A ground loop is essential to protect people from electric shock. For lightning protection, an own grounding device is created, which is not associated with the protective ground loop. For their correct construction, calculation is required.
The grounding device (GD) has a parameter called spreading resistance or simply resistance. It shows how well a given charger is a conductor of electric current. For electrical installations with a line voltage of 380 V, the spreading resistance of the charger should not be more than 30 ohms, at transformer substations - 4 ohms. For ground loops of medical equipment and video surveillance equipment, server rooms, the rate is set individually and ranges from 0.5 to 1 Ohm.
The task of calculating a grounding device is to determine the number and location of vertical and horizontal ground electrodes sufficient to obtain the required resistance.
Determination of soil resistivity
The results of the calculations of the GD are significantly influenced by the characteristic of the soil at the place of its construction, called the resistivity (⍴). For each type of soil, there is a calculated value indicated in the table.
Soil resistance is influenced by humidity and temperature. In winter with maximum freezing and in summer during drought, the resistivity reaches its maximum values. To take into account the influence of weather conditions, corrections for the climatic zone are introduced to the value of ⍴.
If possible, the resistivity is measured before calculations.
Types of ground electrodes and calculation of their resistance
Earthing switches are natural and artificial, and both are used to create a grounding device. Calculate Impact natural earthing(reinforced concrete foundations, piles) by the amount of spreading resistance is difficult, it is easier to do it by measuring on the spot. The resistance of natural ground electrodes with a length of more than 100 m can be found in the table.
If the ⍴ value is different from 100 Ohm ∙ m, the R value is multiplied by the ⍴ / 100 ratio.
As artificial earthing fittings, pipes, angle or strip steel are used. The resistance of each of them is calculated according to its own formula indicated in the table.
Type of earthing switch | Calculation formula |
| Vertical electrode made of round reinforcing steel or pipe. The upper end is below ground level. | 
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| Angle steel vertical electrode. Upper end below ground level | 
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| The vertical electrode of a round reinforcing steel or pipe. Upper end above ground level | 
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| Horizontal strip steel electrode | 
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| Horizontal electrode made of round reinforcing steel or pipe | 
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| Plate electrode (stacked vertically) | 
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| Vertical electrode made of round reinforcing steel or angle steel | |
| Horizontal electrode made of round reinforcing steel or strip steel | 
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Variable values in formulas:
Now the total resistance of the pins of artificial ground electrodes is calculated:
We calculate the resistance of the conductor connecting vertical ground electrodes according to the formula:
And the impedance of the grounding device.
If the calculated resistance of the ground loop turns out to be insufficient, we increase the number of vertical ground electrodes or change their appearance. We repeat the calculation until the required resistance value is obtained.
Standards> Everything about grounding
CALCULATION OF EARTHING DEVICES
The calculation of grounding devices is reduced mainly to the calculation of the actual ground electrode, since grounding conductors in most cases are taken according to the conditions of mechanical strength and resistance to corrosion. The only exceptions are installations with a remote grounding device. In these cases, the resistance of the connecting line and the resistance of the earthing switch are calculated in series so that the total resistance does not exceed the calculated one.
The calculation of the resistance of the ground electrode system is carried out in the following order:
1. The permissible resistance of the grounding device required according to the PUE is established... If the grounding device is common for several electrical installations, then the calculated resistance of the grounding device is the smallest required.
2. The required resistance of the artificial ground electrode is determined, taking into account the use of a natural ground electrode connected in parallel, from the expressions
where - design resistance of the grounding device according to claim 1;- resistance of an artificial ground electrode;- resistance of a natural ground electrode system.
3. The calculated soil resistivity is determined, taking into account the increasing coefficients, taking into account the drying out of the soil in the summer and its freezing in the winter.
In the absence of accurate soil data, you can use table. 12-1, which shows the average data on soil resistance, recommended for preliminary calculations.
