Abstract
A properly designed and constructed ground electrode should perform well for many years. It must be periodically inspected to know its current technical condition. This obligation falls on the installation’s owner. Depending on the function of the ground electrode in the entire installation, it may be a working, protective or lightning protection earth. The choice of a measurement method depends on its function in the installation. The paper presents the results of measurements of the ground electrode resistance of a selected technical facility, performed throughout the year. Repeating the measurements multiple times in different seasons of the year was dictated by assessing the impact of environmental conditions on the results of grounding system resistance measurements. Analysis of the results performed in various environmental conditions may be the basis for recommending those performing this type of measurement to choose the appropriate method and the best date for performing the measurements. Considering the significant impact of the proper value of ground resistance on the effectiveness of protection against electric shock and lightning, its measurements should be carried out with due care.
Keywords: Ground electrode, Ground electrode resistance measurements, Periodic measurement
Subject terms: Electrical and electronic engineering, Energy infrastructure
Introduction
A properly designed and constructed ground electrode of any technical facility should function properly for many years. Its correct operation is primarily based on maintaining its assumed resistance throughout operation at an unchanged level. If the ground electrode is used, for example, as an important element of anti-shock protection, the resistance assumed during its design should not change. To ensure effective protection against electric shock, the resistance of the ground electrode mustn’t exceed a certain limit value.
The ground electrode, like any technical object, is subject to a natural wear-out process during operation. Metal construction elements of the ground electrode are subject to corrosion, especially intensely in acidic soil. This causes an increase in the resistance of the ground electrode as a result of damage to the galvanic coatings. In the further stage of operation of these phenomena, which are unfavorable from the point of view of the proper operation of the ground electrode, some elements of its structure may be completely degraded. The authors deal comprehensively with the issues of proper design, implementation and operation of a grounding system in1,2. An important issue when designing a grounding installation is predicting its operation using available modeling methods. The mentioned issues are described in3–9.
The basic technical parameter of a ground electrode is its resistance10–12 or, under certain conditions, impedance13,14. Its value is influenced by several factors, the dominant one being soil resistivity. This parameter depends, among others, on the temperature and humidity of the soil and may undergo seasonal changes, which was analyzed by the authors in15,16. Therefore, there is a problem in predicting the behavior of soil parameters. To estimate the seasonal variability of soil resistivity, in17 the authors propose using the neural network method.
The primary goal of a working grounding system is to ensure safety for people in the vicinity of the working ground electrode. This is directly related to concepts such as touch or step voltage, which is discussed in18–20. According to Wang et al.21, the best solution would be continuous monitoring of changes in ground electrode resistance. The lack of constant monitoring may result in a lack of reaction of the installation staff to the obvious impact of corrosion of the structure on its resistance value22.
Ground electrode resistance measurements are an important issue affecting the safe operation of the power system as well as installations and individual devices. The basic problem in this case is the selection of the appropriate measurement method23. In this case, the main problem is that the function of the given ground electrode plays in the power system. In the case of working and protective ground electrodes, they are tested using methods based on the analysis of their response to a given power frequency excitation24–26. Other measurement methods are used to test lightning protection ground electrodes because the forcing is based on the nature of a rapidly increasing current pulse simulating the response of the ground electrode to a lightning stroke27–35. In27, Lima et al. present a method of measuring ground electrodes based on the use of a current pulse with a fast rise time and the use of short test leads in measurement circuits. The results obtained with this method were compared with those obtained using traditional methods. Gazzana et al.28 present the results of experimental research on the behavior of the ground electrode in the time and frequency domain. Wind turbine ground electrodes were selected as objects for the research. The problems of measuring ground electrodes at high resistivity using impulse forcing were presented by Alipio et al. in29. The results obtained in this way were compared with simulation studies. The effectiveness of a large grounding network of a 110 kV station in the event of lightning surges was discussed in30 by Yang et al. The authors emphasize the impact of the ground ionization phenomenon on the distribution of electric potential on the ground surface. The impact properties of the ground electrode for areas with high soil resistivity can be significantly improved by using a backfill made of a material with low resistivity, as proven by Tu et al.31. According to the experimental research results they presented, the impact resistance drops from about 25–45% when such a solution is used. According to Grcev32, the properties of the ground electrode under the influence of excitation in the form of a current surge are influenced by the phenomenon of ground ionization and phenomena related to the influence of the ground electrode inductance. The author focuses on the latter in his considerations. The purpose of the considerations contained in33 is to assess the impact of the increase in ground potential in the substation design process. For this purpose, Yu et al. use a multi-layer soil model using CDEGS software dedicated to this purpose34. The presented field test results show that soil ionization and the inductive effect are two important phenomena influencing the behavior of the ground electrode if the ground current is in the form of a current surge. Alipio et al.35 present the ground electrode model for the analysis of transient states of electric potential distributions on the ground surface subjected to lightning strokes. The authors emphasize practical aspects related to the uneven distribution of electric potential along the grounding installation. In36, Vilachá et al. analyze the operation of a ground electrode subjected to lightning impulse.
