Lightning Risk Assessment

The simplified frequency comparison used to decide whether a structure needs lightning protection.

1. What this check does

Computes the equivalent collection area of a rectangular structure, the expected annual number of direct strikes and the tolerable annual number, and recommends protection when the expected exceeds the tolerable.

2. Inputs used

Structure length, width and height.
Ground flash density Ng, or thunderstorm days Td when Ng is unknown.
Location factor C1 and consequence factors C2 to C5, which you select from the tables in the standard you apply.

3. Formulas and assumptions

Collection area

A_e = LW + 6H(L + W) + 9\pi H^2

Area within 3H of the structure.

Flash density

N_g \approx 0.1\,T_d

Only when flash density is unknown.

From satellite data

N_g \approx \dfrac{N_t}{1 + Z}, \qquad Z = 4.16 + 2.16\cos 3\lambda

Nt = total flash density (NASA LIS/OTD), λ = latitude. The default uses Z = 2 (Nt/3), the smallest Z of the formula, which is conservative; the latitude value of Z is offered as an option.

Expected

N_d = N_g\, A_e\, C_1 \times 10^{-6}

Strikes per year.

Tolerable

N_c = \dfrac{1.5 \times 10^{-3}}{C_2 C_3 C_4 C_5}

Protection recommended when Nd > Nc.

Required efficiency

E = 1 - \dfrac{N_c}{N_d}

Level IV if E ≤ 0.80, III ≤ 0.90, II ≤ 0.95, I ≤ 0.98, I plus additional measures above 0.98.

Symbols

$L,\ W,\ H$	Structure length, width, height	m
$A_e$	Equivalent collection area	m²
$N_g$	Ground flash density	flashes/km²/yr
$T_d$	Thunderstorm days	days/yr
$N_t,\ Z,\ \lambda$	Total (cloud + ground) flash density; cloud-to-ground ratio; latitude	flashes/km²/yr, —, °
$C_1$	Location factor (surroundings)
$C_2 \ldots C_5$	Construction, contents, occupancy and consequence factors
$N_d,\ N_c$	Expected and tolerable strike frequency	per yr
$E$	Required interception efficiency of the protection system

Plan view of a rectangular building L by W with a boundary drawn 3H away, with rounded corners of radius 3H. — **Figure 1.** Equivalent collection area: the building footprint extended by 3H on every side.

Flow: Nd compared with Nc; if Nd is greater, E = 1 − Nc/Nd selects protection level I to IV. — **Figure 2.** From expected and tolerable strike frequency to the protection level.

Figure 3. From satellite total lightning to ground flashes: the share that strikes ground by latitude.

Rectangular plan and a flat roof; for complex shapes use the largest applicable area.
The factor values come from the tables of NFPA 780 Annex L or IEC 62305-2. RokBench does not reproduce those tables.
Measured ground flash density should be preferred to the thunderstorm-day estimate.
The coordinate estimate takes the NASA LIS/OTD satellite climatology (total lightning in 0.5° cells, no data beyond 75° latitude) and keeps one third as cloud-to-ground flashes. It is a screening value; a ground-network measurement (for example NLDN in the US) or the map in the applicable standard takes precedence.

4. What is NOT checked

The full IEC 62305-2 risk components (loss of life, service, heritage, economic value).
Strikes to incoming service lines and nearby structures.
Design of the protection system itself (air terminals, down conductors, grounding, surge protection).

5. Reference standards

Pointers only; consult the edition adopted by your authority having jurisdiction.

NFPA 780 (2023), Annex L — lightning risk assessment
IEC 62305-2 — risk management
IEC 61024-1-1:1993 — selection of protection level by efficiency (superseded by IEC 62305, still the common screening method)
NFPA 780 and IEC 62305-3 — protection system design
NASA LIS/OTD 0.5° High Resolution Full Climatology V2.3.2015, GHRC DAAC (doi:10.5067/LIS/LIS-OTD/DATA311)
Prentice, S. A. and Mackerras, D. (1977), J. Appl. Meteor. 16, 545–550 — ratio of cloud to ground flashes

6. Validation cases

These worked examples run through the engine on every change; the expected values come from the cited source. All cases: Validation.

Rectangular structure 30 × 20 m, 10 m high lightning/risk · Hand calculation

Pass

Source: Hand calculation from the published collection-area and frequency equations (NFPA 780 Annex L) — Ae = 30·20 + 6·10·(30+20) + 9π·10² = 6427.4 m²; Nd = 4 · 6427.4 · 1 · 1e-6 = 0.02571; Nc = 1.5e-3. E = 1 − 0.0015/0.025710 = 0.94166 → protection level II.

Quantity	Expected	RokBench	Error	Result
collectionAreaM2	6427.4	6427.4	0%	Pass
expectedFrequency	0.02571	0.02571	0%	Pass
tolerableFrequency	0.0015	0.0015	0%	Pass
requiredEfficiency	0.941656	0.9417	0.005%	Pass

7. Known issues and changes

See the Engineering Notes. Try the tool: Lightning Risk Assessment.