Breaking Capacity. See Interrupting Rating. Current Rating

Transcription

1 Fuse Facs The following Fuse Facs secion will provide a beer undersanding of boh fuses and heir ypical applicaion. The fuses described are curren-sensiive devices ha serve as an inenional weak link in an elecrical circui and provide proecion agains overheaing by reliably meling during curren-overload condiions. They can be used o proec discree componens or complee circuis. lhough his guide provides echnical informaion ha will help you deermine your circuroecion soluion, such as produc daa and design guidelines, i is no inended o be comprehensive. Tesing is srongly recommended and should be conduced o verify applicaion performance. The following fuse parameers and applicaion conceps should be well undersood in order o properly selec a fuse for given circui condiions. In he absence of special requiremens, Lielfuse reserves he righ o make appropriae changes in design, process, and manufacuring locaion wihou noice. mbien Temperaure This refers o he emperaure of he air immediaely surrounding he fuse and should no o be confused wih "room emperaure." The fuse ambien emperaure is appreciably higher in many cases, because i is enclosed (as in a panel moun fuseholder) or mouned near oher hea producing componens, such as resisors, ransformers, ec. reaking apaciy See Inerruping Raing. urren Raing This refers o he nominal ampere value of he fuse. I is esablished by he manufacurer as a value of he curren he fuse can carry, based on a conrolled se of es condiions. (See Reraing). Mos caalog fuse par numbers include series idenificaion and ampere raings. Refer o he Overcurren Selecion Guide secion for guidance on making he proper choice. Reraing For 25 ambien emperaures, i is recommended ha fuses be operaed a no more han 75% of he nominal curren raing esablished using conrolled es condiions. These es condiions are par of UL/S/NE (Mexico) "Fuses for Supplemenary Overcurren Proecion", he primary objecive of which is o specify common es sandards for he coninued conrol of manufacured iems inended for proecion agains fire, ec. Some common variaions of hese sandards include: fully enclosed fuseholders, high conac resisances, air movemen, ransien spikes, and changes in connecing cable size (diameer and lengh). Fuses are essenially emperaure-sensiive devices. Even small variaions from he conrolled es condiions can grealy affec he prediced life of a fuse when i is loaded o is nominal value, usually expressed as 100% of raing. The circui design engineer should clearly undersand ha he purpose of hese conrolled es condiions is o enable fuse manufacurers o mainain unified performance sandards for heir producs, and mus accoun for he variable condiions of he specific applicaion. To compensae for hese variables, he circui design engineer who is designing for rouble-free, long-life fuse proecion, generally loads he fuse no more han 75% of he nominal raing lised by he manufacurer, keeping in mind ha overload and shor circuroecion mus be adequaely provided for. The fuses under discussion are emperaure-sensiive devices whose raings have been esablished in a 25 ambien. The fuse emperaure generaed by curren passing hrough he fuse, increases or decreases wih ambien emperaure change. The ambien emperaure char on page 6 illusraes he effec ha ambien emperaure has on he nominal curren raing of a fuse. Dimensions ll dimensions are given in inches unless oherwise specified. The fuses in his caalog range in size from he 0402 chip size (.041"L x.020"w x.012"h) up o he 5 G, also commonly known as a "MIDGET" fuse (13/32" D x 1 1 /2" L). s new producs have been developed over he years, fuse sizes evolved o fill various elecrical circuroecion needs. The firs fuses were simple, open-wire devices, followed in he 1890's by Edison's enclosure of hin wire in a lamp base o make he firs plug fuse. y 1904, Underwriers Laboraories had esablished size and raing specificaions o mee safey sandards. The renewable ype fuses and auomoive fuses appeared in 1914, and in 1927 Lielfuse sared making very low amperage fuses for he budding elecronics indusry. 3 Elecronic Produc Selecion Guide

