CA1200575A - Current limiting fuse - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
This invention provides a vacuum fuse element serially connected to a current limiting fuse element encased in a common insulating housing whereby the vacuum fuse element is disposed to interrupt low current overloads and the current limiting element is disposed to interrupt high fault currents.

Description

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1 49,364 CURRENT LIMITING FUSE

BACKGROUND OF THE INVENTION
This invention relates generally to current limiting fuses and more particularly to current limiting fuses having plural, diverse, series-connected fuse ele-ments.
DESCRIPTION OF THE PRIOR ART
Generally, current limiting fuses use pure silver fuse elements having multiple links connected by narrow bridges or weak spots that open first under high fault conditions. The multiple bridges open under high fault currents and as each link begins to melt, the im-pedance increases forcing more current into the remaining links until all the links open their ~espective bridges creating multiple arcs in series. The fuse element is encased in ~uartz sand that cools and extinguishes the arcs. This type of design has been effective over the years for fast interruption of high fault currents.
However, for small overload currents the melting point of silver is much too high to melt the silver connecting bridges. Consequently, current limiting ~uses have been modified to provide effective overload protection along with current limiting protection. One method of increasing the overload capability of current limiting fuses that is well known in the art is to provide a small low melting 25 allow in the center of each fuse link which melts when conducting low overload current creating a local area of "", .
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2 49 364 high resistance producing localized temperatures high enough to melt the entire link.
Other designs use auxiliary fuse elements in parallel with the main fuse elements as described in U.S.
Patent No. 3,2~3,542 issued to H. W. Mikuleckey entitled "Current Limiting Fuse", U.S. Patent No. 3,983,526 issued to Koch entitled "Current Limiting Fuse With Auxiliary Element Arcing Clip Spaced By Non-Porous Dielectric Member" , and U.S. Patent No. 4,260,976 issued to H. L.
Miller entitled "Current Limiting Fuse With Auxiliary Element" to facilitate multiple arcing on low current faults.
SU~MARY OF THE INVENTION
There is provided by this invention a novel current limiting fuse employing a vacuum fuse section serially connected to a current limiting fuse section encased in a common housing whereby the vacuum fuse sec-tion provides low current overload protection and the current limiting fuse section provides high fault current protection.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a sectional view of the fuse con-struction incorporating the principles of this invention;
and Figure 2 is a typical time-current curve for a 200 ampere, 15.5 KV fuse incorporating the principles o this invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
There is shown in Figure l a current limiting fuse 10 having a generally tubular hollow electrically insul ating fuse barrel 2 sealed by end terminals 3 and 4 affixed to the insulating housing by means of holding pins 6. Mounted within the insulating fuse barrel 2 is a fuse element assembly 5 having a vacuum fuse section 8 and a current limiting fuse section 9. The current limiting use element 5 is surrounded by silica quartz sand 11 disposed to cool and extinguish arcs generated upon open-ing of the fuse.

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3 49,364 The vacuum fuse section 8 is of the type des-cribed in U.S. Patent ~o. 3,949,341 issued to S. J. Cherry on April 6, 1976. Inasmuch as a detailed descrlption of the vacuum fuse structure is disclosed in the aforemen-tioned patent, the description is limited to parts thatare essential to the invention disclosed herein. The vacuum fuse 8 is generally comprised of a highly evacuated tu~ular insulatin~ envelope 12 formed from glass or suitable ceramic material and a pair o~ metallic end caps 14 and 16 closing off the ends of the insulating envelope 12. Suitable sealing means 24 are provided between the end caps 14 and 16 and insulating envelopes 12 to render the inside of the insulating envelope vacuum tight. The pressure within the insulating envelope 12 under normal conditions is lower than 10 4 Torr. Located within the insulating envelope 12 is a pair of fixed electrodes 18 and 20. A radially disposed ~use link 22 is connected between the inner cylindrical electrode 20 and the outer cylindrical elec-trode 18. A conducting rod 32 is rigidly connected to the end cap 14 by suitable means such as welding or brazing and likewise to the end terminal 3. A lower conducting rod 34 is suitably joined to the end cap 16. Suitably joined ~o the end cap 16 by welding, brazing or other means is an end cap 30 o~ the current limiting fuse section 9. The lower conducting rod 34 passes through apertures in the end cap 16 and the end cap 30 so that the conducting rod 34 is disposed to extend into the current limiting section 9. In another embodiment a common end cap may be utili~ed at 16 and 30 for both the vacuum fuse section 8 and thP current limiting fuse section 9. Four insulating support rods of which 36 and 37 are typical are attached to the end cap 30 and traverse the entire length of ~he current limiting section and ~orm a ~use mandrel or spider around which an elongated main fuse element 31 is wound having one end soldered to ~e conduc~ion rod 34 and the Q .1.~

