Document gDYpkQLkevEk6jkrODBz8ebN9

'? CONSTRUCTION FEATURES OF NONRENEWABLE FUSES' OPERATING TIME ON SHORT CIRCUIT AND OVERLOAD IPlug fuses ore used for branch-circuit protection. Maximum rating Is 30 amp at 123 v. Time*log type, shown ot right, does net open on short-time overload FUSES Indicator shows If fuse opens, making blown fuses eosy to find. Not used on renewable type Ferrule-end cart ridge: 35*60 amp Fuses ore replaced^ safely by. use .of In4; suloted grip. Con& ventlonol fuse pally1 er Is not needed;' _____ 5 Cartridge fuses having knife-end connections are rated 70-600 amp ot 250 v. Similar construction Is used In 600-v fuses but over-oil length is greater 6 Oscillogram at A shows o successful circuit Interruption under short circuit, ftelpnltlon of the ore Is cleoriy Indlcoted In B. de laying the circuit opening. Note voltoge during period of short circuit 4 2 1.0 A fuse serves as e safety valve lp an electrical circuit, opening it shoo too much curreot flone. Uollke most other oleotrical pro tective devices, the same element detects (low of excessive current and Interrupts the primary circuit. Simplicity, loir flret cost and application flexibility Date fuses particularly suitable, to proteot low-voltage low-capacity elreulta. Oeoeral Cttoraoterlstlcs. Fuses consist essentially of two parts: (1) fusible oleoent that carries curreot and melts when the currant exceeds a definite value for e certain length of time (3) casing that encloses and supports the fusible element, or Ifni, end makes eootact with the external electrical circuit. The original fuse was a thin piece of copper wire. This proved unsatisfactory since the element bad to reach on .extremely high temperature before it would blow. Coppor soon gave way to alloyed wire with a lead base. As sine has less tendency to bold an arc. It soon replaced lead. All ordinary fuses have links of sloe alloy, with one or more reduced sections near the center. If an excessive current flows long enough, the link overheats and.the thin portion melts. At first, the gap is not big enough to open the circuit and an arc forms. Further melting Increases the gap to a length over which tho arc canaot be maintained, and the circuit le then Interrupted. The two oscillograph traces, shown at A in Fig. 6, are of voltage and current taken at the point of short circuit. Vhon the short was applied voltage dropped to a near- sero value while current increased to over 3000 amp. The fuee opened the circuit In l/GO of a second. Oscillogram, fl in pig, e. shows rclgnltlon of the ere, delaying circuit Interruption. Here, In creased capacity allowed abortcircuit current to build up to 7680 amp. Time required to doer the circuit waa twice that of A- All fuses posseas aa isuerse tins eheroetritic. This means the amount of ourrent flow that will blow- the fuse decreases as tbs length of time over wblch It Is applied Increases, pig. ?- shows a typical characteristic curve, illustrating ths lnveree-time principle. This curve represents the average blowing time of differ ent makes and sizes of enclosed fuses of both the renewable end the nonreoswahlo types. National Board ot Fire under writers specifies that all tuaos shall carry 110% of rated current indefinitely, and shall blow on 150% of rated current In a speci fied time. Fig. 8 shows time limits withlc which fuses ofaglven rating, blow wbeu carrying 150% rated current. For example.-a 200-eop fuse opens in 6-12 min when carrying 800 amp. Sicca the ability of a fuse to dissipate boat increases with its size, because of increase in the conducting area, blowing time is greater for the larger ruse. - Standard enclosed fuses oust be capable of Interrupting shortcircuit currents of at least 10,000 amp at rated voltage. Tbla test uses direct curreot. We may consider the maximum short-circuit current as that which would flow if- the circuit were shorted at the point where fuses are to be loatalled. puses are plainly marked to show current rating and voltage, cart ridge fuses lo thoO- to 15-amp 25f class have a blue label pasted $ tbo fiber tube. If they are raft at more than is oap they cerrj(i green label.' All 600-v fuses hut their current and voltage rating printed on a red label. fj Fuses differ mainly in the foti of casing. There are throe baalj types, each usually serving a defi nite voltage and current range: M)j plug tunes handle up to and It: eluding 30 amp at 125 v. (*2l Cartridge fuses with ferrule are raeod up to and Including?