DC Traction Power Systems
R.W. Benjamin Stell
T his article reviews the current American and European standards and codes for
maximum permissible rail voltage on dc traction power systems. The principles of
negative grounding device (NGD) operation and its corresponding voltage settings
are also briefly discussed. The negative return portion of a modern dc railway
power system, which includes the running rails (tracks), is normally isolated from earth to
the maximum extent practical. The purpose of this isolation is to prevent stray dc currents
from flowing through the earth and potentially causing corrosion of nearby metallic infra-
structure. The isolation of the tracks from the earth is not perfect. Each track tie and insu-
lated running rail fastener assembly can be electrically represented as a resistor of high-
ohmic value connected between the rails and the earth. With many of these resistors in
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parallel over miles of track, a distributed leakage resist-
ance is established between the rails and earth. However,
for modern dc traction power systems, in particular, this
resistance is high enough for the rails to be considered
essentially ungrounded with respect to local electrical
ground (earth).
The lack of an intentional electrical connection
between the tracks and earth allows voltage differences
to occur along the rails, and between the rails and nearby
structures. These voltage differences are caused by the
flow of current through the running rails back to the sub-
stations. Since the shells of rail vehicles are typically at
the same voltage as the wheels and rails, this voltage dif-
ference could be impressed on a passenger entering or
exiting a train from a grounded platform. In the United
States, these voltage differences have generally been
limited through system design; the North American
standards for substation grounding are typically refer-
enced for design purposes, in particular, IEEE Standard
80, IEEE Guide for Safety in Substation Grounding [1]. In
Europe, a standard has been developed specifically to
address the control of voltages between rails and struc-
tures, International Electrotechnical Commission (IEC)
62128-1 (BS EN 50122-1), Railway Applications—Fixed Instal-
lations—Part 1: Protective Provisions Relating to Electrical
Safety and Earthing [6]. Voltage-limiting equipment that
can be installed in passenger stations and other accessi-
ble locations has been developed in response to the
requirements of IEC 62128-1. These devices quickly con-
nect running rails to the station structure to eliminate
unsafe voltage differences.
If an earth fault occurs (e.g., a broken catenary conduc-
tor falling on the ground), there may not be a low-resist-
ance circuit back to the substation because of the
electrical isolation between running rails and earth
ground. Without a low-resistance path back to the substa-
tion, the resulting low-level short-circuit current flow is
insufficient to operate the substation protective systems.
As a result, the area in the vicinity of the fault may poten-
tially be elevated to unsafe voltage levels. The equipment
intended to detect this condition and connect the substa-
tion negative dc bus to the substation grounding grid is
gradually being incorporated into modern North Ameri-
can dc traction power substation designs. These devices
are known by several names, such as substation ground-
ing contactors, automatic grounding switches, and NGDs.
Devices built to comply with IEC 62128-1 are termed
voltage-limiting devices. IEC 62128-1 includes voltage–time
curves that dictate the maximum permissible magnitudes
and durations for ac and dc voltages, and the equipment
built to EN 50122-1 must clamp (limit) the highest voltages
in no more than 20 ms.
Rail Potential
‘‘Effects of an electric current passing through the vital
parts of the human body depend on the duration, magni-
tude, and frequency of this current. The most dangerous
consequence of such an exposure is a heart condition
known as ventricular fibrillation, resulting in immediate
arrest of blood circulation’’ [1]. Although it is a current
flow that causes this condition, the current flow through a
person’s body is in response to a voltage difference
between two locations on the body. The resulting current
flow is proportional to the equivalent resistance of the
human body and the magnitude of the voltage difference
across the body, or shock voltage, in accordance with
Ohm’s law (body current = voltage difference/body resist-
ance). For this reason, standards for the design of electri-
cal facilities specify maximum permissible voltages,
usually referred to as touch, step, or accessible voltages.
Rail potential is the difference in voltage between the
tracks (steel running rails) and ground. In this instance,
ground means remote earth and earth in the terminologies
of the IEEE Standard 80 [1] and IEC 62128-1 [6]. respec-
tively (a zero potential reference). Rail potential is there-
fore a hypothetical quantity that is difficult to measure
with precision since a direct connection to remote earth
is difficult to achieve in practice. Rail potential is most
often caused by a current flow through the tracks. The
current flow through the electrical resistance of the rails
creates a voltage drop along the rails. This results in a
higher voltage at the location where the current is
injected into the rails, which is why this phenomenon is
sometimes referred to as rail voltage rise.
