Calcolatrice Resistenza di Terra — Formula
## Ground Rod Resistance
The resistance of a ground rod depends on soil resistivity and rod geometry.
### Formula (simplified for vertical rod)
**R = (rho / 2 pi L) ln(4L / d)**
where rho is soil resistivity in ohm-metres, L is rod length, d is rod diameter, and ln is the natural logarithm. Lower resistance is better for safety. Most codes require < 5 ohms for power systems and < 25 ohms for lightning protection.
The resistance of a ground rod depends on soil resistivity and rod geometry.
### Formula (simplified for vertical rod)
**R = (rho / 2 pi L) ln(4L / d)**
where rho is soil resistivity in ohm-metres, L is rod length, d is rod diameter, and ln is the natural logarithm. Lower resistance is better for safety. Most codes require < 5 ohms for power systems and < 25 ohms for lightning protection.
Esempio Risolto
A 3 m x 16 mm ground rod in soil with 100 ohm-m resistivity.
- d = 0.016 m
- ln(4 x 3 / 0.016) = ln(750) = 6.62
- R = 100 / (2 x pi x 3) x 6.62 = 100/18.85 x 6.62 = 35.12 ohm
- This exceeds 5 ohm. Multiple rods or soil treatment may be needed.