Table 12-1 Resistivity of soils
Name of soil |
Resistivity r, Ohm H m |
Name of soil |
Resistivity r, Ohm H m |
Clay (layer 7-10 m, then rock, gravel)
|
70
|
Loam |
100 |
Note: The specific resistance of soils was determined at a moisture content of 10-20% by weight and at a depth of 1.5 m. |
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Raising coefficients k for different climatic zones are given in table. 12-2 for horizontal and vertical electrodes.
4. The resistance to spreading of one vertical electrode is determinedaccording to the formulas from table. 12-3. These formulas are given for rod electrodes made of round steel or pipes. When using angles for vertical electrodes, the equivalent diameter of the angle is substituted as the diameter
where b - width of the sides of the corner.
Table 12-2 Values of the coefficient k for different climatic zones
Data characterizing climatic zones and type of electrodes used |
Climatic zones |
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1. Climatic signs of zones: |
From +16 to +18 |
+18 to +22 |
+22 to +24 |
+24 to +26 |
Table 12-3 Calculation of the spreading resistance of one electrode
Grounding type |
Location of the earthing switch |
Formula |
Explanations |
Vertical near the surface of the earth |
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Vertical below ground level |
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|
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Horizontal extended below ground level |
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b - The width of the line; if the earth is round in diameter d, then b = 2d |
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Lamellar vertical below ground level |
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a and b - dimensions of the sides of the plate |
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Annular horizontal below ground level |
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b -The width of the line; if the earthing switch is round with a diameter d, then b = 2d |
5. The approximate number of vertical ground electrodes is determined n at a previously accepted utilization factor:
where - the required resistance of the artificial ground electrode.
The utilization factors of vertical ground electrodes are given in table. 12-4 in the case of their arrangement in a row and in table. 12-5 in case of their placement along the contour without taking into account the influence of horizontal coupling electrodes.
6. Resistance to spreading of horizontal electrodes is determinedaccording to the formulas from table. 12-3. Utilization rates of horizontal electrodesfor the previously accepted number of vertical electrodes are taken from table. 12-6 when arranging them in a row and according to table. 12-7 when positioned along the contour.
Table 12-4 Utilization factors for vertical electrodes
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2 |
0,84-0,87 |
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2 |
0,90-0,92
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2 |
0,93-0,95 |
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Table 12-5 Utilization factors for vertical electrodes
Distance ratio between vertical
|
Number of vertical electrodes in a row |
|
4 |
0,66-0,72 |
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4 |
0,76-0,80 |
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4 |
0,84-0,86 |
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Table 12-6 Coefficients of utilization of horizontal electrodes
Utilization ratewith the number of vertical electrodes in a row n |
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1 |
0,77 |
0,74 |
0,67 |
0,62 |
0,42 |
0,31
|
0,21 |
0,20 |
Table 12-7 Usage factors for horizontal electrodes
Ratio of dissipation between vertical electrodes to their length |
Utilization ratewith the number of vertical electrodes in the circuit n |
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1 |
0,45 |
0,40 |
0,36 |
0,34 |
0,27 |
0,24 |
0,21 |
0,20 |
0,10
|
7. The required resistance of the vertical electrodes is specified, taking into account the conductivity of the horizontal connecting electrodes from the expressions
where - resistance to spreading of horizontal electrodes, defined in clause 6.
8. The number of vertical electrodes is specified, taking into account the utilization factors according to the table. 12-4 or 12-5:
Finally, the number of vertical electrodes is taken from the placement conditions.
9. For installations above 1000 V with large earth fault currents, the thermal resistance of the connecting conductors is checked according to the formula (12-5).
Example 12-1. It is required to calculate the grounding of a 110/10 kV substation with the following data: the highest current through the ground during ground faults on the 100 kV side 3.2 kA; the highest current through the ground in case of ground faults on the 10 kV side 42 A; the soil at the substation construction site is loam; climatic zone 2; additionally, a system of cables - supports with a grounding resistance of 1.2 Ohm is used as grounding.
