The main and most effective measure for protection of structures against physical damage due to lightning is considered to be the lightning protection system (LPS). An external Lightning Protection System ( https://www.cikit.in/overall-lightning-protection-system/ ) is intended to
1) Intercept a lightning flash to a structure. (Air terminal system)
2) Conduct the lightning current safely towards the earth. (Down conductor
3) Disperse the lightning current into the earth. (Earthing system)
In this article we have explained about down conductor system for Lightning Protection.
The function of a down conductor system is to conduct the lightning impulse from air termination system to the earthing system. The down conductor system should be installed in such a way that the following points are ensured.
1) Several parallel current paths exist
2) Length of current path is kept to minimum.
3) Equipotential bonding to conducting parts is performed.
If the Lightning protection system is isolated from the structure to be protected, it is called Isolated LPS. The down conductor for an isolated LPS can be positioned as follows,
- If the air-termination consists of rods on separate masts which are not made of metal, at least on down conductor is needed for each mast. No additional down conductors are required if the mast is made of metal.
- If the air-termination consists of catenary wires, at least on down conductor is needed at each supporting structure.
For each non-isolated LPS, the number of down conductors shall be not less than two and should be distributed around the perimeter of the structure to be protected. An equal spacing of the down conductors is preferred around the perimeter. The typical values of the distance between the conductors are shown below.
A down conductor should be installed at each exposed corner of the structure and as far as practicable, they form a direct continuation of the air-termination conductors. Down conductors are installed in such a way that they provide the shortest and most direct path to earth.
Down conductors, even if covered in insulating material, shall not be installed in gutters or water spouts. The effects of moisture in the gutters lead to intensive corrosion of the down conductor.
- If the wall is made of non-combustible material, the down conductors may be positioned on the surface or in the wall.
- If the wall is made of readily combustible material the down conductors may be positioned on the surface or in the wall, provided that the temperature rise due to the passage of lightning current is not dangerous for the wall.
- If the wall is made of readily combustible material and the temperature rise of the down conductor is dangerous, the distance between the conductor and the wall should be maintained always greater than 0.1m. Mounting brackets may be in contact with the wall.
When the distance between from down conductor to a combustible material cannot be assured, the cross section of the steel or thermal equivalent conductor shall not be less than 100mm2.
- The metal installations provided that electrical continuity is ensured and the materials have the required dimensions.
- The metal of the electrically continuous reinforced concrete framework of the structure.
- The interconnected framework of steel structures.
- The façade elements, provided that they have electrical continuity and the metal sheets or pipes thickness shall not be less than 0.5mm.
The materials used for air-termination and down conductor system should have the capacity to withstand the heavy impulse current. IEC 62305 suggests the configurations and minimum cross-sectional areas of the materials to be used for these applications and the details are as follows.