BEHAVIOUR AND OPERATION OF THE FUSE

Transcription

1 BEHAVIOUR AND OPERATION OF THE FUSE 1. INTRODUCTION 2. DESCRIPTION 3. FUSE OPERATION UNDER OVERCURRENT FAULTS 3.1. Inroducion o prerc, rc nd he wo ypes of overcurren The shor circui curren (figure 3) Fuse operion on overlods (figure 4) 3.2. The prercing period 3.3. Arcing period 3.4. Two lrge fmilies of fuses 3.5. Inerruping energy, he key pr plyed by he filler 4. THE BEHAVIOUR OF THE FUSE UNDER LOAD CURRENTS 5. DEFINITION OF CURRENT CARRYING CAPABILITY (C.C.C.) OTHER DEFINITION OF RATED CURRENT 6. HOW TO INCREASE THE C.C.C. (Iccc) OF A FUSE IN A GIVEN AMBIENT TEMPERATURE 7. DEFINITIONS Chrles Muler - upded on /11

2 1. INTRODUCTION Fuses cn proec ll kinds of elecricl producs: cbles, moor circuis, rnsformers, beries ec. nd even semiconducors. I is well esblished h power semiconducor, owing o is smll herml cpciy hs o be ssocied o very fs proecive device. The H.R.C. fuse is his bes proecive device becuse i is bsed on he sme herml phenomen s he semiconducor. This pper will del only wih he mos widely used ype of fuse for his kind of proecion: he snd-filled fuse wih compleely enclosed operion. There re herefore no ny exernl phenomen while his kind of fuse is opering. 2. DESCRIPTION fuse-elemens (x4) body snd (inside he body) erminls (x2) Figure 1 The min componens ( see figure 1 ) of such fuse re: The body: mde of insuling meril ( cermic or fibber glss for exmple ). The meril used hs o provide good mechnicl srengh nd good behviour under high emperures. The erminls: mde of good elecricl conducor meril. The fuse elemen: one or severl fuse elemens re linked o he connecing prs. The shpes nd he dimensions of he fuse elemens depend upon he desired chrcerisics nd re mde wih highly conducible meril. Pure silver is he mos ofen used meril becuse of is very low resisiviy, is good mllebiliy nd esy mchining properly nd lso good sbiliy he differen emperures under which i works when submied o is curren ring, or overlods o wihsnd. Along he fuse elemen here re rows of necks (reduced secions) re ccurely cu (figure 2). When shor circui occurs ll necks mel quickly nd simulneously. The shpe of he necks chnge he fuse chrcerisics. Figure 2 : fuse elemen The filler: i is n iner nd nhydrous meril. Silic snd is generlly used. The size of he grins is chosen ccording o he desired rcing performnce, nd he pcking inside he body hs o be very hrd in order o provide consn porosiy. The purpose of he filler is o bsorb he rc energy nd o ensure good insulion fer rc exincion. Chrles Muler - upded on /11

3 3. FUSE OPERATION UNDER OVERCURRENT FAULTS 3.1. Inroducion o prerc, rc nd he wo ypes of overcurren When n overcurren ful is higher hn he minimum fusing curren, he operion of he fuse is composed of wo disinc periods: he prercing period nd he rcing period (figures 3 nd 4) The prercing period: i srs he ime 0 when he overcurren ful is pplied o he fuse, nd i sops he ime p when he meling emperure of he fuse elemens is reched jus prior he very beginning of he rc igniion. Beween ime 0 nd p he circui chrcerisics re no modified by he fuse. During his period one cn herefore esily express he ful curren versus he source volge nd he circui prmeers. The rcing period: i srs excly when one-rc igniion or muli-rcs igniions occurs. This period will end only he ime when he rc complee exincion occurs. The rc exincion (or rc-quenching) depends upon he source volge nd he circui prmeers ogeher wih he fuse chrcerisics. Obviously, he fuse modifies he circui chrcerisics during his period. There re wo min kind of overcurrens: he shor circui curren (lrge vlues) nd he overlod. All fuses opere well on shor circui currens. Mos semiconducor fuses re no designed o provide proecion gins long-durion overlods. Elecronic or oher mens mus be used o swich he circui off when overlods occur. However new syle gr semiconducor fuses (s i is defined by he IEC 60269) cn provide overlod proecion s low s 160% of fuse red curren s gg fuse does when proecing cbles. There re wo lrge fmily of fuses s i is shown on The shor circui curren (figure 3) : All fuses opere he sme wy when hey inerrup shor circui curren. A he end of he prerc he meling emperure of he fuse elemens is reched in ech neck (960 C f or silver). Then fer very shor ime he silver is chnged ino vpour nd rcs sr simulneously ll necks. The curren is limied down o he Ic vlue nd srs o decy owrd he zero vlue. Prerc : during he prerc ime he fuse elemen remins simple resisnce h does no chnge he circui chrcerisics. Arc : n rc is ignied ech neck. There re hen severl rcs in prllel nd in series. Shor circui curren I C A his ime he vporision emperure of he fuse elemens is reched. p = prerc ime = ol clering ime - p = rcing ime I c = pek le hrough curren 0 Prerc p rc Figure 3: shor circui curren Chrles Muler - upded on /11

