Document 91nnQOLere591k41wXRzYQxpe

distribution circuit interruption: breaker* circuit Interruption: fu*e* For top protection, each breakor should have enough interrupting capacity to handle available short-circuit currents at he po/nf in the circuit. Also, breakers in series should have time discrimination so any iault at far end of string wil) be isolated by nearest breaker and service maintained to remaining loads. When you specify both adequate interrupting capacity and time discrimination, you have the recommended selective (ripping scheme. selective m xeicn. npnr, i would be. perhaps, o motor feeder breaker with tripping lament* having both tong time-delay /(or motor-starting duly and overload protection! and in* slanianeou* charaelorislfcs (lor short dr- cults). Breaker* 0 and A would have tripping elements with both tong ond short time-delay characteristic^ lime delay (or A being longer than 8, Tlme-detoy feature provide* nooded time tor ahort-circuli current to trip breaker farther down the tine drat, Thu*, breaker A would trip only lor o fault between A and B. Breaker B would op erate only lor a fault between 0 and C- For a selective tripping system, each breaker must have an Interrupting rating equal to or. better #({((, greater than available shorl-drcuit current at point of application. Also, all breaker*, except thoae with Inalanlaneoua tripping ele ment*. such oa thoae icrlhesl Irom power source, have a hort-timo (30 cycle) rat ing equal to or greater than available Iault current. Then, bo current-time char acteristics wilt not overlap lor adjacent breaker*, current rating* must be select ed with a ratio of at least 2:1 to prevent lolae tripping, Selective tripping has become a muot for essential circuits -- auch Os those supplying power-station auxiliaries. But many engineers find it hard to tusUfy Ik selective tripping on some low-voltage circuit 51 they specify breakers in cascade. Then only breoS closest to source has adequate interrupting capg^? J those down the series Siring are applied beyondtfeJ ratings. H a heavy short occurs, even beyond ^ breaker, main breaker may trip out, killing all ^ Ice. Where properly applied (below), cascading fa practical but not generally recommended. ------------------ J an o*. nected in cascade simply a$ Q wo? ^ cutting first cost. It's possible jw v all three breaksr* (n cascade to op*n y fault current is severe, Sirtc* *to^)tr breaker* are backed by those withhigh er Interrupting rating, their lit* depend) on backup breakers coming to lh* ,*. cue. But where two or mete bookm operate at same fimo, in series, Untied, oal Uee* are \e*s than it each opted alone. This is considered in cascading breokers. Maker* recommend sleqriaoj operation lor cascaded breakers. Where cascading is adopted, power engineers have each breaker 1a setup Inspected offer tripping on a heavy fad( belore reclosing. Heavy Iault cunsst may have damaged breaker to an normal current couldn't be ranted solely. Cascading is usually United to Ques slops o( interrupting rating. Breaksr near est power source. A, must have ode* quato capacity, Breaker next lover to cascade. 8. may have Vi interrupting rating required by maximum avatlabli Iault current, one farthest from source. C, may have maximum rating. Before adopting cascading, wotgh care fully ihe disadvantages of reduced ss- lectlvlty, restricted interrupting-duty cy-* cle, and some oddlilono) roointenonc*. Weigh cascading*s many disadvantages; < 'on't tel price lag atone rule your decision Cascading is tar (>om ideal in breaker application, But teeing that it's with us. and widely used today in industry, rnako sure you know all the facto, before you choose. In a cascade hookup, fowl k*o of connoefed synchronous motor* should not exceed 25% a| supply trqnatormer or gen erator kvo. Main breaker 4 should be set to trip when current through breaker B reaches 60% of its Interrupting rating. This 'insures lhat main breaker will operate whenever short-circuit duty exceeds interrupting rating o( breaker 0. Another general practice of power engineers using the cas cade scheme Is to select breaker* from the same maker, to be sure they have similar characteristic*. Melded-Caee Breakers, b It wise to casoade them? Tests have beon mads by various manufacturers to answer just that question. Tests wars independent but roaults were In subslan. lial agreement. Conclusion: Motded-case breokers