BS EN/IEC 62305 Lightning protection General standard ...€¦ · BS EN/IEC 62305-4 is devoted...

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Guide to BS EN/IEC 62305 Introduction

BS EN/IEC 62305Lightning protectionstandardThe BS EN/IEC 62305 Standard for lightning protectionwas originally published in September 2006, tosupercede the previous standard, BS 6651:1999.

For a finite period, BS EN/IEC 62305 and BS 6651 ran in parallel, but as of August 2008, BS 6651 hasbeen withdrawn and now BS EN/IEC 63205 is the recognised standard for lightning protection.

The BS EN/IEC 62305 standard reflects increasedscientific understanding of lightning and its effectsover the last twenty years, and takes stock of thegrowing impact of technology and electronic systemson our daily activities. More complex and exactingthan its predecessor, BS EN/IEC 62305 includes fourdistinct parts - general principles, risk management,physical damage to structures and life hazard, andelectronic systems protection.

These parts to the standard are introduced here. In2010 these parts underwent periodic technical review,with updated parts 1, 3 and 4 released in 2011.Updated part 2 is currently under discussion and isexpected to be published in late 2012.

Key to BS EN/IEC 62305 is that all considerations forlightning protection are driven by a comprehensiveand complex risk assessment and that this assessmentnot only takes into account the structure to beprotected, but also the services to which the structureis connected. In essence, structural lightning protectioncan no longer be considered in isolation, protectionagainst transient overvoltages or electrical surges isintegral to BS EN/IEC 62305.

Structure of BS EN/IEC 62305The BS EN/IEC 62305 series consists of four parts,all of which need to be taken into consideration.These four parts are outlined below:

Part 1: General principles

BS EN/IEC 62305-1 (part 1) is an introduction to theother parts of the standard and essentially describeshow to design a Lightning Protection System (LPS) inaccordance with the accompanying parts of thestandard.

Part 2: Risk management

BS EN/IEC 62305-2 (part 2) risk management approach,does not concentrate so much on the purely physicaldamage to a structure caused by a lightning discharge,but more on the risk of loss of human life, loss ofservice to the public, loss of cultural heritage andeconomic loss.

Part 3: Physical damage to structures and life hazard

BS EN/IEC 62305-3 (part 3) relates directly to the majorpart of BS 6651. It differs from BS 6651 in as much thatthis new part has four Classes or protection levels ofLPS, as opposed to the basic two (ordinary and high-risk) levels in BS 6651.

Part 4: Electrical and electronic systems within structures

BS EN/IEC 62305-4 (part 4) covers the protection ofelectrical and electronic systems housed withinstructures. It embodies what Annex C in BS 6651conveyed, but with a new zonal approach referred toas Lightning Protection Zones (LPZs). It providesinformation for the design, installation, maintenance &testing of a Lightning Electromagnetic Impulse (LEMP)protection system (now referred to as Surge ProtectionMeasures - SPM) for electrical/electronic systems withina structure.

GeneralPrinciples

BS EN/IEC 62305-1

RiskManagementBS EN/IEC 62305-2

Protectionof the

StructureBS EN/IEC62305-3

ElectronicSystemsProtectionBS EN/IEC62305-4

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Key points Guide to BS EN/IEC 62305

BS 6651 standard (withdrawn August 2008) BS EN/IEC 62305 standard

Document structure

118 page document, including 9 pages devoted to risk assessment

Over 470 pages in 4 parts, including over 150 pagesdevoted to risk assessment (BS EN/IEC 62305-2)

Focus on Protection of Structures against Lightning Broader focus on Protection against Lightning includingthe structure and services connected to the structure

Specific tables relating to choice and dimension of LPS components and conductors

Specific tables relating to sizes and types of conductorand earth electrodes. LPS components - specifically related to BS EN 50164/IEC 62561 testing regimes

Annex B - guidance on application of BS 6651 BS EN/IEC 62305-3 Annex E - extensive guidance givenon application of installation techniques complete withillustrations

Annex C - general advice (recommendation) forprotection of electronic equipment with separate riskassessment

BS EN/IEC 62305-4 is devoted entirely to protection ofelectrical and electronic systems within the structure(integral part of standard) and is implemented throughsingle separate risk assessment (BS EN/IEC 62305-2)

Definition of risk

Risk (of death/injury) level set at 1 in 100,000 (1 x 10-5) based on comparable exposures (smoking, traffic accidents, drowning etc)

3 primary risk levels defined (BS EN 62305):R1 loss of human life 1 in 100,000 (1 x 10-5)R2 loss of service to the public 1 in 10,000 (1 x 10

-4)R3 loss of cultural heritage 1 in 10,000 (1 x 10

-4)

Protection measures

Mesh arrangement is promoted as the commonly used means of air termination network

Mesh arrangement, protective angle method, catenarysystem, extensive use of air finials, all form part of or allof air termination network

2 levels of Lightning Protection mesh design:(20 m x 10 m; 10 m x 5 m)

4 sizes of mesh defined according to structural class of Lightning Protection System:Class I 5 m x 5 m Class II 10 m x 10 mClass III 15 m x 15 m Class IV 20 m x 20 m

2 levels of down conductor spacing:20 m & 10 m

4 levels of down conductor spacing dependent onstructural class of Lightning Protection System:Class I 10 m Class II 10 m Class III 15 m Class IV 20 m

