W + 6 x H x (L + W) + r(3H)' Adp = 20,758.23

A, = L x W + 500 x (L + W) + ~(250) ' A, = 258,349.54

Adla = L x W + 6 x H x (L + W) + ,(3H)' Adla = 4129.56

Comments

L, Wand H should be the maximum elevation dimensions of the structure to be protected as shown in BS EN 62305-2, Table A . l , as the absolute area would be unnecessarily complex to calculate.

L, Wand H are as for the calculation of the area for the structure.

As for Adlb

Risk assessment

Where the structure to be considered is only a part of a larger structure, the dimensions of the part to be protected may be used for determining A&, but only where:

Collection area (m2)

Of the: Aerial power line

Aerial telecom line

Buried power line

Buried telecom line

Near the: Aerial power line

Aerial telecom line

Buried power line

Buried telecom line

the structure to be protected is a separate vertical part of the larger structure;

it does not have a risk of explosion;

Note that in the above formulae, A is the symbol for collection area. This is followed by two subscripted letters: the I and refer to the collection areas of and near the power or telecommunications lines respectively and the (p) and (, refer to power or telecommunications lines respectively. This format continues throughout the calculations of risk.

Multiple cables originating from the same place and sharing the same route should be classed as one cable for the purposes of calculating the collection areas.

Equation and value

A ~(p) = [LC - 3(Ha + Hb)] x 6 x Hc A = Not applicable, line buried.

A = [LC - 3(Ha + H,)] x 6 x Hc A I(T) = Not applicable, line buried.

A I(P) = [LC - 3(Ha + Hb)l x fi A l(p) = 20,750.71

A I(T) = [Lc - 3(Ha + Hd1 x fi A I,,) = 20,750.71

Ai(p) = 1000 x LC Ai(P) = Not applicable, line buried. Ai(, = 1000 x LC Ai(,= Not applicable, line buried. A i ( ~ ) = 25 x LC x ,/j5 Ai(p) = 559,016.99 Ai(, = 25 x LC x fi Ai,, = 559,016.99

propagation of fire between the parts of the whole structure is avoided by means of walls with resistance to fire of 120 minutes or by means of other equivalent protection measures;

Comments

In almost all cases, only two of these four options will be appropriate, for example, there will be either an aerial or buried power cable and an aerial or buried telecom cable.

In almost all cases, only two of these four options will be appropriate, for example, there will be either an aerial or buried power cable and an aerial or buried telecom cable.

surge protection devices (SPDs) are installed at the entrance point of such lines in the structure or by means of other equivalent protection measure.

2.5 Risk assessment stage 3 - assessment of number of dangerous events

As we have now determined the parameters in stage 1 and the collection areas in stage 2, we move forward to calculate the number of dangerous events to and near the structure to be protected ('b'), to and near the power line, to and near the telecommunications line and to and near the structure ('a') at the source end of the services. The calculations to and near the lines, will differ depending upon whether the lines are aerial or buried.


Using the values of the characteristics of the structures and lines and collection areas identified above and inputting them into the appropriate formulae, the number of dangerous events are calculated as shown in Table 2.8.

Table 2.8 - Number of dangerous events

The number of flashes to ground per square kilometre per year, Ng, can be established from Figure 2.3.

Events :

TO structure 'b'

Near s t ~ c t ~ r e 'b'

To the: Aerial power line

Aerial telecorn line

Buried power line

Buried telecorn line

Near the: Aerial power line

Aerial telecorn line

Buried power line

Buried telecorn line

To structure ~t power source a(p)

~t telecorn source a(,

2.6 Risk assessment stage 4 - assessment of risks R, , R2, R3 and R4

We have now determined She assigned values at stage 1, values for the various collection areas at stage 2 and values for She number of dangerous events at stage 3. The next stage is to determine the four risks R1, R2, R3 and R4.

Number of events

ND = Ng X Adlb X Cdlb X 1 o - ~ NDIb = 1.0379 X 10-'

NM = Ng X (A, - Adlb X CdIb) X NM = 2.4797 x lo-'

NL(p) = Ng x A I (P ) x Cd x Ct x NL(p) = Not applicable, line buried NL(, = Ng x AI(T) x Cd x NL(, = Not applicable, line buried NL(p) = Ng x A I (P ) x Cd x Ct x NL(p) = 5.1877 x NL(, = Ng x AI(T) x Cd x NL(, = 5.1877 x 1 0-3

NI (P ) = Ng x Ai(!) x Ce x Ct x 1 0-6 NI(p) = Not appl~cable, line buried Nl(, = N, x A ~ ! ~ x ce x lo4 Nl(, = Not appl~cable, line buried NI (P ) = Ng x Ai (P) x Ce x Ct x NI(p) = 5.5902 x lo-' Nl(, = Ng x Ai(, x Ce x NI(, = 5.5902 x lo-'

N ~ a ( ~ ) = Ng Adla Cd/a

NDa(p) = 2.0648 x 1 o - ~ NDa(, = N~ x x CdIa x NDa,, = 2.0648 x

The four risks are derived from risk components, themselves comprising calculations variously composed of She number of dangerous events and other characteristics relative to the structure as identified earlier and further described in Table 2.9.

Comments

Risk assessment

Figure 2.3 - Map to determine N,


Table 2.9 - Assessment of risk components

Risk component

R A

RB

Rc

RM

R ~ ( ~ )

R ~ ( T )

R ~ ( p )

R ~ ( T )

Formulae and value

RA= N D x P A x r a x L t RA= 0

RE = N D ~ P B ~ ~ p ~ h , ~ r f ~ L f For R1, RE = 4.3592 x For R2, RB = 2.07 x lo-' For R3, RB = 2.0758 x lo-' For R4, RE = 1.0379 x

Rc = N D x P c x L 0 For R2, Rc = 1.0379 x l o 4 For R4, Rc = 1.0379 x l o 4

RM = N M x P M x L 0 For R2, RM = 2.4797 x For R4, RM = 2.4797 x

Ru(P) = (NL(P) + NDa(P)) X PU X ru X Lt For R1, RU(P) = 7.2525 x lo-' Ru(T) = (NL(T) + NDa(T)) X PU X ru X Lt For R,, Rum = 7.2525 x 1 0-'

Rv(P) = (NL(P) + N D ~ ( P ) ) X

P v x r p x * h , x r f x L f For R1, RV(P) = 3.046 x For R2, RV(P) = 1.4505 x lo-' For R3, RV(P) = 1.4505 x lo-' For R4, RV(P) = 7.2525 x lo-' R ~ ( ~ = ( N ~ ( ~ + N ~ a ( ~ ) ) Pv x rp x *h, x r f x Lf For R1, Rv(, = 3.046 x 1 0-5 For R2, RV(T) = 1.4505 x lo-' For R3, RV(T) = 1.4505 x lo-' For R4, RV(T) = 7.2525 x lo-'

Purpose of component and comments

Relates to injury to living beings by touch and step voltages in the zone 3 m from the structure by flashes to the structure. The value in this case is 0, as RA is not applicable as we are only considering the zone inside the structure.

Relates to physical damage by flashes to the structure. For R1, R2 and R4, Lf values are derived from BS EN 62305-2, Tables NC.l, NC.6 and NC.7 respectively. For R3 the Lf value is 0.1. Refer to BS EN 62305-2, NC.4. When calculating values for R2 and R3, the h, factor is disregarded.

Relates to failure of systems due to LEMP by flashes to the structure. Lo is derived from BS EN 62305-2, Table NC.6 for R2 and NC.7 for R4.

Relates to failure of systems due to LEMP by flashes near the structure and is applicable to R2 and R4 only. Lo is derived from BS EN 62305-2, Table NC.6 for R2 and NC.7 for R4.

Relates to injury to living beings by touch voltages by flashes to a service connected to the structure. See determination of parameters schedules, Tables 2.2 to 2.6 above. Pu = PLD where no SPDs are provided for equipotential bonding purposes. Where SPDs provide equipotential bonding, as should be the case where an LPS is provided, Pu is the lower value of PspD (BS EN 62305-2, Table NB.3) or PLD (Table NB.6).

Relates to physical damage (generally caused by dangerous sparking at the entrance point of the line into the structure) by flashes to a service connected to the structure. See determination of parameters schedule above, Pv = PLD where no SPDs are provided for equipotential bonding purposes. Where SPDs provide equipotential bonding, as should be the case where an LPS is provided, Pv is the lower value of PspD (BS EN 62305-2, Table NB.3) or PLD (Table NB.6). For calculation of R, Lf is derived from Table NC.l. For R2 it is derived from Table NC.6. For R3 a value of 0.1 should be used for Lf, see Annex NC.4. For R4, Lf is derived from Table NC.7. h, is not applicable in calculating R2 and R3.

Risk assessment

Risk R, - Risk of loss of human life

Risk component

R ~ ( ~ )

Rw(T)

R z ( ~ )

Rz(T)

Note: When

R1 comprises the following components:

R1 = R~~ + RB + R~~ + R~~ + RU + RV + R~~ + R~~

There are two components (assuming the structure is fed by power and telecommunications), one each for the power and telecommunication lines for risk components U , v, and where they form part of the risk calculation. Those components superscripted ' are only applicable for structures with a risk of explosion and for hospitals with life-saving electrical and electronic equipment or other structures where failure of internal systems immediately endangers human life. That subscripted %s only applicable to the area 3 m outside the structure.

may vary from the values applied to other risks.

Formulae and value

RW(p) = (NL(p) + Nda(p)) x PW x Lo For R2, RW(P) = 7.2525 x For R4, RW(P) = 7.2525 x R W ( ~ = (NL(T) + Nda(T)) x PW x Lo For R2, RW(T) = 7.2525 x For R4, RW(T) = 7.2525 x

RZ(p) = (NI(p) - NL(p)) x PZ x Lo For R2, RZ(P) = 2.0286 x For R4, RZ(P) = 2.0286 x R Z ( ~ = (NI(T) - NL(T)) x PZ x Lo For R2, RZ(T) = 5.0714 x l o4 For R4, R Z ( ~ ) = 5.0714 x l o4

calculating values for R4, refer to Table

So in t e r n of the example we are considering, the R1 risk is as follows:

Ri = RB + RUIP) + RUIT) + RVIP) + RVIT)

R1 = 4.3592 x + 7.2525 x lo-' + 7.2525 x lo-' + 3.046 x + 3.046 x

R1 = 1.0453 x lo3

Purpose of component and comments

Relates to failure of internal systems caused by overvoltages induced onto lines connected to and transmitted to the structure due to flashes to the service. See determination of parameters schedule above. Pw = PLD where no coordinated SPDs are provided. Where coordinated SPDs are provided, Pw is the lower value of PspD (BS EN 62305-2, Table NB.3) or PLD (Table NB.6). Lo is derived for R2 and R4 from Tables NC.6 and NC.7 respectively. Note the values of RW(P) and RW(T) are the same in this example, this would not necessarily be so if the dimensions of the two lines were different.

Relates to failure of internal systems caused by overvoltages induced onto lines connected to and transmitted to the structure due to flashes near the service. See determination of parameters schedule above. PZ = PLI where no coordinated SPDs are provided. Where coordinated SPDs are provided, PZ is the lower value of PspD (BS EN 62305-2, Table NB.3) or PLl (Table NB.7). Lo is derived for R2 and R4 from Tables NC.6 and NC.7 respectively.

NC.7 for values of factors Lf, L, and Lo as these


RP - Risk of loss of service to the public

Rz comprises the following components:

R2 = RB + Rc + RM + Rv + Rw + Rz

As with risk R1 there are two components (assuming the structure is fed by power and telecommunications), one each for the power and telecommunication lines for risk components v, and Z.

So in terms of the example we are considering, the Rz risk is as follows:

R3 - Risk of loss of cultural heritage


R3 = RB + Rv

As with R1 and Rz, risk component R, appears for power and telecommunications lines.

So in terms of the example we are considering, the R3 risk is as follows:

R3 = RB + RVIP) +

R3 = 2.0758 x + 1.4505 x + 1.4505 x

R3 = 4.9768 x

R4 - Risk of loss of economic value


R.4 = RB + RC + RM + R~~ + RV + RW + RZ

Those components superscripted are only applicable for structures where animals may be lost.

Risk components RU, Rv, Rw and RZ appear for both power and telecommunications lines.

So in terms of the example we are considering, the R4 risk is as follows:

Risk assessment

2.7 Risk assessment stage 5 - comparison of calculated and tolerable risk and identifying risk by source of damage

Tolerable levels of risk, as assessed by the UK body having authority, are given in BS EN 62305-2, Annex NK, and shown in Table 2.10.

Table 2.10 - Typical values of tolerable risk RT

There is no assigned tolerable value for Rq. The calculated value of Rq does however work through to later calculations to determine economic costs or benefit from the provision of protection.

Type of loss

Loss of human life or permanent injuries

Loss of service to the public

Loss of cultural heritage

Comparisons of calculated and tolerable values of risk are shown in Table 2.11.

RT 1 o - ~ 1 o4 1 o4

Table 2.11 - Comparisons of calculated and tolerable values of risk

As the risk components have been calculated for all risk types R,, R, and R3, we are now able to further determine the risk in terms of the source of the damage for each risk R1, R, and R3. This will assist in determining appropriate protection measures.

Calculated risks

R1 = 1.0453 x

R2 = 3.4435 x

R 3 = 4 . 9 7 6 8 x 1 0 - '

R4 = 3.4634 x

The source of damage can be split into RD, She risk due to flashes striking the structure (direct strikes) and RI, the risk due to flashes influencing it but not striking the structure (indirect strikes).

Assessment of R, in terms of source of damage

Tolerable risk RT

l o4

l o4 -

The overall risk in terms of source of damage is expressed as:

R = RD + RI

Comment

As R1 > RT protection measures are necessary.

As R2 > RT protection measures are necessary.

As R3 < RT protection measures are not necessary.

This R4 risk is not considered further at this stage. It is further calculated once protection measures have been determined.

where

RD = RA** + RB + Rc*

and

RI = RM* + RU + Rv + Rw* + RZ*


Those components superscripted * are only applicable for structures with a risk of explosion and for hospitals with life-saving electrical and electronic equipment or other structures where failure of internal systems immediately endangers human life and ** is excluded from this example. As in previous calculations, there are two components (assuming the structure is fed by power and telecommunications), one each for the power and telecommunication lines for risk components U, v, and where they form part of the risk calculation.

So our example structure works as follows:

RD = RB = 4.3592 x

As RD = 4.3592 x > RT = 1 x measures to protect against a direct strike to the structure - an external lightning protection system - need to be instigated.

As RI = 6.0935 x > RT = 1 x measures to protect against an indirect strike to the structure need to be instigated.

We can further establish which components represent the largest elements of risk and then aim to mitigate these first by choosing appropriate protection measures. From the results of the R1 risk calculation we see that in excess of 41 % of the risk is likely to be due to physical damage, risk component RB, caused by flashes to the structure and in excess of 29 % of the potential risk is attributed to each of risk components RVIP) and RVIT), leading to physical damage as a result of flashes to services connected to the structure.

Assessment of Rp in terms of source of damage

As with the assessment of R1, in assessing R2 there axe several sources of damage represented as RD = the risk due to flashes to structure and RI = the risk due to flashes near the structure and to or near an incoming service

The overall risk in terms of source of damage is expressed as:

R = RD + RI

where

RD = RB + Rc

and

RI = RM + Rv + Rw + RZ

These components are applicable for all structures when calculating risk R2. As in previous calculations, there are two components (assuming the structure is fed by power and telecommunications), one each for the power and telecommunication lines for risk components v, and Z.

So our example structure works through as follows:

RD = RB + Rc = 2.0758 x + 1.0379 x lo3 = 1.0587 x lo-'

Risk assessment

As RD = 1.0587 x lo-' > RT = 1 x lo-', measures to protect against a direct strike to the structure, such as an internal SPD system, need to be instigated.

As RI = 3.3377 x > RT = 1 x lo-', measures to protect against an indirect strike to the structure or services entering it need to be instigated.

As with the R1 risk, we can further establish which components represent the largest elements of risk and then aim to mitigate these first by choosing appropriate protection measures. From the results of the R2 risk calculation we see that the largest part of the risk, 72 %, is likely to be due to component RM, failure of systems due to LEMP caused by flashes near the structure, with RZIP) and RZIT) representing approximately 6 % and 15 % respectively and the other components representing a much smaller percentage of the overall calculated risk.

Assessment of Rg in terms of source of damage

Unlike R1 and R2, RD and RI for risk R3 comprise only one risk component each:

RD = RB

and

RI = Rv

As in previous calculations, there are two components (assuming the structure is fed by power and telecommunications), one each for the power and telecommunication lines for risk component Rv.

So our example structure works through as follows:

RD = RB = 2.0758 x

As RD = 2.0758 x < RT = 1 x lo-', measures to protect against a direct strike to the structure are not considered necessary by the risk assessment process.

RI = RVIP) + RVIT) = 1.4505 x + 1.4505 x = 2.901 x

As RI = 2.901 x < RT = 1 x lo3, measures to protect against an indirect strike to the structure are not considered necessary by the risk assessment process.

Assessment of R4 in terms of source of damage

For information only at this stage, in terms of source of damage for risk R4, RD and RI comprise as follows:

and


As in previous calculations, there are two components (assuming the structure is fed by power and telecommunications), one each for the power and telecommunication lines for risk component RU, v, and Z.

The R4 assessment will be carried out at the end of the risk assessment once any protection measures and their costs have been determined.

2.8 Risk assessment stage 6 - selection of protection measures

The process of determining whether protection measures are necessary is laborious by long hand, as changing one characteristic, for example fitting a class IV LPS, will impact on many of the risk components and, as has just been demonstrated, the calculation is lengthy. It is unlikely that, for regular practitioners of protection against lightning, anything but the use of detailed bespoke software for the risk assessment will be practical or commercially viable. It is important that any software used uses the appropriate tables within BS EN 62305, takes account of all the permutations of the risk assessment and produces accurate results. To this end, it is recommended that practitioners carry out initial trial calculations using the software and long hand methods to ensure that results are comparable, accurate and representative of the processes of the risk assessment according to BS EN 62305-2.

Following the process shown in Figure 2.2, we are now at the stage where we have identified that R > RT for risks R1 and R2 but R < RT for risk R3 and so under the risk assessment, R3 can now be disregarded from any further considerations.

We now follow the Figure 2.2 process further. The building presently has no LPS and we have identified that both components RD and RI > RT SO the next step is to consider the impact of fitting the lowest, class IV, structural lightning protection system.

Applying measures to reduce R, to a tolerable level

So RB is derived from:

the number of dangerous events to the structure at 'b' end ND;

the class of LPS fitted to the structure PB;

special hazards present h,;

provisions for fire protection r,;

risk of fire rf;

loss to structure due to physical damage Lf.

All these parameters are characteristics of the structure.

Risk assessment

The first step is to allow for a class IV LPS. This then reduces component PB from a value of 1 to 0.2. If we fit a structural LPS, in order to comply with BS EN 62305, we need to fit standard equipotential bonding SPDs at the service line's entrance to the structure and this also reduces components Pv and PU to 0.03 (the lower value between PLD in Table NB.6 and PspD in Table NB.3) for both the power and telecommunications lines.

Therefore the new R1 value is 1.0547 x which still means R1 > RT.

If we then consider installing star * rated SPDs, this will further reduce the values of PU and Pv to 0.003. Recalculating the R1 value on this basis now gives R1 = 8.9013 x Therefore R1 < &.

Recalculating components RB, RU and Rv on this basis gives new values as follows:

RD = 8.7185 x

and

As these new values are less than RT, protection requirements for Rl have been satisfied by fitting a class IV external LPS and level I W star rated equipotential bonding SPDs at the positions where the incoming power and telecommunications lines enter the structure.

Applying measures to reduce R2 to a tolerable level

As we have already applied measures to reduce R1, some of which impact on Rz, we must ensure that the new values of RB and Rv are recalculated to determine the new position of Rz prior to assessing further (if any) measures to reduce Rz.

New values of RD, after applying measures to reduce R1 are:

RD = RB + Rc = 4.1516 x + 1.0379 x lo-' = 1.0421 x lo-'

which is > &

and

which is still > RT.

