Gound control for alternate power

may/june 2003 IEEE power & energy magazine 491540-7977/03/$17.00©2003 IEEE

Ground Control for Alternate Power

© 1998 CORBIS CORP.

WRené Castenschiold

WHEN ENGINE GENERATOR SETS ARE USED AS ANalternate source of power (i.e., emergency or standby

power), it is essential that they be properly groundedand that associated transfer switches be properly

selected. This is necessary to assure safety of per-sonnel, protection of equipment, reliable ground-

fault sensing, and continuity of power toelectrical loads.

Unique circumstances relating to emer-gency power systems require special

grounding considerations. For example,in large building complexes, consid-

eration should be given to possiblepower disruption within the facil-

ity and the need for area protec-tion. Due consideration

should be given to effects ofmultiple ground connec-

tions, need for ground-fault protection, selection of transfer switches, and conformance to

current code requirements. This article applies to most emer-gency and standby systems but does not cover transfer switch-

ing in medium voltage systems or UPS with a solid-stateinverter as an alternative source.

Electric power systems require two types of ground-ing, namely system grounding and equipment ground-

ing, and each has its own functions.

System GroundingSystem grounding pertains to the nature and

location of an intentional interconnectionbetween the electric system conductors and

A number offactors should beconsidered whencoordinatingtransfer switchingand grounding ofengine generatorsets.

50 IEEE power & energy magazine may/june 2003

grounding electrode systems that provide an effective connec-tion to ground (earth). In other words, it relates to how andwhere the grounded conductor is connected to earth.

Purpose of System GroundingAccording to section 250-4 of the U.S. National ElectricalCode (NEC), system and circuit conductors are grounded to:

✔ limit voltages due to lightning, line surges, or uninten-tional contact with higher voltage lines

✔ stabilize the voltage to ground during normal operation.Systems rated 600 V or less are solidly grounded to facili-

tate overcurrent device operation in case of ground faults.

Power SystemsFor maximum reliability, transfer switching is often locatedclose to the point of power utilization (load). In most com-mercial and many industrial installations, critical loads arerated 600 V or less. The following emergency power systemsare commonly used to supply phase-to-neutral loads andshould be solidly grounded:

✔ 240/120 V, single-phase, three-wire system✔ 480/240 V, single-phase, three-wire system✔ 208Y/120 V, three-phase, four-wire, wye system✔ 480Y/277 V, three-phase, four-wire, wye system✔ 600/347 V, three-phase, four-wire, wye system✔ 240/120 V, three-phase, four-wire, delta system.

Solidly Grounded SystemsTo facilitate overcurrent device operation, the equipmentgrounding conductors for solidly grounded systems arerequired to be bonded to the system grounded conductor at theservice equipment and at the source of a separately derivedsystem (standby or emergency power). This bond completesthe ground-fault current return path from the equipmentgrounding conductors to the system grounded conductor. Sys-tem grounding conductors and grounding electrodes are notintended to conduct the ground-fault current that is due to aground fault in equipment, raceways, and other enclosures. Insolidly grounded systems, the ground-fault current flowsthrough the equipment grounding path.

Solidly grounded systems exercise the greatest control ofovervoltages but result in the highest magnitudes of ground-fault current. However, the inherent regulation of alternate-source generators generally limits the available fault current.If line-to-neutral loads must be served, high resistance

grounding of the generator neutral is not permitted, becausethe neutral should never be used as a circuit conductor in sys-tems that are grounded through a high resistance.

Equipment GroundingIt is important that all exposed metallic parts of electricalequipment be bonded to a grounding electrode. This includesmetallic parts, such as engine generator frames, metallic elec-trical conduit and raceway enclosures, cable trays, cableshields, and metallic racks and ladders in vaults.

Purpose of Equipment GroundingThe purpose of equipment grounding is to assure the follow-ing functions:

✔ maintain a low potential difference between nearbymetallic members and thereby protect people in the areafrom electric shock

✔ facilitate the operation of circuit protective devices byproviding low impedance conducting paths for ground-fault currents

✔ provide an effective electrical conductor system overwhich ground-fault currents can flow without creating afire or explosive hazard.

