What is Earth Resistance
The resistance offered by the earth electrode to the flow of current into the ground is known as the earth resistance or resistance to earth. The earth resistance mainly implies the resistance between the electrode and the point of zero potential.
The device used to measure the resistance of the earth is called Earth Resistance Tester. It has a voltage source, a meter to measure resistance in ohms, switches to change instrument range, wires to connect terminal to earth electrode and spikes.
Measurement of Earth Resistance
The earth resistance can be measured by two methods.\\\;\;\;1. Three Point Method\\\;\;\;2.Four Point Method
Three Point Method
In this method the terminals “C1” and “P1” of earth tester are short circuited to each other and connected to the earthing electrode under test. Terminals “C2” and “P2” are connected to two separate spikes. The distance between the electrodes and spikes should be as per the diagram.
Measure the resistance by pressing the test switch on the earth tester and note it as “R1“. Now move the “P2” spike towards earthing electrode almost 10%D and take the measurement and note it as “R2“. Now move the “P2” spike away from electrode almost 10%D and take the measurement and note it as “R3“. Take the average of “R1“, “R2” and “R3“
If measurements “R2” and “R3” are in agreement with the original measurement, within the required level of accuracy, then the test stakes have been correctly positioned and the DC resistance figure can be obtained by averaging the three results.
However, if there is substantial disagreement among any of these results, then it is likely that the stakes have been incorrectly positioned, either by being too close to the ground system being tested, too close to one another or too close to other structures that are interfering with the results. The stakes should be re-positioned at a larger separation distance or in a different direction and the three measurements repeated. This process should be repeated until a satisfactory result is achieved.
Four Point Method
In This Method four spikes are driven in earth in same line at the equal distance. The distance between earth electrodes should be at least 20 times greater than the electrode depth in ground. For Example, if the depth of each earth electrode is 1 foot then the distance between electrodes is greater than 20 feet.
The earth electrode under measurement is connected to C1 Terminal of Earth Tester. Connect the earth tester terminals with the remaining three spikes as shown in figure.
Press START/Test button of Earth tester and read out the resistance value. This is the actual value of the ground Resistance of the electrode under test. If this ground electrode is in parallel or series with other ground rods, the resistance value is the total value of all resistances. take few more readings by repeating above steps by increasing spacing between each electrode at equal distance and measure earth resistance value. Take the average value of all readings.
Calculate Soil Resistivity
The resistance value R can then be converted into a soil resistivity value with the formula: \\\;\;\;ρ=2πAR \\\;\\where “ρ” is measured in ohm-cm and “A” is the spacing of the pins in cm.
How to minimizing Earth Resistance
When the soil resistivity is high, and the resistance to ground exceeds the required values, specific techniques are useful to decrease it. The various ways to reduce the earth resistance are give below
- Avoid corrosive and Loose joints, i.e. Joints should be tight and clean.
- Pour sufficient water in the earth electrode.
- Increase the electrode diameter
- Use multiple rods
- Add salt and charcoal around the electrode
- Increase the width and depth of earth pit.