The calculation of grounding devices comes down mainly to the calculation of the ground electrode itself, since grounding conductors in most cases are accepted according to the conditions mechanical strength and resistance to corrosion according to PTE and PUE. The only exceptions are installations with a remote grounding device. In these cases, the series-connected resistances of the connecting line and the ground electrode are calculated so that their total resistance does not exceed the permissible value.
Particular attention should be paid to the calculation of grounding devices for the polar and northeastern regions of our country. They are characterized by permafrost soils, which have a resistivity of the surface layers one to two orders of magnitude higher than under normal conditions. middle zone THE USSR.
Calculation of the resistance of grounding conductors in other regions of the USSR is carried out in the following order:
1. The permissible resistance of the grounding device r ZM required according to the PUE is established. If the grounding device is common to several electrical installations, then the calculated resistance of the grounding device is the least required.
2. The required resistance of the artificial ground electrode is determined, taking into account the use of natural earth electrodes connected in parallel, from the expressions
(8-14)
where r зм is the permissible resistance of the grounding device according to clause 1, R and is the resistance of the artificial grounding device; R e is the resistance of the natural ground electrode. The calculated soil resistivity is determined taking into account increasing factors that take into account soil drying out in summer and freezing in winter.
In the absence of accurate data on the soil, you can use the table. 8-1, which shows the average soil resistance data recommended for preliminary calculations.
Table 8-1
Average resistivity of soils and waters, recommended for preliminary calculations
Note. The resistivity of soils is determined at a humidity of 10-20% of the soil mass
To obtain more reliable results, resistivity measurements are carried out in the warm season (May - October) in the central zone of the USSR. To the measured value of soil resistivity, depending on the condition of the soil and the amount of precipitation, enter correction factors k, taking into account the change due to drying and freezing of the soil, i.e. P calc = P k
4. The spreading resistance of one vertical electrode R v.o. is determined. formulas table. 8-3. These formulas are given for rod electrodes made of round steel or pipes.
When using vertical electrodes made of angle steel, the equivalent diameter of the angle, calculated from the expression, is substituted in the formula instead of the pipe diameter
(8-15)
where b is the width of the sides of the corner.
5. The approximate number of vertical grounding conductors is determined at a previously accepted utilization factor
(8-16)
where R v.o. - resistance to spreading of one vertical electrode, defined in clause 4; R and is the required resistance of the artificial ground electrode; K i,v,zm - utilization coefficient of vertical grounding conductors.
Table 8-2
The value of the increasing coefficient k for different climatic zones
The coefficients of use of vertical grounding conductors are given in table. 8-4 when arranged in a row and in a table. 8-5 when placing them along the contour
6. The resistance to spreading of horizontal electrodes Rg is determined using the formulas in Table. 8-3. Usage ratios of horizontal electrodes for preliminary accepted number vertical electrodes are taken according to the table. 8-6 when vertical electrodes are arranged in a row and according to the table. 8-7 when vertical electrodes are located along the contour.
7. The required resistance of the vertical electrodes is specified taking into account the conductivity of the horizontal connecting electrodes from the expressions
(8-17)
where R g is the resistance to spreading of horizontal electrodes, defined in paragraph 6; R and is the required resistance of the artificial ground electrode.
Table 8-3
Formulas for determining the resistance to current spreading of various ground electrodes
Table 8-4
Usage factors for vertical grounding electrodes, K and, v, zm, placed in a row, without taking into account the influence of horizontal coupling electrodes
Table 8-5
Usage coefficients of vertical grounding electrodes, K and, v, zm, placed along the contour, without taking into account the influence of horizontal communication electrodes
Table 8-6
Utilization factors K and, g, zm of horizontal connecting electrodes, in a row of vertical electrodes
Table 8-7
Utilization factors K and, g, zm of vertical connecting electrodes in a circuit of vertical electrodes
8. The number of vertical electrodes is specified taking into account the utilization factors according to table. 8-4 and 8-5:
The number of vertical electrodes from the placement conditions is finally accepted.
9. For installations above 1000 V with high ground fault currents, the thermal resistance of the connecting conductors is checked using formula (8-11).