The greatest components of Rz are RM, representing in excess of 75 %, RZIP) and RZIT) representing 6 % and 15 % respectively, and Rc, representing 5 %. These new values require RD and RI to be further reduced below &.

As the level of RB has already been reduced to RB = 4.1516 x as part of the R1 risk reduction, we need to concentrate on reducing the Rc component in order to attempt to


reduce RD below the tolerable level &. Rc refers to the potential failure of internal systems caused by LEMP.

The Rc component is influenced by the:

number of dangerous events ND;

characteristic influenced by the fitting of coordinated surge protection PC;

factor for loss of service to the public Lo found in BS EN 62305-2, Table NC.6.

The next step is then to consider reducing the value of PC by introducing a system of coordinated type IIIAV SPDs. These SPDs need to be of the star rated type in order to coordinate with the equipotential bonding SPDs proposed. This would reduce the value of PC from 1 to 0.003.

Introducing this new PC value into the calculation results in a new value of RD as follows:

RD = RB + Rc = 4.1516 x + 6.218 x = 0.010 x lo-'

As RD and both its components are now less than the tolerable level of RT = lo-', the risk of loss of service to the public from direct sources is now acceptable. However, we also need to consider the indirect losses RI.

The introduction of the co-ordinated SPD set to reduce RD also reduces the risk of losses from indirect sources RI to a value of RI = 0.183 x lo3 and this factor is now also at an acceptable level.

In this example, we do not need to consider other methods of reducing RI as the value is now within the tolerable level, but should the need arise, it may be useful for the reader to understand what influences this factor. RI derives from the components RM, which is influenced by PM (which is derived from the product of KMS = KS1 x KS2 x KS3 x KS3, see Table NB.4), Rv, Rw and RZ, which are influenced by the resistance Rs of the cable screen and the impulse withstand voltage Uw of the equipment, see Table NB.7.

There are several methods of reducing KMS and so PM and consequently RM, and the practical measures for these are covered in more detail in Section 10 of this guide.

Applying measures to reduce Rg to a tolerable level

We have determined in earlier calculations that R3 is at a tolerable level already. However, if we did need to reduce the level of R3 we would follow the same process as for reducing R1 and R2, although the calculations would not be as lengthy:

R3 = RD + RI

where

RD = RB and RI = Rv

This being the case, the only components we would need to influence to reduce the loss of cultural heritage would be RB by fitting structural protection, which by default of the standard requires equipotential bonding SPDs (thus reducing the Pv value), and amending the calculation as per Tables NB.2 and NB.3 to derive the risk level.

Risk assessment

Consideration of risk R4, loss of economic value

This aspect of the risk assessment process is somewhat academic, as within our society the main consideration is to ensure a safe environment for employees and members of the public; indeed employers and providers of services have a legal responsibility to do so.

However, for budgeting and planning purposes it may be a useful academic exercise and can be determined as follows.

Rq = RA* + RB +Rc + RM + Ru* + Rv + Rw + Rz

Components superscripted * only apply where there is a risk that animals may be lost.

It is interesting to compare the Rq values before and after protection measures have been applied, but this part of the exercise forms no part in going on to determine costs or savings as a result of applying measures.

Before applying protection measures:

Rq = 3.4634 x

After applying protection measures:

R4 = 2.108 x

Although this exercise demonstrates that the protection measures we have applied to reduce R1 and R2 to a value lower than RT have also reduced the value of Rq, as the standard has no limit for Rq it is difficult to see what purpose this Rq value actually serves.

The possibly useful part of the exercise determines theoretical costs or savings from applying the protection measures and is applied as follows.

Cost of loss CL with 'no' protection measures applied

The generic equation shown in the standard is:

CL = (RA + Ru) x CA + (RB + Rv) x (CA + CB + Cc + Cs) + (Rc + RM + Rw + Rz) x Cs

All the components of this equation axe values 'before' any protection measures have been applied and components RU, Rv, Rw and RZ have values for both power and telecommunications lines. In addition, RB and Rv are the only two components whose values differ across the four risks. Values relating to Rq for these components should be used.

When only considering one of the three risks R1, R2 or R3, those components not applicable to the particular risk under consideration would be inserted into the formula as a zero or simply excluded from it. As we are considering R1 and R2, all components axe applicable so values will be inserted in the whole of the formula as follows:

where


CA is the cost of any animals and is only applicable where animals are present;

CB is the cost of the structure;

Cc is the cost of the contents (excluding systems);

Cs is the cost of the systems in the structure.

Therefore:

CL = 0 + 127 + 10,316 = f 10,443 = cost of loss without protection measures.

Cost of residual loss CRL 'with' protection measures applied

The formula for determining residual loss is the same as for the loss with no measures applied. However, the values applied to the equation should be those calculated based on the final protection measures applied.

CRL = 0 + 54.04 + 94.27 = f 148.31 = cost of residual loss in spite of protection.

Annual cost of protection measures CpM

The next stage of the process, assuming the cost of protection measures is f20,000, is to calculate CpM using the equation:

where

Cp = cost of protection measures

i = interest rate = 7.25 %

a. = amortization rate = 4%

m = maintenance rate (5 % of cost of protection measures would be a good guide, however for this value of installation we have assumed 3 % for the LPS and LPMS maintenance)

Risk assessment

Annual money saving due to protection measures

Annual saving of money, S:

S = CL - (CPM + CRL)

S = 10,443 - (2,850 + 148.31) = f7,444.69

Note; This value does not include the savings of any consequential losses not incurred.

Protection is considered to be convenient under the standard if S > 0

2.9 Summary of protection measures

The risk assessment process we have followed has determined that we can reduce the risk of loss of human life R1 to below the tolerable level RT = by installing a class IV LPS, which includes fitting type IIMV star-rated equipotential bonding SPDs to the incoming power and telecommunications lines. We can then reduce the risk of loss of service to the public R2 to below the tolerable level RT = lo-' by installing a system of star-rated type 1111 IV coordinated SPDs to the internal electrical and electronic systems.

2.10 Splitting the structure into zones

The previous risk assessment classed the whole of the structure as one zone. In structures where one or more specific areas offer characteristics that materially affect the overall risk, then BS EN 62305 provides a process by which the characteristics of these zones can be individually assessed and appropriately related to the overall risk for the complete structure. However, this is not a simple process.

We now consider a similar structure to that in the first example. This new structure now incorporates a small room, the contents of which presents a high risk of fire. The new value for the risk of fire q derived from BS EN 62305-2, Table NC.4 will now be 0.5, some 50 times greater than the value applied to our original example incorporating an ordinary risk of fire. Without repeating the full processes of the risk assessment, the new value of all four risks, treating this new example as a single zone would have produced the following values:-

The values for R1, R2 and R3 have all been severely adversely affected and have values much higher than their tolerable levels.

Applying the protection measures used in the previous example and applying the new value of rf = 0.5 gives values of:


The value of R1 is still above the tolerable level. Applying a class 1 LPS would further reduce R1 to 7.405 x which is still above the tolerable level which indicates that even with the highest level of protection applied, the residual risk would still be higher than the tolerable risk.

To address this, the next stage is to consider the characteristics of the various areas in order to split the structure into zones.

The structure in question can be split into four zones based upon the following:

the floorlground outside the structure is different to that inside the structure;

the structure is split into three distinct fireproof compartments;

all electrical and electronic systems are common throughout the structure;

the structure has no spatial shielding either internally or on the surface.

Having considered the structure, the zones to be assessed can be defined as:

Z1 - the zone immediately outside the structure;

Z2 - the storage area;

Z3 - the office area;

Z4 - the controlled solvent store.

As before, the first stage is to determine the risk parameters associated with each zone. As the structure has no risk of loss of cultural heritage, this example will only consider risks R1 and R2.

Table 2.12 - Determination of parameters associated with zones

Parameter 1 Symbol 1 value 1 comment

a) Characteristics of zone Z,, the zone immediately outside the structure

Type of ground outside the structure

Loss by touch and step voltages

Number of potentially endangered people in the zone

ra

Lt

"P

tp

b) Characteristics of zone Z2, the storage area

0.00001

Calculation

5

2600

Type of ground inside the structure


Asphalt

Persons outside the building

Number of persons in the zone Hours per year persons present in the zone

ru

Lt

0.01

Calculation

Concrete

Persons inside the building

Risk assessment

Parameter

Loss by physical damage

Loss due to failure of internal systems


Risk of fire

Fire protection

Special hazard

Spatial shield intemal to the structure

Symbol

L f

LO

n~

t,

'-f

'P

h~

Ks2

c) Characteristics of zone 4, Type of ground inside the structure





Risk of fire

Fire protection

Special hazard

Spatial shield intemal to the structure

Value

Calculation

Calculation

10

2600

0.01

0.2

5

1

Comment

To be calculated for each zone as per BS EN 62305-2, Equation NC.l

Only applicable for Rl in explosives or hospital environments. Value to be calculated for R2


Ordinary risk of fire

Automatic alarm or extinguisher system

Average level of panic

No shield

the office area

'-u

Lt

L f

Lo

n~

t,

'-f

'-P

h~

Ks2

d) Characteristics of zone 4, the controlled solvent store

0.0001

Calculation

Calculation

Calculation

20

2600

0.01

0.2

5

1

Type of ground inside the structure



Carpet



Only applicable for R1 in explosives or hospital environments. Value to be calculated for R2


Ordinary risk of fire



No shield

'-u

Lt

L f

0.01

Calculation

Calculation

Concrete




The next stage is to consider the collection areas. As none of the dimensions of the structure or lines have changed, the collection areas will be the same as in the previous example, as shown in Table 2.13.

Parameter



Risk of fire

Fire protection

Special hazard

Spatial shield internal to the structure

Table 2.13 - Collection areas

Symbol

Lo

"P

tp G

r~

h~

Ks2

The collection area will then be used along with the flash density Ng, location factor Cd, line environment factor C, and transformer factor C, to determine the number of dangerous events, as shown in Table 2.14.

Collection area

Structure to be protected


Structure at 'a' end

Of the power line

Of the telecom line

Near the power line

Near the telecom line

Table 2.14 - Number of dangerous events

Value

Calculation

2

2600

0.5

0.2

5

1

Comment

Only applicable for R1 in explosives or hospital environments. Value to be calculated for R2


High risk of fire



No shield

Symbol/equation

AdD = L x W + 6 x H x (L + W) + 1r(3w2

A, = (L x W) + 500 x (L + W) + 1r (250)'

Adla = L x W + 6 x H x (L + W) + 1r(3w2

AI (P) = [LC - 3(Ha + Hb)l x fi A,(, = [LC - 3(Ha + Hb)] x fi A i ( ~ ) = 25 x LC x Jp

Ai(T) = 25 x LC x JT

Value (m2)

20,758.23

258,349.54

4,129.56

20,750.71

20,750.71

559,016.99

559,016.99

Events

To the structure to be protected


To the structure at power source 'a' end

To the structure at telecom source 'a' end

To the power line

To the telecom line

Near the power line

Near the telecom line

Symbol/equation

ND = Ng X Adlb X CdIb X 1 o - ~

NM = Ng X (A, - Adlb X CdIb ) X

N D ~ ( P ) = Ng X Ad/a X Cd/a X

NDa(, = Ng X Adla X CdIa X

NL(p) = Ng x AI (P) x Cd x Ct x

NL(, = Ng x AI(T) x Cd x

NI (P) = Ng x Ai(P) x Ce x Ct x

Nl(, = Ng x A i ( ~ ) x C, x lo-'

Value

NDIb = 1.0379 X

NM = 2.4797 X 10-1

NDa(p) = 2.0648 x

NDa(, = 2.0648 x

NL(p) = 5.1877 x

NL(, = 5.1 877 x 1 o - ~ N I ( ~ ) = 5.5902 x lo-'

NI(T) = 5.5902 x lo-'

Risk assessment

The next part of the process is to evaluate the probability for each of the different types of damage, without protection measures having been applied, for each separate zone, as shown in Table 2.15.

Table 2.15 - Probability of damage

The next part of the process is to determine the loss factors for each zone. In the previous example, the values were chosen from tables within BS EN 62350-2, Annex NC. However, these values relate to the structure as a whole. As an area of the structure now offers a particular characteristic that prevents the statistically derived risk from being reduced to a tolerable level &, we must identify the loss characteristics of each zone in order to derive a more representative risk level.

Pz(T,

As different probabilities and loss factors relate to the different risks, we will now calculate R1 followed by R2. In order to differentiate those values relative to each risk, those values attributable to R1 and R2 will be subscripted or respectively and any common value will have no specific subscript.

The basic principle of the risk assessment is that the risk components RA etc. are derived from the following formula:

R, = N, x P, x L,

Note: - Not applicable to this zone. Refer to Section 2.6 to determine which probability is applicable to risks R, and R2.

1

where

R, is the value of the risk component;

N, is the number of dangerous events calculated in Table 2.14;

P, is the probability of damage derived from BS EN 62350-2, Annex NB;

1

L, is the loss in the component derived from the factors identified in BS EN 62350-2, Annex NC (see also Table 2.9 of this guide for details of the various components making up each risk.)

1


The next task the is the calculation of loss factors Lf, L, and Lo [BS EN 62305-2, NC.2 a,?& NC. 31

All three factors need to be considered when calculating R1, although Lo only needs to be calculated and taken into account where the structure is a hospital or carries a risk of explosion. We can therefore omit Lo from the calculations for R1 in this case. [BS EN 62305-2, NC.21 For calculating R2, only loss factors Lo and Lf apply. [BS EN 62305-2, NC. 31

For the purpose of evaluating R1, values of Lf and L, can be calculated for each zone from the following formula:

L, = (IL~?L,) x (td8760) [BS EN 62305-2, NC. I ]

where

L, = Lf or L, (or Lo if this factor is to be taken into account);

?L, is the number of possible endangered persons;

?L, is the expected total number of persons in the structure;

tp is the time in hours per year for which the persons are present in a dangerous place, outside the structure (L, only) or inside the structure (Lf, L, and Lo; Lo applies only where the structure is a hospital or carries a risk of explosion).

The individually calculated values of L, and Lf are shown in Table 2.16.

Table 2.16 - Results of manual calculation of loss factors

For the purpose of evaluating R2, values of Lf and Lo can be calculated for each zone from the following formula:

L, = ( ~ L I J ~ I ~ x (tl8760) [BS EN 62305-2, NC. 61

Loss factor

Lfl

Ltl

where

?L, is the number of possible endangered persondusers not served;

Zone 1

0.04

?L, is the expected total number of personslusers served;

t is the time in hours per year of loss of service.

Zone 2

0.08

0.08

Quantification of these factors will be difficult and may materially change on a regular basis. Where this information is not available, BS EN 62305-2 advises equally splitting the value identified in Table NC.6 between the four zones. In this case (as will probably be the norm) the example will use values of Lf = 0.025 and Lo = 0.0025 (both being the values in Table NC.3 divided by 4).

Zone 3

0.16

0.16

Zone 4

0.016

0.016

Risk assessment

Calculation of R, - risk of loss of human life

The loss factors are then included in the equations used to calculate the various risk components relevant to the risk R, being considered. The variables forming the various risk components are clearly laid out in Table 2.9 of this guide. These variables, together with the number of dangerous events and probabilities have been calculated and the results for each zone and the zones together are shown in Table 2.17.

Table 2.17 - Calculation of risk R,

The risk components in the zones are added to produce the total risk. This produces a risk R, = 2.737 x lo-', which is greater than the tolerable risk R, and therefore protection measures are necessary.

Risk component

R A ~

RB 1

Ru(P)

Rum

Rv(P)

Rv,, TOTAL

Calculation of Rp - risk of loss of service to the public

The next part of the process is to calculate the various risk components relevant to evaluating Rz. The results of these calculations are shown in Table 2.18.

Zone 1

4.163 x lo-'

4.163 x lo-'

Table 2.18 - Calculation of risk R2

1 TOTAL 1 8.609 x 1 o - ~ 1 8.609 x l o 4 1 9.21 8 x 1 o4 1 2.644 x 1

Zone 2

-

8.326 x 1 0 - 9 . 6 6 5

5.818 x 1 0 - 9 . 1 6 4

5.818 x 1 0 - 9 . 1 6 4

5 . 8 1 8 ~ 1 0 ~ ~ ~ 1 6 4 ~

5 . 8 1 8 ~ 1 0 ~ ~ ~ 1 6 4 ~

3.160 x

The risk components in the zones are added to produce the total risk. This produces a risk Rz = 2.644 x which is greater than the tolerable risk RT and therefore protection measures are necessary.

Zone 3

x

x

x

4.016 x

Zone 4

8.326 x

1.164 x 10-"7.98

1.164 x 10-"7.98

5 . 8 1 8 x 1 0 - ~

5 . 8 1 8 x 1 0 - ~

2.019 x l o 4

Total

4.163 x lo-' 1.082 x l o 4

x lo-" x lo-"

7 . 5 6 3 ~

7 . 5 6 3 ~

2.737 x


Application of protection measures

We now address the protection measures necessary to reduce R1 to the tolerable level, as any measures applied to achieve this may also reduce R2.

Applying a class IV LPS together with type IV equipotential bonding lightning current arrestor SPDs results in the risks within each of the zones and in total being reduced to the values shown in Table 2.19.

Table 2.19 - Calculation of R1 with class IV LPS applied

As can be seen in Table 2.19, the components deriving the indirect risk R, are now all below the tolerable levels. However, the direct risk RD in the form of component RB is still higher than &. If we apply a class I11 LPS, R1 is still greater than RT, SO in order reduce R1 we need to apply a class I1 LPS. Applying a class I1 LPS means that we need to install type I1 equipotential bonding lightning current arrestor SPDs. The results of these changes to the overall risk are shown in Table 2.20.

Risk component

R A ~

RE?

Ru(P)

Ru(T)

Rv(P)

Rv(T) TOTAL

Table 2.20 - Calculation of R1 with class II LPS applied

Zone 1

4.163 x lo-'

4.163 x lo-'

Applying a structural class I1 LPS gives a total risk of R1 = 8.725 x < RT = and, as such, protects against the loss of human life.

Risk component

R A ~

RE?

Ru(P)

Ru(T)

Rv(P)

Rv(T) TOTAL

Risk of loss of service to the public R2 now has to be recalculated on the basis of protection measures taken to reduce R1. The results are given in Table 2.21.

Zone 2

-

1.665 x 1 0 - h . 3 3 0

1.745 x

1.745 x

1.745 x

1.745 x

2.363 x 1 0 - v . 0 3 5

Zone 1

4.163 x lo-'

4.163 x lo-'

Zone 3

x 1 0 - 9 . 6 6 5

3.491 x lo-' 3.491 x lo-' 3.491 x

3.491 x

x 10-".021

Zone 2

-

4.163 x

1 . 1 6 4 ~

1 . 1 6 4 ~

1.164 x

1.164 x

8.817 x

Zone 4

x 1 o - ~ 3.491 x lo-' 3.491 x lo-' 1.745 x 1 0 - 9 . 2 6 9

1.745 x 1 0 - 9 . 2 6 9

x

Total

4.163 x 1 0-'

2.165 x 1 o - ~ 2.129 x

2.129 x

x lo-" x lo-"

2.662 x

Zone 3

8.326 x

2.327x10- '

2.327x10- '

2.327 x

2.327 x

1.303 x 10-"537

Zone 4

4.163 x 1 0 - b . 4 1 2

2.327x10- '

2.327x10- '

1.164 x 1 0 - v . 5 1 3

1.164 x 1 0 - v . 5 1 3

x 1 0 - h . 7 2 5

Total

4.163 x 1 0-'

x lo-" 1 . 4 2 0 x 1 0 - ~

1 . 4 2 0 x 1 0 - ~

x lo-" x lo-" x

Risk assessment

Table 2.21 - Calculation of R2 with class I I LPS applied

1 TOTAL I - 18.597 x 18.597 x l o 4 18.616 x l o 4 12.581 x 1 The recalculated value for R2 = 2.581 x > RT and therefore measures need to be investigated to further reduce the value of R2.

The three components still above a tolerable level are RM2, RZIP) and RZIT). These three components can be reduced directly by applying a system of type II coordinated SPDs. This will also reduce Rc.

Applying these SPDs reduces the values of probabilities PC, PM and PZ to 0.02. Applying these values to the risk calculations produces the values shown in Table 2.22.