2 The fuse sizes in he char below began wih he early "uomobile Glass" fuses, hence he erm "G". The numbers were applied chronologically as differen manufacurers sared making a new size. For example, "3G" was he hird size placed on he marke. Oher non-glass fuse sizes and consrucions were deermined by funcional requiremens, bu hey sill reained he lengh or diameer dimensions of he glass fuses. Their designaion was modified o in place of G, indicaing ha he ouer ube was consruced from akelie, fibre, ceramic, or a similar maerial oher han glass. The larges size fuse shown in he char is he 5G, or "MIDGET", a name adoped from is use by he elecrical indusry and he Naional Elecrical ode range which normally recognizes fuses of 9/16" x 2" as he smalles sandard fuse in use. Fuse Sizes Fuse Lengh Diameer/Widh mm in mm in G G G G G G G Tolerances The dimensions shown in his caalog are nominal. Unless oherwise specified, olerances are applied as follows: ±.010" for dimensions o 2 decimal places. ±.005" for dimensions o 3 decimal places. The facory should be conaced concerning meric sysem and fracional olerances. Tolerances do no apply o lead lenghs. Fuse haracerisics The characerisic of a fuse design refers o how rapidly he fuse responds o various curren overloads. Fuse characerisics can be classified ino four general caegories: very fas-acing, fas-acing, Slo-lo and ime delay (surge oleran) fuses. The disinguishing feaure of Slo-lo fuses is heir addiional hermal ineria and heir abiliy o olerae excessive sar-up or in-rush overload pulses. Fuse onsrucion Inernal fuse consrucion may vary depending on ampere raing. Fuse phoos in his caalog show ypical consrucion of a paricular ampere raing wihin he fuse series. Fuseholders In many applicaions, fuses are insalled in fuseholders. These fuses and heir associaed fuseholders are no inended for operaion as a "swich" for urning power "on" and "off". Inerruping Raing lso known as breaking capaciy or shor circui raing, he inerruping raing is he maximum approved curren which he fuse can safely inerrup a is raed volage. During a faul or shor circui condiion, a fuse may receive an insananeous overload curren many imes greaer han is normal operaing curren. Safe operaion requires ha he fuse remain inac (no explosion or body rupure) and clear he circui. Inerruping raings may vary wih fuse design and range from 35 amperes for some 250V meric size (5 x 20mm) fuses up o 200,000 amperes for he 600V KLK series. Informaion on oher fuse series can be obained from he facory. Fuses lised in accordance wih UL/ S/NE 248 are required o have an inerruping raing of 10,000 amperes a 125 V, wih some excepions (See Sandards secion) which, in many applicaions, provides a safey facor far in excess of he shor circui currens available. Nuisance Opening Nuisance opening is mos ofen caused by an incomplee analysis of he circui under consideraion. Of all he "Selecion Facors" lised in he Fuse Selecion secion, special aenion mus be given o he normal operaing curren, ambien emperaure, and pulses. fuse canno be seleced solely on he basis of normal operaing curren and ambien emperaure. For example, one prevalen cause of nuisance opening in convenional power supplies is he failure o adequaely consider he fuse's nominal meling I 2 raing. In such an applicaion, he fuse's I 2 raing mus also mee he in-rush curren requiremens creaed by inpu impedance of he power supply's smoohing filer. The procedure for convering various waveforms ino I 2 circui values are given in he Fuse Selecion Guide secion. For rouble-free, long-life fuse proecion, i is good design pracice o selec a fuse for which he I 2 of he waveform is no more han 20% of he nominal meling I 2 raing of he fuse. Refer o he secion on PULSES in he Fuse Selecion secion. Resisance The resisance of a fuse is usually an insignifican par of he oal circui resisance. Since he resisance of fracional amperage fuses can be several ohms, his fac should be considered when using hem in low-volage circuis. cual values can be obained from he facory. Mos fuses are manufacured from maerials which have posiive emperaure coefficiens and, herefore, i is common o refer o cold resisance and ho resisance (volage drop a raed curren), wih acual operaion being somewhere in beween. old resisance is he resisance obained using a measuring curren of no more han 10% of he fuse's nominal raed curren. Values shown in his publicaion for cold resisance are nominal and represenaive. The facory should be consuled if his parameer is criical o he design analysis. Ho resisance is he resisance calculaed from he sabilized volage drop across he fuse, wih curren equal o he nominal raed curren flowing hrough i. Resisance daa on all Lielfuse producs is available upon reques. Fuses can be supplied o specified conrolled resisance olerances. Elecronic Produc Selecion Guide 4

