Can we use an Megger or Multimeter for earth resistivity Testing
- We cannot use Megger or Mulitimeter for Earth resistivity Testing.
Insulation Tester (Megger):
- Insulation testers are designed to measure at the opposite end of the resistance by inserting high DC Voltage.
- Insulation testers use high test voltages in the kilovolt range. The area between electrode and ground is charged with high DC Voltage and we do not want grounds that measure in megohms.
- Ground testers use Low Voltage for testing for operator safety, to low voltages.
Multimeter:
- However, a Multimeter or continuity test can use very low Voltage between an installed electrode and a reference ground, which is assumed to have negligible.
- Low voltage DC can produce a resistance reading between ground and an earth electrode but it is not an accurate measurement.
- Multimeter measurement may not be reliable, since reading can be influenced by soil transients, the electrical noise that is generated by utility ground currents trying to get back to the transformer, as well as other sources.
Can Earth resistance reduce by pouring Water around Test Earth Probe
- By pouring water is near test probe reduce contact resistance of between probe and ground at some extent.
- If there is sufficient contact between probe and ground then pouring water near test probe is never decrease earth resistance of the system.
- Earth resistance is the resistance of the ground electrode that is being measured, not that of the test probe. The Test probe is a tool to use measurement of earth resistance.
- If the test setup has adequate spacing, the probes will be far enough away outside of the electrical field of the test ground so that watering them has no influence on the test result.
Test Methods for Measuring Earth Resistance
There are six basic test methods to measure earth resistance
- Four Point Method (Wenner Method)
- Three-terminal Method (Fall-of-potential Method / 68.1 % Method))
- Two-point Method (Dead Earth Method)
- Clamp-on test method
- Slope Method
- Star-Delta Method
(1) Four Point Method (Wenner Method):
- This method is the most commonly used for measuring soil resistivity,
Required Equipments:
- Earth Tester (4 Terminal)
- 4 No’s of Electrodes (Spike)
- 4 No’s of Insulated Wires
- Hammer
- Measuring Tap
Connections:
- First, isolate the grounding electrode under measurement by disconnecting it from the rest of the system.
- Earth tester set has four terminals, two current terminals marked C1 and C2 and two potential terminals marked P1 and P2.
- P1 = Green lead, C1 = Black lead, P2 = Yellow lead, C2 = Red lead
- In this method, four small-sized electrodes are driven into the soil at the same depth and equal distance from one another in a straight line.
- 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.
- Drive another Current Electrode (C2) at depth of 6 to 12 inches from some distance at P2 Earth Electrode and connect to C2 Terminal of Earth Tester by insulted wire.
- Connect the ground tester as shown in the picture.
Testing Procedure:
- Press START and read out the resistance value. 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. For some meters, the RANGE and TEST CURRENT settings may be changed until a combination that provides a stable reading without error indications is reached.
- The Earthing Tester has basically Constant Current generator which injects current into the earth between the two current terminals C1 (E) and C2 (H).
- The potential probes P1 & P2 detect the voltage ΔV (a function of the resistance) due to the current injected in the earth by the current terminals C1 & C2.
- The test set measures both the current and the voltage and internally calculates and then displays the resistance. R=V/I
- If this ground electrode is in parallel or series with other ground rods, the resistance value is the total value of all resistances.
- Ground resistance measurements are often corrupted by the existence of ground currents and their harmonics. To prevent this it is advisable to use Automatic Frequency Control (AFC) System. 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 use for Multiple Depth Testing
Advantage:
- This is most accurate Method.
- 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 2,000 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.
- The Fall-of-Potential method or Three-Terminal method is the most common way to measure earth electrode system resistance, but it requires special procedures when used to measure large electrode systems
- There are three basic fall-of-potential test method.
- Full fall-of-Potential: A number of tests are made at different spaces of Potential Probe “P” and the resistance curve is plotted.
- Simplified Fall-of-Potential: Three measurements are made at defined distance of Potential Probe ”P” and mathematical calculations are used to determine the resistance.
- 8% Rule: A single measurement is made with Potential Probe “P” at a distance 61.8% (62%) of the distance between the electrode under test and “C”.
