Frequency of direct lightning strikes (ND)
Estimate how many lightning strikes hit a building on average per year, based on its dimensions, the local thunderstorm days and its location. This is the first step of a risk assessment — not the final decision about lightning protection.
Pagină în dezvoltare — funcționalitățile se pot schimba sau pot avea erori.
The three terms in Anexa 6.1
ND is obtained by multiplying the ground flash density by the area over which the building “attracts” strikes and by a location factor.
Ng — ground flash density
How many lightning strikes hit a square kilometre of terrain per year. If no Ng map is available, for temperate regions it is estimated from Td, the number of thunderstorm days per year (from keraunic maps, Anexa 6.11).
Ng = 0,1 × Td [trăsnete/km²·an]
Ad — equivalent collection area
The virtual area around the building within which a lightning strike would hit the structure. It grows strongly with height H: a tall building “collects” strikes over a much larger area. L, W and H are entered in metres.
Ad = L×W + 6×H×(L+W) + 9×π×H² [m²]
ND — average annual number of strikes to the structure
The product of density, area and the location factor Cd, scaled by 10⁻⁶ (since Ng is per km² while Ad is in m²). The result is a frequency: usually a very small number, e.g. 2.4·10⁻⁴ strikes per year.
ND = Ng × Ad × Cd × 10⁻⁶ [trăsnete/an]
Cd — location factor
It accounts for the surroundings (table A6.1.2): object surrounded by taller buildings/trees 0.25; of the same height or lower 0.5; isolated object 1; isolated object on a hilltop or mountaintop 2. An isolated, tall object is far more exposed.
Calculate ND for your building
Enter the dimensions, the lightning density source and the location. Results update automatically.
Building dimensions
Lightning density
For the lowland region of Romania a guide value of ~30 thunderstorm days per year is used; check the keraunic map (Anexa 6.11) for the exact value of your area.
Building location
Results
Moderate frequency: on the order of one strike every few hundred years. It is worth continuing the risk assessment, especially if the building houses people or sensitive equipment.
ND is not the protection decision
ND is only the frequency of direct strikes. Whether lightning protection is needed and at what level (LPL I–IV) is determined through the risk assessment R — the sum of the risk components — compared with the tolerable risk RT, per I7-2011 Anexa 6.2 and 6.3. This calculator does not replace that assessment.
The formulas are those from I7-2011 Anexa 6.1: Ng = 0.1 × Td (A6.1.1), Ad = L×W + 6×H×(L+W) + 9×π×H² for an isolated structure on flat ground (A6.1.2) and ND = Ng × Ad × Cd × 10⁻⁶ (A6.1.4). For structures with a complex shape, the area Ad is determined graphically, not with the simple formula.
From ND to the lightning-protection decision
Strike frequency is just one input. The decision is made through the risk assessment per Anexa 6.2 and 6.3 of I7-2011.
Identify the hazardous events
Besides the direct strike to the structure (ND), the risk also accounts for strikes near the structure, on the connected services (power lines, telecom) and near them. Each contributes to the risk components.
Calculate the risk R and compare it with RT
The total risk R is obtained by summing the relevant risk components (loss of human life, public services, heritage, etc.) and is compared with the tolerable risk RT. If R exceeds RT, protective measures are required.
Choose the protection level LPL
If protection is needed, the lightning protection level (LPL I–IV) and the associated measures (external LPS, equipotential bonding, SPDs) are determined. The effectiveness of the SPDs depends on the LPL they were designed for.
The full risk assessment (Anexa 6.2 and 6.3) and the choice of LPL are carried out by the designer of the lightning protection installation. This tool covers only the ND calculation from Anexa 6.1.