1077-2618/10/$26.00©2010 IEEE

Benefits of resistancegrounding and continuous

monitoring of neutral-grounding resistors

ARESISTANCE-GROUNDED POWER

system has a critical element that is often

ignored—the neutral-grounding resistor

(NGR). A resistance-grounded power sys-

tem should have its NGR continuously monitored. Dur-

ing a single-phase-to-ground fault, current flows from the

transformer or generator winding through the faulted-phase

conductor to the fault and to ground,

returning to the sourcewinding through

the ground-return path and the NGR.

When an NGR fails, the failure mode

is usually open circuit, leaving the ground-return path

open. Current-sensing ground-fault protection, which

is the type most commonly employed in a resistance-

grounded system, will not operate with an open resistor,

and the advantages of resistance grounding are unknow-

ingly lost. Inadvertent operation with an ungrounded

system and inoperative ground-fault protection can be

avoided by using a continuous NGR monitor. This arti-

cle reviews the benefits of resistance grounding, compares

continuous resistor monitoring with planned-maintenance

testing and inspection, and defines the characteris-

tics required of an NGR monitor. Case studies are pre-

sented that show the need for continuous monitoring of

the NGR.

NGR Monitoring

Resistance grounding has been used

in three-phase industrial applications for many years. Properly

designed resistance grounding eliminates many of the prob-

lems associated with solidly and ungrounded systems while

retaining their benefits.

Properly applied resistance grounding can limit point-

of-fault damage, eliminate transient overvoltages, reduce

the risk of an arc flash, provide continuity of service with a

ground fault, and provide adequate current for ground-fault

detection and selective coordination.Digital Object Identifier 10.1109/MIAS.2010.937437

BY DONALD SELK I RK ,MERV IN SAVOST IAN IK ,& KENNETH CRAWFORD

© PHOTODISC

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Unfortunately, the advantages of resistance groundingare replaced by the disadvantages of an ungrounded systemwhen an NGR fails. The consequence of inadvertentlyoperating with an ungrounded system can be avoided withthe addition of continuous NGRmonitoring.

In mining, the NGR is recognized as an integral part ofthe ground-return path, and NGR failures occur regularlyenough that the Canadian Standards Association mandatesNGRmonitoring [1].

A well-designed NGR monitor confirms the electricalpath from the transformer or generator neutral through theNGR to the station ground. When powered from a sepa-rate source, the monitor will be active whether or not thesystem is energized and ensures a trip or alarm if an NGRfailure occurs.

At the time of writing this article, there is no commerciallyavailable solution that provides continuous NGR monitoring

for anNGR isolated from the system neutral by a single-phasetransformer. The term resistance grounding is used here torefer to a resistor that is connected directly between thetransformer or generator neutral and the system ground.

NGR Failure ModeThe failure mode of an NGR is usually open circuit. Atypical NGR element (shown in Figure 1) is constructed ofresistance wire or metal strips coiled and wrapped aroundporcelain insulators. These assemblies are grouped as nec-essary for the application. Failure in short circuit is veryunlikely—it is analogous to an incandescent light bulbfailing in short circuit.

An NGR can be short circuited accidentally duringconstruction or maintenance. Unlike an open-circuitedNGR, a short-circuited NGR results in a grounded andstable electrical system. Ground-fault current will flowduring a ground fault, and the fault will be cleared byground-fault or overcurrent protection.

Causes and Frequency of NGR FailureAn NGR is a mechanical component and is subject tomechanical failures. Although NGRs have no moving parts,there are several other factors that affect the integrity of anNGR. A review of NGR manufacturers’ data reveals thatNGRs fail because of lightning, storms, earthquakes, over-loads, or extended service life [2]. Other causes of failure arecorrosive atmospheres, extreme temperature changes, triplenharmonic currents, hail, manufacturing defects, and vibra-tion. Figure 2 shows anNGR thermal failure.

