Ground (electricity)

An earth electrode, also called grounding electrode is uninsulated electrically conductive part and part of a grounding system that is buried as an electrical contact surface into the ground. Due to its nature of the earth electrode is in good contact with the earth and forms with this an electrical connection. This is the grounding of the derivation of electric currents in the ground.

According to the International Electrotechnical Dictionary (IEV number 826-13-05 ) is the grounding defined as: " conductive member into the ground or in another specific conductive medium, eg concrete or coke, which is in electrical contact with the earth is embedded. " (Note: In Germany coke has as a medium for embedding earth electrodes no meaning )

Erderarten

In electrical engineering, a distinction between natural and artificial earth electrodes earth electrodes.

Natural earth electrodes are all components that can be derived electrical currents into the ground though, but have not incorporated for this purpose into the ground. Natural earth electrodes are, for example, metallic piping and structural elements made ​​of concrete with steel reinforcement. Until the late 1980s, it was permitted to use the metallic water piping as protective earth. For this purpose, however, it required the approval of the supplier. Because of adverse effect of the current flow to replace the tubes (corrosion ), and the standard practice of metal tubes by means of plastic pipes, the pipe of the Erderwirkung on time can not be ensured. The use of water and gas pipes as earth electrode for Germany according to DIN VDE 0100-540:2012-06 section 542.2.3 and for Austria in accordance with OVE / ÖNORM E Section 16 expressly prohibits 8001-1:2010-03.

As artificial earth electrode is referred to all the facilities that were introduced for the purpose of grounding into the ground or be. These artificial rods will be divided into:

  • As a ground rod is called grounding, which are laid at greater depths due to their design in general. Earth Rods are made of round steel, pipe and other steel sections and be driven normally in greater depths. The maximum grounding depth is determined by the nature of the ground. In general, ground rod be driven up to 30 meters into the ground, in rocky or stony ground, the depth can be substantially lower. An advantage of the earth rods persistent connection to the groundwater. Ground rod should be at least 9m deep lying in the ground. Here are 0.5 m with a depth not einzuberechnen due to weather-related fluctuations of soil resistivity. However, with adequate depth there is on the surface only a small potential difference, which keeps the step voltage low. Disadvantages of earth rods that were not driven deep enough into the ground, the poor potential distribution and the resulting higher step voltages to the earth electrode.
  • Surface earth are earth electrodes that are with little depth and at least 0.5 meters, but generally up to 1 meter, installed parallel to the earth's surface. This grounding is preferably used in the areas where there are already at the surface highly conductive soil layers. Surface earth are usually made ​​of strip materials, round materials or cables and are combined to form different shapes. An advantage of Oberflächenerdern is the installation without special special tool ( hammer ). The disadvantage is that the spreading resistance is subject to wide fluctuations in Oberflächenerdern due to the changing soil moisture.

A special role is a foundation earth. They are in their roles as steel reinforcement component of the building foundation and therefore a natural earth electrodes in accordance with the DIN standards but they are regarded as artificial earth electrode.

Erderformen

The Erderform has a major impact on the potential behavior of the earth electrode. There are the following Erderformen:

  • Earthing rods are tubes or round steel bars, which are driven into the ground. The individual Erderstäbe can be put into one another and are connected to each other when turning. Ground rod are preferably used in densely populated areas, as there due to lack of space other grounding device usually can not be used.
  • Ground plate made ​​of 3 mm thick sheet steel or copper sheet metal plates. The copper sheet is perforated in the rule. The plate surface is at least 0.5. Ground plate be used as a surface earth as well as a rod. However, they are very rarely used, their use is limited to special cases, such as telecommunication equipment.
  • Earth strip made ​​of steel straps in the extended form. They are often used in the laying of cables, as the trench, which can accommodate the additional earth electrode, already exists. To improve the spreading resistance band earth be buried upright in the ground.
  • Ring earth consisting of a ring of bent steel strip. There are ring earth in different sizes. Earther also be made from steel strip on the ground and then dug around in about 1 meter distance to a building in the ground.
  • Strahlenerder be made ​​of up to 6 Banderdern which are radially moved by one point and connected to each other. The angle between the two beams is thus 60 °. By Strahlenerder you get a grounding device with relatively small propagation resistance connected to a small extent.
  • Meshed earth consist of mesh shape laid and bonded together at the ends Banderdern. The simplest form of a meshed earth is a Strahlenerder in which the beams ends are connected to a ring earth. In order to achieve a greater resistance Konstanz, multiple ground rods are often installed in addition to the meshed earth and electrically connected to the meshed earth.