3 Fuse Facs coninued from previous page Soldering Recommendaions Since mos fuse consrucions incorporae soldered connecions, cauion should be used when insalling fuses inended o be soldered in place. The applicaion of excessive hea can reflow he solder wihin he fuse and change is raing. Fuses are hea-sensiive componens similar o semiconducors, and he use of hea sinks during soldering is recommended. Saisical Process onrol ecause compliance wih cerain specificaions requires desrucive esing, overload ess are performed on a saisical basis for each lo manufacured. Time-urren urve The graphical presenaion of he fusing characerisic (ime-curren curves) is generally an average curve which is presened as a design aid bu is no generally considered par of he fuse specificaion. Time-curren curves are exremely useful in defining a fuse, since fuses wih he same curren raing can be represened by considerably differen ime-curren curves. The fuse specificaion ypically will include a life requiremen a 100% of raing and maximum opening imes a overload poins (usually 135% and 200% of raing). ime-curren curve represens average daa for he design; however, here may be some differences in he values for any one given producion lo. Samples should be esed o verify performance, once he fuse has been seleced. Underwriers Laboraories Reference o Lised by Underwriers Laboraories signifies ha he fuses mee he requiremens of UL/S/ NE Fuses for Supplemenary Overcurren Proecion. Some 32 vol fuses (auomoive) in his caalog are lised under UL Sandard 275. Reference o Recognized Under he omponen Program of Underwriers Laboraories signifies ha he iem is recognized under he componen program of Underwriers Laboraories and applicaion approval is required. Volage Raing The volage raing, as marked on a fuse, indicaes ha he fuse can be relied upon o safely inerrup is raed shor circui curren in a circui where he volage is equal o, or less han, is raed volage. Mos common volage raings used by fuse manufacurers for mos smalldimension and midge fuses are 24, 32, 63, 125, 250, 300, 350 and 600. In elecronic equipmen wih relaively low oupu power supplies, wih circui impedance limiing shor circui currens o values of less han en imes he curren raing of he fuse, i is common pracice o specify fuses wih 125 or 250 vol raings for secondary circuroecion of 500 vols or higher. s menioned previously (See Reraing), fuses are sensiive o changes in curren, no volage, mainaining heir saus quo a any volage from zero o he maximum raing of he fuse. I is no unil he fuse elemen mels and arcing occurs ha he circui volage and available power become an issue. The safe inerrupion of he circui, as i relaes o circui volage and available power, is discussed in he secion on Inerruping Raing. To summarize, under an overload condiion, a fuse may be used a any volage ha is less han is volage raing wihou derimen o is fusing characerisics. Please conac he facory for applicaions a volages greaer han he volage raing. Derivaion of Nominal Meling I 2 Laboraory ess are conduced on each fuse design o deermine he amoun of energy required o mel he fusing elemen. This energy is described as nominal meling I 2 and is expressed as mpere Squared Seconds ( 2 Sec). pulse of curren is applied o he fuse, and a ime measuremen is aken for meling o occur. If meling does no occur wihin a shor duraion of abou 1 millisecond (0.001 seconds) or less for hin-film fuses; 8 milliseconds (0.008 seconds) or less for axial and carridge fuses; he level of pulse curren is increased. This es procedure is repeaed unil meling of he fuse elemen is confined o wihin he 1 or 8 milliseconds ime frame, respecively. The purpose of his procedure is o assure ha he hea creaed has insufficien ime o hermally conduc away from he fuse elemen. Tha is, all of he hea energy (I 2 ) is used o cause meling. Once he measuremens of curren (I) and ime () are deermined, i is a simple maer o calculae meling I 2. When he meling phase reaches compleion, an elecrical arc occurs immediaely prior o he opening of he fuse elemen. learing I 2 = Meling I 2 + arcing I 2. The