IEEE Electrification Magazine - September 2014 - 30

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placing substations near points of maximum train

acceleration
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increasing system nominal voltages
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maintaining electrical continuity in tunnel liners and
reinforcing steel
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coating of rail surface with dielectric insulating material
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filling the entire trough of the embedded rail with
dielectric polyurethane or other suitable material
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cathodic protection
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using rail boots or insulating membrane for embedded rails
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using high-resistivity concrete mix
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using current-collection mats
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maintaining an ongoing maintenance program that
monitors rail-to-earth resistance values and keeps
track-bed areas clean and well drained.
While not all of these methods can be implemented for
every transit system, the research thus
far has indicated that most of the
design parameters, such as traction
power, utility coatings, cathodic protection, substation spacing, insulated fastener clips, spacing of cross bonds, and
train headways for the transit system,
are pretty much standardized, with the
exception of track-to-earth resistance.
The key to attaining the desired trackto-earth resistance is by following the
control at source approach, which in
reality is only achievable for a short
time [11]. This is achieved by completely isolating the rail from other conducting parts, and, generally, dielectric
insulating materials are used to attain
the rail isolation. For the last few decades, the rail boot has
been the most widely used rail isolation technique by transit
agencies for the embedded tracks. This is primarily due to
cost effectiveness of rail boot over other isolation techniques, its ease of installation during construction, and
comparatively less maintenance when compared with
other mitigation methods.
It is very difficult to accomplish perfect insulation
around the rail and maintain track-to-earth resistance at
acceptable levels. Even if perfect insulation is achieved, it
only lasts for a few years if the tracks are not maintained
properly. This generally happens in low-resistivity soils,
high-traffic urban areas, and where utilities and other
metal structures are more concentrated. Where these conditions exist, it is more advantageous to use a combination of mitigation and collection methods to control the
stray-current leakage. This also makes regular testing followed by maintenance of the transit system a must-have
requirement. The testing and maintenance regime is a
two-tier process, where actual testing of the transit system follows the visual inspection. Occasionally, it is difficult for the transit agency(s) to perform a continuous

maintenance and testing regime, and as a result, a combination of mitigation methods is used without realizing
their effectiveness.

stray-Current maintenance and testing
Mitigation methods installed to control stray-current corrosion require periodic inspection and testing during the
operation of the transit system. In the absence of any fixed
industry standards in the United States that would guide a
transit agency on the recommended frequency of these
periodic inspections, most transit agencies have their own
inspection schedule. Some transit agencies do not carry out
any inspections and react only when there is a complaint
by a utility owner. The European standards EN 59162:2004
[15] and EN 50122-2:2010 [16] briefly mention tests and
measurements methods and the principles behind the
continuous and repetitive monitoring.
To understand the various testing
methods and testing programs, several
national and international transit agencies' stray-current-control surveys
(including their findings) have been
analyzed. This study was supplemented by field monitoring and observations. It was observed that only a few
transit agencies carry out regular testing, and the testing methods fluctuate
based on the type of structure under
investigation and the severity level of
the stray-current problems at hand.
Potential and current tests are the most
frequently performed tests for both the
static and dynamic stray currents. The
following are some of the most commonly performed inspections and/or tests conducted by the
transit agencies:
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Visual inspections are conducted in conjunction with
physical measurements of the track elements to
identify any uncharacteristic structure item or fault at
the special track elements.
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Resistance measurements provide an insight to the
magnitude of resistance on the structures and the rail.
These results are then compared with the previous
year's test result to monitor variation.

The use of heavy rail
sections and suitably
bonded rail joints
was one of the
earliest implemented
mitigation methods
for the control of
stray current.

I E E E E l e c t r i f i c ati o n M agaz ine / september 2014

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Track slab current measurement (ground current survey)

provides an insight to the magnitude and direction of
possible current leakage from the rails.
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Structure and/or utility-pipe-to-soil potential measure-

ment help identify whether the structure is influenced
by stray current and the direction of the current flow.
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Track-to-earth resistance test measures the resistance and

helps in locating and isolating the track work discontinuities. American Society for Testing and Materials Standard
G 165-99, 1999 explains the procedure for testing [17].
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Audio frequency signal tracing helps in pinpointing the
local low-resistance areas and is used in conjunction

Table of Contents for the Digital Edition of IEEE Electrification Magazine - September 2014