Erdermaterial

Since rods will be laid in the ground and are subject to certain inherent corrosion, they are made from corrosion-resistant materials. Sufficient corrosion-resistant grounding secure a reliable ground for at least ten years. As Erdermaterialien be used:

  • Galvanized steel is suitable for embedding in concrete as well as in almost all soil types. Most rods will be made ​​of galvanized steel. The zinc coating is at least 70 microns.
  • In Stahlerdern with copper coating ( copper-plated steel) the minimum percentage of the copper deposit is 20% of the steel weight.
  • Copper is very resistant in the ground and is used as Erdermaterial for earthing in electrical power systems: There are grounding pure copper and earth electrodes with electroplated coatings of tin, zinc or lead.
  • Earthing of stainless steel are used in larger cities near subways and trams with DC drive.

When grounding electrodes of different materials, such as galvanized Stahlerdern with Kupfererdern, there will be major problems especially in the vicinity of dc traction due to corrosion phenomena at the earth electrode. The life span of hot-dip galvanized earthing electrodes can fall below five years.

The geometric dimensions of the Erdermaterials be determined in systems with a nominal voltage above 1 kV with impedance-grounded on the required current carrying capacity of the earth electrode. The cross sections depend on the material from which the earth electrode is and must be selected according to VDE 0101 Appendix A. In addition, the DIN VDE 0141 must be followed ( Earthing system for special power installations with nominal voltages above 1 kV).

The minimum cross-sections or minimum diameter of the earth electrode are as follows:

  • Hot dip galvanized steel (tape or profile including plates) 90 mm ²
  • Hot galvanized pipe 25 mm (DM)
  • Galvanized round bar 16 mm for earth (DM, 200 mm ²)
  • Galvanised round wire for surface earth 10 mm (DM, 80 mm ²)
  • Copper coated steel 50 mm ²
  • Copper strip material 50 mm ²
  • Copper cables or copper rods 35 mm ²

Source:

Exception: For ground wires on the secondary side of current and voltage transformers for protected laying a cross section of 2.5 mm ² Cu, or 4 mm ² Cu for unprotected installation.

The minimum dimensions for galvanized flat steel 30 mm * 3.5 mm or 25 mm × 4 mm. In round steel, a minimum diameter of 10 mm is required.

Earth stylus

Rods will be used for various tasks. According to the task it may be:

  • Protective earth
  • Operational earth
  • Lightning protection earth
  • Steuererder
  • Auxiliary earth

As a protective earth ground rod usually one or more may be used. In new buildings, foundation earth can be used.

The interpretation of Betriebserdern is quite extensive. Since high step voltages to the earth electrode arise due to the high ground currents especially at ground rods. Here meshed earth are frequently used, as these grounding electrodes small step potentials arise. Where meshed earth can not be used earther be used. A variant is also used are ground rod with additional Steuererdern.

Lavish rods will be used in HVDC systems and transmitters for frequencies below 3 MHz. In the former case, grounding are sometimes sunk in the sea, in the latter case, several bare metal strips are laid around this to the transmitting antenna, which is called the terrestrial network.

As lightning protection earth ring earth are often installed with a meter away from the building requiring protection into the ground.

Steuererder are earth electrodes, which are used because of their shape and arrangement mainly to control the potential. Reaching a certain spreading resistance is secondary in Steuererdern. As Steuererder earther be used in the rule that to be routed Haupterder. The Steuererder be routed to the Haupterder that the outer Steuererder be laid deeper than the inner ones. All earth electrodes are electrically connected to each other via the Haupterderschiene.

