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 (benjamin.stell@STVinc.com) 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.

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