1. Preface Some people think that the measurement of the grounding resistance is very simple and hastily engaged, and no solution is required; some people think that the measurement of the grounding resistance can not be repeated accurately and put forward the “discarded grounding resistanceâ€. Both attitudes are wrong and undesirable. This article describes in detail the grounding resistance measurement principle and several commonly used measurement methods and experiences to help people master it correctly and obtain accurate measurement results.
2. Definition and expression of grounding resistance 2.1 Definition of grounding resistance Grounding resistance is the resistance exhibited when the current is discharged through the grounding device. It is equal in value to the current flowing through the grounding device to the ground and the voltage drop generated by this current. except.
(1) In the formula, U—ground voltage of the grounding device, that is, the potential difference between the grounding body and the zero potential reference point of the earth.
I - current discharged into the earth through the grounding device.
After calculations, Equation (1) can evolve into an expression of the physical constants of the soil medium:
(2)
Where x is the dielectric constant of the soil;
r - resistivity of the soil, W·m
C - capacitance of the grounding device, F.
From equation (2), it can be seen that in a certain place, because the dielectric coefficient of the soil and the soil resistivity are determined, the magnitude of the grounding resistance depends on the capacitance of the grounding device itself, which in turn depends on the size of the grounding device and structure. In order to obtain a small grounding resistance, the design of the grounding grid is required to have as large a capacitance as possible to ground. On the other hand, the magnitude of the grounding resistance can also be changed by changing the soil (changing soil or using a resistance-reducing agent), decreasing the resistivity of the soil or increasing the dielectric coefficient of the soil.
Several special ground electrode capacitances and their ground resistance examples:
Hemispherical ground electrode, C=2peb,;
Half disk ground electrode, C=4eb,.
In the formula, b is the radius of the hemisphere or half disk.
As can be seen from the above equation, the hemispherical ground electrode of the same radius is approximately 1.6 times the capacitance of the semi-disk ground electrode, and the corresponding grounding resistance is approximately 1.6 times lower. This shows that the use of a spherical or three-dimensional grounding pole is more efficient than using a planar grounding pole, saving material.
2.2 Grounding resistance of a single grounding electrode The grounding resistance of a single grounded electrode depends mainly on its diameter and length. The grounding resistance of a single tube or rod is approximately (3) where L and d are the length and diameter of the tube or rod, respectively, in meters.
From formula (3) we can see that a round steel pipe with a diameter of 50mm is driven into the ground to a depth of 2.5m, and its resistance is about R=0.3r. The lightning protection designer should well remember this 0.3 empirical coefficient, which is very helpful for designing the grounding net.
3. Measurement principle of grounding resistance Measurement of grounding resistance is carried out according to formula (1): Apply a current I to the grounding device (grounding electrode or grounding net) and measure the voltage U, voltage and voltage on the grounding electrode (net). Dividing the current results in a grounding resistance.
Seemingly simple, but how this current is applied correctly, how accurate this voltage is measured, is not known to every measurer. This article introduces readers to this technology and experience.
3.1 Application of measuring current and current auxiliary pole In formula (1), the measuring current refers to the "current flowing through the grounding device into the ground." This current is not the same as the current flowing in the wire. In the wire, current flows along the wire, forming a continuous closed loop whose path is deterministic and predictable. The "current flowing through the grounding device" spreads into the earth and travels far and far. How does it form a closed loop to satisfy the continuity and closure of the current?
If this current is a lightning current formed by a lightning strike, this current flows from the thundercloud through the lightning discharge channel into the air receiver and the down conductor, and then spreads through the grounding device to spread around the earth, and finally passes through the vast space between the clouds and the earth. In the form of a displacement current, it returns to Lei Yun and satisfies the law of continuity and closure of the current, as shown in Fig. 1A. However, in order to be able to inject (apply) the test current to the grounding device during ground resistance testing, we must first solve the problem of return or collection of the current. This is a must to find or artificially create a current loop. In the three-pole straight line method and the triangle method to measure the grounding resistance, it is necessary to temporarily hit an auxiliary current pole at a distant place, and the purpose thereof is to provide a loop for the current. In the clamp metering method, it is not necessary to make a temporary auxiliary current pole. It is not that the current does not need a loop, but that the current A distribution of the lightning current is to be found. B. The current distribution when measuring the ground resistance becomes a circuit that can be used as a loop. Figure 1 depicts the distribution of the current flowing into the earth through the grounding device. Figure 1A shows the field of lightning current, which is a current field that spreads to the periphery, and Figure 1B shows the field of the test current, which is a distortion that spreads to one side. The current field.