Use of bonds promoted to minimise side flashing Extensive sections/explanations provided onequipotential bonding

10 ohm overall earthing requirement, achieved by 10 x number of down conductors

10 ohms overall earthing requirement achieved eitherby Type A arrangement (rods) or Type B arrangement(ring conductor)

Requirement to bond all metallic services, (gas, water,electricity etc) to main earth terminal along with externaldown conductor

Requirement to bond all metallic services to mainequipotential bonding bar. ‘Live’ electrical conductors(e.g. power, data, telecoms) bonded via SurgeProtective Devices (SPDs)

Rolling sphere concept on structures over 20 m tall:20 m sphere used on highly flammable contents/electronic equipment within building60 m sphere all other buildings

4 sizes of rolling sphere concept defined according tostructural class of Lightning Protection System:Class I 20 m Class II 30 m Class III 45 m Class IV 60 m

The following table gives a broad outline as to the key variances between the previous standard, BS 6651, and the BS EN/IEC 62305.

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Guide to BS EN/IEC 62305 BS EN/IEC 62305-1

Damage and lossBS EN/IEC 62305 identifies four main sources ofdamage:

S1 Flashes to the structure

S2 Flashes near to the structure

S3 Flashes to a service

S4 Flashes near to a service

Each source of damage may result in one or more ofthree types of damage:

D1 Injury of living beings due to step and touch voltages

D2 Physical damage (fire, explosion, mechanicaldestruction, chemical release) due to lightningcurrent effects including sparking

D3 Failure of internal systems due to LightningElectromagnetic Impulse (LEMP)

The following types of loss may result from damagedue to lightning:

L1 Loss of human life

L2 Loss of service to the public

L3 Loss of cultural heritage

L4 Loss of economic value

The relationships of all of the above parameters aresummarised in Table 5.

Figure 12 on page 271 depicts the types of damageand loss resulting from lightning.

For a more detailed explanation of the generalprinciples forming part 1 of the BS EN 62305 standard,please refer to our full reference guide ‘A Guide to BS EN 62305.’ Although focused on the BS EN standard, this guide may provide supportinginformation of interest to consultants designing to theIEC equivalent. Please see page 283 for more detailsabout this guide.

Scheme design criteriaThe ideal lightning protection for a structure and itsconnected services would be to enclose the structurewithin an earthed and perfectly conducting metallicshield (box), and in addition provide adequatebonding of any connected services at the entrancepoint into the shield.

This in essence would prevent the penetration of thelightning current and the induced electromagneticfield into the structure.

However, in practice it is not possible or indeed costeffective to go to such lengths.

This standard thus sets out a defined set of lightningcurrent parameters where protection measures,adopted in accordance with its recommendations, willreduce any damage and consequential loss as a resultof a lightning strike. This reduction in damage andconsequential loss is valid provided the lightning strikeparameters fall within defined limits, established asLightning Protection Levels (LPL).

BS EN/IEC 62305-1 General principlesThis opening part of the BS EN/IEC 62305 suite of standards serves as an introduction to the further partsof the standard. It classifies the sources and types of damage to be evaluated and introduces the risks ortypes of loss to be anticipated as a result of lightning activity.

Furthermore, It defines the relationships between damage and loss that form the basis for the riskassessment calculations in part 2 of the standard.

Lightning current parameters are defined. These are used as the basis for the selection andimplementation of the appropriate protection measures detailed in parts 3 and 4 of the standard.

Part 1 of the standard also introduces new concepts for consideration when preparing a lightningprotection scheme, such as Lightning Protection Zones (LPZs) and separation distance.

Point of strike Source ofdamage

Type of damage

Type of loss

Structure S1 D1D2D3

L1, L4**L1, L2, L3, L4L1*, L2, L4

Near a structure

S2 D3 L1*, L2, L4

Serviceconnected tothe structure

S3 D1D2D3

L1, L4**L1, L2, L3, L4L1*, L2, L4

Near a service S4 D3 L1*, L2, L4

* Only for structures with risk of explosion and for hospitals or otherstructures where failures of internal systems immediately endangershuman life.

** Only for properties where animals may be lost.

Table 5: Damage and loss in a structure according to different points oflightning strike (BS EN/IEC 62305-1 Table 2)



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BS EN/IEC 62305-1 Guide to BS EN/IEC 62305

S1 Flash to the structure S3 Flash to a service connectedto the structure

S4 Flash near a service connectedto the structure

Inducedovervoltage

Inducedovervoltage

S2 Flash near tothe structure

LEMP

Lightningcurrent

LEMP

LEMP

LEMP

Injury to people by stepand touch voltages resulting from resistive and inductivecoupling (Source S1)

Fire and/or explosion due to the hotlightning arc itself, due to the resultantohmic heating of conductors, or due to arc erosion ie. melted metal (Source S1)

Injury to people due to touchvoltages inside the structurecaused by lightning currentstransmitted through the connected service (Source S3)

Immediate mechanicaldamage (Source S1)

Failure or malfunction of internalsystems due to overvoltages inducedon connected lines and transmittedto the structure (Source S3 & S4) or by LEMP (Source S1 & S2)

Fire and/or explosion triggered by sparkscaused by overvoltages resulting from resistiveand inductive coupling and to passage of partof the lightning current (Source S1)

Fire and/or explosion triggered by sparks due toovervoltages and lightning currents transmittedthrough the connected service (Source S3)

Figure 12: The types of damage and loss resulting from a lightningstrike on or near a structure

Lightning Protection Levels (LPL)Four protection levels have been determinedbased on parameters obtained from previouslypublished technical papers. Each level has a fixedset of maximum and minimum lightning currentparameters. These parameters are shown inTable 6.