nominal I 2 values given in his publicaion perain o he meling phase porion of he clearing or opening. Fuse Selecion The applicaion guidelines and produc daa in his guide are inended o provide echnical informaion ha will help wih applicaion design. Since hese are only a few of he conribuing parameers, applicaion esing is srongly recommended and should be used o verify performance in he circui/applicaion. Many of he facors involved wih fuse selecion are lised below: Selecion Facors 1. Normal operaing curren 2. pplicaion volage ( or D) 3. mbien emperaure 4. Overload curren and lengh of ime in which he fuse mus open 5. Maximum available faul curren 6. Pulses, Surge urrens, In-rush urrens, Sar-up urrens, and ircui Transiens 5 Elecronic Produc Selecion Guide

4 7. Physical size limiaions, such as lengh, diameer, or heigh 8. gency pprovals required, such as UL, S, VDE, METI, or Miliary 9. onsideraions: mouning ype/form facor, ease of removal, axial leads, visual indicaion, ec. 10. Fuseholder feaures: clips, mouning block, panel moun, p.c. board moun, R.F.I. shielded, ec. Normal Operaing urren The curren raing of a fuse is ypically deraed 25% for operaion a 25 o avoid nuisance blowing. For example, a fuse wih a curren raing of 10 is no usually recommended for operaion a more han 7.5 in a 25 ambien. For addiional deails, see reraing on page 10 secion and mbien Temperaure below. Volage The volage raing of he fuse mus be equal o, or greaer han, he available circui volage. For excepions, see Volage Raing. mbien Temperaure The curren carrying capaciy ess of fuses are performed a 25 and will be affeced by changes in ambien emperaure. The higher he ambien emperaure, he hoer he fuse will operae, and he shorer is life will be. onversely, operaing a a lower emperaure will prolong fuse life. fuse also runs hoer as he normal operaing curren approaches or exceeds he raing of he seleced fuse. Pracical experience indicaes fuses a room emperaure should las indefiniely, if operaed a no more han 75% of caalog fuse raing. * mbien emperaure effecs are in addiion o he normal reraing. Example: Given a normal operaing curren of 2.25 amperes in an applicaion using a Very Fas-cing arridge fuse a room emperaure, hen: aalog Fuse Raing = 2.25 mperes 0.75 x 1.00 Nominal Operaing urren 0.75 x Percen of Raing or = 3 mp Fuse (a 25 ) har 1 Effec of ambien emperaure on curren-carrying capaciy (ypical) har Key PER ENT OF R TING* F urve Thin-Film Fuses and 313 Series (.010 o.150) urve FLT-PK, Telelink, Nano 2, PIO, lade Terminal, Special Purpose and oher leaded and carridge fuses (excep ) urve Reseable PTs ur ve : Re seable P Ts F F Similarly, if ha same fuse were operaed a a very high ambien emperaure of 80, addiional deraing would be necessary. urve of he ambien emperaure char shows he maximum operaing Percen of Raing a 80 o be 95%, in which case; aalog Fuse Raing = 3.15 mp Fuse (a 80 ) = Nominal Operaing urren 0.75 x Percen of Raing or 2.25 mperes 0.75 x 0.95 Overload urren ondiion The curren level for which proecion is required. Faul condiions may be specified, eiher in erms of curren or in erms of boh curren and maximum ime he faul can be oleraed before damage occurs. Time-curren curves should be consuled o ry o mach he fuse characerisic o he circui needs, while keeping in mind ha he curves are based on average daa F 68 F 104 F 140 F 176 F MI ENT TEM PER TUR E F F Maximum faul curren The Inerruping Raing of a fuse mus mee or exceed he Maximum Faul urren of he circui. Pulses The general erm pulses is used in his conex o describe he broad caegory of wave shapes referred o as surge currens, sar-up currens, in-rush currens, and ransiens. Elecrical pulse condiions can vary considerably from one applicaion o anoher. Differen fuse consrucions may no reac he same o a given pulse condiion. Elecrical pulses produce hermal cycling and possible mechanical faigue ha could affec he life of he fuse. Iniial or sar-up pulses are normal for some applicaions and may require he characerisic of a Slo-lo fuse. Slo-lo fuses incorporae a hermal delay design o enable hem o survive normal sar-up pulses and sill provide proecion agains prolonged overloads. The sar-up pulse should be defined and hen compared o Elecronic Produc Selecion Guide 6

