Document 37rDVxor8aYnGZxpmN1eXj1vx

2/C components: conductors First decision, properly made in the early planning stage, is whether primary distribution lines will be overhead or underground. Trend is toward over* head. /( otters a lower oast system that's easier to expand. But don't moke decision on trends alone. An underground system, although it costs more, is unaHeclod by lightning, plant Vires, cranes, etc. And where appearance is a prime ioctor (H your plant is in or near a residential area) you have another vote lor underground. Soil contamination, corrosive fumes, building layouts, distances between buildings, all Influence final decision- utility tunnols. carrying power services between powerhouse and outlying buildings, are used in some plants- All the power services-*steam, water, air, electrical cables -- are included. Prefabricated tunnels are available, oiler installation savings. Type ol conductors, insulation, covering, QQ1I next. Conductor site lor load carrying alone lound from standard tables and vollage*drop But size for short-cfrcuit-curreni carrying is onmkl matter. With wider use ol breakers on feeders!!! selective tripping with its necessary lime delay ( * 106). your cables must be able to stand heating during the delay period. Suing lor this fac} <1 is based on allowable temperature rise ol insulation! and on time delay porlod. 4 Aluminum or copper Is another decision 10 make! early. It won't do lo design the system (or copper,* then swing to aluminum later. Size of conductc**,l pullboxes, etc-, musl be forger lor aluminum. .1 More power engineers see long-term advantages J of standardizing on cable sizes. Even where il means ] versizing, resulting simplification pays oil both dur* With new cable types on lap, the trend is toward overhead high Current-carrying capacity, line losies, voltage drop, provision lor luture neods. alt toes the power engineer ptonntng primary and secondary distribution. Handbooks and manufacturers' catalogs generally give enough Information on this sco/e. Assuming this data Is ol bond, let"* took here o\ the types ol cables finding use today, more specifi cally. eoveral now receiving roafor con sideration. Primary Feeder*. Today's plants, mad* up o( many smallet buildings, spread over large areas. Electrically, that spelts scattered load units and long primary leaders Irom either the primary substa tion or Ihe plant powerhouse. This scat tering ol load over large areas trot been a significant (actor in the trend to over head primaries. Preatsembltd oeriot coble is a lowcost method getting much attention, especially in modernization programs. An existing pole tine, tower line or siructural-stee! framework for conveyors, pipes, ole, can serve as coble support. For a 3-phase circuit, this coble Is mad* up o< three insulated. shielded and jocketed conductors bound to a mes senger. Coble needn'f bo kept dear of towers or buildings, except where dial ing is probable. Close spacing with other cobles and materials, plus mini mum allowable ground clearances, lead lo great freedom In choice of supports. Where neut/a) is grounded, mstsenger may serve as neutral to carry phoneimbalance currents. Ttrminallng and spiking is simplified, slnco the cable is made up ol separate conductors that ore individually insulated and jackelid^' Intotlocked-onnor cable g*U the rtod hom many power engineers for 3.phase primary distribution above ground level. This grown-up version oi long-iamUiar low-veltoge armored coble may be used Indoor* or out. Outer metal armor, elQtsr galvanized steel, bronze or aluminuo, covers a chemical and moisture-resistant sheath. Combination forme flexible amdull and conductors in on* easy-toInstall cable. Bronte ortnor Is a Itoquent choke fn chemical plants. Standard hardware and supporting racks are ovaitable. Terminating and ipJldng methods hove been worked out to cut field lobor to the bone. Users of interlocked-armpr cable re port sharply reduced installation cost once method* are mastered. Where IhU Bu"duCt r,roVK" U*el! " krt key lo compact secondary distribution ? rooAVs eiectiicai siactici SOWII J? wtalfoliori ond alter ihe planl is in operafion. ^<my ^(9 the lo?ch is bowing out lo bolted repression filings lor cable joining. (^ underground ductwork, liber oondulf in con- Is a good bet. Under light traffic. Utile concrete faired artd top ol conduit need nol be more than ^in. below surface (somewhat lower in cold cit ies). For atrefehes exposed lo heovy trcrHlc. lay jult deeper, consider reinforcing.. railability boils down to selecting cotrect l&ble, connectors, insulation, covering. Lead-coverare giving way Jo newer synthetic insulations, his avoids need for cosily lead wiping. Many are adopting new liquid-tight flexible metalRk conduit. Liquid-light covering over flexible con^{1 permits use outdoors, In wet spots indoors. Typi- axf use. leads irom rigid conduit to motor box. High points In coble selection What are on-thc-|ob conditions? Will cable be in conduit, underground duets, buried in oarth, sus pended overhead? Consider load, voltage, location, ambient conditions, hot spots, mechanical inlorforence. Is