Rail voltage rise regularly occurs due to the passage of
trains, the higher values typically corresponding to peri-
ods of peak train acceleration and therefore lasting on the
order of tens of seconds at most. The resulting peak rail
potentials may or may not be significant, being primarily
dependent on the magnitude of the train load currents,
the resistance of the rail return circuit, and the degree of
electrical isolation of the tracks from earth.
Rail voltage rise also occurs as a result of short circuits
between the positive dc supply network [overhead contact
system (OCS) or contact rail] and the tracks. These low-
resistance, higher magnitude faults are typically cleared
rapidly since they are easily detected by substation protec-
tive devices. The resulting short-time rail potentials can be
significant, depending on the location of the fault.
Short circuits from the positive dc supply network to
poorly conducting surfaces (high-resistance ground
faults) can cause voltage in the fault vicinity to rise well
EFFECTS OF AN ELECTRIC CURRENT PASSINGTHROUGH THE VITAL PARTS OF THE HUMANBODY DEPEND ON THE DURATION,MAGNITUDE, AND FREQUENCY OF THISCURRENT.
100 ||| IEEE VEHICULAR TECHNOLOGY MAGAZINE | SEPTEMBER 2011
above that of the rails. This can result in a rail potential
but with a polarity opposite to that of the rail voltage
rises described earlier. Without a low-resistance path-
way back to the substation negative dc bus, these
ground faults can persist for significant lengths of time.
Causes of high-resistance ground faults include broken
OCS conductors, positive cable insulation failures, con-
tact rail insulator failures, and debris touching the con-
tact rail, including snow that has been treated with snow
melting salts.
U.S. Standards for Rail Potential
The IEEE Standard 80, IEEE Guide for Safety in Substation
Grounding [1], is the standard commonly referenced in the
United States for the design of safe electrical facility
grounding. Although other U.S. standards and codes
address grounding methods and requirements, IEEE
Standard 80 is unique among them in establishing safe lim-
its of potential differences (tolerable voltages) between
points that can be contacted by the human body.
The tolerable voltage equations provided in IEEE
Standard 80 are derived from the research work of C.F.
Dalziel, although the works of other authors are also dis-
cussed, including a comparison with the more recent
tolerable body current curve of Biegelmeier and Lee on p.
15. IEEE Standard 80 provides simplified formulas for cal-
culating the 50- and 60-Hz ac voltages that can be tolerated
by 99.5% of the population. IEEE Standard 80 provides
these formulas for two body weights, 110 lb (50 kg) and
155 lb (70 kg), and for touch contact (hand to hand or hand
to feet) and step contact (foot to foot). Persons weighing
155 lb can tolerate approximately 35% higher voltage than
persons weighing 110 lb, and tolerable step voltages are
generally much higher than touch voltages for similar
conditions. For these reasons, the most conservative case
of touch voltages for persons weighing 110 lb will be
addressed below.
A simplified formula for tolerable rms ac touch volt-
age as a function of exposure duration t is provided in
(17) on p. 20 of the IEEE Standard 80, which is also shown
below. This formula is based on an equivalent human
body resistance from hand to feet and hand to hand of
1,000 X. The resistance to remote earth of the human
foot is conservatively represented as an equivalent con-
ducting metallic disc.
Etouch ¼ IB(RB þ 1:5q), (1)
where RB ¼ 1,000 X (equivalent resistance of the human
body), q is the electrical resistivity in ohm meters for the
material on which the person is standing (assumed here
to be a homogeneous material), and IB is the tolerable
body current in amperes for a person weighing 110 lb
(equal to 116=ffiffi
tp
).
It is important to note that the tolerable current in (1)
is based on tests limited to the time range t ¼ 0:03� 3:0 s.
The results for touch voltage are therefore only valid for
this time range; the IEEE Standard 80 does not provide
tolerable voltages for continuous exposure.
