Systems. Power Distribution. Power Distribution Systems Contents

Transcription

1 August Sheet Power Dstrbuton Systems Contents System Desgn Basc Prncples Modern Electrc Power Technologes Goals of System Desgn....- Voltage Classfcatons; BILs Basc Impulse Levels Three-Phase Transformer Wndng Connectons Types of SystemsRadal, Loop, Selectve, Two-Source, Sparng Transformer, Spot Network, Dstrbuton Health Care Faclty Desgn Consderatons....- Generator Systems Generator System Desgn Types of Generators Generator Systems Generator Groundng Generator Controls Generator Short-Crcut Characterstcs Generator Protecton System Analyss Systems Analyss Short-Crcut Currents Fault Current Waveform Relatonshps Fault Current Calculatons and Methods Index Determne X and R from Transformer Loss Data....- Voltage Drop Consderatons System Applcaton Consderatons Capactors and Power Factor Overcurrent Protecton and Coordnaton Protecton of Conductors....- Crcut Breaker Cable Temperature Ratngs Zone Selectve Interlockng..- Ground Fault Protecton....- Suggested Ground Fault Settngs Groundng/Ground Fault Protecton GroundngEqupment, System, MV System, LV System Ground Fault Protecton Lghtnng and Surge Protecton Groundng Electrodes MV Equpment Surge Protecton Consderatons..- Surge Protecton Types of Surge Protecton Devces Power Qualty Terms, Techncal Overvew...- SPD Harmoncs and Nonlnear Loads UPS Other Applcaton Consderatons Secondary Voltage Energy Conservaton Buldng Control Systems...- Dstrbuted Energy Resources Cogeneraton PV System Desgn Consderatons Emergency Power Peak Shavng Sound Levels Reference Data IEEE Protectve Relay Numbers Codes and Standards Motor Protectve Devce Data Chart of Short-Crcut Currents for Transformers...- Transformer Full Load Amperes Impedances Data Transformer Losses, TP- Losses Power Equpment Losses....- NEMA Enclosure Defntons...- Cable R, X, Z Data Conductor Ampactes Conductor Temperature Ratngs Formulas and Terms Sesmc Requrements Desgnng a Dstrbuton System CA000E

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3 August Sheet 00 System Desgn.- Basc Prncples The best dstrbuton system s one that wll, cost-effectvely and safely, supply adequate electrc servce to both present and future probable loadsths secton s ncluded to ad n selectng, desgnng and nstallng such a system. The functon of the electrc power dstrbuton system n a buldng or an nstallaton ste s to receve power at one or more supply ponts and to delver t to the ndvdual lamps, motors and all other electrcally operated devces. The mportance of the dstrbuton system to the functon of a buldng makes t almost mperatve that the best system be desgned and nstalled. In order to desgn the best dstrbuton system, the system desgn engneer must have nformaton concernng the loads and a knowledge of the varous types of dstrbuton systems that are applcable. The varous categores of buldngs have many specfc desgn challenges, but certan basc prncples are common to all. Such prncples, f followed, wll provde a soundly executed desgn. The basc prncples or factors requrng consderaton durng desgn of the power dstrbuton system nclude: Functons of structure, present and future Lfe and flexblty of structure Locatons of servce entrance and dstrbuton equpment, locatons and characterstcs of loads, locatons of unt substatons Demand and dversty factors of loads Sources of power; ncludng normal, standby and emergency (see Tab 0) Contnuty and qualty of power avalable and requred (see Tab ) Energy effcency and management Dstrbuton and utlzaton voltages Bus and/or cable feeders Dstrbuton equpment and motor control Power and lghtng panelboards and motor control centers Types of lghtng systems Installaton methods Power montorng systems Electrc utlty requrements Modern Electrc Power Technologes Several new factors to consder n modern power dstrbuton systems result from two relatvely recent changes. The frst recent change s utlty deregulaton. The tradtonal dependence on the utlty for problem analyss, energy conservaton measurements and technques, and a smplfed cost structure for electrcty has changed. The second change s less obvous to the desgner yet wll have an mpact on the types of equpment and systems beng desgned. It s the dmnshng quantty of qualfed buldng electrcal operators, mantenance departments and faclty engneers. Modern electrc power technologes may be of use to the desgner and buldng owner n addressng these new challenges. The advent of mcroprocessor devces (smart devces) nto power dstrbuton equpment has expanded faclty owners optons and capabltes, allowng for automated communcaton of vtal power system nformaton (both energy data and system operaton nformaton) and electrcal equpment control. These technologes may be grouped as: Power montorng and control Buldng management systems nterfaces Lghtng control Automated energy management Predctve dagnostcs Varous sectons of ths gude cover the applcaton and selecton of such systems and components that may be ncorporated nto the power equpment beng desgned. See Tabs,,, and. CA000E

4 .- System Desgn August Sheet 00 Goals of System Desgn When consderng the desgn of an electrcal dstrbuton system for a gven customer and faclty, the electrcal engneer must consder alternate desgn approaches that best ft the followng overall goals.. Safety: The No. goal s to desgn a power system that wll not present any electrcal hazard to the people who use the faclty, and/or the utlzaton equpment fed from the electrcal system. It s also mportant to desgn a system that s nherently safe for the people who are responsble for electrcal equpment mantenance and upkeep. The Natonal Electrcal Code (NEC ), NFPA 0 and NFPA 0E, as well as local electrcal codes, provde mnmum standards and requrements n the area of wrng desgn and protecton, wrng methods and materals, as well as equpment for general use wth the overall goal of provdng safe electrcal dstrbuton systems and equpment. The NEC also covers mnmum requrements for specal occupances ncludng hazardous locatons and specal use type facltes such as health care facltes, places of assembly, theaters and the lke, and the equpment and systems located n these facltes. Specal equpment and specal condtons such as emergency systems, standby systems and communcaton systems are also covered n the code. It s the responsblty of the desgn engneer to be famlar wth the NFPA and NEC code requrements as well as the customer s faclty, process and operatng procedures; to desgn a system that protects personnel from lve electrcal conductors and uses adequate crcut protectve devces that wll selectvely solate overloaded or faulted crcuts or equpment as quckly as possble.. Mnmum Intal Investment: The owner s overall budget for frst cost purchase and nstallaton of the electrcal dstrbuton system and electrcal utlzaton equpment wll be a key factor n determnng whch of varous alternate system desgns are to be selected. When tryng to mnmze ntal nvestment for electrcal equpment, consderaton should be gven to the cost of nstallaton, floor space requrements and possble extra coolng requrements as well as the ntal purchase prce.. Maxmum Servce Contnuty: The degree of servce contnuty and relablty needed wll vary dependng on the type and use of the faclty as well as the loads or processes beng suppled by the electrcal dstrbuton system. For example, for a smaller commercal offce buldng, a power outage of consderable tme, say several hours, may be acceptable, whereas n a larger commercal buldng or ndustral plant only a few mnutes may be acceptable. In other facltes such as hosptals, many crtcal loads permt a maxmum of seconds outage and certan loads, such as real-tme computers, cannot tolerate a loss of power for even a few cycles. Typcally, servce contnuty and relablty can be ncreased by: A. Supplyng multple utlty power sources or servces. B. Supplyng multple connecton paths to the loads served. C. Usng short-tme rated power crcut breakers. D. Provdng alternate customerowned power sources such as generators or batteres supplyng unnterruptable power supples. E. Selectng the hghest qualty electrcal equpment and conductors. F. Usng the best nstallaton methods. G. Desgnng approprate system alarms, montorng and dagnostcs. H. Selectng preventatve mantenance systems or equpment to alarm before an outage occurs.. Maxmum Flexblty and Expendablty: In many ndustral manufacturng plants, electrcal utlzaton loads are perodcally relocated or changed requrng changes n the electrcal dstrbuton system. Consderaton of the layout and desgn of the electrcal dstrbuton system to accommodate these changes must be consdered. For example, provdng many smaller transformers or loadcenters assocated wth a gven area or specfc groups of machnery may lend more flexblty for future changes than one large transformer; the use of plug-n busways to feed selected equpment n leu of condut and wre may facltate future revsed equpment layouts. In addton, consderaton must be gven to future buldng expanson, and/or ncreased load requrements due to added utlzaton equpment when desgnng the electrcal dstrbuton system. In many cases consderng transformers wth ncreased capacty or fan coolng to serve unexpected loads as well as ncludng spare addtonal protectve devces and/ or provson for future addton of these devces may be desrable. Also to be consdered s ncreasng approprate crcut capactes or quanttes for future growth. Power montorng communcaton systems connected to electronc meterng can provde the trendng and hstorcal data necessary for future capacty growth. CA000E

