CA1125819A - Double tubular time-lag fuse having improved breaking capacity - Google Patents

Abstract

DOUBLE TUBULAR TIME-LAG FUSE HAVING
IMPROVED BREAKING CAPACITY

Abstract of the Technical Disclosure A time-lag fuse with improved breaking capacity is made of double-tubular construction comprising an inner tube made of a material having high thermal conductivity and low thermal impact resistance and an outer tube made of a material with low thermal conductivity but high thermal impact resistance.
No art-quenching material is required and greater safety is afforded when the fuse is subjected to an overload current.

Description

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,., I Field of InventiOn:
I _. __ ___ This invention relates to a time-lag u5e havi~y improved brea~ing capacity and is particularly rela~ed to such a fuse ~hich has a unique double~tuhular c~nstruct;on.
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The Prior Art . - - - --- _,, It is well known that when an overload current, i.e., current is excess of the rated capacity, flows through a fuse, the fuse may explode resulting in fire hazard due to the drawing ~¦ f a long arc between the terminals of the fusible wire element ¦I which is Trequently maint2ined for a lons time. The explosion ~ is often so severe as to cause scattering o fragments of the fuse at a considerable~distance, and when the arc is ~aintained the luse terminals are destroyed.
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' It has long been recognized that the adverse effects ~j and impact of the gases resulting rom explosion of the fuse may ' be reduced by the use of so-called "arc-extinguishing" or "arc-j ~uenching" materials. Thus~ ~s disclosed in a very early patent ~U.S. 550,638, issued December 3, 1895), the fusible element is 1 surrounded with a loose, porous, nor.-combustible material such I
i as asbestos, in order to deaden the sound of explosion of the I fusible element, prevent fraamenta.ion and scattering, absorb 25 1l the smoke and resulting gases and confine the destruction of the fusible el-ment to a relatively small portion of its length. I

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i -1 I The use of arc-q~enching fillers is also rnentioned in, iGerman Patent No. 611,680.
¦ More recent patents have also resorted to the u~e of arc~e~tinguislling materials for this purpose. For examp:le, U. S.
13,069,520, issued on December 18, 1962 to Calnero~ et al aiscloses ithe use of finely divided inert insula~ing ma-Lerial such as sand, disposed within the fuse holder and about the fusible e~ement.
IU S. 3,876,966, issued on April 18, 1975 to Aloysuis J. Fister ,`also discloses the use of "arc-quenching" filler such as quartz Isand, for a similar purpose.
I The use of arc-extinguishing fillers in the manner dis-- I,closed in the aforementioned illustrative patents to im.prove the ! breaking capacity of the fuse has had several disadvantages. As ~ disclosed in the recent patent of Arikawa et al (U.S. 4,100,523, lS lissued on July 11, 1978j, it is difficult to stabilize the fuse ~characteristics when the fuse is filled with arc-extinguîshing ,material as in the prior art. This is due to irregu~arities in ¦~the grain distribution of this material and variations in its ,~illing rate. Moreover, a large amount of heat conduction from the Ifusible element to the arc~extinguishing material causes changes ,in the thermal balance of the fu-sible element and impairs the time lag characteristic of the fuse and, at the sa~e time, heat con- ¦
Iduction from the fusible element to the arc-extinguishing ma~erial, ¦~and then to the fuse tube, ten~s to raise the temperature of the ,'fuse tube excessively. These a~d other disadvant?aes impair the br~aking capacity and time-lay properties of eh~ fuse.

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1 It is therefore an object of thls invention to p~ov.id~
a fuse ~hich has an improved breaX;ng cap~c;ity ana tirne-lag characteristics.
It is another object o this inven tion to providé a ~fuse wlth improved breaking capacity arld ~irne lag charactexist:ics which lS free from the clifficulties and dlsadvantages ~hi~ch are inherent ln ~le prior art fuses ernploying arc-quenching rnaterials.
- The foregolng and other ob~ects of this invention will be more clearly understood from the following detàiled description of the invention, taken in conjunction with the accompanying drawing which is a longitudinal sectional view of a double~tubular fuse constructed in accordance with this invention.

Il - SU~RY OF I~ TION
ll A time-lag ruse having improved breaking capacity is provided which co~prises an inner-tube made of a material having 'high thermal conductivit~-2nd low thermal i~pact resistance and ~an outer tube having low thermal conductivity and high thermal ¦ir~lpact resistance;
¦ - When an overload~current flo~7s through the fusible ele-m~nt which is stretched be~ween the ends of the inner tube an2 an llelectric arc is generated across the terminals of the fuse, the !
;inner tube is fra~mented an~ the resulting heat and vapors will ¦
!be absorbed by the fragmented ~arts Ho~7ever, since ~he outer i,tube is of low thermal impact resista~ce, it will not be fragmented l'and the fracmented pieces ol the inner tube will remain within the ,,outer tube rather than scatter about. Fire hezard is therefore ~1 . - .