The maximum values have been used in the design ofproducts such as lightning protection components andSurge Protective Devices (SPDs).

The minimum values of lightning current have beenused to derive the rolling sphere radius for each level.

LPL I II III IV

Maximumcurrent (kA)

200 150 100 100

Minimumcurrent (kA)

3 5 10 16

Table 6: Lightning current for each LPL based on 10/350 µs waveform

For a more detailed explanation of LightningProtection Levels and maximum/minimum currentparameters please see the Furse Guide to BS EN 62305.


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Lightning Protection Zones (LPZ)The concept of Lightning Protection Zones (LPZ)was introduced within BS EN/IEC 62305particularly to assist in determining the protectionmeasures required to establish protectionmeasures to counter Lightning ElectromagneticImpulse (LEMP) within a structure.

The general principle is that the equipment requiringprotection should be located in an LPZ whoseelectromagnetic characteristics are compatible with theequipment stress withstand or immunity capability.

The concept caters for external zones, with risk of directlightning stroke (LPZ 0A), or risk of partial lightningcurrent occurring (LPZ 0B), and levels of protectionwithin internal zones (LPZ 1 & LPZ 2).

LPZ 0A

LPZ 0B

LPZ 1

LPZ 2

LPZ 3

LPZ 0B

Air termination network

Direct flash, full lightningcurrent, full magnetic field

No direct flash,partial lightningor induced current,full magnetic field

Equipotential bondingby means of SPD

No direct flash, partiallightning or inducedcurrent, damped magnetic field

No direct flash,induced currents,further dampedmagnetic field

Rolling sphere radius

Down conductornetwork

Earth termination network

SPD 0B/1

SPD 0B/1

SPD 0B/1

SPD 0B/1

Figure 13: The LPZ concept

In general the higher the number of the zone (LPZ 2;LPZ 3 etc) the lower the electromagnetic effectsexpected. Typically, any sensitive electronic equipmentshould be located in higher numbered LPZs and beprotected against LEMP by relevant Surge ProtectionMeasures (‘SPM’ as defined in BS EN 62305:2011).

SPM were previously referred to as a LEMP ProtectionMeasures System (LPMS) in BS EN/IEC 62305:2006.

Figure 13 highlights the LPZ concept as applied to the structure and to SPM. The concept is expandedupon in BS EN/IEC 62305-3 and BS EN/IEC 62305-4.

Selection of the most suitable SPM is made using therisk assessment in accordance with BS EN/IEC 62305-2.



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BS EN/IEC 62305-2 is key to the correctimplementation of BS EN/IEC 62305-3 and BS EN/IEC 62305-4. The assessment andmanagement of risk is now significantly more indepth and extensive than the approach of BS 6651.

BS EN/IEC 62305-2 specifically deals with making a riskassessment, the results of which define the level ofLightning Protection System (LPS) required. While BS 6651 devoted 9 pages (including figures) to thesubject of risk assessment, BS EN/IEC 62305-2 currentlycontains over 150 pages.

The first stage of the risk assessment is to identify which of the four types of loss (as identified in BS EN/IEC 62305-1) the structure and its contents canincur. The ultimate aim of the risk assessment is toquantify and if necessary reduce the relevant primaryrisks i.e.:

R1 risk of loss of human life

R2 risk of loss of service to the public

R3 risk of loss of cultural heritage

R4 risk of loss of economic value

For each of the first three primary risks, a tolerable risk(RT) is set. This data can be sourced in Table 7 of IEC 62305-2 or Table NK.1 of the National Annex of BS EN 62305-2.

Each primary risk (Rn) is determined through a longseries of calculations as defined within the standard. Ifthe actual risk (Rn) is less than or equal to the tolerablerisk (RT), then no protection measures are needed. Ifthe actual risk (Rn) is greater than its correspondingtolerable risk (RT), then protection measures must beinstigated. The above process is repeated (using newvalues that relate to the chosen protection measures)until Rn is less than or equal to its corresponding RT.

It is this iterative process as shown in Figure 14 thatdecides the choice or indeed Lightning Protection Level(LPL) of Lightning Protection System (LPS) and SurgeProtective Measures (SPM) to counter LightningElectromagnetic impulse (LEMP).

StrikeRisk risk managementsoftwareAn invaluable tool for those involved in undertakingthe complex risk assessment calculations required by BS EN 62305-2, StrikeRisk facilitates the assessment ofrisk of loss due to lightning strikes and transientovervoltages caused by lightning.

Quick & easy to use, with full reporting capability,StrikeRisk automates risk assessment calculations anddelivers results in minutes, rather than the hours ordays it would take to do the same calculations by hand.

Contact Furse for more details about StrikeRisk.

BS EN/IEC 62305-2 Risk management

NO

YES

NO

YYYYYYYYYEYES

Rn < RT

Figure 14: Procedure for deciding the need for protection (BS EN/IEC 62305-1 Figure 1)


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This part of the suite of standards deals withprotection measures in and around a structureand as such relates directly to the major part ofBS 6651.

The main body of this part of the standard givesguidance on the design of an external LightningProtection System (LPS), internal LPS and maintenanceand inspection programmes.

