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First :
Nd = (Ng) * (Ae) * (C1)
where
Nd = lightning strike frequency
Ng = yearly average flash density in a region
Ae = equivalent collective area of the structure in km2 , or LW + 6H (L+W) + 9H2
C1 = environmental coefficient
Second :
Nc = (1.5x10-3) / C
where
C = (C2) * (C3) * (C4) * (C5)
C2 is the structural coefficient, from 0.5 to 3.0 and based on the contents of the structure (metal, nonmetallic or flammable) and the content of the roof (metal, nonmetallic or flammable)
C3 is a coefficient based on the value of the contents in the structure, their flammability and whether they are replaceable. This value can range from 0.50 to 4.00.
C4 is the coefficient of the occupancy of the structure. The choices for this value are unoccupied, normally occupied and difficult to evacuate or risk of panic. These values can range from 0.5 to 3.00.
C5 is the coefficient of the consequence of a lightning strike
Third
After the calculation of Nd & Nc if Nd is greater than Nc, then lighting protection is recommended. Otherwise, protection is optional.
....................sorry .............i am not electric
As per NFPA risk assessment calculator for a building, the lightning flash annual threat of occurence is calculated (Ng *A*C) and compared with tolerable lightning frequency. If the annual threat of occurence for lightning flash is more than the tolerable limit, then lightning protection method needs to be applied
In the above formula Ng is lightning flash density from map, A- area of collection, C - location factor