The presence of an auxiliary current pole makes the distribution of the current field non-uniform and distorted. The closer the auxiliary current electrode is to the ground electrode (network) to be measured, the greater the distortion of the current field; the farther the auxiliary current electrode is, the smaller the distortion of the current field is, but the larger the test workload is. Therefore, there is a suitable optimal distance here, and the auxiliary current is relatively close to meet the test accuracy requirements.
3.2 Measuring Voltage and Auxiliary Voltage The voltage in equation (1) refers to the voltage between the grounding device (network) and the earth zero potential reference point. Where is the earth zero potential reference point and how to obtain it is another important issue in earth resistance test. Obviously, we can't go to the infinity place to find the zero potential reference point, but instead look for a zero potential reference point in a more acceptable place.
In the grounding resistance measurement, it is necessary to make an auxiliary voltage pole in the place where the zero potential reference point is selected, and the reference potential is taken back by a wire. The difference between the potential of the grounding device (network) and that of the grounding device (network) is the voltage U that we need. . Finding a true zero-potential reference point may not be easy in real-world measurements, but we can find a place as close to zero potential as possible, or where the error is acceptable. If the potential at this point is not a true zero potential, but is a bit larger or smaller than the zero potential, then the resulting voltage and measured ground resistance will have a larger or smaller error. How much error can be accepted by us, which needs to be judged by the measurement results. Here, not only must understand the measurement principle, but also have the corresponding actual measurement experience. In short, in order to ensure the accuracy of the grounding resistance measurement, the key lies in the correctness of the zero potential reference point selection and the judgment of the measurement results.
Where is the earth zero potential reference point? Some people have a misunderstanding that the earth is always at zero potential. They think that ground potential is zero potential, which is incorrect. In fact, as long as there is current flowing in the ground, there is a voltage drop, and the ground here is not zero potential. No current flows through the ground, which is the electrical zero potential ground. Therefore, strictly speaking, zero potential is far away from the grounding device (network) being tested. For a single metal tube grounding electrode, it can be considered as zero potential when the distance from the grounding electrode is more than 20m.
The task of the auxiliary voltage pole is to retrieve the zero potential. Therefore, how to obtain an accurate zero potential point is the key to measuring the grounding resistance.
4. Three-pole straight line method for grounding resistance measurement 4.1 Three-pole straight line method Three-pole straight line method is the most used and most common method for grounding resistance test. The measured grounding grid 1, voltage auxiliary pole 2 and current auxiliary pole are tested. 3 Three points (poles) are arranged in a straight line.
In the three-pole line method measurement, how are the three poles arranged? Specifically, how the distance between the auxiliary voltage pole and the auxiliary current pole and the grounding device (network) to be measured is arranged and controlled is the key to the measurement of the grounding resistance. one. General grounding resistance test instruments and meters are provided with two auxiliary grounding poles and two test wires are matched, one 40m and the other 20m. Some instruments, such as the Hitachi Kyoritsu 4150A watch, have shorter matching leads. Is it possible to obtain accurate measurement results with such a supporting wire? This is a problem that every measurer is very concerned about.
E test power A current meter V voltage meter 1 grounding device under test, 2 voltage pole, 3 current pole D grounding net maximum diagonal size, d13 distance from grounding grid to current pole d12 distance from grounding grid to voltage pole, d23 voltage pole The distance of the current pole Fig. 2 The connection of the three-pole straight-line method to measure the grounding resistance 4.2 The measurement principle of the three-pole straight-line method According to the wiring of Fig. 2, the three-pole voltage equation can be listed:
U1=R1I1+R12I2+R13I3(4a)
U2=R21I1+R2I2+R23I3(4b)
U3=R31I1+R32I2+R3I3(4c)
In the formula, I1 - the current flowing into the grounding device.
I2 - Current flowing into the voltage pole.
I3 - current flowing into the current pole.
R1—self-resistance of the grounding device 1, ie grounded device's measured grounding resistance.
R2 - The self resistance of the voltage pole.
R3 - The self resistance of the current pole.
R23 and R32 - The mutual resistance between the voltage pole and the current pole, they are equal.
R12 and R21 - mutual resistance between grounding device and voltage pole, they are equal.
R13 and R31 - mutual resistance between the grounding device and the current pole, they are equal.
