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Berlin Environmental Atlas

08.05 Electromagnetic fields

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The overhead traction current and the reverse current in the tracks can be determined by simultaneous long-term measurements of magnetic flux density at varying distances (e.g. 5, 10 and 20 m) from the track. For this purpose, the currents - as input parameters in a calculated simulation - are varied until the field strength profile of the magnetic field matches the measurements taken. It is essential to have exact knowledge of the route configuration. The results of the simulation will then have local validity. However, they cannot simply be extrapolated for longer segments of the route, as the magnetic fields caused by railway tracks are dependent on a multitude of parameters. The proportion of reverse current, and thereby compensation of the magnetic field, decreases, for example, as the distance to the next substation increases. To study a length of railway, therefore, we need several profiles composed from long-term measurements. The greater the density of profiles, the more we can deduce from the simulated traction currents.

This technique was used at Savignyplatz in the Berlin borough of Charlottenburg with a total of 15 longitudinal measurements. A typical long-term measurement of magnetic flux density at a frequency of 16 2/3 Hz is shown in Fig. 8.

Fig. 8: Magnetic Flux Density at 11.2 m Distance from the Railtrack in Schlüterstrasse, Berlin-Charlottenburg
Due to the limited storage capacity of the measuring equipment, the duration was restricted to 21 hours, accounting for the gap between 10:11 a.m. and 12:52 p.m.

Typically for railway installations, the mean value for the full measurement period is several magnitudes smaller than the peak values. Each of these peak values was caused by one or more movements of trains between the stations Zoologischer Garten and Savignyplatz, in most cases by trains leaving Zoologischer Garten. The magnetic field is not emitted from the train, but is generated in a circle around the system of catenary and track. As the overhead lines were being fed from Wannsee at the time of measurement, the field only persisted at this site while a train was drawing energy between the point of measurement and Zoologischer Garten. This never lasted longer than five minutes (Plotzke et al. 1995). As the train passed, the field strength dropped suddenly to almost zero. The residual field (base level in Fig. 8) was caused by trains standing at Zoologischer Garten which were drawing energy from the overhead supply for control technology, air conditioning etc.

The individual long-term measurements were used to draw up a profile of maximum field exposure for an ICE train travelling from Zoologischer Garten to Charlottenburg (see Fig. 9, the maximum value of 1.99 µT was measured in a restaurant directly under the viaduct). The fall in magnetic flux density with distance is clearly recognisable. In addition, a numerical calculation of magnetic flux density has been included; its maximum value is based on a simulated overhead current of 226.2 A and a reverse current component through the track of 68 %.

Fig. 9: Peak Magnetic Flux Densities by the Railroad Track - Determined by Simulation
The measured values are marked as dots. The curve on the left represents the magnetic flux density when a train passes on the southern track, the curve on the right corresponds to magnetic flux density on the northern track.

In order to calculate the magnetic flux density on the total track in Charlottenburg, each track was simulated by a three-conductor system (2 tracks, 1 catenary; transversal overhead conductor). Operating current was assumed to be 226 A, the figure yielded for the overhead lines by simulation. A method based on a uniform traction and reverse current along the entire segment is obviously generalising in a manner which is not necessarily realistic.

In Charlottenburg the railroad and the S-Bahn tracks run along a viaduct about 4 m high. As reference points for the observation of magnetic flux density, heights of 1 m and 6 m above the ground were chosen. The height of 1 m is relevant for persons in the vicinity of the railway. The second height of 6 m (or 2 m above the track itself) was chosen to assess the exposure of passengers on the platform or in the trains. For the latter group, the measurements are only of limited value as they ignore the influence of the train on magnetic flux density (possibly a significant reduction, e.g. in the case of the ICE (FGEU 1996)).

It must also be remembered that these are maximum values which only occur during short peaks. Generally the average magnetic flux densities are at least one magnitude lower. The peak values are of relevant interest however, as they are used to determine EMT, or electromagnetic tolerance (e.g. for evaluating screen disturbances).

The overhead conductors for the railroad service (operating voltage 15 kV) also generates an electric field. This was calculated for a height of 2 m above the track. The maximum value of 1.2 kV/m is found in the centre above the track, were passengers are completely shielded by the train's metal body. A person standing directly on the edge of the platform at Savignyplatz is exposed to a maximum field strength of 0.4 kV/m.

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