The physicochemical features of the upper layers of the earth, where the currents of electrical installations flow, affect the state of underground metal structures. When designing and installing parts of pipelines and ground electrodes, knowledge about the electrical conductivity of the soil is required. The index of soil resistivity is of great importance. This parameter determines the level of corrosion hazard for buried metal products.
Content
- General concepts and definitions
- Influence of various factors
- The role of parameter table values
- Influence of soil properties on grounding
- Ways to get the required parameters
- Application in practice
General concepts and definitions
The properties of the soil that allow the conduction of current depend on the structure and content of the various components. The predominant effect on the resistance of ground electrodes is exerted by the upper soil layers at a depth of 20 up to 25 m. Insulators in the form of silica, alumina and limestone are forced to act as a conductor so-called. a soil solution where salts and moisture circulate between the solid parts of dielectrics. This leads to the emergence of ionic conductivity of the soil, and it differs from the electronic conductivity of metals by the provision of greater resistance to electric current.
The corrosive activity of the earth is called its ability to destructive physical and chemical interactions with metals. Moisture, porosity, acidity and permeability of the soil, the presence of organic compounds and waste products bacteria, mineralization, quantitative and qualitative composition of electrolyte salts can increase or decrease this activity.
Soil resistivity, or simply resistance, is denoted by the Greek letter ρ and defines the properties in relation to electrical conductivity. It characterizes the ability of the soil to resist the movement of electric charges (current flow) in a conventional conductor with a cross-sectional area of 1 sq. meter and length 1 meter. The unit of measurement of the indicator is Ohm · m.
To determine the value of soil resistivity, there are two main ways:
- Control electrode method (used in the design of single grounding devices). For this, a sample is made, corresponding to the dimensions of the future grounding installation, and immersed in the test soil. Then a pair of auxiliary electrodes is placed there and the resistance to current spreading from the control device is measured.
- Method of four electrodes. They are lowered into the ground at a distance of 2 to 4 meters from each other to a depth of up to 1/20 of this distance. The value measured in this way corresponds to the depth to which the electrodes are spaced.
There are also special high-precision instruments for analyzing soil activity. They allow you to work not only in the laboratory, but also in the field.
Influence of various factors
Composition of the earth, size, configuration and compactness of placement of its fragments, moisture content and temperature, content of soluble chemical components (salts, acids, alkalis, rotting residues of organic impurities) are reflected in the value of the level electrical conductivity. All these parameters are transformed depending on the season, therefore the properties of the soil also change, and in a wide range.
In a dry and hot summer, the upper soil layers dry out, freeze in winter, in both cases the resistance to current spreading increases significantly. So, at a depth of 30 cm, with a decrease in air temperature from 0 ° C to minus 10 ° C, the specific electrical resistivity of the soil increases 10 times, and at a depth of 50 cm - 3 times. This makes it possible to assess the corrosiveness of the soil and obtain initial data for the selection of an effective grounding structure or the design of electrical protective equipment for an underground structure.
Based on this, the corrosiveness of soils is divided into groups, information about which is given in the table:
| Corrosive activity | Specific electrical resistance, Ohm m |
| Low | more than 100 |
| Average | from 20 to 100 |
| Increased | from 10 to 20 |
| High | from 5 to 10 |
| Very high | up to 5 |
The electrical resistance of the soil directly affects the installation work: the lower its value, the easier it is to install grounding devices, and this reduces money and labor costs.
Indeed, in order to effectively resist the spreading of current when organizing the grounding of an installation for the production of electricity, heating or lightning protection equipment in soil with low resistivity, grounding conductors of significantly smaller size.
The role of parameter table values
When calculating a grounding device, designers are interested in information about the electrically conductive properties of the soil. For a preliminary assessment, their average values are used, but for the needs of a specific construction, the characteristics of ground electrodes are recalculated. Initial data are obtained through control measurements and survey work, specifying parameters of soil resistivity for a specific territory.
The table of approximate values looks like this:
| Name of soil | Average electrical resistivity, Ohm m |
| Basalt | 2 thous. |
| Sandstone | 1 thous. |
| Micaceous shale | 800 |
| Sand | 500 |
| Sandy loam | 300 |
| Porous limestone | 180 |
| Coal | 150 |
| Loam | 80 |
| Clay | 60 |
| Chernozem | 50 |
| Garden land | 40 |
| Il | 30 |
| Peat | 25 |
| Saline | 20 |
Soils such as clay, black soil, loam (i.e. n. good) have low electrical resistivity. Sand indicators largely depend on moisture content and range from 10 to 4 thous. Ohm m. In the case of rocky soils, the count is already in the thousands, for crushed stone - from three to five thousand, and for granite rocks - 20 thousand. Ohm m.