Values of touch voltage for some representative low
values of material resistivity q taken from Tables 7 and 8 of
Standard 80 are provided below for several arbitrary expo-
sure durations. The touch voltages in the metal-to-metal
contact column are highly conservative, intended for
application to hand-to-hand shock situations only. In
order for the metal-to-metal contact values to apply to a
hand-to-foot shock situation, the person’s feet would need
to be in direct contact with remote earth, which may not
be physically possible in dc substation or passenger sta-
tion environments.
The ac touch voltages calculated in Table 1 conserva-
tively assume that hand-and-foot contact resistances are
equal to zero and that glove and shoe resistances are also
equal to zero. In addition, they do not include the benefi-
cial effects of a thin layer of high-resistivity material added
between the above-listed homogenous materials and the
feet of persons to increase their contact resistance, such
as track ballast, asphalt, or platform-insulating materials.
The latter technique, which is addressed in section 7.4 of
TABLE 1 Tolerable ac touch voltages in volts (rms) per (1).
Time (s)
Metal-to-MetalContactq ¼ 0
Wet OrganicSoilq ¼ 10
Wet ConcreteLow-Rangeq ¼ 21
Wet ConcreteHigh-Rangeq ¼ 100
Dry Soilq ¼ 1;000
0.03 670 680 691 770 1,6740.05 519 527 535 597 1,2970.10 367 372 378 422 9170.5 164 167 169 189 4101.0 116 118 120 133 2903.0 67 68 69 77 167
RESEARCH INDICATES THAT THE HUMANBODY CAN TOLERATE SLIGHTLY HIGHER25-HZ CURRENT AND APPROXIMATELY FIVETIMES HIGHER DC.
SEPTEMBER 2011 | IEEE VEHICULAR TECHNOLOGY MAGAZINE ||| 101
the IEEE Standard 80, can significantly increase the tolera-
ble touch voltage.
The IEEE Standard 80 makes some qualifying state-
ments that are important for engineers involved with the
grounding of dc traction power systems to be aware of,
two of which are provided below.
n ‘‘This guide is primarily concerned with outdoor ac
substations. . . ’’ (Scope, p. 1)
n ‘‘This guide is primarily concerned with safe ground-
ing practices for power frequencies in the range of 50–
60 Hz. The problems peculiar to dc substations. . . are
beyond the scope of this guide’’ (Purpose, p. 2).
The tolerable voltages that can be derived from
Standard 80 are therefore applicable for 50–60 Hz frequen-
cies only and for exposure durations between 0.3 and 3.0 s.
The only substantial reference to application at other fre-
quencies can be found on p. 11: ‘‘Research indicates that
the human body can tolerate a slightly higher 25 Hz current
and approximately five times higher direct current.’’
European Standards for Rail Potential
The IEC 62128-1:2003 Standard, Railway Applications—
Fixed Installations—Part 1: Protective Provisions Relating to
Electrical Safety and Earthing [6], is the standard predomi-
nantly referenced in Europe for the design of electrified rail-
way facility grounding. This standard is identical to the
European Standard EN 50122-1:1997 [8] except for page
numbering and formatting, hence all references made to
IEC 62128-1 in this article apply to both standards. The
‘‘Scope’’ section of this standard begins with the following
statement: ‘‘This standard specifies the requirements for
the protective provisions relating to electrical safety in
fixed installations associated with ac and dc traction sys-
tems and to any installations that may be endangered by
the traction supply system. It also applies to all fixed instal-
lations that are necessary to ensure electrical safety during
maintenance work within electric traction systems.’’
