System Neutral Grounding for Chemical

Plant Power Systems

equivalent network may be evolved forthe corroding cell. Once having deter­mined the network, the present-daymethods of circuit analysis were employedto establish the criterion for cathodic pro­tection. One important argument for theuse of such a theoretical approach resultsfrom the fact that the criterion for ca­thodic protection could have been deduced

D. S. BRERETONASSOCIATE MEMBER AlEE

THE chemical industry, because of itscontinuous processes, has always de­

manded reliable performance from itselectrical power system. Observation ofelectrical practices will show that carehas been exercised in the selection andapplication of electrical equipment. Thescope of this paper is to review the in­fluence which system neutral groundinghas on the performance of electrical ap­paratus, something of the history ofgrounded and ungrounded systems andwhy they have been selected, and a dis­cussion of the methods of system neutralgrounding with suggestions on how theover-all performance of the power systemand the connected equipment can be im­proved by the operation of some form ofsystem neutral grounding.

History of Industrial Grounding

System neutral grounding has alwaysbeen applied to some voltage level in theindustrial power field. In the low-volt­age class, the 208-volt (or better knownas the 120/208 Y-volt) system has, for allpractical purposes, been exclusively oper-

Paper 55-689, recommended by the AlEE Indus­trial Power Systems Committee and approved bythe AlEE Committee on Technical Operations forpresentation at the AlEE Fall General Meeting,Chicago, 111., October 3-7, 1955. Manuscript sub­mitted June 8, 1955; made available for printingJuly 20, 1955.

D. S. BRERETON is with the General Electric Com­pany, Schenectady, N. Y., and H. N. HlCKOK iswith the General Electric Company, Houston, Tex.

Acknowledgment is due to J. P. E. Arberry! of thePittsburgh Plate Glass Company, and to C. L.Eichenberg of the Bethlehem Steel Company,whose respective companies confirmed the prin­ciples put forth in this paper by their acceptance ofcorrect grounding practices.

directly from consideration of the equiva­lent network without resort to furtherfield research. I t is likely that the prob­lem of the so-called remote-earth referencecould also be considered in quite an analo­gous manner by the use of a multimeshplanar resistance network which will ap­proximate the field distribution of cur­rents and voltages.

H. N. HICKOKASSOCIATE MEMBER AlEE

ated as a grounded system. For themedium-voltage class, i.e., from 601 to15,000 volts, the voltage levels of 4,160,and 13,800 volts have predominantlybeen operated with the system neutralgrounding. The voltage levels of 480volts and 2,400 volts have experienced aconsiderable degree of ungrounded opera­tion in the past. The last 10 years haveseen a great increase in the application ofsystem grounding at the 480-volt levelwith a decidedly noticeable increase at2,400 volts in the last 3 years. In retro­spect, the most frequent reason given forthe ungrounded operation of these twovoltage levels has been the claim forgreater service reliability because therewill not be a tripout for a single line-to­ground fault. Years of experience inmany industrial fields have shown thatungrounded systems are not as reliable asgrounded systems. Such an example isthe experience cited by Arberry.! Afterstating the importance of continuousservice for the glass industry, he notes,"With continuous process operations thehunting of ground faults is very difficult,and two grounds on the same phase but ontwo different feeders are exceedingly dif­ficult to trace. This is because all thefeeders must be opened at once andclosed one at a time to find the trouble.Our experience is that the first groundfault remains on the system because wecannot open the feeder breakers to huntit. The result is that the system oper­ates with two phases at line-to-line volt­age-to-ground and the operating elec­trician hopes that no other ground occursbefore he has the opportunity to find thefirst one. It was because of our experi­ence such as I have mentioned, and the

References1. R. B. Mears. Journal and Transactions,Electrochemical Society, Baltimore, Md., vol. 95,no. 1, 1949.2. R. B. Mears, Brown. Journal, Ibid., vol. 97,no. 3, 1950. pp. 75-82.