Solution
1. For the 110 kV side, a grounding resistance of 0.5 ohm is required. For the 10 kV side according to the formula (12-6)
where the calculated voltage on the grounding device is taken equal to 125 V, since the grounding device is also used for substation installations up to 1000 V. Thus, the resistance is taken as the calculated one .
2. The resistance of the artificial ground electrode is calculated taking into account the use of the cable-support system;
3. The soil resistivity recommended for preliminary calculations at the site of the earthing switch - loam, according to the above data, is 100 Ohm H m. Increasing coefficients for climatic zone 2 according to table. 12 2 are taken equal to 4.5 for horizontal extended electrodes with a depth of 0.8 m and 1.8 for vertical rod electrodes with a length of 2-3 m with a depth of their apex of 0.5-0.8 m.
Calculated resistivity:
for horizontal electrodes
for vertical electrodes
4. The resistance to spreading of one vertical electrode - corner No. 50 2.5 m long when immersed below ground level by 0.7 m is determined according to the formula from table. 12-3:
where
6. Resistance to spreading of horizontal electrodes is determined - strips 40 X 4 mm2 welded to the upper ends of the corners. The utilization factor of the connecting strip in the contour with the number of corners of the order of 100 and the ratio according to table 12-7 is equal to:.
Resistance to spreading of the strip according to the formula from table. 12-3
7. Refined resistance of vertical electrodes
Taken from table. 12-5 at n = 100 and:
117 corners are finally accepted.
In addition to the contour, a grid of longitudinal strips is arranged on the territory of the substation, located at a distance of 0.8-1 m from the equipment, with cross-links every 6 m.In addition, to equalize the potentials at the entrances and entrances, as well as along the edges of the contour, deepened strips are laid. These unaccounted for horizontal electrodes reduce the total grounding resistance; their conductivity goes into reserve.
9. Thermal resistance of 40 X 4 mm2 strip is checked. The minimum cross-section of the strip from the conditions of thermal resistance at short-circuit. to the ground according to the formula (12-5) at the reduced time of passage of the current short.
Thus, a strip of 40 X 4 mm2 satisfies the condition of thermal resistance.
From the results of example 12-1, it can be seen that with a sufficiently large number of vertical electrodes, the horizontal electrodes connecting the upper ends of the vertical ones have very little effect on the resulting design resistance of the ground loop. This also reveals a defect in the existing calculation method for cases where a sufficiently low loop resistance is required. In the approximate calculation performed, this defect revealed itself in the fact that taking into account the additional conductivity of the circuit from the horizontal connecting strip led not to a decrease in the required number of vertical electrodes, but, on the contrary, to its increase by about 5%. Based on this, it can be recommended in such cases to calculate the required number of vertical electrodes without taking into account the additional conductivity of the connecting and other horizontal strips, assuming that their conductivity will go into the safety margin.
Example 12-2. It is required to calculate the grounding of a substation with two 6 / 0.4 kV transformers with a power of 400 kV H And with the following data: the highest current through the ground in case of a ground fault from the 6 kV side 18 A; the soil at the construction site is clay; climatic zone 3; additionally, a water supply with a spreading resistance of 9 ohms is used as grounding.
Solution
It is planned to construct a ground electrode system on the outside of the building, to which the substation adjoins, with the arrangement of vertical electrodes in one row at a length of 20 m; material - round steel with a diameter of 20 mm, immersion method - screw-in; the upper ends of the vertical rods, immersed to a depth of 0.7 m, are welded to a horizontal electrode made of the same steel.
1. For the 6 kV side, the grounding resistance is required, determined by the formula (12-6):
where the calculated voltage on the grounding device is taken equal to 125 V, since the grounding device is common for sides 6 and 0.4 kV. Further, according to the PUE, the resistance of the ground electrode should not exceed 4 ohms.
