Lightning Protection System Design per NFPA 780
The Lightning Protection Calculator uses the rolling sphere method to design air terminal placement and verify zone of protection (ZOP). Required for hospitals, data centers, and high-risk structures.
Rolling Sphere Method
Imagine a sphere of radius R rolling over the structure. Any point touched = unprotected. Air terminals must intercept the sphere.
R = 150 ft (Level I), 100 ft (II), 75 ft (III), 50 ft (IV)
Zone of Protection
Conical ZOP from air terminal:
Horizontal radius at height h: r = R - √(R² - (R - h)²)
Air Terminal Spacing
| Level | Max Spacing | Min Height |
|---|---|---|
| I | 20 ft | 2 ft |
| II | 25 ft | 2 ft |
| III | 30 ft | 2 ft |
| IV | 40 ft | 2 ft |
Strike Risk Assessment
- Annual strikes = N_g × A_e × C_d × 10⁻⁶
- N_g = ground flash density (flashes/km²/year)
- A_e = equivalent collection area
Examples
- 60 ft building, Level II (R=100 ft), 2 ft terminals → ZOP radius at roof = 89 ft
- 100×50 ft roof → Need 6–8 terminals
Best Practices
- Place terminals at corners, edges, ridges
- Max 20–40 ft spacing
- Bond all metal within 6 ft
- Use UL 96A components
Conclusion
Proper LPS prevents fire, explosion, and equipment damage. Use with Ground Resistance and Surge Protection tools.