3. Nehama, R. M. Wainwright. Report No.3,Cathodic Protection Laboratory, University ofIllinois, Urbana, 111., June 1, 1953.4. ALTERNATING CURRENT CIRCUIT THEORY(book), M. B. Reed. Harper and Brothers, NewYork. N. Y., 1948.

need in our operations for the highestpossible service continuity, that we be­gan to seriously consider the use ofgrounded neutral low-voltage distributionsystems."

Arberry's paper continues with a dis­cussion of how he applied system neutralgrounding and concludes with the follow­ing paragraph: "Our experience withthese [neutral grounded] systems hasbeen very satisfactory. There is noquestion that the service reliability hasgreatly improved. The majcrity of thefaults occur on branch feeders and arecleared by the local branch protection de­vices such as fuses. Troubles are local­ized and promptly repaired. As theelectricians become used to the new sys­tems they are more enthusiastic andquickly learn, for instance, that a singleblown fuse promptly indicates a ground.None of them have expressed any desireto return to non-grounded systems."

I t is the authors' opinion that the mostcomplete and useful information dealingwith the causes and the results of ab­normal overvoltages in industrial sys­tems has been given by R. H. Kauf­mann.! Kaufmann's 1952 paper conven­iently summarized the various causes ofover-voltages and pointed out actual casehistories of damage to a power systemfrom the causes given. The overvoltagesdiscussed in the paper were: 1. lightning,2. static, 3. physical contact with a highervoltage system, 4. resonance effects inseries inductive-capacitive circuits, 5.repetitive restrike (intennittent grounds) ,6. switching surges, 7. forced current zerointerrupting, and 8. autotransformerconnections. Kaufmann states, whendiscussing repetitive restrike on low­voltage systems, "Intermittent groundfault conditions on low-voltage un­grounded neutral systems have been ob­served to create overvoltages of five orsix times normal quite commonly. Anunusual case involved a 480-volt un­grounded system. Line-to-ground poten­tials in excess of 1,200 volts were meas­ured on a test volt meter. The source oftrouble was finally traced to an inter-

NOVEMBER 19;')5 Brereton, Hickok-System Neutral Grounding for Chemical Plants 315

CIRCUIT EQUIVAlANT Icr DIAGRAM

~l~

~T-<~(A) UNGROUNDED i

XG (1' (1' (1'J"ffi\-.o

~ 1 I I

1+ J J J ~ ~ ~~ ~ ~ ~ ~ ~

(8) SOLIDLY GROUNDEDXG T T r T T .r~~

GENERATOR POWERSOUDLV TRANSFORMER GROUNOING ±GROUNDED SOLIDLV TRANSfORMER

RESISTANCE GROUNDED tl GROUNDED SOLIDLYGROUNDING

XG RNSOLID GROUNDING

(C)

~---.:L

en SEC0NDAR'f WINDING

XG xN(D) REACTANCE GROUNDED § 7 ~Y 1) }+ t),

~ ..j;

Fig. 2 (above right). Low-voltage solid-grounding and medium-volt­age lolid-grounding for a small system

Fig. 3 (right). Double line-to-ground fault on ungrounded systemresults in outages of two circuits and high-level fault currents which

can cause severe damage to equipment

't) ~__BOTH OF THESE_ TRIP OUT

~

SECCNDGROUNDFAULT

PATH o­t~-+--- FAULT CL'RRENT~

, / \I~ --4--------+---

Fig. 1 (above). System-neutral circuits and methods of grounding

Xo-e-reectence of gener~tor or transformer used For groundingXN-reactance of grounding reactorRN-resistance of grounding resistor

(E) GROUND FAUL TNEUTRALIZER

mittent ground fault in a motor startingautotransformer, About two hourslapsed while the source was being locatedduring which time between 40 and 50motors broke down."

An excellent comparison of a power sys­tem, part of which operated ungroundedand the other part grounded, is includedin a 1953 AlEE conference paper byC. L. Eichenberg.