The IEC 62128-1 provides tables of maximum tolerable
(permissible) ac and dc voltages versus exposure dura-
tion rather than equations. The substantial technical basis
for these tables is contained in IEC 60479-1:2005, Effects of
Current on Human Being and Livestock—Part 1: General
Aspects [7]. The introduction to the current (fourth)
edition of IEC 60479-1 contains the following informational
statements: ‘‘IEC 60479-1 contains information about body
impedance and body current thresholds for various physi-
ological effects. This information can be combined to
derive estimates of ac and dc touch voltage thresholds for
certain body current pathways, contact moisture condi-
tions, and skin contact areas,’’ and ‘‘On the evidence avail-
able, mostly from animal research, the values are so
conservative that the standard applies to persons of nor-
mal physiological conditions including children, irrespec-
tive of age and weight.’’ The extensive research on which
IEC 60479-1 is based concludes that the impedance of the
human body varies with touch voltage magnitude as well as
with current frequency and duration. For dc current, the
fibrillation threshold is also significantly higher for currents
flowing downward through the body than for currents flow-
ing upward. This results in a more complex electrical model
of the human body than the one incorporated into the IEEE
Standard 80. The research includes investigations with dc
as evidenced by the following statement in the ‘‘Scope’’ sec-
tion: ‘‘Accidents with dc are much less frequent than would
be expected from the number of dc applications, and fatal
accidents occur only under very unfavorable conditions,
for example, in mines. This is partly due to the fact that with
dc, the let-go of parts gripped is less difficult and that for
shock durations longer than the period of the cardiac cycle,
the threshold of ventricular fibrillation is considerably
higher than for alternating current.’’
The tables of permissible voltages in IEC 62128-1 refer to
several key technical terms that reflect the dynamic nature
of an electrified railway environment. An understanding of
these terms is necessary for the correct application of this
standard, and verbatim definitions of these terms from sec-
tion 3 of IEC 62128-1 provided for reference are:
n Rail potential: voltage between running rails and earth
occurring under operating conditions when the run-
ning rails are used for carrying the traction return or
under fault conditions
n Touch voltage: the voltage under fault conditions be-
tween live parts when touched simultaneously
n Accessible voltage: that part of the rail potential under
operating conditions that can be bridged by persons,
the conductive path being conventionally from hand
to both feet through the body or from hand to hand
(horizontal distance of 1 m to a touchable part)
n Short time conditions: �0.5 s (for short circuits)
n Temporary conditions: 0.5 < t � 300 s
n Permanent conditions: >300 s
n Voltage-limiting device: protective device against
permanent existence of an inadmissible high touch/
accessible voltage.
The values of permissible ac and dc voltages contained
in IEC 62128-1 are provided in Tables 2 and 3. These vol-
tages are based on the following assumptions contained in
IEC 60479-1:
n Current path: from one hand to both feet
n Total body impedance: 50% of the population (50th
percentile rank)
n Probability of ventricular fibrillation: 0% (curve c1 on
Figure 22 of 60479-1)
IEC 62128-1 PROVIDES MAXIMUMPERMISSIBLE RAIL POTENTIAL DC VOLTAGESVERSUS EXPOSURE TIME FOR SHORT-TIME,TEMPORARY AND PERMANENT CONDITIONS.
102 ||| IEEE VEHICULAR TECHNOLOGY MAGAZINE | SEPTEMBER 2011
n Direction of current flow: upward (feet positive to
hand negative)
n Hand and foot contact resistance: zero for temporary
and permanent conditions
n Body impedance: it includes an additional 1,000 X for
short time conditions (the equivalent resistance of
old wet shoes)
n Surface layer resistance is not included: for temporary
and permanent conditions, these voltages represent
the metal-to-metal contact scenario in IEEE Standard
80 (in other words, these voltages are highly conserv-
ative for hand-to-feet contact situations since they
assume direct contact of the feet with remote earth).
Control of Rail Potential
IEC 62128-1 provides maximum permissible rail potential
dc voltages versus exposure time for short-time, tempo-
rary, and permanent conditions. These maximum voltages
are based on safety-related considerations only. Clearly,
stray current mitigation considerations must also factor
into the selection of the maximum rail potential levels for
a dc traction power system.
Maximum rail potentials are ideally controlled through
careful system design, involving parameters such as sub-
station spacing, rail return circuit longitudinal and shunt
(leakage) resistances, and the use of electrical insulating
materials at safety-critical locations. However, in many
cases, this ideal approach is not always practical and may
even be cost prohibitive, particularly when abnormal
service conditions such as substation outages and train
bunching (catch-up service) are addressed. In these situa-
tions, rail potential control devices (RPCDs) can be
employed. An RPCD is in essence a modern, electronic ver-
sion of the venerable spark gap, which temporarily shorts
out (clamps) the protected circuit when a voltage time–
current threshold is exceeded. These devices are fre-
quently used in European dc traction power systems,
where they are termed voltage-limiting devices in accord-
ance with the definitions and technical requirements of
IEC 62128-1. Voltage-limiting devices range from simple
thyristor-equipped voltage clamping devices to large thyr-
istor/contactor hybrid assemblies employing numerical
control. However, to qualify as an IEC-compliant voltage-
limiting device, all must provide voltage limiting in accord-
ance with the maximum permissible short-time, tempo-
rary, and permanent voltage tables in IEC-62128-1
(equivalent to Tables 2 and 3). In other words, these time–
current curves must be incorporated into compliant volt-
age-limiting devices.
