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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 partof BS 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.

Diagrams and tables have been included throughoutthe document to aid the readers understanding.

Lightning Protection System (LPS)

BS EN 62305-1 has defined four Lightning ProtectionLevels (LPLs) based on probable minimum andmaximum 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 3. In essence, the greater theLightning Protection Level, the higher class ofLightning Protection System is required.

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

BS EN 62305 defines, through its four LightningProtection Levels, the suitable protection measures tobe applied to mitigate risk. Level I affords thegreatest level of protection and, therefore thegreatest expense, and level IV the least. Throughapplication of protection measures as recommened bythe specific LPL against risk, and reworking of the riskassessment calculations required by BS 62305-2, thedesigner is able to establish the most cost effectiveand acceptable level to protect against risk. Naturally,such a need to rework the risk assessment for best fitcan lead to reasonably long-winded and timeconsuming calculations. Furse has developed our

StrikeRisk software (see previous page) to automateand remove this hassle from risk assessment.

External LPS design considerations

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

Isolated

Non-isolatedAn Isolated LPS is typically chosen when the structureis constructed of combustible materials or presents arisk of explosion.

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

An external LPS consists of:

Air termination system

Down conductor system

Earth termination system

These individual elements of an LPS should be

connected together using appropriate lightningprotection components (LPC) complying withBS EN 50164 series. This will ensure that in the eventof a lightning current discharge to the structure, thecorrect design and choice of components will minimiseany potential damage.

Furse components are fully tested by independenttesting facilities and comply with the requirements ofthe BS EN 50164 series of standards for themanufacture of lightning protection components.

BS EN 62305 part 3 | Introduction

BS EN 62305-3 Physical damage

to structures and life hazard

BS EN 62305 part 3

LPL Class of LPS

I I

II II

III III

IV IV

Table 3: Relation between Lightning Protection Level (LPL)

and Class of LPS (BS EN 62305-3 Table 1)

Furse components are fully tested to BS EN 50164.

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Air termination system

The role of an air termination system is to capture thelightning discharge current and dissipate it harmlessly

to earth via the down conductor and earthtermination system. Therefore it is vitally important touse a correctly designed air termination system.

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

Air rods (or finials) whether they are free standingmasts or linked with conductors to form a meshon the roof (See Figure 4)

Catenary (or suspended) conductors, whether theyare supported by free standing masts or linkedwith conductors to form a mesh on the roof (SeeFigure 5)

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) (See Figure 6)

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:

The rolling sphere method

The protective angle method

The mesh method

Each of these positioning and protection methods willbe briefly described in the following sections.

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 7.

BS EN 62305 part 3

Introduction | BS EN 62305 part 3

Class of LPS Rolling sphere radius

(m)

I 20

II 30

III 45

IV 60

Table 4: Maximum values of rolling sphere radius corresponding

to the Class of LPS

Figure 7: Application of the rolling sphere method

Rollingsphereradius

Air terminationrequired

The rolling sphere method was used in BS 6651, theonly difference being that in BS EN 62305 there are

different radii of the rolling sphere that correspond tothe relevant class of LPS (see Table 4).

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

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The protective angle method is suitable for simple

shaped buildings. However this method is only validup to a height equal to the rolling sphere radius ofthe appropriate LPL.

Air terminationcomponents

Furse offer a full range of highquality air terminationcomponents to suit your needs.For details about the Furserange, seeflattaptape), 53 (solid circular) &59 60 (cable & wire).

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The protective angle method

The protective angle method is a mathematicalsimplification of the rolling sphere method.

The protective angle () is the angle created betweenthe tip (A) of the vertical rod and a line projecteddown to the surface on which the rod sits (seeFigure 8).

The protective angle afforded by an air rod is clearly athree dimensional concept. Therefore a simple air rodis assigned a cone of protection by sweeping the lineAC at the angle of protection a full 360 around theair rod.

The protective angle differs with varying height of theair rod and class of LPS. The protective angle affordedby an air rod is determined from Table 2 ofBS EN 62305-3 (see Figure 10).