The recently observed rapid development of prosumer installations has necessitated interest in a specific type of grounding installations operating with direct current (DC). The possible flow of DC ground current through the installation may cause the appearance of new, previously unknown phenomena37–43. Jayamaha et al. presented a thorough review of DC microgrids based on renewable sources37. The authors draw attention to the new challenges associated with them. Grounding systems in DC networks have been analyzed in detail by Pourmirasghariyan et al. in38. They draw attention to the information gap in this field in the literature and the need to conduct research in this area. de Oliveira et al.39 emphasize the great advantages of DC microgrids in terms of improving the energy efficiency of installations and also note obvious information gaps in existing publications in this field. In40,41 the authors deal with aspects related to the integration of DC microgrids with existing AC networks. The very important problems related to protection against electric shock in DC installations are discussed in41,42. A properly designed ground electrode will always have a significant impact on the shock voltages occurring in this case. When designing DC micro-installations, it is important to have the appropriate level of shock voltages and estimate the value of earth currents, which are the basis for the correct design of the grounding system43–46. Additionally, there is an aspect related to the risk of fire47.
When analyzing the operation of the ground electrode, and especially the required value of its resistance, the expected value of the ground current should be taken into account. These aspects and additionally the influence of the operation of the network neutral point are discussed by the authors in48,49. Specific cases of the analysis of grounding systems include the specific design of ground electrodes for gas-insulated power stations50 and ships moored in ports51. The specificity of measuring ground electrode resistance at high frequencies was discussed by Panicali et al. in52.
This paper presents and analyzes the results of ground electrode resistance measurements for a selected technical facility. The measurements were carried out at different time of the year to estimate the impact of different weather conditions changes on the obtained results. The methods used to measure ground electrode resistance, together with the specifications of the equipment used for this purpose, were discussed in detail in Sect. 2. The obtained measurement results were analyzed in the next stage of work in terms of the influence of the season on the value of soil resistance. Moreover, the results obtained using different methods of measuring ground electrode resistance were compared.
Materials and methods
The ground electrode of an industrial building that has been in use for many years (approximately 50 years) was selected as the research object. The dimensions of the study building are 15 m × 25 m and the grounding system dimensions are 16 m × 26 m. It is made in the form of a ring ground electrode, which is not connected to the foundation grounding system. The tested grounding system is buried into the ground to a depth of 1 m. The four vertical rods are located in the corners of the grounding system (building) and two at half the length of the wider side of the grounding to ensure the appropriate resistance value. The rods used were 3 m long. The grounding system comprises a 30 mm × 4 mm steel flat bar. The tested grounding system is made of galvanized steel, as it was common 50 years ago when the grounding electrode was built. Both, flat bars and vertical rods are made of galvanized steel. The vertical rods are made of solid steel bars. The object’s earthing is located in clayey soil. According to the requirements in Poland, this type of grounding should be equipped with a measuring terminal located on the facade of the building. In the presented case the measuring terminal is in the form of a screw with a nut, and it is a detachable connection between the flat bar and the grounding wire. If the measurement method did not require disconnection of the grounding system, the measuring clamps were clipped onto the grounding conductor during the measurement. If the measurement method required the disconnection of the earthing installation, it was used for this purpose (Fig. 1). The ground is operated in a heavily urbanized area with industrial use. It cannot be clearly stated that there are no other grounding systems in the surroundings. However, there should be no other grounding systems at a distance of at least 30 m. The tested installation is made in TN-C-S system. During the measurements, the PEN wire was disconnected.