Example 1. It is required to calculate the contour grounding system of a 110/10 kV substation with the following data: the highest current through the grounding during ground faults on the 110 kV side is 3.2 kA, the highest current through the grounding during ground faults on the 10 kV side is 42 A; the soil at the substation construction site is loam; climate zone 2; Additionally, a cable-support system with a grounding resistance of 1.2 Ohms 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 formula (8-12) we have:
where the design voltage on the grounding device U calculated is assumed to be 125 V, since the grounding device is also used for substation installations with voltages up to 1000 V.
Thus, the calculated resistance is taken to be rzm = 0.5 Ohm.
2. The resistance of the artificial grounding system is calculated taking into account the use of a cable-support system
3. Recommended for preliminary calculations is the resistivity of the soil at the site of construction of the ground electrode (loam) according to table. 8-1 is 1000 Ohm m. Increasing coefficients k for horizontal extended electrodes at a depth of 0.8 m are equal to 4.5 and, accordingly, 1.8 for vertical rod electrodes 2 - 3 m long at a depth of their top of 0.5 - 0 .8 m.
Calculated resistivities: for horizontal electrodes P calc.g = 4.5x100 = 450 Ohm m; for vertical electrodes calculated in = 1.8x100 = 180 Ohm m.
4. The resistance to spreading of one vertical electrode is determined - angle No. 50 2.5 m long when immersed 0.7 m below ground level using the formula from table. 8-3:
where d= d y,ed= 0.95; b = 0.95x0.95 = 0.0475 m; t =0.7 + 2.5/2 = 1.95 m;
5. The approximate number of vertical grounding conductors is determined with a previously accepted utilization factor K and, in, zm = 0.6:
6. The resistance to spreading of horizontal electrodes (40x4 mm2 strips) welded to the upper ends of the corners is determined. The coefficient of utilization of the connecting strip in the circuit K and, g, zm with the number of corners is approximately 100 and the ratio a/l = 2 according to table. 8-7 is equal to 0.24. Resistance to strip spreading along the perimeter of the contour (l = 500 m) according to the formula from table. 8-3 equals:
7. Improved resistance of vertical electrodes
8. The specified number of vertical electrodes is determined with the utilization coefficient K u, r, zm = 0.52, adopted from table. 8-5 with n = 100 and a/l = 2:
116 corners are finally accepted.
In addition to the circuit, a grid of longitudinal strips is installed on the territory, located at a distance of 0.8-1 m from the equipment, with transverse connections every 6 m. Additionally, to equalize the potentials at the entrances and entrances, as well as along the edges of the circuit, in-depth strips are laid. These unaccounted for horizontal electrodes reduce the overall grounding resistance, their conductivity goes into the safety margin.
9. The thermal resistance of the 40 × 4 mm 2 strip is checked.
Minimum strip section from the conditions thermal resistance at short circuit to the ground in formula (8-11) at the given short-circuit current flow time. tп = 1.1 is equal to:
Thus, a strip of 40 × 4 mm 2 satisfies the thermal resistance condition.
Example 2. It is required to calculate the grounding of a substation with two 6/0.4 kV transformers with a power of 400 kVA with the following data: the maximum current through the grounding during a ground fault on the 6 kV side is 18 A; the soil at the construction site is clay; climate zone 3; Additionally, a water supply with a spreading resistance of 9 Ohms is used as grounding.
Solution. It is planned to construct a grounding system with outside the building to which the substation is adjacent, with vertical electrodes arranged in one row 20 m long; 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, a grounding resistance is required, determined by formula (8-12):
where the design voltage on the grounding device is assumed to be 125 V, since the grounding device is common to the 6 and 0.4 kV sides.
According to the PUE, the grounding resistance should not exceed 4 Ohms. Thus, the calculated grounding resistance is rzm = 4 Ohms.
2. The resistance of the artificial grounding system is calculated taking into account the use of a water supply system as a parallel grounding branch
3. Recommended for calculations is the soil resistance at the site of grounding construction (clay) according to table. 8-1 is 70 Ohm*m. Increasing coefficients k for the 3rd climatic zone according to table. 8-2 are taken equal to 2.2 for horizontal electrodes at a depth of 0.7 m and 1.5 for vertical electrodes 2-3 m long at a depth of their upper end of 0.5-0.8 m.
Calculated soil resistivities:
for horizontal electrodes P calc.g = 2.2 × 70 = 154 Ohm*m;
for vertical electrodes P calc.v = 1.5x70 = 105 Ohm*m.