Lightning Protection System (LPS)BS EN/IEC 62305-1 has defined four LightningProtection Levels (LPLs) based on probable minimumand maximum lightning currents. These LPLs equatedirectly to classes of Lightning Protection System (LPS).

The correlation between the four levels of LPL and LPSis identified in Table 7. In essence, the greater the LPL,the higher class of LPS is required.

The class of LPS to be installed is governed by theresult of the risk assessment calculation highlighted in BS EN/IEC 62305-2.

External LPS design considerationsThe lightning protection designer must initiallyconsider the thermal and explosive effects caused atthe point of a lightning strike and the consequences tothe structure under consideration. Depending uponthe consequences the designer may choose either ofthe following types of external LPS:

l Isolated

l Non-isolated

An Isolated LPS is typically chosen when the structure isconstructed of combustible materials or presents a riskof explosion.

Conversely a non-isolated system may be fitted whereno such danger exists.

An external LPS consists of:

l Air termination system

l Down conductor system

l Earth termination system

These individual elements of an LPS should beconnected together using appropriate lightningprotection components (LPC) complying (in the case ofBS EN 62305) with BS EN 50164 series (note this BS ENseries is due to be superceded by the BS EN/IEC 62561series). This will ensure that in the event of a lightning current discharge to the structure, the correct designand choice of components will minimize any potential damage.

Air termination systemThe role of an air termination system is to capture thelightning discharge current and dissipate it harmlesslyto earth via the down conductor and earth terminationsystem. Therefore it is vitally important to use acorrectly designed air termination system.

BS EN/IEC 62305-3 advocates the following, in anycombination, for the design of the air termination:

l Air rods (or finials) whether they are free standingmasts or linked with conductors to form a mesh onthe roof

l Catenary (or suspended) conductors, whether theyare supported by free standing masts or linkedwith conductors to form a mesh on the roof

l Meshed conductor network that may lie in directcontact with the roof or be suspended above it (inthe event that it is of paramount importance thatthe roof is not exposed to a direct lightningdischarge)

The standard makes it quite clear that all types of airtermination systems that are used shall meet thepositioning requirements laid down in the body of thestandard. It highlights that the air terminationcomponents should be installed on corners, exposedpoints and edges of the structure.

The three basic methods recommended fordetermining the position of the air terminationsystems are:

l The rolling sphere method

l The protective angle method

l The mesh method

These methods are detailed over the following pages.

BS EN/IEC 62305-3 Physical damageto structures and life hazard

LPL Class of LPS

I I

II II

III III

IV IV

Table 7: Relation between Lightning Protection Level (LPL) and Class of LPS (BS EN/IEC 62305-3 Table 1)



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The rolling sphere methodThe rolling sphere method is a simple means ofidentifying areas of a structure that need protection,taking into account the possibility of side strikes to thestructure. The basic concept of applying the rollingsphere to a structure is illustrated in Figure 15.

The rolling sphere method was used in BS 6651, theonly difference being that in BS EN/IEC 62305 there aredifferent radii of the rolling sphere that correspond tothe relevant class of LPS (see Table 8).

This method is suitable for defining zones of protection for all types of structures, particularly those of complex geometry.

The protective angle methodThe protective angle method is a mathematicalsimplification of the rolling sphere method. Theprotective angle (a) is the angle created between thetip (A) of the vertical rod and a line projected down tothe surface on which the rod sits (see Figure 16).

The protective angle afforded by an air rod is clearly athree dimensional concept whereby the rod is assigneda cone of protection by sweeping the line AC at theangle of protection a full 360º around the air rod.

The protective angle differs with varying height of the air rod and class of LPS. The protective angleafforded by an air rod is determined from Table 2 of BS EN/IEC 62305-3 (see Figure 17).

Varying the protection angle is a change to the simple45º zone of protection afforded in most cases in BS 6651. Furthermore the new standard uses theheight of the air termination system above thereference plane, whether that be ground or roof level(See Figure 18).

The protective angle method is suitable for simpleshaped buildings. However this method is only valid upto a height equal to the rolling sphere radius of theappropriate LPL.

Class of LPS Rolling sphere radius(m)

I 20

II 30

III 45

IV 60

Table 8: Maximum values of rolling sphere radius corresponding to the Class of LPS

Figure 15: Application of the rolling sphere method

Rollingsphereradius

Air terminationrequired

Tip of air termination

Reference plane

Protectiveangle

Radius of protected area

Height of an airtermination rodabove the referenceplane of the areato be protected

h

A

C

Figure 16: The protective angle method for a single air rod

h

h2

h1 21

Figure 18: Effect of the height of the reference plane on the protection angle

Figure 17: Determination of the protective angle (BS EN/IEC 62305-3 Table 2)

0 0

2 10 20 30 h(m)

I II III Class of LPS

IV

40 50 60

10

20

30

40

50

60

70

80˚

Note 1 Not applicable beyond the values marked with Only rolling sphere and mesh methods apply in these cases

Note 2 h is the height of air-termination above the reference plane of the area to be protected

Note 3 The angle will not change for values of h below 2m

I II III

IV

50 60

1


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Non-conventional air termination systemsA lot of technical (and commercial) debate has ragedover the years regarding the validity of the claimsmade by the proponents of such systems.

