Stray current control in DC mass transit systems
ABSTRACT Stray current control is essential in direct current (DC) mass transit systems where the rail insulation is not of sufficient quality to prevent a corrosion risk to the rails, supporting and third-party infrastructure. This paper details the principles behind the need for stray current control and examines the relationship between the stray current collection system design and its efficiency. The use of floating return rails is shown to provide a reduction in stray current level in comparison to a grounded system, significantly reducing the corrosion level of the traction system running rails. An increase in conductivity of the stray current collection system or a reduction in the soil resistivity surrounding the traction system is shown to decrease the corrosion risk to the supporting and third party infrastructure.
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ABSTRACT: The ungrounded scheme with overvoltage protection device (OVPD) has been adopted in the construction of Nanjing Metro Line 1. During the operation of Line 1, a high rail potential was observed, which caused the OVPDs installed at some traction supply substations (TSSs) to keep turning on at any moment, and the stray-current leakage increased obviously. Since the magnitude of rail potential and stray current depends on the modes of a running vehicle, including accelerating, coasting, constant speed, or braking, Line 1 suspected that the vehicle running modes were the cause of a potential rise in the rail. This paper presents the results of field tests and numerical simulations to study whether different vehicle running modes have an impact on rail potential and stray current in Line 1. The results show that the rail potential in the acceleration and brake modes is significantly higher than that in the other modes, which are in the opposite direction. Moreover, within the stray-current interference, the maximum shifts of buried-conductor potential may exceed the limit in these two modes. In addition, the high rail potential and buried-conductor potential occur when the running mode changes from acceleration to coasting or the maximum brake power is reached. As a vehicle is running between two different TSSs, the rail potential is highest in its position.IEEE Transactions on Vehicular Technology 01/2013; 62(8):3569-3580. · 2.06 Impact Factor
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ABSTRACT: This paper presents the assessment of the effectiveness of the design provisions adopted for mitigating the electromagnetic interference generated by stray currents in a dc rail traction system. The track characteristics of the Red Line of the Dubai Metro are analysed and the measurements carried out to assess compliance with EN 50122-2 are shown. It is demonstrated that the design of the constituent components of the current return circuit is a key element in attaining overall system EMC.Electromagnetic Compatibility (EMC EUROPE), 2012 International Symposium on; 01/2012
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ABSTRACT: In DC electric railways, while an electric rail car is driving, a part of the working current returned to the substation through rails leaks into the ground. Such a stray current causes railway facilities and metal objects to corrode electrolytically. Therefore, change of stray current needs to be monitored constantly. But so far in domestic, the research on stray current measuring techniques and system adaption are insufficient. To estimate stray current, this paper addresses a method of monitoring the return current that is returned into the negative pole of the substation in real time.Transactions of the Korean Institute of Electrical Engineers 01/2011; 60(4).
722IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 54, NO. 2, MARCH 2005
Stray Current Control in DC Mass Transit Systems
Ian Cotton, Member, IEEE, Charalambos Charalambous, Pete Aylott, and Petra Ernst
Abstract—Stray current control is essential in direct current
(DC) mass transit systems where the rail insulation is not of suf-
ficient quality to prevent a corrosion risk to the rails, supporting
and third-party infrastructure. This paper details the principles
behind the need for stray current control and examines the rela-
tionship between the stray current collection system design and its
efficiency. The use of floating return rails is shown to provide a re-
duction in stray current level in comparison to a grounded system,
significantly reducing the corrosion level of the traction system
running rails. An increase in conductivity of the stray current
collection system or a reduction in the soil resistivity surrounding
the traction system is shown to decrease the corrosion risk to the
supporting and third party infrastructure.