4. The spreading resistance of one rod with a diameter of 20 mm and a length of 2 m is determined when immersed 0.7 m below ground level using the formula from table. 8-3:
5. The approximate number of vertical grounding conductors is determined at the previously accepted utilization factor K and. g. zm = 0.9
6. The spreading resistance 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 use of a horizontal electrode in a row of rods with a number of approximately 6 and the ratio of the distance between the rods to the length of the rods is a/l = 20/5x2 = 2 in accordance with Table. 8-6 is taken equal to 0.85.
The spreading resistance of a horizontal electrode is determined by the formula from table. 8-3 and 8-8:
Table 8-8
Coefficients of increasing resistance in relation to the measured soil resistivity (or grounding resistance) for the central zone of the USSR
Notes: 1) applies to 1 if the measured value P (Rx) corresponds approximately to the minimum value (the soil is wet - the time of measurement was preceded by precipitation large quantity precipitation);
2) k2 is applied if the measured value P (Rx) corresponds approximately to the average value (soil average humidity- the time of measurements was preceded by a small amount of precipitation);
3) k3 is applied if the measured value P (Rx) corresponds approximately highest value(the soil is dry - the time of measurement was preceded by a small amount of precipitation).
7. Improved resistance to spreading of vertical electrodes
8. The specified number of vertical electrodes is determined using the utilization factor K and. g. zm = 0.83, adopted from table. 8-4 with n = 5 and a/l = 20/2x4 = 2.5 (n = 5 instead of 6 is taken from the condition of reducing the number of vertical electrodes while taking into account the conductivity of the horizontal electrode)
Four vertical rods are finally adopted, with the spreading resistance being slightly less than the calculated one.
Excerpt from the Electricity Handbook industrial enterprises
under the general editorship of A. A. Fedorov and G. V. Serbinovsky
Grounding calculations are carried out in order to determine the resistance of the constructed grounding loop during operation, its size and shape. As is known, the grounding loop consists of vertical grounding conductors, horizontal grounding conductors and a grounding conductor. Vertical grounding rods are driven into the soil to a certain depth.
Horizontal grounding conductors connect vertical grounding conductors to each other. The grounding conductor connects the ground loop directly to the electrical panel.
The dimensions and number of these grounding conductors, the distance between them, the soil resistivity - all these parameters directly depend on the grounding resistance.
What does grounding calculation boil down to?
Grounding serves to reduce touch voltage to a safe value. Thanks to grounding, dangerous potential goes into the ground, thereby protecting a person from electric shock.
The magnitude of the current flowing into the ground depends on the resistance of the ground loop. The lower the resistance, the lower the magnitude of the dangerous potential on the body of the damaged electrical installation.
Grounding devices must satisfy certain requirements imposed on them, namely the resistance to current spreading and the distribution of dangerous potential.
Therefore the main calculation of protective grounding is reduced to determine the current spreading resistance of the ground electrode. This resistance depends on the size and number of grounding conductors, the distance between them, their depth and soil conductivity.
Initial data for calculating grounding
1. The main conditions that must be adhered to when constructing grounding devices are the dimensions of the grounding conductors.
1.1. Depending on the material used (angle, strip, round steel) minimum dimensions grounding conductors must be no less than:
- a) strip 12x4 – 48 mm2;
- b) corner 4x4;
- c) round steel – 10 mm2;
- G) steel pipe(wall thickness) – 3.5 mm.
Minimum sizes of fittings used for installation of grounding devices
1.2. The length of the grounding rod must be at least 1.5 - 2 m.
1.3. The distance between the grounding rods is taken from the ratio of their lengths, that is: a = 1xL; a = 2xL; a = 3xL.
Depending on the available area and ease of installation, the grounding rods can be placed in a row or in the form of any shape (triangle, square, etc.).
The purpose of calculating protective grounding.
The main purpose of grounding calculations is to determine the number of grounding rods and the length of the strip that connects them.
Example of grounding calculation
Current spreading resistance of one vertical ground electrode (rod):
where – ρ eq – equivalent soil resistivity, Ohm m; L – rod length, m; d – its diameter, m; T – distance from the ground surface to the middle of the rod, m.
In the case of installing a grounding device in heterogeneous soil (two-layer), the equivalent soil resistivity is found by the formula:
where – Ψ is the seasonal climate coefficient (Table 2); ρ 1, ρ 2 – resistivity of the upper and lower layers of soil, respectively, Ohm m (Table 1); H – thickness of the top soil layer, m; t - depth of the vertical ground electrode (trench depth) t = 0.7 m.
