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

Ae = L·W + 6H(L + W) + 9πH² (area within 3H of the structure) Ng ≈ 0.1 · Td (only when flash density is unknown) Nd = Ng · Ae · C1 · 10⁻⁶ expected strikes per year Nc = 1.5 × 10⁻³ / (C2 · C3 · C4 · C5) tolerable strikes per year Protection recommended when Nd > Nc

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.

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
NFPA 780 and IEC 62305-3 — protection system design

6. Validation cases

Hand-calculated cases run on every change. See Validation.

7. Known issues and changes

See the Changelog. Try the tool: Lightning Risk Assessment.