Every device has a useful service life, and NGRs are nodifferent. Figure 3 shows an NGR in an outdoor location

1An NGR element. (Courtesy of Powerohm Resistors Inc.)

2An NGR thermal failure. (Courtesy of Startco Pty.)

3An NGR in an outdoor mesh enclosure. (Courtesy ofLittelfuseStartco.)54

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and housed in a steel-mesh enclosure. Outdoor-installedequipment is subject to the local extremes of temperatureand humidity. In an installation such as this, the resistorelements, insulators, terminals, welded joints, and wiringare exposed to falling and wind-driven rain, snow, and hail.

A previously unknown cause of an NGR failure wasreported by Labos et al. [3]. The authors reported severalNGR failures on a 26-kV utility substation. The probablecause for the failures is high-frequency (125 kHz) oscilla-tion involving the inherent inductance of the NGR andsystem capacitance, with the oscillation of the systembeing triggered by transients produced by line-to-groundfaults on 26-kV feeder circuits. The overvoltages werefound to be two to four times the system voltage.

While writing this article, one of the authors conducteda study of the NGRs at his facility. The 60-year-old chemicalplant has 80 substations of various ages. Ten are high-resist-ance grounded. Two open NGRs were discovered, yielding a20% failure rate. Table 1 summarizes this information.

Figures 4 and 5 show the failed NGRs. In Figure 4, abroken spot weld resulted in an open NGR. In Figure 5, abroken resistor element wire resulted in an open NGR.

Consequences of an NGR FailureFailure of the NGR converts a resistance-grounded systeminto an ungrounded system. Without continuous NGRmonitoring, there is no indication that the system hasbecome ungrounded. Operators would not be aware thatcurrent-sensing ground-fault protection is no longer opera-tional and that the risk of transient overvoltages exists.

Another potential hazard is that a phase-to-ground voltagetest on an ungrounded systemmay not indicate the presence ofvoltage on an energized conductor. An electrician could thenbelieve that energized phase conductors were safe to touch [4].

NGR Failure Detection MethodsAn open-circuited NGR may not show external signs offailure. In cases where continuousmonitoring is not employed,the system usually continues to operate until the open resistoris discovered following an event.

Occasionally, open resistors are discovered during prevent-ative maintenance. In cases where the maintenance planinvolves measurement of the NGR resistance, an open NGRwould likely be discovered. However, there are no guarantees.There is an example from a maintenance company of anNGR with a weld that had broken. The NGR remainedclosed until a ground fault occurred. I2R heating of theedge-wound NGR coils caused them to expand, whichthen forced the broken weld to open. This illustrates theneed for an NGR monitor to detect an NGR failure on asystem where ground faults are indicated only by an alarm.Measurement of NGR resistance during maintenance onlyprovides confirmation that the NGR was good at the time

when the resistance was measured. The NGR could fail atany time after the measurement is taken or even not bereconnected after the measurement.

Where maintenance involves testing ground-fault relaysby using an intentional ground fault, an open NGR wouldlikely be discovered as the result of an investigation intowhy ground-fault relays failed to operate.

When ground-fault relays are tested by primary currentinjection, as shown in Figure 6, false confirmation of ground-fault relay operation can occur. The ground-fault relays willrespond to the injected current and appear to operate as

5A broken resistor element wire. (Courtesy of DuPont USA.)

TABLE 1. EXPERIENCE OF ONE CHEMICAL PLANT.

n 80 substations

n 10 high-resistance grounded

— 2 open NGRs

— 20% failure rate

4A broken spot weld. (Courtesy of DuPont USA.)

CT

CurrentSource

OpenNGR Ground-Fault

Monitor

6Primary current injection testing.

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designed, despite the presence of an openNGR. This maintenance procedure wouldthen have falsely confirmed the operation ofground-fault protection.

