Earth resistance

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The earth resistance R E is the electrical resistance between the connection point of an earth electrode and the reference earth . The earthing resistance is an important parameter of an earthing measure and should generally be as small as possible.

Composition of earth resistance

The earth resistance is influenced by several variables. It is primarily determined by the specific earth resistance of the soil in the immediate vicinity of the earth electrode and the geometry of the earth electrode. The earth resistance is often also called the propagation resistance. The propagation resistance, also known as earth propagation resistance, basically comprises the resistance (effective resistance of the earth) between the earth electrode and the reference earth. The earth resistance primarily includes the expansion resistance, unless the expansion resistance already includes the resistance of the metal electrode of the earth electrode, as well as any earth conductors connected to the earth electrode or the earthing system. The electrical resistance between the equipotential bonding rail and the reference earth is called the total earth resistance .

Ground impedance

In AC systems, one speaks of earth impedance Z E , this is the alternating current resistance between earth earth and reference earth. In the case of the ground impedance, in addition to the resistance and transverse conductance, the inductance and the capacitance are also taken into account. It results from the parallel connection of the propagation resistances of the connected earth electrodes that are not in the mutual influence area. These include the impedances of the connected earth wires of overhead lines and the metal sheaths of cables.

Specific earth resistance

The specific earth resistance, also known as the specific soil resistance ρ E , is decisive for the level of the earth resistance R E or the expansion resistance R A (also known as the earth expansion resistance). This specific earth resistance corresponds to the resistance of a cube with an edge length of one meter. The prerequisite for the determination of the resistance is that the cube is flowed through from one edge surface to the other edge surface. The level of the specific earth resistance depends on several factors. In addition to the type of soil , the grain size and density of the soil also influence the specific earth resistance. In addition, differences in moisture content affect the level of specific earth resistance. This moisture content is subject to strong seasonal fluctuations. The seasonal fluctuations in the specific earth resistance essentially depend on the depth of the soil. These fluctuations are approximately sinusoidal. The temperature of the ground also has an effect on the level of the specific earth resistance; frozen ground has almost the effect of an insulator .

Specific earth resistance ρ in Ωm
Soil condition Range of values Average
swampy ground 2-50 30th
Brick tone 2-200 40
Alluvial sand, humus
loam sand soil		 20-260 100
Sand and sandy bottom 50-3000 200 (wet)
peat > 100 200
Gravel (wet) 50-3000 1000 (wet)
stony and
rocky soil		 100-8000 2000
Concrete: cement / sand
mixing ratio 1: 5		 50-300 150
Concrete: cement / gravel
mixing ratio 1: 5		 100-8000 400
Concrete: cement / gravel
mixing ratio 1: 7		 50-300

For comparison: The specific resistance ρ of good electrical conductors such as metals is in the range below 10 −6  Ωm.

Source:

Current distribution and potential curve

Two metallic hemispherical earth electrodes as a model and the determination of the earth resistance

The current distribution and the potential curve of the earth electrode and thus also the earth resistance depend on the dimensions and the arrangement of the earth electrode. The larger the surface of the earth electrode, the larger the contact area with the ground. A larger contact area with the ground also means a larger current outlet area. This reduces the earthing resistance. The shape and size of the earth's surface essentially determines the current distribution around the earth near the earth. This influence decreases with increasing distance from the earth electrode.

With a hemispherical earth electrode, the current spreads radially symmetrically in the ground starting from the center of the sphere. However, this is only possible if the soil is homogeneous. The area that is available to the current when it emerges from the earth electrode is initially relatively small, but it increases as the distance from the earth electrode increases. In the sketch opposite, two metallic hemispheres with the radius r are buried in the ground at a distance d from each other. It is assumed that the distance d is significantly larger than the spherical radius r . In this case the resistance R k to be measured at the clamps K is independent of the distance d , this is due to the large cross-section of the ground. Depending on the radius of the earth electrode and the ground resistance ρ in the vicinity of the earth electrode, the resistance R k is determined according to the following equation:

This gives the earthing resistance R of an earth electrode:

In practice, however, hemispherical earth electrodes are not used.

Potential distribution

The potential distribution on the earth's surface depends on the design of the earth electrode. Deep earths have a less favorable potential distribution on the earth's surface than surface earths. The current fed into the ground via the earth electrode creates a voltage funnel around the earth electrode. There are concentric equipotential lines around a hemispherical earth electrode. In the case of earth electrodes used in practice, differently shaped equipotential lines and thus differently shaped voltage funnels result. The potential distribution has a great influence on the step voltage .

Shock earth resistance

In the case of high-frequency processes, such as B. with lightning currents , the earthing resistance can no longer be expected. Here, due to the changed parameters, the impact earthing resistance is used. The surge earth resistance (surge impedance) has a higher value than the earth resistance due to eddy currents and radiation effects . This is because the earth resistance is measured at a frequency of 50 Hertz . For the practical earthing calculation, it is usually sufficient to consider the earthing resistance determined at 50 Hertz.

