Fourth rail electrification, White City, London Underground
Electric railways were introduced towards the end of the 19th century. Electric traction was particularly suitable for use on underground railways where steam traction could be particularly troublesome. Early systems used locally-generated direct current, produced by stationary steam engines.
Largely because of the available materials for generators and motors, working voltages were limited to six or seven hundred volts. At these relatively low voltages, currents are fairly high, so a third rail, the conductor rail, was provided to carry the traction current between the feed point and the train to be powered. One or both running rails were used to return the current to the feed point. The conductor rail was carried on glazed porcelain insulators (invariably called 'pots') mounted on the transverse timbers ('sleepers') to which the running rails were also fixed. The high insulation resistance of the insulators ensured that almost all the current flowing in the conductor rail was that drawn by the train.
The situation was more complex as far as the return current from the train was concerned. The running rails were held at the proper gauge by fixing them rigidly to the sleepers which, in early days, were generally made of wood - a reasonable electrical insulator. To provide resilience against the dynamic loads of passing trains, the sleepers were buried in ballast, often crushed limestone - again a reasonable insulator. Sleepers and ballast were in contact with the mass of the earth. The earth resistance from the running rails may be fairly high in dry conditions but, when wet, earth resistance falls. Although most of the return current would flow from the train to the feed point through the rail, a proportion leaked from rail to sleeper and from sleeper to ballast so that a significant current flowed through the mass of the earth. If there were nearby metallic water or gas pipes, some of this stray current could flow through them, causing damage by electrolytic action.
The situation was even worse in deep tube lines, which were lined with sectional cast iron sections which would not only suffer electrolytic action but, if rings were not bonded together, could display arcing. To mitigate this problem, London's Underground evolved with a 'fourth rail' to carry the return current via a path isolated from earth.
The outer conductor rail is positive at around +420 volts and the inner conductor rail, mounted between the running rails, is negative at around -210 volts. These are the theoretical voltages under dry conditions. Leakage resistance across every porcelain insulator will provide a multitude of paths to earth from both conductor rails which can alter the voltage to earth ratio of the positive and negative conductor rails. To define the 'normal' ratio, London Underground provide bleed resistors every so often connected from each conductor rail to earth. This 'normal' ratio will be subject to significant change on surface lines under varying weather conditions.
Fourth rail electrification with modern concrete slab track
For notes on traction power distribution on London Underground click here.
For an excellent article on conductor rail systems by Russ Elliot click here.
The world's railways use a wide variety of d.c. and a.c. electrification systems employing both conductor rails and overhead conductor systems - to see the Wikipedia article on 'Railway Electrification' click here.
Related posts on this website
All my posts on London Underground can be found here.