Table 2.22 - Calculation of R2 with class I I LPS applied

1 TOTAL I - 13.141 x 13.141 x 13.340 x 19.622 x 1 Applying a system of coordinated type I1 SPDs gives a total risk of R2 = 9.622 x < RT =

lo-' and, as such, protects against the risk of loss of service to the public.

In this example, we have reduced the values of R1 and R2, to below their respective tolerable levels of and lo3, by installing systems of class I1 LPS and coordinated SPDs.

It will be clear from the examples given in Sections 2.6, 2.7, 2.8 and this section that there will always be more than one solution to reducing risks below the tolerable levels.

In order to derive an optimum solution the designer will always need to be provided with full details of the structure and the services feeding it.

Section 3: Protection measures

Protection measures may need to be applied to reduce the risk of damage. Different methods can be applied dependent upon the type of risk identified in the risk assessment.

Measures to reduce injury of living beings due to touch and step voltages include:

adequate insulation of exposed conductive parts;

equipotentialization by means of a meshed earthing system;

physical restrictions and warning notices.

NOTE: Equipotentialization is not effective against touch voltages. Increasing the surface resistivity of the soil inside and outside the structure may reduce the life hazard.

Measures to reduce physical damage include:

lightning protection systems for structures;

shielding wire for services.

Measures to reduce failure of electrical and electronic systems include:

LEMP protection measures systems (LPMS), consisting of the following (these may be used alone or in combination for structures):

0 earthing and bonding measures;

0 magnetic shielding;

0 line routing;

0 coordinated surge protective device (SPD) protection;

SPDs at different locations along the length of the line and at the line termination;

magnetic shields for service cables.

Section 4: Basic criteria for protection of structures

It is often impractical to go to the lengths necessary or impossible to justify the cost of providing absolute protection against lightning and its effects.

Protection measures should be applied to the levels identified by the risk assessment in order to reduce damage and consequential losses.

The application of protection is related to lightning protection zones (LPZs), as shown in Figure 4.1.



LPZs are determined by applied protection measures such as LPSs, shielding wires, magnetic shields and SPDs.

The following LPZs are defined (see Figures 4.1 and 4.2): [Source: BS EN 62305-1, 8.21

LPZ OA - zone where the threat is due to the direct lightning flash and the full lightning electromagnetic field. The internal systems may be subjected to full or partial lightning surge current;

LPZ OB - zone protected against direct lightning flashes but where the threat is the full lightning electromagnetic field. The internal systems may be subjected to partial lightning surge currents;

LPZ 1 - zone where the surge current is limited by current sharing and by SPDs at the boundary. Spatial shielding may attenuate the lightning electromagnetic field;

LPZ 2, ..., ?L - zone where the surge current may be further limited by current sharing and by additional SPDs at the boundary. Additional spatial shielding may be used to further attenuate the lightning electromagnetic field.


SI FLASH TO STRUCTURE

51 FLASH TO THE SERVICE

LEGEND:

r ROLLING SPHERE RADIUS (DEPENDENT ON PROTECTION LEVEL)

S SEPARATION DISTANCE AGAINST DANGEROUS SPARKING

f' AIR TERMINATION AND DOWN CONDUCTOR SYSTEM

- EARTH TERMINATION SYSTEM

INCOMING SYSTEM

SPD LIGHTNING EQUIPOTENTIAL BONDING TYPE I

LPZOn DIRECT FLASH, FULL LIGHTNING CURRENT

LPZ1 NO DIRECT FLASH, LIMITED LIGHTNING OR INDUCED CURRENT

Figure 4.1 - LPZ defined by an LPS

The LPS designer should ensure that the structure to be protected falls inside an LPZ OB or higher. This will be achieved by means of fitting a lightning protection system (LPS).

Basic criteria for ~rotection of structures

SI FLASH TO STRUCTURE (LPS)

LPZ o*

5, FLASH TO THE SERVICE

Protected Volumes LPZ 1 and LPZ 2 must respect

t

LEGEND:

r ROLLING SPHERE RADIUS (DEPENDENT ON PROTECTION LEVEL)

dr SEPARATION DISTANCE AGAINST TOO HIGH MAGNETIC FIELD

- STRUCTURE (SHIELD OF LPZ 1)

r- I

AIR TERMINATION AND DOWN CONDUCTOR SYSTEM

- EARTH TERMINATION SYSTEM

SERVICES CONNECTED TO THE STRUCTURE

SPD LIGHTNING EQUIPOTENTIAL BONDING

LPZ O* DIRECT FLASH, FULL LIGHTNING CURRENT, FULL MAGNETIC FlELD

LPZ 0 8 NO DIRECT FLASH, PARTIAL LIGHTNING OR INDUCED CURRENT, FULL MAGNETIC FlELD

LPZ 1 NO DIRECT FIASH, LIMITED LIGHTNING OR INDUCED CURRENT, DAMPED MAGNETIC FlELD

LPZ 2 NO DIRECT FLASH, LIMITED LIGHTNING OR INDUCED CURRENT, FURTHER DAMPED MAGNETIC FlELD

Figure 4.2 - LPZ defined by protection measures against LEMP

Section 5: Design of structural protection

An LPS is considered to be the most effective measure for protection of structures against physical damage. It usually consists of external and internal elements.

The external element of an LPS is intended to intercept a direct lightning flash to the structure, conduct the lightning current safely towards earth and then disperse it into the earth. The internal elements are intended to prevent dangerous sparking within the structure using either equipotential bonding or a separation distance between the external LPS components and other electrically conducting services or objects internal to the structure.

In addition to protecting the structure, protection measures should be adopted to protect against injury to living beings due to touch and step voltages. Such protection measures are intended to reduce the dangerous current flowing through bodies by insulating exposed conductive parts, increasing the surface soil resistivity or by placing physical restrictions andlor warning notices as appropriate to the individual circumstances.

Early coordination between all contracting parties is important to ensure that the characteristics of the structure to be protected are made available to the LPS designer. This enables proper consideration of possible utilization of metallic parts of the structure as parts of the LPS. The designs of LPS on existing structures shall take into account the physical constraints present, and this will be taken into account when determining the level and positioning of the LPS.

5.1 General considerations

In the majority of cases, it will be appropriate to attach the external LPS directly to the structure to be protected. If the thermal and explosive effects at the point of strike or on the conductors carrying the lightning current may cause damage to the structure or to the contents, an isolated external LPS should be considered. Examples of these minority cases

Design of structural protection

include structures with combustible covering and those structures and environments at risk of explosion and fire. In situations where readily combustible materials are present, such as thatched roofs, a distance of 0.15 m should be maintained between the LPS and the thatch. For other combustible surfaces a distance not smaller than 0.10 m is adequate.

Natural components within and on the structure made of conductive materials complying with material standards covered in parts 5.2, 5.3.1.9 and 5.3.2.4 together with Tables 5.3, 5.5,9.2 and 10.1 within this guide may be used as part of the LPS as long as they will always remain an integral part of the installation.

The installer of the LPS should install all lightning protection conductors and clamps unless the direct client of the LPS contractor instructs otherwise. [BS EN 62305-3, E. 4.3.101

When a structure comprises reinforced sections with expansion joints, and extensive electronic equipment is to be installed, bonding conductors should be provided across the expansion joints between the reinforcement of the structural sections to ensure low- impedance potential equalization and shielding of the internal space. The distance between the bonding conductors should not exceed half the distance between the down- conductors.

[BS EN 62305-3, E. 4.3.121

5.2 Reinforced concrete structures

Steelwork and reinforcing bars within reinforced concrete structures should be considered for use as a natural component of the LPS. These are electrically continuous provided that the major part of interconnections of vertical and horizontal bars are welded or otherwise securely connected. Connections of vertical bars may be welded, clamped or overlapped a minimum of 20 times their diameters and bound or otherwise securely connected. For new structures it will usually be practically and logistically more efficient to use proprietary clamps in accordance with BS EN 50164.

[BS EN 62305-3, E. 4.3.1 to E. 4.3.121

For this type of structure, the electrical continuity of the reinforcing bars should be determined by electrical testing between the uppermost part and ground level and the overall electrical resistance from the highest part to ground level should not be greater than 0.2 ohm. [BS EN 62305-3, Cla.zcse 4.31 In practice, due to the multiplicity of paths offered due to many parts of the reinforced frame being connected together, any continuity measurement will not represent only the resistance of the reinforcing acting as the down- conductor. This fact is accepted by the standard. [BS EN 62305-3, E.4.3. I ] If the requisite resistance is not obtained, external down-conductors should be installed.

Reinforced concrete structures may be used as natural down-conductor and earthing components if the characteristics comply with the requirements for these items covered later in this guide.

In reinforced concrete structures, the conductive reinforcement should form the shell for potential equalization of the internal LPS; it may also serve as an electromagnetic shield,


which assists in protecting electrical and electronic equipment from interference caused by lightning electromagnetic fields.

Locations of cast-in bonding points should be determined early on in the construction phase and coordinated with the electrical and building contractors to ensure that sufficient locations are provided to satisfy lightning protection equipotential bonding.

[BS EN 62305-3, E.4.3.21

If using reinforcing as natural components of the LPS, the continuity should be ensured by clamping or welding joints or tee-offs. Where clamping is used, specially designed clamps should be used for this purpose and where welding is used, the seam should be a minimum of 30 mm long.

[BS EN 62305-3, E.4.3.31

Connections from concrete reinforcing to components outside the concrete may be established variously by a reinforcement rod brought out through the concrete or by a connecting rod or cast-in earth plate passing through the concrete, which is welded or clamped, to the reinforcing rods. Where clamps are used to joint reinforcing bars or to connect reinforcing to components outside the concrete then two clamps should be used. For connecting a bonding conductor to the reinforcing, one conductor connecting to two different reinforcing bars should be used, for safety purposes. All clamps should be treated

Reinforcing bars tied together

Concrete face

Cast in earth plate

Clamps finally wrapped 1 sealed in moisture inhibiting tape 1

compound /

Figure 5.1 - Connections to reinforcing


with corrosion-inhibiting compound and finally sealed with moisture-inhibiting tape or compound.

[BS EN 62305-3, E.4.3.31

The reinforcing within a concrete roof structure may be used as the air-termination network (if all the requirements of BS EN 62305-3 are otherwise complied with) when temporary mechanical damage of the waterproof layer on the roof of the structure is acceptable. [BS EN 62305-3, E. 5.2.4.2.21

Generally a lightning flash to the reinforcement of a concrete roof will damage the waterproof layer; rainwater may then cause corrosion of the steel-reinforcing rods leading to damage.

Unless it is acceptable for temporary damage to the facade to occur, and shattered parts of broken concrete to fall down from the structure, the ring conductor around the perimeter of the roof should consist of a surface-mounted conductor externally sited on the surface of the concrete. [BS EN 62305-3, E.5.2.4.2.21

5.3 External LPS

5.3.1 Air termination

5.3.1 .I Positioning

Air termination systems can be composed of any combination of the following elements:

rods (including free-standing masts);

catenary wires;

meshed conductors.

[BS EN 62305-3, 5.2.11

Air-termination conductors should be located at comers, exposed points and as close as practically possible to edges, especially on the upper level of any facades in order to ensure that the structure and any metallic installations are fully located within the protected volume provided by the LPS, and that the conductors as far as reasonably possible follow the shortest, most direct route. The positioning of air-termination conductors should be determined by one or more of the following methods:

the protection angle method, which is suitable for simply shaped buildings up to certain limits. See Figure 5.2 to determine angles;

the rolling sphere method, which is suitable in all cases. See Table 5.1;

the mesh method, which is suitable where plane surfaces are to be protected. See Table 5.1.

[BS EN 62305-3, 5.2.2 a,?& A ? L ? L ~ ~ A]


80

40

30 CLASS OF LPS IV

20

HEIGHT H m (HEIGHT ABOVE REFERENCE PLANE

THIS METHOD NOT APPLICABLE BEYOND )

Figure 5.2 - Protective angles for various protection levels

Table 5.1 - Dimensions of various protection methods

[BS EN 62305-3, Table 21

Class of LPS

I

I I

Ill

IV

On nonconducting roofs, the air-termination conductor may be placed either under, or preferably over, the roof tiles. Mounting conductors under the tiles has the advantage of simplicity and reduces risk of corrosion, but it is better, where adequate k i n g methods are available, to install it externally. This has the benefit of reducing the risk of damage to the tiles in the event that the conductor receives a direct flash. Installing the conductor above the tiles also simplifies inspection. Conductors placed below the tiles should be provided with short vertical finials or flat strike plates, which protrude above the roof, level throughout the ridge, hips and eaves. These should be spaced at not more than 10 m and 5 m intervals for finials and plates respectively. Finials can be typically 8 mm diameter and protrude approximately 300 mm through the roof surface, and strike plates may typically comprise a flat plate approximately 50 mm square laid flat to the roof surface.

[BS EN 62305-3, E.5.2.4.21

Protection method

5.3.1.2 Protective angle method

Rolling sphere radius, r rn

20

30

45

60

The protective angle method is suitable for simple structures or for small sections of larger structures.

Mesh size, w rn

5 x 5

10 x 10

15 x 15

20 x 20

Protective angle, aO

See Figure 5.2


[BS EN 62305-3, E. 5.2.21

This method of determining volumes of protection is not suitable in situations where the part of the structure to be protected is higher above the reference plane than the radius of the rolling sphere that is applicable to the selected protection level shown in Table 5.1.

Applying the protective angles derived from Table 5.1 provides volumes of protection as shown in Figures 5.3 and 5.4.

5.3.1.3 Mesh method

The mesh method is for general-purpose use and is particularly suitable for the application of an LPS to horizontal or inclined plane surfaces.

[BS EN 62305-3, E. 5.2.21

The mesh method is considered to protect the whole surface if conductors are positioned on roof edge lines, overhangs and on roof ridge lines where the slope of the roof exceeds 1:lO.

On structures up to 60 m in height, only consider fitting an air termination system to the roof and provide protection to points, comers and edges of the structure. No lateral air termination is required regardless of the class of LPS.

On structures between 60 m and 120 m a lateral air termination system should be applied to the top 20% of the structure (typically 24 m) relevant to its class of LPS.

On structures over 120 m high, a lateral air termination should be incorporated into the structure from 120 m upwards relative to its class of LPS. This lateral air termination system will almost certainly be present due to proximity of reinforcing, curtain walling, mullions etc. The presence of these natural air terminations at the surface of the faqade should be ensured such that they fulfil the requirements in terms of exposed strike plates and relevant spacings etc.

[BS EN 62305-3, A.3 a,?& E.5.2.2.31

Table 5.2 - Separation of horizontal ring conductors

The mesh of the air-termination network should have dimensions no greater than those shown in Table 5.1.

Class of LPS

I

I I

Ill

IV

The network should be designed in a way to ensure that any lightning current always has at least two routes to the earth-termination.

Typical ring conductor separation distance rn

10

10

15

20


H = Height of building h, = Height of air terminal 12m B = Reference plane for protection angle determination on roof B1 = Reference plane for protection angle determination on ground a , = 76' determined from "Protective Angle Figure 8" a z 53' determined from "Protective Angle Figure 8" 0 - C = Radius of protected area

Figure 5.3 - Determination of volume protected by a vertical air terminal

5.3.1.4 Rolling sphere method

The rolling sphere method is particularly suitable for identifying protected space and locating conductor positions on structures with complex geometry. It should be used


Figure 5.4 - Determination of volume protected by a catenary air termination system

where the parameters of Figure 5.2 preclude the use of the protective angle method and for the design of the air-termination to protect conductive parts on the roof.

[BS EN 62305-3, E. 5.2.2 a,?~d E. 5.2.4.2.71

The position of the air termination using this method is adequate if no point of the structure to be protected can be touched by a sphere of the radius identified in Table 5.1 rolling in all directions over it. The sphere should only touch the ground and the fitted or proposed air-termination network.

[BS EN 62305-3, A.21

In circumstances where two horizontal LPS air-termination conductors are placed parallel above the horizontal reference plane, as shown in Figure 5.7, the distance that the rolling sphere penetrates below the level of the conductors within the space between the horizontal conductors is:

p = r - [?" - (d12)" l'" [BS EN 62305-3, E.41

A?L a~te?-tm.tive fo?-rnula. that ma,y be easier to work for the practitio?wr a,?~d which gives a,?L identical outcome i s p = r - J?' - (d/2j2

where

p = penetration distance

r = rolling sphere radius

d = distance between the two parallel air terminal rods or conductors.


I I r = radius of volume of protection

I I =joint

1 . --- --- I I I

Figure 5.5 - Positioning of mesh air termination system on flat roof

The penetration distance p should be less than the height of the air terminaVconductor above the roof surface/reference plane, &, minus the height of objects to be protected.

5.3.1.5 Protection for car park roofs

The roof area of this type of structure may be protected by either vertical finials, which may be in the form of metallic structural items (for example lamp posts), horizontal or vertical conductors, air-termination studs, other suitable conductors or extraneous metallic items or a mixture of these. Studs may be connected to the reinforcement steel of a concrete roof. In circumstances where a connection to the reinforcement cannot be made, roof conductors can be laid in the seams of the carriageway slabs and the studs can be located at the mesh joints. The mesh width should not exceed the dimensions corresponding to the relevant protection class given in Table 5.1. In this case, the persons and vehicles on this parking area are not protected against lightning. [BS EN 62305-3, E.5.2.4.2.11

If the uppermost level is to be protected against direct lightning strikes, vertical or horizontal finials and conductors andlor overhead wires should be located to afford zones of protection to the areas to be protected. [BS EN 62305-3, E.5.2.4.2.11

Signs should be provided at the entrances to or on the top levels of car parks drawing attention to the danger of lightning strikes during thunderstorms. [BS EN 62305-3, E.5.2.4.2.11

The touch and step voltages developed on the top level of the car park during a strike may be disregarded if the roof or top level of the car park is covered by a layer of asphalt of at least 5 cm thickness. Additionally, the step voltages may be disregarded if the roof is


Note: For plane surfaces, in this example the upper Et lower roof surfaces, see the mesh method for positioning of air termination network within the perimeter (if necessary).

Figure 5.6 - Positioning of air termination network using the rolling sphere method

constructed of reinforced concrete with interconnected reinforcement steel with continuity provided by welding or clamping as described in Section 5.2. [BS EN 62305-3, E.5.2.4.2.11

5.3.1.6 Protection of roof fixtures

Conductive roof fixtures not protected by air-termination rods can be disregarded if the following three characteristics are satisfied:

their height above the roof level < 0.3 m;

the total area of the fixture < 1.0 m2;

the length of the fixture < 2.0 m.

Non-conductive roof fixtures not within the protected volume of air-termination rods that protrude less than 0.5 m above the surface on which the air-termination system is sited do not require additional protection. [BS EN 62305-3, E.5.2.4.2.41

Where a conducting protruding fixture is present, such as electrical conductors, metallic pipes, etc. the protruding fixtures on the roof surface should be protected by an air- termination system. If protection by means of an air termination system is not possible or not cost-effective, insulated parts, with lengths corresponding to at least twice the


Radius of rolling \ / sphere 'r'

- - - - - - - - - distance Penetration 'p' f

- - - -

Height 'ht' of air Horizontal conductors or tip of termination I

air terminals horizontal conductor

Height of 4, , ltem of plan/ 1 1 1 .bow reference object to be plane omtected

Distance between ~eference plane I conductors or air roof surface terminals 'd'

Figure 5.7 - Penetration of rolling sphere between two air terminals or horizontal conductors

specified separation distance, can be installed in the conductive installations. [BS EN 62305-3, E. 5.2.4.2.41

When a nonconductive chimney falls outside She protective zone of the air-termination system, it should be protected by means of air-termination rods or air-termination conductors. The air termination rod on a chimney should be of such height that the complete chimney lies within She protective space of the rod. [BS EN 62305-3, E.5.2.4.2.41

Metal roof fixtures should be bonded to the air termination system when the necessary clearance for conformity to She separation distance cannot be maintained. [BS EN 62305- 3, E. 5.2.4.2.41

BS EN 62305-3 applies principles of protection that, although technically relevant, taking practical and economic considerations into account are very difficult to achieve.

