Measuring soil resistivity is essential before designing a grounded earthing system for a substation because it determines how well the ground can dissipate fault currents. If the soil resistivity is high, the grounding system needs more dense earthing mesh and more number of conductive electrodes to ensure proper safety. A well-designed earthing system prevents intolerable voltage buildup, protects equipment, and reduces the risk of electric shock. Since soil resistivity varies with depth, moisture, and composition, accurate testing helps in selecting the right grounding method. Without proper measurement, the grounding system may fail to provide adequate protection, leading to dangerous conditions during faults.
Wenner Method of Soil Resistivity Measurement
The Wenner four-electrode method is mostly used for earth resistivity measurement because it gives accurate and reliable results with a simple setup. It uses four equally spaced electrodes, making it easy to apply and analyze. The method helps in measuring resistivity at different depths just by changing the electrode spacing, which is useful for understanding soil layers. It also reduces errors caused by contact resistance, ensuring precise readings. Since this method provides a clear idea of soil resistivity distribution, it is widely preferred for designing effective grounding systems in substations.
The Wenner method works by placing four electrodes in a straight line, equally spaced apart. The outer two electrodes pass a constant electric current into the ground, while the inner two electrodes measure the voltage difference caused by this current. The soil resistivity is then calculated using the formula
$$ \rho = 2\pi\times a \times R $$ Where “a” is the spacing between the electrodes and “R” is the measured resistance.
To perform the test, metal rods (electrodes) are inserted into the ground at a fixed distance. A ground resistance meter is then connected to these electrodes using cables. The meter applies a known current through the outer electrodes, and the inner electrodes measure the resulting voltage drop. This reading is used to determine the soil’s ability to conduct electricity.
The key advantage of the Wenner method is that by increasing the electrode spacing, deeper layers of soil can be tested without needing to dig. This makes it useful for understanding soil resistivity at different depths, which is important when designing grounding systems for substations. The test should be repeated at multiple locations to get an accurate picture of soil conditions across the site.
One important factor to consider during testing is ensuring that the electrodes are properly inserted into the ground and that there is good contact with the soil. If the ground is too dry or rocky, adding water or using conductive gel can help improve contact. Environmental factors like soil moisture and temperature can also affect results, so tests should ideally be done under different conditions to get reliable data.
Overall, the Wenner four-electrode method is a simple yet powerful way to measure soil resistivity. It helps engineers design effective grounding systems that improve electrical safety, prevent faults, and ensure stable operation of substations.
Practical Example of Using the Wenner Four-Electrode Method for Soil Resistivity Measurement
To conduct a soil resistivity test using the Wenner method, consider a substation site where soil conditions need to be analyzed for designing an earthing system. The following standard practices are followed:
Notation of Electrodes: The electrodes are also called probes or spikes.
C1 – The first electrode, where the test current is injected into the ground.
P1 – The second electrode, used to measure the voltage.
P2 – The third electrode, also used for voltage measurement.
C2 – The fourth electrode, which completes the current circuit by returning the current to the meter.
Standard Electrode Spacing (a): The spacing between the electrodes varies depending on how deep into the soil layers the measurement is needed. Common spacing values are 2m, 5m, 10m, 15m, 20m, 25m and so on. The deeper the required resistivity measurement, the larger the spacing between electrodes. The depth of the electrodes should not exceed 1/20th of the spacing between them. All four electrodes must be inserted in the soil in a straight line. Ideally, measurements should be taken in different directions (e.g., North-South, East-West) to account for soil non-uniformity. Sometimes, it is also recommended to measure North-East to South-West and South-East to North-West directions in addition of North to South and East to West directions for more accuracy.
Measurement Process: Insert the electrodes into the soil at the selected distance. Say a = 2 m. The spikes are inserted in the soil along the straight line directed north to south. Connect the electrodes to the resistance meter using test leads. Set the meter to the soil resistivity measurement mode. Inject a known current through C1 and C2 and measure the voltage between P1 and P2. The resistance (R) value is displayed on the meter. Using the formula ρ = 2πaR, calculate the soil resistivity (Ω·m). Now, long the same straight line place the electrodes 5 m away to each other and repeat the process of resistance measurement. Record the value of resistivity calculated by the same formula. Then place the electrodes along same straight line 10 m away from each other and repeat the test and record resistivity. After sufficient and satisfying numbers of measurements change the direction towards east to west and repeat multiple similar measurements. At last from all the resistivity values we have to calculate the overall soil resistivity (Ω·m) in respect of the point or location. This the test for one point or location.
Standard Considerations: The same test should be conducted in multiple locations around the site to get an accurate representation of soil conditions. If the ground is very dry, slightly moistening the soil around the electrodes can help improve contact. Tests should be performed in dry seasons to account for maximum soil resistivity. If this test is conducted in rainy season the result may be misleading.
Example Calculation: Assume the following test values:
Electrode spacing a = 5 m
Measured resistance R = 12 Ω
Using the formula: ρ = 2π×5×12 = 377 Ω·m
This means the soil resistivity of approximate of 5 m depth is 377 Ω·m
Relation between Electrode Spacings and the Depth of the Soil being measured for Resistance
In the Wenner Method of soil resistivity testing, the spacing between the electrodes directly affects how deep the measurement goes. A known current flows into the ground through the two outer electrodes, while the voltage drop is measured between the two inner ones. The key factor here is the electrode spacing (a), which determines the approximate depth of soil being measured.
Simply put:
- If the electrodes are 2 meter apart, the resistivity measurement represents the soil down to about 2 meter.
- If the spacing increases to 5 meters, the reading reflects an average resistivity down to around 5 meters.
This happens because the current spreads out in a semi-spherical pattern. The larger the spacing, the deeper the current penetrates, sampling more soil layers. However, the influence of deeper layers is weaker compared to the shallower ones, meaning the actual depth probed is slightly less than “a” and depends on soil conditions.
To analyze resistivity at different depths, testers gradually increase electrode spacing (e.g., 2m, 5m, 10m, 15m, etc.) and take multiple readings. These readings give a weighted average rather than a precise layer-by-layer breakdown. For more accuracy, additional interpretation techniques (like computer modeling) help separate the influence of shallower layers from deeper ones.
In short: Increasing electrode spacing in the Wenner Method lets you measure deeper soil resistivity, but the result is an average over that depth rather than a direct measurement of the deepest point.