How do I know if my building needs lightning protection?

BS EN 62305-2 requires a formal risk assessment. The key factor is the expected number of lightning strikes per year (Nd) multiplied by risk factors for building type, contents, and consequences. If the calculated risk exceeds the tolerable level (typically 10⁻⁵ per year for loss of human life), lightning protection is required. Tall buildings, isolated buildings, buildings with explosive/flammable contents, hospitals, schools, and churches are more likely to need protection.

What are the four protection levels in BS EN 62305?

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.

What is the rolling sphere method?

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.

How many down conductors does my building need?

The number of down conductors depends on the building perimeter and protection level. Down conductors are spaced equally around the perimeter: Level I and II = 10m maximum spacing, Level III = 15m, Level IV = 20m. 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.

What is the UK lightning flash density?

Lightning flash density (Ng) is the number of lightning flashes per km² per year. The UK average is about 0.5-1.0 fl/km²/yr — relatively low compared to tropical regions. South-East England has the highest density at approximately 1.0, while Scotland averages 0.5. The Met Office maintains lightning location data. Even at UK densities, tall or isolated buildings can expect a strike every few years, making protection important for vulnerable structures.

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Lightning Protection Calculator — BS EN 62305

Assess lightning strike risk per BS EN 62305-2 and determine protection level, down conductor spacing, mesh size, and rolling sphere radius for your building.

Building Length

External building length

Building Width

External building width

Building Height

Height to highest point

UK RegionDetermines flash density (Ng)

Building TypeAffects risk weighting

Protection LevelLevel III standard for most buildings

How We Calculate This

This calculator performs a simplified risk assessment per BS EN 62305-2 and determines the lightning protection system requirements.

Collection Area Formula

Ae = L×W + 2LH + 2WH + πH²

Nd = Ng × Ae × Cd × 10&sup-6;

Protection Levels

Level I: 20m sphere, 5m mesh, 10m down conductors
Level II: 30m sphere, 10m mesh, 10m down conductors
Level III: 45m sphere, 15m mesh, 15m down conductors
Level IV: 60m sphere, 20m mesh, 20m down conductors
Tolerable risk: 10&sup-5; for loss of life

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.