The standard requires that metallic roof fixtures, for example air handling units, should, where space allows, be located within a zone of protection offered in accordance with the angle of protection (or with rolling sphere methods) and maintain a separation distance between the fixture and the protective air-termination equipment to prevent dangerous sparking. This separation distance can be disregarded where the structure is steel framed or reinforced and where all metallic fixtures are mechanically and electrically continuous with the structure.

In practice, taking into account economic, technical and practical considerations, the following solutions are considered to be an acceptable compromise between absolute protection and that achievable within reasonable architectural, economic and practical constraints.

[BS EN 62305-3, E.5.2.4.2.51


Protecting metallic roof fixtures where the fixture cannot withstand direct strike to its casing

Where the casing is not of sufficient cross-section to comply with the thickness requirements of the standard, a system of air rods or suspended conductors should be designed, as covered earlier in Section 5.3.1 of this guide, to ensure that the fixture falls within the zone of protection offered by the air-termination.

A separation distance should be calculated and maintained between the roof fixture and the air-termination to prevent dangerous sparking between the air-termination and roof fixture in the event of a strike. In the event that the separation distance cannot be achieved for whatever reason, the air-termination protection should still be fitted and the fixture should be bonded to the conductor connecting to the air-termination.

Where metallic services route from the fixture into the structure, they should be bonded to the nearest equipotential bonding bar.

Where electrical services route from the fixture into the structure, the armouring or screening should be bonded to the nearest equipotential bonding bar. In addition, where the service feeds into electronic equipment or in cases where the cable has no armouring or screening, the live cores should be bonded to the nearest equipotential bonding bar via Type I1 overvoltage SPDs. Where the separation distance has not been satisfied and a bond has been fitted between the fixture and the air-termination, the SPDs should be Type I equipotential bonding lightning current SPDs.

Protecting metallic roof fixtures where the fixture can withstand direct strike to its casing without risk of puncture

Many roof fixtures are of a substantial size both in terms of volume and thickness of the fixture casing and supports. In the event of a strike to the structure LPS, these fixtures would be affected by the electromagnetic effects of the strike whether they fall within a zone of protection or not. According to the standard, the optimum method of protecting these fixtures is to offer a zone of protection, but this is still likely to require additional SPDs or other measures. On the basis that the casing satisfies thickness requirements in order to prevent puncture, hotspot or ignition, it makes economic sense to consider using the casing of the fixture itself as part of the air-termination, although the electromagnetic effects of a strike are likely to be greater than in the case of using air-termination protection.

There are several issues to consider if adopting this method.

Firstly, the metallic services, ducting etc, feeding to the interior of the structure from the fixture should be bonded to the air-termination network where it leaves the roof and to the nearest equipotential bonding bar on entering the structure.

Secondly, all electrical lines routing from the fixture into the structure should have their armouring or screening directly connected to the nearest equipotential bonding bar and their live cores connected to the same bar via Type 1 equipotential bonding lightning current SPDs.


It is likely that the nearest equipotential bonding bars will be in or aaacent to the distribution boards or control equipment feeding the lines.

While it could be argued that this method may introduce a direct lightning current into the structure, this will be somewhat mitigated by the use of armoured (or similar mechanically protected) cables and screened telecommunications/data lines or lines sited in metallic trunking or conduit, which is customary and practice in the majority of commercial and industrial applications.

The alternative to this approach would be to ensure that all mechanical services are insulated where they enter the structure and all cables are split where they enter the structure and fitted with SPDs at that point. The economic and practical implications of this are obvious and are likely to be prohibitive in all but the most severe or sensitive of cases.

5.3.1.7 Electrical installation protruding from the space to be protected

Ideally any antenna mast located on the roof of a structure should be located within a zone of protection to protect against direct lightning strikes. In practice this will probably not be economically viable and therefore as a minimum the mast should be bonded to the lightning protection system in a minimum of two positions. [BS EN 62305-3, E.5.2.4.2.61

Preferably, all cables, other feeders, wave guides and the like originating on the mast should enter the structure at the common position for all services or near the main lightning protection bonding bar. In practice, the practical convenience of either of these situations being likely is remote and as a minimum, at the point where the services from the mast enter the structure at roof level, all conductive sheaths and conductive mechanical protection should be bonded to the lightning protection air-termination by means of provision of a common earth bar. [BS EN 62305-3, E.5.2.4.2.61

5.3.1.8 Protection of structures covered by soil

Structures with a layer of soil on the roof where people are not regularly present should be fitted with a meshed air-termination system sited on top of the soil. However, k i n g s may present practical problems if using this method. Practically, a permanent k e d mesh may be achieved by burying the conductors a short distance below the surface of the soil, but it should be recognized that the practical benefits of a shallow buried system will lead to a reduction of interception efficiency. Alternatively, air termination rods sited in accordance with the rolling sphere or protective angle method and connected by a buried mesh may be used but this method is less practical to achieve than simple buried conductors. [BS EN 62305-3, E. 5.2.4.2.81

Where people are present, a 5 m x 5 m mesh of bare copper conductor should be installed beneath the surface of the soil to protect against dangerous step potentials. [BS EN 62305- 3, E.5.2.4.2.81 From a practical and sensible approach, strategically placed warning notices should be provided advising people to leave the immediate locality in the event of an approaching thunder storm.


For underground structures containing explosive materials an isolated LPS in addition to a buried mesh should also be fitted over the structure and the earthing systems of both protection measures should be interconnected. [BS EN 62305-3, E.5.2.4.2.81

5.3.1.9 Natural components

The following parts of the structure should be considered for use as part of the LPS.

1. Metal sheets covering the structure or item to be protected, provided that:

the electrical continuity between the various parts is made durable by means of brazing, welding, crimping, seaming, screwing or bolting, for example;

the thickness of the metal sheet is as shown in Table 5.3 taking account of any requirement to prevent puncture and ignition of materials beneath;

they are not clad with insulating material.

2. Metal components of the roof construction underneath non-metallic roofing (for example PVC membrane roofs). However, this will result in damage in the event of a strike and it is recommended that external air-termination should be fitted to prevent this.

3. Metal parts such as ornamentation, railings, pipes, coverings of parapets, etc., with cross-sections not less than that specified for standard air-termination components.

4. Metal pipes and tanks on the roof, provided that they are constructed of material with thicknesses and cross-sections in accordance with Table 5.3.

5. Metal pipes and tanks carrying readily-combustible or explosive mixtures, provided that they are constructed of material with thickness not less than the appropriate value of t given in Table 5.3 and that the temperature rise of the inner surface at the point of strike does not constitute a danger (for detailed information, BS EN 62305-3, Annex E).

[BS EN 62305-3, 5.2.51

Table 5.3 should be consulted and complied with in the use of these components as part of the LPS.

Table 5.3 - Minimum thickness of natural components in air termination systems

[Source: BS EN 62305-3, Ta.ble 31

Class of LPS

I to IV

a t prevents puncture, hot spot or ignition. t'only for metal sheets if it is not important to prevent puncture, hot spot or ignition problems.

Material

Lead

Steel (stainless, galvanized)

Titanium

Copper

Aluminium

Zinc

Thickness a

t mm

4

4

5

7

Thickness t' mm

2.0

0.5

0.5

0.5

0.65

0.7


Metallic parapets intended to be utilized as part of the air-termination network should be electrically and mechanically continuous and comply with the minimum thicknesses of Table 5.3. Air terminations should be connected to any metallic roof parapet covering not being utilized as the air-termination every 20 m along its length and at each down- conductor position. [BS EN 62305-3, E. 5.2.51

Vessels andlor pipework containing gas or liquids under high pressure or flammable gas or liquids should not be used as natural air terminations. [BS EN 62305-3, E.5.2.51

Conductive metallic parts above the roof surface, such as tanks, are often connected to equipment within the structure. To prevent conduction of the full lightning current through the structure, it is necessary to provide a good connection between such natural components of the LPS and the air termination mesh. Where natural conductive parts pass through a metal or reinforced roof structure they should be bonded to it. [BS EN 62305-3, E. 5.2.51

5.3.1.10 Isolated LPS

An isolated external LPS should be applied when damage to the structure or its contents may occur as a result of the flow of lightning current into bonded internal conductive parts and also in cases where there is a need to reduce the electromagnetic field in the protected structure or where there are large numbers of protruding fixtures located on the roof. [BS EN 62305-3, E. 5.2.61

5.3.2.1 General

On a non-isolated LPS there should be a minimum of two down-conductors distributed around the perimeter of the structure to be protected. Equal spacings are preferred, although this requirement may be subject to architectural and practical considerations. Where possible, a down-conductor should be installed at each exposed comer of the structure [BS EN 62305-3, 5.3.31.

Down conductors could present dangerous situations created by touch and step potentials. These hazards are considered to be reduced to tolerable levels if:

1. the probability of people approaching them or the duration of their presence close to them is very low; or

2. the steel or reinforced frame of the structure is used as the down conductor system; or

3. the resistivity of the ground within 3 m of the structure is higher than 5 kRm.

In practice, it is likely that more frequented areas close to downconductors will meet these last two measures without further action being necessary [BS EN 62305-3, 8.1 a,?& 8.21. In the few cases where one of the preceding three measures is not inherently present then posted warning notices are considered to reduce the risk to tolerable levels. Alternatively, the exposed down-conductor should be routed through PVC tubing or capping with walls at least 3 mm thick or with other equivalent insulation [BS EN 62305-3, E.5.4.3.61.


If the air-termination consists of rods on one or more separate masts not made of metal or interconnected reinforcing steel, at least one down-conductor is needed for each mast. No additional down-conductors are required for masts made of metal or interconnected reinforcing steel. If the air-termination consists of one or more catenary wires at least one down-conductor is needed at each supporting structure.

If the air-termination forms a network of conductors, one down-conductor is needed at least at each supporting wire end.

[BS EN 62305-3, 5.3.21

All reasonable steps shall be taken to ensure that down-conductors are installed straight and vertical such that they provide the shortest and most direct path to earth. It is important that the formation of loops is avoided. Where this is not possible, the minimum distance measured across the gap between two points on the conductor and the length I of the conductor between those points should be calculated to ensure that the separation distance s is adequate. See Section 5.4.2 and Figure 5.8.

[BS EN 62305-3, E. 5.3.41

Figure 5.8 - Loops in a down conductor

Due to moisture leading to intense corrosion of the down-conductors, these shall not be routed in gutters or down pipes even if they are covered by insulating material.

[BS EN 62305-3, 5.3.41


Down-conductors of an LPS not isolated from the structure to be protected may be installed as follows:

the down-conductors may be positioned on the surface or in the wall if the wall is made of non-combustible material;

the down-conductors may be positioned on the surface of a wall if the wall is made of readily-combustible material, provided that their temperature rise due to the passage of lightning current is not dangerous for the material of the wall. BS EN 50164 offers guidance;

where down-conductors are sited along or on a wall which is made of readily- combustible material and the temperature rise of the down-conductors is dangerous, they will be fixed in such a way that the distance between them and the wall is always greater than 0.1 m. However, the mounting brackets may be in contact with the wall.

Where the distance from a down-conductor to a combustible material cannot be assured, the cross-section of the down-conductor shall be not less than 100 mm2.

[BS EN 62305-3, 5.3.41

5.3.2.2 Positioning

Down-conductors shall be arranged in such a way that from the point of strike to earth:

a) several parallel paths exist;

b) the length of the down-conductors are kept to a minimum;

c) equipotential bonding to conducting parts of the structure is performed according to BS EN 62305-3, 6.2, and Sections 5 and 6 of this guide. [BS EN 62305-3, 5.3.11

d) the distance between the down-conductor and the services satisfies the distance requirement s covered in Section 5.4.2 of this guide.

Typical values of the distance between down-conductors, subject to architectural and practical constraints, are given in Table 5.4.

Table 5.4 - Typical down-conductor spacings and distance between ring conductors

[BS EN 62305-3, Table 41

Class of LPS

I

I I

Ill

IV

Note that where extensive bonding to the reinforcement is required at different floors for potential equalization and shielding of the inner space of the structure, ring conductors should be installed and interconnected by means of vertical rods at intervals not greater than 10 m. See Sections 5, 6 and 10 of this guide for further detail. [BS EN 62305-3, E.4.3.81 In practice, the requirements for this are likely to be met by virtue of the methods used to construct the reinforced or steel structure. In circumstances where this extensive

Typical distance rn

10

10

15

20


bonding is required, confirmation that vertical and horizontal reinforced or steel sections are connected together and fulfill the requirements of Table 5.4 [BS EN 62305-3, Ta,ble 41 should be sought from the Civils Contractor.

If the separation distance required to avoid dangerous sparking between the down- conductor and the internal services cannot be satisfied, the number of down-conductors should be increased until the required separation distance is met. [BS EN 62305-3, E.5.3.4.11

To take account of architectural and practical considerations, the spacing of down- conductors may be aausted by +20 %. However, the average spacing of all down- conductors must conform to the typical distances for the particular class of LPS. [BS EN 62305-3, E. 5.3.11

Where closed courtyards have a perimeter of more than 30 m, downconductors have to be installed. The spacing of down-conductors should conform to the maximum typical distances for She particular class of LPS. [BS EN 62305-3, E.5.3.11

In large non-conducting, flat structures with dimensions over four times the down- conductor spacing, additional internal down-conductors should be provided, wherever possible, to provide a network of internal down-conductors approximately every 40 m. [BS EN 62305-3, E. 5.3.4.21

In circumstances where it is not practically possible to place down-conductors at a side or part side of the building or where architectural considerations prevent such siting, the down-conductors that should be on that side should be placed as additional compensating down-conductors at the other sides. In this case, the distance between the installed down- conductors should not be less than one-third of the required down-conductor distances dependent upon the class of LPS. [BS EN 62305-3, 5.3.11

5.3.2.3 Cantilevered structure

In situations where cantilevers exist, there is a danger that under certain conditions a flash over may occur from the downconductor to a person standing under the cantilever. To reduce this danger the separation distance, d, in metres, should satisfy She following condition:

d > 2.5 + s

where

s is She separation distance in metres calculated in accordance with Section 5.4.2.

The value 2.5 is representative of the height at the tips of a person's fingers when that person's hand stretches vertically. See Figure 5.9.

[BS EN 62305-3, E.4.2.41

If the separation distance s cannot be satisfied, alternative solutions should be sought. These can include routing the down-conductor through the structure or installing additional down-conductors along the cantilever section in order to reduce the factor kc (see Section 5.4.2 for explanation of kc) and thus s.


Figure 5.9 - Separation distance beneath cantilevers

5.3.2.4 Natural components

The LPS designer should consider the following parts of the structure for use as natural down-conductors:

the metal installations, provided that the electrical continuity between the various parts is made secure by means such as clamping, bolting, brazinglwelding, screwing, crimping or seeming, and their dimensions are at least equal to that specified in Table 5.3;

the metal of the electrically continuous reinforced concrete framework of the structure;

the interconnected steel framework of the structure;

the facade elements, profile rails and metallic sub-constructions of facades, provided that their dimensions conform to the requirements for down-conductors and metal sheets or metal pipes i.e. not less than 0.5 mm thick. Connections along the conductor should be kept to a minimum and where they axe necessary should be made using the methods described in the first bullet point of this Section 5.3.2.4.

Notes

Piping carrying readily combustible or explosive mixtures should not be considered as a natural component down-conductor if the gasket in the flange couplings is not metallic or if the flange-sides are not otherwise properly bonded. Metal installations may be clad with insulating material.


When using reinforcement steel as the down-conductor, care should be taken to ensure that direct electrical continuity is maintained throughout the length of the reinforcing. This can be ensured by using a reinforcement bar sited in the same position throughout the length and carrying out continuity tests at appropriate intervals throughout the erection of the column. When the continuity of a natural down-conductor cannot be guaranteed, additional conductors should be used and these may be securely fixed to the reinforcement steel or installed externally once the column is complete. In either case they should be equipotentially bonded to the reinforcing at the top and bottom at least. [BS EN 62305-3, E. 4.3.71

It is important to establish interconnection points between the reinforcing elements within prefabricated reinforced concrete. In the case of pre-stressed concrete, attention should be paid to the risk of causing unacceptable mechanical damage due either to lightning current or as a result of the connection to the lightning protection system. Ring conductors are not necessary if the metal framework of steel structures or the interconnected reinforcing steel of the structure is used as down-conductors. [BS EN 62305-3, 5.3.51

Where reinforced columns are used as the downconductors they should be interconnected by means of their steel reinforcing rods. If no natural interconnecting conductor is present, the columns should be connected together at their highest and lowest points using external conductors. [BS EN 62305-3, E.4.3.71

Metal reinforcing rods of prefabricated concrete elements and the reinforcing rods of in- situ concrete columns or walls should be connected to the reinforcing rods of floors and roofs before the floors and roofs are cast.

Where reinforced concrete or columns of steel structures are used as down-conductors, every steel column (including those sited internally) should be connected to the air- termination at high level and to the steel reinforcing rods of the concrete foundation by means of cast-in bonding points or other suitable conductors. This will create internal down-conductors, and measures should be taken to guard against corrosion. [BS EN 62305-3, E.4.3.71 It is not made clear in the standard why all columns need to be connected to the air-termination and earth networks, it is thought likely that this requirement is for equipotentialization purposes and not lightning current carrying reasons. Using this assumption as the basis of our interpretation, in practice, on reinforced concrete structures, the frame is likely to be continuous throughout and as such additional connections of internal columns and those located between columns forming down- conductor paths is not necessary. However the LPS contractor should liase with their contractual principal to ensure that the concrete contractor maintains continuity throughout all columns and the reinforcing at low level. On steel framed structures, continuity is likely to be ensured throughout the frame, however the LPS contractor should ensure that the columns are made continuous with the reinforcing at ground level; this may exist by virtue of the method used to fix the steel column to the ground slab/beam/pile. The LPS contractor should liase with his contractual principal to ensure the steelwork contractor satisfies this requirement without the need for additional connections to be provided.

Where down-conductors do not utilize natural components, all internal columns and internal partition walls with conductive parts which do not satisfy the separation distance conditions should be connected to the air-termination system at high level and to the earth- termination system at low level. [BS EN 62305-3, E.5.3.4.21


Steel framed structures generally use steel roof members connected by means of bolted joints. Provided the nuts and bolts are tightened with the force required to achieve the structural mechanical strength, all bolted steel parts may be considered electrically interconnected. [BS EN 62305-3, E. 4.3.71

Where the metal facade is to be used as natural down-conductors, each overlapping vertical joint at each down-conductor position should be bridged by flexible metal strapping. Where the metal faqade is used only for shielding purposes (protection against LEMP), bridging by means of self-tapping screws may be used. [BS EN 62305-3, E.5.2.5 a,?~d Figure E.351 Connections between sheet metal panels should be compatible with the panel material, represent a minimum contact surface area of 50 mm%nd be capable of withstanding the electrodynamic forces of a lightning discharge and the corrosion threats of the environment. [BS EN 62305-3, E.5.6.11

In practice, it may be appropriate to utilize the vertical rails to which the cladding is k e d as the natural down-conductor. In this case, the appropriate sizing and electrical continuity should be ensured from top to bottom. In cases where access to the rear of the faqade is not available, these will need to be addressed individually based on the particular circumstances. In practice, if access is not available to the rear of the faqade, the only type of k i n g available for connections between faqade sheets and where the air-termination or down-conductor tapes attach to it are pop rivets. For these riveted joints, at least four 5 mm diameter rivets should be used and a length of conductor at least 20 mm should be in connection with the faqade being bonded to.

5.3.3 Test joints

Other than in situations where natural downconductors are combined with foundation earth electrodes, a test joint should be fitted on each down-conductor to enable access for measurement of electrode resistance and maintenance. The test joint shall be such that it can only be opened with a tool and shall lie in the closed position during normal use.

[BS EN 62305-3, 5.3.61

5.3.4 Lightning protection earthing

In general, an earthing resistance lower than 10 ohms overall, when measured at low frequency, is recommended.

It is good engineering practice to ensure that as far as is reasonably practicable, each of the individual electrodes on the system is of similar resistance. This ensures that in the event of a strike, the current flowing within the system is reasonably equally distributed throughout the earths and so relative large potential differences do not appear on or between parts of the system. To ensure even distribution of the lightning current, it is recommended that any individual earth electrode has a resistance of no more than 10 times the number of electrodes within the system.

