Practical calculations of lightning strikes frequency

Practical calculations of lightning strikes frequency (on photo: Lightning striking behind the new Manitoba Hydro Building by jolenetara via Flickr)

Frequency of lightning strikes

We distinguish between the following frequencies of lightning strikes which can be relevant for a building or structure:

• N_D – Frequency of direct lightning strikes to the building or structure;·
• N_M – Frequency of close lightning strikes with electromagnetic effects;
• N_L – Frequency of direct lightning strikes in utility lines entering the building or structure;
• N_I – Frequency of lightning strikes adjacent to utility lines entering the building or structure.

The calculation of the frequencies of lightning strikes is given in detail in Annex A of IEC 62305-2 (EN 62305-2).

If a finer grid is used, the local values of the lightning densities can still deviate noticeably from these averages. Owing to the relatively short time of seven years the map has been recorded, and to the large area averaging according to licence plate number areas, the application of a safety factor of 25 % to the values given in Figure 1 is recommended.

Figure 1 - Lightning density in Germany (average of 1999-2005)

For the frequency of directlightning strikes ND to the building or structure we have:

N_D = N_g · A_d · C_d · 10^-6

A_d is the equivalent interception area of the isolated building or structure (Figure 2), C_d a site factor so that the influence of the surroundings (built-up, terrain, trees, etc.) can be taken into account (Table 1).

Similarly, the frequency of close lightning strikes NM can be calculated:

N_M = N_g · A_m · 10^-6

Table 1 – Site factor C_d

Figure 2 - Equivalent interception area Ad for direct lightning strikes into a stand-alone structure

Figure 3 - Equivalent interception areas Ad, Al, Aa for direct lightning strikes into structures / supply lines and Am, Ai for indirect lightning strikes near the structures / supply lines

Table 2 – Equivalent interception areas A_l and A_i in m²

Table 3 – Environment factor C_e

N_l = N_g · A_i · C_t · C_e · 10^-6

The area A_i (Figure 3) is again a function of the type of line (overhead line, underground cable), the length L_C of the line; in the case of cables, it is a function of the earth resistivity ρ; and for overhead lines it is a function of the height H_C of the line above ground level (Table 3).

The correction factors C_t and C_e correspond to those already stated above. The frequency N_l must then also be determined individually for each utility line entering the building or structure.

Close Up Lightning Strike Compilation (VIDEO)

Cant see this video? Click here to watch it on Youtube.

Reference: Lightning Protection Guide – Dehn