IEC 62305 collection area and N_D calculator

Annex A of IEC 62305-2:2024 counts the dangerous events a structure meets in a year. The first of them, and the one every assessment starts from, is N_D: flashes that strike the structure itself, source of damage S1. Three numbers decide it.

The strike density, N_SG. The third edition works in ground strike-point density, the number of points per square kilometre per year where lightning reaches the ground (clause A.1). Older maps and reports often give flash density N_G instead. Equation (A.1) links the two as N_SG = k × N_G, and where the data source gives no k, clause A.1 NOTE 1 says to assume 2. The calculator applies that when you choose flash density, and shows the step.

The collection area, A_D. Clause A.2.1 draws a line falling 1 in 3 from the top of the structure and turns it all the way round. For an isolated rectangular building of length L, width W and height H, equation (A.3) adds three parts: the roof itself, L times W; a band 3H wide along each side, which is 6H times the sum of L and W; and the four rounded corners of radius 3H, which together make a circle of area π times 9H². Height matters far more than plan size, because it appears in every term but the first.

A protrusion on the roof. A mast, chimney or plant room that rises well above the roof collects lightning on its own. Equation (A.4) gives its area as A_D' = π × (3 × H_P)², with H_P measured from the ground to its top (Figure A.2). Clause A.2.1.3 then takes the greater of A_D at the main roof height and A_D'. A low, wide building with a thin mast is usually still governed by A_D; a small building with a tall tower is governed by A_D'.

The setting, C_D. Table A.1 scales the result for what surrounds the structure: below 1 when taller objects shield it, 1 when it stands alone, and above 1 on a hilltop. Then equation (A.5) multiplies the three and divides by a million, which turns the square metres of A_D into the square kilometres N_SG is counted in. The working beside the result shows each figure.

N_D is a frequency, not an answer. IEC 62305-2:2024 multiplies it by a probability of damage and a loss for each risk component from a strike to the structure, then adds the components from strikes near the structure, to its lines and near its lines, which need their own collection areas: A_M, A_L and A_I. The total risk R is compared with the tolerable risk R_T under clause 7.3, and the frequency of damage F with F_T under clause 9.3.

That is where a spreadsheet gets long and errors creep in: the probability tables, the loss values, the line factors and the zones. The IEC 62305-2 risk method guide sets out every component, and the lightning protection level selector shows how the risk turns into a class of LPS.