Since the resistivity of the soil depends on its moisture, to stabilize the resistance of the ground electrode and reduce the influence of climatic conditions on it, the ground electrode is placed at a depth of at least 0.7 m.
The depth of the horizontal ground electrode can be found using the formula:
Installation and installation of grounding must be done in such a way that the grounding rod penetrates upper layer soil completely and partially below.
| Type of ground electrodes | Climate zone | |||
| I | II | III | IV | |
| Rod (vertical) | 1.8 ÷ 2 | 1.5 ÷ 1.8 | 1.4 ÷ 1.6 | 1.2 ÷ 1.4 |
| Strip (horizontal) | 4.5 ÷ 7 | 3.5 ÷ 4.5 | 2 ÷ 2.5 | 1.5 |
| Climatic characteristics of zones | ||||
| Average long-term lowest temperature(January) | from -20+15 | from -14+10 | from -10 to 0 | from 0 to +5 |
| Average long-term highest temperature(July) | from +16 to +18 | from +18 to +22 | from +22 to +24 | from +24 to +26 |
The number of grounding rods without taking into account the resistance of horizontal grounding is determined by the formula:
Rн is the standardized resistance to current spreading of the grounding device, determined based on the rules of PTEEP (Table 3).
| Characteristics of the electrical installation | Soil resistivity ρ, Ohm m | Grounding device resistance, Ohm |
| An artificial grounding conductor to which the neutrals of generators and transformers are connected, as well as repeated grounding conductors of the neutral wire (including in the room inputs) in networks with a grounded neutral for voltage, V: | ||
| 660/380 | up to 100 | 15 |
| over 100 | 0.5 ρ | |
| 380/220 | up to 100 | 30 |
| over 100 | 0.3 ρ | |
| 220/127 | up to 100 | 60 |
| over 100 | 0.6 ρ |
As can be seen from the table, the normalized resistance for our case should be no more than 30 ohms. Therefore, Rн is taken equal to Rн = 30 Ohm.
Current spreading resistance for a horizontal ground electrode:
L g, b – length and width of the ground electrode; Ψ – seasonality coefficient of horizontal ground electrode; η g – demand coefficient of horizontal grounding conductors (Table 4).
We will find the length of the horizontal ground electrode based on the number of ground electrodes:
- in a row; - along the contour.
a is the distance between the grounding rods.
Let us determine the resistance of the vertical grounding conductor taking into account the current spreading resistance of horizontal grounding conductors:
The total number of vertical grounding conductors is determined by the formula:
η in – demand coefficient for vertical grounding conductors (Table 4).
The utilization factor shows how spreading currents from single grounding conductors influence each other at different locations of the latter. When connected in parallel, the spreading currents of single grounding rods have a mutual influence on each other, therefore, the closer the grounding rods are located to each other, the more common ground loop resistance is greater.
The number of grounding conductors obtained in the calculation is rounded to the nearest larger number.
The calculation of grounding according to the above formulas can be automated by using the special program “Electrician v.6.6” for the calculation; you can download it on the Internet for free.
Grounding is necessary to ensure safety in case of damage to electrical devices, insulation of power wiring, or short circuit of conductors. The essence of grounding is to reduce the potential at the point of contact with a grounded electrical installation to the maximum permissible values.
Potential reduction is performed in two ways:
- Grounding – connection of the device body with the neutral conductor going to the substation;
- Grounding - connecting the housing to a grounding loop located in the ground outside the building.
The first option is simpler, but if the neutral conductor is damaged, it ceases to perform its functions, and this is dangerous. Therefore, the presence of a ground loop is prerequisite ensuring security.
Grounding calculations involve determining the resistance of the grounding device, which should not be greater than that specified by technical standards.
Ground loop
The design of the ground loop and the types of materials used are limited by the conditions contained in the documents, for example, in the PUE, the rules for electrical installations.
All electrical installations, without exception, must be grounded, both at the substation and at the enterprise or at home.
The most common design of a ground loop is one or more metal pins (ground electrodes) buried in the ground and welded together. Using a metal conductor, the ground loop is connected to the grounded devices.
Unpainted steel or copper-coated steel materials are used as grounding conductors, the dimensions of which should not be less than those given below:
- Round rolled products – diameter not less than 12 mm;
- Corner – at least 50x50x4 mm;
- Pipes – with a diameter of at least 25 mm and a wall thickness of at least 4 mm.
