Earth Resistance Calculator
Accurate earth resistance calculations for grounding systems
Earth Resistance Calculator
Compliant with IEC 60364 / IEEE 80 Standards
Results:
How to Use the Calculator
1. Soil Data
Enter the Soil Resistivity (ρ) in Ω·m. This is the most critical factor affecting the result.
2. Dimensions
Input the length and diameter of your vertical earth rod. Standard length is 3m.
3. Calculate
Click Calculate to see the estimated earth resistance and a step-by-step breakdown.
How to Calculate Earth Resistance
1. Identify Variables
To calculate the earth resistance of a single vertical rod, you need three key values:
- ρ (Rho): Soil Resistivity in Ω·m
- L: Length of the rod in meters (m)
- d: Diameter of the rod in meters (m)
Ensure all units are consistent (convert mm to m for diameter).
2. Apply Formula
Use the standard IEC / IEEE formula for a driven rod:
This formula assumes the rod is driven into uniform soil.
3. Analyze Result
The result R is the resistance in Ohms (Ω).
Goal: Achieve a resistance value as close to zero as possible. For most electrical systems, a value below 5 Ω is preferred, and below 1 Ω is excellent.
Earth Resistance Chart
Typical resistance values for a single 3m rod in different soil types.
| Soil Type | Typical Resistivity (Ω·m) | Estimated Resistance (Ω) |
|---|---|---|
| Wet Clay / Humus | 10 - 30 | 3 - 10 |
| Moist Loam | 30 - 100 | 10 - 35 |
| Dry Sand | 200 - 1000 | 65 - 330 |
| Gravel / Bedrock | 1000 - 3000+ | 330 - 1000+ |
Earth Resistance FAQs
What is a good earthing resistance?
For general electrical safety (TT systems), a value below 100 Ω is often acceptable if RCDs are used, but IEEE/IEC standards recommend keeping it below 5 Ω for sensitive equipment and telecommunications. Ideally, strive for 1 Ω or less for substations.
How to check earth resistance at home?
You typically need a specialized Earth Tester (Megger) using the 3-point fall-of-potential method. For a rough check, an Earth Clamp Meter can be used if there is a loop. A standard multimeter cannot accurately measure earth resistance directly against the ground itself.
What is the formula for earthing?
For a single vertical rod, the formula is:
R = (ρ / 2πL) × [ln(4L/d) - 1]
Where ρ is soil resistivity, L is length, and d is diameter.
Can I measure earth resistance with a multimeter?
No. A standard multimeter measures resistance between two specific points using low current. Earth resistance testing requires driving test spikes into the ground and injecting a specific test current to measure the potential difference relative to the earth mass.
How many ohms is a good ground?
Under 5 Ohms is considered a good ground for most commercial and industrial applications. Under 25 Ohms is the requirement for a single electrode per NEC (USA) unless a second electrode is added.
What is a good TT earth reading?
In a TT earthing system, the earth resistance `Ra` depends on the RCD rating. It must satisfy Ra × IΔn ≤ 50V. For a 30mA RCD, Ra should be ≤ 1667 Ω. However, to handle soil instability and aging, values below 200 Ω (often 100 Ω) are recommended field practice.
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