5 he ime-curren curve and I 2 raing for he fuse. pplicaion esing is recommended o esablish he abiliy of he fuse design o wihsand he pulse condiions. Nominal meling I 2 is a measure of he energy required o mel he fusing elemen and is expressed as mpere Squared Seconds ( 2 Sec.). This nominal meling I 2, and he energy i represens (wihin a ime duraion of 8 milliseconds [0.008 second] or less and 1 millisecond [0.001 second] or less for hin film fuses), is a value ha is consan for each differen fusing elemen. ecause every fuse ype and raing, as well as is corresponding par number, has a differen fusing elemen, i is necessary o deermine he I 2 for each. This I 2 value is a parameer of he fuse iself and is conrolled by he elemen maerial and he configuraion of he fuse elemen. In addiion o selecing fuses on he basis of Normal Operaing urrens, Reraing, and mbien Temperaure as discussed earlier, i is also necessary o apply he I 2 design approach. This nominal meling I 2 is no only a consan value for each fuse elemen design, bu i is also independen of emperaure and volage. Mos ofen, he nominal meling I 2 mehod of fuse selecion is applied o hose applicaions in which he fuse mus susain large curren pulses of a shor duraion. These high-energy currens are common in many applicaions and are described by a variey of erms, such as surge curren, sar-up curren, in-rush curren, and oher similar circui ransiens ha can be classified in he general caegory of pulses. Laboraory ess are conduced on each fuse design o deermine is nominal meling I 2 raing. The values for I 2 given in his publicaion are nominal and represenaive. The facory should be consuled if his parameer is criical o he design analysis. The following example should assis in providing a beer undersanding of he applicaion of I 2. Example: Selec a 125V, very fas-acing PIO II fuse ha is capable of wihsanding 100,000 pulses of curren (I) of he pulse waveform shown in Figure 1. The normal operaing curren is 0.75 ampere a an ambien emperaure of 25. har 2 D E F Waveshapes Sep 1 Refer o har 2 (op) and selec he appropriae pulse waveform, which is waveform (E) in his example. Place he applicable value for peak pulse curren (ip) and ime () ino he corresponding formula for waveshape (E) and calculae he resul as shown: I 2 = 1 5 ( )2 1 5 x 82 x.004 = Sec. This value is referred o as he Pulse I 2. Sep 2 Deermine he required value of Nominal Meling I 2 by referring o har 3 (page 8). figure of 22% is shown in his char for 100,000 occurrences of he Pulse I 2 calculaed in Sep 1. This Pulse I 2 is convered o is required value of Nominal Meling I 2 as follows: Nom. Mel I 2 = Pulse I 2 /.22 = /.22 = Sec. OR 1 i b Formulas i = k I 2 = ip 2 i = ip -k I 2 = (1/3)(ip 2 + ip i b + ib 2 ) i = ip sin I 2 = (1/2)ip 2 I 2 = (1/3)ip 2 i = k 2 OR i = ip (1-k) 2 I 2 = (1/5)ip 2 i = ip e k ) I 2 (1/2)ip 2 1 urren (mperes) Normal Operaing urren l 2 Pulse Energy Time (Seconds) Figure Figure 1 1 Sep 3 Examine he I 2 raing daa for he PIO II, 125V, very fas-acing fuse. The par number , 1 ampere design is raed a Sec., which is he minimum fuse raing ha will accommodae he Sec. value calculaed in Sep 2. This 1 ampere fuse will also accommodae he specified 7 Elecronic Produc Selecion Guide

6 0.75 ampere normal operaing curren, when a 25% deraing facor is applied o he 1 ampere raing, as previously described. Tesing: The above facors should be considered in selecing a fuse for a given applicaion. The nex sep is o verify he selecion by requesing samples for esing in he acual circui. efore evaluaing he samples, make sure he fuse is properly mouned wih good elecrical connecions, using adequaely sized wires or races. The esing should include life ess under normal condiions and overload ess under faul condiions, o ensure ha he fuse will operae properly in he circui. har 3 100,000 Pulses 10,000 Pulses 1,000 Pulses 100 Pulses Number of Pulses Pulse I 2 = 22% of Nominal Meling I 2 Pulse I 2 = 29% of Nominal Meling I 2 Pulse I 2 = 38% of Nominal Meling I 2 Pulse I 2 = 48% of Nominal Meling l % 100% Pulse I 2 / verage Meling I 2 Noe: dequae ime (10 seconds) mus exis beween pulse evens o allow hea from he previous even o dissipae. Fuseholder Selecion Reraing: For 25 ambien emperaures, i is recommended ha fuseholders be operaed a no more han 60% of he nominal curren raing esablished using he conrolled es condiions specified by Underwriers Laboraories. The primary objecive of hese UL es condiions is o specify common es sandards