Fig. 1.
Ground electrode resistance (impedance) measurement methods (a) 4p method; (b) 3p + clamp method, (c) 2 clamp method, (d) impact method (CP- Current Probe, PP-Potential Probe, G – Ground Electrode, GS – Auxiliary Ground Electrode).
To facilitate the process of performing cyclic measurements of ground electrode resistance, it is equipped with a measuring clamp. Known measurement methods were used to measure the ground electrode resistance: four-pole method (4p), three-pole method (3p) + clamps, and 2-times clamp method. Additionally, a measurement using the impact method was also performed. The earth resistance measurements were carried out following the recommendations in the manufacturer’s standard guide53. A graphic illustration of the methods used to measure ground resistance (impedance) is shown in Fig. 1. The MRU-200-GPS grounding system resistance meter was used for the measurements.
The four-pole method requires the greatest amount of work among those analyzed while ensuring the highest accuracy. An additional measurement lead is used to compensate for the influence of the resistance of the test leads. The disadvantage of this method is the need to disconnect the measuring clamp for the duration of the measurement, which is not always possible. The 3p + clamp method is very commonly used in practice. The current probe should be placed in the ground at a distance of approximately 40 m from the ground electrode and the voltage probe at half this distance. This method is sensitive to potential errors in the arrangement of measurement electrodes, which significantly affect the measurement results. When technical conditions do not allow placing measurement probes in the ground, the double clamp method is used. This method involves generating voltage and measuring the resistance of the ground electrode based on the voltage drop between the conductors in a closed circuit. For this purpose, transmitting clamps are used, which generate voltage on the circuit, and receiving clamps, which measure the value of the current flowing in the circuit2,23,54.
If it is necessary to measure the resistance of the lightning protection ground electrode, the impact method is used for this purpose. It involves analyzing the response of the ground electrode in the form of the maximum value of the voltage drop on the ground electrode to the current pulse generated for that purpose. The measurement of the impact resistance (impedance) of the ground electrode is performed without disconnecting the test clamp. A ground electrode subjected to a current impulse with a fast rise time behaves differently than in the case of power frequency excitation2,23,54.
If a surge impulse flows through the ground electrode, part of it may be discharged to the ground not only by the ground electrode structure but e.g. through the foundation of the building, near which there is a lightning protection ground electrode, usually made in the form of a ring ground electrode embedded around the building’s foundation. The influence of other cables or underground installations made of conductive materials is also important. Usually, the further the above-mentioned elements are located from the grounding system structure, the smaller part of the ground current they dissipate. To compare the behavior of the ground electrode in various operating conditions, the impact coefficient ku was introduced. This quantity is defined as the ratio of the impact resistance Ru to its static resistance Rst and can be described by formula (1)2.
 |
1 |
Due to the value of this coefficient, ground electrodes can be divided into simple ones in the form of, e.g., a vertical rod, where the value of the ku coefficient is close to unity, and ground electrodes with a more complex structure, e.g., ring-shaped ones in the form of a complex grid with many meshes, for which the value of the impact coefficient is from several to several dozen. A very important aspect is the fact that the value of this coefficient and thus the behavior of the ground electrode under the influence of a current surge with a fast rise time is strictly dependent on external factors. Such important factors in this case are temperature and soil humidity. If it is necessary to measure the behavior of the ground electrode under the influence of impact excitation, the recommendations contained in the operating instructions of the measuring device should be strictly followed due to the measurement procedure. The choice of the date of measurement is important due to the variability of environmental conditions2,23,54.