Index Terms—Corrosion, heavy rail, light rail, rail transporta-
tion, stray current, transit.
ning rails as a mechanical support/guideway and as the return
circuit for the traction supply current. Since the rails have a fi-
nite longitudinal, or series, resistance—around 40–80 m
m of rail—and a poor insulation from earth—typ-
ically from 2 to 100
km—a proportion of the traction current
returning along them will leak to earth and flow along parallel
circuits (either directly through the soil or through buried con-
ductors) before returning onto the rail and the negative terminal
of the DC rectifier. It should be noted that, in a DC system, the
current loss is by direct leakage. Induced effects found in alter-
nating current (ac) systems are less significant in terms of cor-
Given that current flow in a metallic conductor is electronic,
while that through electrolytes such as the earth, concrete, etc.,
is ionic, it follows that there must be an electron to ion transfer
as current leaves the rails to earth. Where a current leaves the
rail to earth there will, therefore, be an oxidation, or electron-
URRENT leakage from directional coupler (DC) railway
systemsis aninevitableconsequenceof theuse oftherun-
This reaction is visible after time as corrosion damage. For
the current to return onto the rail, there must be a reduction
Manuscript received April 22, 2002; revised April 15, 2003, September 23,
2004, and October 12, 2004. The review of this paper was coordinated by Prof.
I. Cotton and C. Charalambous are with the School of Electrical Engineering
and Electronics, University of Manchester, Manchester, M60 1QD, U.K.
(e-mail: email@example.com; firstname.lastname@example.org).
P. Aylott and P. Ernst are with CAPCIS Systems, Ltd., Manchester, M1 7DP,
U.K. (e-mail: email@example.com; firstname.lastname@example.org).
Digital Object Identifier 10.1109/TVT.2004.842462
or electron-consuming reaction. In an oxygenated environment,
this will typically be
It should be noted that the iron-reduction reaction is not ther-
modynamically preferred and that iron does not plate back onto
Corrosion of metallic objects will, therefore, occur from each
point that current transfers from a metallic conductor, such as a
reinforcement bar in concrete, to the electrolyte (i.e., the con-
to both the rails and any other surrounding metallic elements. In
a few extreme cases, severe structural damage has occurred as
a result of stray current leakage.
imize the impact of the stray current on the rail system, sup-
porting infrastructure, and third-party infrastructure. It is sig-
nificant to note that stray current has not always been perceived
as a problem and has been positively encouraged. Schwalm and
Scandor  produced a paper detailing such a view that states
that rails are generally not insulated from the earth so that part
of the return current travels through the earth and makes use of
any metallic underground path in the vicinity that provides con-
This paper illustrates how the stray current magnitude varies
before, comparing the performance of stray current collection
systems of different constructions and placed in different soils.
II. IMPACT OF RUNNING RAILS AND POWER-SYSTEM DESIGN
ON STRAY CURRENT LEVELS
The essential elements of a transit system are the rails, power
supply, and vehicles. The design and placement of these ele-
ments of the transit system dictates the stray current perfor-
mance in terms of the total stray current leaving the rails. If
the total stray current for a given design of a system is high,
a stray current collection system may be needed to control the
path through which this stray current returns to the substation
considerable corrosion of the supporting infrastructure and of
third-party infrastructure may occur.
However, as stated by Schaffer et al. , no stray current col-
lection system will be needed if the rail insulation and power-
system design themselves can keep stray current levels below
a “damage-causing” value. It is, therefore, obviously desirable
to eliminate the need for any stray current collection system
by controlling the level of stray current being produced by the
transit system. Means to reduce stray current levels below a
damage-causing value may include measures such as increased
power system cross-bonding, increased rail to earth resistances
0018-9545/$20.00 © 2005 IEEE
COTTON et al.: STRAY CURRENT CONTROL IN DC MASS TRANSIT SYSTEMS723
Fig. 1.Section of model to illustrate stray current production.
Fig. 2.Rail-to-earth voltage profiles for a floating and grounded rail system.
(by use of better coatings/insulating supports), and the encase-
ment of the track slab by an insulating membrane.
The determination of the need for a stray current collec-
tion system is, therefore, initially based on examination of
the rail-to-earth voltage profiles during the operation of the
transit system (determined by the power-supply design and the
electrical performance of trains themselves), the rail-to-earth
insulation levels, and the resulting stray current leakage profile.
A. Impact of Floating/Grounded Running Rails on Stray
Fig. 1 shows a 1-km section of track used to illustrate the
rail-to-earth voltage profile when a train draws current from a
substation. This 1-km section is representative of a symmetrical
2-km section of track with a single train at the center and a
substation at each end. The 1000 A that has been produced by
a substation at the far end of the track is being drawn by a train
placed at 0 m.
For every 1 m
km of track resistance, there will be a re-
sulting voltage drop of 1 V/km along the rail. Take a case where
the resistance of a single rail is 40 m
track). For 1000 A current, the resulting voltage difference be-
tween the two ends of the track will be 20 V.
