Lightning protection under IEC 62305 uses the rolling sphere method: a sphere of a defined radius is imagined rolling across and around the structure. Wherever the sphere can touch, a strike could land, so an air terminal (lightning rod) is needed there; the region the sphere cannot reach is protected. The sphere radius depends on the chosen protection level, which comes from a risk assessment. A taller rod pushes the sphere up and protects a wider circle on the ground.
Reviewed: June 19, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: IEC 62305-3 (physical damage) and NFPA 780.
Safety notice. Lightning protection is life-safety and fire-safety engineering. This single-rod estimate is for orientation only; a compliant IEC 62305 / NFPA 780 design — including bonding, surge protection and earthing — must be carried out by a qualified specialist. See our disclaimer.
The rolling sphere formula
Here h is the height of the air terminal above the surface being protected and r is the rolling sphere radius. The protected radius grows with height, reaching a maximum of r when h equals the sphere radius. For structures taller than r the sphere can touch the sides, so corners and edges also need terminals — a single rod no longer covers everything.
Worked example — protecting a rooftop unit
Scenario: An LPL III installation (r = 45 m) with a 6 m mast protecting a rooftop HVAC unit.
So a 6 m mast under LPL III shields any equipment within about 22 m on the same plane. Pair this with a 15 × 15 m roof mesh and down-conductors at ~15 m spacing, all bonded to an earth system — size the electrode with the earth pit resistance calculator.
Frequently Asked Questions
You roll an imaginary sphere of fixed radius over the structure. Where it touches needs an air terminal; what it can't reach is protected. Radius: 20 m (LPL I), 30 (II), 45 (III), 60 (IV).
rp = √(2rh − h²) for h ≤ r. A 10 m rod under LPL II (r = 30) protects √(600 − 100) = √500 ≈ 22.4 m at ground.
LPL I (sphere 20 m, mesh 5×5, ~99% efficiency), II (30 m, 10×10), III (45 m, 15×15), IV (60 m, 20×20). Chosen from a structure risk assessment.
About 10 m for LPL I and II, 15 m for III, 20 m for IV around the perimeter — giving the current parallel paths and reducing side-flash risk.
Up to h = r, yes — the protected radius peaks at r. Beyond that the sphere touches the sides of tall structures, so corners and edges need their own terminals.