During the test, the current flows from the grounding device to the ground, and from the current flow to the power supply. Take the direction of the current flowing into the earth as positive, then I1=-I3. Because the current flowing through the voltage pole is extremely small, I2=0 can be considered. And because three pairs of mutual resistance are equal: R23 = R32, R12 = R21, R13 = R31, the equations can be rewritten as:
(5a)
(5b)
(5c)
From Formula (5a)-Formula (5b), U
(6)
This results in grounded resistance measurements:
(7)
To the right of the equation of equation (7), the first item R1 is the true value of the grounding resistance of the grounding device. Therefore, the last three items of mutual resistance (R23-R12-R13) are the measurement errors:
(8)
From Equations (7) and (8), the measurement error consists of three mutual resistances, which are caused by the relative positions of the electrodes, depending on the arrangement of the electrode positions. The correct placement of the electrodes should be such that (9)
Then the measurement error can be equal to or close to zero.
If the soil resistivity around the electrode is uniform, the mutual resistance of the two poles is proportional to the soil resistivity and inversely proportional to the distance between the two electrodes:
(10)
(11)
(12)
In the formula, Ï—the soil resistivity around the electrode;
α=d12/d13;
D12, d13 and d23 are the distances between the grounding device and the voltage pole, the grounding device and the current pole, and the voltage pole and the current pole, respectively.
Substituting (10), (11), and (13) into (9) yields:
This equation has two solutions, leaving no meaningful negative solution, and finally: α = 0.618.
That is, in order to make the measurement error equal to zero, the auxiliary voltage pole should be placed at a distance of 0.618d13 from the edge of the grounding device. This method is called 0.618 measurement method, or compensation method. In fact, due to various reasons in the scene, it is difficult to ensure that the voltage pole is in this accurate position. Considering the conditions of site selection by d13, it can be calculated that the specific range of α is shown in Table 1 under the conditions of different measurement tolerances and d13.
As can be seen from Table 1, the shorter the distance d13 is, that is, the closer the current assist pole is located, the smaller the α interval required to ensure the measurement accuracy, the harder the accurate position of the voltage assist pole is.
Allowable measurement error δ
% of the range of α values ​​below the distance d13 of 5D3D2D
50.56 to 0.670.59 to 0.650.59 to 0.63
100.50 to 0.710.55 to 0.680.58 to 0.66
Note: D is the maximum diagonal length of the grounding device.
4.3 Adjustment of voltage pole position by three-pole linear method The above describes the arrangement of the three electrodes for accurately measuring the ground resistance requirements. The range of α listed in Table 1 is the position of the voltage pole when measuring. However, in the actual field conditions, the soil resistivity measured in the area may not be uniform. Due to various channels, rocks, and various metal pipes in the ground, they will affect the distribution of the current field and give the measurement results. To error.
The adjustment of the voltage pole position in a specific measurement is the search for the exact position of the zero potential.
The heuristic method is usually used to find the exact location of earth zero potential points. The method is that in the straight line connecting three poles, in a region slightly larger than the range of α listed in Table 1, for example, within the range of (0.5-0.7)d13, 3% of d13 is used as the distance, and 5 to consecutive 7 voltage auxiliary poles, measuring 5 to 7 points. In the specific operation, you can make a little test, pull up the voltage pole and then lay down a bit to measure the next data. For each point of the voltage pole, a ground resistance value can be measured.
4.4 Determination of grounding resistance test results Using the grounding resistance as the ordinate and the distance as the abscissa, several measured grounding resistance values ​​are plotted on a coordinate diagram to form a grounding resistance curve. If the trend of the connection line with at least three resistance values ​​is flat, the value of the grounding resistance corresponding to this position is the exact value. It can be judged directly without drawing. If there is a relative error between three or more resistance values ​​less than 3% among all the measured values, the average of these values ​​is used as the final measurement result.
5. Tripolar triangle method of grounding resistance measurement In some cases, when measuring the grounding resistance of a large grounding net, due to the topography, it is difficult to hit the current (3 to 5) D far. In order to shorten the distance of the current pole, the ground resistance can be measured using the triangle method.
The triangle method is to arrange the auxiliary voltage electrode and the auxiliary current electrode in two directions at an included angle q. The grounding device, the voltage electrode, and the current electrode have isosceles triangles at three points, as shown in FIG. 3 . Since d12=d13, R12=R13, substituting this relationship into (9) yields:
R23=2R13
therefore,
This calculates the apex angle θ=29° of the isosceles triangle.
Theoretical calculations and actual measurements show that when d12=d13≥2D and θ=30°, the measurement error is δ≈±10%.
6, on-site grounding resistance measurement experience bit by bit 6.1, the wrong test operation According to the author's observation, in the measurement of grounding resistance, there are often the following incorrect operation.