5 August Sheet 00 System Desgn.-. Maxmum Electrcal Effcency (Mnmum Operatng Costs): Electrcal effcency can generally be maxmzed by desgnng systems that mnmze the losses n conductors, transformers and utlzaton equpment. Proper voltage level selecton plays a key factor n ths area and wll be dscussed later. Selectng equpment, such as transformers, wth lower operatng losses, generally means hgher frst cost and ncreased floor space requrements; thus, there s a balance to be consdered between the owner s utlty energy change for the losses n the transformer or other equpment versus the owner s frst cost budget and cost of money.. Mnmum Mantenance Cost: Usually the smpler the electrcal system desgn and the smpler the electrcal equpment, the less the assocated mantenance costs and operator errors. As electrcal systems and equpment become more complcated to provde greater servce contnuty or flexblty, the mantenance costs and chance for operator error ncreases. The systems should be desgned wth an alternate power crcut to take electrcal equpment (requrng perodc mantenance) out of servce wthout droppng essental loads. Use of drawout type protectve devces such as breakers and combnaton starters can also mnmze mantenance cost and out-of-servce tme.. Maxmum Power Qualty: The power nput requrements of all utlzaton equpment has to be consdered ncludng the acceptable operatng range of the equpment and the electrcal dstrbuton system has to be desgned to meet these needs. For example, what s the requred nput voltage, current, power factor requrement? Consderaton to whether the loads are affected by harmoncs (multples of the basc 0 Hz sne wave) or generate harmoncs must be taken nto account as well as transent voltage phenomena. The above goals are nterrelated and n some ways contradctory. As more redundancy s added to the electrcal system desgn along wth the best qualty equpment to maxmze servce contnuty, flexblty and expandablty, and power qualty, the more ntal nvestment and mantenance are ncreased. Thus, the desgner must wegh each factor based on the type of faclty, the loads to be served, the owner s past experence and crtera. Summary It s to be expected that the engneer wll never have complete load nformaton avalable when the system s desgned. The engneer wll have to expand the nformaton made avalable to hm on the bass of experence wth smlar problems. Of course, t s desrable that the engneer has as much defnte nformaton as possble concernng the functon, requrements, and characterstcs of the utlzaton devces. The engneer should know whether certan loads functon separately or together as a unt, the magntude of the demand of the loads vewed separately and as unts, the rated voltage and frequency of the devces, ther physcal locaton wth respect to each other and wth respect to the source and the probablty and possblty of the relocaton of load devces and addton of loads n the future. Coupled wth ths nformaton, a knowledge of the major types of electrc power dstrbuton systems equps the engneers to arrve at the best system desgn for the partcular buldng. It s beyond the scope of ths gude to present a detaled dscusson of loads that mght be found n each of several types of buldngs. Assumng that the desgn engneer has assembled the necessary load data, the followng pages dscuss some of the varous types of electrcal dstrbuton systems that can be used. The descrpton of types of systems, and the dagrams used to explan the types of systems on the followng pages omts the locaton of utlty revenue meterng equpment for clarty. A dscusson of short-crcut calculatons, coordnaton, voltage selecton, voltage drop, ground fault protecton, motor protecton and other specfc equpment protecton s also presented. CA000E

6 .- System Desgn August Sheet 00 Voltage Classfcatons ANSI and IEEE standards defne varous voltage classfcatons for sngle-phase and three-phase systems. The termnology used dvdes voltage classes nto: Low voltage Medum voltage Hgh voltage Extra-hgh voltage Ultra-hgh voltage Table.- presents the nomnal system voltages for these classfcatons. Table.-. Standard Nomnal System Voltages and Voltage Ranges (From IEEE Standard -) Voltage Nomnal System Voltage Class Three-Wre Four-Wre Low voltage Medum voltage Hgh voltage Extra-hgh voltage Ultra-hgh voltage 0/ ,0,00,000,00,000,000,000,000,000,000,000 00,000,000 BILBasc Impulse Levels Y/ 0/ 0Y/ ANSI standards defne recommended and requred BIL levels for: Metal-clad swtchgear (typcally vacuum breakers) Metal-enclosed swtchgear (typcally load nterrupters, swtches) Lqud mmersed transformers Dry-type transformers Table.- through Table.- contan those values.,0,000 Y/00 Y/00 00Y/0 0Y/0 0Y/ 00Y/0 0Y/00 0Y/0 0Y/00 00Y/ Table.-. Metal-Clad Swtchgear Voltage and Insulaton Levels (From ANSI/IEEE C..-) Rated Maxmum Voltage (kv rms) Impulse Wthstand (kv) 0 0 Table.-. Metal-Enclosed Swtchgear Voltage and Insulaton Levels (From ANSI C..-) Rated Maxmum Voltage (kv rms) Impulse Wthstand (kv) 0 0 Table.-. Lqud-Immersed Transformers Voltage and Basc Lghtnng Impulse Insulaton Levels (BIL) (From ANSI/IEEE C ) Applcaton Dstrbuton Nomnal System Voltage (kv rms) Power BIL (kv Crest) BIL values n bold typeface are lsted as standard. Others lsted are n common use. Table.-. Dry-Type Transformers Voltage and Basc Lghtnng Impulse Insulaton Levels (BIL)From ANSI/IEEE C..0-) Nomnal BIL (kv Crest) System Voltage (kv rms) BIL values n bold typeface are lsted as standard. Others lsted are n common use. Optonal hgher levels used where exposure to overvoltage occurs and hgher protecton margns are requred. Lower levels where surge arrester protectve devces can be appled wth lower spark-over levels. Voltage Recommendatons by Motor Horsepower Some factors affectng the selecton of motor operatng voltage nclude: Motor, motor starter and cable frst cost Motor, motor starter and cable nstallaton cost Motor and cable losses Motor avalablty Voltage drop Qualfcatons of the buldng operatng staff; and many more The followng table s based n part on the above factors and experence. Because all the factors affectng the selecton are rarely known, t s only an approxmate gudelne. Table.-. Selecton of Motor Horsepower Ratngs as a Functon of System Voltage Motor Voltage (Volts) ,0 Motor hp Range up to 00 0 to 00 0 to to 000 above 00 System Voltage ,00 CA000E

7 August Sheet 00 System Desgn.- Table.-. Three-Phase Transformer Wndng Connectons Phasor Dagram DELTA-DELTA Connecton Phasor Dagram: H X Notes. Sutable for both ungrounded and effectvely grounded sources.. Sutable for a three-wre servce or a four-wre servce wth a md-tap ground. H H X X Angular Dsplacement (Degrees): 0 DELTA-WYE Connecton Phasor Dagram: H H H X Angular Dsplacement (Degrees): 0 WYE-DELTA Connecton Phasor Dagram: H H H X Angular Dsplacement (Degrees): 0 WYE-WYE Connecton Phasor Dagram: H H H X X X0 X X X X X0 X. Sutable for both ungrounded and effectvely grounded sources.. Sutable for a three-wre servce or a four-wre grounded servce wth XO grounded.. Wth XO grounded, the transformer acts as a ground source for the secondary system.. Fundamental and harmonc frequency zero-sequence currents n the secondary lnes suppled by the transformer do not flow n the prmary lnes. Instead the zero sequence currents crculate n the closed delta prmary wndngs.. When suppled from an effectvely grounded prmary system does not see load unbalances and ground faults n the secondary system.. Sutable for both ungrounded and effectvely grounded sources.. Sutable for a three-wre servce or a four-wre delta servce wth a md-tap ground.. Groundng the prmary neutral of ths connecton would create a ground source for the prmary system. Ths could subject the transformer to severe overloadng durng a prmary system dsturbance or load unbalance.. Frequently nstalled wth md-tap ground on one leg when supplyng combnaton three-phase and sngle-phase load where the three-phase load s much larger than sngle-phase load.. When used n kv and kv three-phase four-wre prmary systems, ferroresonance can occur when energzng or de-energzng the transformer usng sngle-pole swtches located at the prmary termnals. Wth smaller kva transformers the probablty of ferroresonance s hgher.. Sutable for both ungrounded and effectvely grounded sources.. Sutable for a three-wre servce only, even f XO s grounded.. Ths connecton s ncapable of furnshng a stablzed neutral and ts use may result n phase-to-neutral overvoltage (neutral shft) as a result of unbalanced phase-to-neutral load.. If a three-phase unt s bult on a three-legged core, the neutral pont of the prmary wndngs s practcally locked at ground potental. Angular Dsplacement (Degrees): 0 GROUNDED WYE-WYE Connecton Phasor Dagram: H H0 H H X Angular Dsplacement (Degrees): 0 DELTA-DELTA Connecton wth Tap Phasor Dagram: H X X X0 X X. Sutable for four-wre effectvely grounded source only.. Sutable for a three-wre servce or for four-wre grounded servce wth XO grounded.. Three-phase transformers wth ths connecton may experence stray flux tank heatng durng certan external system unbalances unless the core confguraton (four or fve legged) used provdes a return path for the flux.. Fundamental and harmonc frequency zero-sequence currents n the secondary lnes suppled by the transformer also flow n the prmary lnes (and prmary neutral conductor).. Ground relay for the prmary system may see load unbalances and ground faults n the secondary system. Ths must be consdered when coordnatng overcurrent protectve devces.. Three-phase transformers wth the neutral ponts of the hgh voltage and low voltage wndngs connected together nternally and brought out through an HOXO bushng should not be operated wth the HOXO bushng ungrounded (floatng). To do so can result n very hgh voltages n the secondary systems.. Sutable for both ungrounded and effectvely grounded sources.. Sutable for a three-wre servce or a four-wre servce wth a md-tap ground.. When usng the tap for sngle-phase crcuts, the sngle-phase load kva should not exceed % of the three-phase kva ratng of the transformer. The three-phase ratng of the transformer s also substantally reduced. H H X Angular Dsplacement (Degrees): 0 X CA000E

