Practical calculations of lightning strikes frequency

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).

For practical calculations it is recommendable to take the annual density of the cloud-to-earth flashes N_g for the region under consideration from Figure 1.

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

Relative site of the building or structure	C_d
Object is surrounded by higher objects or trees	0.25
Object is surrounded by objects or trees of the same or lower height	0.5
Free-standing object: no further objects near by (within a distance of 3H)	1
Free-standing object on top of a moutain or a rounded hilltop	2

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

	Overhead line	Underground cable
A_l	[ L_c – 3 · (H_a + H_b) ] · 6 · H_c	[ L_c – 3 · (H_a + H_b) ] · 6 · √ρ
A_i	100 · L_c	25 · L_c · √ρ

Table 3 – Environment factor C_e

Environment	C_e
Urban with high buildings or structures (higher than 20 m)	0
Urban (buildings or structures of heights between 10 m and 20 m)	0.1
Suburban (buildings or structures not higher than 10 m)	0.5
Rural	1

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 same worst-case scenario applies. The area A_i is usually significantly larger than A_l. This makes allowance for the fact that surges resulting in faults or damage to electrical and electronic systems can also be caused by lightning strikes further away from the line.

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)

Reference: Lightning Protection Guide – Dehn

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.

One Comment

kiran Kumar Avadhanula
Feb 08, 2017
Dear Edward,
can you share any guidelines for the lightning protection for metal chimney having 64,86 mtrs height how to decide size of the air terminals , horizontal terminals and down comers etc [email protected] is my email id
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Practical calculations of lightning strikes frequency