CA1088600A - High continuous current capacity oil expulsion fuse - Google Patents

Abstract

HIGH CONTINUOUS CURRENT CAPACITY OIL
EXPULSION FUSE
ABSTRACT OF THE DISCLOSURE

An oil expulsion fuse has multiple rupture-resistant fuse wire chambers for providing fault current protection in high voltage distribution circuits in the 25-35 KV range.
In preferred forms, the fuse has a Teflon (a Trademark) core defining three discrete, small diameter cylindrical chambers, each containing a respective fuse wire, whereby sufficient deionizing gas flow is generated upon fusing of the wires to effectively extinguish arcs formed in the chambers, even in t he face of high recovery voltage rates associated with distribution voltages above 15 KV. The ampacity of the fuse per unit cross-sectional area of fuse wire is increased over single wire fuses by virtue of greater heat loss by transverse convection from the multiple wires to oil in the chambers.

Description

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HIGH CONTINUOUS CURRENT CAPACITY OIL
EXPULSION FUSE
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Background This invention relates to oil expulsion fuses in general and more particularly, it is concerned with a fuse of this variety having greater ampacity as well as improved arc-quenching capability such that the fuse is rendered suitable for use in high voltage distribution circuits (i.e.
greater than 15 KV).
Oil expulsion fuses have long been used in distri-bution circuits to protect electrical equipment from the deleterious effects of fault currents. Such fuses typically comprise a tubular fuse cartridge having conductive terminal caps at opposed ends, and adapted to receive an expendable fuse link comprising an elongate fusible element contained within an auxiliary tube having an inner liner exhibiting ,...
arc extinguishing properties. When an overcurrent or fault current is experienced in the circuit containing the fuse, the fusible element is caused to melt, whereupon arcing occurs within the auxiliary tube between the severed segments of the fusible element. Heat produced by the arc vaporizes . the inner liner of the auxiliary tube producing pressurized deionizing gas which vents at opposite ends of the fuse cartridge. As the venting gasses flow past the arc, they serve to cool and deionize the latter such that the arc is effectively extinguished. The disabled fuse may subsequently - be returned to service by simply replacing the expendable fuse link contained within the fuse cartridge.
While oil expulsion fuses have heretofore proved satisfactory for use in distribution circuits, it has been found that there are some difficulties associated with their ~0 -1- ': . .
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use in high voltage applications in the 25~35 KV range. In this regard, it has been discovered that when conventional oil expulsion fuses are constructed to provide the required current capacity for use in the higher distribution voltage circuits, the fuses do not reliably interrupt against higher transient recovery voltage rates associated with the higher distribution vol~ages. Of course, this is a highly undesira-ble characteristic since it represents a failure to clear the overcurrent and could result in serious damage to elec-10 trical apparatus relying upon the fuse for current protection.
While it has not been conclusively determined why larger conventional oil expulsion fuses are not adapted for use in high voltage distribution circuits, one theory is that the larger internal diameter of the auxiliary tube required to - accomodate the desired ampacity fuse element, precludes ; development of deionizing gas flow sufficient to adequately extinguish the arc against the higher transient recovery voltages. In any event, there simply is not commercially available a refusible oil expulsion fuse capable of reliably ? 20 interrupting fault currents or harmful overcurrents in a 25 KV or 35 KV distribution system.
While certain nonconventional expulsion fuse --designs have been proposed, such as those shown in U.S.
Letters Patent 2,156,058 to Lohausen and 2,291,341 to Lincks, none of the known prior art devices appears to have practical application in solving the aforementioned problem. Both of the patents identified show an expulsion fuse having a - plurality of fusible elements, each element being provided with its own auxiliary tube. While these fuses may exhibit 30 limited increased current-carrying capacity and, at first glance, appear to distinguish over the conventional single