Measurements of ground electrode resistance and ground electrode impedance were carried out periodically over 12 months. When performing measurements, special attention was paid to maintain the same arrangement of measurement electrodes. Based on the manufacturer’s recommendations, each measurement was repeated three times. Based on the results of three measurements, the average RAVGE value was calculated (using formula 2). The calculated average value was taken as the value of the ground electrode resistance. Additionally, calculating the average value made it possible to compare whether the measured resistance value does not deviate from the average value by more than 3%, which is also suggested by the manufacturer of the device.
 |
2 |
where respectively: RE, i – – measured resistance value in the i-th measurement, n – number of measurements.
The percentage deviation sE, i of the measurement result from the average value was calculated using the formula (3).
 |
3 |
Results
Tables 1, 2 and 3 include the results of ground electrode resistance measurements obtained using the following methods: 3p + clamp, 4p and 2x clamp, respectively.
Table 1.
Results of ground electrode resistance measurements using the 3p method + clamps.
| No | Month | Ground electrode resistance RE (Ω) | Percentage deviation σE (%) | ||||||
|---|---|---|---|---|---|---|---|---|---|
| RE1 (Ω) | RE2 (Ω) | RE3 (Ω) | REAVG,3p (Ω) | σE1 (%) | σE2 (%) | σE3 (%) | |||
| 1 | July | 0.278 | 0.278 | 0.271 | 0.276 | 0.725 | 0.725 | 1.812 | |
| 2 | August | 0.161 | 0.160 | 0.158 | 0.160 | 0.625 | 0.000 | 1.250 | |
| 3 | September | 0.256 | 0.255 | 0.256 | 0.256 | 0.000 | 0.391 | 0.000 | |
| 4 | October | 0.368 | 0.371 | 0.373 | 0.371 | 0.809 | 0.000 | 0.539 | |
| 5 | November | 0.411 | 0.413 | 0.415 | 0.413 | 0.484 | 0.000 | 0.484 | |
| 6 | December | 0.560 | 0.560 | 0.558 | 0.559 | 0.179 | 0.179 | 0.179 | |
| 7 | January | 0.543 | 0.548 | 0.551 | 0.547 | 0.731 | 0.183 | 0.731 | |
| 8 | February | 0.456 | 0.468 | 0.471 | 0.465 | 1.935 | 0.645 | 1.290 | |
| 9 | March | 0.660 | 0.661 | 0.664 | 0.662 | 0.302 | 0.151 | 0.302 | |
| 10 | April | 0.652 | 0.655 | 0.648 | 0.652 | 0.000 | 0.460 | 0.613 | |
| 11 | May | 0.267 | 0.269 | 0.264 | 0.267 | 0.000 | 0.749 | 1.124 | |
| 12 | June | 0.451 | 0.453 | 0.459 | 0.454 | 0.661 | 0.220 | 1.101 | |
Table 2.
Results of ground electrode resistance measurements using the 4p method.
