Question

Describe with neat circuit diagram the measurement procedure of earth resistance for an installation.

Answer 1

To measure the earth resistance for an installation, we typically use the Wheatstone Bridge method or the Fall-of-Potential method. However, one of the most common methods is the Fall-of-Potential method (also called the "three-point method"). This method is based on measuring the voltage drop as current flows through the earth and determining the resistance. Below is an explanation of how to measure earth resistance using this method, along with a circuit diagram.

Procedure for Measuring Earth Resistance Using the Fall-of-Potential Method:

1. Circuit Diagram:

Here's the general circuit diagram used for measuring earth resistance with the Fall-of-Potential method:

`
          +------------------------+
          |                        |
         [P1]---->----[A]---(Earth Electrode)----[B]----<----[P2]
          |                        |
          |                        |
         [C]                      [D]
        (Source)                (Voltmeter)
          |
         (R) (Current electrode)
`

1. [P1] and [P2]: These are the potential probes that are used to create a circuit with the earth and measure the voltage drop.
   

1. [A] and [B]: These are the current injection probes placed at a known distance from the earth electrode.
   

1. [R]: The current electrode through which a known current is injected into the earth.
   

1. [C]: Power supply (source) to provide a known current for injection.
   

1. [D]: Voltmeter to measure the potential difference between probes.
   

2. Setting up the Test:

1. Prepare the earth electrode: The earth electrode (the ground rod or plate you are measuring) should be connected to the installation's earthing system. This is the electrode for which you will measure the resistance.
   

1. Place the current electrode (R): A current electrode (typically a metal rod or plate) is placed at a known distance away from the earth electrode. The distance between the current electrode and the earth electrode is critical to the accuracy of the measurement.
   

1. Place the potential probes (P1 and P2): These probes are placed at varying distances from the earth electrode along the line of the current electrode. The voltage between these probes will be measured.
   

1. Use a power supply: The power supply [C] is used to inject a known current through the earth. This current passes through the earth from the current electrode (R) to the earth electrode. This allows for the measurement of the voltage drop between the potential probes.
   

3. Taking Measurements:

1. Inject Current: Activate the power supply to inject a constant current through the ground. The current flows through the earth between the current electrode and the earth electrode.
   

1. Measure Voltage: Measure the voltage drop between the potential probes [P1] and [P2] using the voltmeter. The voltage drop is proportional to the earth resistance.
   

1. Change Position of Probes: The potential probes are moved step by step farther from the earth electrode, and the voltage readings are taken at each position.
   

1. Plot the Voltage vs Distance Curve: After taking the measurements at different distances, plot the voltage readings against the distance of the probes from the earth electrode.
   

1. Find the Earth Resistance: From the plot, the point where the voltage stabilizes (no longer changing significantly with distance) indicates the fall of potential curve, which helps determine the earth resistance.
   

4. Formula to Calculate Earth Resistance:

Once you have the voltage and current measurements, the resistance of the earth electrode can be calculated using Ohm’s Law:

\[
R_{\text{earth}} = \frac{V}{I}
\]

Where:

1. \( V \) is the voltage drop measured between the potential probes.
   

1. \( I \) is the injected current.
   

Alternatively, the earth resistance can be calculated using the following formula if you know the resistivity of the soil and other factors:

\[
R_{\text{earth}} = \frac{\rho}{2 \pi L} \cdot \ln \left( \frac{D}{r} \right)
\]

Where:

1. \( \rho \) = resistivity of the soil
   

1. \( L \) = length of the earth electrode
   

1. \( D \) = distance between the current and potential probes
   

1. \( r \) = radius of the earth electrode
   

5. Results and Interpretation:

The final value obtained from the Fall-of-Potential method gives the earth resistance, which is crucial for ensuring that the installation is properly grounded. For safety reasons, the earth resistance should generally be below 10 ohms for industrial and commercial installations, although this may vary depending on the local regulations and soil conditions.

Precautions:

1. Ensure that the measurement is conducted on a dry day since wet soil can alter the resistance significantly.
   

1. The distance between the electrodes should be sufficient to avoid interference between the electrodes.
   

This method provides a simple yet effective way to measure the earth resistance and is widely used in grounding system testing.