Transmission Line Grounding Design: Tower Footing Resistance, Lightning Performance, and IEEE/CIGRE Standards

An authoritative engineering guide covering tower grounding, soil resistivity, grounding techniques, backflashover reduction, and lightning performance optimization.

1. Purpose of Transmission Line Grounding

• Control tower voltage rise during lightning
• Minimize backflashover probability
• Dissipate earth fault current
• Ensure touch and step voltage safety

2. Soil Resistivity Measurement (IEEE 81)

The Wenner 4-pin method calculates soil resistivity:

ρ = 2πaR

Where a = pin spacing, R = measured resistance.

3. Soil Modeling

• Single-layer soil
• Two-layer soil
• Multi-layer soil (CIGRE)

4. Tower Footing Resistance (TFR) Targets

Voltage	Target TFR
69 kV	25 Ω
115–138 kV	15–20 Ω
230 kV	10 Ω
500 kV	5–7 Ω

5. Tower Grounding Methods

• Counterpoise wires
• Ground rods (multiple)
• Deep rods
• Chemical-enhanced rods
• Ground boosters

6. Lightning Current Distribution

Tower voltage rise:

V = I * Rt

7. Backflashover Reduction

• Lower TFR
• Increase insulator string length
• Improve shielding angle
• Install line arresters

8. CIGRE Methods for LLP

CIGRE defines statistical computations for backflashover and shielding failure probability.

9. Numerical Example (230-kV Tower)

• Soil resistivity = 400 Ω·m
• Single counterpoise 60 m
• Final TFR achieved = 12 Ω

10. Measuring Footing Resistance

Use the Fall-of-Potential method:

Rt = V / I

11. Ground Boosting Techniques

• Parallel counterpoise
• Additional rods
• Soil conditioning
• Mesh grounding