Artificial grounding. Scheme, description

ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Artificial grounding. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Grounding and grounding

More often artificial grounding is a steel conductor laid horizontally or vertically (obliquely) in the ground, or a group of such conductors interconnected. In the latter case, the ground electrode is called complex, and if the electrodes form a circuit, then such a complex earth electrode is called a ground circuit.

The name "horizontal" and "vertical" ground electrodes is rather conditional. Strict adherence to horizontality in the first case is not necessary, it is important that the electrodes are in the ground at the right depth, without being damaged during the operation of the machines.

Since the surface of the earth in ravines, on slopes and in a number of other places may not be horizontal, then extended (beam) ground electrodes will follow the curvature of the surface. For vertical electrodes, strict adherence to verticality is also not necessary.

Horizontal earthing switches they are laid at a depth of 0,5 m, on arable land - at least 1 m. They are rational in cases where the electrical conductivity of the upper soil layer provides the desired conductivity. The installation of such ground electrodes is mechanized and performed with minimal manual labor, however, the upper soil layers often have a higher electrical resistance than the deep ones. In addition, close to the earth's surface, the current does not spread uniformly in all directions, as it does at depth. Therefore, the resistance of horizontal electrodes is usually greater than that of vertical electrodes of the same mass. Therefore, it is vertical electrodes that are most widely used as grounding conductors.

Deep vertical electrodes the most economical, they reach well-conductive soil layers.

Grounding electrodes mounted in the ground, jumpers between them and leads from ground electrodes to the surface must have the following minimum dimensions:

round steel - diameter not less than 10 mm;
round galvanized steel - diameter not less than 6 mm;
corner steel - shelf thickness not less than 4 mm;
total cross section for lightning protection grounding (lightning protection) - not less than 160 mm2;
strip steel - thickness not less than 4 mm with a cross section not less than 48 mm2 (for grounding lines - not less than 100 mm, for lightning protection - not less than 160 mm);
rejected pipes - wall thickness of at least 3,5 mm.

The minimum dimensions of the electrodes are used mainly for temporary electrical installations, where corrosion conditions are not critical. For permanent installations, the cross-section of ground electrodes is selected with a margin for corrosion damage. In terms of resistance to corrosion, round steel is preferable, since corrosion of the electrode by rust is proportional to the surface area of the electrode in contact with the ground, and the area of the electrode of circular cross section is the smallest of all profiles.

In order to ensure reliable operation of the ground electrode for 40-50 years in favorable soil conditions, it is sufficient to increase the diameter of the rod electrode against the minimum by only 2-3 mm; in wet soils, it is necessary to double the diameter of the ground electrode.

From a grounded element of an electrical installation, for example, from an overhead power line support, horizontal beams are laid in two opposite directions or, if there are not 2, but 3-4 beams, they are spaced at an angle in terms of 120 ° or 90 °. This is necessary for the efficient use of the metal being laid, since the adjacent ground electrodes are mutually shielded and their efficiency is reduced many times over. For the same reason, vertical ground electrodes must be removed from each other as far as possible, equal to at least the length of the electrode. For example, if ten vertical electrodes 5 m long are placed in one line at a distance of 5 m from each other, then the coefficient of their use will be 0,47, and if the same electrodes are placed in a closed triangle or quadrangle to save space, then their coefficient usage will be even lower. The same applies to the use of inclined electrodes, which are spaced at equal angles similar to horizontal and immersed in the ground at an angle of about 45 ° for best use.

The uneven distribution of potentials on the earth's surface above the ground electrode and around it creates dangerous step and touch voltages. To equalize potentials in such cases, the grounding conductor can be made in the form of a grid of horizontal elements laid in the ground along and across the territory of the electrical installation and connected by welding at intersections. The cell size of such a grid is usually from 6x6 to 10x10 m.

Around the overhead line support, the potentials can be equalized by a ground electrode made in the form of concentric rings embedded in the ground and connected to the support.