BS EN 62305-3, E.4.2.3.1 states that soil resistivity tests should be performed preferably prior to finalizing the design of an LPS and should take into consideration the seasonal variations of soil resistivity. Soil resistivity data should be provided to the lightning protection contractor at tender stage so that there is a consistent value available on which


to base the risk assessment that will ultimately determine the class of protection. If soil resistivity data is not available at tender stage, a value of 500 Rm should be assumed and confirmed in data provided with the initial design specification. On the basis that the resistance is obtained on site once electrodes have been installed, it will be possible to calculate the soil resistivity from the resistance readings without the need to visit the site further.

A single integrated structure earth termination system suitable for all purposes including lightning protection, power systems and telecommunication systems is preferable.

[BS EN 62305-3, 5.4.11

The embedded depth of earth electrodes shall be such so as to minimize the effects of soil drying, freezing and corrosion and thereby stabilize the earth resistance to the effects of seasonal variations.

[BS EN 62305-3, 5.4.31

Structure foundations of interconnected steel-reinforced concrete can be used as foundation earth electrodes where practically and economically possible. When the foundations are not used, an earth termination system consisting of one or a mixture of the ones described later in this section may be used.

When excavations for earth electrodes are back filled, no fly ash, lumps of coal or building rubble should be in direct contact with the earth electrode. [BS EN 62305-3, E.5.4.3.21

Lightning protection earthing equipment may typically consist of any configuration of 16 mm (nominal 14.2 mm shank diameter) copper-bonded or 15 mm diameter copper or stainless steel electrodes, lattice mats of 25 x 3 mm copper conductor, copper plates or 25 x 3 mm copper conductor or other materials for earthing components that comply with the requirements of the BS EN 50164 series of standards. [BS EN 62305-3, E.5.4.3.21

The three types of earthing system configuration recommended for use with lightning protection systems are detailed as follows.

5.3.4.1 Type A arrangement

A type A earth termination arrangement is suitable for low structures (below 20 m), existing structures or an LPS with rods or stretched wires or for an isolated LPS. [BS EN 62305-3, E.5.4.2.11 If the requirements of BS EN 62305-4 are to be considered, a type A earth may be used in structures where only electrical systems and non-extensive electronic systems are provided. However, in structures with extensive electronic systems a type B arrangement is recommended. [BS EN 62305-3, 5.4.31

A type A arrangement comprises vertical or horizontal earth electrodes, or for practical reasons a combination of both, connected to each downconductor and usually installed outside the structure to be protected. The total number of earth electrodes shall be not less than two, regardless of the perimeter of the structure.

[BS EN 62305-3, 5.4.2.11


Class I

Class II

Class Ill-IV

Figure 5.10 - Minimum length I, of each earth electrode according to class of LPS

The minimum length of each earth electrode at the base of each down-conductor can be established from Figure 5.10. It is ll for horizontal electrodes, or 0.511 for vertical (or inclined) electrodes. [BS EN 62305-3, 5.4.2.11 If using lattice mats as horizontal electrodes, the total length of conductor within the mat should be greater than the value of l l . If using solid plates as an electrode, the surface area of the plate should be at least equal to the surface area of the length of conductor that would need to be used to satisfy the 0.511 or l l requirement.

For combined vertical and horizontal electrodes, the individual electrode lengths should follow the 0.511 and l l principle respectively.

Type A earth electrodes should be installed such that the depth of the upper end is at least 0.5 m below the ground surface and distributed as uniformly as possible to minimize electrical coupling effects in the earth. This requirement is to reduce ground potentials at the surface. It is customary and good practice to mount electrodes with an inspection pit. For ease of use and safety reasons, it is common practice to use plastic pits. As this material insulates the top of the rod electrode from the ground, fitting a plastic inspection pit over the rod electrode negates the need for the rod head to be 0.5 m below the surface of the ground.

The embedded depth of earth electrodes should also minimize the effects of soil drying, freezing and corrosion and thereby stabilize the earth resistance to the effects of seasonal variations.

BS EN 62305-3, 5.4.2.1 states, 'The minimum lengths stated in Figure 2 may be disregarded provided that an earthing resistance of the earth termination system less than 10 ohms (measured at a frequency different from the power frequency and its multiple in order to avoid interference) is achieved'. Due to the physical make-up of ground, it is good practice to install vertical electrodes with a depth of at least 2.4 m in any event.


BS EN 62305-3, 5.4.3 states, 'It is recommended that the upper part of a vertical earth electrode equal to the depth of freezing soil should not be regarded as being effective under frost conditions.

NOTE Hence, for every vertical electrode, 0.5 m should be added to the value of the length I, calculated in 5.4.2.1 and 5.4.2.2'.

In practice, the UK does not suffer from the extremes of cold to the point where permafrost is an issue for earthing systems. In practice, the most common form of type A electrode is the 1.2 m long extendable vertical rod. It is not considered necessary to discount the top 0.5 m of electrode, as it is good practice to mount the electrode with an inspection pit (thus reducing the implications of risk component RA) which installs to depths of approximately 300 mm, thus taking account of any detrimental impact of ground freezing. Experience shows that the 2.4 m of driven electrode is likely to achieve the resistance necessary, on all but the smallest installation or highest resistivity ground, to satisfy the overall 10 ohms resistance and to ensure the integrity of the mechanical and electrical properties of the electrode are maintained. Regular testing at intervals of 11 months will demonstrate the effects of ground freezing and drying out or flooding throughout the seasons of the year.

5.3.4.2 Type B arrangement

Where structures are either located on bare solid rock, incorporate extensive electronic systems or present a high risk of fire, the type B arrangement is recommended and preferable. [BS EN 62305-3, 5.4.2.2 a,?~d 5.4.31

A type B arrangement comprises either a ring conductor external to the structure to be protected, which should be in contact with the soil for at least 80 % of its total length, or a foundation earth electrode; these earth electrodes may be meshed.

[BS EN 62305-3, 5.4.2.21

If the requirements of BS EN 62305-4 are being considered, the type B earth around the structure, or if sited in the concrete, at the perimeter of the foundation, should be integrated with a meshed network with a width of typically 5 m under and around the structure for shielding purposes. The reinforced concrete floor slab can be used for this purpose if the mesh is clearly defined. [BS EN 62305-4, 5.11

Where the resistance of a type B arrangement needs to be reduced, vertical or radial electrodes may be incorporated. [BS EN 62305-3, E. 5.4.3.41

Where a type B earth system is provided, it is good engineering practice to arrange it such that an inspection pit with integral earth bar will be installed at the junction where the legs of the ring and conductor routing onto the ring from the each test clamp join. This type of configuration facilitates future access and enables more accurate testing.

For a type B arrangement, the mean radius of the area enclosed by the ring or foundation electrode shall be not less than II. [BS EN 62305-3, 5.4.2.21 In practice, where the soil resistivity is 500 Rm or less and the dimensions of the structure are greater than 8.86 m x 8.86 m (approximately 78 m", the effective radius will always comply. Where the structure requires a class I LPS and the soil resistivity is higher than 500 Rm, or where the structure


Plastic inspection pit

Type 'B' clamp,

25x3 HDHC copper

Leg of type 'B' ring conductor 25 x 3 n

Spare way 2 / I I I Leg of type 'B' ring conductor 25 x 3 mm bare copper

Additional vertical 16mm /' copperbond electrode installed if needed to reduce resistance

'ii, Connection to down-

conductor, 25 x 3 mm PVC covered copper

Figure 5.11 - Example of ring conductor to allow future inspection


is to be fitted with a level II LPS and the soil resistivity is greater than 800 Rm, individual calculations will need to be made to determine the effective radius specific to the structure to be protected, as in these situations the minimum length of electrode increases with the soil resistivity.

Should calculations need to be carried out, to aid understanding, below are examples of effective radius for a structure 10 m wide x 30 m long, at higher levels of soil resistivity and for a level I LPS.

Example 1 - Soil resistivity = 600 Rm

According to Figure 5.10, the minimum length Il = 8 m

Ground floor plan area of 10 x 30, A = 300 m"

The effective radius of the structure re = J(AI(r) = J(300/(r) = 9.772 m

The requirement that re is greater than Il is therefore satisfied and, other than the installation of the ringlfoundation electrode, no further measures relative to the earth are necessary.

Example 2 - Soil resistivity = 900 Rm

According to Figure 5.10, the minimum length Il = 17 m

Ground floor plan area of 10 x 30, A = 300 m"

The effective radius of the structure re = J(AI(r) = J(300/(r) = 9.772 m

The requirement that re is greater than Il is therefore not satisfied and additional earthing is required.

In this case, additional vertical, horizontal or inclined electrodes should be added in order to satisfy the requirement for re to be greater than or equal to Il.

Additional individual lengths of horizontal (I,) andlor vertical (C) electrodes should be added to the ringlfoundation earth in order to satisfy the requirement for re to be greater than or equal to Il. These additional electrode lengths are determined as follows.

For additional horizontal electrodes:

I, = Il - re = 17 - 9.772 = 7.228 m of additional horizontal electrode required to be connected to the ringlfoundation at each down-conductor position. [BS EN 62305-3, 5.4.2.21

For additional vertical electrodes:

I, = (Il - re)12 = (17 - 9.772)lz = 3.114 m of additional vertical electrode required to be connected to the ringlfoundation at each down-conductor position. [BS EN 62305-3, 5.4.2.21

Where it is not possible to install a ring earth electrode fully surrounding the structure due to directly abutting structures etc, the efficiency of the earth termination system may be


somewhat reduced, as the ring conductor acts as a type B electrode, a foundation earth and an equipotential bonding conductor. This situation can be overcome by continuing the ring within the foundations of the building along the perimeter of the inside walls of the structure where access is not available outside in order to maintain continuity in the ring. If internal downconductors are appropriate along these walls, these should also be connected to the internal foundation ring via test clamps. [BS EN 62305-3, E.5.4.3.41

5.3.4.3 Foundation earth electrodes

Where a foundation is used as an earth-termination, the installation of clamped or welded connections between lengths of reinforcing bar and sections of foundation is required, in addition to interconnection of the reinforcing rods by wire-lashing. Alternatively, the installation of an additional meshed network of conductors is required to ensure good continuity. Any additional network should also be lashed to the reinforcement steel and connected to it by clamps or welds every 20 m throughout the earthing layout. The earthing layout, whether reinforcing or additional conductors, should be connected to each external down-conductor and internal column, which in turn should be connected to the air-termination network. [BS EN 62305-3, E.4.3.91 The foundation earth electrode shall consist of a network of foundation reinforcing, or other conductors complying with the requirements of BS EN 50164, with a mesh size not exceeding 10 m, and shall be installed within the reinforced concrete base below the waterproof membrane. [BS EN 62305-3, E.5.4.3.21 The use of the foundation as an earth electrode is only allowable if the reinforcement falls within a section of concrete that is below any insulating or waterproof membrane.

In most cases where foundation earth electrodes are utilized, it is not possible to measure the earthing resistance of the foundation earth in isolation once all services have been connected together. Where utilizing the foundation, a reference earth electrode should be installed at approximately 100 m intervals around the perimeter of the structure close to the structure. This will provide a method of monitoring changes in the environment of the earthing system by measurement of the reference electrode. [BS EN 62305-3, E.4.3.91

At the perimeter of the structure foundation, a connection from the reinforcing should be routed upwards to a test clamp or cast-in earth point for connection to the down- conductor. [BS EN 62305-3, E. 5.4.3.21

5.3.4.4 Earthing of structures on rocky soil

During construction of structures on rock or rocky soil, a foundation earth electrode should be built into the concrete foundation and connected to test clamps. [BS EN 62305- 3, E. 5.4.3.51

Where a foundation earth electrode is not installed, a type B earth arrangement should be used instead, augmented by radial electrodes if necessary. If the ground is hard such that the earth electrodes (vertical or horizontal) cannot be installed within it, the electrode should be mounted on the surface but in this case it should be provided with mechanical protection. [BS EN 62305-3, E. 5.4.3.51

When earthing structures on rocky ground, the 10 ohm requirement is not applicable.


5.3.4.5 Earth termination systems in large areas

Due to the large amounts of power, other electrical and electronic systems and other electrically conducting paths within and around large installations, earth termination systems around such structures are particularly important for the protection of the electrical and electronic systems. The provision of a common low impedance earth system reduces the potential difference between the structures and so reduces the interference injected into the electrical interfaces.

A suitable low earth impedance may be achieved by providing the structures with foundation earth electrodes and type B and type A earth arrangements as necessary to obtain a resistance to earth of at least 10 R. By interconnecting a number of the aaacent structures, a mesh earthing system is obtained. [BS EN 62305-3, E.5.4.3.61

An earthing conductor or, in the case of wider cable trenches, a number of earthing conductors should be installed above the cables to reduce the probability of a direct lightning flash to the cables in the ground. [BS EN 62305-3, E.5.4.3.61 These conductors should be connected to the lightning protection earthing system either at a test clamp aaacent to where the cable route enters the structure or onto the earth bar within an aaacent inspection pit at either end of the cable route.

Interconnections between the earth electrodes, foundation earth and the down-conductors should be installed at the test joints. Some of the test joints should also be connected to the equipotential bars of the internal LPS; quantities and locations of these connections will be determined by the layout of the internal LPS. In cases where there are more than two conductors at any one test clamp position, for practical purposes, it may be preferable for the interconnections to be made onto an integral earth bar within an inspection pit. [BS EN 62305-3, E. 5.4.3.61

Step and touch voltages should be mitigated by ensuring that internal down-conductors andlor internal structural parts used as down-conductors, are connected to an earth electrode and the reinforcement steel of the floor. [BS EN 62305-3, E. 6.2. I ] If internal down-conductors are to be located near expansion joints in the concrete floor, these joints should be bridged as near to the internal down-conductor as possible in order to ensure continuity. [BS EN 62305-3, E. 5.4.3.61

5.4 Internal lightning protection system

Application of internal lightning protection is very important in order to achieve adequate lightning protection. [BS EN 62305-3, E. 6.11

The application of internal lightning protection is identical for all protection levels other than for the determination of separation distances, which is dependent upon the lightning protection level. [BS EN 62305-3, E. 6.11

The function of the internal LPS is to avoid the occurrence of dangerous sparking within the structure to be protected.

Dangerous sparking may occur due to lightning current flowing in the external LPS or in other conductive parts of the structure creating differences in potential between the


external LPS and other components such as metal installations, the electrical systems or external conductive parts and lines connected to the structure.

An adequate separation distance s, determined later in this section, should be maintained between the external LPS and all conductive parts connected to the equipotential bonding of the structure. [BS EN 62305-3, E. 6.1.11 Avoidance of dangerous sparking between different parts may be achieved by means of equipotential bonding or electrical insulation between the parts.

In the case of lines or external conductive parts connected to the structure, it is always necessary to ensure lightning equipotential bonding by direct connection or connection by SPDs at their point of entry to the structure. [BS EN 62305-3, 6.1 a,?& 6.21

5.4.1 Avoiding dangerous sparking using lightning equipotential bonding to internal parts and systems

It is important to achieve equipotentialization by interconnecting the LPS with structural metallic parts, metal installations, internal systems, external conductive parts and service lines connected to the structure.

[BS EN 62305-3, 6.2.11

When lightning equipotential bonding is established to internal systems, part of the lightning current may flow into such systems and this effect shall be taken into account.

Physical bonding can be established by bonding conductors where the electrical continuity is not provided by natural bonding and surge protective devices (SPDs), where a direct connection with live conductors is not feasible. [BS EN 62305-3, 6.2.11 In every case where an LPS is fitted to the structure, type I equipotential bonding lightning current SPDs should be fitted to the live cores of services entering the structure at the service entrance positions. The purpose of these SPDs is to provide an equipotential bond between the service and the lightning protection system in order to avoid dangerous sparking; they are not employed and may not be suitable to protect electrical and electronic systems.

Where there is an isolated LPS, bonding shall take place only at ground level. [BS EN 62305-3, 6.2.21

Where a directly attached non-isolated LPS is fitted, the main bonding barlposition should be located in the basement or approximately at ground level. The bonding bar should be connected to the lightning protection earthing system in addition to the items mentioned earlier in this Section 5.4.1. This bonding bar should be combined with and form part of the bonding bars required by the IEE regulations/BS 7671. It is common practice in the UK for the electrical contractor to undertake all the required equipotential bonding. However, it is vital that detailed coordination with the electrical contractor takes place to ensure that any equipotential bonding requirement under BS EN 62305 and BS 7671 are neither overlooked nor duplicated.

Table 5.5 identifies the minimum cross-sectional areas of bonding cables required to comply with BS EN 62305. However, it is possible that BS 7671 may require different sizes. In this case the electrical contractor should install the largest size conductor required by the two standards.


Table 5.5 - Minimum sizes of conductor for bonding purposes

[BS EN 62305-3, Ta,bks 8 a,?~d 9, a,?~d BS EN 62305-4, Ta,ble I ]

Minimum dimensions of conductors From bonding bars connecting to: a) different bonding bars or the earth-termination system; or b) internal metal installations.

Where internal gas lines or water pipes have insulated sections, Class I SPDs should be fitted to bridge the insulation. [BS EN 62305-3, 6.2.21

Class of LPS

I to IV

5.4.2 Avoiding dangerous sparking using electrical insulation of external LPS

Avoiding dangerous sparking between parts of the LPS and structural metallic parts, metal installations and internal systems may be achieved by providing a distance between the LPS and the parts systems greater than the minimum separation distance s required to provide electrical insulation such that dangerous sparking will not occur. In structures with metallic or electrically continuous, connected, reinforced concrete framework, a separation distance is not required.

Material

Copper

Aluminium

Steel

The distance s can be determined from the following:

s = ki x x I

Connecting SPDs (class of test)

where

ki depends on the selected class of LPS;

Cross-section a) mm2

14

22

50

I

I I

Ill

kc depends on the lightning current flowing on the down-conductors;

Cross-section b) mm2

5

8

16

k , depends on the electrical insulation material;

Note: Where the sizes given above are non-industry standard, the next largest standard size should be chosen.

Copper

Copper

Copper

I is the length, in metres, along the air-termination or the down-conductor, from the point where the separation distance is to be considered, to the nearest equipotential bonding point.

5

3

1

[BS EN 62305-3, 6.3)

It may be necessary to evaluate the separation distance for several positions.


When the distance between a down-conductor and the internal installations cannot be increased above the calculated separation distance, bonding should be provided at She most distant point. [BS EN 62305-3, Figure E.431 In the case of electrical conductors, they should either be re-sited beyond the required separation distance or installed within for example a conductive shield, armouring, metal conduit or trunking, and this shield should be bonded to the LPS at the equipotential bonding point and at the furthest point. The separation distance is then deemed to be satisfied along the whole length of the conductor. [BS EN 62305-3, E. 6.1.11

It is good practice to make the equipotential bond in a position where the LPS will not be disconnected from the equipotential bonding bar during periodic testing. See item (6) in Figure 5.12.

Table 5.6 gives values for ki, kc and k,.

Table 5.6 - Values for factors ki, kc and k,,,

[BS EN 62305-3, 6.3 a,?& A?L?L~: C]

Class of LPS

I

I I

Ill & IV

Number of down-conductors, n

1

2

4 or more

4 or more connected with horizontal ring conductors

Material

Air

Concrete, bricks

5.4.3 Lightning equipotential bonding to external parts and lines connected to the structure

Where external conductive parts enter the structure to be protected, lightning equipotential bonding should be established as near as possible to the point at which they enter. Bonding should be made using conductors complying with Table 5.5 or, if direct bonding is not possible or appropriate, as in the case of live conductors, by means of type I

h = Height of down-conductor c = Distance between down-conductors n = Total number of down-conductors

Where more than one insulating material is present, the lower value of k, should be used.

If the single type A electrodes do not have comparable resistances, kc = 1 should be assumed.

ki 0.08

0.06

0.04

kc

Type A earths

1

0.66

0.44

0.44

Type B earths

1

= ( h + c)/(2h + c)

= (1/2n) + 0.1 + 0.2 x

J (c lh ) = (1/2n) + 0.1 + 0.2 x

J (c /h)

k, 1

0.5


'd ' Actual distance 'I' Length for evaluation of separation distance 's' 1 Electrically conductive building service 2 Brick and wooden walllroof 3 Lightning protection system 4 Lightning protection system earth 5 Equipotential bonding bar 6 Equipotential bonding connection above test clamp

Equipotential bond

Note: If 'd' does not satisfy the separation distance requirement, the service should be bonded to the lightning protection system at a point where the recalculation of 's' at that point is satisfactory.