necessary for he coninued conrol of manufacured iems inended for proecion agains fire, ec. copper dummy fuse is insered in he fuseholder by Underwriers Laboraories, and hen he curren is increased unil a cerain emperaure rise occurs. The majoriy of he hea is produced by he conac resisance of he fuseholder clips. This value of curren is considered o be he raed curren of he fuseholder, expressed as 100% of raing. Some of he more common, everyday applicaions may differ from hese UL es condiions as follows: fully enclosed fuseholders, high conac resisance, air movemen, ransien spikes, and changes in connecing cable size (diameer and lengh). Even small variaions from he conrolled es condiions can grealy affec he raings of he fuseholder. For his reason, i is recommended ha fuseholders be deraed by 40% (operaed a no more han 60% of he nominal curren raing esablished using he Underwrier Laboraories es condiions, as previously saed). Sandards UL Lised UL Lised fuse mees all he requiremens of he UL/S Sandard. Following are some of he requiremens. UL ampere raing ess are conduced a 100%, 135%, and 200% of raed curren. The fuse mus carry 100% of is ampere raing and mus sabilize a a emperaure ha does no exceed a 75 rise a 100%. The fuse mus open a 135% of raed curren wihin one hour. I also mus open a 200% of raed curren wihin 2 minues for 0-30 ampere raings and 4 minues for ampere raings. The inerruping raing of a UL Lised fuse is 10,000 amperes a 125 vols minimum. Fuses raed a 250 vols may be lised as inerruping 10,000 amperes a 125 vols and, a leas, he minimum values shown below a 250 vols. mpere Raing of Fuse Inerruping Raing In mperes Volage 0 o V 1.1 o V 3.6 o V 10.1 o V 15.1 o V UL 275 uomoive Glass Tube Fuses (32 Vols) UL Lised UL ampere raings ess are conduced a 110%, 135%, and 200%. Inerruping raing ess are no required. Recognized Under he omponen Program of Underwriers Laboraories anadian Recognized omponen Mark The Recognized omponens Program of UL is differen from UL Lising. For recogniion, UL will es a fuse o a specificaion requesed by he manufacurer. The es poins can be differen from he UL Lising requiremens for fuses ha have been designed for a specific applicaion. pplicaion approval is required by UL for he use of fuses recognized under he omponen Program. S S erificaion in anada is equivalen o UL Lising in he Unied Saes. S omponen ccepance Program The omponen ccepance Program of S is equivalen o he Recogniion Program a UL. This S Program allows he manufacurer o declare a specificaion. S hen ess o his specificaion. METI pproval METI approval in Japan uses similar requiremens as hose covered in he UL/S/NE METI also uses special esing similar o ha covered in he IE sandards. Elecronic Produc Selecion Guide 8

7 Inernaional Elecroechnical ommission (IE) IE , Shee 1, 2, 3, 5, 6 (250 Vols) The IE is differen from UL and S, since IE is an inernaional organizaion ha wries specificaions and does no cerify. UL and S wrie specificaions and are responsible for esing and cerificaion in he US and anada, respecively. erificaion o IE specificaions are given by such naional organizaions as SEMKO (Swedish Insiue of Tesing and pprovals of Elecrical Equipmen) and SI (riish Sandards Insiue), as well as UL and S. IE defines hree breaking capaciy levels (inerruping raing). Low breaking capaciy fuses mus pass a es of 35 amperes or en imes raed curren, whichever is greaer, while enhanced breaking capaciy fuses mus pass a es of 150 amperes and high breaking capaciy fuses mus pass a es of 1500 amperes. Shee 1 Type F Quick cing, High reaking apaciy Shee 2 Type F Quick cing, Low reaking apaciy Shee 3 Type T Time Lag, Low reaking apaciy Shee 5 Type T Time Lag, High reaking apaciy Shee 6 Type T Time Lag, Enhanced reaking apaciy The leers 'F' and 'T' represen he imecurren characerisic of he fas-acing and ime delay fuses. One of hese leers will be marked on he end cap of he fuse. The newes addiion o IE is par 4 which covers UMF (Universal Modular Fuse) producs for boh hrough-hole and surface moun fuse ypes. The sandard allows for boh hrough-hole and surface moun fuses wih volage raings of 32, 63, 125 and 250 vols. reaking capaciies for he 32, 63, and 125 vol fuses are he same as low breaking capaciy fuses covered by IE Par 2. The 250 vol UMF fuse is available in a low breaking capaciy (100), inermediae breaking capaciy (500), and