The annual number of strikes to a structure is ND = NG · AD · CD · 10⁻⁶. NG is the ground flash density per km² per year; AD is the collection area, which the standard draws with a 1:3 slope line from the highest parts of the structure; CD is a location factor between 0.25 and 2. Other equations give the events near the structure (NM) and on the incoming lines (NL and NI).
Short answer
The number of dangerous events ND per year for an isolated structure is the product of the ground flash density NG, the collection area AD and a location factor CD, with the factor 10⁻⁶ to convert square meters to square kilometers. Part 2 of IEC 62305 gives the full equation in its Annex A, with analogous equations for strikes near the structure and for those that hit the connected lines or fall near them.
ND = NG × AD × CD × 10⁻⁶
Ground flash density NG
NG is the number of ground flashes per square kilometer per year. The standard says this value is available from lightning location networks in many regions. If there is no NG map, in temperate regions it can be estimated with:
NG ≈ 0.1 × TD
where TD is the thunderstorm days per year, taken from an isokeraunic map. For other regions, local data should be used. This guide gives no NG values for any country, because the standard does not provide them.
Collection area AD
For an isolated structure on flat ground, AD is the area obtained by drawing a straight line of 1:3 slope (a rise of 1 for every 3 of horizontal run) that passes through the highest parts of the structure and turns around it. Its intersection with the ground defines AD. It can be found graphically or mathematically.
For a rectangular structure of length L, width W and height H, all in meters:
AD = L × W + 2 × (3H) × (L + W) + π × (3H)²
That is, the building footprint, plus a strip of width 3H along each side, plus four circular corners that add up to a circle of radius 3H.
Example of the equation: a building 40 m long, 20 m wide and 10 m high has 3H = 30 m. Then AD = 800 + 2 × 30 × 60 + π × 900 = 800 + 3,600 + 2,827 ≈ 7,227 m². That is nine times the building footprint, and for that reason height weighs so heavily on the number of strikes. This example is purely arithmetic; it is not a real case.
Structures of complex shape
If the roof has higher parts, a graphical method is used. An acceptable approximation is the larger of two values: the area AD,MIN from the equation above with the minimum height of the structure, and the area of the high part, AD′ = π × (3HP)², where HP is the height of the protrusion.
A part of a building
The structure can be just a part of a building B, with the dimensions of that part, if these conditions are met:
- it is a vertical part separated from the building;
- the building has no explosion risk;
- fire propagation between that part and the rest is prevented by walls with a fire resistance of 120 min (REI 120) or other equivalent protection;
- the propagation of overvoltages along common lines is prevented by SPDs at the entrance of those lines, or other equivalent protection.
If they are not met, the dimensions of the whole building are used.
Location factor CD
The factor CD corrects for the objects surrounding the structure:
| Relative location | CD |
|---|---|
| Surrounded by taller objects | 0.25 |
| Surrounded by objects of the same height or lower | 0.5 |
| Isolated, no other objects nearby | 1 |
| Isolated on a hilltop or knoll | 2 |
Strikes near the structure: NM
Strikes that fall nearby can damage internal systems through the electromagnetic impulse without touching the structure. They are calculated with the area AM, which extends to a line located 500 m from the perimeter:
NM = NG × AM × 10⁻⁶
AM = 2 × 500 × (L + W) + π × 500²
For the building in the example, AM = 2 × 500 × 60 + π × 500² ≈ 845,400 m².
Strikes on the lines: NL, NI and NDJ
For each line section entering the structure the following are calculated:
- NL, strikes to the line: NL = NG × AL × CI × CE × CT × 10⁻⁶, with AL = 40 × LL.
- NI, strikes near the line: NI = NG × AI × CI × CE × CT × 10⁻⁶, with AI = 4,000 × LL.
- NDJ, strikes to the structure at the other end of the line: NDJ = NG × ADJ × CDJ × CT × 10⁻⁶.
LL is the section length in meters. If unknown, 1,000 m is assumed. NL and NI are the annual number of overvoltages of 1 kV or more in the section.
The line correction factors are these:
| Factor | Condition | Value |
|---|---|---|
| CI (installation) | Overhead | 1 |
| Buried | 0.5 | |
| Buried within a meshed earthing system | 0.01 | |
| CT (line type) | Low voltage, telecommunication or data | 1 |
| High voltage (with an HV/LV transformer) | 0.2 | |
| CE (environment) | Rural | 1 |
| Suburban | 0.5 | |
| Urban | 0.1 | |
| Urban with buildings taller than 20 m | 0.01 |
Soil resistivity affects the collection area of buried sections: the standard indicates that, in general, the higher the resistivity the larger the area, and that the CI value in the table is based on a resistivity of 400 Ω·m.
What is done with ND, NM, NL and NI
These numbers are the first factor of each risk component: ND for RA, RB and RC; NM for RM; NL and NDJ for RU, RV and RW; NI for RZ. They are multiplied by the probability of damage and by the loss to obtain each component.
Guides in the same series
ND enters the components described in risk components RA to RZ and the procedure in lightning risk management. The probability that a strike causes damage is estimated in probability of lightning damage. The protection level and the system that meets it are in protection levels and LPS, classes I to IV.
When to contact us
If you need the number of strikes for a building or plant with its power and communication lines, or need to support it before an authority, write to us with the drawings and the line data. Learn about our engineering design and studies service and write to us through contact.
Related guides
- IEC 62305 lightning protection: what each part covers
- Lightning risk management: when protection is needed
- Lightning risk components in IEC 62305-2: RA to RZ
- Probability of lightning damage: PB, PSPD and KS factors
- Lightning shielding in substations
References
- IEC 62305-2:2010, Protection against lightning – Part 2: Risk management, Annex A, clauses A.1 to A.5, equations A.1 to A.11, tables A.1 to A.4 and figures A.1 to A.5.
Frequently asked questions
Where does the ground flash density NG come from?
From lightning location networks, which exist in many regions of the world. The standard adds that, if there is no NG map and the area is temperate, it can be estimated as NG ≈ 0.1 · TD, where TD is the thunderstorm days per year from an isokeraunic map. For other areas, local data should be used.
Why is the collection area larger than the building footprint?
Because the collection area is the ground surface whose strikes are counted as strikes to the structure. The standard draws it with a 1:3 slope line that touches the highest parts of the structure and turns around it; the area that line marks out on the ground is AD, and it grows with the height H.
What does the value of NL and NI on a line mean?
It is the annual number of overvoltages with an amplitude of not less than 1 kV in the line section considered. NL corresponds to strikes to the line and NI to strikes near it. If the section length is not known, the standard asks for LL = 1,000 m to be used.