8 .- System Desgn August Sheet 00 Types of Systems In many cases, power s suppled by the utlty to a buldng at the utlzaton voltage. In these cases, the dstrbuton of power wthn the buldng s acheved through the use of a smple radal dstrbuton system. In cases where the utlty servce voltage s at some voltage hgher than the utlzaton voltage wthn the buldng, the system desgn engneer has a choce of a number of types of systems that may be used. Ths dscusson covers several major types of dstrbuton systems and practcal modfcatons of them.. Smple radal. Loop-prmary system radal secondary system. Prmary selectve system secondary radal system. Two-source prmary secondary selectve system. Sparng transformer system. Smple spot network. Medum voltage dstrbuton system desgn. Smple Radal System The conventonal smple radal system receves power at the utlty supply voltage at a sngle substaton and steps the voltage down to the utlzaton level. In those cases where the customer receves hs supply from the prmary system and owns the prmary swtch and transformer along wth the secondary low voltage swtchboard or swtchgear, the equpment may take the form of a separate prmary swtch, separate transformer, and separate low voltage swtchgear or swtchboard. Ths equpment may be combned n the form of an outdoor pad-mounted transformer wth nternal prmary fused swtch and secondary man breaker feedng an ndoor swtchboard. Another alternatve would be a secondary unt substaton where the prmary fused swtch, transformer and secondary swtchgear or swtchboard are desgned and nstalled as a close-coupled sngle assembly. In those cases where the utlty owns the prmary equpment and transformer, the supply to the customer s at the utlzaton voltage, and the servce equpment then becomes low voltage man dstrbuton swtchgear or a swtchboard. Low voltage feeder crcuts run from the swtchgear or swtchboard assembles to panelboards that are located closer to ther respectve loads as shown n Fgure.-. Each feeder s connected to the swtchgear or swtchboard bus through a crcut breaker or other overcurrent protectve devce. A relatvely small number of crcuts are used to dstrbute power to the loads from the swtchgear or swtchboard assembles and panelboards. Because the entre load s served from a sngle source, full advantage can be taken of the dversty among the loads. Ths makes t possble to mnmze the nstalled transformer capacty. However, the voltage regulaton and effcency of ths system may be poor because of the low voltage feeders and sngle source. The cost of the low voltagefeeder crcuts and ther assocated crcut breakers are hgh when the feeders are long and the peak demand s above 00 kva. Prmary Fused Swtch Dstrbuton Panel Transformer Fgure.-. Smple Radal System MCC Fgure.-. Prmary and Secondary Smple Radal System A fault on the secondary low voltage bus or n the source transformer wll nterrupt servce to all loads. Servce cannot be restored untl the necessary repars have been made. A low voltage feeder crcut fault wll nterrupt servce to all loads suppled over that feeder. A modern and mproved form of the conventonal smple radal system dstrbutes power at a prmary voltage. The voltage s stepped down to utlzaton level n the several load areas wthn the buldng typcally through secondary unt substaton transformers. The transformers are usually connected to ther assocated load bus through a crcut breaker, as shown n Fgure.-. Each secondary unt substaton s an assembled unt consstng of a three-phase, lqudflled or ar-cooled transformer, an ntegrally connected prmary fused swtch, and low voltage swtchgear or swtchboard wth crcut breakers or fused swtches. Crcuts are run to the loads from these low voltage protectve devces. Dstrbuton Panel 00V Class Swtchboard Prmary Man Breaker Secondary Unt Substaton Dstrbuton Dry-Type Transformer Lghtng Panelboard Prmary Feeder Breakers Prmary Cables CA000E

9 August Sheet 00 System Desgn.- Because each transformer s located wthn a specfc load area, t must have suffcent capacty to carry the peak load of that area. Consequently, f any dversty exsts among the load area, ths modfed prmary radal system requres more transformer capacty than the basc form of the smple radal system. However, because power s dstrbuted to the load areas at a prmary voltage, losses are reduced, voltage regulaton s mproved, feeder crcut costs are reduced substantally, and large low voltage feeder crcut breakers are elmnated. In many cases the nterruptng duty mposed on the load crcut breakers s reduced. Ths modern form of the smple radal system wll usually be lower n ntal nvestment than most other types of prmary dstrbuton systems for buldngs n whch the peak load s above 00 kva. A fault on a prmary feeder crcut or n one transformer wll cause an outage to only those secondary loads served by that feeder or transformer. In the case of a prmary man bus fault or a utlty servce outage, servce s nterrupted to all loads untl the trouble s elmnated. Reducng the number of transformers per prmary feeder by addng more prmary feeder crcuts wll mprove the flexblty and servce contnuty of ths system; the ultmate beng one secondary unt substaton per prmary feeder crcut. Ths of course ncreases the nvestment n the system but mnmzes the extent of an outage resultng from a transformer or prmary feeder fault. Prmary connectons from one secondary unt substaton to the next secondary unt substaton can be made wth double lugs on the unt substaton prmary swtch as shown, or wth separable connectors made n manholes or other locatons. Dependng on the load kva connected to each prmary crcut and f no ground fault protecton s desred for ether the prmary feeder conductors and transformers connected to that feeder or the man bus, the prmary man and/or feeder breakers may be changed to prmary fused swtches. Ths wll sgnfcantly reduce the frst cost, but also decrease the level of conductor and equpment protecton. Thus, should a fault or overload condton occur, downtme ncreases sgnfcantly and hgher costs assocated wth ncreased damage levels and the need for fuse replacement s typcally encountered. CA000E In addton, f only one prmary fuse on a crcut opens, the secondary loads are then sngle phased, causng damage to low voltage motors. Another approach to reducng costs s to elmnate the prmary feeder breakers completely, and use a sngle prmary man breaker or fused swtch for protecton of a sngle prmary feeder crcut wth all secondary unt substatons suppled from ths crcut. Although ths system results n less ntal equpment cost, system relablty s reduced drastcally because a sngle fault n any part of the prmary conductor would cause an outage to all loads wthn the faclty.. Loop Prmary System Radal Secondary System Ths system conssts of one or more PRIMARY LOOPS wth two or more transformers connected on the loop. Ths system s typcally most effectve when two servces are avalable from the utlty as shown n Fgure.-. Each prmary loop s operated such that one of the loop sectonalzng swtches s kept open to prevent parallel operaton of the sources. When secondary unt substatons are used, each transformer has ts own duplex (-load Prmary Man Breaker Loop A NC Fgure.-. Loop PrmaryRadal Secondary System NC Loop B NC NO Te Breaker Secondary Unt Substatons Consstng of: Duplex Prmary Swtches/Fused Prmary Swtches/ Transformer and Secondary Man Feeder Breakers break swtches wth load sde bus connecton) sectonalzng swtches and prmary load break fused swtch as shown n Fgure.-. When pad-mounted compartmentalzed transformers are used, they are furnshed wth loop-feed ol-mmersed gang-operated load break sectonalzng swtches and drawout current lmtng fuses n dry wells as shown n Fgure.-. By operatng the approprate sectonalzng swtches, t s possble to dsconnect any secton of the loop conductors from the rest of the system. In addton, by openng the transformer prmary swtch (or removng the load break drawout fuses n the pad-mounted transformer) t s possble to dsconnect any transformer from the loop. A key nterlockng scheme s normally recommended to prevent closng all sectonalzng devces n the loop. Each prmary loop sectonalzng swtch and the feeder breakers to the loop are nterlocked such that to be closed they requre a key (whch s held captve untl the swtch or breaker s opened) and one less key than the number of key nterlock cylnders s furnshed. An extra key s provded to defeat the nterlock under qualfed supervson. NC NC Prmary Man Breaker NO NC NC NC Loop Feeder Breaker Fault Sensors

