📋 Case Study

South African Coal Mine Lightning-Resistant Grounding System

Average 12 direct lightning strikes/year on HV lines; repeated transformer failures and relay misoperations

🏗️ Project Overview

Sasol Secunda Coal-to-Liquids Plant

🎯 Challenge

Average 12 direct lightning strikes/year on HV lines; repeated transformer failures and relay misoperations

🔧 Design Approach

Ring-type counterpoise grounding with 30 m radial rods + exothermic welded connections + surge arresters on all 33 kV terminations + soil resistivity-driven enhancement (bentonite + graphite backfill)

📐 Key Calculations

Step Potential Limit

E_step = ρ × I_fault × K_s / (2π × s)

Result: 1,140 V

Must remain <1,500 V for personnel safety

Ground Impedance Target

Z_g ≤ 5 Ω (IEEE Std 80)

Result: 3.2 Ω

Validated via fall-of-potential testing

📊 Results

Zero lightning-induced equipment damage in 3 years; relay false-trip rate dropped from 4.7 to 0.1 per month

💡 Lessons Learned

• •Exothermic welding critical for long-term joint integrity
• •Soil enhancement increased test repeatability by 94%

✅ Key Takeaways

• 1Exothermic welding critical for long-term joint integrity
• 2Soil enhancement increased test repeatability by 94%