BS EN 62305 part 3 | Introduction

Tip of air termination

Referenceplane

Protectiveangle

Radius ofprotected area

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

h

A

C

Figure 8: The protective angle method for a single air rod

h

h2

h1 21

Figure 11: Effect of the height of the reference plane on the

protection angle

Figure 9: Application of the protection angle method to air rods in a

non-isolated system

Figure 10: Determination of the protective angle

(BS EN 62305-3 Table 2)

0

0 2 10 20 30

h(m)

I II IIIClass of LPS

IV

40 50 60

10

20

30

40

50

60

70

80

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

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

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

Figure 9 shows how a series of air rods in position canprovide complete coverage and effective protectionagainst lightning strikes.

Varying the protection angle is a change to the simple45 zone of protection afforded in most cases inBS 6651. Furthermore the new standard uses theheight of the air termination system above thereference plane, whether that be ground or roof level(See Figure 11).

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As in BS 6651, this 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 than10m apart and if strike plates are used as analternative, these should be strategically placed overthe roof area not more than 5m apart.

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The mesh method

This is the method that is most commonly used underthe recommendations of BS 6651. Again, for the new

standard four different air termination mesh sizes aredefined and correspond to the relevant class of LPS(see Table 5).

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

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

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.

BS EN 62305 part 3

Introduction | BS EN 62305 part 3

Class of LPS Mesh size

(m)

I 5 x 5

II 10 x 10

III 15 x 15

IV 20 x 20

Table 5: Maximum values of mesh size corresponding to

the Class of LPS

Figure 12 shows clearly the zone of protection provided by a

mesh conductor network appropriately installed above a

flat roofed structure.

Figure 13: Concealed air termination network

Vertical airterminationor strike plate Horizontal

conductor

Cross section of roof ridge

Roof pitch

Concealedconductor

Vertical airtermination

Down conductor

Air termination conductors& fixings

For details of the Furse rangeof air termination conductors,seeconductors.

For details of the Furse rangeof air termination fixings andconnection components, see

lightning protection.54 58 (solid circular) &61 62 (cable & wire).

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Down conductors

Down conductors should within the bounds ofpractical constraints take the most direct route from

the 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 intervalsis also encouraged. The down conductor/ringconductor spacing should correspond with therelevant class of LPS (see Table 7).

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 as

research has shown these to carry the major part ofthe lightning current.

Non-conventional airtermination systems

A lot of technical (and commercial) debate has raged

over the years regarding the validity of the claimsmade by the proponents of such systems. This topicwas discussed extensively within the technical workinggroups that compiled BS EN 62305. The outcome wasto remain with the information housed within thisstandard. Typically, Annex A (normative) whichdiscusses the positioning of the air rods (finials) statesunequivocally that the volume or zone of protectionafforded by the air termination system (e.g. air rod)shall be determined only by the real physicaldimension of the air termination system. Typically ifthe air rod is 5m tall then the only claim for the zoneof protection afforded by this air rod would be based

on 5m and the relevant class of LPS and not anyenhanced dimension claimed by some non-conventional air rods.

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

BS 63205 part 3 | Introduction

Class of LPS Material Thickness(1)

t (mm)

Thickness(2)

t (mm)

I to IV

Lead - 2.0

Steel (stainless,

galvanized) 4 0.5

Copper 5 0.5

Aluminium 7 0.65

Zinc - 0.7

Table 6: Minimum thickness of metal sheets or metal pipes in air

termination systems (BS EN 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.

Class of LPS Typical distances (m)

I 10

II 10

III 15

IV 20

Table 7: Typical values of the distance between down conductors

and between ring conductors according to the Class of LPS

(BS EN 62305-3 Table 4)

Natural components

When metallic roofs are being considered as a naturalair termination arrangement, then BS 6651 givesguidance on the minimum thickness and type ofmaterial under consideration. BS EN 62305-3 givessimilar guidance as well as additional information ifthe roof has to be considered puncture proof from a

lightning discharge (see Table 6).

Down conductors & fixings

For details of the Furse rangeof down conductors, seeconductors.

For details of the Furse rangeof relevant fixings andconnection components,seelightning protection.54 58 (solid circular) &61 62 (cable & wire).

... there should always be a minimum oftwo down conductors distributed around

the perimeter of the structure ...