10 .- System Desgn August Sheet 0 Loop Feeder Loop Feeder Load Break Loop Swtches Fused Dsconnect Swtch Fgure.-. Secondary Unt Substaton Loop Swtchng Loop Feeder Loop Feeder Load Break Loop Swtches Load Break Drawout Fuses Fgure.-. Pad-Mounted Transformer Loop Swtchng In addton, the two prmary man breakers, whch are normally closed, and prmary te breaker, whch s normally open, are ether mechancally or electrcally nterlocked to prevent parallelng the ncomng source lnes. For slghtly added cost, an automatc throw-over scheme can be added between the two man breakers and te breaker. Durng the more common event of a utlty outage, the automatc transfer scheme provdes sgnfcantly reduced power outage tme. The system n Fgure.- has hgher costs than n Fgure.-, but offers ncreased relablty and quck restoraton of servce when ) a utlty outage occurs, ) a prmary feeder conductor fault occurs, or ) a transformer fault or overload occurs. Should a utlty outage occur on one of the ncomng lnes, the assocated prmary man breaker s opened and the te breaker closed ether manually or through an automatc transfer scheme. When a prmary feeder conductor fault occurs, the assocated loop feeder breaker opens and nterrupts servce to all loads up to the normally open prmary loop load break swtch (typcally half of the loads). Once t s determned whch secton of prmary cable has been faulted, the loop sectonalzng swtches on each sde of the faulted conductor can be opened, the loop sectonalzng swtch that had been prevously left open then closed and servce restored to all secondary unt substatons whle the faulted conductor s replaced. If the fault should occur n a conductor drectly on the load sde of one of the loop feeder breakers, the loop feeder breaker s kept open after trppng and the next load sde loop sectonalzng swtch manually opened so that the faulted conductor can be sectonalzed and replaced. Note: Under ths condton, all secondary unt substatons are suppled through the other loop feeder crcut breaker, and thus all conductors around the loop should be szed to carry the entre load connected to the loop. Increasng the number of prmary loops (two loops shown n Fgure.-) wll reduce the extent of the outage from a conductor fault, but wll also ncrease the system nvestment. When a transformer fault or overload occurs, the transformer prmary fuses open, and the transformer prmary swtch manually opened, dsconnectng the transformer from the loop, and leavng all other secondary unt substaton loads unaffected. Prmary Metal-Clad Swtchgear Lneup Bus A Feeder A Loop A Fgure.-. Basc Prmary SelectveRadal Secondary System NO NC NO NC NO NC Feeder B Loop A In cases where only one prmary lne s avalable, the use of a sngle prmary breaker provdes the loop connectons to the loads as shown here. Fgure.-. Sngle Prmary Feeder Loop System A basc prmary loop system that uses a sngle prmary feeder breaker connected drectly to two loop feeder swtches whch n turn then feed the loop s shown n Fgure.-. In ths basc system, the loop may be normally operated wth one of the loop sectonalzng swtches open as descrbed above or wth all loop sectonalzng swtches closed. If a fault occurs n the basc prmary loop system, the sngle loop feeder breaker trps, and secondary loads are lost untl the faulted conductor s found and elmnated from the loop by openng the approprate loop sectonalzng swtches and then reclosng the breaker.. Prmary Selectve System Secondary Radal System The prmary selectvesecondary radal system, as shown n Fgure.-, dffers from those prevously descrbed n that t employs at least two prmary feeder crcuts n each load area. It s Prmary Man Breaker Bus B Prmary Feeder Breaker Feeder B Feeder A To Other Substatons Typcal Secondary Unt Substaton Duplex Prmary Swtch/Fuses Transformer/00V Class Secondary Swtchgear CA000E

11 August Sheet 0 System Desgn.- desgned so that when one prmary crcut s out of servce, the remanng feeder or feeders have suffcent capacty to carry the total load. Half of the transformers are normally connected to each of the two feeders. When a fault occurs on one of the prmary feeders, only half of the load n the buldng s dropped. Duplex fused swtches as shown n Fgure.- and detaled n Fgure.- are the normal choce for ths type of system. Each duplex fused swtch conssts of two () load break threepole swtches each n ther own separate structure, connected together by bus bars on the load sde. Typcally, the load break swtch closest to the transformer ncludes a fuse assembly wth fuses. Mechancal and/or key nterlockng s furnshed such that both swtches cannot be closed at the same tme (to prevent parallel operaton) and nterlockng such that access to ether swtch or fuse assembly cannot be obtaned unless both swtches are opened. Prmary Feeders Load Break Swtches Fuses Fgure.-. Duplex Fused Swtch n Two Structures As an alternate to the duplex swtch arrangement, a non-load break selector swtch mechancally nterlocked wth a load break fused swtch can be used as shown n Fgure.-. The non-load break selector swtch s physcally located n the rear of the load break fused swtch, thus only requrng one structure and a lower cost and floor space savngs over the duplex arrangement. The non-load break swtch s mechancally nterlocked to prevent ts operaton unless the load break swtch s opened. The man dsadvantage of the selector swtch s that conductors from both crcuts are termnated n the same structure. Prmary Feeders Interlock Non-Load Break Selector Swtches Load Break Dsconnect Fuses Fgure.-. Fused Selector Swtch n One Structure Ths means lmted cable space especally f double lugs are furnshed for each lne as shown n Fgure.- and should a faulted prmary conductor have to be changed, both lnes would have to be de-energzed for safe changng of the faulted conductors. In Fgure.- when a prmary feeder fault occurs, the assocated feeder breaker opens and the transformers normally suppled from the faulted feeder are out of servce. Then manually, each prmary swtch connected to the faulted lne must be opened and then the alternate lne prmary swtch can be closed connectng the transformer to the lve feeder, thus restorng servce to all loads. Note that each of the prmary crcut conductors for Feeder A and B must be szed to handle the sum of the loads normally connected to both A and B. Smlar szng of Feeders A and B, etc., s requred. If a fault occurs n one transformer, the assocated prmary fuses blow and nterrupt the servce to just the load served by that transformer. Servce cannot be restored to the loads normally served by the faulted transformer untl the transformer s repared or replaced. Cost of the prmary selectve secondary radal system s greater than that of the smple prmary radal system of Fgure.- because of the addtonal prmary man breakers, te breaker, two-sources, ncreased number of feeder breakers, the use of prmaryduplex or selector swtches, and the greater amount of prmary feeder cable requred. The benefts from the reducton n the amount of load lost when a prmary feeder s faulted, plus the quck restoraton of servce to all or most of the loads, may more than offset the greater cost. Havng two sources allows for ether manual or automatc transfer of the two prmary man breakers and te breaker should one of the sources become unavalable. The prmary selectve-secondary radal system, however, may be less costly or more costly than a prmary loop secondary radal system of Fgure.- dependng on the physcal locaton of the transformers whle offerng comparable downtme and relablty. The cost of conductors for the two types of systems may vary greatly dependng on the locaton of the transformers and loads wthn the faclty and greatly overrde prmary swtchng equpment cost dfferences between the two systems.. Two-Source Prmary Secondary Selectve System Ths system uses the same prncple of duplcate sources from the power supply pont usng two prmary man breakers and a prmary te breaker. The two prmary man breakers and prmary te breaker beng ether manually or electrcally nterlocked to prevent closng all three at the same tme and parallelng the sources. Upon loss of voltage on one source, a manual or automatc transfer to the alternate source lne may be used to restore power to all prmary loads. Each transformer secondary s arranged n a typcal double-ended unt substaton arrangement as shown n Fgure.-. The two secondary man breakers and secondary te breaker of each unt substaton are agan ether mechancally or electrcally nterlocked to prevent parallel operaton. Upon loss of secondary source voltage on one sde, manual or automatc transfer may be used to transfer the loads to the other sde, thus restorng power to all secondary loads. Ths arrangement permts quck restoraton of servce to all loads when a prmary feeder or transformer fault occurs by openng the assocated secondary man and closng the secondary te breaker. If the loss of secondary voltage has occurred because of a prmary feeder fault wth the assocated prmary feeder breaker openng, then all secondary loads normally served by the faulted feeder would have to be transferred to the opposte prmary feeder. Ths means each prmary feeder conductor must be szed to carry the load on both sdes of all the secondary buses t s CA000E