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88~0 element design, it is noted that both patents require con-structing the auxiliary tubes in suc~ a manner that, upon encountering high pressure, they "open or ~urst so as to provide communication between adjacent cham~ers". Hence, in actual practice these devices are the equivalent of a single element, large bore fuse and consequently, are not appropriate - for utilization in high voltage distribution circuits.
.. Moreover, the Lincks patent is an air expulsion fuse involving . significantly different design characteristics than fuses of 10 the oil expulsion ~ariety.
- Summary . In general terms, the present invention provides an `i~ oil expulsion fuse adapted for interrupting low range fault ~; and overload currents in a high voltage distribution circuit, ;.
said fuse comprising: a pair of spaced, electrically conductive `. terminals adapted to be interposed in said circuit; a noncon-. ductive elongate member spanning the distance between said terminals, said member having a plurality of discrete chambers extending the full length of the member in parallel relation ,~
to the longitudinal axis thereof and being symmetrically arrangedaround said longitudinal axis; means for permitting .. oil flow into the chambers; and a number of fusable elements each disposed in a respective one of said chambers, and having . its opposite ends coupled with respective said terminals, the . walls of said chambers being sufficiently rigid to preclude . rupture from high internal pressures generated by arc form-ation in the chambers upon fusing of the respective elements, : said elements and said chambers, collectiYley defining a number of se~regated current-intexxupting links between : 30 said terminals wh.ere~y the fuse is p~.ovided with increased ampacity as we.Il as the capa~ility o~ clearing low range ~ault currents in said distri~ution circuit notwithstanding the greater transient recovery voltage rates associated with current interruption at higù distribution voltages.

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The chambers are preferably cylindrical and equal in diameter. In accordance ~ith anot~er feature of the invention, there are three of sa~d chambers and a corresponding num~er of said elements. The oil expulsion fuse as defined above may further include a rigid tubular fuse cartridge configured to complementally receive said member whereby to provide additional support for said c~ambers against rupture from high internal pressures developed after fusing of said elements.
In another aspect, the present invention provides, in combination with high voltage electrical distribution apparatus of the type having a reservoir of dielectric liquid, an expulsion fuse electrically coupled with said ; apparatus and ~ubmerged in said reservoir, said fuse including:
a pair of spaced, electrically conductive terminals, a nonconductive elongate member spanning the distance between ; said terminals, said member having a plurality of discrete chambers extending the full length of the member in parallel ~-relation to the longitudinal axis thereof; means permitting said dielectric liquid to flow into said chambers when the fuse is submerged; and a number of fusible elements each - disposed in a respective one of said chambers, and having its opposite ends coupled with respective said terminals, ;~ the walls of said chambers being sufficiently rigid to -preclude rupture from high internal pressures generated by arc formation in the chambers upon fusing of the respective elements, said elements and said chambers, collectively defining a number of segregated current-interrupting links between said terminals whereby the fuse is provided with increased ampacity as well as the capabilit~ of clearing 1QW range fault currents in said d;stribution circuit notwith-standing tne greater transient recovery voltage rates ,' .
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associated with current interruption at high distribution voltages.
The instant invention thus presents an oil expulsion fuse su;table for use in high voltage distri~ution circuits by virtue of the provision of multiple fuse wires each disposed within a discrete, rupture-resistant chamber such that the , fuse has the desired increased ampacity as well as exhibits arc-extinguishing capability sufficient to overcome the ' transient recovery voltage rates associated with the interruption of high voltage circuits. More specifically, the fuse includes a synthetic resin, rod-like insert having three elongate, cylindrical chambers formed therein, each chamber being ; sufficiently small in diameter to establish a flow rate of deionized gas adequate to extinguish arcs formed within the chamber upon melting of fuse wire carried therewithin.
Though each of the fuse wires is substantially identical in size and material to others in the fuse, inherent material and physical differences assure sequential fusing of the wires and arcing in the chambers such that the overcurrent 20 i9 cleared withir the l~st cha~oer to arc.