Thus, the calculated grounding resistance is .
2. The resistance of an artificial ground electrode is calculated taking into account the use of a water supply system as a parallel grounding branch:
3. Recommended soil resistance for calculations in the place of construction of the ground electrode - clay according to table. 12-1 is 70 Ohm H m. Increasing coefficients for climatic zone 3 but tab. 12-2 are taken equal to 2.2 for horizontal electrodes with a depth of 0.8 m and 1.5 for vertical electrodes with a length of 2-3 m with a depth of their apex of 0.5-0.8 m.
Calculated soil resistivity:
for horizontal electrodes
for vertical electrodes
4. The resistance to spreading of one rod with a diameter of 20 mm and a length of 2 m is determined when immersed below ground level by 0.7 m according to the formula from table. 12-3:
5. The approximate number of vertical ground electrodes is determined with the previously adopted utilization factor:
6. The resistance to spreading of a horizontal electrode made of round steel with a diameter of 20 mm, welded to the upper ends of the vertical rods, is determined. The coefficient of using a horizontal electrode in a row of rods when their number is approximately equal to 5 and the ratio of the distance between the rods to the length of the rod 
in accordance with table. 12-6 is taken equal to 0.86.
Resistance to spreading of a horizontal electrode according to the formula from table. 12-3
7. Refined resistance to spreading of vertical electrodes
8. The specified number of vertical electrodes is determined at the utilization factor taken from table. 12-4 at n = 4 and 
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section prepared according to the standard design SERIES 3.407-150
Grounding devices
basics of electricity supply
Requirements for grounding devices
basics of power supply
Calculation of grounding devices
basics of power supply
Electrocorrosion of underground networks by stray currents
basics of power supply
Re-grounding of the neutral wire at the entrance to an individual residential building
Purpose of work: familiarize yourself with the algorithm for calculating protective grounding using the method of utilization rates of ground electrodes (electrodes) according to the permissible resistance of the grounding system to current spreading.
The purpose of the calculation: determination of the main parameters of grounding (the number, size and location of single vertical grounding conductors and horizontal grounding conductors)
1. Brief theoretical information.
Protective earth- deliberate electrical connection to earth or its equivalent of non-conductive metal parts that may be energized.
Purpose of protective grounding- elimination of the risk of injury to people by electric shock when voltage appears on the structural parts of electrical equipment, i.e. when closed to the case.
Functional principle of protective grounding- reduction to safe values of touch and step voltages caused by a short circuit to the case. This is achieved by reducing the potential of the grounded equipment, as well as equalizing the potentials by raising the potential of the foundation on which the person stands, to a potential close to the intended potential of the grounded equipment.
Grounding device is called a set of vertical grounding conductors - metal conductors in direct contact with the ground, and horizontal grounding conductors connecting the grounded parts of the electrical installation with the grounding electrode.
Indoors, potential equalization occurs naturally through metal structures, pipelines, cables and similar conductive objects connected to a branched grounding network.
Metal non-current-carrying parts of the equipment are subject to protective grounding, which, due to insulation failure, may be energized and which people can touch. At the same time, in a room with increased danger and especially dangerous due to the conditions of electric shock, as well as in outdoor installations, grounding is mandatory at a rated voltage of an electrical installation above 42V AC and above 110V DC, and in rooms without increased danger - at a voltage of 380V and above AC 440V and above DC. Only in hazardous areas, grounding is carried out regardless of the purpose of the installation.
Distinguish between ground electrodes artificial intended solely for grounding purposes, and natural- metal objects in the ground for other purposes (metal water pipes laid in the ground; pipes of artesian wells; metal frames of buildings and structures, etc.). It is forbidden to use pipelines of flammable liquids, combustible and explosive gases, as well as pipelines covered with insulation to protect against corrosion, as natural grounding conductors. Natural ground electrodes have, as a rule, low resistance to current spreading, and therefore their use for grounding purposes gives great savings. The disadvantages of natural ground electrodes are their availability and the possibility of disrupting the continuity of the connection of extended ground electrodes.