Planned maintenance is not the idealmeans of determining NGR health. Anopen NGR is not a condition that shouldbe allowed to remain on the system for anylength of time. Further, there have beencases where NGRs disconnected for testingwere not reconnected. In an example froma large industrial facility, eight NGRs weredisconnected for transformer testing. For-tunately, shortly before the system was tobe reenergized, it was discovered that fiveof the NGRs had not been reconnected.

Examination of NGR preventativemaintenance suggests that an automaticmonitoring device is a better solution.

A continuous NGRmonitor, as shownin Figures 7 and 8, detects an open NGRwhen the failure occurs. A continuousNGR monitor is active when controlpower is applied and indicates NGR health whether or notthe system is energized, with or without a ground fault.

Continuous Monitoring RequirementsThe requirements of a continuous NGR monitor are thefollowing [5]:

n An NGR monitor must be capable of detecting anNGR failure whether or not there is a ground-faulton the system.

n The monitor must work in tripping or alarm-onlysystems.

n The monitor must not be exposed to neutral volt-age during a ground fault.

n Monitoring the NGR should include monitoringthe neutral and ground connections.

n The monitor should not be capaci-tively or inductively coupled to theNGR as this could contribute toferroresonance if the resistor fails.

n If ground-fault protection is in-cluded, the monitor should becapable of detecting a ground faultwhen the NGR is open.

A potential transformer and a time-delay voltage relay connected across anNGR is not continuous NGR monitor-ing. This is a voltage-based ground-faultdetector, and it does not monitor NGRcontinuity. Rather, it monitors neutralvoltage and will not operate until aground fault occurs, regardless of thecondition of the NGR.

A similar approach is to apply an over-voltage measurement (59 N), an overcur-rent measurement (51 N), and a logiccircuit to the NGR as shown in Figure 9.The combination of these devices candetect an NGR failure only if there is a

ground fault on the system. If the voltage measurementindicates the presence of voltage and the current measure-ment indicates the presence of current, then there is aground fault on the system. Since there is current flowingthrough the NGR, it must be continuous. If the voltagemeasurement indicates the presence of voltage but themeasured current in the NGR is zero, then the NGR mustbe open. This design is not continuous NGR monitoringbecause it relies on the presence of a ground fault to operate.

This method has been tested in industry. A device simi-lar to that shown in Figure 9 was applied on a 69-kV sys-tem in a Canadian mining operation. Without groundfault on the system, the device was not able to determineNGR continuity.

Another possibility is the use of a resistance measure-ment, similar to an ohmmeter, and anoutput contact as an NGR monitor(shown in Figure 10). Problems withthe resistance measurement methodare the possibility for interference byexternal dc influences, as shown inFigure 11, and the possibility thatcontinuity through a ground faultmay be recognized as NGR continu-ity, as shown in Figure 12.

The resistance measurement canbe influenced by dc currents in themeasuring circuit. There are manypossible sources of stray dc voltage inindustrial electrical systems: dc canbe impressed on the system by sour-ces such as a ground fault on the dcbus of an adjustable speed drive andmore obscure sources such as atmos-pheric electrical conditions. Figure 11shows a charged cloud that impresses adc voltage on the system. Under theright conditions, this charged cloudcan interfere with the resistance

NGR

51 N

59 N

Ω

7Continuous NGR-monitormeasurements.

L1

L2

NGR Monitor

SensingResistor

CT

NGR

Neutral

NGR Enclosure

Transformer or Generator

GroundFault

ResistorFault

8Continuous NGR monitor.

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measurement to a degree that wouldcause it to fail to operate as designed.

Another problem with the resist-ance measurement method is the possi-bility of measuring continuity througha ground fault. When a ground faultoccurs, there are two parallel pathspresented to the resistance-measuringdevice. There is a path through theNGR and another path through thetransformer or generator winding of thefaulted phase to, the fault, and back tothemeasuring device through ground.

If the NGR fails during a groundfault, the resistance-measuring devicecould measure a resistance similar tothe NGR through the ground fault.Figure 12 shows how a resistance-monitoring device could be satisfiedby measuring resistance through aground fault. Under these circum-stances, a resistance measurementwould falsely indicate that the NGR was continuous.