shouldn't b used in cascade arrangement. Here's why: Various types and sites of motded-case units were tested and nearly alt those casoaded were damaged-covet* broken- contacts and operating mechanism destroyed, otc. So there's general agreement that motded-case breakers should be u^ wllhln their interrupting capacity limits, not coscadedCascading Disadvantage*. forgetting Initial east tor o momsnk look into major limitations ol cascaded aaangemeni. Realif* (hat. although you had reliability and service continuity up permost in mind when you selected a circuit arrangement (pp 88-91). these values can be cancelled quickly with car* coding. Unlike the sefectfve-lrfppi'ng scheme, whenever o leader Iault draws current greater than 60% ol Interrupting rating o! loader breoker, main breaker will trip. Then servle* to all leader* served by main breaker Is lost and will eioy lost until main breaker is roefossd. tn briei, cascading to being usod and proving totlalaciort In many industries. Your Job to to weigh its disadvantage* _ against Its lower Initial cost and determine which ol the l* tripping arrangements (its youro*er-o)J neods bast. KM TODAri mCTIICAl SIACTICf fOWII ifalion or so back, the fuse vs circuit breaker often discussed. But the breaker has 2*pa|j*dn developed to a point where few engineers r,:.`^--jqkl,,u*mrraeafcfojoefrinnfolljlneeyyeaeeigagotsurrusseep,, lhafinnocweItehh.i.neve..e..cJlro..io.rw.Icw.h.u-.-ev.vi.to.of.u.plHt.s.ara.oeggteeehcatffsifioeenmfldd...ovBBeaadcckkinfooolf mCflva (h9 high-interrupiing-copacily luso. It tenmip0Vm oopoallM In Im--o---w--itJh--thJ #wn..eed> o( mV,oTMde!m. plants. Its chief boast is that it will interrupt a short circuit before current reaches ihe dangerous values possible. Some (use manufacturers look on it as an "anticipator". On this page and next, you'll find in/ormafion to help you seek proper spots in your plant for this fuse xtr tr-type. Since (here is for Irom firm agreement on where good engine..e...r.i.n...g...j..u...d...9. menl. -------^-- yUr `........ nni I'' 1 .'n"l..ril' ll|r||;i;.,:i|-^ ;.).||,.<. SfaopUtt ol (uses has but a tingle eh- 1 Here (use fink capes with heavy over- bsL On heavy overload etpeufi, weak spots In the fink or short me ud> | toads and short circuits, while a thermal 1 cutout lakes care ol sustained over danfy heated to a high temperature and loads. Current Row generates heat In vaporise. 6scause length ol these weak link. W|lh minor overload*, where there tpots (i relatively short, any arc lanoed is ample time lor thermal conduction, would continue. But arc* are in series heal is carried Into thermal cutout fn and voltage across each is low. to arc (use center, fusible alloy melts at prop* u stopped and efreu/t opened. Only one weal spot may open an minor overload. or point, permitting Intortupler to move undor spring tension and open circufl. Hfgh-lfttemrptlng-eapacity fuse fs a lube with silver elements In efflro filler. Silver Is used because: <11 Conductivity is not Impaired by continuous operation or currant surges yielding temperatures fust under molting point. (2) Having tow specific heat, sliver can be raUed to voporiration (osier. (3) Its vaporisation temperature Is much below temperature at which the vapor readily Ionite*. High-inlerrupling-capacity fuses offer sound low-cost protection tics nd^J^e^V *^^Tpv.j1 ^' A hlgh-intemjpttng.capaelty current-limiting (use Is designed to open o short wilhin the firs! quarter cycle, before serious damage can be done. This last microsecond action In lace ol Ugh available short-circuit currents (up to Infinity) Beds nw (pets in Industry lor Ihit loss type. Magnetic stresses circuit equipment must stand are a func tion ol actual ''lot-through" current squarod. Thermal stress, or equlpoisnl healing, depends on Jet-through current squared multiplier py clearing time. Tie these thoughts to the sketch obov# and you'll see how current-limiting action of this fuse holds In check tho dangerous eilects o( a heavy short. A symmetrical ehort-clrcufl is sketchod above, Bui ll the short hit at time sera, de component would give an asymmetrical v<rve about a displaced higher-value axis. Curve to right is plotted lor available fully displaced rms ampere* (asymmetrical or taro] rms amp). This fs on (he basis thru maximum ollsol rms amp equal symmetrical amp mullipliad by (.4. Curves are peak iet-through values. OCTOMt 1950 TODAfS IlICtHCAl MACTICI