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.
Page under development — features may change or may contain errors.
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
Anexa 6.11 is a drawn plate with isolines, split into eight zones in the legend; the standard has no county → Td table, so the zone is read off the map. As a rough guide: the Carpathian ridge falls in zones 1–2, the Romanian Plain and the Bucharest area in 5–6, and south-eastern Dobrogea and the coast in 7–8. The calculation uses the middle of the selected range.
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.
Is protection needed? R1 vs RT
Risk of loss of human life (R1) from DIRECT strikes on the structure, compared with the tolerable risk RT = 10⁻⁵ (I7 Annex 6.2/6.3)
High: combustible materials / >800 MJ/m². Medium: 400–800 MJ/m². Low: <400 MJ/m² or occasional combustibles.
None = unprotected structure. Pick a level to see how the RB risk drops.
R1 ≤ RT — protection not required (for the direct components)
The structure’s direct contribution to R1 is below the tolerable threshold. See the limits of this assessment below.
What this assessment covers (and what it does not)
Only the components from DIRECT strikes on the structure (RA + RB) are calculated. The full R1 in Anexa 6 also adds the components from strikes on or near the connected lines (RU/RV/RW/RZ), which depend on the power-line data and are assessed by the lightning protection designer. For an ordinary residential building, with no risk of explosion, the loss Lo is not defined in Tabelul A6.3.1, so RC/RM/RW/RZ come out zero and RA + RB cover R1. In hospitals, however, Lo = 10⁻³, so the result here is an underestimate — there a full assessment is mandatory. Near the threshold, ask for a full assessment anyway.
The factor values (PA, PB, ra, rp, rf, hz, Lf, Lt) are taken verbatim from I7-2011, Anexele 6.2 and 6.3 (the SR EN 62305-2 method), and the tolerable risk RT = 10⁻⁵ for loss of human life is the one in Tabelul 6.10. The result is indicative and does not replace a lightning protection design.
The separation distance s
How far the lightning protection has to stay from the metal parts of the building — and what you do when there is not that much room.
The lightning current running down a rod raises the potential of the down-conductor to hundreds of kilovolts relative to the rest of the house. If a pipe, a gutter, the panel structure or the inverter cable passes too close, the spark jumps from the down-conductor straight into the installation — that is flashover. Electrical insulation is ensured if the actual distance d between the parts is greater than the separation distance s.
s = ki × kc × l / km [m]
Measured along the air-termination or the down-conductor, from the point being examined to the nearest point of the equipotential bond.
Tabelul A6.8.4 gives ranges for the type B arrangement: 0.5…1 with two down-conductors, 0.25…0.5 with four or more. The calculator always takes the conservative end; the exact value is set by the lightning protection designer, from the geometry of the building.
The distance is sufficient
d is greater than s, so the insulation is ensured by the material. Here you do not bond the lightning protection to the metal part — and it is actually better not to, so that you do not carry the lightning current into the installation of the building.
The exception that spares you the whole calculation
“In reinforced concrete structures with interconnected metal reinforcement, having metallic or electrical continuity, a separation distance is not required” (Anexa 6.8). Continuity is not assumed, it is measured: the resistance between the topmost part and ground level must not exceed 0.2 Ω.
1.Take the three factors from the tables in Anexa 6.8
formula:ki = f(SPT) [A6.8.1] · kc = f(n, A/B) [A6.8.4] · km [A6.8.3]
with your numbers:ki = 0.04 · kc = 0.50 · km = 1.0
gives: ki = 0.04, kc = 0.50, km = 1.0
ki from the SPT class (A6.8.1), kc from the number of down-conductors and the earth electrode arrangement (A6.8.4, the conservative end of the range), km from the material between the parts (A6.8.3).
2.Calculate the separation distance
formula:s = ki × kc × l / km
with your numbers:s = 0.04 × 0.50 × 12.00 / 1.0
gives: s = 0.24 m
3.Compare it with the actual distance between the parts
formula:d > s
with your numbers:0.50 m > 0.24 m
gives: The distance is sufficient
Only the two distances are compared: if d > s, the material between the parts provides the insulation and nothing is bonded; if d ≤ s, you make the equipotential bond.