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The ~ynthetic resin insert is adapted to be comple-mentally received within a rigid tubular fuse cartridge such that the latter provides additional strength to protect against rupturing of the fuse wire chamber~. Vent-defining threaded fittings at opposite ends of the fuse cartridge `~ permit oil flow into the elongate chambers and gas venting therefrom, while at the same time preclude expulsion of the insert when overcurrents are encountered.
Convective heat transfer from the fuse wires to the surrounding oil in the respective chambers is responsible for an increase in the rurrent-carrying capacity per unit cross-section of the present in~ention over single element oil expulsion fuses. Thus, the multiple wire design provides desired ampacity while also permitting the provision of a number of wholly separate fuse ch$mbers each of a ~uffi-ciently small diameter to assure proper clearing of fault - currents notwith~tanding the higher transient voltage re-covery rates a~sociated with high voltage distribution circuits.
2Q Detailed Descri~tion Of The Drawin~

Fig. 1 is a perspective view of a ~ad mounted double fused vacuum switchgear incorporating the prese~t invention;

- Fig. 2 is a longitudinal cross-sectional view of a high continuous current capacity oil expulsion fuse con-structed in accordance with the principles of the present invention;
Fig. 3 is an elevational Vi2W, with portions shown in cross-section, of the expulsion fuse link which forms a part of the fuse illustrated in Fig. 2;
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' - 10886~)0 Fig. 4 is an enlarged bottom end ~iew of the link shown in Fi~. 3;
'' Fig. 5 is an enlarged cross~sectional view taken along line 5-5 of Fi~. 3; and Fig. 6 is a top end vie~ o$ the link shown inlFig. 3.
., ~'~ Detailed Description There is shown in Fig. 2 an oil expulsion fuse 10 comprising a rigid, nonconductive, tubular fuse cartridge 12;
a pair of conductive end caps 14 enclosing opposite ends of ' 10 cartridge 12; and an expulsion fuse link 16 disposed within the cartridge 12 intermediate the caps 14.
The fuse link 16 includes a cylindrical rod-like insert 18 constructed of a synthetic resin material such as Teflon, (a Trademark) and dimensioned to be complementally received by the cartridge 12. Three cylindrical chambers are formed in the insert 18, extending parallel to and symmetrically arranged around the longitudinal axis thereof. The chambers 20 are of relatively small diameter in comparison with the ' diameter of the insert 18; in the preferred embodiment, chambers '' 20 20 are .133 inches in diameter whereas the insert 18 is .432 '~ inches in diameter. The insert 18 has secured on opposite ends respectively a flanged metal contact 22 and an opposed slotted contact 24.
As shown in Fig. 5, for example, the chambers 20 are positioned within the insert 18 in a manner to maximize the . minumum wall thickness of the chambers 20. This construction, combined with the inherent strength of the base material for insert 18, renders the chambers 2Q su~stantially nonburstible under the influence of deionizin~ gas build-up in t~e 3Q respective cham~ers 20 generated upon operation of the , , .: -.