(1) The current pole and the voltage pole are within the grounding network. I had lectured in a large factory and visited the factory electrical test class before the lecture. See the test report of the grounding resistance of the lightning rod made by the tester. The person in charge of the electrical test of the plant introduced to me that they bought a new grounding resistance test meter, Hitachi Kyoritsu 4150A type, and used it to measure the grounding resistance quickly and accurately. The self-contained current and voltage auxiliary pole leads of this type of instrument are only 25m and 15m respectively. Due to the short wire, the test workload is small, and the test results are not changed very much. It is very accurate. He also said that they have measured this way in recent years and have never had any problems. I saw a few pages of test reports that the results of the grounding resistance of dozens of lightning rod measurement points were all around 0.2 Euro. I told them that there is no problem with the lightning rod in your factory, but there is a problem with your measurement method. Your so-called measurement result is not the grounding resistance of the lightning rod at all, but only the connection resistance between the lightning rod and the factory grounding net. Because the current and voltage auxiliary poles you are measuring are all located in the factory grounding network, they have not exceeded the scope of the grounding grid. The measuring current flows only in the grounding grid and has not yet flowed into the surrounding soil. Not grasping the correct measurement method is not only a problem of uncertainty, but it is not the grounding resistance that is measured at all. Measured can only be the resistance of the connection between the lightning rod and the grounding grid.
(2) Regardless of the size of the grounding grid, measurements are made by 40m and 20m. Many people do not ask for the size of the grounding grid when measuring the grounding resistance. They are operated according to the instructions of the grounding instrument. The current and voltage leads of the instrument are only 40m and 20m respectively, and the current and voltage poles are only 40m and 20m away.
For a small grounding grid with a diagonal size of not more than 10 m, the current and voltage poles are 40m and 20m. But for large grounding grids, the measurement error is large.
(3) Lack of operation to find zero potential Many measurement operators only use one position of the voltage pole, and measuring the value of a grounding resistance will end up doing nothing. They did not seek zero potential through the movement of the voltage poles. The results obtained were correct and there was little error. They did not know for themselves.
6.2 How to reduce the grounding resistance of the auxiliary current pole itself If the grounding resistance of the auxiliary current pole itself is too large, the measurement current will be very small under a certain measurement voltage, which not only affects the measurement sensitivity, but also the measurement error. Sometimes even measurement instruments or meters are not reflected and no results can be measured.
Methods to reduce the current-grounding resistance include: increasing the diameter of the grounding electrode, increasing the length, using multiple current-poles in parallel, water injection around the current pole, and injecting salt water to reduce its grounding resistance.
If there is a tree around the current pole, the tree can be used cleverly as a current pole: cut the bark of multiple trees lightly, wound with bare copper wire, connected together, and connected in parallel with the current pole to form an auxiliary current. Extreme system. In some special occasions, this method can save time and work.
6.3. Experience in a variety of soil resistivity areas When the soil resistivity in a measured area is not uniform, it will affect the search for the location of the zero potential reference point. If the resistivity of the primary soil is Ï1, there is a layer of high soil resistivity Ï2 between the grounding grid and the voltage pole, Ï2>Ï1, such as the depression of the dry channel trench, the zero potential point distance from the grounding grid is less than 0.618 D13. As the Ï2/Ï1 ratio increases, the zero potential point is closer to the grounding grid.
When there is a high soil rate stratum between the voltage pole and the current pole, the position of the zero potential point will be larger than 0.618d13. As the Ï2/Ï1 ratio increases, the potential of the zero point is further from the grounding device.
When the soil in the measured area is a two-layer structure, ie, when there are two different soil resistivities, the distance of the current pole should be appropriately increased.
6.4. If the concrete cannot be used as a measuring electrode in a concrete field, if it is difficult to hit auxiliary current poles or voltage poles around the urban areas, 25×25 cm2 steel plates can be placed on the cement floor and poured with salt water instead of measuring electrodes. Under normal circumstances, Measurements can be made.
7. Summary (1) In any case, the grounding resistance is measurable, as long as the correct measurement method is mastered, it can be calibrated.
(2) To accurately measure the grounding resistance, the distance d13 of the auxiliary current pole from the grounding device to be measured must not be too small, at least more than 3 times the maximum diagonal size of the grounding device. The position of the voltage pole is at 0.618d13, but the voltage pole should be moved back and forth from 5 to 7 points before and after measurement. The value of 5 to 7 grounding resistances is measured. Select at least three data whose mutual error is less than 3%. The average is the final measurement.