This topic was discussed extensively within the technical working groups that compiled BS EN/IEC 62305. The outcome was to remain with the information housed within this standard.

BS EN/IEC 62305 states unequivocally that the volumeor zone of protection afforded by the air terminationsystem (e.g. air rod) shall be determined only by thereal physical dimension of the air termination system.

This statement is reinforced within the 2011 version ofBS EN 62305, by being incorporated in the body of thestandard, rather than forming part of an Annex(Annex A of BS EN/IEC 62305-3:2006).

Typically if the air rod is 5 m tall then the only claim forthe zone of protection afforded by this air rod wouldbe based on 5 m and the relevant class of LPS and notany enhanced dimension claimed by some non-conventional air rods.

There is no other standard being contemplated to runin parallel with this standard BS EN/IEC 62305.

Natural componentsWhen metallic roofs are being considered as a naturalair termination arrangement, then BS 6651 gaveguidance on the minimum thickness and type ofmaterial under consideration.

BS EN/IEC 62305-3 gives similar guidance as well asadditional information if the roof has to be considered puncture proof from a lightning discharge(see Table 10).

The mesh methodThis is the method that was most commonly usedunder the recommendations of BS 6651. Again, withinBS EN/IEC 62305 four different air termination meshsizes are defined and correspond to the relevant classof LPS (see Table 9).

This method is suitable where plain surfaces requireprotection if the following conditions are met:

l Air termination conductors must be positioned atroof edges, on roof overhangs and on the ridges ofroof with a pitch in excess of 1 in 10 (5.7º)

l No metal installation protrudes above the airtermination system

Modern research on lightning inflicted damage hasshown that the edges and corners of roofs are mostsusceptible to damage.

So on all structures particularly with flat roofs,perimeter conductors should be installed as close tothe outer edges of the roof as is practicable.

As in BS 6651, the current standard permits the use ofconductors (whether they be fortuitous metalwork ordedicated LP conductors) under the roof. Vertical airrods (finials) or strike plates should be mounted abovethe roof and connected to the conductor systembeneath. The air rods should be spaced not more than10 m apart and if strike plates are used as analternative, these should be strategically placed overthe roof area not more than 5 m apart.

Class of LPS Mesh size(m)

I 5 x 5

II 10 x 10

III 15 x 15

IV 20 x 20

Table 9: Maximum values of mesh size corresponding to the Class of LPS

Figure 19: Concealed air termination network

Concealed conductor

Vertical airtermination

Down conductor

Class of LPS MaterialThickness(1)

t (mm)Thickness(2)

t’ (mm)

I to IV

Lead - 2.0

Steel (stainless,galvanized)

4 0.5

Titanium 4 0.5

Copper 5 0.5

Aluminium 7 0.65

Zinc - 0.7

Table 10: Minimum thickness of metal sheets or metal pipes in airtermination systems (BS EN/IEC 62305-3 Table 3)

(1) Thickness t prevents puncture, hot spot or ignition.(2) Thickness t’ only for metal sheets if it is not important to prevent

puncture, hot spot or ignition problems.



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can be decided at the early construction stage by usingdedicated reinforcing bars or alternatively to run adedicated copper conductor from the top of thestructure to the foundation prior to the pouring of theconcrete. This dedicated copper conductor should bebonded to the adjoining/adjacent reinforcing barsperiodically.

If there is doubt as to the route and continuity of thereinforcing bars within existing structures then anexternal down conductor system should be installed.These should ideally be bonded into the reinforcingnetwork of the structures at the top and bottom of the structure.

Down conductorsDown conductors should within the bounds ofpractical constraints take the most direct route fromthe air termination system to the earth terminationsystem. The greater the number of down conductorsthe better the lightning current is shared betweenthem. This is enhanced further by equipotentialbonding to the conductive parts of the structure.

Lateral connections sometimes referred to as coronalbands or ring conductors provided either by fortuitousmetalwork or external conductors at regular intervalsare also encouraged. The down conductor spacingshould correspond with the relevant class of LPS (seeTable 11).

There should always be a minimum of two downconductors distributed around the perimeter of thestructure. Down conductors should wherever possiblebe installed at each exposed corner of the structure asresearch has shown these to carry the major part of thelightning current.

Natural componentsBS EN/IEC 62305, like BS 6651, encourages the use offortuitous metal parts on or within the structure to beincorporated into the LPS.

Where BS 6651 encouraged an electrical continuity whenusing reinforcing bars located in concrete structures, sotoo does BS EN/IEC 62305-3. Additionally, it states thatreinforcing bars are welded, clamped with suitableconnection components or overlapped a minimum of 20times the rebar diameter. This is to ensure that thosereinforcing bars likely to carry lightning currents havesecure connections from one length to the next.

When internal reinforcing bars are required to beconnected to external down conductors or earthingnetwork either of the arrangements shown in Figure 20 is suitable. If the connection from thebonding conductor to the rebar is to be encased inconcrete then the standard recommends that twoclamps are used, one connected to one length of rebarand the other to a different length of rebar. The jointsshould then be encased by a moisture inhibitingcompound such as Denso tape.