The formula s = ki × kc × l / km and the three factors come from I7-2011, Anexa 6.8 (tables A6.8.1, A6.8.3 and A6.8.4). The calculator gives the conservative case; the design of the lightning protection installation remains the job of the specialist designer.
The rolling sphere method
How far the sphere dips between two air terminations — the check that decides whether the panels really are protected.
The rolling sphere is rolled over the building in every direction; wherever it touches, lightning can strike. Between two neighbouring air terminations the sphere dips below the line joining them by the penetration depth p. Anything sticking up above that point — a PV module, a chimney, an aerial — is no longer in the protected volume, even if it “sits between the rods”.
p = r − √(r² − (d/2)²) [m]
The distance between the two air-termination rods or conductors the modules sit between.
Zero means the module is right at the level of the tips, so it is certainly exposed.
The object stays in the protected volume
The top of the object is below the lowest point of the sphere, so the sphere does not touch it. Check the separation distance separately as well: “protected” does not mean it is allowed to touch the down-conductor.
1.The sphere radius, from the class of the installation
formula:r = f(IPT) [Tab. 6.15]
with your numbers:r = 45 m
gives: r = 45 m, Mesh size of the air-termination network 15 × 15 m
2.How far the sphere dips between the two air terminations
formula:p = r − √(r² − (d/2)²)
with your numbers:p = 45 − √(45² − 5.00²) = 45 − 44.72
gives: p = 0.28 m
3.Check whether the object stays below the lowest point of the sphere
formula:Δh ≥ p
with your numbers:0.50 m ≥ 0.28 m
gives: The object stays in the protected volume
Δh is how far below the tips of the air terminations the top of the object sits — the value you entered in the form. If Δh ≥ p, the top stays below the lowest point of the sphere.
The rolling sphere radii (20 / 30 / 45 / 60 m) and the mesh sizes of the air-termination network (5 / 10 / 15 / 20 m) are those in Tabelul 6.15 of I7-2011, and the penetration depth follows geometrically from the rolling sphere method (art. 6.2.3.7.2). The PV installer course material gives exactly the same depths, but puts class III at 40 m; in the standard the class III radius is 45 m — and the figures in their own table fit 45, not 40.
Panels on a roof with lightning protection
Five rules that are applied in this order, not in the order the problems turn up on site.
The PV system goes up exactly where the lightning protection installation sits: on the most exposed surface of the house. The accredited PV installer course says it plainly — when the generator comes out of the building’s protected zone, a lightning protection installation is needed, and the system can be damaged even without a direct strike.
Keep the whole generator in the protected volume
On a house with lightning protection — a conductor along the ridge and down-conductors on each side — the PV plant can be covered entirely by the existing installation, but only if every part of the generator is inside the protected zone. You check that with the rolling sphere, not by eye.
Calculate the separation distance, do not assume it
The distance s is kept between the panels, the structure and the lightning protection components. In practice, on the small systems found on houses, at least 0.5 m has proved sufficient — but the number that counts is the calculated s, which grows with the length of the route and with the class chosen.
If you do not have the distance, make the bond deliberately
When the minimum distance cannot be achieved, the PV generator and the protection system are connected together, precisely in order to limit the consequences of flashover. The bond is made in copper of at least 16 mm² — the cross-section below which the conductor melts at the specific energy of a lightning strike; in aluminium that corresponds to 25 mm², and in steel to 50 mm² (the same values from Tabelul 6.20, nota 9). Best of all, the bond runs from the mounting frame to the equipotential bonding of the house.
Cable loops as small as possible
“In order to minimise the voltages induced by lightning, the area of all cable loops must be as small as possible.” The plus and the minus of a string are run together, not by different routes. By the same logic, equipotential bonding conductors are run parallel to and in the closest possible contact with the DC and AC cables (art. 7.11.25).