| No | Month | Ground electrode resistance RE (Ω) | Percentage deviation σE (%) | |||||
|---|---|---|---|---|---|---|---|---|
| RE1 (Ω) | RE2 (Ω) | RE3 (Ω) | REAVG,4p (Ω) | σE1 (%) | σE2 (%) | σE3 (%) | ||
| 1 | July | 0.200 | 0.205 | 0.207 | 0.204 | 1.961 | 0.490 | 1.471 |
| 2 | August | 0.119 | 0.121 | 0.120 | 0.120 | 0.833 | 0.833 | 0.000 |
| 3 | September | 0.135 | 0.136 | 0.136 | 0.136 | 0.735 | 0.000 | 0.000 |
| 4 | October | 0.845 | 0.852 | 0.847 | 0.848 | 0.354 | 0.472 | 0.118 |
| 5 | November | 0.845 | 0.842 | 0.840 | 0.842 | 0.356 | 0.000 | 0.238 |
| 6 | December | 0.450 | 0.456 | 0.449 | 0.452 | 0.442 | 0.885 | 0.664 |
| 7 | January | 0.465 | 0.470 | 0.471 | 0.469 | 0.853 | 0.213 | 0.426 |
| 8 | February | 0.360 | 0.364 | 0.367 | 0.364 | 1.099 | 0.000 | 0.824 |
| 9 | March | 0.745 | 0.747 | 0.747 | 0.746 | 0.134 | 0.134 | 0.134 |
| 10 | April | 0.871 | 0.922 | 0.878 | 0.890 | 2.135 | 3.596 | 1.348 |
| 11 | May | 0.135 | 0.139 | 0.138 | 0.137 | 1.460 | 1.460 | 0.730 |
| 12 | June | 0.118 | 0.121 | 0.115 | 0.118 | 0.000 | 2.542 | 2.542 |
Table 3.
Results of ground electrode resistance measurements using the method 2p clamps.
| No | Month | Ground electrode resistance RE (Ω) | Percentage deviation σE (%) | ||||||
|---|---|---|---|---|---|---|---|---|---|
| RE1 (Ω) | RE2 (Ω) | RE3 (Ω) | REAVG,2p (Ω) | σE1 (%) | σE2 (%) | σE3 (%) | |||
| 1 | July | 0.170 | 0.170 | 0.170 | 0.170 | 0.000 | 0.000 | 0.000 | |
| 2 | August | 0.170 | 0.170 | 0.170 | 0.170 | 0.000 | 0.000 | 0.000 | |
| 3 | September | 0.180 | 0.180 | 0.180 | 0.180 | 0.000 | 0.000 | 0.000 | |
| 4 | October | 0.250 | 0.250 | 0.260 | 0.253 | 1.186 | 1.186 | 2.767 | |
| 5 | November | 0.650 | 0.650 | 0.650 | 0.650 | 0.000 | 0.000 | 0.000 | |
| 6 | December | 0.300 | 0.300 | 0.300 | 0.300 | 0.000 | 0.000 | 0.000 | |
| 7 | January | 0.300 | 0.300 | 0.300 | 0.300 | 0.000 | 0.000 | 0.000 | |
| 8 | February | 0.310 | 0.310 | 0.310 | 0.310 | 0.000 | 0.000 | 0.000 | |
| 9 | March | 1.810 | 1.810 | 1.810 | 1.810 | 0.000 | 0.000 | 0.000 | |
| 10 | April | 1.920 | 1.920 | 1.920 | 1.920 | 0.000 | 0.000 | 0.000 | |
| 11 | May | 0.160 | 0.160 | 0.170 | 0.163 | 1.875 | 1.875 | 4.924 | |
| 12 | June | 0.170 | 0.170 | 0.170 | 0.170 | 0.000 | 0.000 | 0.000 | |
For the 3p + clamps method, the percentage deviation from the average value did not exceed 3%. The highest resistance values occurred in March and April and equaled 0.662 Ω and 0.648 Ω for the average value in March and 0.648 Ω respectively. The lowest value of ground electrode resistance occurred in August and amounted to 0.160 Ω, which is more than three times lower than in March or April. This was influenced by high soil humidity in August. For measurements made using the 4p method, the assumed value of the percentage deviation of the ground electrode resistance from the average value was exceeded only for measurements made in April. The highest values of average earth resistance were obtained in October, November, March and April. They are 0.848 Ω, 0.842 Ω, 0.746 Ω and 0.890 Ω, respectively. Similarly to the 3p + clamp method, the minimum value of earth resistance occurred in August and amounted to 0.120 Ω. For this method, the dispersion in the results obtained is greater than for the 3p + clamps method, exceeding six times the value of the results obtained in different months. For the 2-times clamp method, the highest value of ground electrode resistance was obtained in March and April. The obtained resistance values are 1.810 Ω and 1.920 Ω, respectively. The lowest resistance value of 0.168 Ω was obtained in May. A similar resistance value of 0.170 Ω was obtained in July, August and June.