If the reinforcing bars (or structural steel frames) are tobe used as down conductors then electrical continuityshould be ascertained from the air termination systemto the earthing system. For new build structures this

Class of LPS Typical distances (m)

I 10

II 10

III 15

IV 20

Table 11: Typical values of the distance between down conductors according to the Class of LPS (BS EN/IEC 62305-3 Table 4)

Figure 20: Typical methods of bonding to steel reinforcement within concrete

Stranded copper cable(70 mm2 PVC insulated)

Cast innon-ferrousbondingpoint

Bonding conductor

Clamped cable to rebarconnection

Steel reinforcement withinconcrete (rebar)


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Earth termination systemThe earth termination system is vital for the dispersion of lightning current safely and effectivelyinto the ground.

In line with BS 6651, the new standard recommends asingle integrated earth termination system for astructure, combining lightning protection, power andtelecommunication systems. The agreement of theoperating authority or owner of the relevant systemsshould be obtained prior to any bonding taking place.

A good earth connection should possess the followingcharacteristics:

l Low electrical resistance between the electrodeand the earth. The lower the earth electroderesistance the more likely the lightning current willchoose to flow down that path in preference toany other, allowing the current to be conductedsafely to and dissipated in the earth

l Good corrosion resistance. The choice of materialfor the earth electrode and its connections is ofvital importance. It will be buried in soil for manyyears so has to be totally dependable

The standard advocates a low earthing resistancerequirement and points out that it can be achievedwith an overall earth termination system of 10 ohms or less.

Three basic earth electrode arrangements are used.

l Type A arrangement

l Type B arrangement

l Foundation earth electrodes

Type A arrangementThis consists of horizontal or vertical earth electrodes,connected to each down conductor fixed on theoutside of the structure. This is in essence the earthingsystem used in BS 6651, where each down conductorhas an earth electrode (rod) connected to it.

Type B arrangementThis arrangement is essentially a fully connected ringearth electrode that is sited around the periphery ofthe structure and is in contact with the surroundingsoil for a minimum 80% of its total length (i.e. 20% of its overall length may be housed in say thebasement of the structure and not in direct contactwith the earth).

Foundation earth electrodesThis is essentially a type B earthing arrangement. Itcomprises conductors that are installed in the concretefoundation of the structure. If any additional lengthsof electrodes are required they need to meet the samecriteria as those for type B arrangement. Foundationearth electrodes can be used to augment the steelreinforcing foundation mesh.

Separation (isolation) distance ofthe external LPSA separation distance (i.e. the electrical insulation)between the external LPS and the structural metalparts is essentially required. This will minimise anychance of partial lightning current being introducedinternally in the structure.

This can be achieved by placing lightning conductorssufficiently far away from any conductive parts thathave routes leading into the structure. So, if thelightning discharge strikes the lightning conductor, itcannot `bridge the gap’ and flash over to the adjacent metalwork.

A sample of Furse high quality earthing components.

BS EN/IEC 62305 recommends a singleintegrated earth termination system for astructure, combining lightning protection,power and telecommunication systems.



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Internal LPS design considerationsThe fundamental role of the internal LPS is to ensurethe avoidance of dangerous sparking occurring withinthe structure to be protected. This could be due,following a lightning discharge, to lightning currentflowing in the external LPS or indeed other conductiveparts of the structure and attempting to flash or sparkover to internal metallic installations.

Carrying out appropriate equipotential bondingmeasures or ensuring there is a sufficient electricalinsulation distance between the metallic parts canavoid dangerous sparking between different metallic parts.

Lightning equipotential bondingEquipotential bonding is simply the electricalinterconnection of all appropriate metallicinstallations/parts, such that in the event of lightningcurrents flowing, no metallic part is at a differentvoltage potential with respect to one another. If themetallic parts are essentially at the same potential thenthe risk of sparking or flashover is nullified.

This electrical interconnection can be achieved bynatural/fortuitous bonding or by using specificbonding conductors that are sized according to Tables8 and 9 of BS EN/IEC 62305-3.

Bonding can also be accomplished by the use of surgeprotective devices (SPDs) where the direct connectionwith bonding conductors is not suitable.

Figure 21 (which is based on BS EN/IEC 62305-3 figE.43) shows a typical example of an equipotentialbonding arrangement. The gas, water and centralheating system are all bonded directly to theequipotential bonding bar located inside but close toan outer wall near ground level. The power cable isbonded via a suitable SPD, upstream from the electricmeter, to the equipotential bonding bar. This bondingbar should be located close to the main distributionboard (MDB) and also closely connected to the earthtermination system with short length conductors. Inlarger or extended structures several bonding bars maybe required but they should all be interconnected witheach other.

The screen of any antenna cable along with anyshielded power supply to electronic appliances beingrouted into the structure should also be bonded at theequipotential bar.

Further guidance relating to equipotential bonding,meshed interconnection earthing systems and SPDselection can be found in the Furse guidebook.

Figure 21: Example of main equipotential bonding

Equipotentialbonding bar

Structural lightning protection system

Central heating system

Screen of antenna cable

Electronic appliances

Power from utility

Meter

Meter

Gas

Water

Electricitymeter

Consumer unit/fuseboard

SPD

ON

OFF

Neutral bar

Live bar

N


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Electronic systems now pervade almost every aspect ofour lives, from the work environment, through fillingthe car with petrol and even shopping at the localsupermarket. As a society, we are now heavily relianton the continuous and efficient running of suchsystems. The use of computers, electronic processcontrols and telecommunications has exploded duringthe last two decades. Not only are there more systemsin existence, the physical size of the electronicsinvolved has reduced considerably (smaller size meansless energy required to damage circuits).