12 .- System Desgn August Sheet 0 servng under secondary emergency transfer. If the loss of voltage was due to a falure of one of the transformers n the double-ended unt substaton, then the assocated prmary fuses would open takng only the faled transformer out of servce, and then only the secondary loads normally served by the faulted transformer would have to be transferred to the opposte transformer. In ether of the above To Other Substatons Typcal Double-Ended Unt Substaton Prmary Fused Swtch Fgure.-. Two-Source PrmarySecondary Selectve System Fgure.-. Sparng Transformer System K K K Transformer K K K emergency condtons, the n-servce transformer of a double-ended unt substaton would have to have the capablty of servng the loads on both sdes of the te breaker. For ths reason, transformers used n ths applcaton have equal kva ratng on each sde of the double-ended unt substaton and the normal operatng maxmum load on each transformer s typcally about / base nameplate kva ratng. Typcally these Te Breaker Prmary Man Breakers Prmary Feeder Breakers To Other Substatons Secondary Man Breaker K Sparng Transformer Typcal Secondary Busway Loop Typcal Sngle-Ended Substaton transformers are furnshed wth fan-coolng and/or lower than normal temperature rse such that under emergency condtons they can carry on a contnuous bass the maxmum load on both sdes of the secondary te breaker. Because of ths spare transformer capacty, the voltage regulaton provded by the double-ended unt substaton system under normal condtons s better than that of the systems prevously dscussed. The double-ended unt substaton arrangement can be used n conjuncton wth any of the prevous systems dscussed, whch nvolve two prmary sources. Although not recommended, f allowed by the utlty, momentary re-transfer of loads to the restored source may be made closed transton (ant-parallel nterlock schemes would have to be defeated) for ether the prmary or secondary systems. Under ths condton, all equpment nterruptng and momentary ratngs should be sutable for the fault current avalable from both sources. For double-ended unt substatons equpped wth ground fault systems specal consderaton to transformer neutral groundng and equpment operaton should be madesee Groundng and Ground Fault Protecton n Secton.. Where two sngle-ended unt substatons are connected together by external te conductors, t s recommended that a te breaker be furnshed at each end of the te conductors.. Sparng Transformer System The sparng transformer system concept came nto use as an alternatve to the captal cost ntensve double-ended secondary unt substaton dstrbuton system (see Two-Source Prmary Secondary Selectve System). It essentally replaces double-ended substatons wth sngle-ended substatons and one or more sparng transformer substatons all nterconnected on a common secondary bus (see Fgure.-). Generally no more than three to fve sngle-ended substatons are on a sparng loop. The essence of ths desgn phlosophy s that conservatvely desgned and loaded transformers are hghly relable electrcal devces and rarely fal. Therefore, ths desgn provdes a sngle common backup transformer for a group of transformers n leu of a backup transformer for each and every transformer. Ths system desgn stll mantans a hgh degree of contnuty of servce. CA000E

13 August Sheet 0 System Desgn.- Referrng to Fgure.-, t s apparent that the sparng concept backs up prmary swtch and prmary cable falure as well. Restoraton of lost or faled utlty power s accomplshed smlarly to prmary selectve scheme prevously dscussed. It s therefore mportant to use an automatc throwover system n a two source lneup of prmary swtchgear to restore utlty power as dscussed n the Two-Source Prmary schemesee Fgure.-. A major advantage of the sparng transformer system s the typcally lower total base kva of transformaton. In a double-ended substaton desgn, each transformer must be rated to carry the sum of the loads of two busses and usually requres the addton of coolng fans to accomplsh ths ratng. In the sparng concept, each transformer carres only ts own load, whch s typcally not a fan-cooled ratng. Major space savngs s also a beneft of ths system n addton to frst cost savngs. The sparng transformer system operates as follows: All man breakers, ncludng the sparng man breaker, are normally closed; the te breakers are normally open Once a transformer (or prmary cable or prmary swtch/fuse) fals, the assocated secondary man breaker s opened. The assocated te breaker s then closed, whch restores power to the sngle-ended substaton bus Schemes that requre the man to be opened before the te s closed ( open transton ), and that allow any te to be closed before the substaton man s opened, ( closed transton ) are possble Wth a closed transton scheme, t s common to add a tmer functon that opens the te breaker unless ether man breaker s opened wthn a tme nterval. Ths closed transton allows power to be transferred to the sparng transformer wthout nterrupton, such as for routne mantenance, and then back to the substaton. Ths closed transton transfer has an advantage n some facltes; however, approprate nterruptng capactes and bus bracng must be specfed sutable for the momentary parallel operaton. In facltes wthout qualfed electrcal power operators, an open transton wth key nterlockng s often a prudent desgn. Note: Each par of man breaker/te breaker key cylnders should be unquely keyed to prevent any paralleled source operatons. Careful szng of these transformers as well as careful specfcaton of the transformers s requred for relablty. Low temperature rse specfed wth contnuous overload capacty or upgraded types of transformers should be consdered. One dsadvantage to ths system s the external secondary te system, see Fgure.-. As shown, all sngleended substatons are ted together on the secondary wth a te busway or cable system. Locaton of substatons s therefore lmted because of voltage drop and cost consderatons. Routng of busway, f used, must be carefully layed out. It should also be noted, that a te busway or cable fault wll essentally prevent the use of the sparng transformer untl t s repared. Commonly, the sngle-ended substatons and the sparng transformer must be clustered. Ths can also be an advantage, as more kva can be supported from a more compact space layout.. Smple Spot Network Systems The AC secondary network system s the system that has been used for many years to dstrbute electrc power n the hgh-densty, downtown areas of ctes, usually n the form of utlty grds. Modfcatons of ths type of system make t applcable to serve loads wthn buldngs. Prmary Crcut Network Transformer Network Protector Optonal Man, 0/ Relayng and/or Network Dsconnect LV Feeder Customer Loads Fuses Te NC Fgure.-. Three-Source Spot Network The major advantage of the secondary network system s contnuty of servce. No sngle fault anywhere on the prmary system wll nterrupt servce to any of the system s loads. Most faults wll be cleared wthout nterruptng servce to any load. Another outstandng advantage that the network system offers s ts flexblty to meet changng and growng load condtons at mnmum cost and mnmum nterrupton n servce to other loads on the network. In addton to flexblty and servce relablty, the secondary network system provdes exceptonally unform and good voltage regulaton, and ts hgh effcency materally reduces the costs of system losses. Three major dfferences between the network system and the smple radal system account for the outstandng advantages of the network. Frst, a network protector s connected n the secondary leads of each network transformer n place of, or n addton to, the secondary man breaker, as shown n Fgure.-. Also, the secondares of each transformer n a gven locaton (spot) are connected together by a swtchgear or rng bus from whch the loads are served over short radal feeder crcuts. Fnally, the prmary supply has suffcent capacty to carry the entre buldng load wthout overloadng when any one prmary feeder s out of servce. A network protector s a specally desgned heavy-duty ar power breaker, sprng close wth electrcal motor-charged mechansm, wth a network relay to control the status of the protector (trpped or closed). The network relay s usually a sold-state mcroprocessorbased component ntegrated Customer Loads Te NC Customer Loads Typcal Feeder To Other Networks Drawout Low Voltage Swtchgear CA000E