Resistance to earth in practice

In the case of practical grounding electrodes, there are additional resistance components such as those of the connection line and the connecting elements (terminals). These resistances are mostly negligible as long as the connecting elements are well fixed, since the components are usually made of highly conductive metals. With other geometries of the earth electrode that deviate significantly from the spherical shape, such as tape earth electrodes, there are other relationships between the shape of the earth electrode and resistance, which are often determined not analytically but by practical measurements.

Measurement of earth resistance

Arrangement for the practical measurement of the earth resistance

For the measurement of earth resistance, there are several different methods. Important aids are ground spikes as auxiliary earth electrodes and probes as well as current clamps for feeding and measuring the earth currents. The measurement with an alternating current as test current is actually always a measurement of impedances. In the case of very low measuring frequencies, however, this is usually largely (very close) to the ohmic value.

Practical use

Earthing systems with a spatial extension of several kilometers in diameter can be designed with a particularly low earthing resistance in the range around and just under 1 Ω. These are used in monopolar high-voltage direct current transmissions , in which the operating earth carries currents of up to a few kA in normal operation. An example of such a facility is the Pacific DC Intertie in the USA.

Neutral point treatment

The term earthing resistance as a component (technical component) is also used to denote neutral point earthing resistances , which in electrical power engineering such as medium-voltage systems are connected between the neutral point of a power transformer or generator and the earthing system in order to limit the earth-fault current in the event of a fault .

Norms

  • DIN VDE 0100-540 (VDE 0100-540: 2012-06) Construction of low-voltage systems - Part 5-54: Selection and installation of electrical equipment - Earthing systems and protective conductors
  • EN 50522: 2010-11 (VDE 0101-2: 2011-11) Earthing of power systems with nominal alternating voltages above 1 kV

Web links

Wiktionary: Earthing resistance  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. ^ A b Herbert Schmolke: Equipotential bonding, foundation earth electrodes, risk of corrosion . 7th completely revised edition, VDE Verlag GmbH, Berlin Offenbach 2009, ISBN 978-3-8007-3139-8 , p. 19.
  2. ↑ a b c d Wilfried Knies, Klaus Schierack: Electrical systems technology; Power plants, networks, switchgear, protective devices. 5th edition. Hanser Fachbuchverlag, 2006, ISBN 3-446-40574-7 .
  3. ↑ a b c Hennig Gremmel, Gerald Kopatsch: switchgear book (ABB). 11th edition. Cornelsen, Berlin 2008, ISBN 978-3-589-24102-6 .
  4. ↑ a b Valentin Crastan : Electrical energy supply 1. Springer Verlag, Berlin / Heidelberg 2007, ISBN 978-3-540-69439-7 .
  5. ↑ a b c d Gerhard Kiefer: VDE 0100 and the practice. 1st edition. VDE-Verlag, Berlin / Offenbach, 1984, ISBN 3-8007-1359-4 , pp. 50-52, 148-158.
  6. ↑ a b c Enno Hering: Measurements and tests on earthing systems. German Copper Institute. ( online , accessed on July 18, 2011; PDF; 364 kB).
  7. ↑ a b c d e f Anton Gabbauer: A contribution to the computational determination of grounding impedances, grounding currents and grounding voltages of electrical systems in networks with low-ohmic neutral point grounding. Thesis. ( online , accessed on July 18, 2011; PDF; 1.7 MB).
  8. ↑ a b c Johann Frei: Measurement of the impedance of extensive earth systems and their calculation. Thesis. Graz University of Technology. ( online , accessed on July 18, 2011; PDF; 2.9 MB).
  9. ^ A b Friedhelm Noack: Introduction to electrical energy technology . Carl Hanser Verlag, Munich / Vienna 2003, ISBN 3-446-21527-1 .
  10. ↑ Wilhelm cabinet: Protection against touch voltages. Third revised edition, Springer Verlag, Berlin / Heidelberg 1958, pp. 56–62.
  11. ↑ Oskar Lobl: Earthing, zeroing and protective circuit along with explanations of the earthing guidelines . Published by Julius Springer, Berlin 1933, pp. 27-29.
  12. ↑ Hans-Günter Boy, Uwe Dunkhase: The master's examination in electrical installation technology. 12th edition. Vogel Buchverlag, Oldenburg / Würzburg 2007, ISBN 978-3-8343-3079-6 .
  13. ^ Klaus Heuck, Klaus-Dieter Dettmann, Detlef Schulz: Electrical energy supply. 7th edition. Friedrich Vieweg & Sohn Verlag, Wiesbaden 2007, ISBN 978-3-8348-0217-0 .
  14. ↑ GINO GmbH: neutral point earthing resistors . Online (accessed February 20, 2017; PDF; 1.3 MB).