Railway grounding
The railway ground is a ground measure that is applied in the field of electrically operated railways. When working, it serves to protect people and equipment in the overhead contact line or conductor rail and in the pantograph area . Railway earthing is regulated for Germany in DIN EN 50122-1. The railway grounding concepts are regulated differently in the respective countries due to the different ground conditions.
Basics
If the rails are used as a power return line in electrically operated railways , a potential difference arises between the rails and the earth. This tension can be tapped completely or partially by people. The level of the rail potential depends on various factors. Depending on the longitudinal resistance of the return line, the level of the operating current, the distance between the substations and the driving operation in the adjacent lines, the rail potential can also reach impermissibly high values. Rail earthing is used to protect against this inadmissibly high rail potential. The railway grounding has a number of significant characteristics compared to the grounding of public power supplies. The railway grounding is connected to the widely branched and widely grounded rail system. In addition, operating currents in the form of track reverse currents also flow via the rail ground . Due to the significantly lower frequency of the traction current of 16.7 Hertz , the earth currents spread differently than with a network frequency of 50 Hertz.
Railway earth
The basis for the railway grounding is the railway grounding, it consists of the rails serving as traction current conductors and all of the lines , vehicles and system parts connected to them . The main element here is formed by the running rails. To increase the conductance, these must be connected to one another with good conductivity in the longitudinal and transverse directions. To connect the rails of a track, strap connections are suitable fasteners. The rail joints must be bridged lengthways. In the case of adjacent tracks with a track spacing of 30 meters and less, the respective tracks must be connected to one another using track connectors. In addition, to improve the return line conditions, separate lines are installed on the masts of the overhead line as return line cables and connected to the railroad earth. These earth ropes must have a sufficiently large cross-section. As a rule, earth ropes with a cross section of 95 mm 2 are sufficient. In the case of rocky or poorly conductive ground and in the area of direct current railways, the cross-section must be larger. Several earth ropes laid in parallel are then used here. The purpose of using earth ropes is to lead the return current component mainly through the earth rope. To do this, the earth ropes must have a significantly lower resistance than the running rails and the earth. In addition to the return current, the earth ropes also serve as protective earthing and to reduce the potential difference between the rails and the ground. The earthing ropes are connected to the catenary masts if possible. In order to enable the greatest possible current over the earth cables, cross connections must be made at intervals of 250 to 300 meters. These cross connections connect the running rails to the catenary masts in an electrically conductive manner. The cross connections also serve as equipotential bonding. To protect against damage, the cross connections are buried at least 25 centimeters deep in the gravel . The running rails and the electrically conductive parts connected to them are specifically earthed to the ground.
Building earthing
Building earthing is used as earthing in tunnels or other engineering structures. All electrically conductive metal parts such as B. Reinforcements , metal structures of tunnels or retaining walls and other buildings in the area of the railway line are connected to one another in an electrically conductive manner. This building grounding forms a system that is initially metallically separated from the railway ground and the ground of the public network. The building earth can either remain separate from the railway earth or be connected to it in an electrically conductive manner. If the railway earth and building earth are laid separately, a clear separation between the two earthing systems must be entered and maintained in the plans. This separation must then also be ensured over the life of the structure. It is up to the respective planners whether the respective earthing systems are laid separately or connected. In the case of direct-current railways, a consistent separation between the protective earth of the low-voltage network and the building earth and especially the rail earth must be observed. In the case of separate installation, personal protection must be ensured by other measures such as B. isolated sidewalks can be achieved. In the event of an impermissibly high potential difference, automatic earthing short- circuiters are used for the temporary connection of the earthing systems , which conductively connect the building earth to the rail earth if necessary.
Rail ground problems
Carryover of potential
Due to the railway grounding, potential transfer can occur under certain conditions. This can lead to reverse currents being introduced into the public utility network. This can affect the electrical systems in the VNB network. At the frequency of 16.7 Hz, the current displacement effects are less pronounced than at 50 Hz. As a result, the currents can penetrate further into the ground. As a result, the earth return flow is less closely tied to the line route. The potential of the railway earth is carried over in particular when objects lying outside the railway line are conductively connected to the railway earth. As a result, the potential funnel , which arises on the right and left parallel to the railway line, is moved outwards from the railway line. This leads to a scattering of train frequencies in the public network. But even in densely built-up areas there is a coupling of the traction current and the mains current due to the low grounding impedances . In the area of train stations, the low voltage for the electrical systems is usually obtained from the public network; This can lead to an unwanted, but also to an intentional connection of the two earthing systems. In order to achieve equipotential bonding between the two earths, the rail earth is then connected to the protective earth. This can lead to disruptive 16.7 Hertz currents being coupled into the low-voltage network.
