Document pek29op829Q0GKqxOJokMgdpD

TYPICAL RENEWABLE FUSES SHOWING CONSTRUCTION AND LINK DESIGN n Series of thin sections decreases Interrupt* Ing lime of fuse. $hort*<lr* Cult Current results In series of arcs thot Quickly go out 4 Q Asbestos powder surrounding the I renewable, link olds In quenching the ore. Blade-end cortrldge fuse shown is in the 70*600 amp, 250*v class. Most renewable elements are type In Fig. 13 w LTbd 4 fl Renewable links common* I V |y used are shown. The time*log element to right has blowing areas along the edge. Center area provides time-log 4 M Good clip con * tact reduces fuse heat. Clomps eompen* sote for lock of tension 4 C Clomp holding * v ferrule clip snug Is easily removed for fuse replacement. by unscrewing the cep 4QO 9Fu*s ossembly showing element, cortrldge cate and end blades. Renewable element Is Held Hmt by washers and heavy bolts, reducing contoct resistance to o minimum a A Extro time* III lag Is provid ed by log plates. Narrow sections of the link ere kept from opening under momentary over* lood by the thermal capacity of plates In replacing ordinary fuses with two arcs in series cannot bo main* time-lags, the site often had to bo taloed at tho sane voltage as easll/j reduced. This Is Important In motor as one, fast Interruption results.' protection since ovorfuslns with Hence less natal Is vaporised, rerll tine-lags reduces ootor oeerload ducing pressure developed wlthle~ protectloo. Siae reduction an/ be the fuse. Becauso of this condition', made elthout changing the original fuse holder. Reducers ore available that fit Into knife clips If It Is necessary to change to ferrule-end fuses. Reneeable Tine-Lag Link. The there is less possibility of the] fuse exploding or bolchlng fire ool short circuit. On gradual tenpera-^l turo rise, duo to prolonged over-P load, tho lag platoe cannot absorb] the extra heat and so the reduced/ renewable link, at the far right in center eeotlon opens. 1 Pig. 13, consists of a eerlea of narrow- and otdo-eleneot sections, vide portions are called the thereat fby puses Blow, a blown fuse] areas and act as heat-storage com* indicates trouble In a circuit; partnents. Uoder short-tine over There are several reasons for blot*L loads, these soetions absorb the sudden beat froo surrounding tbln spots. Thus the fuse Is prevented from bloving, on momentary overloads. lng in addition to a short circuit! and overload, poor electrical! contact at the fuso holder causey overheating, which lowers the fuse'.!., Bbould over-current continue, the current ratiog. decoloration o| heat-absorbing capacity of the tho copper results, making tbiaf tbornal areas decreases, allowing trouble east to spot. i reduced sections to nolt. Od abort circuit, thin bloving areas open since tine does not pernlt heat dissipation to tbornal sections. Large link-end connections provide points of heat dissipation through eopper-fuse tornlnsls besides assuring nlolnun contact resistance. Tlno-lag link In Pig. 10 provides an extra degree-of tine delay, two heavy lag plates separato three thin blowing sections. Heavy portions Puses are ofteo used In note] circuits for short-circuit protec tloo. while a thermal devl'c connected In series with the 11 L operates if notor Is overloaded I Here fuse rating Is well abo! I full-load current rating of ootor ( allow for heavy starting current* If fuses are used for overload * well as short-oircutt protection select the time-lag type to prerei blowing on ootor etartlog. { If a fuse is at a point whet provide tlno lag by drawing beat ambient temperature is high, froe tho adjoining thin spots, cannot carry Its rated load, sin4 principal blowing parts are the two temperature changes offoct fui thin sections, near the terelnals. operation, they should be Install! which open on short circuit, since when possible where the fuse ` POWER July 1*41 subject to tho same ambient tempera ture as the protected equipment. Observe this precaution generally In applying protective devices that operate on tho thernal principle. Ovsrfualng. overfuaing Is not the proper romedy for annoying circuit Interruptions. There fuso blowing is frequent due to over* loads, the blowing provents motor or wiring burnout. Too large a.foeo reduces tho degree of proteetlon against overload and defeats the purpose of fusing. Drilling holes In the sides of fuse cortridgcs, to cool the eleaent. is a dangerous practlco. On short circuit a high preasura Is developed within the fuse that aidB In quonchlng the ore. Small vents at each endofthe cartridge relieve Pressure after It has swept the ere. Roles in the tube reduce pressuro, lowering rupturing capacity of tho fuse. They also allow emission of hot gas ami molten metal, both of which are hazardous. Good Contact, importance of good electrical contact at fuse clips esanot be overstressed. Keep In mind that a fuse blows when heat'de veloped excoeda the element's eelttng point, since total current Passes through the fuse clipe, any resistance at those points causes additional heat to be generated, as a result, the fuse blows below Its reted curreot value. Poor contact at fuse enda reduces spring-tension of clips duo to tho jest developed. Tension loss Intro duces additional resistance, further increasing the best, to Insure good 10 20 30 . .40 .30- 60 . Time, seconds.. 1G Typkal lirrie*lqg-characteristic curves-for sev eral renewable links, showing high log at etart fuse contact, terminal clamps. pig, 14 and 13, may be used. Replace soy spring clips that may have lost their tension. tt is good practice to check fuse holders periodically. keeping the above points In mind, puses In critical circuits, which normally carry near rated current, should be replaced at regular intervals. At time of replacooont, check fuse clips for tension. Puses taken out of service any be returnod to stock after Inspection. Puses la Parallel. Puses aro ordinarily made with current ratings up to 600 amp. Those of higher rat ing are nado on ordor. To proteet circuits carrying more than 600 amp, fuses in the standard range are often paralleled, when so arranged, any additional resistance Introduced In one fuee path results In the other fuse carryingoproportionate ly greeter current, if fuses in par allel blow frequently, tho oauas may often be traced to poor contact at one fuee clip. Increased resistance at this point reduces current flow through that fuso; tbs fuse la parallel blows slneo it assumes a major portion of the circuit load. Curront then shifts back to the first fuse, causing It to opeo on overload. There practicable, avoid paral leling of fuses. On parallel Installations, it is good practice to meesure.the current through each fuee periodically. A split-core asneter serves .this pur pose In an alternatlng-current system. An appreciable unbalance should be corrected. Renewable Links. Note these pre cautions lo connection with renew able links: (1) After repeated blow ing there Is a tendency to end the annoyance by using two links in parallel ora link of higher current rating. This merely removes the symptom temporarily while cause of failure remains. (2) Renewable lick often overheats and blows because of poor element contact at olther end. When renewing, be sure element and link terminal are clean before acrowtng on end caps. July 1948 (407) <3