According to the form of arrangement of grounding electrodes, grounding can be contour and remote.
V outline grounding, all electrodes are located around the perimeter of the protected area. V remote(concentrated or focal) - earthing switches are located at a distance from each other not less than the length of the electrode.
In accordance with the requirements of mechanical strength and permissible heating by earth fault currents in installations with voltages above 1000V, grounding steel main conductors must have a cross section of at least 120 mm 2, and in installations up to 1000V - at least 100 mm 2.
Additional information (extracts from the PUE - "Electrical Installation Rules", 2000) is given in Appendix 2.
2. Calculation procedure.
2.1 Determine the estimated short-circuit current by the formula:
I 3 = U l ∙ (35 l To + l v ) / 350, A, (1)
2.2 Calculate the required resistance of the grounding device R s in accordance with table. eleven . If R s more than the permissible value, then in further calculations R s take equal to the admissible value.
2.3 Determine the design soil resistivity ρ R :
ρ R = ρ rev ∙ , Ohm ∙ m (2)
where ρ rev- electrical resistivity of the soil, obtained by measurement or from reference literature (Table 2); - seasonality coefficient , the value of which depends on the climatic zone; (for the fourth climatic zone with average low temperatures in January from 0 to - 5 0 С and higher temperatures in July from +23 to +26 0 С = 1,3 ).
With a high resistivity of the earth, methods of artificial reduction are used ρ rev in order to reduce the size and number of electrodes used and the area occupied by the ground electrode system. A significant result is achieved by chemical treatment of the area around the ground electrodes using electrolytes, or by laying the ground electrodes in pits with bulk coal, coke, and clay.
Normalized resistance to current spreading into the ground (permissible for a given soil) Normalized resistanceNormalized resistance to current spreading of the grounding device in accordance with the Electrical Installation Rules (PUE). Dimension - Ohm.
In accordance with the PUE, the permissible resistance of the grounding device Rн is set. If the grounding device is common for installations for different voltages, then the lowest of the permissible values is taken as the design resistance of the grounding device.
The resistance of the grounding device, to which the neutrals of the generator or transformer or the terminals of the single-phase current source are connected, at any time of the year should be no more than 2, 4 and 8 Ohms, respectively, at line voltages of 660, 380 and 220 V of the three-phase current source or 380, 220 and 127 In a single-phase current source. This resistance must be ensured taking into account the use of natural grounding conductors, as well as grounding electrodes of repeated grounding of the PEN- or PE-conductor of overhead lines with a voltage of up to 1 kV with the number of outgoing lines at least two. The resistance of the ground electrode located in the immediate vicinity of the neutral of a generator or transformer or the output of a single-phase current source should be no more than 15, 30 and 60 Ohms, respectively, at line voltages of 660, 380 and 220 V of a three-phase current source or 380, 220 and 127 V of a single-phase source current.
At line voltages of 660, 380 and 220 V of a three-phase current source or 380, 220 and 127 V of a single-phase current source in case the earth resistivity p> 100 Ohm * m, it is allowed to increase the indicated norms by 0.01 p times, but not more than ten times.
Grounding devices for electrical installations with voltages up to 1 kV in networks with insulated neutral, used for protective grounding of exposed conductive parts in the IT system, must comply with the condition:
where R is the resistance of the grounding device, Ohm;
Upr - touch voltage, the value of which is assumed to be 50 V (see also 1.7.53 PUE);
I - total earth fault current, A.
As a rule, it is not required to accept the resistance value of the grounding device less than 4 ohms. The resistance of the grounding device is allowed up to 10 Ohm, if the above condition is met, and the power of generators or transformers does not exceed 100 kVA, including the total power of generators or transformers operating in parallel.