The better the conductivity of the grounding electrodes, the more efficiently the grounding works, so the most preferable option is to use copper electrodes, but in practice this does not occur due to the high cost of copper.
Uncoated steel has a high corrosion ability, especially at the interface between moist soil and air, therefore it is determined minimum thickness metal walls (4 mm).
Galvanized metal resists corrosion well, but not in the case of current flow. Even the smallest current will cause an electrochemical process, resulting in thin layer zinc will last a minimum time.
Modern grounding systems are based on copper-plated steel. Since the amount of copper for production is low, the cost finished materials slightly exceeds steel ones, and the service life increases many times over.
The most common ground loop designs are triangular or row electrode placement. The distance between adjacent electrodes should be 1.2-2 m, and the laying depth should be 2-3 m. The laying depth (electrode length) largely depends on the characteristics of the soil. The higher it is electrical resistance, the deeper the electrodes should lie. In any case, this depth must exceed the freezing depth of the soil, since frozen soil has a high ohmic resistance. The same applies to areas of land with low humidity.
Where high currents may flow, for example, in a substation or an enterprise with powerful equipment, the approach to choosing the design of the ground loop and its calculation are of great importance for safety.
Grounding Resistance Factors
The calculation of a protective grounding device depends on many conditions, among which the main ones can be identified, which are used in further calculations:
- Soil resistance;
- Electrode material;
- Electrode laying depth;
- Location of grounding conductors relative to each other;
- Weather.
Soil resistance
The soil itself, with a few exceptions, has low electrical conductivity. This characteristic varies depending on the moisture content, since water with salts dissolved in it is a good conductor. Thus, the electrical properties of the soil depend on the amount of moisture contained, the salt composition and the properties of the soil to retain moisture.
Common soil types and their characteristics
| Soil type | Specific resistance ρ, Ohm m |
|---|---|
| Rock | 4000 |
| Loam | 100 |
| Chernozem | 30 |
| Sand | 500 |
| Sandy loam | 300 |
| Limestone | 2000 |
| garden soil | 50 |
| Clay | 70 |
The table shows that the resistivity can differ by several orders of magnitude. In real conditions, the situation is complicated by the fact that at different depths the type of soil can be different and without clearly defined boundaries between the layers.
Electrode material
This part of the calculations is the simplest, since only a few types of materials are used in the manufacture of grounding:
- Steel;
- Copper;
- Copper-plated steel;
- Cink Steel.
Copper in pure form not used due to high cost, the most commonly used materials are pure and galvanized steel. IN Lately Grounding systems that use steel coated with a layer of copper have become increasingly common. Such electrodes have the lowest resistance, which has good stability over time, since the copper layer resists corrosion well.
Uncoated steel has the worst characteristics, since a layer of corrosion (rust) increases the contact resistance at the electrode-soil interface.
Bookmark depth
The linear length of the boundary between the electrode and the ground and the size of the earth layer that participates in the current flow circuit depend on the depth of placement of the electrodes. The larger this layer, the lower the resistance value it will have.
On a note. In addition, when installing electrodes, it should be borne in mind that the deeper they are located, the closer they will be to the aquifer.
Electrode placement
This characteristic is the least obvious and difficult to understand. You should know that each grounding electrode has some influence on its neighbors, and the closer they are located, the less effective they will be. The exact justification of the effect is quite complex; you just need to take it into account during calculations and construction.
It is easier to explain the dependence of efficiency on the number of electrodes. Here we can give an analogy with parallel connected resistors. The more there are, the lower the total resistance.
Weather
The grounding device has the best parameters when high humidity soil. In dry and frosty weather, soil resistance increases sharply and when certain conditions are reached ( complete drying or freezing) takes on the maximum value.
Note! In order to minimize the influence of weather conditions, the depth of laying the electrodes should be lower maximum depth freezing in winter or reach the aquifer to prevent drying out.
Important! Subsequent calculations must be made for the worst operating conditions, since in all other cases the grounding resistance will decrease.
Calculation method
The main calculation parameter is the required value of grounding resistance, which is regulated regulatory documents, depending on the supply voltage, the type of electrical installations, and the conditions of their use.
There is no strict calculation of protective grounding that gives the number and length of electrodes, so it is performed on the basis of some approximate data and tolerances.