When they are needed, voltage-limiting devices are typ-
ically installed at safety-critical locations such as station
platforms and train storage/maintenance and shop areas.
At station platforms, they are connected between the sta-
tion platform grounding system and the running rails in
accordance with a practice that is defined in IEC 62128-1
as open traction system earthing: the ‘‘connection of con-
ductive parts to the traction system earth (running rails)
by a voltage-limiting device or by circuit breakers, which
make a conductive connection either temporarily or per-
manently if the limited value of the voltage is exceeded.’’
They can also be connected to individual conductive
structures if deemed necessary (IEC 62128-1 defines an
‘‘overhead contact line zone’’ and a ‘‘pantograph zone’’
within which wholly or partially conductive structures
must be protected from impermissible touch voltages).
When an impermissible voltage between the rails and
the platform ground is sensed by the voltage-limiting
device, it shorts the platform ground to the rails within the
time requirements of IEC 62128-1, equalizing the voltage
between them. This voltage could be the result of an opera-
tional current (a rail voltage rise) or a positive-to-earth
ground fault (local earth voltage rise). If the voltage-limiting
device is to operate for both conditions, it must have bidir-
ectional capability. It reopens after a time delay if the
conducted current is below an acceptability threshold.
TABLE 2 Maximum permissible touch voltages.
Time (s) Volts rms ac Volts rms dc
0.02 940 9400.05 935 7700.10 842 6600.20 670 5350.30 497 4800.40 305 4350.50 225 395
TABLE 3 Maximum permissible accessible voltages.
Time (s) Volts rms ac Volts rms dc
0.60 160 3100.70 130 2700.80 110 2400.90 90 2001.00 80 170�300 65 150>300* 60 120
* IEC 62128 1 notes that accessible voltages in workshops shallnot exceed 25 Vac or 60 Vdc. These lower values are intendedto lessen the chance that a nonlethal shock to a worker usingshop equipment could result in the worker being injured by theequipment, rather than by the shock itself.
IN EUROPE, A STANDARD HAS BEENDEVELOPED SPECIFICALLY TO ADDRESS THECONTROL OF VOLTAGES BETWEEN RAILSAND STRUCTURES.
SEPTEMBER 2011 | IEEE VEHICULAR TECHNOLOGY MAGAZINE ||| 103
The need for and location of dc RPCDs are best deter-
mined by a traction power load flow simulation. A dc rail-
way traction power simulation program that correctly
models the electrical interaction (leakage resistance)
between the negative return system, remote earth, and
the substation grounding system under peak service con-
ditions can determine maximum rail potentials with and
without the use of RPCDs [2]. It can also be used to deter-
mine the most appropriate locations of these devices
(their zones of influence on the right of way) and the
impacts of different voltage threshold triggering settings
on nearby accessible voltages and the potential stray dc
currents that may result during device closure.
Negative Grounding Devices
In the United States, at present, the use of RPCDs has been
primarily limited to dc traction substation locations.
When installed in substations, RPCDs are connected
between the dc negative bus and the substation grounding
grid. For this reason, RPCDs located in traction substa-
tions are commonly referred to as NGDs, a term which will
be used herein.