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4 4g,364 other end soldered to the end cap ~. The fuse element 31 has V-shaped notches for providing areas of reduced cross-sectional to facilitate multiple arcing in the event of heavy fault current. Silicon quartz sand 11 is also disposed within the current limiting section 9 to absorb heat and extinguish the arcs generated.
The current limiting fuse 10 thus has a current path from the end cap 3 to the conduction rod 32, cylinder 18, fuse link 22, cylinder 20, end cap 16, conduction rod 34, fuse link 31, and end cap 4. When low overLoad cur-rent flows through this current path fuse link 22 is melted or vaporized and an arc is formed in the arcing gap area between the electrodes 18 and 20. The arc is quickly extinguished in the evacuated volume. Also in the worst condition where heavy fault current may flow numerous arcs are generated in the V-shaped notches 35 to provide current limiting action, the heat from which is absorbed by the sand which extinguishes the arc in the well-known conven-tional manner.
In Figure 2 there is shown a time current char-acteristic curve which might pertain to a typical 200 amp, 15.5 KV current limiting section that incorporates the principles of this invention. It can be seen that this new fuse construction provides an overload protective device with a composite time current characteristic curve admirably suited for transformer protection applications.
From the foregoing, it can readily be seen that there is provided by this invention a novel fuse construc-tion comprised generally of a dual, serially connected fuse element having a vacuum fuse section to provide low current overload protection and a current limiting fuse section to provide heavy duty short circuit current pro-tection.
Although there has been illustrated and des-c~ibed specific structures, it is to be clearly understoodthat the same ~ere merely for purposes of illustration and ~hat changes and modifications may readily be made therein ~2~575 49,364 by those skilled in the art without departing from the scope of this invention.

Claims (3)

CLAIMS:

1. A fuse, comprising:
(a) an insulating fuse housing;
(b) spaced terminal means disposed at either end of said fuse housing to seal the ends thereof; and (c) a fuse element disposed within said insulat-ing fuse housing having a vacuum fuse means connected to one end of said spaced terminal means and a current limit-ing fuse means serially connected to the vacuum fuse means and connected to the spaced terminal means at the opposite end of the fuse housing.

2. A fuse as recited in claim 1 wherein said vacuum fuse means is generally comrised of:
(a) a highly evacuated elongated insulating en-velope;
(b) a pair of end plates attached in a sealing relationship to the ends of said highly evacuated insulat-ing envelope rendering it vacuum tight;
(c) a first hollow cylindrical electrode, electrically and mechanically connected to one of said end plates and extending longitudinally within said highly evacuated elongated insulated envelope, (d) a second electrode, electrically and mechanically connected to said end plate opposite the end plate to which said first electrode is connected extending longitudinally within said highly evacuated elongated insulating envelope and partially within said first hollow cylindrical electrode; and (e) a fuse link disposed generally radially with respect to said highly evacuated insulating envelope electrically connecting said first hollow cylindrical electrode as said second electrode.

3. A fuse as recited in claim 2 wherein said current limiting fuse means is generally comprised of:
(a) an end plate electrically and mechanically connected to said vacuum fuse means;
(b) insulating support rods connected to said end plate and extending longitudinally to traverse the entire length of said current limiting fuse section to form a fuse mandrel or spider;
(c) an elongated main fuse element wound around said insulating support rods having one end electrically connected to said vacuum fuse means and the other end electrically connected to the opposite spaced terminal means.