In electrical installations with a voltage higher than 1 kV of a network with an insulated neutral, the resistance of the grounding device during the passage of the rated earth fault current at any time of the year, taking into account the resistance of natural grounding conductors, must be
but not more than 10 Ohm, where I is the rated earth fault current, A.
The calculated current is taken as follows:
1) in networks without compensation of capacitive currents - earth fault current;
2) in networks with compensation of capacitive currents:
for grounding devices to which compensating devices are connected - a current equal to 125% of the rated current of the most powerful of these devices; for grounding devices to which compensation devices are not connected - the earth fault current passing in this network when the most powerful of these devices is disconnected compensating devices.
The estimated earth fault current must be determined for the one of the possible network circuits in operation, at which this current has the highest value. ")" Onmouseout = "hide_info (this)" src = "/ pics / help.gif">
The most important function of grounding is electrical safety. Before installing it in a private house, at a substation and in other places, it is necessary to calculate the grounding.
What does the grounding of a private house look like?
An electrical contact with the ground is created by a metal structure immersed in the ground made of electrodes together with connected wires - all this is a grounding device (GD).
The points of connection with the memory of the conductor, protective conductor or cable shield are called grounding points. The figure below shows the grounding from one vertical metal conductor 2500 mm long, dug into the ground. Its upper part is placed at a depth of 750 mm in a trench, the width of which is 500 mm in the lower part and 800 mm in the upper part. The conductor can be connected by welding with other similar grounding conductors into a contour with horizontal plates.
View of the simplest grounding of the room
After installing the earthing switch, the trench is covered with soil, and one of the electrodes should come out. A wire above the ground is connected to it, which goes to the ground bus in the electrical control panel.
When the equipment is in normal conditions at the ground points, the voltage will be zero. Ideally, in the event of a short circuit, the resistance of the charger will be zero.
When a potential appears at a grounded point, it should be zeroed. If we consider any example of calculation, we can see that the short-circuit current I s has a certain value and cannot be infinitely large. The soil has a resistance to current spreading R z from points with zero potential to the ground electrode:
R s = U s / I s, where U s is the voltage on the ground electrode.
Solving the problem of correct calculation of grounding is especially important for a power plant or substation, where a lot of equipment operating under high voltage is concentrated.
The quantityRsdetermined by the characteristics of the surrounding soil: humidity, density, salt content. Here, too, important parameters are the construction of ground electrodes, the immersion depth and the diameter of the connected wire, which must be the same as that of the wires of the electrical wiring. The minimum cross section for bare copper wire is 4 mm 2 and for insulated wire 1.5 mm 2.
If the phase wire touches the housing of the electrical appliance, the voltage drop across it is determined by the values of R s and the maximum possible current. The touch voltage U pr will always be less than U z, since it is reduced by footwear and clothing of a person, as well as the distance to grounding conductors.
On the surface of the earth, where the current flows, there is also a potential difference. If it is high, a person can fall under a step voltage U w, which is life-threatening. The farther from the ground electrodes, the smaller it is.
The value of U s must have a permissible value in order to ensure the safety of a person.
It is possible to reduce the values of U pr and U w if you reduce R s, due to which the current flowing through the human body will also decrease.
If the voltage of an electrical installation exceeds 1 kV (for example, substations at industrial enterprises), an underground structure is created from a closed loop in the form of rows of metal rods driven into the ground and welded together using steel strips. Due to this, potential equalization is carried out between adjacent points of the surface.
Safe work with power grids is ensured not only due to the presence of grounding of electrical appliances. This still requires fuses, circuit breakers and RCDs.
Grounding not only ensures the potential difference to a safe level, but also creates a leakage current, which must be sufficient for the protection to operate.
It is impractical to connect each electrical appliance to the ground electrode. Connections are made through the bus located in the apartment panel. The input for it is a ground wire or a PE wire laid from the substation to the consumer, for example, through the TN-S system.
