Protection Against Lightning - A UK Guide to the Practical Application of BS en 62305-BSI Standards Ltd. (2007)

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

Protection Against Lightning - A UK Guide to the Practical Application of BS en 62305-BSI Standards Ltd. (2007)

Documents

protection measures

design of protection

lightning section

basic protection measures

protection measures

r4 risk assessment stage

zones section

risk of explosion