Equipment Grounding PathsOnly by providing adequate equipment grounding paths formetallic enclosures, frame of an engine generator set, etc., ina manner that assures adequate current-carrying capability andan adequately low value of ground-fault circuit impedance,can both electric-shock hazard and fire hazard be avoided.Technical investigations have pointed out that it is importantto have good electric junctions between sections of conduit ormetal raceways that are used as equipment grounding pathsand to assure adequate cross-sectional area and conductivityof these grounding paths. Where systems are solidly ground-ed, equipment grounding conductors are bonded to the systemgrounded conductor and the grounding electrode conductor atthe service equipment and at the source of a separately derivedsystem in accordance with the requirements of NEC sections250-30, 250-64 and 250-130.

The same network of equipment grounding paths isrequired for systems that are solidly grounded, high resistancegrounded, or ungrounded. The equipment grounding paths inhigh-resistance grounded or ungrounded systems provideshock protection and a conducting path for phase-to-phase

Proper grounding and selection of transfer switches assures safety of personnel, protection of equipment, reliableground-fault sensing, and continuity of power to electrical loads.

may/june 2003 IEEE power & energy magazine

fault current if two ground faults occur simultaneously on dif-ferent phase conductors.

Engine Generator FrameThe engine generator frame, automatic transfer switch enclo-sure, conduits, and other exposed components of an emer-gency power system should be permanently bonded togetherand connected to the grounded conductor of the service equip-ment providing normal power and/or the grounded conductorof the engine generator set. By means of grounding electrodeconductors, these grounded conductors are connected togrounding electrodes at their respective locations.

NEC permits the grounding electrode to be a metal under-ground water pipe that may be part of a municipal water pip-ing system or a well on the premises.However, buried portions of a waterpipe system less than 10 feet inlength or that may include plastic orcement materials should not be used.For small emergency power systemswhere the ground currents are of rel-atively low magnitude, existingburied water pipe systems are usual-ly preferred as electrodes, becausethey are economical in first cost.Nevertheless, before reliance can beplaced on any existing electrodes, itis essential that their resistance toearth be measured to ensure thatsome unforeseen discontinuity hasnot seriously affected its suitability.Care should be exercised to ensurethat all parts that might become disconnected are effectivelybonded with flexible bonding jumpers.

Transfer SwitchingAn engine generator set may or may not be considered as partof a “separately derived system” as defined by NEC article100: “Separately Derived Systems: A premises wiring systemwhose power is derived from a battery, a solar photovoltaicsystem, or from a generator, transformer, or converter wind-ings and that has no direct electrical connection, including asolidly connected grounded circuit conductor, to supply con-ductors originating in another system.”

An engine generator set can only be considered to be a partof a separately derived system with its neutral terminal sepa-rately grounded when the associated transfer switch providesswitching of the neutral conductor. In order to comply with theNEC definition for separately derived systems, grounding ofthe engine generator’s neutral terminal could, therefore, bedone only by one of two methods:

✔ Permanently ground the neutral terminal of the enginegenerator at only the grounding electrode of the incom-ing service for the normal utility source: There wouldbe no switching of the neutral conductor, and the emer-

gency or standby source would not, therefore, be con-sidered a separately derived system. It later becameapparent that this method did not resolve all ground-fault sensing problems.

✔ Permanently ground the neutral terminal of the enginegenerator at its location: This would require transferswitching of the neutral conductor, and the emergencysource would be defined as a separately derived system.

Providing reliable ground-fault protection in electrical sys-tems having alternate emergency or standby power can be dif-ficult unless adequate equipment is selected and properlycoordinated. Multiple neutral-to-ground connections may pre-vent adequate sensing of ground-fault currents and may alsocause nuisance tripping of circuit breakers. To assure proper

ground-fault protection, understanding code requirements canbe helpful, including NEC sections 230-95, 250-30, and 445.3.

There are three approaches to consider in meeting the cur-rent requirements of NEC. For a three-phase, four-wiregrounded system, they include using a:

✔ three-pole transfer switch with the engine generator setconsidered as a nonseparately derived source

✔ four-pole transfer switch with the engine generator setconsidered as a separately derived source

✔ three-pole transfer switch with overlapping neutral con-tacts with the engine generator set considered as a sep-arately derived source.

However, just meeting the minimal requirements of thecode does not necessarily provide the degree of reliabilityneeded for a good emergency power system. The systemdesigner should consider the following: An engine generatorset is often remotely located from the grounded utility serviceentrance, and the ground potentials of the two locations maynot be the same.

Good engineering practice requires the automatic transferswitch to be located as close to the load as possible to providemaximum protection against power failure due to cable orequipment failures within the facility (area protection). Thus,

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figure 1. No area protection with ungrounded engine generator set providingpower to the load.

