A simple geometric check — how far from the base of a 10 m mast does an IEC 62305 Class III lightning protection zone actually extend?
| LPS protection class | Class III |
| Rolling sphere radius for Class III | 45 m |
| Air terminal (mast) height | 10 m |
This is the radius at ground level within which the sphere cannot touch the ground without first touching the mast tip — geometrically, a sphere resting on the ground and just grazing the top of the mast.
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Mast height within sphere radius | h ≤ R | 10 m ≤ 45 m | ✓ PASS |
| Protection radius at ground level | n/a (this is the computed result) | 28.28 m | ✓ PASS |
Key insight: Protection radius grows with mast height, but not linearly — doubling the mast height doesn't double the protection radius, because the geometry is governed by a square-root relationship. There are real diminishing returns to just building a taller mast, which is why multi-terminal or mesh-conductor systems are typically more practical than a single very tall mast for protecting a large area.
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Open LPS Rolling Sphere Method calculator →The simple single-mast formula stops being valid once mast height exceeds the sphere radius for the chosen LPS class — physically, the sphere can then roll past the tip and touch a wider area at a different geometry, which requires a full multi-terminal or mesh-conductor rolling-sphere study rather than the single closed-form equation used here.
A smaller rolling sphere radius represents a more stringent protection level — it means the model assumes lightning can strike from a shorter final jump distance, which is associated with lower-current, more localized strikes that are harder to intercept. Class I (20 m radius) is therefore the most protective and demanding classification, while Class IV (60 m) is the least stringent.