Calculation of the grounding device
The calculation consists in determining R z. To do this, you need to know the soil resistivity ρ, measured in Ohm * m. Its average values are taken as a basis, which are summarized in a table.
Determination of soil resistivity
| Priming | Priming | Resistivity p, Ohm * m | |
|---|---|---|---|
| Sand with a water depth of less than 5 m | 500 | Garden land | 40 |
| Sand with a water depth of less than 6 and 10 m | 1000 | Chernozem | 50 |
| Water-saturated sandy loam (fluid) | 40 | Coke | 3 |
| Wet water-saturated sandy loam (lamellar) | 150 | Granite | 1100 |
| Low-moisture water-saturated sandy loam (solid) | 300 | Coal | 130 |
| Plastic clay | 20 | chalk | 60 |
| Semisolid clay | 60 | Wet loam | 30 |
| Loam | 100 | Marl clay | 50 |
| Peat | 20 | Porous limestone | 180 |
From the values given in the table, it can be seen that the value of ρ depends not only on the composition of the soil, but also on moisture.
In addition, the tabular values of the resistivity are multiplied by the seasonality coefficient K m, taking into account the freezing of the soil. Depending on the lowest temperature (0 С), its values can be as follows:
- from 0 to +5 - K m = 1.3 / 1.8;
- from -10 to 0 - K m = 1.5 / 2.3;
- from -15 to -10 - K m = 1.7 / 4.0;
- from -20 to -15 - K m = 1.9 / 5.8.
The values of the coefficient K m depend on the method of laying the ground electrodes. The numerator shows its values for vertical immersion of ground electrodes (with the tops laid at a depth of 0.5-0.7 m), and in the denominator - for a horizontal arrangement (at a depth of 0.3-0.8 m).
In the selected area, the soil ρ can differ significantly from the average tabular values due to technogenic or natural factors.
When approximate calculations are carried out, for a single vertical ground electrode R z ≈ 0.3 ∙ ρ ∙ K m.
The exact calculation of protective grounding is carried out according to the formula:
R z = ρ / 2πl ∙ (ln (2l / d) + 0.5ln ((4h + l) / (4h-l)), where:
- l is the length of the electrode;
- d is the diameter of the rod;
- h - the depth of the midpoint of the ground electrodes.
For n vertical electrodes connected from above by welding, R n = R s / (n ∙ K isp), where K isp is the electrode utilization factor, taking into account the shielding effect of neighboring ones (determined from the table).
Location of grounding electrodes
There are many formulas for calculating grounding. It is advisable to apply the method for artificial ground electrodes with geometric characteristics in accordance with the PUE. The supply voltage is 380 V for a three-phase current source or 220 V for a single-phase one.
The normalized resistance of the ground electrode, which should be guided by, is no more than 30 Ohm for private houses, 4 Ohm - for a current source at a voltage of 380 V, and for a 110 kV substation - 0.5 Ohm.
For group storage, a hot-rolled corner with a shelf of at least 50 mm is selected. A strip with a cross section of 40x4 mm is used as horizontal connecting jumpers.
Having decided on the composition of the soil, its resistivity is selected from the table. In accordance with the region, the increasing seasonality coefficient K m is selected.
The number and method of arrangement of the charger electrodes are selected. They can be installed in a row or in a closed loop.
Closed ground loop in a private house
In this case, their shielding effect on each other arises. It is the more, the closer the ground electrodes are located. The values of the coefficients of the use of earthing switches K isp for the circuit or located in a row are different.
