Lightning Protection Calculator — BS EN 62305

BS EN 62305-2 requires a formal risk assessment by a competent person. It compares the calculated risk R1 — the sum of components Nx×Px×Lx covering the number of dangerous events, the probability of damage and the resulting loss — against the tolerable risk RT, which is 10⁻⁵ per year for loss of human life. If R1 exceeds RT, protection measures are required to bring it back below RT. This calculator gives an indicative screening only: it derives the collection area and number of dangerous events (Nd) per the standard but uses a simplified rule-of-thumb risk figure, so it cannot replace the full assessment. Tall, isolated or hilltop buildings, and those with explosive/flammable contents, hospitals, schools and churches, are more likely to need protection.

Level I — Highest protection (>98% efficiency), 20m rolling sphere radius, 5m mesh, 10m down conductor spacing. Used for explosive stores, critical infrastructure. Level II — 30m sphere, 10m mesh, 10m spacing. Used for hospitals, data centres. Level III — 45m sphere, 15m mesh, 15m spacing. Standard for most commercial buildings. Level IV — 60m sphere, 20m mesh, 20m spacing. Basic protection for low-risk buildings.

The rolling sphere method is a design technique where an imaginary sphere (radius depends on protection level) is rolled over the building and surrounding area. Any point on the building touched by the sphere could be struck by lightning and needs an air termination (conductor) at that point. The sphere represents the maximum distance a lightning leader can jump. Smaller sphere radius = more air terminations needed = better protection.

Per BS EN 62305-3 the number of down conductors depends on the building perimeter and protection level. Down conductors are spaced equally around the perimeter at the typical distance for the class of LPS: Level I and II = 10m, Level III = 15m, Level IV = 20m. A minimum of two down conductors is required for any structure. For a building with 80m perimeter at Level III: 80/15 = 5.3, round up to 6 down conductors. They should be as straight as possible, avoiding sharp bends, and connected to the earth termination system at the base. Use our Earthing System Calculator to design the earth termination network.

Lightning flash density (Ng) is the number of lightning flashes per km² per year. Across the UK it is broadly in the range 0.5-1.0 fl/km²/yr — low compared to tropical regions — with the south and east tending higher than Scotland and the far north. The regional figures in this tool are indicative averages for a quick screen only; a formal BS EN 62305-2 assessment must use the measured local flash density for the actual site, taken from the Met Office or a recognised lightning-density map, not a fixed regional average. Even at UK densities, tall or isolated buildings can expect a strike every few years, making protection important for vulnerable structures.