A better solution is the combination of an overvoltagemeasurement (59 N), an overcurrent measurement (51N), and a resistance measurement as shown in Figure 7.This solution is better because it continuously monitorsNGR continuity.

A continuous NGR monitor combines measured NGRcurrent, transformer or generator neutral voltage, andNGR resistance to continuously determine the health ofthe NGR. Figure 8 shows a typical NGR monitor and theconnections to the NGR.

The resistance measurement is the sum of the resistancefrom the sensing resistor to the neutral point, through theNGR to ground, and through ground back to the monitor.Connecting the sensing resistor to a separate lug on theneutral bus assures that the NGR connection to the neutralpoint is part of the monitored loop.

When there is no ground fault on the system, ameasurement of NGR resistance is enough to confirmNGR continuity. The monitor determines the presence of aground fault through the voltage andcurrent measurements. Voltage on theneutral and current in the NGR indi-cates a ground fault.

When a ground fault is present, aresistance measurement is not suffi-cient to confirm NGR continuitybecause of the possibility of measuringcontinuity through the fault as shownin Figure 12. Because a resistancemeasurement alone is not sufficient toconfirm NGR continuity, the monitorconstantly evaluates resistance, cur-rent, and voltage measurements.

When neutral voltage is elevatedand current is flowing through theNGR, the NGR must be continuous.When the neutral voltage is elevatedbut no current flows through theNGR, the NGR must be open. The

ability to detect an open NGR in thepresence of a ground fault is particu-larly important in alarm-only sys-tems where ground faults can remainon the system for long periods.

When a ground fault occurs in aresistance-grounded system, voltageappears on the system neutral. In thecase of a bolted fault, the transformeror generator neutral elevates to line-to-neutral voltage. An NGR monitorthat is directly connected to the sys-tem neutral brings a conductor withline-to-neutral voltage during a groundfault into a low-voltage control cubicle.This is not acceptable in many applica-tions. The sensing resistor, as shown inFigure 8, connects the monitor to thepower system while isolating it fromneutral voltage. The sensing resistorlimits the voltage transfer from thesystem neutral to the NGR monitor.

The resistive elements of the sensing resistor will not contrib-ute to ferroresonance. Ferroresonance is a condition in whichmagnetic saturation of an iron core working in conjunctionwith system capacitance results in voltage oscillations. Theseoscillations can produce overvoltages, which may exceed twoto three times system voltage [6]. A failure of the sensingresistor results in a resistor-fault trip because the resistancemeasurement reads an open circuit.

NGR Failure Case Studies

Mine Safety and Health Administration Fatality ReportThe following event occurred at an underground coal mine inVirginia on 11 November 1991. Several factors contributed toa fatality. The victim was not qualified to perform electricalwork, the circuit was not locked and tagged out, a trailing-cablemonitor was defeated, there was an existing ground fault,and theNGRwas open.

A loaded, cable-powered shuttle car was trammingaway from a continuous mining machine when its cable-

reel-motor drive chain came off thesprocket. The cable reel locked, andthe forward momentum of the trampulled the portable power cable apartat a splice. The victim touched anexposed phase conductor with his barehand several times and then groundedthe exposed phase conductor againstthe mine rib to determine whether thecable was energized. The victim wasthen electrocuted when he contactedthe energized conductor and the minefloor, by current flowing phase to phasethough the existing ground fault,ground, and the victim [7].

There were several unsafe conditionsthat, had they been corrected, wouldhave saved the victim; the primary ofwhich is the failure to lock-out and tag-out the circuit breaker.

Logic

NGR

51 N

59 N

9Ineffective NGR monitoring method.

NGRΩ

10Resistance measurement as an NGRmonitor.

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Had the mine employed continuous NGR monitoring,the NGR-monitoring relay would have deenergized thecircuit when the NGR opened.