(3) The measurement of the grounding resistance of a large grounding device (grid) is more difficult, but not measurable. When that kind of difficulty is encountered, it gives up measurement, and even puts forward the idea of ​​“discarding the grounding resistance†because it is a lazy Chinese thinking because of tired food.
2. Definition and expression of grounding resistance 2.1 Definition of grounding resistance Grounding resistance is the resistance exhibited when the current is discharged through the grounding device. It is equal in value to the current flowing through the grounding device to the ground and the voltage drop generated by this current. except.
(1) In the formula, U—ground voltage of the grounding device, that is, the potential difference between the grounding body and the zero potential reference point of the earth.
I - current discharged into the earth through the grounding device.
After calculations, Equation (1) can evolve into an expression of the physical constants of the soil medium:
(2)
Where x is the dielectric constant of the soil;
r - resistivity of the soil, W·m
C - capacitance of the grounding device, F.
From equation (2), it can be seen that in a certain place, because the dielectric coefficient of the soil and the soil resistivity are determined, the magnitude of the grounding resistance depends on the capacitance of the grounding device itself, which in turn depends on the size of the grounding device and structure. In order to obtain a small grounding resistance, the design of the grounding grid is required to have as large a capacitance as possible to ground. On the other hand, the magnitude of the grounding resistance can also be changed by changing the soil (changing soil or using a resistance-reducing agent), decreasing the resistivity of the soil or increasing the dielectric coefficient of the soil.
Several special ground electrode capacitances and their ground resistance examples:
Hemispherical ground electrode, C=2peb,;
Half disk ground electrode, C=4eb,.
In the formula, b is the radius of the hemisphere or half disk.
As can be seen from the above equation, the hemispherical ground electrode of the same radius is approximately 1.6 times the capacitance of the semi-disk ground electrode, and the corresponding grounding resistance is approximately 1.6 times lower. This shows that the use of a spherical or three-dimensional grounding pole is more efficient than using a planar grounding pole, saving material.
2.2 Grounding resistance of a single grounding electrode The grounding resistance of a single grounded electrode depends mainly on its diameter and length. The grounding resistance of a single tube or rod is approximately (3) where L and d are the length and diameter of the tube or rod, respectively, in meters.
From formula (3) we can see that a round steel pipe with a diameter of 50mm is driven into the ground to a depth of 2.5m, and its resistance is about R=0.3r. The lightning protection designer should well remember this 0.3 empirical coefficient, which is very helpful for designing the grounding net.
3. Measurement principle of grounding resistance Measurement of grounding resistance is carried out according to formula (1): Apply a current I to the grounding device (grounding electrode or grounding net) and measure the voltage U, voltage and voltage on the grounding electrode (net). Dividing the current results in a grounding resistance.
Seemingly simple, but how this current is applied correctly, how accurate this voltage is measured, is not known to every measurer. This article introduces readers to this technology and experience.
3.1 Application of measuring current and current auxiliary pole In formula (1), the measuring current refers to the "current flowing through the grounding device into the ground." This current is not the same as the current flowing in the wire. In the wire, current flows along the wire, forming a continuous closed loop whose path is deterministic and predictable. The "current flowing through the grounding device" spreads into the earth and travels far and far. How does it form a closed loop to satisfy the continuity and closure of the current?
If this current is a lightning current formed by a lightning strike, this current flows from the thundercloud through the lightning discharge channel into the air receiver and the down conductor, and then spreads through the grounding device to spread around the earth, and finally passes through the vast space between the clouds and the earth. In the form of a displacement current, it returns to Lei Yun and satisfies the law of continuity and closure of the current, as shown in Fig. 1A. However, in order to be able to inject (apply) the test current to the grounding device during ground resistance testing, we must first solve the problem of return or collection of the current. This is a must to find or artificially create a current loop. In the three-pole straight line method and the triangle method to measure the grounding resistance, it is necessary to temporarily hit an auxiliary current pole at a distant place, and the purpose thereof is to provide a loop for the current. In the clamp metering method, it is not necessary to make a temporary auxiliary current pole. It is not that the current does not need a loop, but that the current A distribution of the lightning current is to be found. B. The current distribution when measuring the ground resistance becomes a circuit that can be used as a loop. Figure 1 depicts the distribution of the current flowing into the earth through the grounding device. Figure 1A shows the field of lightning current, which is a current field that spreads to the periphery, and Figure 1B shows the field of the test current, which is a distortion that spreads to one side. The current field.
