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Earthing and lightning sytems is dependent on power distribution system, and designed same as power distribution systems. with every power cable there will be grounding cable and size will be depending on power cable or protection device it is connected to.
The best resource for info on this topic is National Electric Code (NEC) Article250.
hope it will help
They are2 types of earthing one is lightning protection earthing one is system earthing. For system earthing (power distribution earthing) calculation is same as cable size calculation of distribution system.
A Thumb rule is that upto16 sqmm the same size of earthing cable will be use. above16 sqmm half of the distribution cable size will be used. For lightning protection system there is series of calculation, usually in real feild furse will provide this calculation. Again thumb rule is that for down for down conductor25X3 mm Copper tape will be used. for Airterminal25 square mm rod will be use and for earthing25 mm of rod3mtrs of lenght wil be used. again numbers of eafrthing rod, airteminals again its depend upon earth resiistence if more is the earth more rod need to install to get lowest resitance path for follow of electric charges.