This voltage will appear on the system in one of two ways.
In a floating system where the running rails (and, hence, the
DC negative bus) are allowed to float with respect to earth, the
voltage will appear on the rails as 10 V to remote earth near the
10 V to remote earth near the substation.
In a grounded rail system, where the running rails are effec-
tively bonded to earth (via a stray current collection system or
km (20 mkm for the
any reinforced concrete/metallic structure around the track such
as a tunnel) at the substation, the voltage will appear on the rails
as 20 V to remote earth at the train and 0 V to remote earth at
the substation. Fig. 2 shows the rail to earth voltage in a floating
and grounded system.
leaks out of the rails into the earth. For the negative voltage
case, the current leaks back into the rails. The magnitude of the
current leaking from the rails is determined by the voltage to
remote earth at any point along the track and the resistance to
remote earth will be 0 V (implying no current leakage in either
direction). In the floating system, stray current will, therefore,
leave the rails in the region 0–500 m and then re-enter the rails
in the region 500–1000 m. This is shown in Fig. 3.
In the case of a grounded rail system, where the voltage is al-
ways positive with respect to earth, stray current leaves the rails
along their full length and returns to the traction system power
supply at the substation earth bond (i.e., through the substation
earthing system and any metallic components connected to it).
For the two forms of system described, the overall stray cur-
rent level can be described using the following equations. These
equations are based on the single-train case shown in Fig. 3
with a uniform rail coating. In the floating system, stray cur-
rent leaves the track over the first 500 m, returning to the track
over the final 500 m. It can be shown that the total stray current
leaking from the system can be described as
724IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 54, NO. 2, MARCH 2005
Fig. 3. Basic model of floating rail system, illustrating stray current leakage.
Fig. 4.Positive (corrosive) stray current charge from running rails in dynamic simulation of the (left) grounded and (right) floating system.
the track (i.e., two parallel rails) in ohms per kilometer, l is the
distance between the train and substation in kilometers, and
is the resistance to earth of the tracks.
For a grounded system, this equation can be rewritten as
is the traction current in amps, is the resistance of
The increase in stray current level by a factor of four on the
grounded system arises from the doubling in the peak rail-to-
earth voltage in combination with a halving of the resistance
through which stray current can leak by a factor of two (due
to doubling the amount of track at positive rail to earth poten-
tial). It would, therefore, seem that floating running rails are the
best option if stray current is to be minimized. This conclusion
is shared by Yu and Bomar. Yu  states that the floating rail
system is the best option for the reduction of stray current levels
of stray current was observed in a system where the rails were
directly bonded to a ground mat at the traction substations.
currents, it is shown that the “factor of four” is generally a low
estimate of the increase in stray current level from a grounded
mented in MATLAB. The simulation determines the train posi-
tion, velocity, current requirement, and rail voltages as a func-
tion of time .
Fig. 4 is based on a dynamic simulation and shows the sum-
mated positive (i.e., corrosive) stray current charge produced by
a train running between two stations at a 1200-m interval. Sub-
stations are located at 0 and 1200 m, i.e., at the two end stations.
Fig. 5 then shows the ratio of the grounded system to floating
system summated positive stray current charge along the rail
While these results clearly demonstrate the advantages of
a floating rail system, it must be proven that unsafe levels of
track-to-earth voltages will not develop during fault conditions
(such as the conductor rail coming into contact with earth).
As safety is the prime concern in the design of mass transit
power systems, grounded systems may occasionally be the only
choice. Modern protective devices do, however, allow faults to
be detected and cleared with relative ease.
An oft-proposed variation on these systems is the use of a
diode-bonded approach, in which the rail is connected to the
COTTON et al.: STRAY CURRENT CONTROL IN DC MASS TRANSIT SYSTEMS725
Fig. 5. Ratio of grounded to floating system summated positive stray current charge along the rail length.
Fig. 6.Variation of maximum rail stray current density as a function of rail coating and base material resistivity.
ground mat via a diode. This diode will prevent stray currents
passing directly from a ground mat to the rail. When the rail
is at a negative potential with respect to earth, the system is,
therefore, floating. The diode will, however, appear as a short
circuit when the rail potential moves positive with respect to
earth and the general effect is to increase stray current levels in
comparison to a floating system , .