Similarly to the two-clamp method, there is no need to analyze deviations from the average value of individual measurement results for the impact method. The measurement results are summarized in Table 4. The highest impedance values occurred in April and November and were 0.955 Ω and 0.656 Ω, respectively.
Table 4.
Results of ground electrode impedance measurements using the impact method.
| No | Month | Ground electrode impedance ZE (Ω) | ||||
|---|---|---|---|---|---|---|
| ZE1 (Ω) | ZE2 (Ω) | ZE3 (Ω) | ZEAVG,2p (Ω) | |||
| 1 | July | 0.173 | 0.173 | 0.174 | 0.173 | |
| 2 | August | 0.101 | 0.100 | 0.105 | 0.102 | |
| 3 | September | 0.126 | 0.123 | 0.121 | 0.123 | |
| 4 | October | 0.650 | 0.651 | 0.653 | 0.651 | |
| 5 | November | 0.661 | 0.658 | 0.649 | 0.656 | |
| 6 | December | 0.321 | 0.321 | 0.320 | 0.321 | |
| 7 | January | 0.330 | 0.323 | 0.321 | 0.325 | |
| 8 | February | 0.285 | 0.286 | 0.289 | 0.287 | |
| 9 | March | 0.655 | 0.651 | 0.647 | 0.651 | |
| 10 | April | 0.961 | 0.949 | 0.954 | 0.955 | |
| 11 | May | 0.211 | 0.213 | 0.214 | 0.213 | |
| 12 | June | 0.116 | 0.115 | 0.114 | 0.115 | |
Additionally, the measured average values of the ground electrode resistance are presented in Fig. 2.
Fig. 2.
Measured average values of ground electrode resistance with different point methods (a) and impact method (b).
Discussion
To measure the resistance of the earth system three methods were used described in the Materials and methods section. Each of them has its advantages and disadvantages described in53. To evaluate the obtained results, the differences between the obtained measurements were calculated according to Eqs. (3), (4) and (5).
 |
3 |
 |
4 |
 |
5 |
The values of differences calculated using the above-defined dependencies (3), (4) and (5) were calculated on an annual basis separately for each month. The results obtained in this way are placed in Table 5.
Table 5.
Results of calculations of differences using the 4p, 3p and 2p methods.
| No | Month | Differences (Ω) | ||
|---|---|---|---|---|
| R 1 ERR | R 2 ERR | R 3 ERR | ||
| 1 | July | 0.07 | 0.03 | 0.11 |
| 2 | August | 0.04 | 0.05 | 0.01 |
| 3 | September | 0.12 | 0.04 | 0.08 |
| 4 | October | 0.48 | 0.60 | 0.12 |
| 5 | November | 0.43 | 0.19 | 0.24 |
| 6 | December | 0.11 | 0.15 | 0.26 |
| 7 | January | 0.08 | 0.17 | 0.25 |
| 8 | February | 0.10 | 0.05 | 0.16 |
| 9 | March | 0.08 | 1.06 | 1.15 |
| 10 | April | 0.24 | 1.03 | 1.27 |
| 11 | May | 0.13 | 0.03 | 0.10 |
| 12 | June | 0.34 | 0.05 | 0.28 |
The smallest value of the difference defined as (3) occurs for August and is 0.04.Ω. For the difference defined as (4), the smallest value occurs for May, June and July and is 0.03 Ω. But for the difference defined as (5), the lowest value of 0.01 Ω occurred for August. For the difference defined as (3) its highest value occurs in October and is 0.48 Ω, while in March the difference described by the relation (4) is as high as 1.06 Ω. For the difference (5) its maximum value was observed in April and is 1.27 Ω. There is no month in which all these predefined differences are larger.
