BS EN 62305 – Earthing & Lightning Protection System Design

Published 10 Jul 2018

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Design of Lightning Protection System

The following information has been provided courtesy of AN Wallis, leading UK manufacturers of Earthing & Lightning Protection Systems.

The LPS (Lightning Protection System) is required to:

  • Intercept the lightning strike (the air termination network)
  • Conduct the lightning strike safely to ground (using down conductors, such as copper earth tapes)
  • Disperse the strike safely into the earth (earthing)
  • Whilst the structural protection is there to conduct a strike safely to earth this is normally combined with internal protection to prevent sparking within the structure ensuring all metallic services are at equipotential (bonding)

The designer of the LPS should ensure that:

  • The safest path to earth is the LPS
  • The risk of sparking whilst the strike is conducted safely to earth is minimised (separation distance/s)
  • The risk of voltage differential whilst the strike is being dissipated in the ground safely is minimised (step & touch potentials)

The designer of the LPS has to gather all the relevant information to ensure the earthing system design is as safe as possible within any economic restraints:

  • A designer may find it impractical to fully install the desired LPS
  • A designer may not be able to justify the cost of providing the desired LPS
  • A designer may consider using the metal roof or reinforcing bars within a building as the safest and most economic design
  • A designer may consider extra bonding and surge protection devices are required to protect the internal space, especially if the space houses sensitive electronic equipment
  • A designer may consider a building of such a high risk that additional measures are taken to ensure safety, possibly a flour factory or a building with a combustible roof, in these cases the LPS system may have to stand off the building
Earth Tapes

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Criteria For The Protection Of Structures

The level of protection/Lightning Protection Level (LPL) applied to the structure is identified by the risk assessment.

Lightning Protection System (LPS) Level

  • LPL I requires a Class I
  • LPS LPL II requires a Class II
  • LPS LPL III requires a Class III
  • LPS LPL IV requires a Class IV LPS

Design of The LPS General Considerations

To help the earthing system designer, the threat of lightning to a structure or building can be defined in lightning protection zones requiring protection and the type of lightning strike likely to enter the building shown in Figure 2.

S1 – Strike directly to the structure

S2 – Strike on the ground near the structure

S3 – Strike to a service connected to the structure

S4 – Strike on the ground near a service connected to the structure

Design of Structural Lightning Protection

LPZ1 – The protected zone inside the building, the zone where current is limited by current sharing and SPD’s at the boundary (less the separation distance)

LPZ Oa – At risk from the full lightning strike and the full lightning electromagnetic field

LPZ Ob – Not at risk from a direct lightning strike considering the protected area through the rolling sphere but at risk from the full lightning electromagnetic impulse. (LEMP)

LPZ 2 – Protected zone with further dampened magnetic field

Design of Structural Lightning Protection

The LPS designer should ensure everything to be protected falls inside the LPZ Ob range in figure 2.

  • The bonding measures employed need consideration at the design stage
  • The earthing design should consider fully the step and touch potential risks
  • The requirements for Surge Protection Devices (SPDs) on incoming mains and conductive services should be considered in accordance with the risk assessment carried out for the structure LPS requirements
  • Where combustible wooden type materials are present a distance of 0.15 m should be maintained between the LPS conductors and the roof, for any other combustible surfaces a distance not less than 0.10 m is required
  • Some structures will have reinforced sections with expansion joints, if the designer of the LPS considers electronic equipment within the building is at risk then bonding conductors should be provided across the joints to provide low-impedance potential equalization. The separation distance between the bonds should not be more than half the distance between the down conductors
  • Natural components within/part of the structure such as the rebars can be made use of provided they will always remain an integral part of the structure conforming to the requirements below
Earth Tapes

Manufacturer by high conductivity and purity copper the range of earth tapes provide effective protection to buildings and substations

Using natural conductors as part of the LPS

The building’s natural components, metal roof, rebar, steelwork etc can be considered as part of the LPS provided they meet the minimum criteria shown in Table 1.

