IEEE Electrification Magazine - September 2014 - 24

(a)

(b)

(c)

Figure 1. Examples of different track types: (a) a direct-fixation track, (b) an embedded track, and (c) a ballasted track.

article focuses only on the types of corrosion caused by
stray current from a dc-powered transit system. We provide a synopsis of the technical methods that have been
used to control stray current over the years and the maintenance and testing plans used by transit agencies in the
United States and globally.

Stray Current
In a dc transit system, the current flows through the overhead catenary or the third rail to the vehicle and returns to
the substation through various conductors that run along
the path of the transit system. Ideally, the return current
should return to the substation through the running rail.
However, since perfect insulation does not exist and because
the rail has a finite resistance, a percentage of the return current leaks into the earth and takes the path of least resistance on the way to the substation. The path(s) of least resistance that this return current may take can include the
metallic utility lines, the reinforcement in the slab structure,
other metallic structures, and the soil. This current that leaks
and finds the path of least resistance is called the stray current, and the corrosion it causes on its way to the substation

Notation
R N Resistance of negative return circuit
R P Resistance of positive circuit
R L Track-to-earth resistance at the load end
R S Track-to-earth resistance at the source end
IT

Train operating current

IN

Current return through the rails

IL

Leakage current to earth at the load end

IS

Current returning to substation through earth

VS Substation voltage
VGL Track-to-earth voltage at the load end
VGS Track-to-earth voltage at the substation location
VN Voltage developed across R N by I N .

24

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

is called the stray-current corrosion. Figure 2 shows the path
of the return current in a dc transit system.
Stray currents are hard to detect since they are irregular
because rails carry dynamic traffic, and thus, the conventional method is to record the pipeline potential in the suspicious areas for at least 24 h. Figure 3 is a simple circuit model
demonstrating the basic components affecting the levels of
stray currents generated by a dc traction power system [7].
The relationship between the voltages VN, VGL, and VGS are
presented by (for a list of symbols used, see "Notation") [7]:
R
VGL . R +L R # VN
L
S

(1)

R
VGS . R +S R # VN,
L
S

(2)

VN = I N # R N .

(3)

where

History of Stray-Current Corrosion
and Methods of Mitigation
A review of the literature was conducted to gain insight
and understanding of the physical principles and scientific significance that lead to the development of the various
stray-current mitigation methods adapted to date by various rail transit agencies. The purpose was to understand
the evolution of stray-current corrosion mitigation methods from the early days of the introduction of the electrified rail systems to the present-day design. On the basis of
this literature review, it would be safe to say that with the
inclusion of technological advances, most of the technical
methods, conclusions, and assertions made in the early
1900s are still being used by transit providers in the present-day design and construction of stray-current control.
To help understand the longevity of the stray-current
issue, it must be noted that there had been regulations
defined in the United Kingdom since 1894, in France since
1911, and in Germany since 1910, limiting the maximum
potential difference between the points on the rail system
and the maximum potential between tramway rails and
neighboring underground structures [4]. In the United States
in 1916, the Corrosion Society distributed recommendations



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

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