BS EN/IEC 62305 accepts that we now live in theelectronic age, making LEMP (LightningElectromagnetic Impulse) protection for electronic andelectrical systems integral to the standard through part 4. LEMP is the term given to the overallelectromagnetic effects of lightning, includingconducted surges (transient overvoltages and currents)and radiated electromagnetic field effects.

LEMP damage is so prevalent such that it is identifiedas one of the specific types (D3) to be protectedagainst and that LEMP damage can occur from ALLstrike points to the structure or connected services -direct or indirect - for further reference to the types ofdamage caused by lightning see Table 5 on page 270. This extended approach also takes intoaccount the danger of fire or explosion associated withservices connected to the structure, e.g. power,telecoms and other metallic lines.

Lightning is not the only threat…Transient overvoltages caused by electrical switchingevents are very common and can be a source ofconsiderable interference. Current flowing through a

conductor creates a magnetic field in which energy isstored. When the current is interrupted or switchedoff, the energy in the magnetic field is suddenlyreleased. In an attempt to dissipate itself it becomes ahigh voltage transient.

The more stored energy, the larger the resultingtransient. Higher currents and longer lengths ofconductor both contribute to more energy stored and also released!

This is why inductive loads such as motors,transformers and electrical drives are all commoncauses of switching transients.

Significance of BS EN/IEC 62305-4Previously transient overvoltage or surge protectionwas included as an advisory annex in the BS 6651standard, with a separate risk assessment. As a resultprotection was often fitted after equipment damagewas suffered, often through obligation to insurancecompanies. However, the single risk assessment inBS EN/IEC 62305 dictates whether structural and/orLEMP protection is required hence structural lightningprotection cannot now be considered in isolation fromtransient overvoltage protection - known as SurgeProtective Devices (SPDs) within this new standard. Thisin itself is a significant deviation from that of BS 6651.

Motors create switching events

BS EN/IEC 62305-4 Electrical andelectronic systems within structures



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Indeed, as per BS EN/IEC 62305-3, an LPS system can nolonger be fitted without lightning current orequipotential bonding SPDs to incoming metallicservices that have “live cores” - such as power andtelecoms cables - which cannot be directly bonded to earth. Such SPDs are required to protect against the risk of loss of human life by preventing dangerous sparking that could present fire or electricshock hazards.

Lightning current or equipotential bonding SPDs arealso used on overhead service lines feeding thestructure that are at risk from a direct strike. However,the use of these SPDs alone “provides no effectiveprotection against failure of sensitive electrical orelectronic systems”, to quote BS EN/IEC 62305 part 4,which is specifically dedicated to the protection ofelectrical and electronic systems within structures.

Lightning current SPDs form one part of a coordinatedset of SPDs that include overvoltage SPDs - which areneeded in total to effectively protect sensitiveelectrical and electronic systems from both lightningand switching transients.

Lightning Protection Zones (LPZs)Whilst BS 6651 recognised a concept of zoning in Annex C (Location Categories A, B and C), BS EN/IEC 62305-4 defines the concept of LightningProtection Zones (LPZs). Figure 22 illustrates the basicLPZ concept defined by protection measures againstLEMP as detailed within part 4.

Boundary of LPZ 2(shielded room)

Boundaryof LPZ 1(LPS)

Antenna

Electricalpower line

Water pipe

Gas pipe

Telecomsline

Mast orrailing

LPZ 2

B

B

B

B

LPZ 1

Criticalequipment

Equipment

SPD 1/2 - Overvoltage protection

Connected service directly bonded

SPD 0/1 - Lightning current protection

Equipment

LPZ 0

Figure 22: Basic LPZ concept - BS EN/IEC 62305-4

Within a structure a series of LPZs are created to have,or identified as already having, successively lessexposure to the effects of lightning.

Successive zones use a combination of bonding,shielding and coordinated SPDs to achieve a significantreduction in LEMP severity, from conducted surgecurrents and transient overvoltages, as well as radiatedmagnetic field effects. Designers coordinate theselevels so that the more sensitive equipment is sited inthe more protected zones.

The LPZs can be split into two categories - 2 externalzones (LPZ 0A, LPZ 0B) and usually 2 internal zones (LPZ 1, 2) although further zones can be introduced fora further reduction of the electromagnetic field andlightning current if required.

External zonesLPZ 0A is the area subject to direct lightning strokesand therefore may have to carry up to the fulllightning current.

This is typically the roof area of a structure. The fullelectromagnetic field occurs here.

LPZ 0B is the area not subject to direct lightning strokesand is typically the sidewalls of a structure.

However the full electromagnetic field still occurs hereand conducted partial lightning currents and switchingsurges can occur here.

Internal zonesLPZ 1 is the internal area that is subject to partiallightning currents. The conducted lightning currentsand/or switching surges are reduced compared withthe external zones LPZ 0A, LPZ 0B.

This is typically the area where services enter thestructure or where the main power switchboard islocated.

LPZ 2 is an internal area that is further located insidethe structure where the remnants of lightning impulsecurrents and/or switching surges are reduced comparedwith LPZ 1.

This is typically a screened room or, for mains power, atthe sub-distribution board area.

Protection levels within a zone must be coordinatedwith the immunity characteristics of the equipment tobe protected, i.e., the more sensitive the equipment,the more protected the zone required.

The existing fabric and layout of a building may makereadily apparent zones, or LPZ techniques may have tobe applied to create the required zones.