The presence of an auxiliary current pole makes the distribution of the current field non-uniform and distorted. The closer the auxiliary current electrode is to the ground electrode (network) to be measured, the greater the distortion of the current field; the farther the auxiliary current electrode is, the smaller the distortion of the current field is, but the larger the test workload is. Therefore, there is a suitable optimal distance here, and the auxiliary current is relatively close to meet the test accuracy requirements.
3.2 Measuring Voltage and Auxiliary Voltage The voltage in equation (1) refers to the voltage between the grounding device (network) and the earth zero potential reference point. Where is the earth zero potential reference point and how to obtain it is another important issue in earth resistance test. Obviously, we can't go to the infinity place to find the zero potential reference point, but instead look for a zero potential reference point in a more acceptable place.
In the grounding resistance measurement, it is necessary to make an auxiliary voltage pole in the place where the zero potential reference point is selected, and the reference potential is taken back by a wire. The difference between the potential of the grounding device (network) and that of the grounding device (network) is the voltage U that we need. . Finding a true zero-potential reference point may not be easy in real-world measurements, but we can find a place as close to zero potential as possible, or where the error is acceptable. If the potential at this point is not a true zero potential, but is a bit larger or smaller than the zero potential, then the resulting voltage and measured ground resistance will have a larger or smaller error. How much error can be accepted by us, which needs to be judged by the measurement results. Here, not only must understand the measurement principle, but also have the corresponding actual measurement experience. In short, in order to ensure the accuracy of the grounding resistance measurement, the key lies in the correctness of the zero potential reference point selection and the judgment of the measurement results.
Where is the earth zero potential reference point? Some people have a misunderstanding that the earth is always at zero potential. They think that ground potential is zero potential, which is incorrect. In fact, as long as there is current flowing in the ground, there is a voltage drop, and the ground here is not zero potential. No current flows through the ground, which is the electrical zero potential ground. Therefore, strictly speaking, zero potential is far away from the grounding device (network) being tested. For a single metal tube grounding electrode, it can be considered as zero potential when the distance from the grounding electrode is more than 20m.
The task of the auxiliary voltage pole is to retrieve the zero potential. Therefore, how to obtain an accurate zero potential point is the key to measuring the grounding resistance.
4. Three-pole straight line method for grounding resistance measurement 4.1 Three-pole straight line method Three-pole straight line method is the most used and most common method for grounding resistance test. The measured grounding grid 1, voltage auxiliary pole 2 and current auxiliary pole are tested. 3 Three points (poles) are arranged in a straight line.
In the three-pole line method measurement, how are the three poles arranged? Specifically, how the distance between the auxiliary voltage pole and the auxiliary current pole and the grounding device (network) to be measured is arranged and controlled is the key to the measurement of the grounding resistance. one. General grounding resistance test instruments and meters are provided with two auxiliary grounding poles and two test wires are matched, one 40m and the other 20m. Some instruments, such as the Hitachi Kyoritsu 4150A watch, have shorter matching leads. Is it possible to obtain accurate measurement results with such a supporting wire? This is a problem that every measurer is very concerned about.
E test power A current meter V voltage meter 1 grounding device under test, 2 voltage pole, 3 current pole D grounding net maximum diagonal size, d13 distance from grounding grid to current pole d12 distance from grounding grid to voltage pole, d23 voltage pole The distance of the current pole Fig. 2 The connection of the three-pole straight-line method to measure the grounding resistance 4.2 The measurement principle of the three-pole straight-line method According to the wiring of Fig. 2, the three-pole voltage equation can be listed:
U1=R1I1+R12I2+R13I3(4a)
U2=R21I1+R2I2+R23I3(4b)
U3=R31I1+R32I2+R3I3(4c)
In the formula, I1 - the current flowing into the grounding device.
I2 - Current flowing into the voltage pole.
I3 - current flowing into the current pole.
R1—self-resistance of the grounding device 1, ie grounded device's measured grounding resistance.
R2 - The self resistance of the voltage pole.
R3 - The self resistance of the current pole.
R23 and R32 - The mutual resistance between the voltage pole and the current pole, they are equal.
R12 and R21 - mutual resistance between grounding device and voltage pole, they are equal.
R13 and R31 - mutual resistance between the grounding device and the current pole, they are equal.