In addition to limiting the rail potential in the vicinity of
the substation, NGDs can assist in the detection and clear-
ing of positive-to-earth ground faults external to the dc
switchgear. A very simplified circuit diagram illustrating a
typical NGD arrangement for a 750 Vdc nominal system is
shown in Figure 1. Figure 1 illustrates the result of a OCS-
to-ground fault, although the result will be the same for
any form of dc ground fault. The NGD is normally in an
open state (nonconducting). As long as it remains open,
significant fault currents cannot flow back to the substa-
tion dc negative bus since there is only a very high-resist-
ance return path available to it. A small amount of fault
current will flow into the rails near the fault via the leak-
age/shunt resistance of the rails in proportion to how well
they are insulated from earth. Some fault current may also
return to the negative bus through stray current drainage
circuits, but drainage circuits are typically avoided on
modern dc traction systems. After the NGD senses a trig-
gering voltage difference across it and closes, the fault cur-
rent will flow through the earth into the substation
grounding grid. The fault current will be limited by the
resistance of the grounding grid to remote earth, Rg . For
example, if the grid resistance is 1 X and the other typi-
cally smaller circuit resistances are neglected, the ground
fault current will be 750 V/1 X or 750 Adc or 750 Amperes
dc. This is a low value of current for purposes of protec-
tive relaying; it is clear from this example that the substa-
tion grounding grid resistance Rg must be made as low as
practicable for the NGD to work effectively.
Negative Grounding Device Application
Voltage Threshold Settings
When safety is the primary consideration, NGD voltage
threshold settings in accordance with IEC 62128-1 appear
most appropriate since the U.S. standards do not address
dc traction power system rail potential at present. Refer-
ence [2] indicates that NGD settings that are too low (on
the order of 50 Vdc) will result in no decrease in rail poten-
tial as well as increased stray dc currents. Reference [2]
also notes that the effectiveness of NGDs in controlling
accessible voltages is greatly improved by a low ground-
ing grid resistance Rg ; a maximum Rg of 0.5 X is recom-
mended by these authors for NGD effectiveness, and they
also note that a dramatic improvement occurs with an Rg
of 0.10 X or less. Reference [2] describes a threshold crite-
rion termed rail voltage limit, which can be determined by
the load flow simulation of a dc traction power system;
this is the setting below which NGDs will offer no reduc-
tion in accessible voltage. As noted above, the voltage
sensing and device operation should be bidirectional to
accommodate the various modalities of rail potential.
The design criteria for several transit agencies in the
United States that use NGDs specify triggering thresholds
as low as 50 Vdc. With respect to the information given
750 Vdc OCS or Contact Rail
Faul
tC
urre
nt
Gro
und
Faul
tC
urre
nt l f
Ground Fault
+
Vf
Running Rails
750 VdcPositive Bus
DC NegativeBus
Any Fault CurrentReturning ThroughRails Bypasses theNGD
NegativeGrounding
Device
Substation Grounding Grid
Rg (Grounding Grid Resistance toRemote Earth)
Vg+
Ground Fault Current Returning to SubstationThrough Grounding Grid Encounters TypicalGrounding Grid Resistance of 0.5–2 Ω.
750 VdcRectifier
+ –
–
–
FIGURE 1 NGD response to a ground fault.
RAIL POTENTIAL IS THE DIFFERENCEIN VOLTAGE BETWEEN THE TRACKS(STEEL RUNNING RAILS) AND GROUND(REMOTE EARTH).
104 ||| IEEE VEHICULAR TECHNOLOGY MAGAZINE | SEPTEMBER 2011
earlier, this low of a threshold may not be necessary for
safety reasons, may not actually reduce rail potential, and
may be contributing to excessive stray current flow due to
sustained and/or frequent operation.
Short-Circuit Current Rating
The NGD temporarily connects the substation negative
bus to the substation grounding grid when in operation.
For this reason, it must be able to close into, and with-
stand (but not interrupt), the worst-case ground fault cur-
rent; ground fault current interruption is performed by
the substation feeder breakers rather than the NGD. The
highest ground fault current through an NGD will normally
occur when the fault is near a substation, with approxi-
mately 100% of the resulting fault current returning to the
substation grounding grid through the earth (any fault
current returning through the rails would not pass
through a closed NGD, as can be seen in Figure 1).
If the source, feeder, OCS and fault (arc) resistances are
neglected, the resulting worst-case ground fault current
through the NGD would be approximately equal to the dc
bus voltage divided by the grounding grid resistance Rg .
This assumes that there are no alternate lower resistance
paths back to the substation ground grid, such as structure
rebar or stray current drainage circuits. If these exist, then
the magnitude of ground fault current through the NGD
could be higher. Calculation of Rg for new substations, and
the measurement of Rg for existing substations, is
addressed in IEEE Standards 80 [1], [3].