B. Variation of Rail Leakage Current as a Function of Rail
Insulation Level and Soil Resistivity
An important parameter in (3) and (4) is the rail resistance
to earth. If near-perfect insulation was placed around the rails,
any level of rail voltage could be tolerated with minimal stray
currenteffects (althoughitshouldbe notedthatotherconsidera-
allowed in a traction system).
The rail resistance to earth usually is a function of the insu-
sistivity of the base material (e.g., concrete or ballast) on which
the rails are laid. In normal circumstances, the resistivity of the
rail insulation/the pads upon which the rail is mounted is more
significant than the resistivity of the material upon which they
are placed (such as concrete).
Fig. 6 shows the variation in the maximum stray current
leakage density of the floating system previously described in
Fig. 1 as a function of the resistivity of the base material and
resistance of the insulating pads used to fix the track to the
ground at regular intervals. The stray current leakage density is
expressed in A/m, i.e., the stray current leaving a 1-m section
of rail. The maximum stray current leakage density is found at
the location of the train or substation in the case of the floating
system where the rail voltage is at a peak.
In the CDEGS software  used to carry out this modeling,
the resistance of insulating pads used in a rail system must be
converted to a coating of a given resistivity that is placed uni-
formly along the rails. This simplifies the modeling require-
ments. Altering the resistivity of a 10-mm thickness coating
placed around the rail varies the value of insulating pad resis-
tance. A rail coating resistivity of 100 M m (the last point on
the -axis) is equivalent to an insulating pad resistance of 340
km (produced by all the insulating pads found in 1 km being
placed in parallel).
Fig. 6 shows that the resistivity of the material the rail is laid
on does not have an effect on the stray current leakage den-
sity until the rail coating resistivity drops significantly lower
m, equivalent to an insulating pad resistance of 3.4
726 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 54, NO. 2, MARCH 2005
Fig. 7.Path of stray current leaving the running rails.
Fig. 8.Path of stray current returning to the running rails.
km. The base material resistivity would only become signif-
icant in conditions where the rail insulation was rendered in-
effective (for example, for light rail systems running on streets
that are gritted with salt during winter).
The key point illustrated by the graph is the illustration of the
importance of the rail insulation. If the rail insulation resistance
can be maximized, then the resulting stray current leaving the
running rail will be minimized.
In practice, the production and operation of a transit system
with a high rail-to-earth resistance is possible in the short term
after construction. Regular maintenance of the insulation is re-
quired thereafter to ensure that there is no decrease in resis-
tance and resulting risk to the transit system or third-party in-
frastructure. However, it may not be possible to maintain the
rail-to-earth insulation of surface transit systems running on
roadways, since the insulation will quickly become coated with
dirt and/or salt during winter weather.
C. Impact of Stray Current on Rails, Supporting and
As previouslydetailed, corrosion willoccur ateach pointthat
current transfers from a metallic conductor, such as a reinforce-
ment bar in concrete, to the electrolyte (i.e., the concrete). The
corrosion must occur unless they are perfectly insulated from
ning rails; this should be considered at the start of any analysis.
Should the running rails be placed in an area where there are
no other metallic conductors, the only corrosion risk is to the
rails. However, a transit system may be placed in a tunnel made
with some form of reinforced concrete or for urban transit sys-
tems close to power cables, gas mains, and similar utilities.
For a tunnel system, the primary corrosion risk is to the rails,
their fixings, and the tunnel walls. The exact nature of the risk
struction, and longitudinal continuity of the construction. There
also is a secondary risk to structures outside the tunnel.