Material for LPS levelI to IV Prevents puncture, hot spots or ignition. minimum thickness (mm) (ta) requirement  Only for metal sheets where preventing puncture, hot spots or ignition is not important. minimum thickness (mm) requirement (tb)
Lead 2.00
Stainless Steel 4 0.50
Titanium 4 0.50
Copper 5 0.50
Aluminium 7 0.65
Zinc 0.70

The reinforcing bars within the concrete structure can be used as a natural component of the LPS provided they are electrically continuous by either welding or clamping the joints.

The re-bars are considered as electrically continuous provided that the major part of interconnections of vertical and horizontal bars are welded or otherwise securely connected by clamps conforming to BS EN 50164 standards.

The connecting rebar must overlap and be clamped using rebar clamps or welded to a minimum of 20 times the diameter of the rebar as shown in figure 3. (Welding to be done on either side of the rebars.)

Example of a rebar joined by clamps

Example of a rebar joined by clamps

To test the continuity of the reinforcing bars the resistance between the re-bar connection to the air termination network and the rebar connection to the earthing network should be measured, the resistance should not exceed 0.252, otherwise proprietary down conductors will be required.

In order to provide a connection to the rebar from outside the concrete a cast-in earth plate can be used as shown in figure 4, the earth point sits in the wall (or within an enclosure) providing a connection to the re-bar with a welded copper tail attached to the earth point and to the re-bar with propriety clamps.

Earth point sits in the wall providing a connection to the rebar

Earth point sits in the wall providing a connection to the rebar

The designer of the structural LPS has 4 main criteria to consider:

  • The roof termination system
  • The down conductor configuration
  • The Earth Termination network including equipotentialization and the risk of step and touch potential (equipotentialization on its own is not effective in reducing the risk against touch voltages)
  • Bonding (creating a euipotential zone across all zones, Oa, Ob, Z1, Z2)

The diameter of the sphere depends on the class of LPS selected/determined.

Class LPS Sphere Radius
I 20
II 30
III 45
IV 60
Earth Bars

Complete range of earth bars with connection options and number of cable termination ways to provide effective common isolation point.

Methods Of Designing The Air Termination Network

1 – The rolling sphere

2 – The protective angle design

3 – The mesh design

The Rolling Sphere Method

This method simply rolls a sphere around the building to be protected, wherever the sphere touches the building dictates where the protection measure is to be applied, where the sphere does not touch the building, this is accepted as a protected area, this method can be used to design the LPS on complex structures or where the LPS has to be isolated.

The rolling sphere method is especially relevant on complex structures with many different levels, this method easily identifies the protected space and where protection measures should be applied to the structure.

Examples of the air termination system using the rolling sphere technique

Examples of the air termination system using the rolling sphere technique


Desigining The Lightning Protection System (LPS)

Designing The Lightning Protection System (LPS)

The Protective angle design

The Protective Angle method in figure 10 is only used on simple structures or for small sections of larger structures.

The Protective Angle design method cannot be used where the part of the structure/service to be protected is higher than the radius of the rolling sphere corresponding to the class of LPS.

The level of LPS dictates the angle of protection depending on the reference height, see figure 9.

This method of earthing system design is an alternative method based on the rolling sphere and is not offered to give a wider range of protection than the rolling sphere.

In figure 9 the height limits for designers are clear and correspond to the radius of the rolling sphere.

Protective Angle Design

Protective Angle Design

The Mesh design

The most commonly used method, is usually employed where the structure is simple, a square or rectangular building or typical house or block of apartments with a sloping roof, the mesh method is for protection in zone OA.

The mesh design protects the whole area if conductors are positioned on the edge of the roof where the slope of the roof exceeds 1:10.

Protective angle design to protect free standing equipment on the roof of a building

Protective angle design to protect free standing equipment on the roof of a building

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

On structures higher than 60 metres lateral air termination systems should be applied to the top 20% of the structure relevant to its class of LPS (or at least conforming to class IV LPS).

The mesh of earth conductors are installed on the roof, the earth conductor must be at the edge of the area to be protected and for metal items such as air conditioning units that protrude above the conductor, the protective angle design should be applied for protection.

The size of earth mesh required is defined by the level of LPS determined/selected

LPS Class Mesh Size (M)
I 5 x 5
II 10 x 10
III 15 x 15
IV 20 x 20


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