To begin with, the type of soil is taken into account, and the approximate length of the grounding electrodes, their material and quantity are determined. Next, a calculation is performed, the order of which is as follows:
- The current spreading resistance for one electrode is determined;
- The number of vertical grounding conductors is calculated taking into account their relative position.
Single ground electrode
We calculate the current spreading resistance according to the formula:
In this expression:
ρ – specific equivalent soil resistance;
l – electrode length;
d – diameter;
t is the distance from the earth’s surface to the center of the electrode.
When using a corner instead of a pipe or rolled product, take:
d = b·0.95, where b is the width of the corner flange.
Equivalent resistance of multilayer soil:
- ρ1 and ρ2 – resistivity of soil layers;
- H – thickness of the top layer;
- Ψ – seasonal coefficient.
The seasonal coefficient depends on the climate zone. Amendments are also made to it, depending on the number of electrodes used. Approximate values of the seasonal coefficient range from 1.0 to 1.5.
Number of electrodes
The required number of electrodes is determined from the expression:
n = Rз/(К·R), where:
- Rз – permissible maximum resistance of the grounding device;
- K – utilization factor.
The utilization rate is selectable. in accordance with the selected number of grounding conductors, their relative position and distance between them.
Row arrangement of electrodes
| Quantity electrodes | Coefficient | |
|---|---|---|
| 1 | 4 6 10 | 0,66-0,72 0,58-0,65 0,52-0,58 |
| 2 | 4 6 10 | 0,76-0,8 0,71-0,75 0,66-0,71 |
| 3 | 4 6 10 | 0,84-0,86 0,78-0,82 0,74-0,78 |
Contour placementelectrodes
| Ratio of the distance between the electrodes to their length | Quantity electrodes | Coefficient |
|---|---|---|
| 1 | 4 6 10 | 0,84-0,87 0,76-0,80 0,67-0,72 |
| 2 | 4 6 10 | 0,90-0,92 0,85-0,88 0,79-0,83 |
| 3 | 4 6 10 | 0,93-0,95 0,90-0,92 0,85-0,88 |
The calculation of the grounding loop does not always give the required value, so it may need to be done several times, changing the number and geometric dimensions of the grounding electrodes.
Ground Measurement
To measure grounding resistance, special measuring instruments. Organizations with the appropriate permit have the right to measure grounding. Usually this energy organizations and laboratories. The measured parameters are entered into the measurement protocol and stored at the enterprise (in the workshop, at the substation).
Ground resistance calculation represents difficult task, in which it is necessary to take into account many conditions, so it is more rational to use the help of organizations that specialize in this area. To solve the problem, you can make calculations using an online calculator, an example of which can be found freely available on the Internet. The calculator program itself will tell you what data needs to be taken into account when calculating.
Video
IN modern world, we cannot imagine our life without the use of electricity. It is all around us and it is precisely this that has allowed humanity to move to a completely new level development. It is impossible to overestimate its importance, however, for all its positive qualities, behind its harmlessness and simplicity, hides colossal energy that poses a mortal danger.
In order to secure premises where people are constantly present, a special device was created - a grounding switch. This is a set of conductors that are designed to drain electrical energy from devices to the ground, thereby eliminating electric shock to a person. It consists of grounding rods (horizontal and vertical rods) and grounding conductors.
Our service offers you to perform grounding calculations using a convenient online calculator. Based on the type of soil, climate zone and types of ground electrodes, the program will provide results on the resistance of individual rods, as well as the overall resistance to spreading. We work only on the latest current data; the following sources were used:
- rules for electrical installations;
- standards for the construction of grounding networks;
- grounding devices for electrical installations - Karyakin R. N.;
- reference book on the design of electrical networks and electrical equipment - Yu. G. Barybina;
- reference book on power supply of industrial enterprises - Fedorov A. A. and Serbinovsky G. V.
Grounding calculator
In order to simplify the calculations, we suggest you use a simple and accurate calculator grounding calculations.
Our online grounding calculator takes into account all correction factors and works based on the given formulas. In order to perform a reliable calculation, you need to fill out the program fields correctly.
- Priming. Specify the top and bottom soil layers, as well as the depth.