high breaking capaciy (1500). PT Facs Overcurren circuroecion can be accomplished wih he use of eiher a radiional one ime fuse or he more recenly developed reseable PT. oh devices funcion by reacing o hea generaed by he excessive curren flow in he circui. The fuse elemen mels open, inerruping he curren flow, while he PT changes from low resisance o high resisance o limi curren flow. Undersanding he differences in performance beween he wo ypes of devices will make he bes circuroecion choice easier. The mos obvious difference is ha he PT is reseable. The general procedure for reseing afer an overload has occurred is o remove power and allow he device o cool down. There are several oher operaing characerisics ha differeniae he wo ypes of producs. The erminology used for PTs is ofen similar bu no he same as for fuses. Two parameers ha fall ino his caegory are leakage curren and inerruping raing. Leakage urren PT is said o have "ripped" when i has ransiioned from he low resisance sae o he high resisance sae due o an overload (see figure 2). Proecion is accomplished by limiing he curren flow o some low leakage level. Leakage curren can range from less han a hundred milliamps a raed volage up o a few hundred milliamps a lower volages. The fuse, on he oher hand, compleely inerrups he curren flow and his open circui resuls in no leakage curren afer being subjeced o an overload. Log resisance (ohms) Tem peraure ( ) Figure 2 Tr Poin Inerruping Raing PTs are raed for a maximum shor circui curren a raed volage. This faul curren level is he maximum curren ha he device can wihsand keeping in mind ha a ypical PT shor circui raing is 40. PTs will no acually inerrup he curren flow (see Leakage urren above), whereas fuses do inerrup he curren flow in response o he overload. The range of inerruping raings for fuses varies from ens of amperes up o amperes a raed volage. The circuarameers may dicae he componen choice based on ypical device raing differences. Operaing Volage Raing General use PTs are no raed above 72V while fuses are raed up o 600V. urren Raing The operaing curren raing for PTs can be up o 11 while he maximum level for fuses is 60, in accordance wih UL/S/ NE Temperaure Raing The useful upper limi for a PT is generally 85 while he maximum operaing emperaure for fuses is 125. The emperaure reraing curves (see char 4) ha compare PTs o fuses illusrae ha more reraing is required for a PT a a given emperaure. ddiional operaing characerisics can be reviewed by he circui designer in making he decision o choose a PT or a fuse for overcurren proecion. gency pprovals PTs are Recognized under he omponen Program of Underwriers Laboraories o UL Thermisor Sandard These devices have also been cerified under he S omponen ccepance Program. Resisance Reviewing produc specificaions indicaes ha similarly raed PTs have abou wice (someimes more) he resisance of fuses. 9 Elecronic Produc Selecion Guide

8 Time-urren haracerisic omparing he ime-curren curves of PTs o ime-curren curves of fuses show ha he speed of response for a PT is similar o he ime delay of a Slo-lo fuse. (see figure 3) Summary Many circuroecion issues are a maer of preference, bu here is an imporan area of applicaion where he use of reseable PTs is becoming a requiremen. Much of he design work for personal compuers and peripheral devices is srongly influenced by he Microsof and Inel Sysem Design Guide which saes ha "Using a fuse ha mus be replaced each ime an overcurren condiion occurs is unaccepable." In addiion, he Plug and Play SSI (Small ompuer Sysems Inerface) Specificaion for his large marke includes he saemen, "...mus provide a self-reseing device o limi he maximum amoun of curren sourced." selecion guide work-shee appears on he following page as an aid in choosing he bes circuroecion componen and deermining when PTs may be he appropriae choice for providing overcurren circuroecion. har 4 Temperaure Reraing urves omparing PTs o Fuses har Key PER ENT OF R TING* F urve Thin-Film Fuses and 313 Series (.010 o.150) urve FLT-PK, Telelink, Nano 2, PIO, lade Terminal, Special Purpose and oher leaded and carridge fuses (excep ) urve ur Reseable ve : RePTs seable P Ts F 1.5 PT 1.5 Fuse F F 68 F 104 F 140 F 176 F MI ENT TEM PER TUR E F F 10 TIME IN SEONDS L Series PT 430 Series Slo-lo Thin-Film fuse URRENT IN MPERES Figure 3 Elecronic Produc Selecion Guide 10