4 Fuse operion on overlods When he ful curren is low here re 2 kinds of problems: The fuse body is dmged: When he prerc ime is longer hn 1 second here is n excessive heing of he fuse elemens (severl hundred degrees) resuling in dmges in he body like: crcks of he bodies mde of cermic crbonision of he bodies mde of fibre glss impregned wih oher merils (melmine, silicone, epoxy ). These dmges occur before he fuse elemens mel. The CC curve (see 3.3.) ploed on he ime curren curve of some fuses shows he insn when he fuse body is dmged. All fuses wih he CC curve re ype fuses in he IEC sndrd. The rc goes ou of he fuse hrough he erminls: When he prerc ime is long ll rows of necks my no mel simulneously. One rc only is iniied incresing quickly is lengh s i ges he whole volge of he circui. Then he rc exincion mus be chieved before he rc reches he ends of he fuse. Fuses no ble o inerrup overlods high enough o mel he fuse elemens re s well ype fuses in he IEC sndrd. For boh bove resons noher proecing device mus be ssocied o he ype fuses. Such fuses hve minimum inerruping cpciy round wo o more hn six imes he fuse curren ring. 0 rc Prerc ime = 0.1s o 4 hours P Tol ime Figure 4: overlod The IEC menions: for imes longer hn 0.1 s, for prcicl purposes he difference beween pre-rcing nd opering ime is negligible. Chrles Muler - upded on /11

5 3.2. The prercing period During his period, here is heing process of he fuse elemens (nd even of he whole fuse if he prerc ime p is long enough) s consequence of he Joules effec due o he curren flowing hrough he fuse elemens. Therefore, he fuse elemens emperure rises up o he meling poin (960 C for silver), hen here is conversion of he solid phse ino liquid phse, hen noher emperure rises up o he vporision poin nd finlly he conversion ino he vpour phse. During he solid phse, pr of he herml energy is sored o he fuse elemen iself, he oher pr is rnsmied o he surroundings of he fuse elemen. The prerc I 2 my be reduced by mking consricions long he fuse elemens while he elecricl resisnce keeps low vlue (see figure 5). This provides differen ime curren curve shpes. differen kind of consricions = differen shpes of he ime / curren curve Figure 5: differen kind of consricions 3.3. The rcing period i Resisnce R Inducnce L G e fuse u figure 6 During he rcing period, he use of differenil equions resuling from Kirchoff s lw for given circui (figure 6) leds o he following equion: Wih: e = volge supplied by he generor u = rc volge of he fuse R = resisnce of he circui L = inducnce of he circui di e = Ri + L + d u Chrles Muler - upded on /11

6 he momen of he rc igniion, he pek vlue Ic of he curren pssing hrough he fuse is deermined by boh he circui prmeers during he prercing period nd by he prercing I² of he fuse Sring from he I c vlue, he curren will rech he zero vlue wihin durion depending upon he rc volge ( figure 7) I c Curren limied by he fuse di Therefore he d is negive during p he rc exincion process Since he ge: di 1 = ( e u ) d L R i vlue is very smll we Volge cross he fuse Volge supplied by he generor di This shows clerly he is negive d only when u > e Arc volge Figure 7: oscillogrm Conclusion: i is bsoluely necessry o genere n rc volge higher hn he volge of he power supply in order o sop he curren nd o open he circui Two lrge fmilies of fuses Anoher echnique found by Mr METCALF is o use low meling poin lloy or in (see figure 8) o force he opening in he middle of he fuse elemen when here is low overlod in he ful circui. This concep llowed o cree wo lrge fmilies of fuses. ype fuse: M, URD, A70QS ec.) Generlly very fs cing fuse g ype fuse (grb, gg, ec. ) wih M-effec fuse elemen Figure 8: wo min kind of fuse elemens for wo lrge fmilies of fuses Chrles Muler - upded on /11