During the test, the current flows from the grounding device to the ground, and from the current flow to the power supply. Take the direction of the current flowing into the earth as positive, then I1=-I3. Because the current flowing through the voltage pole is extremely small, I2=0 can be considered. And because three pairs of mutual resistance are equal: R23 = R32, R12 = R21, R13 = R31, the equations can be rewritten as:
(5a)
(5b)
(5c)
From Formula (5a)-Formula (5b), U
(6)
This results in grounded resistance measurements:
(7)
To the right of the equation of equation (7), the first item R1 is the true value of the grounding resistance of the grounding device. Therefore, the last three items of mutual resistance (R23-R12-R13) are the measurement errors:
(8)
From Equations (7) and (8), the measurement error consists of three mutual resistances, which are caused by the relative positions of the electrodes, depending on the arrangement of the electrode positions. The correct placement of the electrodes should be such that (9)
Then the measurement error can be equal to or close to zero.
If the soil resistivity around the electrode is uniform, the mutual resistance of the two poles is proportional to the soil resistivity and inversely proportional to the distance between the two electrodes:
(10)
(11)
(12)
In the formula, Ï—the soil resistivity around the electrode;
α=d12/d13;
D12, d13 and d23 are the distances between the grounding device and the voltage pole, the grounding device and the current pole, and the voltage pole and the current pole, respectively.
Substituting (10), (11), and (13) into (9) yields:
This equation has two solutions, leaving no meaningful negative solution, and finally: α = 0.618.
That is, in order to make the measurement error equal to zero, the auxiliary voltage pole should be placed at a distance of 0.618d13 from the edge of the grounding device. This method is called 0.618 measurement method, or compensation method. In fact, due to various reasons in the scene, it is difficult to ensure that the voltage pole is in this accurate position. Considering the conditions of site selection by d13, it can be calculated that the specific range of α is shown in Table 1 under the conditions of different measurement tolerances and d13.
As can be seen from Table 1, the shorter the distance d13 is, that is, the closer the current assist pole is located, the smaller the α interval required to ensure the measurement accuracy, the harder the accurate position of the voltage assist pole is.
Allowable measurement error δ
% of the range of α values ​​below the distance d13 of 5D3D2D
50.56 to 0.670.59 to 0.650.59 to 0.63
100.50 to 0.710.55 to 0.680.58 to 0.66
Note: D is the maximum diagonal length of the grounding device.
4.3 Adjustment of voltage pole position by three-pole linear method The above describes the arrangement of the three electrodes for accurately measuring the ground resistance requirements. The range of α listed in Table 1 is the position of the voltage pole when measuring. However, in the actual field conditions, the soil resistivity measured in the area may not be uniform. Due to various channels, rocks, and various metal pipes in the ground, they will affect the distribution of the current field and give the measurement results. To error.
The adjustment of the voltage pole position in a specific measurement is the search for the exact position of the zero potential.
The heuristic method is usually used to find the exact location of earth zero potential points. The method is that in the straight line connecting three poles, in a region slightly larger than the range of α listed in Table 1, for example, within the range of (0.5-0.7)d13, 3% of d13 is used as the distance, and 5 to consecutive 7 voltage auxiliary poles, measuring 5 to 7 points. In the specific operation, you can make a little test, pull up the voltage pole and then lay down a bit to measure the next data. For each point of the voltage pole, a ground resistance value can be measured.
4.4 Determination of grounding resistance test results Using the grounding resistance as the ordinate and the distance as the abscissa, several measured grounding resistance values ​​are plotted on a coordinate diagram to form a grounding resistance curve. If the trend of the connection line with at least three resistance values ​​is flat, the value of the grounding resistance corresponding to this position is the exact value. It can be judged directly without drawing. If there is a relative error between three or more resistance values ​​less than 3% among all the measured values, the average of these values ​​is used as the final measurement result.
5. Tripolar triangle method of grounding resistance measurement In some cases, when measuring the grounding resistance of a large grounding net, due to the topography, it is difficult to hit the current (3 to 5) D far. In order to shorten the distance of the current pole, the ground resistance can be measured using the triangle method.
The triangle method is to arrange the auxiliary voltage electrode and the auxiliary current electrode in two directions at an included angle q. The grounding device, the voltage electrode, and the current electrode have isosceles triangles at three points, as shown in FIG. 3 . Since d12=d13, R12=R13, substituting this relationship into (9) yields:
R23=2R13
therefore,
This calculates the apex angle θ=29° of the isosceles triangle.
Theoretical calculations and actual measurements show that when d12=d13≥2D and θ=30°, the measurement error is δ≈±10%.
6, on-site grounding resistance measurement experience bit by bit 6.1, the wrong test operation According to the author's observation, in the measurement of grounding resistance, there are often the following incorrect operation.