electrolysis a potential problem? What i* best Insolation and covering? Get to know the newer insulation materials and conditions lor which each is best suited. Whal ora li/niiotions? Determine maximum currentcarrying capacity under Ihe cable'* operating con ditions. Study effect of your cable selection on Installation. Simple exomple: Two, or more, single cobles, ip parallel, often prove more economical and easier lo Install than one large cable. voltage primary distribution: low installed cost, easy expansion {gif (he cos*, (rouble can bo traced lo lock oi advance planning to lake full advantage ol this cable's economy. Since conduit Isn't needed, there's no confined air space around conductor*: (Us translates into 20% greater current capacity than for samo cable In conduit. Corfsrgmund Distribution. For this, many ngineers are shifting to butyl-bate *yntbsllc cable insulation. This serves fn conduit, underground ducts, tunnel*, also directly In the earth. Cable can stand rough treatmont. survive nearly any atmospheric condition, operate In the 60-QS C range with proper jacket. Varnished-cambric lead cable is an eld standby and still first choice of many for underground distribution. Labor costs lor handling and terminating are high, lead sheath offers virtually positive as surance that moisture will not enter cable. Also, sheath's low resistance oilers good grounding protection; Its thermal capadiy helps absorb heat from shorl-llme overloads. In spite of theso lime-proved advantages, high Installed cost of lead-covered cable cause* Indus try to took to newor and lees costly cable types. Secondary Feeders. Selecting cable lor hot or damp locations has always been a Jieadacho. But a relative newcomer In this country promises to be (he best so lution in some cases. This sell-contained wiring systom comprises copper locketlag enclosing conductors spaced and insulated by noncombustible mineral Insulation. It Is completely motstureproel, can eland continuous operation in am bient temperatures up to 2$0C. There is no need (or conduct capper ehedrh usually offore sullident mechanical pro tection. Aluminum may be used lor sheathing fn the future. This wirfng syeiem is rigid yet flexible, so hand-pressure bending Is usually enough lo ehape to any needed con tour, Once formed, wiring stays put when fastened with straps. Fittings seal both ends ol run at terminal boxes. Be cause ol all-mineral insulation, system it claimed (a be immune to etge-detedonation. Insulation Is stable up to 21100 C. Power engineers are looking into (hi* wiring lor fobs where moislure. heat or restricted space are problems. Inlorlocted-armor cable alto **** use lor low-voltage feeder*, with tame ad vantage* as lor high voltages. Unique virtues af bus duct are cited below. . since power loads are pluggcd-in, changing machine location becomes simple, last You'd be out ol June with modern secondary distribution methods if you didn't consider bus duet where shifting of production machines is common. Bus duct Is a metal-enclosed system of bus bars (generally copper, but aluminum Is com ing in) available In prefabricated sections, lor simple instal lation. Prefab fittings - lees, elbows, cresses --moke running duct In any direction and around obstruction* a Deld assembly lob rather than a major construction problem. Plag-ln feature pormlts last connection and disconnection ol machines on production floor. Plante can be retooled, build ings revamped, without mq)or wiring changes since bus duct, Unlike conduit and cable, con be taken down and re assembled with minimum tabor, using seme ports again. Users of bus duel cite Ih/ee major advantages that lie in nieely with any modernization program: 0) Maximum flexi bility meets changing future demands. (2) Sfmpfe installation chalks up labor savings. (31 There's minimum disruption of service during changes and It's easy to ilo In with other secondary distribution methods. There ore four distinct type* el bus duct available today (hat generally meet all distribution needs. Plug-in duct, mast used for branch runs, provides outlets (or plugglng-ln switch ing equipment about every loot. Where machines are to b* swopped around, you need only unplug switch and push It into another outlet at new moehine location. Low-lmpsdance dud is logical choice lor tong (oedor* where voltage drop musl be held down and currents are heavy. Even lor short runs whore impedance Is Important, as with wsfdar toads, low.Impedance duct is a good bet. Weatherproof lesder dud, lor outdoor runs, has gasketed steal enclosure and weather-resistant hardware. Where voltage drop must be kept to a minimum on outdoor runs, use weatherproof low-fmpedonce bus duel. Aluminum Is hor* (or bus dud. Aluminum-oxide probiom (see next page) la being challenged with stnc-copper-sllver plating. Maker knows this will temove aluminum oxide and predicts It will prevent oxide reforming when duct is fn operation. At Joints, pressure-compensating spring washers give pressure, claimed permanent, ol over 1000 psl. Early figures point to 41% weight advantage over copper. OCTOIU 19)) TOOAV'I lllCTtlCAl HaCIICE . `1 J