For a transit system placed above ground, the primary corro-
sion risk is again to the rails and their fixings as well as to any
nearby utility cables/pipes. Again, the risk is determined by the
and the utility cables/pipes, and the longitudinal conductivity of
ments of the supporting infrastructure or third-party infrastruc-
ture due to stray current flow. Fig. 7 shows the effect of stray
current leaving the rails; this case is found in the sections of rail
systems where the rail potential is positive with respect to earth.
fasteners themselves. If present, the stray current will then enter
any civil/stray current mat found in rail systems where the rails
are placed on a concrete base. The entry of the stray current to
this mat of reinforcement does not cause corrosion itself, but
the limited conductivity of the mat results in the further leakage
of a proportion of the stray current into the surrounding soil
and into any buried services (or the longitudinal conductors of
any tunnel). The only corrosion risk at this time is to the mat
reinforcement due to the leakage of current off the mat.
rent mat, where current flows from the metallic object into the
COTTON et al.: STRAY CURRENT CONTROL IN DC MASS TRANSIT SYSTEMS727
soil. Of particular significance is the fact that, in both cases de-
scribed, the civil/stray current control mat has had stray current
leaving the metallic reinforcement bar to enter an electrolyte
causing corrosion. The same conclusion could be applied to
segments of an underground tunnel constructed with reinforced
In a symmetrical system where the running rails remain
floating with respect to earth, half of the system could be taken
to be operating as in Fig. 7 while the other half will be operating
as in Fig. 8.
In a system where the running rails are grounded to earth at
the substation, the stray current leakage is generally from the
rails, unless regeneration is taking place by a train. The stray
current leaves the rails and will again pass into any civil/stray
current mat and partly pass through any tunnel segments, buried
via the substation earthing grid or the civil/stray current control
mat (usually bonded directly to the DC negative busbar in such
a case). Stray current would also not leak from the civil/stray
current mat to the rails. It would, instead, pass directly through
busbar mentioned before.
Due to the obvious corrosion risks resulting from the stray
current flows detailed before, an analysis of the stray current
flows within a system, and the impact on the rails, the traction
system supporting infrastructure and any third-party infrastruc-
ture should be carried out. This assessment would usually iden-
tify the components that are vulnerable to stray current attack
and calculate their lifetime based on the level of corrosion that
they will suffer. The calculation would involve an application
of Faradays law that can relate the total charge leakage from a
metallic component with the amount of metal oxidized.
If the risk to any component/structure is too much to tolerate
in terms of a reduction in lifetime or through safety consider-
ations, further steps must be taken to control the level of stray
current. There are a number of options, such as increases in the
rail-insulation level, an increase in the operating voltage of the
traction system, and reducing the substation spacing. However,
a stray current collection system is often used to “catch” the
stray current leaving the rails and provide a conductive path
through which it can flow without a risk of damage to sup-
III. STRAY CURRENT COLLECTION SYSTEM
A stray current collection system can be constructed under
to the segments. Such collection systems usually take the form
of reinforcement in the concrete track bed of a traction system.
This reinforcement is bonded along its length to provide a con-
tinuous and relatively low resistance path. The stray current
leaking from the running rails is intended to flow into this col-
lection system and be captured upon it, as opposed to flowing
through the tunnel construction or other local conductors such
as utilitypipes/cables. Forthisstrategy to succeed,themat must
offer a significantly lower resistance path than segment rein-
forcement in a tunnel, buried services, and the surrounding soil
itself. In a floating system, the stray current collection system
will not be bonded to the running rails.
Figs. 7 and 8 show an example stray current collection
system. The reinforced concrete mat placed underneath the
rails is used for both structural support and as a conductive path
for a stray current. Connected to this mat is an insulated cable,
generally copper, that increases the overall conductivity of the
stray current collection circuit relative to the alternative stray
current paths in the soil and other buried objects. Stray current
control mats have generally been constructed in 100-m sections
with the starts/ends of each section being electrically connected
to each other and to the stray current collector cable producing
a continuous stray current path. If a designer considered that
stray current was likely to be a problem in a specific region
of the transit system, local stray current collection systems
could be used but careful attention would have to be given to
the design of the terminations of the system in which severe
corrosion could occur.
A. Model for the Prediction of the Stray Current Collection
fore construction of a transit system is essential to ensure that
the stray current levels will not have an adverse effect on the
time is broadly based on the amount of time the system would
operate far before corrosion started to render the structural el-
ements of the system unsafe. Consideration of the effect of the
stray current system on other buried services must also be taken
A CDEGS  model of the system described is used to il-
lustrate the impact of factors such as soil resistivity and size of
the stray current control mat on the efficiency of the stray cur-
rent collection system. CDEGS allows a geometrically accurate
model of the system to be constructed and allows the investi-
gation of the performance of a stray current collection system
along its length. It is, however, restricted to the simulation of
nondynamic situations within the transit system.
