Lightning Protection Basics: Rolling Sphere and Mesh Method

A lightning protection system is often drawn as “a rod on the roof and a cable to the ground”. Then the design review asks which parts of the roof are protected, and nobody can answer. The rolling sphere and the mesh method give a clear yes or no for every edge, parapet and rooftop unit.

Step by step

1. Set the class. Run the IEC 62305-2 risk assessment, or follow the national rule for the building type. Why: the class drives every other number, and “class I everywhere” wastes money.
2. Draw the sphere on sections and elevations. In CAD, draw a circle of radius r and roll it over the building outline, touching the ground and the structure. Why: it shows exactly which edges, corners and tall parts can be struck.
3. Put air terminations wherever the sphere touches. On a flat roof, that means conductors on all edges, parapets and ridges, plus a mesh no larger than the class allows. Why: most strikes hit corners and edges, not the middle of the roof.
4. Protect rooftop equipment with rods. Between two rods or conductors the sphere sags by p = r − √(r² − (d/2)²), where d is the spacing. The equipment must stay below the rod tips minus p. Why: a unit that sticks out of the protected zone becomes the air terminal.
5. Keep the separation distance. Calculate s = ki × (kc / km) × l between the LPS and metal parts or cables that are not bonded to it. Why: otherwise the strike flashes over into the building’s services.
6. Run the down conductors. Use at least two, spread evenly and preferably at the corners, at the class spacing. Fit a test joint 0.3–1 m above ground on each one. Why: several paths split the current and lower the voltages along each conductor.
7. Earth and bond. A type B ring about 0.5 m deep and about 1 m from the walls, or a foundation electrode, bonded to the main earthing terminal. Add a Type 1 SPD at the origin of the installation. Why: part of the lightning current enters the building through the incoming services.

Worked example: rods over a rooftop unit

Common slips

• Forgetting the walls of tall buildings. On a building taller than r, the sphere touches the facade above height r. Those upper edges and corners need air terminations too.
• Using “special” air terminals to cover a large radius. Early streamer emission devices are not recognised by IEC 62305. Design with the sphere, mesh or angle method.
• Undersized conductors. Copper 50 mm² (Ø 8 mm round) and hot-dip galvanised steel 50 mm² are the usual minimums for air terminations and down conductors.
• No bonding of roof metalwork. Handrails, ducts and cable trays within the separation distance must be bonded or moved.
• Earth ring not bonded to the main earth. The LPS earth and the electrical earth must be one system. See earthing systems explained and how to test the earth electrode.
• No surge protection. An LPS without a Type 1 SPD at the origin lets part of the strike into the boards. See how to connect an SPD.

US projects follow NFPA 780, which also uses a rolling sphere, typically 46 m (150 ft) for ordinary structures, with air terminals at most about 6 m (20 ft) apart along roof edges.

Typical values — verify with the edition adopted locally.

FAQ

What rolling sphere radius should I use?

Under IEC 62305 the radius follows the protection class: 20 m for class I, 30 m for II, 45 m for III and 60 m for IV. The class comes from the risk assessment or the national rule for the building type. NFPA 780 uses 46 m (150 ft) for ordinary structures.

How do I calculate the sag of the sphere between two rods?

Use p = r − √(r² − (d/2)²), where d is the distance between the rods. For r = 45 m and d = 10 m, p is about 0.28 m, so equipment between the rods must stay at least 0.28 m below the rod tips.

When should I use the mesh method instead of the rolling sphere?

Use a mesh on flat or gently sloping roofs, with conductors on all edges and a cell size no larger than the class allows (5 to 20 m). Use the rolling sphere for complex shapes, tall parts and rooftop equipment. Most designs use both.