In his comparison between the un­grounded and grounded sections of his6,900-volt system Eichenberg says, "Theoperating record of the system since thegrounded neutral was installed is mostgratifying. The ground faults experi­enced show a marked reduction in numberand severity. For instance, during theyear 1944 the number of ground indica­tions recorded totaled 34.. Of these 34indications, 19 resulted in equipment

failures such as grounded motor coils orflashed-over bushings. During the year1951 (with the system grounded) therewere two ground relay operations result­ing in one equipment failure, and thefirst fifty weeks of 1952 show a similarrecord. Particular attention has beenpaid to the severity of the damage causedby these ground faults. In each instanceit appears that the relaying has been fastenough to clear the fault before any de­structive burning resulted."

These examples show that it has takenactual field experience to indicate thatimproved system continuity and reli­ability can be obtained from the groundedover the ungrounded system. This isvery important to the chemical industrysince such a substantial percentage of theutilization equipment is now supplied by480- and 2,400-volt ungrounded systems.

Description of Various GroundingMethods

Many different means have been em­ployed in the chemical and other indus­tries to ground the system neutral. Thevarious methods of grounding, as well asungrounded operation, are shown in Fig.1. The general trend in industry is to­ward solid grounding for low-voltagesystems and resistance grounding formedium-voltage systems.

UNGROUNDED OPERATION

The operation of the system with theneutral ungrounded, Fig. l(A), has beenproposed as a method of maintainingservice to essential loads on the occur­rence of the first line-to-ground fault.This desire for not tripping on the oc­currence of the first line-to-ground fault

31G Brereton, Hickok-System Neutral Grounding for Chemical Plants NOVEMBER 1955

TABLE r. SYSTEM CHARACTERISTICS WITH VARIOUSGROUNDING METHODS

IReport of insignificant number of failures showing multiple

outages.Multiple Faults Many case studies

reported showingmultiple failures

Not more than 1-1/2 to 2 times normal

Outage

Solid Grounding

Outap:eYes

'5f. to 2f11, IAbout 100%

Low ResistanceGrounding

High ResistanceGrounding

NoNo outage

tess than 0 .1~

Ungrounded

Up to 6 times 1

Properties during faultsCurrent for a phase-to-groundfault in percent of 3 phasefault currentTransient overvoltagesAutomatic segregation offaulty circuit and equipmentCircuit out- One ph. to groundage forvarious types Phase-to-phase)of sys tem Two ph. to ground)faults Three phase )

Power System PropertiesTransformer· winding connection Delta Wye or delta wit}}. grounding transformer

Fault Location Method

Have to take part or all of the system System does not have to be taken outout of service or use ground fault locator service because the faulty equipmentto find ground faults been automatically isolated

ofhas

If ground fault is not removed, may lose two Ground faults are localized and tripcircuits due to another ground fault off immediately

Including ~roJnd de- Somewhat higher dueMedium voltage tector equipment to high resistancesystem delta is slightly grounding equipment

lower in costMaintenance costRating of lightning arresters

Application of groundingmethod

Lowest, in thatvye and deltatransformers costabout the sameWye connectedsubstationslightly higherthan delta

Grounded-neutraltype

are easily located

Somewhat higher dueto low resistancegrounding equipment

Applied on medium-volt Applied on low­age sys"tems i.e., 2.4, voltage systems4.16, 6.9, or 13.8 KV i.e., 208, 240,

480, or 600-YSome applicationon small mediumvoltage systems

Ungrounded neutral type

Applied on low or me­dium voltage systemswhen sys tem not per­mitted to be tripPedfor first ground fault

Takes time and equipment to find grounds Ground faults

Delta Connected Sub- Slightly higher due to Not generally applied,station with ground high resistance resis- but would be slightlydetector generally tor and ground highereos ts more than vs« indicator

less and less fre­Fluently applied

Low voltagesystems

First Cost

has resulted in a wide variety of ground­fault locating equipment to locate morefreely and to remove the fault from thesystem. Experience has indicated thatungrounded operation permits abnonn­ally high transient overvoltages whichmay cause damage to the connectedelectrical apparatus.