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Natural components

BS EN 62305, like BS 6651, encourages the use offortuitous metal parts on or within the structure to be

incorporated into the LPS.Where BS 6651 requires an electrical continuity whenusing reinforcing bars located in concrete structures,so too does BS EN 62305-3. Additionally, it states thatreinforcing bars are welded, clamped with suitableconnection components or overlapped a minimum of20 times the rebar diameter. This is to ensure thatthose reinforcing bars likely to carry lightning currentshave secure 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 inFigure 14 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. Thejoints should then be encased by a moisture inhibitingcompound such as Denso tape.

If the reinforcing bars (or structural steel frames) areto be used as down conductors then electricalcontinuity should be ascertained from the airtermination system to the earthing system. For newbuild structures this can be decided at the earlyconstruction stage by using dedicated reinforcing barsor alternatively to run a dedicated copper conductorfrom the top of the structure to the foundation priorto the pouring of the concrete. This dedicated copperconductor should be bonded to the adjoining/adjacentreinforcing bars periodically.

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 ofthe structure.

Rebar components

For details of the Furse rangeof rebar connectioncomponents, seeEarthing.8

Figure 14: Typical methods of bonding to steel reinforcement

within concrete

Stranded copper cable(70mm2 PVC insulated)

Cast innon-ferrousbondingpoint

Bonding conductor

Clamped cable to rebarconnection

Steel reinforcement withinconcrete (rebar)

BS EN 62305 part 3

Introduction | BS EN 62305 part 3

BS EN 62305

encourages the useof fortuitous metal

parts on or within

the structure

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www.furse.com BS EN 62305 part 3 | Introduction

Earth termination system

The earth termination system is vital for the dispersionof lightning current safely and effectively into 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:

Low electrical resistance between the electrodeand the earth. The lower the earth electroderesistance the more likely the lightning currentwill choose to flow down that path in preferenceto any other, allowing the current to be conductedsafely to and dissipated in the earth

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 ohmsor less.

Three basic earth electrode arrangements are used.

Type A arrangement

Type B arrangement

Foundation earth electrodes

Type A arrangement

This 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 arrangement

This arrangement is essentially a ring earthelectrode that is sited around the periphery of thestructure and is in contact with the surroundingsoil for a minimum 80% of its total length(i.e. 20% of its overall length may be housedin say the basement of the structure andnot in direct contact with the earth).

Foundation earth electrodes

This is essentially a type B earthing arrangement. Itcomprises conductors that are installed in the concrete

foundation 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.

Earth rods & plates

For details of the Furse rangeof extensible earth rods andearth plates, seeEarthing.

Earthing conductors &FurseWELD exothermicwelding

For details of the Furse rangeof earthing conductors, see

conductors.

For details of the FurseWELDexothermic welding system,seeFurseWELD.

Separation (isolation) distance of

the 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 byplacing lightning conductors sufficiently far away fromany conductive parts that have routes leading into thestructure. So, if the lightning discharge strikes thelightning conductor, it cannot bridge the gap andflash over to the adjacent metalwork.

A sample of Furse high quality earthing components.

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Internal LPS design considerations

The fundamental role of the internal LPS is to ensurethe avoidance of dangerous sparking occurring within

the 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 can avoiddangerous sparking between different metallic parts.

Lightning equipotential bonding

Equipotential bonding is simply the electrical

interconnection 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 potentialthen the risk of sparking or flash over 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 62305-3.

Earth bonds & clamps

For details of the Furse range

of earth bonds and clamps, seeEarthing.

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

Figure 15 (BS EN 62305-3 fig E.45) shows a typicalexample of an equipotential bonding arrangement.The gas, water and central heating system are allbonded directly to the equipotential bonding barlocated inside but close to an outer wall near groundlevel. The power cable is bonded via a suitable SPD,upstream from the electric meter, to the equipotentialbonding bar. This bonding bar should be located closeto the main distribution board (MDB) and also closelyconnected to the earth termination system with shortlength conductors. In larger or extended structuresseveral bonding bars may be required but they shouldall be interconnected with each 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.

Equipotential bonding SPDs

For details of the Furse rangeof equipotential bonding,lightning current SPDs, see ourESP section.

Figure 15: Example of main equipotential bonding

BS EN 62305 part 3

Introduction | BS EN 62305 part 3

Equipotentialbonding bar

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

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