SPDs, not just bonding
On a building with an external lightning protection installation, protection starts with a Type 1 SPD — a lightning current arrester — followed by a Type 2 in the consumer unit. Bonds carry the current, but they do not limit the overvoltage that reaches the inverter and the rest of the installation.
The earth electrode: separate or joined, but not left to chance
The dispersion resistance of an earth electrode serving only the lightning protection installation must be no more than 10 Ω. The lightning protection electrode and the earthing of the photovoltaic system are treated separately; where separation is not possible they are joined — but then you have a single electrode and the stricter requirement applies. When measuring, the course recommends a margin: multiply the reading by about 1.3, because in summer, in dry ground, the resistance rises.
Early streamer emission air terminals (PDA) wear out
Installations with an early streamer emission device have a chapter of their own in the standard (cap. 6.3): the protection radius is calculated from the triggering advance declared by the manufacturer, not with the rolling sphere. What you cannot see from the ground: after a number of discharges — from a few to a few dozen, depending on the model — the device loses its parameters. Without a strike counter you have no way of knowing it has got there, so the counter is not an optional extra.
Two things that get mixed up on the DC side
Bonding the metal structure to the lightning protection installation is a measure against lightning, not protection against indirect contact: on the DC side “protective measures using non-conducting locations and local equipotential bonding are not permitted” (art. 7.11.10). And one for your own safety when working: PV equipment on the DC side must be considered live even when the system is disconnected on the AC side (art. 7.11.4).
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.
Practise the assessment
Three levels, on the formulas from Anexa 6.
Choose the correct option
Design decisions, no calculation.
What ND actually tells you
A house in a rural area comes out of the assessment with ND = 4.5·10⁻³ per year.
Step 1: What does this value mean?
The location factor Cd
The same house, but moved to the top of a hill, with nothing around it.
Step 1: Which Cd do you use?
Panels mounted between two rods
An array of modules on the roof, between two air-termination rods, class III installation.
Step 1: What do you check before saying the panels are protected?
When there is no room for the separation distance
The panel structure passes 20 cm from the down-conductor, and the calculated s is 0.45 m.
Step 1: What do you do?
Calculate step by step
There are hints if you get stuck.
ND for an ordinary house
A house 12 × 10 m, 8 m high, in an area with 30 thunderstorm days a year, surrounded by buildings of the same height (Cd = 0.5).
Step 1: What is the ground flash density Ng?
Separation distance on a house with four down-conductors
Class II installation, four down-conductors, ring earth electrode (type B arrangement), a 12 m route to the equipotential bonding, air between the parts.
Step 1: Which kc value do you take?
How far the sphere dips between the rods
Two air-termination rods, 12 m apart, with modules mounted between them.
Step 1: The penetration depth for class III (r = 45 m)?
Solve it on your own
A single final answer.
ND for an isolated building
A hall 20 × 15 m, 10 m high, in an area with 40 thunderstorm days a year, isolated object (Cd = 1). What is ND, expressed in thousandths (×10⁻³)?
Separation for class I, through concrete
Class I installation, two down-conductors, type A earth electrode, a 15 m route, and concrete between the parts. What is the separation distance s?
Penetration depth for class I
Class I installation (r = 20 m), air terminations 16 m apart. How far does the sphere dip below the line between them?
Where the figures come from
I7-2011 (Anexa 6 and ch. 7.11) plus the accredited PV installer course
The ND formulas (Anexa 6.1), the risk tables (Anexele 6.2 and 6.3), the separation distance (Anexa 6.8), the rolling sphere radii (Tabelul 6.15), the minimum cross-sections (Tabelul 6.20) and the articles in ch. 7.11 are taken from the text of the standard. The eight keraunic zones are the ranges on the legend of the plate in Anexa 6.11. The rules for coexistence with the PV system — the practical 0.5 m distance and the 16 mm² copper bond — come from the material of the specialist course “Instalator sisteme fotovoltaice solare”, checked against the standard wherever the standard says something on the subject.
Discussion
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