SOLID GROUNDING

Low-voltage services, 600 volts andbelow, are more and more frequentlysolidly grounded; see Fig. l(B). Suchservices are generally supplied from load­center unit substations, of which the cir­cuit breakers employ direct-acting tripdevices. Solid grounding provides ap­proximately the same amount of groundfault current for a 3-phase fault or a line­to-ground fault; thus, the phase-con­nected protective devices can provideproper protection. The medium-voltagesystem might be solidly grounded if theline-to-ground fault current is fairly low,in the order of 3,000 amperes. Methodsof solid grounding are shown in Fig. 2.REACTANCE GROUNDING

The most frequent application of re-

actance grounding, Fig. l(D), has beenfor low-voltage generators. A generatoris usually braced to withstand only its 3­phase fault current. Generally its zero­phase sequence reactance will be lessthan the positive- or negative-phase se­quence reactances resulting in a line-to­ground fault current greater than the 3­phase fault current if the generator issolidly grounded. A reactor may be em­ployed in the system neutral to providesufficient reactance to limit the line-to­ground faul t current to the 3-phase faultcurrent. Generally speaking, if a smallimpedance is desired in the neutral, areactor will be found most economical andif a large impedance is required, a resist­ance will be used.

GROUND-FAULT NEUTRALIZER

The ground-fault neutralizer, Fig. 1(E),has been applied in a very limited numberof cases; principally it has been in sys­tems where the plant operator does notdesire to trip a circuit on the occurrenceof the first ground fault. Another ap­plication has been where a degree ofground-fault protection was desired in

large existing systems having only twocurrent transformers per circuit, makingthe application of the residually connectedground .relay difficult. The ground-faultneutralizer has also been applied wherethe zero-phase sequence charging currenthas been high.

RESISTANCE GROUNDING

The general trend in industries is forall of the voltage classes of medium-volt­age systems, such as 2.4, 4.16, 6.9, and13.8-kv, to apply a resistance groundedsystem, Fig. 1(C). These systems usu­ally incorporate power circuit breakersand relays, and can include a residuallyconnected ground relay which can pro­vide much faster and more sensitive pro­tection than a phase-connected devicefor a line-to-ground fault. This is par­ticularly important in the medium-volt­age system because it will have a higherlevel of 3-phase fault kva as comparedto the low-voltage system. It is im­portant to realize that the additionalequipment used to ground a power sys­tem is a very small percentage of theelectrical system cost. Further, the least

NOVEMBER 1955 Brereton, Hickok-System Neutral Grounding for Chemical Plants 317

expensive way of substantially limitingline-to-ground fault current is by use of aresistor.

Reference has been made to resistancegrounding for the industrial system.Nearly all of the published information"dealing with this fonn of system neutralgrounding will apply a resistor to limit theline-to-ground fault current from. 5 to 20per cent (%) of the 3-phase fault current.This type of system will be so arrangedthat the ground relays will trip the circuitif a fault should occur. This type of re­sistance grounding will now be referred toas "low-resistance grounding." A recenttype of resistance grounding, called"high-resistance grounding" will apply tothe system when the largest practicalvalue of resistance is applied in the systemneutral. This will result in a line-to­ground fault current of less than 0.1%of the 3-phase fault current. No meansare provided for removing a faulted cir­cuit for a single line- to-ground fault.

