Lightning Protection: From LPS Class to Roof Geometry

This is a design-geometry and quantity tool for a conventional building lightning protection system. Once an LPS class has been established, it converts that class into the practical geometry that appears on drawings: rolling-sphere radius, maximum mesh dimension, typical down-conductor spacing, a first-pass separation distance and conductor quantities.

It deliberately does not claim to perform the full lightning risk assessment. IEC 62305-2 was technically revised in 2024, including a move to lightning ground strike-point density N_SG in the risk procedure. Risk is also more than strike frequency. It depends on loss, occupancy, fire consequences, incoming lines, protection measures and other project-specific factors. Use the selected class here only after that assessment or another valid project requirement has established it.

These are the geometry values used by the calculator. A more severe protection class uses a smaller rolling sphere and tighter mesh. The class is not a quality grade to choose by instinct. It is a design consequence of the risk assessment or another valid requirement.

The plan view in the calculator rounds the mesh into equal cells that do not exceed the selected maximum dimension. The down-conductor count is then rounded up so the average perimeter spacing does not exceed the class spacing used by the tool.

The mesh method is efficient on simple roof planes. Conductors follow the perimeter and divide the roof into cells no larger than the class limit. It is a geometric rule, not proof that every rooftop object is protected.

The rolling-sphere method is the more general geometric test. Imagine a sphere of the class radius rolling over the structure. Any point the sphere can touch is a potential strike point and needs to be intercepted or otherwise protected. Smaller spheres therefore produce stricter protection.

Air rods create local interception points. The calculator shows the theoretical reach at a flat protected plane using the rolling-sphere geometry. That circle is useful for intuition, but adjacent rods, parapets, tanks, PV frames and changes in roof level require a full geometric check.

A lightning conductor can rise to a very high transient potential while carrying current to earth. If nearby internal metalwork, cable containment, pipework or structural steel sits too close, the current can jump the gap. The separation distance is therefore not a decorative clearance. It is part of the insulation coordination of the lightning protection system.

The calculator uses the familiar simplified structure s = k_i × k_c × l / k_m. Here k_i follows the selected LPS class, k_c represents current sharing, k_m represents the insulating medium, and l is the relevant conductor length. The 2024 revision of IEC 62305-3 specifically clarifies use of general and simplified separation-distance methods, so do not treat a default k_c or k_m as a universal constant.

For a current Kenyan design basis, check the applicable Kenya Standards adoption and project requirements. KEBS lists KS IEC 62305-1:2024, KS IEC 62305-3:2024 and KS IEC 62305-4:2024 in its standards catalogue. Internationally, IEC 62305-2:2024 is the current risk-management part.

• Part 1: general principles and lightning parameters.
• Part 2: risk management and selection of protection measures.
• Part 3: physical damage, life hazard and the external LPS.
• Part 4: surge protection measures for electrical and electronic systems.

This web tool is an engineering aid. It does not reproduce the standard and does not replace the licensed standard, project specifications, statutory requirements, competent design review, installation inspection or testing.

Lightning protection interfaces directly with the building earthing and bonding system, surge protective devices, main distribution board and sensitive electronic loads. Use the protective device tools for the LV distribution design, and keep the complete suite under EEE Kenya calculators.