Railway stray currents

Corrosion due to stray current—the metal is attacked at the point where the current leaves. Typically, this kind of damage can be observed in buried stmctures in the vicinity of cathodic protection systems or the DC stray current can stem from railway traction sources. 

The Berlin City electrical engineer M. Kallmann reported in 1899 on a system for controlling stray currents of electric railways [64]. As early as 1894, the Board of Trade in London issued a safety regulation for the British electric railways which specified a potential differential of not more than 1.5 V where the pipeline was positive to the rails, but 4.5 V with the rails positive. Extensive research was undertaken on reducing the risk of stray current in the soil by metallic connections from pipes to rails. However, as one writer noted, a procedure on these lines should definitely be discouraged as it carries the seed of its own destruction [64]. 

If the projected pipeline is situated in an area with dc railway lines, rail/soil potential measurements should be carried out at crossing points and where the lines run parallel a short distance apart, particularly in the neighborhood of substations, in order to ascertain the influence of stray currents. Potential differences at the soil surface can give information on the magnitude of stray current effects in the vicinity of dc railway lines. It is recommended that with existing pipelines the measurements be recorded synchronously (see Section 15.5) and taken into account during design. 

If the protection current becomes too high due to this connection in cathodi-cally protected tank installations, then insulating joints are usually installed in the pipeline from the filling nozzle. Care must be taken that the continuity bond is not broken. If there is a danger of stray currents with dc railways due to a permanent connection between track and filling equipment, the continuity bond should be applied only during the filling process. 

Direct current installations that are grounded in several places cause stray currents in the soil which can interfere with other installations (see Section 9.2). All dc railways are sources of stray currents. Protection methods that can be applied in the same way to cables are described in Chapter 15. 

Stray Currents from dc Railways 15.2.1 Regulations for dc Railways... 

The railway lines on bridges are often electrically connected with steel or reinforced steel structures which usually have a very low grounding resistance. In new installations, an electrical separation of the rails from the bridge structure is required according to the grounding resistance of the structure and the type of rail bed. Independent of this, pipelines and metal sheathing of cables are always electrically separated from the structure in order to exclude direct transmission of stray current from the rails in these conductors. 

Frequently, measures for dc installations are not sufficient to limit stray currents. This applies particularly to dc railways. In many cases, additional protective measures for the affected installations are advisable [1] or even necessary [11]. 

Fig. 15-5 Stray current interference in the region of dc railway (a) Polarization of the railway lines, (b) voltage between the soil in the vicinity of the rails against a remote ground. Polarization of the pipeline (c) without stray current drainage, (d) with stray current drainage without a resistor, (e) with stray current drainage via a resistor R. Current in the pipeline (f) without stray current drainage, (g) with stray current drainage.

Fig. 15-8 Synchronous current, voltage and potential recording with stray current interference from dc railways (a) Without protective measures, (b) direct stray current drainage to the rails, (c) rectified stray current drainage to the rails, (d) forced stray current drainage with uncontrolled protection rectifier, (e) forced stray current drainage with galvanostatically controlled protection rectifier (constant current), (f) forced stray current drainage with potentiostatically controlled protection rectifier (constant potential), (g) forced stray current drainage with potentiostatically controlled protection rectifier and superimposed constant current.

Stray current effects with reinforced concrete are not likely from the usual causes, but are possible with roadways and bridges over which dc railways pass. In... 

The corrosion of underground pipes and cables caused by the electrolytic action of stray currents from d.c. electric railway and tramway systems has long been a serious problem. Regulations limiting the maximum potential between tramway rails and neighbouring buried structures and the maximum potential difference between points on the rail systems have been in operation in the UK since 1894 and in Germany since 1910 . 

An electric railway or tramway system with an adjacent buried pipeline or cable which may cross the running rails at intervals is illustrated in Fig. 10.35 in which the arrows indicate the general flow of stray currents when one vehicle is in service. Rapid variations of current and potentials will occur as the tram or train moves along the rails. Corrosion will occur at points near the sub-station or near negative feeders where the stray current leaves the buried structure to return to the negative busbar at the sub-station. 

The foregoing paragraphs refer particularly to stray d.c. from railways, tramways, trolley vehicles, mobile cranes and similar systems where the running rails or metallic supports are in direct contact with the earth. Underground d.c. tube railways usually operate on the fourth rail system and may be enclosed within cast-iron tube linings so that there is little or no stray current. 

Stray currents Railway lines Cable break Arc welding... 

At points where the current enters the structure, the site will become cathodic in nature because of changes in potential, while the area where the current leaves the metal will become anodic. Electric railways, cathodic protection, electrical welding machines, and grounded DC electrical sources are subject to stray current corrosion. (Craig)5... 

Corrosion plays a high risk underground, in particular to aluminum which is totally unacceptable. The electrolytic properties of some soils cause corrosion to all these metals, as do stray currents produced by DC railway lines on DC high voltage systems where the earth is used as a return path. Cathodic protection can help eliminate this type of problem. 

Figure 9.1 Example of stray current from a DC railway line picked up by a buried pipeline...

G. Cavallero, D. Melodia, F. Panaro, Monitoring of stray current interference in the reinforced-concrete structures of the Turin underground railway loop , 54 NACE Corrosion 99, 1999. 

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