Introduction:
The components of Lightning Protection System (LPS) are exposed to direct lightning strikes and corrosive atmosphere. The materials should have high current withstanding capacity and they should be less corrosive. The current carrying capacity depends on the cross sectional area and the materials should be selected based on the local environment.
A material which is most preferable for some site conditions might be the least preferred material for other site conditions due to its chemical properties. Apart from the corrosion of materials due to the local environment, the contact of two dissimilar materials also leads to galvanic corrosion. Hence, IS/IEC 62305 suggests the materials that can be used for different corrosive environments and it has provided the minimum cross sectional area required for different materials of LPS Components.
Minimum Cross sectional area of different materials:
The minimum cross-sectional area of different materials suggested by IS/IEC 62305-3 are as follows.
- Copper, tin plated copper strip, cable, Copper coated steel should have a minimum cross-sectional area of 50mm². When mechanical strength of the material is not important then the cross sectional area of the material can be reduced up to 25mm²
- For air terminals the minimum cross sectional area should be 176mm² for all material and if the mechanical stress due to wind loading is not critical then the cross sectional area can be reduced up to 70mm²
- Normal cross sectional area of stainless steel strip, and conductor is 50 mm² and it can be increased upto 75 mm² when thermal & mechanical factors are considered.
Protection against corrosion:
- The components of LPS shall be made of corrosion resistant materials like copper. aluminium, stainless steel and galvanized steel. Among these materials aluminium shall be used only above the ground level.
- Connections between different materials should be avoided as it leads to galvanic corrosion. Hence the material of the air-termination rods should be compatible with the connecting and mounting element materials.
- Copper parts should not be directly installed over the aluminium material without any protection against the galvanic corrosion.
- Aluminium conductors should not be directly used in concrete limestone surfaces and soil.
- Lead-sheathed steel conductors should not be used as earth conductors.
- Lead-sheathed copper conductors are not suitable for concrete and high calcium content soil.
Aluminium has very good electrical conductivity but it more prone to corrosion on soil and concrete medium. Hence, they can be used for air terminal and down conductor systems above the ground level and connected to earthing system of Gl/Copper/SS using proper bimetallic connectors. The fasteners or sleeves for aluminium conductors should be made of similar metal and it should have adequate cross-sectional area to withstand the adverse weather conditions.
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