Stray currents
With direct current railways stray currents can occur in the ground. As a result, pipelines or other metallic components that are laid in the ground are destroyed by corrosion. There is also the risk of cables being thermally overloaded.
But AC railways can also suffer from impairments. In the case of cathodically protected pipelines that are laid in the ground in the immediate vicinity, parallel to the railway line, inductive coupling occurs. The risk of corrosion is greatest when the alternating current density exceeds the critical value of 30 amperes per square meter and the defect is around 1 square centimeter. This can even lead to pitting at the imperfections in the pipelines.
Remedies
There are different approaches to avoiding the dragging of the railway potential. If possible, the rail earth is not to be connected to the network earth. In the area of train stations, the mains supply should be carried out via separate transformers , so that the railway earth is safely separated from the network earth. If possible, no longer electrically conductive objects, such as B. pipelines, crash barriers or the like, are laid along the railway line. Fences or walls that are located along the railway line are not to be connected to the railway ground. If possible, protection against accidental contact should be preferred to protective earth. In order to effectively separate the return current of the direct-current railways from the ground, the rails of the direct-current railways must be laid so that they are insulated from the ground, especially in areas that are likely to be influenced by stray currents. If possible, a close-meshed equipotential bonding should be created to avoid potential differences. Pipelines that are laid along the railway line should be provided with an insulating layer to protect against pitting. Full protection pipes with optimum corrosion protection and cement mortar coating are particularly suitable here. These pipes have no connection to the railway or network earth, but are nonetheless connected to one another with good electrical conductivity. This means that there can be no potential differences.
Individual evidence
- ↑ a b Lothar Fendrich (Ed.): Manual Railway Infrastructure. Springer Verlag, Berlin Heidelberg 2007, ISBN 3-540-29581-X
- ↑ Klaus Kruse: The railway earthing of the overhead line serves to protect the emergency services. In: Eisenbahn-Unfallkasse (EUK) (Ed.): Bahn Praxis E , 1/2006, Bahn Fachverlag GmbH, 55013 Mainz, Printing and Design Master Printing, pp. 3–5
- ↑ a b Christian Budde: Revision of EN 50122: Railway applications - Fixed installations - Electrical safety, earthing and return current conduction . In Eisenbahn-Unfallkasse (EUK) (Ed.): Bahn Praxis E , 2/2011, Bahn Fachverlag GmbH, 55013 Mainz, Printing and Design Master Printing, p. 3
- ^ Christian Budde: Comparison of the rail grounding concepts of different 16.7 Hz railways . In Eisenbahn-Unfallkasse (EUK) (ed.): Bahn Praxis Spezial E1, 2007, Bahn Fachverlag GmbH, 55013 Mainz, Druck und Gestaltung Meister Druck, pp. 3–5 online (accessed on July 22, 2011; PDF; 495 kB )
- ↑ a b c Christoph Rützel: Railway earthing and return current conduction . In Eisenbahn-Unfallkasse (EUK) (ed.): Bahn Praxis Spezial, 11th 2007, Bahn Fachverlag GmbH, 55013 Mainz, Printing and Design Master Printing, pp. 125–128
- ↑ a b Reinhold Bräunlich, Günther Storf, Max Sigg: Earthing measurements in substations of the Swiss Federal Railways Online ( Memento of the original from April 3, 2015 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (accessed on July 21, 2011; PDF; 482 kB)
- ↑ a b c d e f Swiss Association of Road and Transport Experts : Earthing Manual. Railway technology regulations, Bern 2008
- ↑ a b Ulrich Bette, Markus Büchler: Pocket book for cathodic corrosion protection. 8th edition, Vulkan-Verlag GmbH, Essen 2010, ISBN 978-3-8027-2556-2
- ↑ Wv Baeckmann, W. Schwenk: Handbook of cathodic corrosion protection. 4th completely revised edition, WILEY-VCH GmbH, Weinheim 1999, ISBN 3-527-29586-0
- ↑ Rene Mathys: Merger of Bahn- und Netzerdung Online ( Memento of December 12, 2011 in the Internet Archive ) (accessed on July 22, 2011; PDF; 519 kB)