Reduces step and touch voltages to acceptable values over the entire area it occupies, the mesh ground electrode system, however, outside the mesh, the danger may persist. Therefore, in dangerous places, for example, at the approaches to the territory of substations or around the foundations of overhead lines, additional ground electrodes are laid at a gradually increasing depth and connected to the main ground electrodes.

The area allotted for the ground electrode and the consumption of metal can be reduced by a protective insulating fence built around the ground electrode. The simplest fence made of dielectric material prevents the current from spreading over the earth's surface and reduces the step voltage compared to the voltage on the ground electrode by at least 100 times and equalizes the potential outside the ground electrode.

The vertical part of the fence from the surface level is located at 0,4-0,6 m from the depth of the top of the earth electrode.

Flanging of the fence performed at an angle of 90-95° to the vertical and has a length of

(S - ground electrode area). Any inexpensive dielectric material that has sufficient mechanical strength and has an electrical strength of at least 1 MV/m can be used for fencing (bitumen-based insulating materials, for example, brizol, produced from production waste and having a strength of at least 20 MV/m).

When current drains from a grounding conductor, for example, from a grounding grid, an electric field is formed around it. An electric potential arises on the surface of the earth, and the step voltage can reach dangerous values directly outside the ground electrode, even when using known methods of potential equalization. Therefore, the geometrical parameters of the fence are established as a result of the analysis of the electric field formed by the ground electrode together with the dielectric leveling fence, and meet the safety requirements. The device can be used for grounding conductors of any design and for any soil structures.

Often earthing switches made of profile steel do not meet the requirements for grounding devices. For example, in arid places it is difficult to achieve stable conductivity of such ground electrodes, in rocky soils they are difficult to mount, and in aggressive soils it is difficult to provide corrosion protection and a long service life. For such situations, designs of special ground electrode systems have been developed.

For arid regions, the ground electrode can be made, for example, in the form of a reinforced concrete tank, installed below the ground and filled with water through a removable hatch. The grounding conductor is supplied with a water distribution system in the form of segments of metal pipes with holes for water drainage, located evenly along the entire length of the pipes. The pipes are covered with a layer of moisture-absorbing material (concrete, cement). The rate of moisture filtration through concrete into the ground is set by selecting the brand of concrete, which makes it possible to avoid frequent dampening adjustments and reduce labor costs associated with the need for regular moistening. The output from the reinforced concrete tank to the grounded equipment, for example, to the neutral of the transformer, is connected to the steel reinforcement bars of the reinforced concrete.

Let's pay attention to the design of the ground electrode, proposed abroad. The purpose of this development is to reduce metal consumption and facilitate driving into the ground. The earthing switch has a thin-walled (1-2 mm) metal tube, into which a semi-rigid rod made of plastic material is pressed, having a rigidity sufficient to be a support for an elastic thin-walled tube. This quality provides the possibility of some bending of the electrode to bypass the obstacles encountered when driving it into the ground. To increase the service life, i.e. to reduce corrosion, stainless steel is offered as a material for the tube. The tip at the lower end of the electrode is only needed for driving, so there is no need to make it from an anti-corrosion material. The shape of the tip can be sharp or rounded for better sliding off obstacles encountered in the ground. Instead of making a tip, you can crimp the end of the tube with filler.

A typical tube diameter is 15 mm. The preliminary diameter of the core, which is pressed into the tube, must be slightly larger than the inner diameter of the tube. The tube may be filled (optionally) with an internally hardening fluid material such as epoxy, polyurethane or elastomer. Semi-rigid filler is located inside the steel tube along the entire length. Stiffer materials and thicker tube walls reduce the rod's flexibility and reduce the electrode's ability to bypass obstacles in the ground, leading to breakage. On the other hand, overly ductile materials do not provide sufficient wall strength for driving to a sufficient depth (about 2,3 m). To drive the electrode, a removable anvil is provided, which has a shoulder resting against the end of the tube, and a ledge that mates with the inner diameter of the tube and the core.

Author: Bannikov E.A.

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