Coefficient valuesKispwith different positions of the electrodes
| The amount will ground. n (pcs.) | |||
| 1 | 2 | 3 | |
| 2 | 0.85 | 0.91 | 0.94 |
| 4 | 0.73 | 0.83 | 0.89 |
| 6 | 0.65 | 0.77 | 0.85 |
| 10 | 0.59 | 0.74 | 0.81 |
| 20 | 0.48 | 0.67 | 0.76 |
| Arrangement of electrodes in a row | |||
| The amount will ground. n (pcs.) | The ratio of the distance between ground electrodes to their length | ||
| 4 | 0.69 | 0.78 | 0.85 |
| 6 | 0.61 | 0.73 | 0.8 |
| 10 | 0.56 | 0.68 | 0.76 |
| 20 | 0.47 | 0.63 | 0.71 |
The influence of horizontal bridges is insignificant and may not be taken into account in estimated calculations.
Examples of calculating the ground loop
For a better understanding of the methods of calculating grounding, it is better to consider an example, or better - several.
Example 1
Earthing switches are often made by hand from a steel corner 50x50 mm 2.5 m long. The distance between them is chosen equal to the length - h = 2.5 m. For clay soil, ρ = 60 Ohm ∙ m. The seasonality coefficient for the middle band, selected from the tables, is 1.45. Taking it into account, ρ = 60 ∙ 1.45 = 87 Ohm ∙ m.
For grounding, a trench 0.5 m deep is dug along the contour and a corner is hammered into the bottom.
The size of the angle shelf is reduced to the nominal diameter of the electrode:
d = 0.95 ∙ p = 0.995 ∙ 0.05 = 87 Ohm ∙ m.
The depth of the midpoint of the corner will be:
h = 0.5l + t = 0.5 ∙ 2.5 + 0.5 = 1.75 m.
Substituting the values in the previously given formula, you can determine the resistance of one ground electrode: R = 27.58 ohms.
According to the approximate formula R = 0.3 ∙ 87 = 26.1 Ohm. From the calculation it follows that one rod will obviously not be enough, since according to the requirements of the PUE, the value of the normalized resistance is R norms = 4 ohms (for a 220 V mains voltage).
The number of electrodes is determined by the approximation method according to the formula:
n = R 1 / (k isp R norms) = 27.58 / (1 ∙ 4) = 7 pcs.
Here, at first, k isp = 1. According to the tables, we find for 7 earthing switches k isp = 0.59. If you substitute this value into the previous formula and recalculate again, you get the number of electrodes n = 12 pcs. Then a new recalculation is made for 12 electrodes, where again, according to the table, k isp = 0.54 is found. Substituting this value into the same formula, we get n = 13.
Thus, for 13 corners R n = R s / (n * η) = 27.58 / (13 ∙ 0.53) = 4 Ohm.
Example 2
It is necessary to make an artificial grounding with a resistance R norm = 4 Ohm, if ρ = 110 Ohm ∙ m.
The earthing switch is made of rods with a diameter of 12 mm and a length of 5 m. The seasonality factor according to the table is 1.35. You can also take into account the condition of the soil k g. Measurements of its resistance were made during the dry period. Therefore, the coefficient was k g = 0.95.
Based on the data obtained, the following value is taken as the calculated value of the earth resistivity:
ρ = 1.35 ∙ 0.95 ∙ 110 = 141 Ohm ∙ m.
For a single rod, R = ρ / l = 141/5 = 28.2 ohms.
The electrodes are arranged in a row. The distance between them should be no less than the length. Then the utilization factor will be according to the tables: k isp = 0.56.
Find the number of rods to getRnorms= 4 Ohm:
n = R 1 / (k isp R norms) = 28.2 / (0.56 ∙ 4) = 12 pcs.
After earthing is installed, measurements of electrical parameters are made on site. If the actual R value is higher, more electrodes are added.
If there are natural grounding conductors nearby, they can be used.
This is most often done in a substation where the lowest R value is required. The equipment is used as much as possible here: underground pipelines, power line supports, etc. If this is not enough, artificial grounding is added.
Independent grounding calculations are estimates. After its installation, additional electrical measurements should be made, for which specialists are invited. If the ground is dry, long electrodes must be used due to poor conductivity. In wet soil, the cross-section of the electrodes should be taken as large as possible due to increased corrosion.