Transformer

GFP

ATS

N

Cable Failure Load

EngineGenset

the distance of cable between incoming service and the trans-fer switch and then to the engine generator set may be sub-stantial. Consider what would happen if a cable failureoccurred, as in Figure 1, with the engine generator not ground-ed to its own grounding electrode. The load would be auto-matically transferred to an ungrounded emergency powersystem. This, in turn, could jeopardize emergency-servicecontinuity and possibly lead to additional failures. Concurrentfailure of equipment or cable failure (breakdown between lineand equipment ground) after transfer to emergency may not bedetected. Thus, the generator frame may approach line poten-tial, causing a substantial voltage difference between the gen-erator frame and the grounded conductor (neutral).

Some local codes require ground-fault protection while theengine generator is operating. This may present a sensingproblem if the neutral conductor of the generator is not con-nected to a grounding electrode at the generator site and prop-er isolation of neutrals is not provided. Furthermore, somelocal and state codes (Pennsylvania and Massachusetts)require area protection that may create additional problemsunless the neutral conductor of the generator is isolated andgrounded at the generator location.

When the transfer switch is in the emergency position,other problems may occur if the engine generator set is notproperly grounded. For example, a ground-fault condition,could cause nuisance tripping of a normal source ground-faultcircuit breaker even though load current is not flowing through

the breaker. This would prevent automatic retransfer to thenormal source and isolation of the fault condition. This wouldnot be in accord with NEC section 230-95(C), FPN 3, whichstates, “Where ground-fault protection is provided for theservice disconnecting means and interconnection is made withanother supply system by a transfer device, means or devicesmay be needed to assure proper ground-fault sensing by theground-fault protection equipment.” Furthermore, both thenormal neutral conductor and the emergency neutral conduc-tor would be simultaneously vulnerable to the same ground-fault current. Thus, a single fault could jeopardize power tocritical loads even though both utility and emergency powerare available. Such a condition may be in violation of codesrequiring independent wiring and separate emergency feeders.

Three-Pole Transfer SwitchingUsing three-pole transfer switches is the least expensiveapproach and requires less space, and the system is not con-sidered as a separately derived system. However, ground-faultcurrents may trip the normal source breaker when the load isconnected to the engine generator set (Figure 2). Use of three-pole transfer switches does not provide area protection withinthe facility, and this could lead to an ungrounded power sys-tem in the event of cable failure (Figure 1). With the neutralconductor connected to the incoming normal service, it is dif-ficult to provide ground-fault sensing on the emergency sideas required to meet NEC section 700-7(D).


figure 2.Improper ground-fault sensing.

Transformer

GFS

ATS

N

Load

Ground FaultNeutral Conductor

Equipment Grounding Conductor

EngineGenset

Techniques for equipment grounding and system grounding should be such as to provide optimum safety and assure maximumcontinuity of power to essential loads.

Four-Pole Transfer SwitchingA better solution is to use a four-pole transfer switch, whichprovides complete isolation of service and generator neutralconductors. This eliminates possible improper ground-faultsensing and nuisance tripping caused by multiple neutral-to-ground connections. When this is done, the generator willcomply with the current NEC definition of a separatelyderived system. With the neutrals thusisolated, ground-fault protection withconventional sensors can be added tothe generator output.

Four-pole transfer switches havebeen satisfactorily applied on applica-tions where the loads are passive andrelatively balanced. However, unbal-anced loads may cause abnormal volt-ages for as long as 10 to 15 ms when theneutral conductor is momentarilyopened during transfer of the load.Transfer switches are frequently calledupon to operate during total load unbal-ance caused by a single phasing condi-tion. Inductive loads may causeadditional high transient voltages in themicrosecond range. It should beremembered that the contacts of thefourth switch pole interrupt neutral cur-rents and are, therefore, subject to arcing and contact ero-sion. However, contact erosion of the fourth pole can beminimized if the fourth pole is designed to open last (after thethree other poles interrupt the load current) and make firstwithout contact bounce. In any event, a good maintenanceprogram must reaffirm at intervals the integrity of the fourthpole as a current-carrying member with sufficiently lowimpedance.

Both NEC and the Underwriters Laboratories, Inc. (UL)recognize the use of four-pole transfer switches for three-phase, four-wire systems. In some European countries, includ-ing Germany and France, it is mandatory to use four-poleswitching devices.