Figure 5.12 - Calculation of separation distance s at the point of closest proximity


SPDs. All protective earth conductors entering with the lines should be bonded to the main bonding bar either directly or via the SPD.

[BS EN 62305-3, 6.2.51

Where incoming lines are screened or routed in metal conduits, trunking or the like, the screens or conduit shall be bonded near to where they enter the building.

[BS EN 62305-3, 6.2.51

Conductors within the screen or conduit do not need to be bonded provided that the cross- sectional area of the screen or conduit is not lower than:

semi, = (If x p, x LC x 106)luw

where

If is the current flowing in the screen;

p, is the resistivity of the screen in Rm;

LC is the cable length in m;

Uw is the impulse withstand voltage of the electrical or electronic system fed by the cable in kV.

The limits of the value of If should be stated as:

If = 8 x S, or If = 8 x 1 ~ ' x S', for shielded and unshielded cables respectively

where

1 ~ ' is the current on the screen in kA;

S, is the cross-sectional area of the screen in mm2;

S', is the cross-sectional area of each conductor in mm"

The values for LC are given in Table 5.7.

Table 5.7 - Values for LC

[BS EN 62305-3, A ? L ? L ~ ~ B]

Condition of the screen

In contact with a soil with resistivity p (Om).

Insulated from the soil or in air.

Extreme caution should be exercised if using this principle, as it will not always be certain that the screening is uniform throughout the cable length or has not been compromised in other ways such as jointing. Unless detailed information regarding cable lengths and screen resistivity are available and are considered to be accurate, and it is certain that the integrity of the screen has not been compromised, it is recommended that type I SPDs are fitted at the point of entry to the structure.

LC

LC 5 8 x Jp

LC = distance between the structure and the closest earthing point of the screen.


5.4.4 Structure shielding

If there is a requirement to protect the internal electrical and electronic systems against the electromagnetic effects of lightning, the external walls and roof of a structure may be used as an electromagnetic shield. [BS EN 62305-3, E.5.2.4.2.31 If consideration is given to this protection, reference should be made to Section 10 of this guide, which deals with design of protection for electrical and electronic systems within a structure.

Within the domain of the air-termination system on the roof, all conductive parts with at least one dimension larger than 1 m should be interconnected to form a mesh. The meshed shield should be connected to the air-termination system at the roof edge and also at other points within the roof area in accordance with BS EN 62305-3, 6.2. [BS EN 62305-3, E. 5.2.4.2.31

Due to the reduced mesh size of reinforced concrete and steel structures, the lightning current is distributed over several parallel conductors, resulting in a lower electromagnetic impedance. Consequently the separation distances are reduced and the necessary separation distances between the installations and the LPS are much easier to obtain. [BS EN 62305-3, E. 5.2.4.2.31

When the roof is constructed of non-conductive elements, it is possible to improve shielding by reducing the spacing of the roof conductors. [BS EN 62305-3, E.5.2.4.2.31

Section 6 Joints, bonding and connections

For lightning current-carrying connections, such as in down-conductor applications, clamping or welding are the preferred methods of establishing acceptable continuity of reinforcing. Lashing as a connection is only suitable for additional conductors for equipotentialization and for electromagnetic compatibility (EMC) purposes.

Where connections to reinforcing within concrete are welded, the weld should be at least 30 mm long; it is not acceptable to merely weld for a few millimetres where conductors cross.

Horizontal reinforcing at ground and each floor level may satisfy the requirement to ensure an interconnected equipotentialization bar is fitted, provided that the conductive bars of the walls, floors and columns are electrically and mechanically continuous. Where an equipotentialization bar is not present by virtue of these continuous reinforcing bars, a separate equipotential bar ring conductor should be fitted at ground and each floor level, and these should be connected to the conductive parts in the outer walls, columns and floor. If the equipotentialization bar is in the form of an external conductor, where possible, a connection should be made to the steel reinforcement in the floor, column or wall at each down-conductor position. The connection should be made to at least three reinforcing rods. [BS EN 62305-3, E.4.3.71 In large structures, where the reinforcing acts as the equipotentialization bar ring conductor, cast-in earth points or other suitable connections should be made to the steel reinforcing bars every 10 m for future connections. [BS EN 62305-3, E.4.3.81 It may be commercially sensible from a client's perspective if cast-in earth points were installed in positions that were determined based upon the characteristics and use of the structure and not necessarily every 10 m regardless.

Materials used for joints between conductors buried in the soil should have corrosion characteristics compatible to that of the earth termination conductors. Generally, joints below ground should be made by welding (conventional or exothermic methods are both suitable) or brazing. In circumstances where it is not safely feasible to weld or braze, clamped joints may have to be used. Where clamped joints have to be used, they should be

Joints, bonding and connections

provided with effective corrosion protection after making the joint. [BS EN 62305-3, E.5.6.2.2.11

In the case of an isolated external LPS, the equipotential bonding is established only at ground level. [BS EN 62305-3, E. 6.2.11

For buildings higher than 30 m, it is recommended to repeat the equipotential bonding at a level of 20 m and every 20 m above that. However, in all circumstances the separation distance should be maintained. This means that on those levels where this repeat equipotential bonding is provided, the external down-conductor, any internal down- conductors and metal parts should be bonded. Live conductors should be bonded via SPDS. [BS EN 62305-3, E. 6.2.11

6.1 Equipotential bonding of internal conductive parts

Where the requirements of BS EN 62305-4 are being given consideration in order to protect electrical and electronic systems within the structure, requirements additional to those in this section are necessary. For these additional requirements, refer to Section 10 of this guide covering the design of protection for electrical and electronic systems within a structure.

Where the requirements of BS EN 62305-3 only are being considered, the following will apply.

A main equipotential bonding bar should be installed near ground level close to the main incoming electricity distribution board. Metal installations such as water, gas, heating, pipes, lift shafts and other extraneous structural metal shall be bonded together at this point and to the LPS above the test clamp and to the earth-termination (foundation earth, ring earth and other natural earths as applicable) at ground level. [BS EN 62305-3, E. 6.2.21 Typically, provision of this earth bar and equipotential bonding will be carried out by the electrical contractor under the requirements of BS 7671. However, the LPS contractor should liaise with the electrical contractor to identify that the requirements of BS EN 62305 are satisfied. The electrical contractor should be advised at tender stage that separation distances are important and of the possible additional bonding requirements should the mechanical and electrical systems be located closer to the LPS than permissible under the calculation for the separation distance s.

Where the LPS contractor is acting as principal, for example on an existing building, due allowance should be given to complying with the requirements to bond all services to an equipotential earth bar. Bonding conductors from the main equipotential earth bar should be kept as short as possible. [BS EN 62305-3, E.6.2.21

In large structures, several equipotential earth bonding bars may be required. If so, these should be interconnected. Reinforcing may be used for equipotential bonding purposes; if this is the case, continuity should be ensured.

Where a type A earthing arrangement is fitted, the bonding bars should be connected to the individual aaacent down-conductor above any test clamp. In addition, the earth bars should be interconnected by an internal ring conductor or an internal conductor forming a partial ring. [BS EN 62305-3, E. 6.2.51


Main incoming earth cable

Figure 6.1 - Example of main equipotential bonding arrangement

For entries of external services above the earth surface, She bonding bars should be connected to a horizontal ring conductor inside or outside the outer wall bonded to the down-conductors of the LPS and to the metallic reinforcement of the structure, if applicable. [BS EN 62305-3, E.6.2.51 The ring conductor should be bonded to the steel frame or reinforcement at intervals corresponding to the downconductor spacings. [BS EN 62305-3, E. 6.2.51

E 3 S .- u 0 b 5 3 .- u .-

d

I-0

In structures designed for data or communications applications or where a low level of LEMP induction effects is desirable, the ring should be bonded to the steel or reinforcement at approximately 5 m centres. [BS EN 62305-3, E. 6.2.51 For She bonding of external services in reinforced concrete buildings that contain large communication or computer installations, and for structures where EMC demands are severe, a ground plane with multiple connections to the metallic reinforcement of the structure or other metallic elements should be used. In this case the ground plane should consist of a superimposed 5 m mesh connecting to the reinforcement of the floor slab every 1 m. [BS EN 62305-3, E. 6.2.5, a,?& BS EN 62305-4, 5.2 a,?~d Figure 71

- .- e 0, 9 a - b - 8

6

6.2 Corrosion

A

% .- a kl - $ .- u - -

8

6

A

6

Whether in air, the ground or other containment, corrosion of metals will occur variously at rates depending on the type of metal and the nature of its environment. Factors such as moisture, dissolved salts, degree of aeration, temperature, gasses, other contaminants and the extent of movement of electrolyte all combine to make this condition a very complex one. [BS EN 62305-3, E.5.6.2.2.11 Corrosion may be mitigated by using materials compliant with the BS EN 50164 series. Typical materials are aluminium or copper air- termination and down-conductor networks (either bare, PVC-covered or lead-covered copper tape in aggressive gaseous or sulphurous environments, i.e. the top of chimneys) and PVCcovered copper conductors for connecting down-conductors to the earth- termination and copper, or copper bonded, electrodes either in the ground or in other containments such as concrete. Where the ground is contaminated andlor presents an environment where excessive corrosion may be possible, reference to BSI PD 6484:1979 for guidance on metal and electrolytic compatibility with various corrosive mediums should be made; typically, stainless steel electrodes may be appropriate in many

A A

.- a VI

.- - - i t s -

I-0

6

if 4 u " E " e E

I-0

6

A

E 2 f C .-

I ° $ : , - g

z m .- - 8

6

A A

z - ;; * - 5 0, - .- a 6 E 2 cO

6

Joints, bonding and connections

5 .- Y 111 U .- C z .- d

Connection to steel frame natural down- Main incoming earth - conductor or conventional lightning $ bonded back to main protection system above test clamp 2 equipotential earth bar

Type 'B' earth -------------

b

metalwork SPD connected to live Main conductors and bonded

equipotential back to main earth bar bonding bar

0" . . . . Building steel frame

- - vO

3 f

Figure 6.2 - Example of bonding arrangement in a structure with multiple service entry points using external type 13 electrode for interconnection

circumstances. In locations where cathodic protection systems are present, specialist advice should be obtained, as electrodes of certain metals may cause galvanic damage to ferrous metals.

The material of the LPS should be electrochemically compatible throughout and with the material of the connection and elements of the structural parts utilized as part of the LPS. [BS EN 62305-3, E. 5.6.21


Connections between dissimilar metals should be avoided where possible. If unavoidable, precautions should be taken, such as the use of proprietary bi-metallic clamps, tinning of copper surfaces and application of corrosion-inhibiting compound, and wrapping or sheathing with suitable tape to prevent the ingress of moisture and ensure joints are protected. [BS EN 62305-3, E.5.6.2.21 Conductors should be configured such that copper equipment or conductors are not installed above aluminium or galvanized equipment, unless appropriate protection against corrosion is provided, as this will create an unacceptable corrosive situation. Bare aluminium conductors should not be attached directly to surfaces such as plaster, cement, concrete limestone or the like; PVC-covered aluminium may be used in such cases. Aluminium conductors should never be buried in the ground irrespective of whether PVC sheathing is fitted or not. [BS EN 62305-3, E. 5.6.2.21

Where bonding conductors are brought through the surface of a concrete wall, there is no corrosion risk if a solid copper conductor, proprietary bonding point, PVC covering or insulated wire is used. Where stainless steel bonding conductors are used, no corrosion prevention measures need to be used. Where steel reinforcement bonding conductors are used, the simplest corrosion protection measure is the application of a bitumen (or similar) finish for a minimum of 50 mm within the wall and 50 mm where the conductor exits the wall.

In circumstances where the corrosive atmosphere is particularly aggressive, it is recommended that the bonding to any reinforcement is made using proprietary gunmetal cast-in earthing points or stainless steel conductors. [BS EN 62305-3, E.4.3.51

Section 7 Requirements for structures with risk of explosion, in addition to standard requirements

7.1 General requirements

Unless specifically instructed by the client, structures with a risk of explosion should be fitted with at least a class I1 LPS including provision of SPDs, irrespective of the outcome of a risk assessment in accordance with BS EN 62305-2. [BS EN 62305-3, 0 . 1 a,?~d 0 .41

As any risk of explosion may be due to the presence of many different materials in many different environments, it is vital that detailed consultation takes place with the client and if necessary their specialist advisors so that any local, national or international legislation or lesser requirements are taken into account and all structural and other layout arrangements may be taken into account. [BS 66511

BS EN 62305-3, D.3.1 states 'The lightning protection system should be designed and installed in such a manner that, in case of a direct lightning flash, there are no melting or spraying effects except at the striking point'. Due to this possibility it is good practice to install isolated LPS systems on structures with a risk of explosion. If, due to architectural or other practical or economic reasons, it is not possible to install an isolated LPS, detailed consultation with the client and their specialists should be undertaken in order to derive the optimum alternative solution, which will inevitably be non-isolated in its form. [BS EN 62305-3, 0 .3 .1 a,?& 0 .41


7.2 Structures containing solid explosive materials

Although an isolated external LPS is encouraged, structures totally contained within a metallic shell of 5 mm thickness steel or equivalent (7 mm for aluminium structures) may be considered protected by a natural air-termination system. [BS EN 62305-3, 0.41

A type B earth-termination system is preferred for all lightning protection systems on structures with danger of explosion. [BS EN 62305-3, 0.3.31

Lightning equipotential bonding at ground level and where the distance between the conductive parts is less than the separation distance s should be assured inside hazardous areas and locations where solid explosive materials may be present. For structures with a risk of explosion, kc = 1 should be used to calculate s. However, additional equipotential bonding should be installed to ensure no dangerous discharges will take place in those areas where an explosive atmosphere is present frequently, for long periods or continuously, e.g. in the form of a cloud of combustible dust in air (zone 20), or a mixture of air and flammable substances in the form of gas, vapour or mist (zone 0). [BS EN 62305- 3, 0.3.41

7.3 Structures containing hazardous areas

Where possible, all parts of the external LPS should be at least 1 m away from a hazardous zone. [BS EN 62305-3, 0.5.11

SPDs should be positioned outside the hazardous zone where practicable. SPDs positioned inside a hazardous zone should be approved for the zone in which they are installed or should be contained within an enclosure and both should be approved for this purpose. [BS EN 62305-3, 0.5.1.11

Structures in zones 1, 2, 21 and 22 as defined below may not require additional air- termination systems if the structure is made of metal complying with the thickness requirements of Table 5.3. However, they should be earthed using a type B arrangement in accordance with the class of protection. [BS EN 62305-3, 0.5.2 a,?~d 0.5.31

Zone 1 - place in which an explosive atmosphere consisting of a mixture of air and flammable substances in the form of gas, vapour or mist is likely to occur in normal operation occasionally

Zone 2 - place in which an explosive atmosphere consisting of a mixture of air and flammable substances in the form of gas, vapour or mist is not likely to occur in normal operation but if it does will persist for a short period only (zone 2)

Zone 21 -place in which an explosive atmosphere in the form of a cloud of combustible dust in air is likely to occur in normal operation occasionally (zone 21)

Zone 22 -place in which an explosive atmosphere in the form of a cloud of combustible dust in air is not likely to occur in normal operation but if it does will persist for a short period only (zone 22),

This waiving of a requirement for an additional isolated air-termination network on structures within zones 1, 2, 21 and 22 may also be applicable for outdoor facilities defined

Reauirernents for structures with risk of ex~losion

as zones 0 or 20. In this case, where closed steel containers with areas defined as zones 0 and 20 inside are used, they should have a wall thickness of at least 5 mm at the possible lightning striking points, otherwise an additional air-termination network should be installed. [BS EN 62305-3, 0.5.41

Section 8 Protection measures touch and step voltages

In She event of a lightning strike to a LPS, dangerous voltages may be propagated. Hazard could be present outside the building near to the down-conductors, even if the LPS has been designed and constructed in accordance with BS EN 62305.

The potential hazards derived from dangerous touch and step potentials will be reduced to tolerable levels if one of the following conditions is satisfied:

1. She probability of persons approaching, or the duration of their presence outside the structure close to the down-conductors, is very low;

2. the natural down-conductor system consists of several columns of the extensive metal framework of the structure or of several paths of interconnected steel of the structure, ensuring that the electrical continuity is assured;

3. She resistivity of the surface layer of She soil, within 3 m of the down-conductor, is not less than 5000 Rm (a layer of insulating material, asphalt for example, of 5 cm thickness or a layer of gravel approximately 15 cm thick generally reduces the hazard to a tolerable level);

If none of the above conditions are satisfied, the following protection measures shall be considered and applied against injury to living beings due to touch and step voltages:

insulation of the exposed down-conductor should be provided giving a 100 kV, 1.2150 ps impulse withstand voltage, e.g. at least 3 mm cross-linked polyethylene;

physical restrictions andlor warning notices should be provided in order to minimize She probability of access to within 3 m of the down-conductors to prevent them being touched;

equipotentialization by providing a meshed earthing system.

Section 9 Components and materials

Components used in the LPS shall withstand the electromagnetic effects of lightning current and predictable accidental stresses without being damaged. This requirement is satisfied by choosing components that have been successfully tested in accordance with BS EN 50164.

Acceptable materials are described in Tables 9.1 and 9.2.

Table 9.1 - Materials commonly used and compliant with BS EN 62305

Material Configuration

Air-termination and down-conductors

Minimum cross- sectional area rnrn2

Copper

Tin plated copper')

Aluminium

Aluminium alloy

Comments

Solid tape Solid round 7, Stranded

Solid round 3), 4,

Solid tape Solid round 7, Stranded

Solid tape Solid round Stranded

Solid tape Solid round Stranded

Solid round 3,

50 50 50

200

50 50 50

70 50 50

50 50 50

200

2 rnrn rnin. thickness 8 rnrn diameter 1.7 rnrn rnin. diameter of each strand. 16 rnrn diameter

2 rnrn rnin. thickness 8 rnrn diameter 1.7 rnrn rnin. diameter of each strand

3 rnrn rnin. thickness 8 rnrn diameter 1.7 rnrn rnin. diameter of each strand

2.5 rnrn rnin. thickness 8 rnrn diameter 1.7 rnrn rnin. diameter of each strand. 16 rnrn diameter


Table 9.2 - Material, configuration and minimum dimensions of earth electrodes

Material

Hot dipped galvanized steel ')

Stainless steel 5,

1) Hot dipped or electroplated minimum thickness coating of 1 prn. 2) The coating should be smooth, continuous and free from flux stains with a minimum thickness coating of 50 prn. 3) Applicable for air-termination rods only. For applications where mechanical stress such as wind loading is not critical, a 10 rnrn diameter, 1 rn long maximum air-termination rod with an additional fixing may be used. 4) Applicable to earth lead-in rods only. 5) Chromium 2 16 %, nickel 2 8 %, carbon 1 0.07 %. 6) For stainless steel embedded in concrete, and/or in direct contact with flammable material, the minimum sizes should be increased to 78 rnrn2 (10 rnrn diameter) for solid round and 75 rnrn2 (3 rnrn minimum thickness) for solid tape. 7) 50 rnrn2 (8 rnrn diameter) may be reduced to 28 rnrn2 (6 rnrn diameter) in certain applications where mechanical strength is not an essential requirement. Consideration should, in this case, be given to reducing the spacing of the fasteners. 8) If thermal and mechanical considerations are important, these dimensions can be increased to 60 rnrn2 for solid tape and to 78 rnrn2 for solid round. 9) The minimum cross-section to avoid melting is 16 rnrn2 (copper), 25 rnrn2 (aluminium), 50 rnrn2 (steel) and 50 rnrn2 (stainless steel) for a specific energy of 10 000 kJ/O. For further information see BS EN 62305-3, Annex E. 10) Thickness, width and diameter are defined at + 10 %.