- Climate coefficient. Adjustment in calculations based on climate zone:
- Zone I - from -20 to -15°C (January); from +16 to +18°С (July);
- Zone II - from -14 to -10°C (January); from +18 to +22°С (July);
- Zone III - from -10 to 0°C (January); from +22 to +24°С (July);
- Zone IV - from 0 to +5°C (January); from +24 to +26°С (July);
- Vertical grounding conductors. The number of vertical ground electrodes (we assume any number, default is 5), their length and diameter.
- Horizontal grounding conductors. Laying depth horizontal stripe, shelf width and rod length (taken at a ratio of 1:3, 1:2 or 1:1 to the length of the vertical ground electrode - the longer the better).
- electrical resistivity of soil;
- resistance of a single vertical ground electrode;
- length of horizontal grounding conductor;
- horizontal grounding resistance;
- total flow resistance electric current.
The last parameter is defining. Make sure that the standard resistance (2 Ohms - for 380 volts; 4 Ohms - for 220 volts; 8 Ohms - for 127 volts) is in electrical networks was always more than calculated.
An example of grounding calculation on a calculator
Let's assume that our house is located on chernozem soils with a layer thickness of 0.5 m. We live in the south of Russia in the fourth climatic zone. Presumably, 5 vertical electrodes with a diameter of 0.025 m and a length of 2 m will be used as grounding electrodes, horizontal rods at a depth of 0.5 m - 2 m long with a shelf width of 0.05 m.
Then, by transferring all the values into the grounding calculator, we get a total spreading resistance of 4.134 Ohms.
If in our private home single-phase network with a voltage of 220 W, then this value is unacceptable, since this grounding will not be enough.
Let's add another vertical electrode and get a value of 3.568 Ohms. This value is quite suitable for us, which means that such grounding is guaranteed to protect your building and its inhabitants.
If you get a value close to critical, then it is better to increase the number or size of electrodes. Remember that calculating the ground loop is extremely important for safety!
How to calculate grounding in a private house manually
As you already understood, the main parameter that needs to be calculated is the total resistance to spreading, i.e. it is necessary to select such a configuration of electrodes so that the resistance of the grounding device does not exceed the standard one. According to the provisions of the rules for electrical installation devices (PEU), certain maximum currents must be observed:
- 2 Ohm - for 380 volts;
- 4 Ohm - for 220 volts;
- 8 Ohms - for 127 volts.
Correct calculation begins with counting optimal size and the number of rods. To do this manually, the easiest way is to use the simplified formulas below.
- R o - rod resistance, Ohm;
- L - electrode length, m;
- d - electrode diameter, m;
- T is the distance from the middle of the electrode to the surface, m;
- p eq - soil resistance, Ohm;
- ln — natural logarithm;
- π is a constant (3.14).
- R n - standardized resistance of the grounding device (2, 4 or 8 Ohms).
- ψ - climatic correction factor for soil resistance (1.3, 1.45, 1.7, 1.9, depending on the zone).
It is also very important that when choosing the depth and length of the grounding rods, the lower end passes below the freezing level, since when negative temperatures Soil resistance increases sharply, and certain difficulties arise.
A protective contour created around any object that is supplied with electricity will ensure drainage high voltage into the ground via specially installed electrodes. Such designs protect expensive equipment from short circuit and burnouts due to power surges. The installation of the structure must be carried out in accordance with the results of calculations of the level of electrical conductivity of conductors.
Purpose of calculation
Before installing it on a residential or other facility, it is necessary to determine its standard dimensions. This design consists of:
- elements installed vertically to the surface of the earth;
- conductor;
- stripes connecting the contour in the horizontal plane.
The electrodes are dug in and connected to each other using a horizontal ground electrode. After this, the created protection system is connected to the electrical panel.
Such artificial structures are used in power networks with different voltage levels:
- variable from 380 V;
- constant from 440 V;
at hazardous production facilities.
Protective systems are installed in different places of the equipment. Depending on the installation location, they can be remote or contour. IN open structures The elements are connected directly to the grounding element. In contour devices, placement occurs along the outer perimeter or inside the device. For each type protective installations it is necessary to carry out a calculation to establish the resistance value of the vertical grounding conductors, the number of required rods and the length of the strips for connecting them.
Except special devices natural systems can be used:
- communications from metal pipes;
- metal structures;
- substations;
- supports;
- metal cable sheath;
- casing.