7 100 A fuse URD IEC R clss minimum breking curren IEC ype fuses: M (moor) R (semi conducor) Above he CC curve i is forbidden o use he fuse (ssociion wih noher proecive device is bsoluely necessry) As soon s he overcurren cn cuse he meling of he fuse (even in 4 hours), «g» fuses cn brek ll currens. 1h à 4h 110 % IEC «g» ype fuses : generl purpose: gg, gm, gn, gd semiconducor: gs & gr 100 A fuses grb IEC gr clss UL 248 pr 13 fuses: proecion of semi conducors (componen «UL Recognized» ) Dul elemen fuses (or Time Dely fus FERRAZ SHAWMUT exmples: PSC 690 V URD (semiconducors) M fuses (moors) Form 101 fuses (A50P, A70P, A70QS, ) Figure 9 : wo lrge fmily of fuses FERRAZ SHAWMUT exmples: PSC 690 V grb (semi conducors) gg (generl) AJT fuses (clsse J Time Dely) Comprison of he ime curren curve of 4 IEC fuse ypes UL 248 fuses: clsses J, L, RK5, RK1, T, K5, K,H..( componen «UL Lised» ) IEC clss gg (Full rnge fuse) or generl purpose fuse essenilly for conducor proecion IEC clss M fuse(pril rnge fuse) or bck up fuse for moor circui proecion Pril rnge fuse for semi-conducor proecion Full rnge fuse for conducor nd semiconducor proecion Figure 10: comprison of he ime curren curve of 4 differen IEC fuses Chrles Muler - upded on /11

8 3.5. Inerruping energy, he key pr plyed by he filler The inerruping energy dissiped inside he fuse is he energy produced during he rcing period: W = W L + W S W R Wih W W = L S = u i d = rc energy p 1 2 LI C 2 p = energy sored in he inducnce he end of he prercing period W = e id = energy provided by he source power during he rc quenching process W = R R i 2 d = energy los in he circui resisnce p Fuse elemen during high curren inerrupion Prerc: ll noches of he fuse elemen mel simulneously for high mgniude currens. The energy is negligible becuse he volge drop cross he fuse is very smll. Arc: muliple rcs in series (4 in his exmple) re creed. The energy is very high becuse his insn he curren hs high pek vlue nd he sme ime he volge cross he fuse is higher hn he circui volge. The grins of snd in conc wih he rc mel nd form glssy body known s «fulgurie» Figure 11 Meled snd = fulgurie The snd confines he rc nd crees pressure helping o exinguish he rc bu he sme momen he snd porosiy prevens he pressure o become oo high oherwise he fuse cn explode The snd porosiy llows s well he vpour of silver (when he fuse elemen is mde of silver) o escpe from he rc region hrough he gps beween he grins of snd helping he exincion of he rc. Afer he rc exincion he snd deermines he insuling resisnce of he fuse. Figure 12 The picure (figure 12) shows how looks fuse elemen fer he breking of shor circui curren. The significn moun of meled snd indices he rc energy ws lrge. Chrles Muler - upded on /11

9 4. THE BEHAVIOUR OF THE FUSE UNDER LOAD CURRENTS If he speed of fuse (clering I 2 ) is essenil o he semiconducor proecion, one should no forge he behviour of he fuse under norml lods i mus wihsnd under rel working condiions is very imporn s well. I hs lrge effec on he opering emperure of he equipmen nd moreover on he life durion of he equipmen. The mximum curren crrying cpbiliy of fuse depends widely upon wo groups of prmeers influencing is behviour: Surrounding condiions : (mbien emperure, ir cooling, connecions ec.). These condiions re mos of he ime fr wy from he condiions defined in he IEC or he UL sndrds. Lod curren vriions nd overlods. In opposiion o semiconducor of which he knowledge of he herml resisnce beween he juncion nd he cse is sufficien, he he flow genered by he fuse elemen goes o he mbien ir hrough severl wys (figure 13) such s: connecions: hey re no generlly symmericl. Very ofen you cn even see one connecion cooled by wer cooled he sink nd he oher connecion heed by he flexible of he semiconducor. he body of he fuse: is ino direc conc wih he mbien ir emperure (no lwys uniform). AMBIENT AIR He dissiped o mbien ir hrough he body I coninuous He produced by he fuse elemen θ mx he dissiped o erminls Silic-snd filler helps in he eliminion of he Figure 13 I is herefore impossible o deermine emperure vlue for he body or he connecing prs in order o hve conrol on he cooling condiions of he fuse elemens. Anoher imporn difference beween fuse nd semiconducor is he herml ime consn. This prmeer hs high enough vlue in fuse o llow lile sensibiliy o curren vriions due o he wve shpes such s sinusoidl ones or recified ones in converers. On he oher hnd, he life durion of he fuse is widely ffeced by cyclic vrible lods he period of which is beween few seconds nd few hours. Despie ll he precuions nd echnology improvemens, he conrcions nd exensions king plce successively in he fuse elemens becuse of successive coolings nd heings cuse mel geing which my led o n undesired operion of he fuse. Chrles Muler - upded on /11