Overlapping Neutral ContactsAnother method for isolating the normal and emergencysource neutrals is for a three-pole automatic transfer switch toinclude overlapping neutral transfer contacts. This featureprovides the necessary isolation between neutrals and, at thesame time, minimizes abnormal switching voltages. Bymeans of overlapping contacts, the only time the neutrals of

the normal and emergency power sources are connectedtogether is during transfer and retransfer. With a solenoid-operated, conventional, double-throw transfer switch, thisduration can be less than the operating time of the ground-fault sensor, which is usually set anywhere from six to 24cycles (100 to 400 ms).

Figure 3 shows a typical system utilizing a three-poletransfer switch with overlapping neutral transfer contacts forisolating the neutral conductors. Note that there is no possi-ble flow of fault current through the neutral conductor thatwould detract from or effectively reduce ground-fault detec-tion. Furthermore, there is no possible flow of unbalancedcurrent through the generator neutral to alter the response

level of the ground-fault sensor and possiblycause nuisance breaker tripping. As with fourpole transfer switches, conventional ground-fault sensing can be readily added to the emer-gency side. As shown in Figure 3, ground-faultsensing on the emergency side is often usedfor actuating an alarm circuit rather than trip-ping a breaker, such as required per NEC sec-tion 700-7(D).

With overlapping neutral transfer contacts,the load neutral is always connected to onesource of power. In that there is no momentaryopening of the neutral conductor when thetransfer switch operates, abnormal and tran-sient voltages are kept to a minimum. Anoth-

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figure 3. System with overlapping neutral contacts.

Transformer

GFS

ATS Alarm

Load

Ground FaultNeutral Conductor

Overlapping Contacts

Equipment-Grounding Conductor

EngineGenset

Questions Yes NoA Is the application for emergency power per Article

700 of the National Electrical Code?B Is the incoming service of the normal source

greater than 1,000 A?C Is the service solidly grounded wye and rated

more than 150 V to ground?D If the engine generator set is located remote

from the normal incoming service, is area protection required?

E Is sensing and alarm indication of a ground fault on the emergency or standby source required?

table 1. Guide for selecting the type of transfer switch.


er advantage is that there is no erosion of the overlappingcontacts due to arcing, thus assuring current-carrying integri-ty and no increase in impedance of the neutral circuit. Itshould be noted that a three-pole transfer switch with over-lapping neutral contacts should not be specified as a four-pole switch according to UL-1008 and NEC.

Selecting Transfer Switching EquipmentFor three-phase, four-wire systems involving one transferswitch and one engine generator set, response to the questionsin Table 1 serve as a guide in selecting the type of transferswitch needed for a particular application.

If the response to the first three questions (A, B, and C) areall “yes,” it is necessary to use a four-pole transfer switch or athree-pole transfer switch with overlapping neutral contacts inorder to satisfy the requirements of the 1999 NEC. If theanswer is “no” to any one of these questions, then proceedwith questions D and E.

If the response is “yes” to either question D or E, use afour-pole transfer switch or a three-pole transfer switch withoverlapping neutral contacts in order to satisfy the require-ments of NEC. If the answer is “no” to both questions, a less

expensive three-pole transfer switch without overlapping neu-tral contacts can be used; then, the neutral terminal of the gen-erator should be grounded at the incoming service of thenormal source.

Although the questions in Table 1 apply to three-phase,four-wire systems, the same approach can be taken for single-phase, three-wire systems to determine the transfer switchingequipment. However, most single-phase systems are 400 A orless and, therefore, can be satisfied with two-pole transferswitches. It should be kept in mind that the this selectionapproach only applies to applications with one engine genera-tor set and one transfer switch.

Multiple Transfer SwitchesMultiple transfer switches, located close to the loads, are oftenused rather than one transfer switch for the entire load. In suchcases, consideration should be given to the possibility of cableor equipment failure between the service equipment and thetransfer switches, thus possibly causing an emergency orstandby power system to become ungrounded. This is partic-ularly important if a solidly interconnected neutral conductoris grounded at the service equipment only.


figure 4.Multiple automatic transferswitches with improperground-fault sensing.

Transformer

GFS

GFS

N

N

ATS

ATS

Load No. 1

Load No. 2

Neutral Conductor

Neutral Conductor Note: Main Circuit Breakers Not Shown.

EngineGenset

With many emergency and standby power systems being updated, the concern regarding safety and product liabilityshould not be treated lightly.