# up it 125. 250 and 500 . (l| Knlfo-end cartridge fuses head}* currents from 70 to 600. amp, Ml 125. 250 and 600 v. Oyer-all leniV of the 600-v cartridge fuse?!! greater than of .those designed w| 230-r circuits. *1 *.'*jfl* Plug Fuses, plug fusee, pig.$r are llmltod to 125-v application They are commonly used on lightly circuits and for protection of sail motors. A screw-ehell base aerjl for both electrical contact x> mechanical support. Fusible eleoffi is visible through a window of elS glass or other transparent maten* la the cap. V National Electrical Code requlit that 0- to 15-amp plug fuses;" distinguished from others of lwj rating by a hexagonal window in^L cap or some other prominent hexagofi feature. Plug fuses are ovaiUjil with a symbol on ths transport top Indicating the link's condltie When it opens the symbol is stroyed. making it easy to pick' the blown fuse. ifl Both industry and the home *? wide uoo of plug fuses. Mlsusojg units of this clans is common- prevent overfusing, bridging other abuses, the Type 6 nontamper- able fuee has been developed, it cao be installed In ite own special fuse holder or any Bdlaon-base fuaed-cutout by use of an adapter. Type B cornea in two sizes: o-15 and 16-30 amp. Because of dlfforanoe In base .sizes, Typo 8 fuses of one rating .are not Interchangeable with those of the othor. The adaptors are so Bade that once they are screwed Into a standard-base fuse holder they cannot be removed. At the present lime, use of Type 8 fusos Is not asndatory although the National Board of Plre Underwriters has the tetter under consideration. Cartridge puses, in cartridge types, the link la enclosed in a hard-fiber tube. The element of oonrenewablo fuses is surrounded *ltb heat-resistant powder, which aide in quenching the arc by mech anically displacing It and cooling the metallic vapor. This vapor, produced by the funlble element under short circuit, generates considerable pressure on the cart-' r*uie. This is relieved and the gaa cooled by vents In tho oart- *!* ends. Typical links and eutrldge constructions are shown. then a fuse blows, only tho fink is destroyed, plber tube and opper ends remain Intact. Developmeat of the renewable fuse came as natural outgrowth of tbe desire 5 rtaln these undomagod parts, -k rnl renewable-link designs are ;;*n In Fig. 13. pig. u MIub- n renewable element that Is ftr"Quenchlng powder. ho,,T5,5rne'ra*5]0 fuses cost less and ei ar ldely used to protect Maa* . !h8ro fault nr* unlikely, tn/h thse fuses may be returned lo the taker to refill at small oost. The renewable type represents a larger initial Investment', but usually pays over ao extended period since only tbe . link is replaced each time the fuse blows. Time Lag. Tho tlns-log fuse was developed to eliminate blowing on momentary overloads, on short circuit, It opens wltb the same speed as a regular fuse, such a unit Is well suited to protect motor circuits where the large starting currents would blow a regular fuse. This type Is available In both the plug and cartridge designs for the standard voltage ranges, pig. (3 illustrates the regular and time-lag renewable llnke. in addition to those -shown, another style time-lag fuse conslats of a regular link In series wltb a thermal cutout, puse link opens on short circuit but will not blow on short-time overloads. Overloads that persist cause the thermal cutout to-operate. This cutout Is designed to trip under prolonged loads that are within 500% of the fuse rating. Copper Link blows on overloads and short circuits drawing over 500% rated current, a helical spring is con nected to the Junction of fuse link and thermal cutout, on dangeroue overloads solder In the cutout molts, allowing the spring'to pull the fuse link and open circuit. Panelboard overheating may often be tracod to accumulated heat doveloped by fuses. It is reduced wltb time-lag fuses, which have lower resistance than ordinary units. Reduced resistance comes through the use of copper as tbe blowing element. 80. (4041 50WI2 July ij '0V" July 1945 U05) 81 a6 0.4 0.20 O.lO 006 004 0.02 aoi Tima* norms! rated eurtwei 7 O Typfcol blowing characteristic for * V various fuses It shown above. Open ing time on 50% overibod is shown below 16 E .0 i 0 CO 800 300 400 900 600 .Fuss roilftq.emp