Continuous Current Rating
The NGD requires a continuous current rating, at least,
equal to the expected stray current that will return to the
substation when the substation’s negative bus and
grounding grid are connected via the NGD. A continuous
rating is needed for the situation in which the NGD either
fails closed or locks out and therefore remains closed for a
substantial period of time.
Dielectric Withstand Ratings
When the NGD is open, it will have the substation ground
potential rise (GPR) voltage across its terminals when a
substation ac ground fault occurs. The NGD must there-
fore be insulated for this dielectric withstand require-
ment. Calculations of substation GPR during ac ground
faults is addressed in the IEEE Standard 367 [4].
Monitoring
When equipped with recording capability, NGDs can
provide useful data related to traction power system
behavior under normal service, abnormal service, and
equipment contingency conditions. This data include rail
potential levels, ground fault current magnitudes, and
stray current activity and can be used to verify design
assumptions and criteria. Frequent operation can also
serve as an indicator of gradual breakdowns in positive or
negative return system insulation or as an indicator that
the triggering thresholds may be too low.
Conclusions
Presently, the U.S. standards do not address the electrical
safety and grounding aspects of rail potential specific to dc
traction power systems. This lack of standardization may be
contributing to uncertainty in the United States about
acceptable levels of rail potential, as well as the need for,
and the application of NGDs. International Standard IEC
62128-1:2003, Railway Applications—Fixed Installations—Part
1: Protective Provisions Relating to Electrical Safety and Earth-
ing, is a comprehensive, mature standard that provides the
necessary guidance specific to dc traction power systems.
Until then, as the U.S. standard is developed for this applica-
tion, it is suggested that IEC 62128-1 be referenced.
Author Information
R.W. Benjamin Stell ([email protected]) re-
ceived his bachelor’s and master’s degrees in electrical
power engineering from Northeastern University in 1985
and 1994, respectively. He is a specialist in the planning,
design, and construction of railway electrical systems
for STV, Inc. in Philadelphia, Pennsylvania. He is cur-
rently the chair of IEEE Traction Power Substation Sub-
committee Working Group 22, Traction Power Rectifiers.
His professional experiences include the development of
load flow analysis programs and the performance of sys-
tem planning studies for light- and heavy-rail traction
power systems.
References[1] IEEE Guide for Safety in Substation Grounding, IEEE Standard 80-2000.[2] M. T. Soylemez, S. Acikbas, and A. Kaypmaz, ‘‘Controlling rail poten-
tial of DC supplied rail traction systems,’’ Turk. J. Electr. Eng., vol. 14,no. 3, pp. 475–484, 2006.
[3] IEEE Guide for Measuring Earth Resistivity, Ground Impedance, andEarth Surface Potentials of a Ground System, IEEE Standard 81, 1983.
[4] IEEE Recommended Practice for Determining the Electric Power StationGround Potential Rise and Induced Voltage From a Power Fault, IEEE
Standard 367, 1996.[5] Railway Applications—Fixed Installations—Part 1: Protective Provisions
Relating to Electrical Safety and Earthing, European Standard EN50122-1, 1997.
[6] Railway Applications—Fixed Installations—Part 1: Protective ProvisionsRelating to Electrical Safety and Earthing, International Standard IEC62128-1, 2003.
[7] Effects of Current on Human Being and Livestock—Part 1: GeneralAspects, International Standard IEC 60479-1-1, 2005.
[8] Railway Applications—Fixed Installations—Electrical Safety, Earthingand Bonding—Part 1: Protective Provisions Relating to Electrical Safetyand Earthing, British Standard European Standard 50122-1, 1998.
[9] IEEE Guide for Measurement of Impedance and Safety Characteristics ofLarge, Extended or Interconnected Grounding Systems, IEEE Standard81.2-1991.
RAIL POTENTIAL CONTROL DEVICES BUILT TOCOMPLY WITH IEC 62128-1 ARE TERMEDVOLTAGE-LIMITING DEVICES.
SEPTEMBER 2011 | IEEE VEHICULAR TECHNOLOGY MAGAZINE ||| 105
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