Ground electrode resistance measurements were performed monthly for a year. The observed changes in resistance value depend primarily on changes in soil resistivity with changes in climatic conditions. To estimate the trends in changes in grounding resistance with changes in the seasons, the changes can be calculated on a month-to-month basis. The relative change in the ground electrode resistance value is defined as the ratio of the ground electrode resistance in the current month to the resistance in the previous month. This quantity can be calculated from the Eq. (6).
 |
6 |
The monthly relative changes in the resistance value obtained by the three measurement methods used, calculated based on (6), are presented in Table 6.
Table 6.
Monthly change in resistance value month to month (%).
| No | Month | Change in the grounding resistance value in a specific method in relation to the previous month’s DRMonthE [%] | ||
|---|---|---|---|---|
| 3-point | 4-point | 2-point | ||
| 2 | August | -42.08 | -41.18 | 0.00 |
| 3 | September | 60.13 | 13.06 | 5.88 |
| 4 | October | 44.98 | 525.06 | 40.74 |
| 5 | November | 11.42 | -0.67 | 156.58 |
| 6 | December | 35.43 | -46.38 | -53.85 |
| 7 | January | -2.15 | 3.76 | 0.00 |
| 8 | February | -15.04 | -22.40 | 3.33 |
| 9 | March | 42.29 | 105.22 | 483.87 |
| 10 | April | -1.51 | 19.29 | 6.08 |
| 11 | May | -59.08 | -84.58 | -91.49 |
| 12 | June | 70.38 | -14.08 | 4.08 |
In the case of the 3p measurement method, the largest changes in the ground electrode resistance were observed in September and June and equaled 60.13% and 70.38%. During these months, month-to-month changes in resistance were positive but a clear negative resistance change of 59% was observed in May.
The value of the ground electrode resistance is influenced by environmental conditions such as humidity and temperature. Grounding standards do not recommend the use of correction factors related to environmental conditions. Correction factors taking into account seasonal changes in ground electrode resistance were proposed in2 and described in Table 7.
Table 7.
Coefficients of seasonal changes in equivalent soil resistivity according to2.
| Type of ground electrode | The value of the correction factor kp depending on the degree of soil moisture | ||
|---|---|---|---|
| Dry ground 1) | Damp ground 3) | Wet ground 2) | |
| Horizontal ground electrode buried at a depth of 0.6–1.0 m | 1.4 | 2.2 | 3.0 |
| Vertical ground electrode with a length of 2.5 m to 5 m | 1.2 | 1.6 | 2.0 |
| Vertical ground electrode with a length of more than 5 m | 1.1 | 1.2 | 1.3 |
| Compound ground electrode | It is recommended to determine the influence of the resistance of horizontal and vertical ground electrodes on the resultant groundingresistance | ||
1) Ground can be classified for the period from June to September inclusive. The exception is the three days after long-term heavy rainfall.
2) Ground can be classified in a period beyond the defined period in point 1).
3) Ground is classified in this category if the conditions cannot be assigned to cases 1) and 2).
The above factors were used to correct the measured values of ground electrode resistance. The results are presented in Table 8. The value of the corrected resistance RkE and impedance (for the impact method) ZkE is calculated as the quotient of the average value of the average ground electrode resistance and the value of the kp coefficient (formulas 7 and 8).
 |
7 |
 |
8 |
Table 8.