Fig. 9 shows the perspective view of a CDEGS model. A
simplified model of the stray current control mat is constructed
using 12 longitudinal conductors and hoops at regular intervals.
This is a reduction in the number of conductors that would be
present in the real reinforcement mat. This simplification is,
however, necessary since the number of conductors that would
be required to model a complete reinforced concrete mat would
result in excessive memory and time requirements in computa-
tions. The simplified model has the same longitudinal conduc-
tivity of the real mat and tests have shown that the simplified
model performs with accuracy comparable to a more complex
The stray current collector cable is connected directly to the
stray current control mat at 100-m intervals, at which point the
model,thesizeof thestray currentcollectorcablecan bealtered
to assess the impact it has on the system performance.
The running rails are placed above the mat, the separation
of the reinforcement bar and the rails being equivalent to
728IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 54, NO. 2, MARCH 2005
Fig. 9. CDEGS model of stray current collection system.
Fig. 10. Efficiency of the stray current collection system against soil resistivity.
that in a real system. The running rails are simulated by a
cylindrical conductor having the same longitudinal resistance
as an actual rail. The rails are also coated with a resistive
coating to model the insulated pads on which the rails are
placed in a real system.
within wet (30
m) or dry (180
soil resistivity can then in turn be placed around this concrete.
Buried services, tunnel segments, and other conductive objects
that may need to be considered in any particular case can also
be simulated if necessary.
m) concrete, while a varying
B. Results of Stray Current Collection System Modeling
1) VariationofEfficiencyWithSoilResistivity: Amodelofa
1-km section of floating rail system is initially used to illustrate
theeffectof soilresistivityonthe performanceofa stray current
collectionsystem. Thecopperstray currentcollectorcablehas a
cross-sectional area of 120 mm , the steel stray current mat has
a cross-sectional area of 1600 mm , and the concrete resistivity
m. The soil resistivity is 100
The rail voltage profile due to the injection of 1000 A into the
track is as shown in Fig. 2. The total stray current leaving the
rails in the first 500-m section is 26 mA (equivalent to a track
to earth resistance of 96
km). This 26 mA, 13 mA from each
of the running rails, will flow into the stray current collection
system. It will then either remain in the stray current collection
system or will flow into the soil surrounding the reinforced con-
crete slab. Measurements of the current flows are taken at the
500-m point where, for this symmetrical floating rail system,
no stray current is entering or leaving the rails. A plot of the ef-
ficiency (i.e., the percentage of stray current found on the stray
current collection system at 500 m compared to the total stray
current) of the stray current collection system against the re-
sistivity of the soil surrounding the reinforced concrete slab is
shown in Fig. 10.
As the soil resistivity increases, the percentage of the stray
current retained on the stray current collection system is in-
creased. This is due to the reduction in the conductivity of the
alternative path through the soil. The ratio of the current carried
by the stray current mat in comparison to the collector cable is
approximately 1:1 (not shown on the graph).
While the resistivity of the soil is an important factor in the
determination of the stray current collection system efficiency,
it may in fact vary throughout the year and a worst case value
should be taken. Factors that can be controlled are the stray cur-
rent collector cable cross-sectional area and the diameter of the
reinforcement bar used for the stray current control mat.
2) VariationofEfficiencyWithMat/CableSize: Inthemodel
described, the total cross-sectional area of the steel within the
mat is 1600 mm . Adjusting this cross-sectional area to take
COTTON et al.: STRAY CURRENT CONTROL IN DC MASS TRANSIT SYSTEMS729
Fig. 11.Efficiency of the stray current collection system against stray current collector cable cross-sectional area (10 ?m soil around the dry concrete base).
Fig. 12. Efficiency of the stray current collection system for wet and dry concrete.
into account the fact that the steel used has a resistivity 13.1
times that of copper, this would be equal to 123 mm of copper.