Auxiliary electrodes are usually about 1 meter long upward conical ground rod needed to ground measurement. The length of the earth electrode varies depending on the terrain. The auxiliary electrodes are inserted either as earth ground stakes into the ground or turned with a wood screw at the top of the soil. The auxiliary electrodes are brought to the measurement with a greater distance (40 m) from the Haupterder into the ground and removed again after the measurement.

Erdereinbau

All surface earth and ground plate (also called a ground rod ) are buried in the ground. For this purpose, the grown floor is senior and slurried or pulped after the installation of earth electrode. The disadvantage of this earth electrodes that the exact propagation resistance can be measured only after setting the soil.

Earth Rods are taken as ground rod or Kreuzerder with a pile hammer into the ground. The individual Erderstäbe have a length of 1.5 meters and be plugged into each other. When driving the rods to connect them independently to each other. Become a ground plate used as ground rods, they are as well as surface earth buried.

Electrical properties

The electrical properties of the ground depend on the following factors:

  • Earth resistance
  • Design of the earth electrode

The earth resistance shall be composed of the spreading resistance and the resistance of the earth electrode and the ground. Since the resistance is substantially less than the spreading resistance, it will be neglected in practical calculations. The spreading resistance is thus composed of the soil resistivity and the dimensions and arrangement of the earth electrode.

The specific earth resistance is the resistance of a cubic meter of soil in cubic shape with an edge length of one meter. The unit of measurement for the specific earth resistance is the specific earth resistance is dependent on the soil type, soil texture and moisture content of the soil.

Since the moisture content of the soil varies, the calculation can be of the grounding resistance carried out only with moderate accuracy. The calculation of the spreading resistance is therefore only a rough design in practice and it is checked by subsequent ground measurement.

The spreading resistance for grounding strip can be determined using the following formula.

For a ground rod is the formula:

Source:

If several () ground rod connected in parallel, the spreading resistance is determined by the formula:

The constant, depending on soil moisture

A meshed earth with the area may be closer ... as calculated according to the formula:

Source:

Approximate formula applies for a ring earth with a diameter:

The constant is in this case:

The spreading resistance of a Plattenerders with the edge length is determined according to the formula:

Source:

If the drive only with a surface having conductive ground in conjunction formula applies:

Improvement of the spreading resistance

To reduce the spread of resistance, there are several methods:

  • Using specific filler materials
  • Use of mineral or chemical ground electrode
  • Use of several parallel connected earth electrodes

In soils with varying soil moisture eg rocky or sandy soils, a special filling material is used to improve contact, which is used to improve contact of the earth electrode with soil. The rods will be embedded in the filling material, and then covered with soil.

Improve soil conductivity special ground electrodes are used, which are filled with a mixture of salts. The ground electrodes being provided with a plurality of holes through which the salt solution can migrate into the soil. Fillers or chemical ground electrodes are used mainly for Blitzschutzerdern.

Due to the parallel connection of multiple earth electrodes, the contact area is increased with soil, thus reducing the spread of resistance. In order to influence the grounding not mutually they are laid at intervals of two Erderlängen.

To further improve the grounding effect of so-called chain conductors are connected, eg metal sheaths of cables or ground wires for overhead lines. In larger buildings and structures ( industrial plants ) are often the grounds are connected to each other so as to achieve a meshed earthing system. By combining the individual earthing systems, the potential differences between the grounding electrodes are on the one hand and on the other reduce the grounding resistance of the entire system is reduced.

Legal regulations and other rules

  • DIN VDE 0100-540 Erection of low -voltage equipment Part 5-54: Selection and erection of electrical equipment - Earthing arrangements, protective conductors and protective bonding conductors
  • DIN VDE 0101 power installations with nominal voltages above 1 kV Part 1: General Provisions
  • DIN VDE 0141 Earthing system for special power installations with nominal voltages above 1 kV
  • DIN VDE 0151 Material and minimum of earth electrodes with respect to the corrosion
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