Corrected ground electrode resistance and impedance values.
| Month | kp | Grounding system resistance RkE (Ω) | Grounding system impedance RkE (Ω) | Average temperature | Note* | ||
|---|---|---|---|---|---|---|---|
| 4p | 3p | 2p | Impact method | °C | |||
| July | 1.4 | 0.286 | 0.386 | 0.238 | 0.243 | 19 | DM |
| August | 3.0 | 0.360 | 0.479 | 0.510 | 0.306 | 15 | DM, HR |
| September | 1.4 | 0.190 | 0.358 | 0.252 | 0.173 | 15 | DM |
| October | 2.2 | 1.866 | 0.815 | 0.557 | 1.433 | 8 | HM |
| November | 2.2 | 1.853 | 0.909 | 1.430 | 1.443 | 0 | HM |
| December | 2.2 | 0.994 | 1.231 | 0.660 | 0.705 | 2 | HM |
| January | 2.2 | 1.031 | 1.204 | 0.660 | 0.714 | -1 | HM |
| February | 2.2 | 0.800 | 1.203 | 0.682 | 0.631 | 3 | HM |
| March | 2.2 | 1.642 | 1.456 | 3.982 | 1.432 | 13 | HM |
| April | 2.2 | 1.959 | 1.434 | 4.224 | 2.100 | 7 | HM |
| May | 3.0 | 0.412 | 0.800 | 0.490 | 0.638 | 11 | HM, HR |
| June | 1.4 | 0.165 | 0.636 | 0.238 | 0.636 | 20 | DM |
*DM - “dry” month, HR - heavy rainfall, HM - “humid” month.
If the measurements were performed after heavy rainfall occurred within 1–2 days, i.e. the water column increased by at least 10 mm, the value of the kp coefficient was assumed to be 3.0. This situation occurred in August and May. In the first case, the day before measurements were taken, there was very heavy rainfall (20 mm of water column). In the second case, it rained for 3 days before the measurement (2–3 mm of water column per day) and on the day of the measurement (1 mm of water column).
The facility owner usually sets permissible grounding resistance values. An acceptable grounding resistance value of 5 Ω was adopted for the tested building based on the requirements set. The low grounding resistance value is due to the type of manufacturing carried out. For other locations operated by the facility owner, the earth resistance requirements are more stringent. For these facilities, the acceptable value of earth resistance should not be greater than 3 Ω.
The use of correction factors generally increases the ground electrode resistance values. This provides an additional margin of safety when assessing the resistance of the ground electrode. If the permissible value of ground electrode resistance is equal to 5 Ω is assumed, this condition is met for all methods. Therefore, the ground electrode can be used. However, if the permissible resistance of the ground electrode was 3Ω, such a value would be exceeded in March and April if the measurement was made using the double clamp method.
Conclusion
A properly designed and constructed ground electrode should maintain the assumed resistance value during operation. Any increase in its resistance may indicate a continuing degradation process or a change in the physical and chemical properties of the soil. The basic parameter of each ground electrode, its resistance, should be verified by performing measurements. The choice of measurement method depends on technical possibilities and the type of ground electrode. The paper presents and analyzes the results of ground electrode measurements of a selected object, performed using four methods. Due to the influence of environmental conditions on the results obtained, measurements were carried out cyclically every month throughout the year. This enabled the analysis of seasonal changes in the grounding resistance value. The 4p method is the most accurate one. It results from the measurement method and the measurement accuracy class included in the meter manufacturer’s documentation53. Based on the measurements and calculations, taking into account the climatic conditions, the value of the grounding resistance does not exceed the 5 Ω value that was required by the building owner. It can therefore be concluded that the grounding system meets all requirements for fault protection and atmospheric influences over the year. To obtain reliable results of ground electrode resistance measurements, it can be recommended that the measurements should be performed at least four times a year in representative months for each season.
Author contributions
Conceptualization: R.S., A.W., P.M. and E.K.; methodology: R.S., A.W., P.M.; formal analysis: R.S., A.W., P.M. and EK; investigation: R.S., A.W., P.M. and E.K.; resources: R.S., A.W. and P.M.; data curation: R.S., A.W.; writing—original draft preparation: R.S., A.W., P.M. and E.K.; writing—review and editing: R.S., A.W., P.M. and E.K. All authors reviewed the manuscript.
Data availability
Data is provided within the manuscript.
Declarations
Competing interests
The authors declare no competing interests.
Footnotes
Publisher’s note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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Associated Data
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Data Availability Statement
Data is provided within the manuscript.