Soft Starter FailureA soft starter manufacturer reported that its soft startersoperated in an unexpected manner when an NGRwas open.

The soft starter responded to reference pulses from eachof the three line-to-ground voltage signals. With the neu-tral properly grounded, all three voltage signals are bal-anced. Failure of the NGR allows the neutral voltage tofloat and the three voltage signals to become unbalanced.The unbalanced voltages were interpreted by the softstarter as a single-phase condition that caused a trip.

Upon studying the problem the manufacturer con-cluded that the problem was due to unbalanced referencevoltages. Through further study and customer interviews,

it was determined that an open NGR was responsible forthe unbalanced reference voltages.

Had the customer employed continuous NGRmonitor-ing, the open NGR would have been discovered before itcaused problems with the soft starter.

Loose Connections on an NGRIn Canadian mining operations, continuous NGR moni-toring is required. The following incident occurred at amine in Eastern Canada on a 200-A, 4,160-V NGR.

The NGR monitoring relay tripped on a resistor fault.Upon inspection, electricians noticed a loose connection onthe NGR. By moving the loose connection, the electricianswere able to reset the resistor fault on the monitor and thencause the monitor to trip on resistor fault again. The looseconnection was tightened, and the rest of the NGR con-nections was inspected. Eight other loose connections werefound and repaired. The equipment was restarted, and theresistor fault trip did not reoccur.

ConclusionsThe information presented here has shown that an openNGR is an undesirable situation. A system with an openNGR is subject to transient overvoltages, and so current-sensing ground-fault protection will not indicate the pres-ence of a ground fault. A ground fault then remains on thesystem and might escalate to a phase-to-phase fault.

NGRs are subject to failures related to thermal overload,lightning, storms, earthquakes, wildlife, extended servicelife, manufacturing defects, vibration, corrosion, and improperspecification or installation.

A well-designed NGR monitor provides continuousprotection against failures that previously renderedground-fault protection, coordination, and annunciationsystems inoperative, as well as leaving the system exposedto damaging transient overvoltages. An NGR monitorprovides confidence that current-sensing ground-fault pro-tection will operate as designed on the next ground fault.

References[1] CSA International, “M421-00 use of electricity in mines,” 2000.[2] P. Glover, “Neutral grounding resistors installation and maintenance

instructions,” PGR Document #NG111-06.[3] W. Labos, A. Mannarino, G. Drobnjak, S. Ihara, and J. Skliutas, “A

possible mechanism for neutral grounding resistor failures,” in Proc.Power Engineering Society General Meeting, June 2005.

[4] K. S. Crawford and N. K. Haggerty, “Test before touch—Easiersaid than done,” IEEE Ind. Applicat. Mag., vol. 14, no. 3, pp. 32–39,May/June 2008.

[5] G. E. Paulson, “Monitoring neutral-grounding resistors,” in Proc. IEEEPulp and Paper Industry Committee Conf., June 1999, p. 240.

[6] D. Beeman, Industrial Power System Handbook, 1st ed. New York:McGraw-Hill, 1955, pp. 281–284.

[7] L. T. Ward and G. Deel, “Accident investigation report (undergroundcoal mine),” United States Department of Mine Safety and HealthAdministration, Norton, VA, Nov. 1991.

Donald Selkirk (don.selkirk@ieee.org) and Mervin Savostianikare with Littelfuse Startco in Saskatoon, Saskatchewan. KennethCrawford is with DuPont Company in Washington, WestVirginia. Selkirk, Savostianik, and Crawford are Members of theIEEE. This article first appeared as “Why Neutral-GroundingResistors Need Continuous Monitoring” at the 2008 IEEE Petro-leum and Chemical Industry Technical Conference.

OpenNGR

RFaultΩ

+

−

12Continuity through a ground fault.

NGRΩ

++

+ + +++

+ ++ +

+++

+

−

11Stray dc on the system.

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