Current conductivity calculations are made for artificial structures. Their arrangement at the place of use of power plants ensures the drainage of electric current into the ground, protecting people and equipment from large discharges as a result of a voltage surge. The lower the electrical conductivity, the lower the level of electric current flowing through the protective structure.
Step-by-step calculation of the ground loop
Calculations must be carried out taking into account the number of elements, their distance from each other, the conductivity of the soil and the depth of digging of the vertical ground electrode. Using these parameters, it will be possible to accurately calculate the protective grounding.
First, use the table to determine the type of soil. After that select suitable materials for construction. Then calculations are carried out using special formulas that determine the number of all elements, as well as their ability to conduct electrically.
Based on the results obtained, the entire system is installed, after which control measurements are taken to determine its conductivity.
Initial data
When calculating the force value, you should make a ratio of their number, the length of the connecting strips and the distance at which digging is carried out.
In addition, it will be necessary to take into account the soil resistivity, which is determined by its moisture level. To achieve a stable value, it is necessary to bury the electrodes into the soil to a depth of at least 0.7 meters. It is also important not to deviate from the size of the protective device itself established by GOST. When performing calculations, you need to use ready-made tables with existing indicators for the materials used and the electrical conductivity of certain types of soil.
Table of conductivity indicators for various soils
The required depth to which a vertical electrode is buried in the ground is calculated using the formula:
When installing a protective structure, you need to ensure that the metal rods completely enter the top layer of the earth and partially into its lower levels. During calculations, it will be necessary to use the average coefficients of the soil electrical conductivity level in different seasons in certain seasons. climatic zones presented in this table:
Soil resistance in different climatic zones
To accurately determine the number of vertical elements in the assembled structure, without taking into account the indicators for the narrow strips connecting them, you need to use the formula:
In it Rн, indicating the strength of the current spreading through the soil certain type, the resistance coefficient for which is taken from the table.
To calculate the physical parameters of the material, the dimensions of the system elements used should be taken into account:
- for strips 12x4 - 48 mm2;
- at the corners 4x4 mm;
- for a steel circle – 10 mm2;
- for pipes whose walls are 3.5 mm thick.
Example of grounding calculation
Calculation of the conductivity of the conductors used, taking into account the characteristics of the soil, is necessary for each electrode separately according to the formula:
Wherein:
- Ψ is the climatic coefficient, which is taken from reference literature;
- ρ1, ρ2 – conductivity value of the upper and lower layers of the earth;
- H – thickness of the top layer of soil;
- t – depth of location vertical element in the trenches.
Rods for such structures are buried at a level of no less than 0.7 meters, according to current standards.
What should we have at the end of the calculation?
After carrying out calculations using the formulas used, it is possible to obtain the exact resistance of an artificial type grounding device. Measure these indicators natural systems often fails due to the inability to obtain the exact dimensions of buried communications, tracks, cables or already installed metal structures.
At the end of the calculations, it is possible to obtain the exact number of rods and strips for the contour, which will help create reliable system protection for the equipment used and the entire facility as a whole. Calculations will also help to establish the exact length of the strips connecting the rods. The main result of all the calculations performed will be obtaining the final value of the properties of the conductors used in the created circuit, which determines the strength of the electric current passing through them. This is the most important PES standard, which has certain values for networks with different voltage levels.
Permissible values of grounding resistance, according to standards
There are uniform standard values according to which the current flow resistance for an electrical network with a certain voltage value should not exceed the established GOST standards. In networks with a voltage of 220 V, it should not be more than 8 ohms. At a voltage of 380 V, its value should not be higher than 4 ohms.
To calculate the indicators of the entire circuit, you can use the formula R= R0/ ηв*N, in which:
- R0 current conductivity level for one electrode;
- R - indication of the level of obstruction to the passage of current for the entire system;
- ηв - utilization factor of the protective device;
- N is the number of electrodes in the entire circuit.
Material required for circuit design
You can assemble the circuit from metal material:
- corner,
- strips having certain sizes.
Afterwards, it must be checked by an expert from an independent measurement laboratory. Construction fittings can be used as a natural contour if available in load-bearing structures building. The PES contains a special list of structures that can be used as a natural contour when creating protective systems.
To check the operation of the entire structure it is necessary general meaning and check the resistance of vertical grounding conductors and the entire system with special devices. This work should be entrusted to independent experts from the electrical laboratory. In order for the structure to reliably protect the entire object, measurements should be taken regularly, checking their value to established standards.