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fuse 10. Hence, the discrete nature o~ the chambers 20 is at all times maintained, significantly contri~uting to the ability of the fuse lQ to reIiably clear fault currents in high voltage distribution circuits as descri~ed hereinbelow.
Respective fuse wires 26 extend through each chamber 20 and are secured at opposite ends to the contacts 22, 24 by soldering or other suitable means. In the preferred embodiment, the fuse wires 26 are .075 inches in diameter and are comprised of eutectic solid wire solder having a composition of 49.8 percent tin, 32 percent lead, and 18.2 percent cadmium.
The fuse wires 26, in combination with their respective chambers 20, define separate current-interrupting links which ; operate in response to a current of predetermined magnitude.
The end caps 14 are securely coupled with the cartridge 12. Each cap 14 has a central bore 28 extending therethrough including shoulder 30 adapted to engage a respective contact - 22, 24 of the fuse link 16. In this connection, when the link 16 is positioned within the cartridge 12, the flange on ; contact 22 seats against the shoulder 30 on one of the caps 14 and the slotted contact 24 is deformed in such a manner as to seat against the shoulder 30 of the opposite contact 14.
Thus, the fuse link 16 is firmly secured within the cartridge ; 12 and positive electrical contact is established between the contacts 22, 24 and respective end caps 14.
Each bore 28 on the end caps 14 is adapted to receive a removable, threaded contact plug 32 whereby the fuse link 16 is positively locked within the cartridge 12. A plurality of vent ports 34 are provided in the plugs 32 for the purpose of perml~tting oil flow into t~e chambers 20 of :,.
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insert 18, as well as to allow venting of gasses upon melting of the fuse wires 26.
Referring now to Fig. 1, there is shown a preferred ~! environment of use for the oil expulsion fuse 10 of the present invention. A pad mounted, double fused switchgear 36 has a number of removable fuse assemblies 38 each including an oil expulsion fuse 10 in series combination with a con-ventional current-limiting fuse 40. The fuse assembly 38 is coupled in series between a switch shown schematically in the drawing and broadly designated 42, and a tap line 44, in a manner well known in this art. Though not shown, the switch-gear 36 is provided with a switch 42 and fuse assembly 38 for each of the six tap lines 44 illustrated in Fig. 1.
Typically, the housing of the switchgear 36 contains a large reservoir of dielectric liquid, such as mineral transformer - oil, the assemblies 38 being submersed in the liquid.
Under normal operating conditions, electrical loads are carried and switched through the switchgear 36, and the distribution current is conducted through the fuse assembly 38. Should a fault current be experienced on the load side of the switchgear 36, the fuse assembly 38 functions to protect both the source and load sides from the overcurrent.
In this regard, the current-limiting fuse 40 actuates in response to high level fault currents to quickly clear the . . .
~ fault before damage occurs, while the expulsion fuse 10 -; clears low range faults by melting of the fuse links 26.
` When a low level fault current is experienced in the distribution circuit, the temperature of the fuse wires 26 rises as a result of the increased current until the melting point of the wires 26 is reached whereupon the latter are caused to melt and sever. As explained herein-above, the severing of the wires 26 will occur sequentially due to inherent minuscule differences in size and material. ;~

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Of course, there will be created an arcs between the respective segments of the severed wires 26, which arcs must be quickly extinguished in order to effectively clear the fault current. In this regard, heat generated by the arcs vaporizes material from the walls of the chambers 20 defined by the insert 18 to produce deionizing gas therewithin.
Pressure in the chambers 20 rises rapidly with the formation of the deionizing gas such that the latter seeks to vent through the open ends of the chambers 20. The pressurized deionizing gas flows past the arc as it vents through ports 34 thereby cooling and effectively extinguishing the arc in a desired manner. It is to be noted that the relatively small diameter of the chambers 20 assures that a sufficient gas flow is established to extinguish virtually any arc formed therein against the higher transient recovery voltage expected at the higher distribution voltages.
The disabled fuse 10 may subsequently be returned to service after replacing the spent fuse link 16. In this regard, assembly 38 is simply removed from the housing of switchgear 36, and the contact plugs 34 are subsequently unscrewed to provide access into the interior of the fuse cartridge 12. Slotted contact 24 is then restraightened such that the spent fuse link 16 may be removed from the cartridge 12 and a new fuse link 16 substituted therefor.
During normal service, the fuse 10 is capable of conducting high continuous currents without melting of the fuse wires 26 such that the fuse assembly 38 is suited for , use in heavy service, high voltage distribution circuits.
: Further in this regard, it is noted that the oil within the - 30 housing of switchgear 36 is permitted to flow into the chambers 20 through the vent ports 34. Hence, each of the fuse wires 26 transfers heat by convection transversely or radially to the oil thereby increasing the ampacity of the . 8 .
wires 26. Thi~ heat transfer accounts for the fact that the three fuse wires 26 have a greater combined ampacity than a single fuse wire presenting the same cross-sectional area.
In other words, the multiple fuse wire design of the present invention results in an increase in ampacity per unit cross-sectional area of the fuse wire over conventional single wire designs.
The increase in ampacity due to the provision of multiple fuse wires, while only negligable in air expulsion fuses, is dramatic in oil expulsion fuses. In actual tests using a .075 inch diameter single fuse wire and two .050 inch diameter multiple fuse wires, ampacity was shown to increase by only 5 percent in an air expulsion fuse whereas a 30 percent increase in ampacity was realized in an oil expulsion fuse. This startling difference, heretofore unrecognized, may possibly be explained by the different means by which heat is transferred from air fuses as compared with heat transfer from oil fuses. More specifically, heat loss in an air fuse is primarily by axial conduction to the relatively large . ,:
; 20 terminals on opposite ends of the fuse, whereas heat loss in an oil fuse is primarily by transverse or radial convection to the surrounding oil medium. It has been found that the : ampacity per unit of fuse element cross-sectional area increases as the fuse element diameter decreases. Thus, the plurality of small diameter fuse elements results in signifi-cantly greater ampacity than a single fuse element of similar total cross-sectional area.
It is to be understood that the fuse 10 may be tailored to meet particular service demands by eliminating ` 30 one or more of the fuse wires 26. In this regard, the -ampacity of the tailored fuse 10 is directly proportional to the number of fuse wires 26 utilized (even though the insert 18 may still contain three chambers 20).