(1) The current pole and the voltage pole are within the grounding network. I had lectured in a large factory and visited the factory electrical test class before the lecture. See the test report of the grounding resistance of the lightning rod made by the tester. The person in charge of the electrical test of the plant introduced to me that they bought a new grounding resistance test meter, Hitachi Kyoritsu 4150A type, and used it to measure the grounding resistance quickly and accurately. The self-contained current and voltage auxiliary pole leads of this type of instrument are only 25m and 15m respectively. Due to the short wire, the test workload is small, and the test results are not changed very much. It is very accurate. He also said that they have measured this way in recent years and have never had any problems. I saw a few pages of test reports that the results of the grounding resistance of dozens of lightning rod measurement points were all around 0.2 Euro. I told them that there is no problem with the lightning rod in your factory, but there is a problem with your measurement method. Your so-called measurement result is not the grounding resistance of the lightning rod at all, but only the connection resistance between the lightning rod and the factory grounding net. Because the current and voltage auxiliary poles you are measuring are all located in the factory grounding network, they have not exceeded the scope of the grounding grid. The measuring current flows only in the grounding grid and has not yet flowed into the surrounding soil. Not grasping the correct measurement method is not only a problem of uncertainty, but it is not the grounding resistance that is measured at all. Measured can only be the resistance of the connection between the lightning rod and the grounding grid.
(2) Regardless of the size of the grounding grid, measurements are made by 40m and 20m. Many people do not ask for the size of the grounding grid when measuring the grounding resistance. They are operated according to the instructions of the grounding instrument. The current and voltage leads of the instrument are only 40m and 20m respectively, and the current and voltage poles are only 40m and 20m away.
For a small grounding grid with a diagonal size of not more than 10 m, the current and voltage poles are 40m and 20m. But for large grounding grids, the measurement error is large.
(3) Lack of operation to find zero potential Many measurement operators only use one position of the voltage pole, and measuring the value of a grounding resistance will end up doing nothing. They did not seek zero potential through the movement of the voltage poles. The results obtained were correct and there was little error. They did not know for themselves.
6.2 How to reduce the grounding resistance of the auxiliary current pole itself If the grounding resistance of the auxiliary current pole itself is too large, the measurement current will be very small under a certain measurement voltage, which not only affects the measurement sensitivity, but also the measurement error. Sometimes even measurement instruments or meters are not reflected and no results can be measured.
Methods to reduce the current-grounding resistance include: increasing the diameter of the grounding electrode, increasing the length, using multiple current-poles in parallel, water injection around the current pole, and injecting salt water to reduce its grounding resistance.
If there is a tree around the current pole, the tree can be used cleverly as a current pole: cut the bark of multiple trees lightly, wound with bare copper wire, connected together, and connected in parallel with the current pole to form an auxiliary current. Extreme system. In some special occasions, this method can save time and work.
6.3. Experience in a variety of soil resistivity areas When the soil resistivity in a measured area is not uniform, it will affect the search for the location of the zero potential reference point. If the resistivity of the primary soil is Ï1, there is a layer of high soil resistivity Ï2 between the grounding grid and the voltage pole, Ï2>Ï1, such as the depression of the dry channel trench, the zero potential point distance from the grounding grid is less than 0.618 D13. As the Ï2/Ï1 ratio increases, the zero potential point is closer to the grounding grid.
When there is a high soil rate stratum between the voltage pole and the current pole, the position of the zero potential point will be larger than 0.618d13. As the Ï2/Ï1 ratio increases, the potential of the zero point is further from the grounding device.
When the soil in the measured area is a two-layer structure, ie, when there are two different soil resistivities, the distance of the current pole should be appropriately increased.
6.4. If the concrete cannot be used as a measuring electrode in a concrete field, if it is difficult to hit auxiliary current poles or voltage poles around the urban areas, 25×25 cm2 steel plates can be placed on the cement floor and poured with salt water instead of measuring electrodes. Under normal circumstances, Measurements can be made.
7. Summary (1) In any case, the grounding resistance is measurable, as long as the correct measurement method is mastered, it can be calibrated.
(2) To accurately measure the grounding resistance, the distance d13 of the auxiliary current pole from the grounding device to be measured must not be too small, at least more than 3 times the maximum diagonal size of the grounding device. The position of the voltage pole is at 0.618d13, but the voltage pole should be moved back and forth from 5 to 7 points before and after measurement. The value of 5 to 7 grounding resistances is measured. Select at least three data whose mutual error is less than 3%. The average is the final measurement.
(3) The measurement of the grounding resistance of a large grounding device (grid) is more difficult, but not measurable. When that kind of difficulty is encountered, it gives up measurement, and even puts forward the idea of ​​“discarding the grounding resistance†because it is a lazy Chinese thinking because of tired food.
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