The simulations carried out for Fig. 11 show the relationship of
thestray current collectionsystem efficiencytothestray current
collector cable cross-sectional area. This plot relates to a sur-
rounding soil resistivity of 10
of the system is relatively low (69% for a 120 mm collection
cable as compared to 90% in 100
A 120-mm collector cable results in an approximately equal
current flow in both the mat and collector cable. This confirms
that the 1600 mm of steel within the mat is equivalent to ap-
proximately 120 mm of copper. As the cross-sectional area of
the stray current collector cable is increased, the percentage of
the total stray current flowing through the collector cable also
increases, but the current flowing through the stray current mat
m where the overall efficiency
In the case where the stray current collector cable size
changes from 120 mm to 240 mm , there is an increase in
the conductivity of the total stray current collection circuit by
50%. The efficiency of the stray current collection system is,
however, only increased by a relatively small 3.9%. It can,
therefore, be concluded that control of stray current levels using
stray current collection systems can, therefore, be difficult
in areas where there is a low soil resistivity or other highly
3) Variation of Efficiency With Base Material Resis-
tivity: The base material resistivity may change, particularly
in the case of concrete placed above ground, where it may
vary between wet and dry states. Fig. 12 shows the impact of
changing the base material resistivity from 180
when the system is placed in 100
material resistivity does result in a small change in system
m to 30m
m soil. Changing the base
730IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 54, NO. 2, MARCH 2005
efficiency, but is not significant in comparison to the effect of a
changing soil resistivity.
The total stray current produced by a DC mass transit system
will adversely affect the transit system itself and third-part in-
a DC mass transit system can be limited in a number of ways,
including the reduction of substation separation, rail resistance,
and operating currents and an increase in the rail resistance to
earth. A system where the running rails float with respect to
earth produces roughly four times less stray current in compar-
ison to an equivalent grounded system when static simulations
can actually result in even higher local reductions in stray cur-
rent level in comparison to a grounded system.
Reducing stray current levels is best done by careful con-
trol of factors, such as substation spacing, rail-to-earth resis-
tance, and rail resistance. However, if after analysis the stray
current level produced by a transit system is too high and may
affect supporting or third-party infrastructure, a stray current
collection system may have to be considered. The role of the
stray current collection system is to collect the stray current
leaving the rails and to conduct it along the traction system to
the point where it re-enters the running rails. In this way, corro-
sion damage to supporting and third-party infrastructure might
The performance of a stray current collection system is, how-
ever, highly dependant on the conductivity of the system itself
and of the neighboring soil. Extremely high efficiencies can be
achieved when the material surrounding the stray current col-
lection system is highly resistive. At lower soil resistivities, the
results showed the difficulty in achieving a stray current collec-
tion system with a high efficiency. In such cases, it may be more
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Ian Cotton (M’98) was born in Cheshire, U.K., in
1974. He received a Class I B.Eng. (Hons) degree
in electrical engineering from the University of
Sheffield, Sheffield, U.K., in 1995 and the Ph.D.
degree in electrical engineering from the University
of Manchester Institute of Science and Technology
(UMIST), Manchester, U.K., in 1998.
He now is a Lecturer in the Electrical Energy and
Power Systems Group, School of Electrical Engi-
neering and Electronics, University of Manchester,
Manchester, U.K. His current research interests in-
clude condition monitoring, the electrical control and analysis of DC corrosion,
AC interference, and lightning protection.
Charalambos Charalambous was born in Nicosia,
Cyprus, in 1979. He received the Class I B.Eng.
(Hons.) degree in electrical and electronic engi-
neering from the University of Manchester Institute
of Science and Technology (UMIST), Manchester,
U.K., in 2002 and is currently working toward
the Ph.D. degree under the O.R.S scheme within
the Electrical Energy and Power Systems Group,
University of Manchester, Manchester, U.K.
rosion for DC mass transit systems.
Pete Aylott was born in London, U.K., in 1959. He
received the M.A. (Hons) degree in natural sciences
from the University of Cambridge, Cambridge, U.K.,
In 1985, he joined CAPCIS Systems, Ltd.,
Manchester, U.K., where he is now a Director for
Infrastructure Projects. His main area of work relates
to management of stray current on light rail and
heavy rail underground systems and the interactions
between these and utility systems, both across the
United Kingdom and overseas.
Petra Ernst was born in Bremen, Germany, in
1969. She received the Dipl. degree in materials
from Fachhochschule Osnabrück, Germany, the
M.Phil. degree in corrosion from Sheffield Hallam
University, Sheffield, U.K., and the Ph.D. degree
in corrosion from the University of Manchester
Institute of Science and Technology (UMIST),
She now is a Project Engineer at CAPCIS, Ltd.,