Configuration

Solid tape Solid round ') Stranded

Solid round 3), 4), ')

Solid tape ')

Solid round ') Stranded

Solid round 3), 4,

Material

Copper

Steel

Minimum cross- sectional area rnrn2

50 50 50

200

50 50 70

200

Configuration

Stranded 3,

Solid round 3, Solid tape 3,

Solid round Pipe Solid plate Lattice plate

Galvanized solid round ') ')

Galvanized pipe ') ')

Galvanized solid tape ')

Comments

2.5 rnrn rnin. thickness 8 rnrn diameter 1.7 rnrn rnin. diameter of each strand 16 rnrn diameter

2 rnrn rnin. thickness 8 rnrn diameter 1.7 rnrn rnin. diameter of each strand 16 rnrn diameter

Comments

1.7 rnrn rnin diameter of each strand 8 rnrn diameter 2 rnrn rnin thickness

2 rnrn rnin. wall thickness 2 rnrn rnin. thickness 25 rnrn x 2 rnrn section Minimum length of lattice configuration: 4.8 rn

2 rnrn rnin. wall thickness

3 rnrn rnin. thickness

Minimum

Earth rod 0 rnrn

15 20

16 ')

25

dimensions

Earth conductor

50 rnrn2

50 rnrn2 50 rnrn2

10 rnrn 0

90 rnrn2

Earth plate rnrn

500 x 500 600 x 600

Corn~onents and materials

Material

Stainless steel 7,

Configuration

Galvanized solid plate ') Galvanized lattice plate ') Copper coated solid round 4,

Bare solid round ') Bare or galvanized solid tape ') 6,

Galvanized stranded ') ')

Galvanized cross profile ')

Solid round Solid tape

1) The coating shall be smooth, continuous and free from flux stains with a minimum thickness of 50 prn for round and 70 prn for flat material. 2) Threads shall be machined prior to galvanizing. 3) May also be tin-plated. 4) The copper should be intrinsically bonded to the steel. 5) Only allowed when completely embedded in concrete. 6) Only allowed when correctly connected together at least every 5 rn with the natural reinforcement steel of the earth touching part of the foundation. 7) Chromium 2 16 %, nickel 2 5 %, molybdenum 2 2 %, carbon 5 0.08 %. 8) In some countries 12 rnrn is allowed. 9) Earth lead in rods are used in some countries to connect the down-conductor to the point where it enters the ground.

Where an excessive rise in temperature is of concern because She surface on which the components are to be attached is flammable or has a low melting point, conductors with larger cross-sections should be specified or alternative safety precautions should be considered, such as the use of stand-off fittings or fire-resistant layers.

BS EN 62305-3, Table E.l shows suggested k i n g centres for conductors on various planes. The dimensions given in this table are not particularly relevant in the UK, as the k i n g centres suggested appear to be suitable for hard or semi-hard drawn conductors. The conductors typically used in the UK are generally soft drawn, thus making them easier to apply in practice. In this case the dimensions given in Table 9.3 are more appropriate.

Table 9.3 - Recommended fixing centres for conductors

Arrangement

Horizontal conductors on horizontal surfaces

Horizontal conductors on vertical surfaces

Comments


30 rnrn x 3 rnrn section

250 prn minimum radial Copper coating 99.9 % copper content


1.7 rnrn rnin. diameter of each strand

2 rnrn rnin thick

Minimum

Earth rod 0 rnrn

14

50 x 50 x 3

15

Fixing centres for tape and stranded conductors rnrn

1000 (500)

500 (500)

dimensions

Earth conductor

10rnrn0

75 rnrn2

70 rnrn2

10rnrn0 100 rnrn2

Fixing centres for round solid conductors rnrn

750 (1 000)

500 (1 000)

Earth plate rnrn

500 x 500

600 x 600

-


Arrangement Fixing centres for tape and Fixing centres for round solid stranded conductors conductors I mm I mm

I Vertical conductors from the around to 20 m

I I000 ( I 000)

NOTE 1 This table does not apply to built-in type fixings, which may require special consideration. NOTE 2 Assessment of environmental conditions should be undertaken and it may be found that fixing centres different from those recommended are necessary. NOTE 3 For reference, the dimensions shown in brackets are those given in BS EN 62305-3, Table

Vertical conductors from 20 m and thereafter

E.1. There appears to be no logic to the spacings given in Table E. l and the assumption of the author of this guide and the bodies endorsing it is that there have been editorial errors in this table. It is expected that the values in Table E. l will be addressed during the first amendments of the standard in order to state those values necessary to ensure the conductors are fixed in a mechanically secure manner reflected by the recommended fixing centres of this Table 9.3.

[BS EN 62305-3, Table E. I ]

500 (500) 500 (1 000)

Section 10 Design of protection for electrical and electronic systems within a structure

This Section is intended to give an overview of the practical application of SPDs compliant with the requirements of BS EN 62305-4. It is not intended to offer any guidance on the design of the devices themselves or the suitability of equipment produced by different manufacturers to be used in conjunction with each other. This will be dependent upon the characteristics of the products and systems to be protected. In all cases, SPD manufacturers' equipment specifications should be consulted, as their characteristics and applications may vary.

10.1 Design and installation of lightning electromagnetic pulse protection measures system (LPMS)

Lightning flashes release energy many magnitudes greater than the energy that can be absorbed by sensitive electrical and electronic systems and equipment. Due to the increasing sensitivity and cost of such systems, in most cases it will make economic sense to assess protection measures to reduce the risk of permanent failures of these and subsequent loss of service to the public.

BS EN 62305-4, Clause 4 states the following.

Permanent failure of electrical and electronic systems can be caused by the lightning electromagnetic impulse (LEMP) via:

conducted and induced surges transmitted to apparatus via connecting wiring;

the effects of radiated electromagnetic fields directly into apparatus itself.


Surges to the structure can be generated externally or internally:

surges external to the structure are created by lightning flashes striking incoming lines or the nearby ground, and are transmitted to electrical and electronic systems via these lines;

surges internal to the structure are created by lightning flashes striking the structure or the nearby ground.

The coupling can arise from different mechanisms:

resistive coupling (e.g. the earth impedance of the earth-termination system or the cable shield resistance);

magnetic field coupling (e.g. caused by wiring loops in the electrical and electronic system or by inductance of bonding conductors);

electric field coupling (e.g. caused by rod antenna reception).

Note that the effects of electric field coupling are generally very small when compared to the magnetic field coupling and can be disregarded.

Radiated electromagnetic fields can be generated via:

the direct lightning current flowing in the lightning channel;

the partial lightning current flowing in conductors (e.g. in the down-conductors of an external LPS according to IEC 62305-3 or in an external spatial shield according to this standard)." [BS EN 62305-4, I?~troductio?~]

The purpose of the LPMS is to limit any conducted and induced surges transmitted to apparatus via connecting wiring and the effects of radiated electromagnetic fields directly into the apparatus itself to levels that the equipment can withstand without being damaged.

The general principle of protection against LEMP uses the lightning protection zone (LPZ) concept.

The rooms or volumes to be protected are classified and divided into LPZs.

BS EN 62305-4, Clause 4 states that: 'These zones are theoretically assigned volumes of space where the LEMP severity is compatible with the withstand level of the internal systems enclosed. Successive zones are characterized by significant changes in the LEMP severity. The boundary of an LPZ is defined by the protection measures employed'.

Figure 10.1 shows the theoretical division of LPZs.

For each zone considered it is important for the LPS contractor to liaise with the manufacturer of the equipment or system to be protected in order to ascertain the magnitude of electromagnetic field strength and the rated impulse withstand levels of the equipment within which tolerances the equipment should not suffer damage. These factors can then be taken into account to determine appropriate LPZs. Within the normal contracting environment of the UK, on new-build contracts in particular, it is unlikely that the LPS contractor will be provided with sufficient detail regarding the characteristics, quantities and location of the internal equipment to enable detailed designs to be derived

Design of protection for electrical and electronic systems within a structure

Bonding of services where they enter the structure or move from zone to zone. Bonding is carried out directly to metal services andlor by surge protection devices to live conductors. Equipment should be sited away from the comers of the structure and LPS down conductors

LPZl

Figure 10.1 - Theoretical division of lightning protection zones

at tender stage. Consulting engineers and others responsible for the overall building services and contents should engage a lightning protection specialist at a stage prior to the tendering of the project so that designs or other parameters can be established and LPS contractors can offer quotations andlor designs to appropriate predetermined criteria. If lightning protection contractors are not involved in the project prior to tender stage, it is unlikely that optimum solutions will be offered. Based on only outline tender information, this is likely to make coordination of the LPMS difficult and correct integration with other building services and their own physical location very difficult. This could be to the detriment of the efficiency of the installed LPMS.

Main panel Main or similar equipotential o, o controller bonding bar .g .E .g with SPD

8 8 fitted z 0 k 5 2's .- .- c c r' r .M ry d

s t - c E .g 3 8 g % 8 U

Data lnout Equipotential bonding Large amounts bars connected back t electronic equipment main equipotential ba:

Server Room Vital equipment LPZ3 /\

Protection against conducted surges is afforded by installing coordinated SPDs on cables and equipment feeding any equipment or system requiring protection. Protection against

Office

Single PCs

C3 Cl

LPZZ

Off ice

Single PCs


radiated electromagnetic fields is afforded by installing coordinated spatial shields, around cables, equipment, and sometimes the room itself. This may take the form of building structural members (if their characteristics are appropriate), purpose-installed spatial shields or equipment enclosures.

Where two or more LPZs of the same level are interconnected and the cable routes through a lower level LPZ, SPDs should be fitted at the entry or exit to or from the zones in question, or the cables should be shielded and their shields equipotentially bonded to an equipotential earth bar located aaacent to where the cables enter or exit the zone. [BS EN 62305-4, 4.21 The benefit of installing SPDs in this situation is that correctly chosen SPDs should protect against both damage to and malfunction of the equipment, whereas simply providing shields should protect against damage but would still allow system malfunction. This earth bar should be connected back to the main earthing terminal of the distribution board that feeds the equipment to be protected, using earth connections as short and straight as possible, with copper conductor of at least 16 mm? BS EN 62305-4, Table 1 states minimum sizes of conductors connecting SPDs as 5 mm" 3 mmm" and 1 mmvor class I, I1 and I11 protectors respectively. Care should be taken when applying these sizes. SPD manufacturers' information will give detailed advice and instructions on connection and earthing of the devices to ensure that the full protection specification of the SPD is achieved. Failure to comply with these instructions (especially the earthing requirements of the devices) could result in a protection level below that required to protect the device or system.

Figure 10.2 shows the general concept. However, all applications will be different depending upon cable and signal type, line voltage, number of cores, etc.

10.2 Basic protection measures

The first stage of the process for existing and new structures is to carry out a risk assessment in accordance with BS EN 62305-2 in order to determine required protection levels. See Section 2 of this guide for details of risk assessment inputs.

Existing structures and their services will almost certainly be laid out such that they impose restrictions on the type and efficiency of protection measures that can be provided; each structure will provide different characteristics in this respect. The risk assessment described in Section 2 should be used to derive the best technical and economic compromise between the provision of or reducing the dimensions of any spatial shielding and provision of coordinated surge protection. Any restrictions that existing structures and services impose on the efficiency of a retrospectively fitted LPMS should be discussed with the client so that any continuing risks are understood and accepted.

For an LPMS to be effective, the following basic considerations need to be taken into account by the building services professionals responsible for coordinating services on new build or refurbishment projects, or by the client where retrospective protection is considered.

The earthing system should be adequate, as it conducts and consequently disperses the lightning current into the earth.

The bonding of all services should always be ensured, including provision of equipotential bonding of the power and telecommunications line conductors at their


LPZ 1

Surge protection device

LPZ 2

vf Cable shield or surge protection device to local earth bar

LrL u

Ah Unshielded cable

Bonding to screens may sometimes affect the system. System providers should be consulted.

LPZ 1

..

LPZ I

Shielded cable

Figure 10.2 - Examples of interconnecting lightning protection zones

..

LPZ 2

point of entry to the structure using SPDs, as this minimizes potential differences and may reduce radiated magnetic fields.

Spatial shielding, which could be in the form of the external LPS, structural conductive metalwork or some other appropriate conductive medium, as this attenuates the magnetic field within the LPZ arising from a lightning strike to or nearby the structure, and helps reduce internal surges.

Shielding of internal cables and lines, by using cables with inherent screening or routing through metallic conduits, trunking, ducts or the like, minimizes internal induced surges.

Routing the internal power, telecommunications or data cables away from the external walls or roof of the structure and running cables feeding common equipment together (or as close together as the operating characteristics of the cable equipment allow) will minimize the induction effects from the lightning current flowing in the external LPS, induction loops and minimize internal surges.

Protection of low voltage electrical systems can be provided with relative ease as these are generally delivered at a common voltage. However, telecommunications and data communications in particular offer very many more permutations and these systems need much more involvement to determine not only the characteristics of the electronic system itself, but also the implications of introducing the characteristics of the SPD onto the system.


In every case where an LPS is fitted to the structure, type I equipotential bonding lightning current SPDs should be fitted to the live cores of services entering the structure at the service entrance positions. The purpose of these SPDs is to provide an equipotential bond between the service and the lightning protection system in order to avoid dangerous sparking; they are not employed and may not be suitable to protect electrical and electronic systems.

10.3 Earthing and bonding

For sizes of bonding conductors, see Table 5.5. For detailed arrangements of layout and connections to the earthing arrangement, refer to Section 5.3.4. Minimum standard, commercially available sizes of conductors are shown in Table 10.1.

Table 10.1 - Minimum sizes of commercially available bonding components

It is technically correct for connections of equipment and protective earth conductors to be made using the configurations shown in Figure 10.3. However, SPD manufacturers' information and application guides should recommend how to achieve the maximum specification of their SPDs. This is highly dependent on correct earthing of the device. Modem computer rooms tend to have an earth mesh system installed under the removable floor. This allows for short, straight connections to be made to the SPD earth terminal. Retro fit would usually be made to a star point connection again using a connection as short and straight as possible. Part of the surge protection survey should take into account the correct siting of the SPD, this being highly influenced by the cable routing feeding the equipment requiring protection. A natural interface would be chosen where no (or minimum) inductive loops exist and the maximum number of cables can be protected in a cluster. This cluster would have its own 'local' earth bar for SPD use. The surge survey should consider and recommend that any inductive loops identified as part of the survey must be removed (by cable rerouting or rewiring) to ensure that induced voltages are not generated across input and output due to inductive or capacitive coupling.

Configuration "S" is only suitable for use where all lines enter the room at one point only and where the LPZ is relatively small, possibly up to 10 to 12 m? In this configuration, all metal components and equipment should only be connected to the earthing system at one common point and all lines connecting equipment should be run parallel with the bonding conductors in the star configuration in order to avoid induction loops.

Bonding component

Bonding bars

Connecting conductors from bonding bars to the earthing system or to other bonding bars

Connecting conductors from internal metal installations to bonding bars

Note: Materials used other than copper should have a cross-sectional area that provides the same conductivity as copper.

Material

Cu

Cu

Cu

Cu Connecting conductors for SPDs

Size mm2

50

16

6

6 4 1

Type 1 Type II Type Ill


----- Bonding network

Bonding conductor

0 Equipment

Bonding point to the bonding network

ERP Earth reference point

Sr Star point configuration integrated by star point

h Meshed configuration integrated by mesh

Figure 10.3 - Bonding of electronic systems into the bonding network


Configuration "M" is preferable where the network covers zones larger than 10 to 12 m" or where it extends over the whole structure and lines enter the structure at several locations allowing lines and equipment to be connected to the bonding network at no less than two points.

The bonding network can be axranged as a three-dimensional meshed structure with a typical mesh width of 5 m maximum. This requires multiple interconnections of metal components in and on the structure (such as concrete reinforcement, elevator rails, cranes, metal roofs, metal facades, metal frames of windows and doors, metal floor frames, service pipes and cable trays). Bonding bars (e.g. ring bonding bars, several bonding bars at different levels of the structure) and magnetic shields of the LPZ should be integrated in the same way. If a room contains a standalone piece of sensitive equipment that fully meets all of the relevant CE EMC standards, the equipment should not be susceptible to radiated magnetic fields. The problem arises when two or more pieces of equipment are connected together. At this point it becomes a 'system', and could then be susceptible to induced currents and voltages entering via connecting cables, etc. The exception to this is if the system has been certified under the CE EMC standards as such, and has been tested as such.

Bonding bars should be located as close to the LPZ boundary as practicably possible and connected to the earthing system using conductors not longer than 0.5 m. Where this distance cannot be maintained, two conductors should be run in parallel following the shortest practical route possible, avoiding loops and sharp bends in order to reduce the volt drop along the conductor. [BS EN 62305-4, E. 5.4 a,?& 5.31

The LPMS bonding network should not be used as part of the power system or signal return path. It is however acceptable for a functional earth to be connected to it. [BS EN 62305-4, B. 81

At the position where incoming lines enter an LPZ, an equipotential earth bar should be provided to which equipotential bonding SPDs will be connected. [BS EN 62305-4, 5.41 This earth bar will be connected to the bonding network at the closest point practicable.

10.4 Magnetic shielding and line routing [BS EN 62305-4, Clause 6, Clause 4, B. 6 and B. 71

Magnetic shielding and the appropriate routing of power, telecommunications and data lines can reduce the magnitude of the internal electromagnetic field and induced surges.

Shielding may be in the form of cable shields and metallic enclosures for equipment, or where it is more practical may take the form of a defined LPZ. [BS EN 62305-4, 6.1 a,?& 6.21 In either case, if protection of internal electrical and electronic systems is required, it is vital that the services' consulting engineers consider the need and provide for appropriate cabling, enclosures or tray, etc. or the integration of natural components to form a spatial shield at the early planning stages. This early consideration will enable simpler and more economic provision of an appropriate shield.

Natural components of a shield to form a lightning protection zone should be in the form of a grid or be added to in order to provide a grid with maximum dimensions of 5 m x 5 m.


Examples of natural components for shielding purposes, which can be used alone or in conjunction with each other are:

the external LPS;

electrically continuous reinforcing within concrete roofs and walls, facades and metal sheets covering the structure;

metal components of the roof construction including those underneath non-metallic roofing;

metal parts such as ornamentation, railings, pipes, coverings of parapets;

metal pipes and tanks on the roof.

Consideration and care should be given to the routing of the power, telecommunications or data lines to minimize induction loops and surge voltages inside the structure. A practical solution to this is to route lines away from external walls and the external LPS and close to (or such that they are shrouded by) bonded or earthed components, and also to route together power and communications lines feeding the same equipment. In the latter case, discussions should take place with the telecommunications or data communications supplier to ensure appropriate measures are provided, such as siting the power and signal lines at least 20 cm away from each other, providing screened cables, compartmentalized metallic trunking or tray, in order to avoid interference. [BS EN 62305-4, 6.31

Conductors for the purpose of creating a spatial shield at the boundary of LPZ OA and LPZ 1 should comply with the requirements for those used to provide an external LPS as described in Section 5 of this guide unless the source of damage due to lightning flashes to the structure (risk component RD) is negligible. Conductors for the purpose of creating a spatial shield at the boundary of LPZ 1 and 2 or higher do not need to meet the requirements for the external LPS conductors, as they are not intended to carry lightning currents. [BS EN 62305-4, 6.51 BS EN 62305-4 does not state a minimum size for conductors forming the magnetic shield, however it is recommended to apply a minimum size of 5 mm"6 mm%ll be applied in practice) as stated in BS EN 62305-4, Table 1 for a conductor connecting SPDs tested to class 111.

10.5 Externally sited equipment

Wherever possible, external equipment should fall within LPZ OB established in accordance with BS EN 62305-3 and Section 5.3.1 of this guide. All equipment, except some types of aerials, can be protected in this manner. Aerials sometimes have to be placed in exposed positions to avoid their performance being adversely affected by nearby lightning conductors. Some aerial designs are inherently self-protecting because only wellearthed conductive elements are exposed to lightning flash. Others might require SPDs to be installed on their feeder cables to prevent excessive transients from flowing down the cable to the receiver or the transmitter. When an external LPS is available the aerial supports should be bonded to it.