Under such conditions, there is a possibility of tripping theground-fault circuits when no ground fault exists. Figure 4illustrates a typical system with multiple three-pole transferswitches. When power is supplied from the normal source tothe load through a transfer switch, the neutral current willdivide between two neutral conductors. Some of the currentwill return by the normal return path. The remainder of thecurrent will flow to the neutral terminal of the engine genera-tor and through the neutral conductor of the other transferswitch to the normal source. This can cause tripping of eitherground-fault circuit breaker, even when no ground fault exists.The problem can be corrected by the use of the three-poletransfer switches with overlapping neutral or four-pole trans-fer switches.

Multiple Engine Generator SetsWhen multiple engine generator sets are connected in paralleland serve as a common source of power, each generator neu-tral is usually connected to a common neutral bus within theparalleling switchgear which, in turn, is grounded. The asso-ciated switchgear containing the neutral bus should be locatedin the vicinity of the generator sets. A single system-ground-ing conductor between the neutral bus and ground simplifiesthe addition of ground-fault sensing equipment.

However, engine generator sets that are physically separat-ed and used for isolated loads may necessitate additional neu-tral-to-ground connections. By using multiple four-poletransfer switches or three-pole switches with overlapping neu-tral contacts, proper isolation and ground-fault sensing can beobtained.

Upgrading Transfer Switching EquipmentUpgrading older transfer switch equipment to present codesand standards can be a real challenge, particularly for largerand more complex industrial and commercial facilities.Awareness that three-pole transfer switches may not alwaysprovide complete isolation and proper ground-fault sensinghas not been very apparent in the past. With many emer-gency and standby power systems being updated, the con-cern regarding safety and product liability should not betreated lightly. This concern provides another reason forupgraded installations to include adequate grounding andpreferably new transfer switches that provide switching ofthe neutral conductor. Regardless of whether or not ground-fault sensing is required, proper switching of the neutral con-ductor is good engineering practice because of the systemisolation, improved grounding and the extra safety it canprovide.

Optimum Safety and Continuity of PowerThere are a number of factors that should be considered whencoordinating transfer switching and grounding of engine gen-erator sets. Simply conforming to minimum code require-ments will not necessarily assure the degree of reliabilityrequired for such systems. Thorough consideration should be

given to protecting against power disruption within the build-ing or facility and providing adequate ground-fault protection.Techniques for equipment grounding and system groundingshould be such as to provide optimum safety and assure max-imum continuity of power to essential loads. This includesproper grounding and ground-fault sensing when the transferswitch is in the emergency position as well as the normal posi-tion.

As a general rule, for most applications requiring ground-fault sensing, area protection, multiple transfer switches, ormultiple engine generator sets, the on-site power should beconsidered as a separately derived system. In such cases, theneutral of the engine generator set is grounded at its location.

It is not intended for the scope of this article to cover indetail all aspects of grounding on-site power systems. Thoserequiring more detailed information may find the followinglist of publications helpful.

Further ReadingOn-Site Power Generation: A Reference Book, 4th ed., CoralSprings, FL: Electrical Generating Systems Association,2002.

National Electrical Code, NFPA No. 70-2002, NationalFire Protection Association, 2002.

IEEE Recommended Practice for Grounding of Industrialand Commercial Power Systems (Green Book), Standard 142-1991, IEEE, 1991.

Standard for Automatic Transfer Switches, UL-1008,Underwriters Laboratories, Inc.

R. Castenschiold and G.S. Johnson, “Proper grounding ofon-site electrical power systems,” IEEE Ind. Appl. Mag.,Mar./Apr. 2001.

Recommended Practice for Emergency and Standby PowerSystems (Orange Book), Standard 446-1995, IEEE, 1995.

BiographyRené Castenschiold received his B.S. degree in electricalengineering from Pratt Institute and is president of LCR Con-sulting Engineers, P.A., Green Village, New Jersey, a firm spe-cializing in electroforensic engineering. Prior to LCR, he wasexecutive engineering manager for the Automatic SwitchCompany, Florham Park, New Jersey. He has pioneered in theresearch, design, and standardization of automatic transferswitches and emergency generator controls. His accomplish-ments have increased reliability of alternate power for criticalelectrical loads, such as in hospitals, airports, and communi-cation facilities. As the recipient of 14 patents and author of 50published papers and articles, he successfully advancedclosed-transition switching for computer and other essentialloads, which are vulnerable to momentary losses of power. Inrecognition of his research and standardization of standby andemergency power systems, the National Electrical Manufac-turers Association (NEMA) presented him with the presti-gious James H. McGraw Award in 1986, and IEEE presentedhim with the 1990 Richard Harold Kaufmann Award.

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Gound control for alternate power