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Kingsmill Industries (UK) - Lightning Protection Design Guide

Air Termination Network - Conductors - Earthing - The Earth Network

Air Termination Network

The Air Termination Network is a vital part of any structural lightning protection system.

This network is the part that arrests the lightning strike. An air termination network can be designed using the “protective angle method”, the “rolling sphere” or a combination of both. The adoption of which method(s) to use is very much determined by the nature of the structure to be protected.

An air termination network may consist of:

  • Conductors laid in a mesh over the structure
  • Vertical air terminals (or lightning masts)
  • A combination of the two, particularly useful when, for example, protecting roof mounted plant.

The lightning protection system can be fixed to the building (the most common form), it can use elements of the structure to conduct lightning, for example, reinforcing bar or steelwork (but special conditions relate to the use of such “fortuitous conductors” in BS:EN 62305) or it can be an isolated system where separation distances need to be maintained between the lightning protection system and the structure being protected, in order to avoid “flash over” to that structure, for example, in the case of stores containing explosive material.

Traditionally, conventional small air terminals have been widely utilised. However, there are now a range of innovative solutions available, including:

  • Conductors and fittings for roof mesh conductor systems
  • Lightweight lightning masts
  • Free standing air terminals
  • Insulated lightning conductor used in conjunction with associated air terminals and supports
  • Catenary wire systems

Conductors

Conductors are an essential component of a Lightning Protection System.

They form part of:

  • The Air Termination Network – arresting the lightning strike
  • The Down Conductor System – taking the lightning discharge to ground
  • The Earth Termination/Network – safely dissipating the lightning discharge into the ground

Conductors are also an integral and important component of an earthing system, whether this be for a power station, sub-station, cell site, solar PV array etc.

Conductors provide a low impedance path and they must be:

  • Sized to take the fault current
  • Sufficiently robust to resist mechanical damage
  • Able to resist the effects of corrosion

Guide to Conductor Selection

When choosing the material for “above ground” Lightning Conductors, it is important to take into account the avoidance of any bi-metallic corrosion.

For example, one should avoid the installation of copper conductors over aluminium or steel cladding.

  • Galvanic corrosion – happens when there is a potential difference between dissimilar metals
  • When the materials are in contact in the presence of an electrolyte, corrosion occurs. One metal becomes the anode, the other the cathode. The anode will tend to go into solution and therefore corrode. The electrolyte can be water with impurities from the air.
  • In the instance of water run-off from copper – the water carries fine copper corrosion particles which, when they drip on to aluminium or zinc, may result in corrosion. The direct contact of the dissimilar metals is not required for corrosion to occur in such a case.

Whilst there are cautions with respect to the use of aluminium, these can generally be overcome with the use of PVC covered aluminium conductors, in an above ground Lightning Conductor setting.

The most common cross-section areas for above ground lightning protection conductors are 8mm dia (50mm² – BS:EN 62305 min) and 25mm x 3mm rectangular section.

PVC covering is often used as a decorative cover when blending in external conductors to building aesthetics. However, thought must be given to using such conductors in areas of high UV, due to colour bleaching and life of modern UV stabilisation compounds.

Copper Conductor Ratings: Fault Current

The following conductor ratings are based upon the recommendations of BS 7430 with an initial conductor temperature of 30°C and a maximum temperature of 250°C.

copper conductor ratings

Earthing

Copper is the recommended material to use in earthing applications, whether below or above ground.

Materials used in earthing systems must be able to:

  • Withstand mechanical damage
  • Resist corrosion
  • Provide a low impedance path to earth
  • Carry the maximum fault condition for the application/installation

The Earth Network (Earth Termination or Grounding System)

It is important that lightning current is carried and dispersed through the ground in a safe manner that will last the life of the building.

With this in mind, an earth system (grounding system) must be capable of:

  • Providing a low resistivity path to earth (ground)
  • Allow circuit protection devices to operate correctly
  • Have excellent corrosion resistance properties

Soil resistivity readings should be taken prior to designing an earth system (grounding system). The reason for this is that the nature of the ground (soil) has a major influence over the final resistivity of the earth electrode (ground electrode) system. Additionally, the ground (soil) composition also affects the soil resistivity. The presence of salts can significantly reduce resistivity, but salt and some other elements can be extremely corrosive to the material used in an earthing system.

Once the soil resistivity readings have been obtained, the Earthing System can be designed. Corrosion resistance is of paramount importance.

Kingsmill recommends the use of copper earth electrode systems. This can be achieved through either:

  • Solid Copper – stranded wire, solid section tape/circular conductors
  • Copper Covered Steel – earth (ground) rods, bi-metallic wire

When using copper covered steel, it is important to specify a minimum copper thickness of 0.254 microns on the ground rod, as specified by UL (Underwriters Laboratory). This measurement results from research carried out in the USA, that studied the weight loss of copper (corrosion) under differing soil conditions. It was determined that 0.254mm (or 254 microns) will provide a lifetime of 20+ years, in average soil conditions.

Kingsmill do not recommend the use of galvanised, zinc plated or bare steel, for two reasons:

  • High resistivity of steel compared to copper
  • Poor corrosion resistance when compared to copper plated steel or solid copper

Earth connection components must also be corrosion-resistant and carry the nominated fault rating of the conductor.

Click the PDF image to view the technical information in full

Kingsmill Industries

Request Kingsmill Industries (UK) Info

Kingsmill Industries (UK) Ltd
Unit 4 Plymouth Avenue
Pinxton
Derbyshire
NG16 6NS

Email Kingsmill Industries (UK)

Tel: 01773 510001
Fax: 01773 417850

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