IEEE Electrification Magazine - September 2014 - 28
(b)
(a)
Figure 6. Examples of (a) a rail rubber boot and (b) steel reinforcement with a collection mat.
pipe drainage, the interconnection of the affected structures and the rail return circuit, and keeping the new utility
construction at a greater distance from the rail lines and
avoiding the crossing of rail lines if at all possible or keeping the depth as large as possible, where the utilities must
cross the tracks [4], [6].
However, like any other mitigation method, these procedures had limitations; some of the methods proved
effective and required more testing and development and
thus were studied and investigated further. While some
methods that were originally recommended by the corrosion committee in 1921 are still in use, the drainage-bond
mitigation technique was widely criticized later by the
engineering community because of the variation of the
conductivity of different types of pipes under different
conditions, their material properties, and various joint
types. In fact, it was observed that drainage bond would
increase the overall leakage of the stray current, and
because of that, some European countries, such as
Germany and England, did not even recommend the use of
the pipe drainage [9].
1960s-1990s
Using the latest technological advances, further enhancements were made to the design and construction principles of the earlier era to control the leakage of stray current. Baseline surveys were introduced and conducted
before the construction of transit systems. As the name
suggests, baseline surveys were used to accumulate inception data on soil characteristics, utility locations, and other
relevant tests, based on the transit agency location. This
helped the transit agency and the corrosion consultants in
setting up the design parameters for the transit agency,
including the maintenance guidelines.
Advancements were made in the areas of track-toearth resistance, rail return circuit resistance, traction
28
I E E E E l e c t r i f i c ati o n M agaz ine / september 2014
power substation distance, the conductance of negative
conductors, the spacing and location of track cross bonds,
and the magnitude of propulsion current [10]. Design solutions and methods, including the use of nonmetallic pipes
and electrically continuous metallic pipelines as well as
the installation of testing locations, were implemented by
the transit agencies and the utility providers. Further
research was conducted to find the appropriate earthing
scheme that would minimize the leakage of the traction
current. The experiments included the comparative analysis of three different earthing schemes and weighed the
benefits of each of the following:
xx
solidly bonded or grounded systems
xx
floating or ungrounded systems
xx
diode-bonded systems.
Studies conducted showed that the floating system
with ungrounded substations provided the best earthing
system and kept the potential within limits [12]. For the
new transit system, stray-current leakage was kept in
check by decreasing and regulating the rail return circuit
resistance and increasing the resistance of the rail-toearth leakage path [10]. Decreasing the resistance of the
rail return path was achieved by undertaking:
xx
the use of standard-sized rail with a longitudinal
resistance of around 40-80 m X /km [12] (the crosssectional area or size of the rail was increased)
xx
the use of continuously welded rails and cable bonds
to achieve a continuous electrical path for the negative current
xx
by providing closely spaced traction power substations (spacing between 1 and 2 mi) to reduce the voltage drop between the two substations.
Increasing the resistance of the rail-to-earth leakage
path, which is considered the most useful approach to mitigate the stray-current leakage, was accomplished by undertaking the following measures [13]:
Table of Contents for the Digital Edition of IEEE Electrification Magazine - September 2014
IEEE Electrification Magazine - September 2014 - Cover1
IEEE Electrification Magazine - September 2014 - Cover2
IEEE Electrification Magazine - September 2014 - 1
IEEE Electrification Magazine - September 2014 - 2
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IEEE Electrification Magazine - September 2014 - Cover3
IEEE Electrification Magazine - September 2014 - Cover4
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