IEEE Electrification Magazine - September 2014 - 29
table 1. the stray-Current-mitigation methods Used for Corrosion Control.
Description
Corrosion Committee (1921)
Currently Used
Rail size (cross-sectional area)
X
X
Rail bonds
X
X
Cross-bonding
X
X
Parallel conductors
X
X (with modification)
Traction power substation
X
X
Drainage bonds (case-by-case basis)
X
X
Track-to-earth resistance
X
X
Ungrounded traction power substation
X
X
Storage yard/mainline isolation
X
X
Decreasing the resistance of the rail return path
Increasing resistance of the earth-to-rail leakage path
xx
the use of well-graded, well-drained, and clean ballast
at an increased distance from the rail and the use of
insulated track fasteners
xx
the use of a sealing compound or rail boot (Figure 6)
for embedded rail sections
xx
maintaining an ungrounded or diode-grounded negative circuit
xx
isolating the main track from the tracks in the yards
and storage areas.
Suggestions were made for regular inspections, testing, and
maintenance programs after severe weather changes, street
and pavement repairs, and track and substation repair work to
minimize the slippage of stray-current leaks and to avoid hefty
cost repercussions in the form of utility-line corrosion.
2000 to the Present
By now, it is obvious to the transit agencies that the design
of earthing installations, substation spacing, track-to-earth
resistance, return circuits for dc transit systems, and a regular maintenance and testing regime are the most important
defense mechanisms for controlling stray current and
touch potentials. Learning from past experiences, most of
the newer rail transit agencies started designing their tracks
with provisions for the control of stray current within the
limits of their transit system. Baseline surveys and track-toearth resistance testing are carried out before construction.
During the process, transit agencies have also incorporated
testing and maintenance plans in their design-criteria
document. The idea of control at the source is idealized by
most of the corrosion consultants for the embedded tracks.
The use of rail boots, dielectric polyurethane, rail coatings,
cathodic protection, isolating the rail in troughs, and combinations of various mitigation methods are adopted by the
transit agencies to control the stray-current leakage. Additionally, for tracks in low-resistivity soils, wet climates, and/
or with high stray-current leakage activity, mitigation
methods were augmented with the use of currentcollection methods. This includes the use of steel reinforcement meshes or collection mats (Figure 6) provided in the
concrete below the tracks to provide a low-resistancecontrolled path for the fugitive current. The collection mats
are continuously bonded together along their length, and
insulated cables are connected to the mesh at regular interval to carry the current to the substation.
Table 1 lists some critical control measures/principles
that were identified in 1921 by the corrosion committee in
the United States and are still being used in present
designs, supplemented by some advancements and
recent developments. Furthermore, the following are various methods and/or techniques that are used alone or in
combination with each other to achieve manageable limits of stray current on a rail transit system and to protect
the utility lines and transportation infrastructure [4], [5],
[8], [11], [12]:
xx
floating and diode earthed schemes
xx
grounded systems and substations
xx
insulating pads
xx
insulating direct-fixation fasteners
xx
minimizing the stray-current leakage path through rail/
ballast contact by maintaining the ballast at a minimum of 1 in below the bottom of the rails
xx
cross-bonding between rails and between tracks to
maintain equal potentials of all rails
xx
bonding rail jumpers at mechanical rail connections
for special track work
xx
insulating switch machines at the switch rods
xx
using separate traction power substations for the
main line, yard, and shop
xx
insulating the impedance bond tap connections from
the housing case
xx
maintaining as close substation spacing as practicable
and cost effective
IEEE Elec trific ation Magazine / s ep t em be r 2 0 1 4
29
Table of Contents for the Digital Edition of IEEE Electrification Magazine - September 2014
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