HIGH-RESISTANCE GROUNDING

High-resistance grounding, defined as aresistance nearly equal to, but not greaterthan, the zero-phase sequence capacitivereactance of a system, 1/3 X co, may be ap­plied in the chemical or any other in­dustry where it is desired to limit tran­sient overvoltages and not trip out a cir­cuit or a piece of equipment upon the oc­currence of a single line-to-ground fault.The range of zero-phase sequence react­ance for the typical chemical industrypower system may he from 50 to 500oluns for a 480-volt system and from 500to 10,000 ohms for a 2,400-volt system.Since the high-resistance grounded sys­tem does not have all the properties of thelow-resistance grounded system, such asbeing able automatically to isolate faultyequipment, it is desirable to state thecharacteristics of this additional fonn ofsystem-neutral grounding. Simply statedthis means that continuous process in­dustries, such as the chemical industry,may propose that an attempt be made tokeep a faulted circuit in service, but thatsuch a fault should not cause damage tothe other connected apparatus or causeserious overvoltages that may cause mul­tiple circuit failures. Kaufmann, pre­viously mentioned, explains the manypossible ways that transient overvoltagesmay occur on the ungrounded system. Asevidence is kept and accumulated on theoperation of ungrounded systems, it soonbecomes apparent that multiple failuresfrequently occur on ungrounded systems.It has been shown that transient over­voltages, in the neighborhood of sixtimes normal, can be sustained on the

ungrounded system. This stress is im­posed on all the insulation of all motors,cable, and other electrical apparatus con­nected to the same metallic circuit for theduration of the fault. High-resistancegrounding moderates to ineffective valuesthese transient overvoltages for a singleline- to-ground fault.

Comparison of System PropertiesDuring Faults for VariousGrounding Methods

A comparison of system characteristicswith various grounding methods is givenin Table 1. The first group of data inthis table deals with the properties andperformance of the system during a fault.I t should be noted that although tran­sient overvoltages are eliminated withhigh-resistance grounding, as against anungrounded system, neither of thesesystems provides automatic segregationof the faulty circuit and equipment. Auto­matic segregation would occur for twofeeder services in the case of a secondline-to-ground fault as shown in Fig. 3.Experience has indicated that the mostfrequent type of fault occurring in theindustrial system is the line-to-groundfault. Neither the high-resistancegrounded nor ungrounded system will re­move the first faulted circuit, should afault occur in an electrical equipment.Should any other type of fault occur, suchas a phase-to-phase or 2-phase-to-groundor a 3-phase fault, the faulty equipmentwill automatically be isolated.

Comparison of Power SystemProperties for Various GroundingMethods

Under the section in Table I dealingwith power system properties, a com­parison is given of the various fault loca­tion methods for the four types of systemoperation. Low-resistance grounding orsolid grounding automatically isolates thefaulted equipment. Determining thecircuit on which the fault occurs can bevery time-consuming and costly whensome methods of operation are used.This is particularly true if a fault shouldoccur on the same phase of two differentservices for that voltage level.

For low-voltage systems, the com­parison of first cost shows that the solidlygrounded neutral is lower because thedelta- and wye-connected transformersare nearly the same price and the delta­connected transformer requires the addi­tional expense of ground indicator equip­ment to tell when a ground fault exists onthe system. High-resistance grounding,

with a ground resistor and a ground in­dicating relay, may cost a few dollars1110re than an ungrounded system withground indicators; see Fig. 4. In com­paring the various methods of groundingfor a medium-voltage system, the un­grounded, or delta-connected systemmay be slightly lower in cost because ofthe fourth bushing required for thewye-connected transformer, When high­resistance grounding is applied on themedium-voltage system, two methodsshould be checked to determine the low­est first cost. A medium-voltage high­resistance resistor m.ay be inserted di­rectly in the neutral of the wye-connectedtransformer, Fig ..5, or, and this may bemore economical, a small distributiontransformer may be applied in the neutralcircuit and loaded with a resistor to pro­vide an equivalent resistance in theneutral circuit; see Fig. 4. In the caseof both low- and medium-voltage ex­isting ungrounded systems high-resist­ance grounding may be economically ap­plied by the use of a 3-phase 2-windingtransformer or three single-phase 2­winding transformers. This is shown inFig. 6. Indication of a line-to-groundfault is provided by the voltage relay ap­plied across the resistor. Protection ofthe transformers can conveniently beprovided by the delta-connected currenttransformers. This current transformerconnection will circulate zero-phase se­quence currents, as appearing duringground faults, and permit the detectionof positive- or negative-phase sequencecomponents of current by the time-over­current relays. It should be emphasizedthat the slight additional cost for equip­ment used in providing system neutralgrounding is a very small portion of thetotal cost of the electrical system and issmall in proportion to the benefits ofadded service reliability obtained from it.