14 .- System Desgn August Sheet 0 nto the protector enclosure that functons to automatcally close the protector only when the voltage condtons are such that ts assocated transformer wll supply power to the secondary network loads, and to automatcally open the protector when power flows from the secondary to the network transformer. The purpose of the network protector s to protect the ntegrty of the network bus voltage and the loads served from t aganst transformer and prmary feeder faults by quckly dsconnectng the defectve feeder-transformer par from the network when backfeed occurs. The smple spot network system resembles the secondary-selectve radal system n that each load area s suppled over two or more prmary feeders through two or more transformers. In network systems, the transformers are connected through network protectors to a common bus, as shown n Fgure.-, from whch loads are served. Because the transformers are connected n parallel, a prmary feeder or transformer fault does not cause any servce nterrupton to the loads. The paralleled transformers supplyng each load bus wll normally carry equal load currents, whereas equal loadng of the two separate transformers supplyng a substaton n the secondaryselectve radal system s dffcult to obtan. The nterruptng duty mposed on the outgong feeder breakers n the network wll be greater wth the spot network system. The optmum sze and number of prmary feeders can be used n the spot network system because the loss of any prmary feeder and ts assocated transformers does not result n the loss of any load even for an nstant. In spte of the spare capacty usually suppled n network systems, savngs n prmary swtchgear and secondary swtchgear costs often result when compared to a radal system desgn wth smlar spare capacty. Ths occurs n many radal systems because more and smaller feeders are often used n order to mnmze the extent of any outage when a prmary fault event occurs. In spot networks, when a fault occurs on a prmary feeder or n a transformer, the fault s solated from the system through the automatc trppng of the prmary feeder crcut breaker and all of the network protectors assocated wth that feeder crcut. Ths operaton does not nterrupt servce to any loads. After the necessary repars have been made, the system can be restored to normal operatng condtons by closng the prmary feeder breaker. All network protectors assocated wth that feeder wll close automatcally. The chef purpose of the network bus normally closed tes s to provde for the sharng of loads and a balancng of load currents for each prmary servce and transformer regardless of the condton of the prmary servces. Also, the tes provde a means for solatng and sectonalzng ground fault events wthn the swtchgear network bus, thereby savng a porton of the loads from servce nterruptons, yet solatng the faulted porton for correctve acton. The use of spot network systems provdes users wth several mportant advantages. Frst, they save transformer capacty. Spot networks permt equal loadng of transformers under all condtons. Also, networks yeld lower system losses and greatly mprove voltage condtons. The voltage regulaton on a network system s such that both lghts and power can be fed from the same load bus. Much larger motors can be started across-the-lne than on a smple radal system. Ths can result n smplfed motor control and permts the use of relatvely large low voltage motors wth ther less expensve control. Fnally, network systems provde a greater degree of flexblty n addng future loads; they can be connected to the closest spot network bus. Spot network systems are economcal for buldngs that have heavy concentratons of loads coverng small areas, wth consderable dstance between areas, and lght loads wthn the dstances separatng the concentrated loads. They are commonly used n hosptals, hgh rse offce buldngs, and nsttutonal buldngs where a hgh degree of servce relablty s requred from the utlty sources. Spot network systems are especally economcal where three or more prmary feeders are avalable. Prncpally, ths s due to supplyng each load bus through three or more transformers and the reducton n spare cable and transformer capacty requred. They are also economcal when compared to two transformer doubleended substatons wth normally opened te breakers. Emergency power should be connected to network loads downstream from the network, or upstream at prmary voltage, not at the network bus tself.. Medum Voltage Dstrbuton System Desgn A. Sngle Bus, Fgure.- The sources (utlty and/or generator(s)) are connected to a sngle bus. All feeders are connected to the same bus. Utlty Man Bus Fgure.-. Sngle Bus Ths confguraton s the smplest system; however, outage of the utlty results n total outage. Normally the generator does not have adequate capacty for the entre load. A properly relayed system equpped wth load sheddng, automatc voltage/ frequency control may be able to mantan partal system operaton. Any future addton of breaker sectons to the bus wll requre a shutdown of the bus, because there s no te breaker. B. Sngle Bus wth Two Sources from the Utlty, Fgure.- Same as the sngle bus, except that two utlty sources are avalable. Ths system s operated normally wth the man breaker to one source open. Upon loss of the normal servce, the transfer to the standby normally open (NO) breaker can be automatc or manual. Automatc transfer s preferred for rapd servce restoraton especally n unattended statons. G One of Several Feeders CA000E

15 August Sheet 0 System Desgn.- Utlty # Normal Fgure.-. Sngle Bus wth Two-Sources Retransfer to the Normal can be closed transton subject to the approval of the utlty. Closed transton momentarly ( cycles) parallels both utlty sources. Cauton: when both sources are paralleled, the fault current avalable on the load sde of the man devce s the sum of the avalable fault current from each source plus the motor fault contrbuton. It s recommended that the short-crcut ratngs of the bus, feeder breakers and all load sde equpment are rated for the ncreased avalable fault current. If the utlty requres open transfer, the dsconnecton of motors from the bus must be ensured by means of sutable tme delay on reclosng as well as supervson of the bus voltage and ts phase wth respect to the ncomng source voltage. Ths busng scheme does not preclude the use of cogeneraton, but requres the use of sophstcated automatc synchronzng and synchronsm checkng controls, n addton to the prevously mentoned load sheddng, automatc frequency and voltage controls. Ths confguraton s more expensve than the scheme shown n Fgure.-, but servce restoraton s qucker. Agan, a utlty outage results n total outage to the load untl transfer occurs. Extenson of the bus or addng breakers requres a shutdown of the bus. If parallelng sources, reverse current, reverse power and other approprate relayng protecton should be added as requested by the utlty. C. Multple Sources wth Te Breaker, Fgure.- and Fgure.- Ths confguraton s smlar to confguraton B. It dffers sgnfcantly n that both utlty sources normally carry the loads and also by the ncorporaton of a normally open te breaker. The outage to the system load for a utlty outage s lmted to half of the system. CA000E Utlty # Standby NC NO Loads Agan, the closng of the te breaker can be manual or automatc. The statements made for the retransfer of scheme B apply to ths scheme also. Utlty # NC NC Load Utlty # NO Bus # Bus # Load Fgure.-. Two-Source Utlty wth Te Breaker If looped or prmary selectve dstrbuton system for the loads s used, the buses can be extended wthout a shutdown by closng the te breaker and transferrng the loads to the other bus. Ths confguraton s more expensve than B. The system s not lmted to two buses only. Another advantage s that the desgn may ncorporate momentary parallelng of buses on retransfer after the faled lne has been restored to prevent another outage. See the Cauton for Fgures.-,.- and.-. In Fgure.-, closng of the te breaker followng the openng of a man breaker can be manual or automatc. However, because a bus can be fed through two te breakers, the control scheme should be desgned to make the selecton. Utlty # Utlty # NO Fgure.-. Trple-Ended Arrangement The thrd te breaker allows any bus to be fed from any utlty source. Cauton for Fgures.-,.- and.-: If contnuous parallelng of sources s planned, reverse current, reverse power and other approprate relayng protecton should be added. When both sources are paralleled for any amount of tme, the fault current avalable on the load sde of the man devce s the sum of the avalable fault current from each source plus the motor fault contrbuton. It s requred that bus bracng, feeder breakers and all load sde equpment s rated for the ncreased avalable fault current. Summary The medum voltage system confguratons shown are based on usng metalclad drawout swtchgear. The servce contnuty requred from electrcal systems makes the use of sngle-source systems mpractcal. In the desgn of a modern medum voltage system, the engneer should:. Desgn a system as smple as possble.. Lmt an outage to as small a porton of the system as possble.. Provde means for expandng the system wthout major shutdowns.. Relay the system so that only the faulted part s removed from servce, and damage to t s mnmzed consstent wth selectvty.. Specfy and apply all equpment wthn ts publshed ratngs and natonal standards pertanng to the equpment and ts nstallaton. NC NC NC NO Bus # Bus # Te Busway NO Typcal Feeder Utlty # Bus # NO