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From the foregoing, it is clear that the present invention offers a unique expulsion fuse suitable for service ~ in high voltage distribution circuits. The multi-chamber : design results in increased ampacity as well as provides the fuse with the ability to effectively clear fault currents even against the high transient recovery rates experienced in interrupting high voltage distribution circuits.
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Claims (11)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An oil expulsion fuse adapted for interrupting low range fault and overload currents in a high voltage distribution circuit, said fuse comprising: a pair of spaced, electrically conductive terminals adapted to be interposed in said circuit; a nonconductive elongate member spanning the distance between said terminals, said member having a plurality of discrete chambers extending the full length of the member in parallel relation to the longitudinal axis thereof and being symmetrically arranged around said long-itudinal axis; means for permitting oil flow into the chambers;
and a number of fusable elements each disposed in a respective one of said chambers, and having its opposite ends coupled with respective said terminals, the walls of said chambers being sufficiently rigid to preclude rupture from high internal pressures generated by arc formation in the chambers upon fusing of the respective elements, said elements and said chambers, collectively defining a number of segregated current-interrupting links between said terminals whereby the fuse is provided with increased ampacity as well as the capability of clearing low range fault currents in said distribution circuit notwithstanding the greater transient recovery voltage rates associated with current interruption at high distribution voltages.

2. The oil expulsion fuse of Claim 1, said chambers being cylindrical and equal in diameter.

3. The oil expulsion fuse of Claim 2, said chamber diameter being less than .150 in.

4. The oil expulsion fuse of Claim 3, said elements being substantially equal in diameter and comprised of the same material.

5. The oil expulsion fuse of Claim 1, there being three of said chambers and a corresponding number of said elements.

6. The oil expulsion fuse of Claim 1, and a rigid tubular fuse cartridge configured to complementally receive said member whereby to provide additional support for said chambers against rupture from high internal pressures developed after fusing of said elements.

7. In combination with high voltage electrical distribution apparatus of the type having a reservoir of dielectric liquid, an expulsion fuse electrically coupled with said apparatus and submerged in said reservoir, said fuse including: a pair of spaced, electrically conductive terminals, a nonconductive elongate member spanning the distance between said terminals, said member having a plurality of discrete chambers extending the full length of the member in parallel relation to the longitudinal axis thereof; means permitting said dielectric liquid to flow into said chambers when the fuse is submerged; and a number of fusible elements each disposed in a respective one of said chambers, and having its opposite ends coupled with respective said terminals, the walls of said chambers being sufficiently rigid to preclude rupture from high internal pressures generated by arc formation in the chambers upon fusing of the respective elements, said elements and said chambers, collectively defining a number of segregated current-interrupting links between said terminals whereby the fuse is provided with increased ampacity as well as the capability of clearing low range fault currents in said distribution circuit notwithstand-ing the greater transient recovery voltage rates associated with current interruption at high distribution voltages.

8. The combination of Claim 7, said chambers being cylindrical and having equal diameters.

9. The combination of Claim 7, said chambers being symmetrically arranged around the longitudinal axis of said member.

10. The combination of Claim 7, there being three of said chambers and a corresponding number of said elements.

11. The combination of Claim 7, and a rigid tubular fuse cartridge configured to complementally receive said member whereby to provide additional support for said chambers against rupture from high internal pressures developed after fusing of said elements.