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.,,~ I ., . 1 eliminated and sa~e operation of the fuse is insured by this I I unlque aouble tubular structure. ' DETAILED DES:~IPIl~N OF 1~ .'N~C~IN
. . It has no~ been discovered that the Aisadvar.t~yes - ¦ inherent in fuses which contain art-quenching fillerb ln the i tllse'tube can be eliminated by a novel and unique fuse construc-¦ tion which'does not require the use of arc; quenching fillers.
- !I Yet, the fuse of this inventi-on exhibits e~cellent breaking . 10 ~ capacity and time-lag behavior, and an overload current can be' interrupted in a fraction o~ a second without explosion of the ¦ tube and while maintainlng the stability of the fuse.
~j ~ Referring now to the drawin~, the novel fuse co~.prises i' a ~usible element 1 r such 2S a conductor metal wire and the like, '15 1~ disposed within an inner tubular member 3 ~hich is, in.turn, !~ disposeh within an outer tubular-me~ber 5 which is coterminus with the inner tubular member. The ends of the tubular members j, are capped with a pair of ferrules 7, 7 to seal the double-¦l tubular ~use while maintaining electrical contact with fhe ¦. fusible element. The ~usi~ble element is thus aisposed within the empty tubular member 3 and no.arc-quenching filler is . employed. - I ¦
For reasons which are hereinafter e~plaine~, the inner l~ tubular me~ber 3 must be made of a material which has a high I thermal conductivity but low thermai impact resistance so as to ~be lragm~nted when an overload current is passed through the' fusible element 1. On the other hand, the outer tubular niember 5 ¦

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1 must 'oe made of a material which has a low thermal conduct:ivity I but hlgll thermal impact resistarlce to withstand fragrrlentation~
... ... . Thus, when an overload current Elo~s through the f~e and an electric arc is generated across t~e fusible el~Jn~n~ 1, this element will melt and generate me-tal vapors in the tubular Inember;
3, as in the prior art fuses. ~!o~ever, and un1ike the prior art ¦
fuses, the metal vapors and the heat generated in the tublllar --~
member are not absorbed by arc-quenching fillers, but rather, - ¦ the electric arc and the resultin~ heat will cause fraymentation I of the inner tubular member 3 since, as it was previously ~en-I tioned~ the inner tube is made of material which has high thermal ¦ conductivity but low impact resistance~ Once the inner tube - 11 is fragmented, the electric arc and metal vapors wili diffuse ~! into the gaps and spa~es bet~Yeen the broken and fragmented pieces !
¦l and the vapor is thus cooled do~n. ~he electric arc energy is 1 accordingly consumed in the inner tube causing a rapid rise in the inter-electrode vol.age, and the current is auickly interrupt ed.
Since the outer`tubular member 5 is ma~è of a material I of high thermal impact resistance but low thermal conductivity, ¦I the outer tubular member withstands fragmentation under the !~ aforementioned conditions, znd the fragmented inner tubular member, l 1l! will thus be confined within the outer tube. There is, therefore,~ ¦
¦I no likelihood of fragmentation and scattering of the tube, nor is ` - I! there the danger of fire hazard as in the prior art fuses where 1 hot, fragmented pieces of the fuse tube are scattered about 1~ the surr unding environme-t Il -. . . . 1 I . , , . .1l ~ !

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`~ l 1 ¦ Although several materials suggest themselves to one skilled in the art on the basis of the disclosure and require~ents set E~rth for the construction of the lnner and outer tubes o the fuse OL -this invention, nevertheless, and by way o~ e~.amples, it has been found that ceramic materials such forstexit~, ste~tite - and talc are eminently suitable for the construction of the inner tube, while cordierite and llthlum ceramics constitute excellent materials of constructlon for the outer tubular member. Again, it jmust be emphasi~ed that these materials are ~erely examp1ary and .
iare no, intended to be all inclusive. ~
The properties o forsterite used in making the inner Itube and cordierite used in making the outer tube are given in the ¦Ifoll~wing table: : ¦

"Material Forsterite Cordierite ¦~ Composition, Wt.% ;
¦ 2MgO.SiO2 99.5 2MgO 2A12~3 5SiO2 X2O 0.12 - 0.50 ! Na2 : o.38 . o.20 ISpeciIic ~ravity 2.~7 ` 2.40 jThermal Expansion , 10.5 x 1~ 6 3.2 x lD
jCoe~ficient o' . ~ ~
Thermal Conductivity, 0.0082 0 0045 IICal cm/cm2 sec. C
!~ Bendiny Strength, kg/cm2 1470 ` 1260 1 ¦I Softing Point, C in air 1350 1380 I ¦
1~ ~ater Absorption 0.000 o 000 ;~Electrical Resistance Value 2.5 x 107 - 7.2 x 107 -, , ', - "' ,:''~, , ~. .
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1 I Thus, in ~ccordance with this invention-, a unique ~use . Iconstruction is providecl which dispenses wi-th ~il].ing the fu~e tube with arc-~uenching materi.als and ~hich af~ords exce].lent n ,-; I , - . .

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Claims (4)

WHAT IS CLAIMED IS:

1. A time lag fuse having improved current-interrupting characteristics which comprises an inner tubular member, an outer tubular member encasing said inner tubular member and coeterminus therewith, a fusible element disposed between the ends of said inner tubular member and means for enclosing the ends of said tubular members while establishing electrical con-tact with said fusible element, said inner tubular member being made of a material having high thermal conductivity and low thermal impact resistance so as to be fragmented by the passage of an overload electrical current through said fusible element, and said outer tubular member being made of a material having low thermal conductivity and high thermal impact resistance to resist fragmentation during the passage said overload electrical current.

2. A time lag fuse as in claim 1 wherein said means for enclosing the ends of said tubular member is ferrules.

3. A time lag fuse as in claim 1 wherein said inner tubular member is made of forsterite, steatite, talc, or mixtures thereof and said outer tubular member is made of cordierite ceramic, lithium ceramic or mixtures thereof

4. A time lag fuse as in claim 2 wherein said inner tubular member is made of forsterike, steatite, talc, or mixtures thereof and said outer tubular member is made of cordierite ceramic, lithium ceramic or mixtures thereof.