The situation is especially difficult with permafrost soils, because a decrease in temperature dramatically increases their resistivity. For example, for the same loam at +10 ° C it is equal to 80 Ohm m, and at minus 10 ° C it already reaches 1 thousand. Ohm m. In winter, the soil monolith freezes to a depth of kilometers, and in summer thawing of the upper layers occurs only a few meters.
Influence of soil properties on grounding
A decrease in the values of the specific electrical resistance of the soil creates more favorable conditions for the spreading of an electric charge. Absorption of leakage currents and lightning discharges reliably protects buried metal structures. This prevents electrical injuries to workers and malfunctions of other devices.
Communication facilities and networks, electrical substations and medical institutions with energy-intensive equipment require lower values of the resistance of ground electrodes than the components of the electrical network in the form of power lines and simple residential at home. Their installation and safe use is regulated by the PUE and numerous industry standards, and the norms are indicated in the accompanying documentation for the installed devices.
In all climatic zones, the same natural phenomena affect the soil in different ways, which is reflected in the special coefficients of freezing, moisture and seasonality. When the soil gets wet, its resistivity decreases several times, and when it freezes, it increases. The moisture coefficient has a significant effect on the specific electrical resistivity of the soil. It is used to correct measurements in the places of the planned grounding device. in some cases:
-
The soil is oversaturated with moisture - a lot of precipitation has fallen. The measured indicator corresponds to the minimum possible. - The soil has an average moisture content - precipitation was not numerous. Measurements are also average.
- The soil is dry - little precipitation. The result of soil resistance measurements is maximum.
The growth in the size of grounding devices reduces the dependence of the structure on climatic phenomena.
This is due to the fact that the current spreads to a depth corresponding to the horizontal dimensions of the ground electrode, and the main impact falls on the inner layers of the soil, which have a deliberately low specific resistance.
Ways to get the required parameters
Earthing switches of traditional design consist of a set of vertical and horizontal electrodes and are mounted in problem-free, "good" soils. Vertical electrodes have many advantages, because with increasing depth:
- soil characteristics are more stable;
- seasonal fluctuations make themselves felt less;
- the moisture content rises and also reduces the resistance.
Horizontal electrodes are used for connection needs, but they can also be used as independent elements, when it is impossible to normally mount vertical earthing switches or a device of a certain constructions. In critical conditions of permafrost or heavy soils, the installation of classical grounding is ineffective. The specific situation of the area will require gigantic grounding devices, and as a result of the ejection phenomenon, the electrodes will last in the soil for no more than a year.
To solve these problems specialists have developed a number of techniques:
-
The required volumes of “bad” soils are removed and replaced with “good” ones: coal or clay. In the case of permafrost, the effect of this will be short-term, since the substitute soil also hardens sooner or later. - In areas with low soil resistivity, remote grounding installations are mounted at a distance of up to 2 km from the main source.
- Chemical compounds are used - salts and electrolytes. Sodium chloride (common table salt), calcium chloride, copper sulfate (copper sulfate) reduce resistance of freezing soil, but require renewal after a short time (from 2 to 4 years), because. are susceptible to washout.
The best solution to the problem is to create an electrolytic grounding complex. It favorably combines chemical soil tillage and soil replacement. For this, electrolytic electrodes are used, which are filled with a prepared mixture of mineral salts and are evenly distributed over the working space. The process of leaching of reagents becomes more stable due to the use of a special near-electrode filler, which increases the area of contact with the soil. This makes it possible to solve the problems of installing traditional ground electrodes, significantly reduces the size and quantity of equipment, and reduces the volume of general construction work.
Application in practice
The level of electrical conductivity of the earth is a variable value. Its value is influenced by various factors, among which the main ones are humidity, temperature, structure and air permeability. When installing a grounding device, reliable information about the places of construction work is required. To ensure that the resistance of the ground electrode does not exceed the permissible norm, it is necessary to accurately designate the limits within which it can change.
All data for design needs are obtained using geological surveys and measurements at a specific object. The results obtained are subject to correction taking into account the season, because the normalized values must be ensured under the most critical conditions. And only if it turns out that there is no possibility of binding to the terrain for various reasons, they use reference tables, while the calculation will always be indicative.