An important comparison will be thatof maintenance costs. The high-re­sistance grounded system has the verygreat advantage over an ungroundedsystem of reducing transient overvolt­ages, thus lengthening the life of all theapparatus connected at that voltage level,but still requiring the same time andequipment to find ground faults as doesthe ungrounded system. ·The eliminationof multiple failures caused by transientovervoltages inherently means that lessapparatus is damaged, resulting in muchlower maintenance and replacement costs.

Application of High-ResistanceGrounding Method

One of the most important comparisonsin Table I deals with the application of

318 Brereton, Hickok-System Neutral Grounding for Chemical Plants NOVEMBER 1955

System Line-to­Neutral ground

Grounding PaultResistor Current

Ohms Amperes

Connected Loadat Voltage Level

Kva

Table II. Application of High-ResistanceGrounding

with third-harmonic compensation, maybe applied, such as the General ElectricfA V51D. This relay is designed so thatits third-harmonic voltage pickup is ap­proximately three times greater than itsfundamental zero-phase sequence voltagepickup. An overcurrent relay could alsobe applied, but it is not readily availablewith third-bannonic compensation.

The high-resistance system does notpermit sufficient ground-fault current totrip protective devices. To prevent theground fault from being maintained onthe system for an extensive period, atiming relay might be operated from thevoltage relay, as well as an alarm, Oneplant at present applies a timing relayset for a maximum of 2 hours to permitother equipment to be substituted for thefaulty equipment before the timing relayinitiates the tripping circuit.

A chemical plant operating a 2,400­volt ungrounded system may choose toprovide the neutral by use of a zigzaggrounding transformer: see Fig. 5. Ashas been stated, either a medium-voltagehigh-resistance resistor may be applied,

SystemVoltage

Volts

I

f1uL~

~11 g

n TT IT~ ~T T Tl£GENERATOR POWER GROUNDING

NEUTRAL TRANSFORMER TRANSFORMERNEUTRAL NEUTRAL

RESISTANCE GROUNDING

Fig. 5 (right). Low-voltagehigh-resistance grounding andmedium-voltage low- and

high-resistance grounding

Fig. 4 (left). Use of i-wind.ing transformer for medium·,oltage high-resistance ground.ing when system neutral is

available

done in the investigation of recoveryvoltages. Data are currently being ac­cumulated to determine parameters deal­ing with the charging current for motors.This is particularly important in the ap­plication of high-resistance grounding be­cause motors are the major contributorof charging current in the modern indus­trial power system.

Table II, showing the results of the in­vestigation of the zero-phase sequencecapacitive property of the industrialsystem, may be used as a general guidefor high-resistance grounding. It dealswith the application of high-resistancegrounding, and is based on the operationof a load-center type of distribution sys­tem with approximately 100% connectedmotor load. An extensive amount ofcable would decrease the zero-phase se­quence capacitive reactance (increasingthe charging current) and a lower per­centage of motors would increase thecapacitive reactance (decreasing thecharging current). The value of thegrounding resistor suggested in the tablepermits some allowance for system ex­pansion, but does not provide for the ap­plication of surge capacitor equipment tomotors. Where rotating machine pro­tective capacitors are applied, it will besufficiently accurate to add this addi­tional capacitive current to the currentgiven in Table II to determine the lowerohmic rating of the system neutral ground­ing resistor.

grounding methods. There is every in­dication that ungrounded operation israpidly declining in usage. When a por­tion of an operation in a chemical plant,or other such industry, will not permittripping on the first ground fault, a high­resistance grounded system may be em­ployed. In general, the preferred ap­plication is for low-resistance groundingon medium-voltage systems and solidgrounding on low-voltage systems toisolate a fault automatically, which saveshunting for it.