10 5. DEFINITION OF CURRENT CARRYING CAPABILITY (C.C.C.) OTHER DEFINITION OF RATED CURRENT. As shown on figure 13 he mximum dmissible emperure θ mx is in he necks closed o he middle of he fuse elemen. For he semi conducor fuses he vlue of his emperure is deermined by he fuse mnufcurer depending upon he choice of : he merils for he body nd erminls he design of he fuse elemen he expeced life ime. For generl purpose fuses he sndrds specify some emperure rises on erminls nd specify s well he power loss. The curren I is incresed up o he momen θ mx (fer herml equilibrium) is reched. When he opering condiions re differen from he esing condiions defined in IEC 269 his vlue of I is he C.C.C. of he fuse (Iccc) The red curren (or nominl curren) is he C.C.C. obined under condiions defined by sndrds such s he IEC 269 nd UL HOW TO INCREASE THE C.C.C. (Iccc) OF A FUSE IN A GIVEN AMBIENT TEMPERATURE The fuse cn crry more hn is red curren In wih good ir flow or by mouning he fuse on low emperure connecions (liquid cooled) The dvnge of one of he wo soluions depends on he fuse lengh s shown on he ble below. FUSE So clled shor fuses Un<700v So clled long fuses Un = 1000 V o 2000V Very long fuse Un >3000v Evcuion of he he 80% connecions 20 % body 20% connecions 80% body 1% connecions 99% body Influence on he Iccc Air flow 5m/s Necessry on boh body nd connecions Iccc =1,25 In Necessry on body Iccc = 1.25 In Necessry on body Iccc = 1.25 In Curren Crrying Cpciy Less hn 60 C on fuse connecions The mbien does no hve ny longer influence Iccc = 1,25 In o 1,35 In Iccc = 1,05 o 1.25 In Iccc = In These rios re quie ineresing for sedy se or smll vriions curren. When he fuse ring is essenilly deermined by he wihsnding of overlods, he forced cooling does no bring rel dvnge. The curren ring selecion lefle shows how o selec he fuse red curren when i mus wihsnd overlods or in cse of cyclic lods. Chrles Muler - upded on /11

11 7. DEFINITIONS Red volge: for IEC fuses oher hn 690 V re esed beween 110% o 115% of heir red volge Fuses red 690 V re esed beween 105% nd 110% of heir red volge i.e. les 725 V. In Norh Americ fuses re esed s per UL sndrd 100% o 105% of heir red volge. Red curren: vlue of curren h he fuse cn crry coninuously wihou deeriorion under specified condiions. Prospecive curren of circui (Avilble curren in Norh Americ): curren h would flow in circui if fuse siued herein were replced by link of negligible impednce. The vlue of he prospecive curren in AC is he R.M.S. vlue of he AC componen. Breking cpciy: vlue of prospecive curren (for.c. he r.m.s. vlue of he.c. componen) h fuse link is cpble of breking sed volge under prescribed condiions of use nd behviour. Convenionl non fusing curren ( Inf ): vlue of curren specified s h which he fuse is cpble of crrying for specified ime (convenionl ime) wihou meling. Convenionl fusing curren ( If ): vlue of curren specified s h which cuses operion of he fuse wihin specified ime (convenionl ime) g fuse ( formerly generl purpose fuse): curren limiing fuse cpble of breking under specified condiions ll currens which cuse meling of he fuse elemen up o is red breking cpciy. fuse ( formerly bck-up fuse): curren limiing fuse cpble of breking under specified condiions ll currens beween he lowes curren indiced on is opering ime-curren chrcerisic nd is red breking cpciy. Chrles Muler - upded on /11