Lightning Protection Risk Calculator

1. Attractive collection area
   Ac = LxW + 2xH x (L+W) + pi x H^2
   L, W, H are building length, width, height in meters.
2. Expected strikes per year
   Nd = Ng x Ac x 10^-6
   Ng is lightning density in flashes per km^2 per year.
3. Risk index
   R = Nd x Pf x Lf x Cf x Rf x Sf x Gf x Hazard x Services x Downtime x People x Mf
   Factors represent occupancy, construction, roof, exposure, earthing, hazards, and mitigation.

• Measure the building length, width, and overall roof height.
• Enter local lightning density Ng from trusted regional sources.
• Choose occupancy, construction, roof type, and surroundings exposure.
• Select soil difficulty to reflect grounding design challenges.
• Add hazard and service modifiers if your site demands it.
• Press Calculate Risk, then export CSV or PDF.

Lightning density and strike frequency

The calculator starts with local lightning density (Ng), expressed as flashes per km^2 per year. Higher Ng increases the expected annual strike count (Nd) almost linearly. When comparing sites, keep Ng consistent with the same data source and averaging period. Use conservative values when the project is safety-critical or outages are unacceptable.

Building geometry and collection area

Larger and taller structures intercept more leaders, so the attractive collection area (Ac) grows with plan size and height. The screening formula combines roof footprint, side exposure, and a height term to capture this effect. Small changes in height can shift Ac meaningfully on compact buildings, especially in open terrain.

Occupancy, loss, and operational sensitivity

Risk is not only about being struck; it is also about what happens afterward. Occupancy and loss factors scale consequences for people, property, and mission. Facilities with vulnerable occupants, irreplaceable assets, or continuous operations typically justify stronger protection. The downtime and people modifiers help quantify those priorities in a consistent way.

Construction, roofing, and surrounding shielding

Materials and layout influence ignition likelihood, touch and step voltages, and the chance of side flashes. Combustible roofs or rooftop PV can increase exposure, while urban shielding can reduce effective interception. Select surroundings carefully: an isolated structure on a ridge behaves very differently than a building among taller neighbors.

Mitigation selection and documentation

The mitigation factor represents how external lightning protection systems and surge protection devices reduce overall risk. Use the suggested class as a starting point, then align it with codes, insurer requirements, and equipment tolerance. Export the CSV or PDF to document assumptions, compare alternatives, and support design decisions during reviews.

1) What does the risk index represent?

It is a screening indicator combining expected strikes with consequence and mitigation factors. It helps compare options and prioritize protection, but it does not replace a full standards-based assessment.

2) Where can I get Ng for my site?

Use national meteorological sources, lightning detection network maps, or regional climatology reports. Prefer multi-year averages, and document the source used for consistency across projects.

3) Why does height change results so much?

Height increases the collection area through side exposure and the height term in Ac. Taller structures intercept more leaders, especially when surrounding shielding is limited.

4) Which mitigation option should I select?

Select what currently exists on the structure. If you are evaluating upgrades, run multiple scenarios and compare risk levels and recommendations side by side.

5) Does soil type affect lightning strikes?

Soil does not change lightning occurrence, but it influences earthing design difficulty and potential voltage rise. Higher resistivity typically increases the importance of robust grounding and bonding.

6) Can I use this output for compliance reports?

Use it for planning and documentation, then perform the required code or standard analysis for compliance. Attach exports as supporting evidence for design reviews and discussions.