Earth tester set has four terminals, two current terminals marked C1 and C2 and two potential terminals marked P1 and P2. The distance between earth electrodes should be at least 20 times greater than the electrode depth in ground. 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.
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Earth tester set has four terminals, two current terminals marked C1 and C2 and two potential terminals marked P1 and P2. The distance between earth electrodes should be at least 20 times greater than the electrode depth in ground. 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. Drive another potential Earth terminal P1 at depth of 6 to 12 inches from some distance at C1 Earth Electrode and connect to P1 Terminal of Earth Tester by insulted wire.
Drive another potential Earth terminal P2 at depth of 6 to 12 inches from some distance at P1 Earth Electrode and connect to P2 Terminal of Earth Tester by insulted wire. Connect the ground tester as shown in the picture. This is the actual value of the ground Resistance of the electrode under test. Record the reading on the Field Sheet at the appropriate location. If the reading is not stable or displays an error indication, double check the connections.
The test set measures both the current and the voltage and internally calculates and then displays the resistance. Ground resistance measurements are often corrupted by the existence of ground currents and their harmonics. This automatically selects the testing frequency with the least amount of noise enabling you to get a clear reading. Repeat above steps by increasing spacing between each electrode at equal distance and measure earth resistance value.
Average the all readings An effective way of decreasing the electrode resistance to ground is by pouring water around it. The addition of moisture is insignificant for the reading; it will only achieve a better electrical connection and will not influence the overall results.
Also a longer probe or multiple probes within a short distance may help. Application: It is advisable for Medium or Large electrode System. It is Quick, easy method. Extremely reliable conforms to IEEE 81; Disadvantage: There need to turn off the equipment power or disconnect the earth electrode.
One major drawback to this method is that it requires a large distance for measurement. This distance can range up to 2, feet or more for ground systems covering a large area or of very low resistance. Time consuming and labor intensive 2 Three Point Fall-of-potential Method. Drive another Current Electrode C2 into the earth to feet at depth of 6 to 12 inches from the center of the electrode and connect to C2 Terminal of earth tester. Place the potential electrode P2 This is the actual value of the ground electrode under test.
Move the potential electrode 10 feet farther away from the electrode and make a second Measurement. Move the potential probe 10 feet closer to the electrode and make a third measurement.
If the three measurements agree with each other within a few percent of their average, then the average of the three measurements may be used as the electrode resistance. If the three measurements disagree by more than a few percent from their average, then additional measurement procedures are required.
The electrode center location seldom is known. In this case, at least three sets of measurements are made, each with the current probe a different distance from the electrode, preferably in different directions. When space is not available and it prevent measurements in different directions, suitable measurements can be made by moving the current probe in a line away from or closer to the electrode.
For example, the measurement may be made with the current probe located , and feet along a line from the electrode. Each set of measurements involves placing the current probe and then moving the potential probe in 10 feet increments toward or away from the electrode. The starting point is not critical but should be 20 to 30 feet from the electrode connection point, in which case the potential probe is moved in 10 feet increments toward the current probe, or 20 to 30 feet from the current probe, in which case the potential probe is moved in 10 feet increments back toward the electrode.
The spacing between successive potential probe locations is not particularly critical, and does not have to be 10 feet, as long as the measurements are taken at equal intervals along a line between the electrode connection and the current probe. Larger spacing means quicker measurements with fewer data points. Once all measurements have been made, the data is plotted with the distance from the electrode on the horizontal scale and the measured resistance on the vertical scale.
Importance of Position of Current Electrode C2 : Fall-of-Potential measurements are based on the distance of the current and potential probes from the center of the electrode under test. For highest degree of accuracy, it is necessary that the probe is placed outside the sphere of influence of the ground electrode under test and the auxiliary earth.
If we Place Current Electrode C2 too near to Earth Electrode C1 then the sphere of influence, the effective areas of resistance will overlap and invalidate measurements taken. For the accurate results and to ensure that the ground stakes are outside the spheres of influence.
Reposition the inner Potation Electrode P1 1meter in either direction and take a fresh measurement. The best distance for the current probe is at least 10 to 20 times the largest dimension of the electrode. Because measurement results are often distorted by underground pieces of metal, underground aquifers, etc so re measurements are done by changing axis of earth spike by 90 degrees, by changing the depth and distance several times, these results can be a suitable ground resistance system.
The table is a guide for appropriately setting the probe inner stake and auxiliary ground outer stake.
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