16 .- System Desgn August Sheet 0 Health Care Facltes Health care facltes are defned by NFPA (Natonal Fre Protecton Agency) as Buldngs or portons of buldngs n whch medcal, dental, psychatrc, nursng, obstetrcal, or surgcal care are provded. Due to the crtcal nature of the care beng provded at these facltes and ther ncreasng dependence on electrcal equpment for preservaton of lfe, health care facltes have specal requrements for the desgn of ther electrcal dstrbuton systems. These requrements are typcally much more strngent than commercal or ndustral facltes. The followng secton summarzes some of the unque requrements of health care faclty desgn. There are several agences and organzatons that develop requrements for health care electrcal dstrbuton system desgn. The followng s a lstng of some of the specfc NFPA (Natonal Fre Protecton Agency) standards that affect health care faclty desgn and mplementaton: NFPA -Standard for Statonary Combuston Engnes and Gas Turbnes NFPA 0-Natonal Electrcal Code NFPA -0Health Care Facltes Normal Source Non-Essental Loads NFPA -0Lfe Safety Code NFPA -Standard for Emergency and Standby Power Systems NFPA -Standard on Stored Electrcal Energy Emergency and Standby Power Systems These NFPA gudelnes represent the most ndustry recognzed standard requrements for health care electrcal desgn. However, the electrcal desgn engneer should consult wth the authortes havng jursdcton over the local regon for specfc electrcal dstrbuton requrements. Health Care Electrcal System Requrements Health care electrcal systems usually consst of two parts:. Non-essental or normal electrcal system.. Essental electrcal system. All electrcal power n a health care faclty s mportant, though some loads are not crtcal to the safe operaton of the faclty. These non-essental or normal loads nclude thngs such as general lghtng, general lab equpment, non-crtcal servce equpment, patent care areas, etc. These loads are not requred to be fed from an alternate source of power. Normal Source Non-Essental Loads Normal Source The electrcal system requrements for the essental electrcal system (EES) vary accordng to the type of health care faclty. Health care facltes are categorzed by NFPA as Type, Type or Type facltes. Some example health care facltes, classfed by type, are summarzed n the followng Table.-. Table.-. Health Care Facltes Descrpton Defnton EES Type Hosptals Nursng homes Lmted care facltes NFPA Chap. NFPA Chap. NFPA Chap. Ambulatory surgcal facltes NFPA Chap. Type Other health care facltes NFPA Chap. Type If electrcal lfe support or crtcal care areas are present, then faclty s classfed as Type. Type Essental Electrcal Systems (EES) Type Type Type Type essental electrcal systems (EES) have the most strngent requrements for provdng contnuty of electrcal servce and wll, therefore, be the focus of ths secton. Type EES requrements meet or exceed the requrements for Type and Type facltes. Emergency Power Supply G Manual Transfer Swtch Delayed Automatc Transfer Swtch Automatc (Non-Delayng) Transfer Swtch Fgure.-. Typcal Large Hosptal Electrcal SystemType Faclty Equpment System Lfe Safety Branch Crtcal Branch Emergency System Essental Electrcal System CA000E

17 August Sheet 0 System Desgn.- Sources: Type systems are requred to have a mnmum of two ndependent sources of electrcal powera normal source that generally supples the entre faclty and one or more alternate sources that supply power when the normal source s nterrupted. The alternate source(s) must be an on-ste generator drven by a prme mover unless a generator(s) exsts as the normal power source. In the case where a generator(s) s used as the normal source, t s permssble for the alternate source to be a utlty feed. Alternate source generators must be classfed as Type, Class X, Level gensets per NFPA Tables.(a) and.(b) that are capable of provdng power to the load n a maxmum of seconds. Typcally, the alternate sources of power are suppled to the loads through a seres of automatc and/or manual transfer swtches (see Tab ). The transfer swtches can be non-delayed automatc, delayed automatc or manual transfer dependng on the requrements of the specfc branch of the EES that they are feedng. It s permssble to feed multple branches or systems of the EES from a sngle automatc transfer swtch provded that the maxmum demand on the EES does not exceed 0 kva. Ths confguraton s typcally seen n smaller health care facltes that must meet Type EES requrements (see Fgure.-). Normal Source Non-Essental Loads Alternate Source Fgure.-. Small Hosptal Electrcal SystemSngle EES Transfer Swtch G Entre Essental Electrc System (0 kva or Less) Table.-. Type EES Applcable Codes Descrpton Standard Secton Desgn Sources Uses Emergency Power Supply Classfcaton NFPA NFPA NFPA NFPA Dstrbuton NFPA NEC thru (-)...0 Systems and Branches of Servce: The Type EES conssts of two separate power systems capable of supplyng power consdered essental for lfe safety and effectve faclty operaton durng an nterrupton of the normal power source. They are the emergency system and the equpment system.. Emergency systemconssts of crcuts essental to lfe safety and crtcal patent care. The emergency system s an electrcal sub-system that must be fed from an automatc transfer swtch or seres of automatc transfer swtches. Ths emergency system conssts of two mandatory branches that provde power to systems and functons essental to lfe safety and crtcal patent care. A. Lfe safety branchsupples power for lghtng, receptacles and equpment to perform the followng functons:. Illumnaton of means of egress.. Ext sgns and ext drecton sgns.. Alarms and alertng systems.. Emergency communcatons systems.. Task llumnaton, battery chargers for battery powered lghtng, and select receptacles at the generator.. Elevator lghtng control, communcaton and sgnal systems.. Automatc doors used for egress. These are the only functons permtted to be on the lfe safety branch. Lfe safety branch equpment and wrng must be entrely ndependent of all other loads and branches of servce. Ths ncludes separaton of raceways, boxes or cabnets. Power must be suppled to the lfe safety branch from a non-delayed automatc transfer swtch. B. Crtcal branchsupples power for task llumnaton, fxed equpment, selected receptacles and selected power crcuts for areas related to patent care. The purpose of the crtcal branch s to provde power to a lmted number of receptacles and locatons to reduce load and mnmze the chances of fault condtons. The transfer swtch(es) feedng the crtcal branch must be automatc type. They are permtted to have approprate tme delays that wll follow the restoraton of the lfe safety branch, but should have power restored wthn seconds of normal source power loss. The crtcal branch provdes power to crcuts servng the followng areas and functons:. Crtcal care areas.. Isolated power systems n specal envronments.. Task llumnaton and selected receptacles n the followng patent care areas: nfant nurseres, medcaton prep areas, pharmacy, selected acute nursng areas, psychatrc bed areas, ward treatment rooms, nurses statons.. Specalzed patent care task llumnaton, where needed.. Nurse call systems.. Blood, bone and tssue banks.. Telephone equpment rooms and closets.. Task llumnaton, selected receptacles and selected power crcuts for the followng: general care beds (at least one duplex receptacle), angographc labs, cardac catheterzaton labs, coronary care unts, hemodalyss rooms, selected emergency room treatment areas, human physology labs, ntensve care unts, selected postoperatve recovery rooms.. Addtonal crcuts and snglephase fracton motors as needed for effectve faclty operaton. CA000E

18 .- System Desgn August Sheet 0 Table.-. Type Emergency System Applcable Codes Descrpton Standard Secton General NFPA NEC Lfe safety branch NFPA NEC Crtcal branch NFPA NEC Wrng NFPA NEC (C). Equpment systemconssts of major electrcal equpment necessary for patent care and Type operaton. The equpment system s a subsystem of the EES that conssts of large electrcal equpment loads needed for patent care and basc hosptal operaton. Loads on the equpment system that are essental to generator operaton are requred to be fed by a non-delayed automatc transfer swtch. The followng equpment must be arranged for delayed automatc transfer to the emergency power supply:. Central sucton systems for medcal and surgcal functons.. Sump pumps and other equpment requred for the safe operaton of a major apparatus.. Compressed ar systems for medcal and surgcal functons.. Smoke control and star pressurzaton systems.. Ktchen hood supply and exhaust systems, f requred to operate durng a fre. The followng equpment must be arranged for delayed automatc or manual transfer to the emergency power supply:. Select heatng equpment.. Select elevators.. Supply, return and exhaust ventlatng systems for surgcal, obstetrcal, ntensve care, coronary care, nurseres and emergency treatment areas.. Supply, return and exhaust ventlatng systems for arborne nfectous/solaton rooms, labs and medcal areas where hazardous materals are used.. Hyperbarc facltes.. Hypobarc facltes.. Autoclavng equpment.. Controls for equpment lsted above.. Other selected equpment n ktchens, laundres, radology rooms and central refrgeraton as selected. Table.-. Type Equpment System Applcable Codes Descrpton Standard Secton General NFPA NEC Equpment NFPA NEC (-).(A)-(B) Any loads served by the generator that are not approved as outlned above as part of the essental electrcal system must be connected through a separate transfer swtch. These transfer swtches must be confgured such that the loads wll not cause the generator to overload and must be shed n the event the generator enters an overload condton. Ground fault protectonper NFPA 0 NEC Artcle., ground fault protecton s requred on any feeder or servce dsconnect 00A or larger on systems wth lne to ground voltages of 0V or greater and phase-to-phase voltages of 00V or less. For health care facltes (of any type), a second level of ground fault protecton s requred to be on the next level of feeder downstream. Ths second level of ground fault s only requred for feeders that serve patent care areas and equpment ntended to support lfe. 0% selectve coordnaton of the two levels of ground fault protecton must be acheved wth a mnmum sx-cycle separaton between the upstream and downstream devce. New n the NEC, ground fault protecton s now allowed between the generator(s) and the EES transfer swtch(es). However, NEC.(B) prohbts the nstallaton of ground fault protecton on the load sde of a transfer swtch feedng EES crcuts (see Fgure.-addtonal level of ground fault). Careful consderaton should be used n applyng ground fault protecton on the essental electrcal system to prevent a ground fault that causes a trp of the normal source to also cause a trp on the emergency source. Such an event could result n complete power loss of both normal and emergency power sources and could not be recovered untl the source of the ground fault was located and solated from the system. To prevent ths condton, NEC 00. removes the ground fault protecton requrement for the Normal Source 0/V ➀ 00 A GF or Larger GF GF GF GF GF Non-Essental Loads Servce Entrance Addtonal Level of Ground Fault Protecton 0/V ➀ 00 A or Larger GF Normal Source(s) Servce Entrance GF GF GF Non-Essental Loads 0/V ➀ 00 A GF or Larger GF GF GF Servce Entrance G Generator Breakers are Suppled wth Ground Fault Alarm Only. (NEC 00.) Ground Fault s Permtted and EES Transfer Swtches. (NEC.(B)) GF = Ground Fault Protecton Requred Essental Electrcal System Addtonal Level of Ground Fault s not Transfer Swtches. (NEC.a()) Fgure.-. Addtonal Level of Ground Fault Protecton Ground fault protecton s requred for servce dsconnects 00A and larger or systems wth less than 00V phase-to-phase and greater than 0V to ground per NEC.. CA000E

