Lightning protection earthing

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The earthing of a lightning protection system is called lightning protection earthing . The lightning protection earthing has the task of safely diverting the lightning current flowing from the safety gear into the earth .

Basics

A weft forming in the lightning protection system Lightning causes electric current having an average peak value of 20 kilo amperes . An earth resistance of 10 ohms creates an overvoltage of 200 kilovolts in the lightning rod for a period of 10 to 50 microseconds . The extremely high values ​​can lead to breakdowns due to foreign approaches in the electrical installation. So that the high lightning currents can be safely discharged into the ground, the earthing must be implemented accordingly. The high current density of the lightning current generates a high electrical field strength, which causes electrical discharges in the ground, which lower the ground resistance in the area around the ground electrode.

Earth design

In order to safely divert lightning currents into the ground, the earthing system must have a low impedance . While the inductivity of the grounding electrodes can be neglected when designing a grounding system for mains frequency , this inductance must be taken into account when providing lightning protection. Although the value of the inductance with one microhenry per meter of earth electrode length is relatively small, the inductance with lightning currents has a strong influence on the effective earth electrode length due to the rapid increase in the lightning current and leads to a high inductive voltage drop. The part of the earth electrode that is remote from the earth connection only has a limited effect on lightning current discharge. In the case of lightning currents, the earth resistance increases compared to the values measured using the earth measurement . The maximum value of the lightning current and the level of the specific earthing resistance determine the level of the shock propagation resistance. From the so-called critical length , a further lengthening of the earth electrode no longer affects the earth resistance.

Usable earth electrodes

Foundation earth electrodes and other metal parts embedded in the ground, such as the reinforcement of reinforced concrete foundations, can be used as earth electrodes for lightning protection earthing . Sheet piling and steel parts of steel frame structures can also be used. In contrast, active pipelines may no longer be used. Alternatively, a closed ring earth electrode can be provided. This is laid in the ground around the building to be protected at a distance of one meter at a depth of approx. 50 centimeters. Individual earth electrodes with a minimum length of 20 meters and rod earth electrodes with a minimum length of nine meters can also be considered. Since the shock resistance of the earth electrode is no longer significantly reduced from an earth electrode length of 30 meters, deep earth electrodes should no longer be implemented. The earthing resistance can be reduced by connecting several shorter individual earths in parallel . So that the earth electrodes do not influence each other in their effectiveness, the distance between the individual earth electrodes should be at least as large as the effective earth length. If the individual earth electrodes are closer to one another, the actual total resistance to expansion of the earth electrodes is greater than the calculated value.

Execution of the earthing system

Particular attention must be paid to the shape and dimensions of the earthing system. In order for the earthing system to remain functional for a long time, the earth electrodes must be protected in such a way that they withstand the corrosive effects of the earth. If possible, the connections to the earth electrode must be made via detachable separation points so that subsequent earthing measurements can be carried out. If there is a connection between the potential equalization of the building and the lightning protection earth, a specific earth resistance is not required. If there is no connection, the earthing resistance in ohms must not be greater than five times the minimum distance in meters between the above-ground parts of the lightning protection system and other conductive parts of the building. Special lightning protection equipotential bonding must be used as equipotential bonding. The decisive factor for the function of the earthing system is not the level of the expansion resistance of the earthing system, but the consequent equipotential bonding. This means that the lightning current can be safely distributed in the ground. At least 80 percent of the earth electrode must be sensitive to the earth, i.e. have contact with the ground. In the case of foundation earthers, if the foundation is designed in a certain way, this value may not be achieved. Foundations in cohesive soil or in the groundwater are usually designed as white or black tanks . Sealing membranes as well as base and perimeter insulation in the basement area have an insulating effect and reduce the sensitivity to earth. In this case, additional earth electrodes must be provided outside the insulation and sealing layers.

Meshed earthing systems

While it is sufficient in residential buildings to earth them individually, in buildings of power plants and industrial plants it is often necessary to connect the earths to one another in an electrically conductive manner. Conductive connections from neighboring buildings significantly reduce the potential difference between the buildings. As a result, electrical and electronic connection lines that run between the buildings experience a significantly lower voltage load.

The earthing system usually consists of a ring earth that is laid around the respective building. In addition, a mesh-shaped earthing network will be laid between the buildings. The earthing meshes are connected to the respective ring earthing at a distance of ten meters. The earthing meshes are 20 to 30 meters long and are reconnected as directly as possible to the ring earthing of the neighboring building. In the case of larger distances between the buildings, the individual grounding meshes are connected to one another at special nodes. In power plants, the mesh earthing network is created from copper ropes which, depending on the nature of the soil, can be coated with tin, zinc or lead. The intermeshing of the earthing system increases the conductivity of the earthing system for the lightning current, which can enter the earth over a larger area. The lightning current is routed around the respective building via several connections.

High voltage overhead lines

High-voltage overhead lines with two earth wires each in the upper mast area

Due to their height, high-voltage overhead lines are particularly at risk from lightning. On overhead lines, lightning can strike either directly into one of the high-voltage pylons or into one of the outer conductors . In order to protect overhead lines as much as possible from lightning strikes, two types of lightning protection earthing are used. In order to safely divert lightning strikes to earth, the masts are earthed at the base points with the lowest possible propagation resistance. The resistance to propagation depends on the nature of the soil and on non-linear processes in the soil. The mast geometry determines the shock resistance of the mast. Depending on the nature of the soil, either deep earth electrodes or radiation earth electrodes are used. If the soil moisture increases with increasing depth, then deep earth electrodes are about 1.5 times as effective as radiation earth electrodes. However, the maximum effective length of the earth rod must not be exceeded. Radiant earth electrodes are used for other floors. To protect the external conductors, an earth wire is stretched from top to top of the mast at the top of the overhead line . The effectiveness of the earth ropes depends on the height at which the earth ropes hang above the ground. With an appropriate construction and design of the earth wire, lightning strikes are diverted via the earth wire before they can reach an outer conductor.

Individual evidence

  1. ↑ a b c Johann Pröpster: Lightning protection systems. Online (accessed July 7, 2011; PDF; 206 kB)
  2. ↑ 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
  3. ^ Paul Waldner: Fundamentals of electrotechnical and electronic building equipment. Werner-Verlag 1998, ISBN 3-8041-3983-3 .
  4. ↑ Reya Venhuizen: Earthing with a system. Deutsches Kupferinstitut Online ( Memento from January 15, 2015 in the Internet Archive ) (English).
  5. ↑ a b DEHN + Söhne GmbH + Co.KG .: Blitzplaner. 2nd updated edition, Neumarkt 2007, ISBN 978-3-00-021115-7
  6. ^ A b 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 .
  7. ↑ Franz Pigler: EMC and lightning protection process control plants. Siemens Aktiengesellschaft, Publicis Corporate Publishing, 2001, ISBN 3-8009-1565-0 .
  8. ^ Friedrich Kießling, Peter Nefzger, Ulf Kaintzyk: Overhead lines. 5th edition. Springer-Verlag, Berlin / Heidelberg / New York 2001, ISBN 3-540-42255-2 .

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