Where externally sited equipment does not fall within a zone of protection offered by the LPS, for example in situations where equipment protrudes above the mesh protecting a flat roof, if this equipment has a metallic casing that complies with the dimensions shown in


Table 5.3 [BS EN 62305-3, Ta,ble 31 then this equipment may be bonded directly to the nearest lightning part of the LPS. If this equipment has connected metallic services then these services should be bonded to the nearest equipotential bonding bar either directly or by type I lightning current SPDs.

Where externally sited equipment falls within a zone of protection offered by the LPS and an appropriate separation distance is maintained then no further direct bonding is required to the service, but type I1 overvoltage SPDs are required to be fitted. The exact location of these SPDs will be determined by the siting and layout of cabling and internal control equipment.

10.6 Coordinated SPD protection

Where a structure is supplied by an overhead service, if fitted with a LPS or not, then the service entrance SPD should be of a type I lightning current arrestor, as this device may see the direct lightning current. Where a service is supplied underground, if the structure is fitted with a LPS then the SPD should be a type I lightning current arrestor. Wherever the structure has no LPS the SPD can be of type 11.

Where more than one SPD is installed in the same circuit, they must be coordinated such that they share the energy between them and individually do not become overstressed. It is important that the manufacturer's technical data is consulted in the selection of SPDs to ensure that the characteristics of the individual SPDs under consideration provide suitable coordination. [BS EN 62305-4, C. I ]

Depending upon the withstand levels of the equipment internally, it is possible that protection can be afforded by one layer of SPDs. In this case further coordinated SPDs would serve no useful purpose. SPDs will normally be installed in the following positions.

a) At the line entrance into the structure (at the boundary of LPZ 011, at the main distribution board for example) consisting of either:

type I SPD tested with Ii,, (typical waveform 101350, SPD tested according to class I should be used for lines entering from a LPZ OA, for example overhead lines);

type II SPD tested with In (typical waveform 8120, SPD tested according to class II may be used for lines entering from a LPZ OB, for example underground cables).

b) Close to the equipment to be protected (at the boundary of LPZ 112 and higher, at secondary distribution boards, or at a socket outlet or local power outlet):

type II SPD tested with In (typical waveform 8120, SPD tested according to class 11);

type III SPD tested with a combination wave (typical current waveform 8120, SPD tested according to class 110. [BS EN 62305-1, E.4 a,?& BS EN 62305-4, C. I ]

BS EN 62305-1 states that 'for direct lightning flashes to connected services, partitioning of the lightning current in both directions of the service and the breakdown of insulation must be taken into account'.

In practice, this means that 50 % of the lightning current is understood to flow into the LPS or earth system, and the other 50 % through the internal services. On a three-phase power


system fed with a TN-C-S configuration, this statement manifests into the relationship between the lightning protection level (LPL) and the maximum current seen by a type I SPD, per mode shown in Table 10.2.

Table 10.2 - Relationship between LPLs and maximum current per mode for type I SPD

In practice these magnitudes are likely to be overstated for the UK. Table 10.3 [BS EN 62305-4, Ta,ble E.21 shows more realistic expected surge overcurrents on power and telecommunications or data lines due to lightning flashes from various sources of damage, defined in Section 2.1 of this guide.

LPL/Class of LPS

I

I I

III/IV

Table 10.3 - Expected surge currents to lines by source of damage

Maximum lightning current in kA at 10/350 ps

200

150

100

In an LPMS using the lightning protection zones concept with more than one LPZ (LPZ 1, LPZ 2 and higher), SPDs shall be located at the line entrance into each LPZ (see Figure 10.2). In an LPMS using LPZ 1 only, SPDs shall be located at the line entrance into LPZ 1 at least. In both cases, additional SPDs may be required if the distance between the location of the SPD and the equipment being protected is long. [BS EN 62305-4, Clause 71 The distances will depend upon the type of system being protected, cable types, earthing arrangements etc. It is important to refer to the SPD manufacturer installation data to ensure that the full SPD specification is satisfied.

Type I SPD Maximum current per mode in kA

at 10/350 ps

25

18.75

12.5

LPL

Ill-IV

1-11

Where more than one LPZ is established, it is important to ensure sufficient distance between SPDs (decoupling). The manufacturer's specifications should be consulted in order to consider what practical measures are needed to take account of these requirements; depending upon the type of equipment and manufacturer it may be that a decoupling element is already built into certain equipment. Where decoupling is not built into equipment, minimum lengths of cables between SPDs of types I, I1 and I11 should be typically 15, 5 and 5 m respectively, depending upon the SPD technology being applied; again, the manufacturer's data should be consulted in this respect.

Low voltage systems

Flash to the service

Source of damage S3 (direct flash) Waveform: 10/350 LLS

(MI

5

10

Telecommunication lines

Flash to the service

Source of damage S3 (direct flash) waveform: 101350 LLS

(kA)

1

2

Flash near the service

Source of damage S4 (indirect flash) Waveform: 8/20 LLS

(MI

2.5

5

Near to, or on the structure

Source of damage S1 or S2 (induced current only for S1) Waveform: 8/20 LLS

(kA) 0.1

0.2

Flash near the service

Source of damage S4 (indirect flash) measured: 51300 LLS

(estimated: 8/20 LLS) (kA) 0.01 (0.05)

0.02(0.1)

Near to, or on the structure

Source of damage S2 (induced current) Waveform: 8/20 ps (kA)

0.05

0.1


BONDING BY DIRECT CONNECTION TO SCREENING I ARMOURING AND BY SPD's TO LIVE CONDUCTORS

Figure 10.4 - Typical siting of coordinated SPDs

10.7 Connections between structures

Where connections between structures comprise metal free or fibre optic cables, these will not transmit lightning currents and so pose no problems in this respect to equipment sited at each end of the lines.

Where metallic conductors link structures, provision for the bonding of the cable shield and cores either directly or by SPDs at the entry to both structures as described in Section 10.3 should be made. This would protect the equipment inside each building connected to the lines, although the external lines themselves will remain unprotected.

For protection of external cable in the ground, refer to Section 5.3.4.5. However, even if the measures in this section are adopted, metal ducts between the structures for the routing of the lines are recommended. [BS EN 62305-4, B. 10.21

Section 11 Inspection, testing and maintenance of LPS and LPMS

11 .I Inspection, testing and maintenance of LPMS

After the original installation of a new LPMS is complete, a visual inspection should be carried out to ensure that the system as installed complies with the design. An inspection log should be completed to confirm that:

the installation complies with the design and the standard;

there are no loose connections;

bonding conductors and shields are properly terminated and installed;

any indicator lights show the SPDs are fully operational;

line routings are appropriate;

safety distances to spatial shields are adequate.

The inspection log should also detail:

any deviations from the design or standard;

the values of any measurements needed on those parts of the bonding network that are not visible;

other issues specific to the installation to enable accurate future inspections.

any electrical testing, which should be confirmed and recorded on Electrical Contractors Association (ECA) or National Inspection Council for Electrical Installation Contractors (NICEIC) test certificates.

This completed inspection log should be provided to the person responsible for the upkeep of the system for inclusion into the building health and safety file for future reference.


Further visual inspections and any necessary measurements of hidden connections should be carried out periodically at frequencies dependent upon regulation, the local environment, the type of protection measures employed and the sensitivity of and consequential losses to the installation, but preferably at the same time as the LPS inspection. [BS EN 62305-4, 8.21 Visual inspections should also be carried out after alteration of the LPMS or if there has been a reported lightning strike directly to or nearby the structure or lines feeding it.

Subsequent inspections should cover the areas of the initial inspection and report on any corrosion that may have occurred, especially at ground level.

After inspection, all defects noted shall be corrected without delay. If necessary, the technical documentation shall be updated. [BS EN 62305-4, 8.31

11.2 Inspection, testing and maintenance of the LPS

Where no specific requirements are identified by the authority having jurisdiction, BS EN 62305-3, Table E.2 defines differing periods between visual and complete inspections. However, these contradict an explicit statement in BS EN 62305-3, E.7.1 which states 'The LPS should be visually inspected at least annually'. The statutory instrument applicable in the UK and published by the authority having jurisdiction is the Electricity at Work Regulations 1989 ( E m ) .

Inspection, testing and maintenance of lightning protection systems in the UK should be carried out at intervals not exceeding 12 months. In addition, an LPS should be fully inspected and tested after receiving a lightning strike. On very small systems with no more than two or three earths, for example church towers, where empirical evidence exists to technically justify low levels of risk and consequential effects, it may be appropriate for the duty holder under the E m to consider and determine different periodicities.

Existing systems designed and installed to the requirements of BS 6651 should be inspected, tested and maintained in accordance with that standard.

Where an LPS is fitted to a structure with a risk of explosion, a visual inspection should be carried out every 6 months and electrical testing at maximum intervals of 12 months. [BS EN 62305-3, E. 7.11

In order to account for any variations in the resistance of the earth system due to seasonal variations, it is recommended that systems are inspected and tested at 11 monthly intervals. Applying this method will ensure that over a twelve-year period, the LPS will have been inspected and tested throughout each month of the year and any detrimental effects over the year due to seasonal variations will have been identified.

Due to the specialist nature of BS EN 62305, inspections and tests should only be carried out by competent experienced personnel.

The client should ensure that they are in possession of completion documentation for the LPS installation. This should be provided to the LPS specialist so that the original design criteria and any previous inspection and maintenance reports are taken into account. This is a particularly important aspect of the inspection process and the building owner should

Inspection, testing and maintenance of LPS and LPMS

have this information available on site as part of the site health and safety folder derived under the Construction, Design and Management Regulations 1994. [BS EN 62305-3, E. 7. I]

Inspection

The objective of inspection is to determine and confirm that:

the LPS conforms to the design based on BS EN 62305;

all components of the LPS are in good condition and capable of performing their designed functions, and there is no corrosion;

any recently added services or structures are incorporated into the LPS.

[BS EN 62305-3, 7. I]

Inspections should be carried out:

during the construction of the structure, especially in order to check the embedded electrodes and associated clamps and bonds;

after installation of the LPS;

periodically at such intervals as determined with regard to the nature of the structure to be protected;

after alterations or repairs, or when it is known that the structure has been struck by lightning.

It is particularly important to check the following during the periodic inspection:

deterioration and corrosion of air termination elements, conductors and connections;

corrosion of earth electrodes;

earthing resistance value for the earth termination system;

condition of connections, equipotential bonding and kings. [BS EN 62305-3, 7.21

Visual inspections should be made to ascertain that:

the design conforms to BS EN 62305 by checking the original technical design documentation where available;

the LPS is in good condition;

there are no visible loose connections or accidental breaks in the LPS conductors and joints;

no accessible part of the system has been weakened by corrosion, especially at ground level;

all visible earth connections are intact;

all visible conductors and system components are fastened to the mounting surfaces, and components which provide mechanical protection are intact and in the right place;

there have not been any additions or alterations to the protected structure which would require additional protection;


there has been no indication of damage to the LPS, or by reference to any indicators to SPDs;

correct equipotential bonding has been established for any new services or additions which have been made to the interior of the structure since the last inspection, and that continuity tests have been performed for these new additions where practically possible;

bonding conductors and connections inside the structure are present and intact where practically possible.

separation distances are maintained, subject to availability of original design information;

bonding conductors, joints, shielding devices, cable routing and SPDs have been checked and tested where practicable and subject to availability of original design information. [BS EN 62305-3, E. 7.2.31

In order to check for obvious defects caused by theft of or vandalism to down-conductors and earths, it may be prudent for the client's representative responsible for a building's lightning protection system to agree with the lightning protection specialist a routine where the client carries out visual inspections of certain key parts of the system at more frequent intervals than the specialist inspections. This will ensure that any remedial action as a result of obvious theft or vandalism is identified and carried out in a timely manner.

Testing

Testing of the LPS should comprise:

performing random continuity tests, especially continuity of those parts of the LPS which were not visible for inspection during the initial installation and are not subsequently available for visual inspection;

conducting earth resistance tests of the earth-termination system. The following isolated and combined earth measurements and checks should be made and the results recorded in an LPS inspection report:

the resistance to earth of each local earth electrode and, where practical, the resistance to earth of the complete earth-termination system. Each local earth electrode should be measured in isolation with the test point between the down- conductor and earth electrode in the disconnected position (isolated measurement). If the resistance to earth of the earth-termination system as a whole exceeds 10 R, additional electrodes should be installed, where practicable, to obtain the requisite resistance value. If there is a significant increase in the value of the earth resistance from previously measured values, additional investigations should be made to determine the reason for the increase and measures taken to improve the situation. For earth electrodes in rocky soil, the requirements of Section 5.3.4.4 should be followed; the 10 R requirement is not applicable in this case.

the results of a visual check of all conductors, bonds and joints or their measured electrical continuity. [BS EN 62305-3, E. 7.2.41

If the earth-termination system does not conform to these requirements, or checking the requirements is not possible because of a lack of information, the earth-termination system


should be improved by installing extra earth electrodes or installing a new earth- termination system. [BS EN 62305-3, E. 7.2.41

The preferred method of testing the resistance of earth electrodes is by the 'fall of potential' (FOP) method. This method has severe practical limitations, however, especially in built-up areas or where access to sufficient areas of open virgin ground is unavailable for the insertion of test electrodes. For these reasons, if the FOP method cannot practically be applied, the 'deadknown earth' method may be used in substitution. This method is suitable only for relatively high resistance earths, such as lightning protection earths, as the result derived is the series resistance of the earth under test and the 'deadknown earth', the resistance of which is not always known. The 'deadknown earth' method is only suitable for use by experienced engineers competent to test using this method and with suitable knowledge of lightning protection systems.

Where a foundation earth is provided, there will be no individual electrodes within the network capable of being tested in isolation. However, the reference electrodes in this case should be tested. Any variation in the resistance of the reference electrode should be considered to be representative of variations likely within the aaacent foundation electrode. Should this assessment raise concerns that the resistance of the foundation electrode may have risen above the 10 R overall requirement, further additional detailed modelling may be required to assess whether additional electrodes are necessary.

Where a type A arrangement is installed, the electrodes should preferably be tested by the FOP method. If, as is very often likely to be the case, the FOP method is impractical, the 'deadknown earth' method may be used. In this case it is useful to utilize the LPS as the 'deadknown earth' source, as this satisfies the requirement for performing certain continuity tests and confirms good continuity, or otherwise, along the downconductor and through the joints onto the air-termination network. It is impractical to test the overall resistance of the earthing system in isolation from all other services. This is due to there being many building services that are likely to be connected to other earths offering a multiplicity of parallel paths, which would corrupt any overall measurement. A mathematical calculation of the overall parallel resistance of the individual earths should be carried out to ensure the overall 10 R requirement is satisfied. It is good engineering practice to ensure that as far as reasonably possible each of the individual electrodes on the system is of a similar resistance. This ensures that in the event of a strike, the current flowing within the system is reasonably equally distributed throughout the earths and so relative large potential differences do not appear at certain parts of the system. To ensure even distribution of the lightning current, it is recommended that any individual earth electrode has a resistance of no more than ten times the number of electrodes within the system. Where the test of the electrode is by the FOP method and thus the continuity of the down-conductor onto the air-termination is not verified, it is good practice and satisfies the need for random testing, to take a measurement of the electrode with the test joint open and a subsequent test with the test joint closed. This second test would verify the continuity through the down-conductor to the rest of the system. In addition, it may be prudent to test for continuity from end to end and side to side on the system.

Where a type B arrangement is installed, the electrodes should preferably be tested by the FOP method. If, as with other cases the FOP method is impractical, the 'deadknown earth' method may be used but in order to ensure that earth resistance and not simply continuity tests are carried out, the type B arrangement should be configured such that, where practically possible, each leg of the ring between each downconductor can be isolated from the rest of the system for testing. This can be achieved by installing earth bars within


inspection pits at positions where each down-conductor joins the ring configuration. At each earth bar, labels should be fitted to identify the various legs of the ring and connections onto it. If each leg of the ring has a resistance of more than ten times the number of electrodes within the system (legs on the ring), additional vertical electrodes can simply be installed within the inspection pit and connected to the earth bar to reduce the resistance of the particular leg to an acceptable level.

If the earth-termination system does not conform to these requirements, or checking the requirements is not possible because of a lack of information, the earth-termination system should be improved by installing extra earth electrodes or installing a new earth- termination system.

Inspection documentation [BS EN 62305-3, E. 7.2.51

The LPS specialist should prepare inspection checklists or guides in a manner that facilitates consistent and comprehensive LPS inspections. These checklists should be designed in such a way to logically guide the inspector through the inspection process so that all areas of importance are documented for immediate consideration and reference at future inspections or tests.

The LPS inspection report should typically contain the following information as a minimum:

general conditions of air-termination conductors, and other air-termination components;

general level of corrosion and the condition of the corrosion protection;

security of attachment of the LPS conductors and components;

security of bonding, taking note of the main equipotential bonding bars and any newly installed plant, equipment or other services. If bonding cannot be identified visually, as a minimum, a continuity test should be carried out between the LPS and an item of electrical plant to identify if a low resistance exists between the two;

earth resistance measurements of individual earth-terminations;

earth resistance measurements of the earth-termination system if practically possible;

any deviation from the requirements of BS EN 62305; this requirement is extensive in nature and it should be recognized that, other than the preceding items, this will normally not form part of an annual inspection but should form part of a separate exercise.

documentation of all changes and extension of the LPS and any changes to the structure;

the results of the tests performed;

method of LPS installation (type of air-termination method, down-conductor type, earthing arrangement etc.; a general description of the system as fitted);

the type and condition of the LPS components;

verification of electrical continuity of the LPS installation;


verification of SPD indicators;

test methods;

proper recording of the test data obtained;

any other comments deemed necessary by the LPS engineer.

In addition, the LPS construction drawings and original LPS design description should be reviewed if available on site. If this information is not made available to the LPS engineer this should be noted on the report and the client should ensure that the information is available for the next inspection.

The client should be provided with a copy of the detailed inspection and test report so that a file can be maintained on site. This file should include the initial LPS design report, which on new buildings should be present within the health and safety file required under the Construction, Design and Management Regulations 1994, together with any previously compiled LPS inspection, test and maintenance reports.

Maintenance

Regular inspections are among the fundamental conditions for reliable maintenance and so effective performance of an LPS. The property owner shall be advised of all identified faults and they shall be repaired without delay.

[BS EN 62305-3, 7.31

The LPS maintenance programme should ensure a continuous updating of the LPS to the current issue of this standard. [BS EN 62305-3, E. 7.31

On completion of any repairs to the system, the client should be advised and the on-site file updated so that remedial action can be considered by the LPS engineer during the next inspection and test. If any repairs involve the earthing system, the individual electrodes repaired or enhanced should be retested and their resistances advised to the client for logging in the on-site file.

Introduction

The application of measures to protect against lightning and its effects are essential, as there are no devices able to change the natural weather phenomena in order to prevent lightning discharges. Lightning striking a structure, directly or nearby, can be hazardous to people, structures and their contents and services.

Initially the client or their engineering professionals should consider which risk they wish to protect against:

R1 - loss of human life;

Rz - loss of service to the public;

R3 - loss of cultural heritage;

R4 - loss of economic value.

The risks and need for protection measures should be determined using the risk assessment methodology in Section 2 of this guide or by means of commercially available software written specifically for the purpose. If using commercially available software, care should be taken to ensure that all indices used within the software are consistent with those in BS EN 62305, as some IEC or European versions for use in other countries may have different indices set by their own respective national authorities. Protection measures according to Sections 3 to 9 of this guide should be applied to reduce the risk of life hazard and physical damage and the risk of failure of electrical and electronic systems within structures as required.

Risk assessment together with the determination and application of measures to reduce the risk of damage to structure, services and life hazard should all be considered in order to comply with the requirements of BS EN 62305.

This guide covers the assessment and reduction of risk below tolerable levels, together with techniques for the protection against lightning and touch and step voltages of:

structures, their services, contents and persons within;

services connected to the structure.

Introduction

Railway systems, vehicles, ships, aircraft, offshore installations, underground high pressure pipelines, piping and telecommunication lines not connected to the structure, distribution stations, storage tanks and pipelines are not covered by this guide. For details on these specialist areas, refer to BS EN 62305 and industry specific standards.

Where appropriate, references to the BS EN 62305 series are made throughout this guide adjacent to the relevant subject matter. This enables easier cross-referencing to the standard if a more detailed review of any subject matter is required.

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IEC Protection Against Lightning

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