19 August Sheet 0 System Desgn.- emergency system source. Typcally, the emergency system generator(s) are equpped wth ground fault alarms that do not automatcally dsconnect power durng a ground fault. Table.-. Ground Fault Protecton Applcable Codes Descrpton Standard Secton Servces Feeders Addtonal level Alternate source NEC NEC NEC NFPA NEC NEC Wet procedure locatonsa wet procedure locaton n a health care faclty s any patent care area that s normally subject to wet condtons whle patents are present. Typcal wet procedure locatons can nclude operatng rooms, anesthetzng locatons, dalyss locatons, etc. (Patent beds, tolets and snks are not consdered wet locatons.) These wet procedure locatons requre specal protecton to guard aganst electrc shock. The ground fault current n these areas must be lmted to not exceed ma. In areas where the nterrupton of power s permssble, ground fault crcut nterrupters (GFCI) can be employed. GFCIs wll nterrupt a crcut when ground fault current exceeds ma (± ma). In areas where the nterrupton of power cannot be tolerated, protecton from ground fault currents s accomplshed through the use of an solated power system. Isolated power systems provde power to an area that s solated from ground (or ungrounded). Ths type of system lmts the amount of current that flows to ground n the event of a sngle lne-to-ground fault and mantans crcut contnuty. Electronc lne solaton montors (LIM) are used to montor and dsplay leakage currents to ground. When leakage current thresholds are exceeded, vsble and/or audble alarms are ntated to alert occupants of a possble hazardous condton. Ths alarm occurs wthout nterruptng power to allow for the safe concluson of crtcal procedures. Table.-. Wet Procedure Locaton Applcable Codes Descrpton Standard Secton General NFPA NEC Isolated power systems NFPA NEC Mantenance and Testng Regular mantenance and testng of the electrcal dstrbuton system n a health care faclty s necessary to ensure proper operaton n an emergency and, n some cases, to mantan government accredtaton. Any health care faclty recevng Medcare or Medcad rembursement from the government must be accredted by the Jont Commsson on Accredtaton of Health Care Organzatons (JCAHO). JCAHO has establshed a group of standards called the Envronment of Care, whch must be met for health care faclty accredtaton. Included n these standards s the regular testng of the emergency (alternate) power system(s). Desel-powered EPS nstallatons must be tested monthly n accordance wth NFPA Standard for Emergency and Standby Power Systems. Generators must be tested for a mnmum of 0 mnutes under the crtera defned n NFPA. One method to automate the task of monthly generator tests s through the use of Power Xpert communcatons. Wth the Power Xpert ntegrated meterng, montorng and control system, a faclty mantenance drector can ntate a generator test, control/montor loads, meter/montor generator test ponts and create a JCAHO complant report automatcally from a central PC. The report contans all metered values, test results, date/tme nformaton, etc. necessary to satsfy JCAHO requrements. Ths automated generator testng procedure reduces the labor, tranng and naccuraces that occur durng manual emergency power system tests. (See Power Montorng Tab.) Table.-. Mantenance and Testng Applcable Codes Descrpton Standard Secton Groundng NFPA... Emergency power system NFPA JCAHO... EC..(d) Generator NFPA. Transfer swtches NFPA..,.. Breakers NFPA NFPA..... Routne mantenance should be performed on crcut breakers, transfer swtches, swtchgear, generator equpment, etc. by traned professonals to ensure the most relable electrcal system possble. See Tab for Eaton s Electrcal Servces & Systems (EESS), whch provdes engneers, traned n development and executon of annual preventatve mantenance procedures of health care faclty electrcal dstrbuton systems. Parallelng Emergency Generators Wthout Utlty Parallelng In many health care facltes (and other large facltes wth crtcal loads), the demand for standby emergency power s large enough to requre multple generator sets to power all of the requred essental electrcal system (EES) loads. In many cases, t becomes more flexble and easer to operate the requred multple generators from a sngle locaton usng generator parallelng swtchgear. Fgure.- on Page.- shows an example of a typcal one-lne for a parallelng swtchgear lneup feedng the EES. A typcal abbrevated sequence of operaton for a multple emergency generator and ATS system follows. Note that other modes of operaton such as generator demand prorty and automated testng modes are avalable but are not ncluded below. (Reference Tab for complete detaled sequences of operaton.). Enterng emergency mode a. Upon loss of normal source, automatc transfer swtches send generator control system a run request. b. All avalable generators are started. The frst generator up to voltage and frequency s closed to the bus. c. Unsheddable loads and load shed Prorty loads are powered n less than seconds. d. The remanng generators are synchronzed and paralleled to the bus as they come up to voltage and frequency. e. As addtonal generators are paralleled to the emergency bus, load shed prorty levels are added, powerng ther assocated loads. f. The system s now n emergency mode.. Ext from emergency mode a. Automatc transfer swtches sense the utlty source s wthn acceptable operatonal tolerances for a tme duraton set at the automatc transfer swtch. CA000E

20 .- System Desgn August Sheet 0 b. As each automatc transfer swtch transfers back to utlty power, t removes ts run request from the generator plant. c. When the last automatc transfer swtch has retransferred to the utlty and all run requests have been removed from the generator plant, all generator crcut breakers are opened. d. The generators are allowed to run for ther programmed cool-down perod. e. The system s now back n automatc/standby mode. Wth Utlty Parallelng Today, many utltes are offerng ther customers excellent fnancal ncentves to use ther on-ste generaton capacty to remove load from the utlty grd. These ncentves are sometmes referred to as lmted nterruptble rates (LIP). Under these ncentves, utltes wll greatly reduce or elmnate kwhr or kw demand charges to ther customers wth on-ste generaton capabltes. In exchange, durng tmes of peak loadng of the utlty grd, the utlty can ask ther LIP rate customers to drop load from the grd by usng ther on-ste generaton capabltes. Health care facltes are deally suted to take advantage of these programs because they already have sgnfcant on-ste generaton capabltes due to the code requrements descrbed. Many health care facltes are takng advantage of these utlty ncentves by addng generator capacty over and above the NFPA requrements. Fgure.- on Page.- shows an example one-lne of a health care faclty wth complete generator backup and utlty nterconnect. NFPA requrements state that the normal and emergency sources must be separated by a fre-rated wall. The ntent of ths requrement s so that a fre n one locaton cannot take out both sources of power. To meet ths requrement, the parallelng swtchgear must be splt nto separate sectons wth a te bus through a fre-rated wall. For more nformaton on generator parallelng swtchgear, see Tab 0. Transformer Servce Man Utlty Utlty Meterng Typcal Generator Breaker Generators X = Number of Unts G G Gx Normal Bus Emergency Bus Optonal Electrcally Operated Stored Energy Breakers F F Fx EF EF EFx Optonal Electrcally Operated Stored Energy Breakers Non-Essental Loads Equpment ATS # Lfe Safety ATS # EP EP Typcal EPX Panelboards Fgure.-. Typcal One-Lne for a Parallelng Swtchgear Lneup Feedng the Essental Electrcal System (EES) Crtcal ATS # X Load Shed/Load Add ATS Unts Optonal Closed Transton Parallelng of Generators and Utlty CA000E