How to Calculate Earthing Resistance
What is Earthing Resistance?
Measures electrical resistance of grounding system. Low resistance ensures safety by quickly dissipating faults to ground.
Step-by-Step Guide
- 1Rod length and diameter affect resistance
- 2Soil resistance varies (clay 10, loam 50, sand 1000+ ohms)
- 3Resistance ≈ (ρ ÷ L) × (ln(8L÷d) - 2)
- 4Target <5 ohms for safety (varies by code)
- 5Multiple rods in parallel reduce resistance
Worked Examples
Input
20 ohm rod, 2 meter deep
Result
5-10ohm
Common Mistakes to Avoid
- ✕Not measuring resistance after installation
- ✕Using single rod in high-resistance soil
Frequently Asked Questions
Why is low earthing resistance critical?
Allows fault current to return safely; high resistance causes dangerous voltages.
How do I improve earthing resistance?
Longer rods, lower soil resistance (add bentonite), multiple rods, better soil moisture.
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