PRIMARY WINDINGRATED SYSTEMUNE-TO-liNE VOLTAGE

SECONDARY WINDINGRATED IZ0/240V

NEUTRALSYSTEM

LINE-TO-GROUND CURRENT FOR HIGH­

RESISTANCE GROUNDING

Investigation has shown that there is avery limited amount of information deal­ing with the zero-phase sequence charg­ing current of motors4• A good deal of in­formations is available on the chargingcurrent for transformers, autotrans­formers, high-voltage potential trans­formers, high-voltage current transform­ers, induction regulators, current-limit­ing reactors, power circuit breakers andbushings, insulators, and lightning ar­resters, as well as other apparatus, be­cause of the extensive amount of work

Fig. 6. Use of i-winding transformer(s) forlow- or medium-voltage high-resistancegroundins when system neutral is not available

PRIMARY WINDINGRATED SYSTEM

L1NE-TO-L1NEVOLTAGE

SECONDARY WINDINGRATED 120 VOLTS

J~~

PROTECTION OF EQUIPMENT FOR HIGH­

RESISTANCE GROUNDING

The method of detecting when a groundfault occurs in a system will be to ob­serve the zero-phase sequence voltageacross the high-resistance resistor in thesystem neutral or, as may be applied inthe medium-voltage system, across theloading resistor on the secondary of thesingle-phase distribution transformer. Aninduction-type voltage relay, provided

480 .. { 1 ,000 kva or under. . .. 90...... 31,500 to 3,000 kva 45 6

2400* .. J 2,500 kva or under 280. . . . .. 5l3,750 to 7,000 kva 140 10

*The standard rotating machine protective capaci­tor is rated 0.5 microfarad per pole. For a line-to­ground fault, a set of capacitors would contribute0.785 ampere to the capacitive charging current.

3Ico = (Vl-n)(211"fC) = (1388)(377)(3 XO.5)/101-=0.785 ampere

If ten motors on the 2,400-volt system had pro­tective capacitors, the resistor size should bedecreased to permit an increased current of ap­proximately 8 amperes.

NOVEMBER 1955 Brereton, Hickok-System Neutral Grounding for Chemical Plants 319

A. v. DASBURGMEMBER AlEE

Automation For Gravity Freight

Classification Yards

or a single- phase distribution transformerwith a loading resistor to provide anequivalent resistance in the system neu­tral; see Fig. 4. Normally, no voltage(with the possible exception of a smallvalue of third-harmonic voltage) wouldexist across the secondary of the distri­bution transformer. Should the single­phase secondary of this transformer be­come short-circuited, there is no con­venient means of detection prior to aground fault appearing in the power sys­tem. To assure proper protection undersuch circumstances, it is suggested that acurrent transformer be placed betweenthe distribution transformer or high-re­sistance resistor to operate an overcurrentrelay (shown as the IA C51A in Fig. 4) toprovide protection against a short circuitof the secondary or primary of the dis­tribution transformer, The current trans­former in this circuit should have a me­chanicallimit greater than the maximum

Synopsis: Recently developed speed­measuring and control systems automati­cally retard cars and guide them to classi­fication tracks in modem gravity yards.Uncoupled from the train at the crest,"cuts" consisting of one car, or severalcoupled cars, having wide variations inrolling resistance select prior establishedroutes and determine their own releasingspeeds as they move by gravity to couplesafely with other cars in the yard.

TH E simplest form of freight carclassification yard is the original form

of a group of parallel tracks, connectedon both ends, on which a locomotiveshuttles back and forth to sort cars intoproper order before making them up intotrains. Bringing mechanization andautomation to this basic operation hasbeen a step-by-step process.

Increases in freight traffic requiredfaster sorting of cars, and in 1883 thefirst step was taken by placing one endof the yard on a rise or "hump." As alocomotive steadily pushed cars to thecrest they were uncoupled and ran bygravity to the classification tracks.Brakemen rode the cars to prevent them

symmetrical line-to-ground fault cur­rent.