Required Equipment:
- Earth Tester (4 Terminal or 3 Terminal)
- 4 No’s of Electrodes (Spike)
- 4 No’s of Insulated Wires
- Hammer
- Measuring Tap
Connections:
- First, isolate the grounding electrode under measurement by disconnecting it from the rest of the system.
- For Small System:
- For 4 Terminal Earth Tester Short Current Terminal (C1) and Potential Terminal (P1) together with a short jumper on the earth tester and connect it to earthing electrode under test.
- For 3 Terminal Earth Tester Connect current terminal (C1) to the earth electrode under measurement.
- Drive another Current Electrode (C2) into the earth 100 to 200 feet at depth of 6 to 12 inches from the center of the electrode and connect to C2 Terminal of earth tester.
- Drive another potential terminal (P2) at depth of 6 to 12 inches into the earth midway between the Current Electrode (C1) and Current Electrode (C2) and connect to Earth Tester on P2
- For Large System
- Place the current electrode (C2) 400 to 600 feet from the measuring Earth Current Electrode (C1)
- Place the potential electrode (P1)8% of the distance from the Earth Current Electrode (C1)
- Measure the resistance
- Move the current electrode (C2) farther 50 to 100 Feet away from its present position.
- Place the potential electrode (P2) 61.8% of the distance from the Earth Current Electrode (C1).
- Spike length in the earth should not be more than 1/20th distance between two spikes.
Testing Procedure:
- Press START and read out the resistance value. 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 200, 300 and 400 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. smaller spacing means more data points with slower measurements.
- 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. If there is a significant change in the reading (30 %), we need to increase the distance between the ground rod under test, the inner stake (probe) and the outer stake (auxiliary ground) until the measured values remain fairly constant when repositioning the inner stake (probe).
- 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).
|
Distance of Probe |
||
| Depth of the ground electrode | Distance to the inner stake |
Distance to the outer stake |
| 2 m | 15 m | 25 m |
| 3 m | 20 m | 30 m |
| 6 m | 25 m | 40 m |
| 10 m | 30 m | 50 m |
Application:
- It is advisable for High Electrical Load.
- It is suitable for small and medium electrodes system (1 or 2 rods/plates). .
- It is useful for homogeneous Soil
Advantage:
- The three-point method is the most reliable test method;
- This test is the most suitable test for large grounding systems.
- Three-terminal is the quicker and simpler, with one less lead to string Spacing For Current Probe
Disadvantage:
- Individual ground electrodes must be disconnected from the system to be measured.
- It is extremely time consuming and labor intensive.
- There are situations where disconnection is not possible.
- Knowledge of location of center probe is necessary
- Time consuming and labor intensive Ineffective if the electrical center is unknown.
- If less measurements are being made then less accurate than full Fall of Potential
61.8% Rule:
- It is proven that the actual electrode resistance is measured when the potential probe is located 61.8% of the distance between the center of the electrode and the current probe. For example, if the current probe is located 400 feet from the electrode center, then the resistance can be measured with the potential probe located 61.8% x 400 = 247 feet from the electrode center.
- The 61.8% measurement point assumes the current and potential probes are located in a straight line and the soil is homogeneous (same type of soil surrounding the electrode area and to a depth equal to 10 times the largest electrode dimension).
- The 61.8% measurement point still provides suitable accuracy for most measurements.
- Suppose, the distance of Current Spike from Earth Electrode D = 60 ft, Then, distance of Potential Spike would be 62 % of D = 0.62D i.e. 0.62 x 60 ft = 37 ft.
Application:
- It is suitable for small and medium electrodes system.
- It is useful for homogeneous Soil
Advantage:
- Simplest to carry out.
- Required minimum calculation;
- Fewest number of test probe moves.
Disadvantage:
- Soil must be homogeneous.
- Less accurate
- Susceptible for non-homogeneous soil
View more at https://electricalnotes.wordpress.com/2020/03/01/methods-of-earth-resistance-testing-part-2/.
Published by Department of EEE, ADBU: tinyurl.com/eee-adbu