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Surge Protection Measures (SPM)Some areas of a structure, such as a screened room, arenaturally better protected from lightning than othersand it is possible to extend the more protected zonesby careful design of the LPS, earth bonding of metallicservices such as water and gas, and cabling techniques.However it is the correct installation of coordinatedSurge Protective Devices (SPDs) that protect equipmentfrom damage as well as ensuring continuity of itsoperation - critical for eliminating downtime. Thesemeasures in total are referred to as Surge ProtectionMeasures (SPM) (formerly LEMP Protection MeasuresSystem (LPMS)).

When applying bonding, shielding and SPDs, technicalexcellence must be balanced with economic necessity.For new builds, bonding and screening measures canbe integrally designed to form part of the completeSPM. However, for an existing structure, retrofitting aset of coordinated SPDs is likely to be the easiest andmost cost-effective solution.

Coordinated SPDsBS EN/IEC 62305-4 emphasises the use of coordinatedSPDs for the protection of equipment within theirenvironment. This simply means a series of SPDs whoselocations and LEMP handling attributes arecoordinated in such a way as to protect the equipmentin their environment by reducing the LEMP effects to asafe level. So there may be a heavy duty lightningcurrent SPD at the service entrance to handle themajority of the surge energy (partial lightning currentfrom an LPS and/or overhead lines) with the respectivetransient overvoltage controlled to safe levels bycoordinated plus downstream overvoltage SPDs toprotect terminal equipment including potentialdamage by switching sources, e.g. large inductivemotors. Appropriate SPDs should be fitted whereverservices cross from one LPZ to another.

Coordinated SPDs have to effectively operate togetheras a cascaded system to protect equipment in theirenvironment. For example the lightning current SPD atthe service entrance should handle the majority ofsurge energy, sufficiently relieving the downstreamovervoltage SPDs to control the overvoltage.

Poor coordination could mean that the overvoltageSPDs are subject to too much surge energy puttingboth itself and potentially equipment at risk fromdamage.

Furthermore, voltage protection levels or let-throughvoltages of installed SPDs must be coordinated withthe insulating withstand voltage of the parts of theinstallation and the immunity withstand voltage ofelectronic equipment.

Enhanced SPDsWhilst outright damage to equipment is not desirable,the need to minimize downtime as a result of loss ofoperation or malfunction of equipment can also becritical. This is particularly important for industries thatserve the public, be they hospitals, financial institutions,manufacturing plants or commercial businesses, wherethe inability to provide their service due to the loss ofoperation of equipment would result in significanthealth and safety and/or financial consequences.

Standard SPDs may only protect against common modesurges (between live conductors and earth), providingeffective protection against outright damage but notagainst downtime due to system disruption.

BS EN 62305 therefore considers the use of enhancedSPDs (SPD*) that further reduce the risk of damage andmalfunction to critical equipment where continuousoperation is required. Installers will therefore need tobe much more aware of the application andinstallation requirements of SPDs than perhaps theymay have been previously.

Superior or enhanced SPDs provide lower (better) let-through voltage protection against surges in bothcommon mode and differential mode (between liveconductors) and therefore also provide additionalprotection over bonding and shielding measures.

Such enhanced SPDs can even offer up to mains Type1+2+3 or data/telecom Test Cat D+C+B protectionwithin one unit. As terminal equipment, e.g.computers, tends to be more vulnerable to differentialmode surges, this additional protection can be a vitalconsideration.

Appropriate SPDsshould be fittedwherever servicescross from one LPZ

to another



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ConclusionLightning poses a clear threat to a structure but agrowing threat to the systems within the structure dueto the increased use and reliance of electrical andelectronic equipment. The BS EN/IEC 62305 series ofstandards clearly acknowledge this. Structurallightning protection can no longer be in isolation from transient overvoltage or surge protection ofequipment. The use of enhanced SPDs provides apractical cost-effective means of protection allowingcontinuous operation of critical systems during LEMP activity.

All Furse SPDs offerenhanced SPD

performance withindustry leading low let-through voltage

Furthermore, the capacity to protect against commonand differential mode surges permits equipment toremain in continued operation during surge activity -offering considerable benefit to commercial, industrialand public service organisations alike.

All Furse SPDs offer enhanced SPD performance with industry leading low let-through voltages(voltage protection level, Up), as this is the best choiceto achieve cost-effective, maintenance-free repeatedprotection in addition to preventing costly systemdowntime. Low let-through voltage protection in allcommon and differential modes means fewer units arerequired to provide protection, which saves on unitand installation costs, as well as installation time.

A Guide to BS EN 62305 Protection Against Lightning

Further to this summary on BS EN/IEC 62305, we have available a comprehensive guide to the BS EN 62305 standard for those interested inlearning more about the new developmentsgoverning lightning protection design andinstallation. This A4 Guide helps to explain in clearterms the requirements of BS EN 62305. Followingthe 4 sections of the standard (Part 1 - Generalprinciples; Part 2 - Risk management; Part 3 - Physicaldamage to structures and life hazard; and Part 4 -Electrical and electronic systems within structures)the Guide provides the information necessary toenable the reader to identify all risks and calculatethe required level of protection in accordance withBS EN 62305.

To request your free of charge copy - contact usdirectly at any of the addresses given on the backcover or visit www.furse.com.

A Guide to BS EN 62305:2006

Protection Against Lightning2nd edition


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