Conclusions

The chemical industry has contributedmuch to the electrical art. As this in­dustry expands and applies newer meth­ods, it is hoped that this review of sys­tem neutral grounding will not only pro­vide a review but present suggestionsthat will permit improved performanceof the electrical power system throughthe grounding of power systems at allvoltage levels. The grounding methodshould be selected that best suits theprocess and operating conditions. Whentripouts are permitted, a solidly groundedor low-resistance grounding system shouldbe applied. When tripouts are not de­sired, the high-resistance grounded sys­tem may be applied. The increased

from coupling at damaging speeds inthe classification tracks. Other men 'wereassigned the duty of operating switchesalong routes followed by the cars.

Power-operated switches were intro­duced in 1891, permitting one man tocontrol several switches. In 1924, carretarders (sometimes called rail brakes)were perfected and these have largelyreplaced the car riders.

A simple form of gravity yard designedfor retarder operation is shown in Fig.1, and its profile diagram in Fig. 2.The scale permits automatic weighingof cars as they roll down the grade.Gravity operation is now used extensivelyfor major yards, but flat yards continueto be used for small-volume sortingoperations.

Automatic control of retarders toprovide a fixed releasing speed was intro­duced in 1941 and expanded in 1951to provide multiple speed selection. Thisscheme uses a series of short electric trackcircuits. When shunted by th~ wheelsthey detect the position of a car in theretarder. Speed is determined by timingthe intervals between shuntings. Control

availability of ground-fault detectionequipment, which more easily permits thelocation of a round fault,' makes prac­tical the high-resistance grounded sys­tem.

References

1. AlEE POWBR CONFBRENCB: POWER GENERA­TION AND INDUSTRIAL POWER SYSTEMS. AlEESpecial Publication S-38, "The Use of 600 Voltand 460 Volt Power Systems with GroundedNeutrals," James P. E. Arberry. Aug. 19;')0,pp. 187-88.

2. NEUTRAL GROUNDING OP LOW-VOLTAGB SYS­

TEMS, R. H. Kaufmann. Iron and Steel Engineer,Pittsburgh, Pa., Feb. 1952, pp. 96-103.

3. INDUSTRIAL POWER SYSTEMS HANDBOOK,edited by D. L. Beeman. McGraw-Hill BookCompany, New York, N. Y., 1955.

4. CAPACITANCE OF SYNCHRONOUs-MACHINEARMATURB WINDINGS DETERMINED FOR HIGH­POTENTIALTEST, R. W. Wieseman. General ElectricReuies», Schenectady, N. Y., July 1947, pp. 26-30.

5. POWER SYSTEM OVERVOLTAGES PRODUCED BYFAULTS AND SWITCHING OPERATION, AlEE Com­mittee Report. AlEE Transactions, vol. 67, pt.II, 1948, pp. 912-22.

of retarder pressure is derived from thisand a release speed setting chosen by theretarder operator.

The development of automatic routeswitching in 1950 relieved the retarderoperator of switch operation and per­mitted control, by one operator, of ayard with any practical number oftracks. Routes are established on arelay network by pressing destinationbuttons on a route selection machine(Fig. 3) located at the crest of the hump.As the cuts roll forward they are de­tected by track circuits associated witheach switch and automatically call forroutes corresponding to the destinationsselected. Route storage for an entiretrain can be added to this system whencircumstances warrant. Usually theroute machine is controlled by a memberof the engine crew at the hump. Thusthe retarder operator has only the taskof adjusting retarder pressure so as tomaintain enough separation betweencars to permit switch operation, and torelease cars at speeds required for safecoupIing on the classification tracks.

Solutions under manual operation aredeveloped intuitively by an operatorfrom the experience of handling thousandsof cars of various types under a variety

Paper 55-754, recommended by the AlEE LandTransportation Committee and approved by theAlEE Committee on Technical Operations forpresentation at the AlEE Fall General Meeting.Chicago. 111., October 3-7, 1955. Manuscriptsubmitted June 6, 1955; made available forprinting August 12, 1955.

A. V. DASBURG is with the General Railway SignalCompany, Rochester. N. Y.

320 Dasburg-Automation for Gravity Freight Classification Yards NOVEMBER 1955