US 3679938 A
A disconnector for lightning arresters is provided with a simplified configuration wherein a cup-shaped breakable housing has within it a substantially concentric insulating support member that is recessed to accommodate an explosive containing metal case resting on an end portion of a ground terminal. The insulating support defines a fixed spark gap spacing between the metal case and an electrode making direct contact with a lightning arrester. A flexible grading resistor is provided in parallel with the spark gap and comprises a flexible insulating core on which is wound a plurality of turns of resistive wire, the resistor encircling the insulating support in the space between it and the inner housing wall.
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Description (OCR text may contain errors)
United States Patent Carothers et al.
 ELECTRICAL DISCONNECTOR  Inventors: Charles H. Carothers; Robert M. Kelly,
both of Bloomington, 1nd.
Westinghouse Electric Corporation, Pittsburgh, Pa.
22 Filed: Sept. 29, 1970 211 Appl.No.: 76,573
[ 1 July 25, 1972 Attorney-A. T. Stratton, F. P. Lyle and Gordon H. Telfer 57 ABSTRACT A disconnector for lightning arresters is provided with a simplified configuration wherein a cup-shaped breakable housing has within it a substantially concentric insulating support member that is recessed to accommodate an explosive containing metal case resting on an end portion of a ground terminal. The insulating support defines a fixed spark gap spacing between the metal case and an electrode making direct contact with a lightning arrester. A flexible grading resistor is provided in parallel with the spark gap and comprises a flexible insulating core on which is wound a plurality of turns of resistive wire, the resistor encircling the insulating support in the space between it and the inner housing wall.
6 Claims, 3 Drawing Figures PATENTEDJUL25I97Z 3.679.938
INVENTORS Charles H. Coro'rhers&
Robert M. Kelly.
BY QM @551 ATTORNEY BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to explosive circuit opening devices (sometimes called disconnectors) for excess voltage protection devices such as lightning arresters.
2. Prior Art Disconnectors are known types of electrical devices used in association with devices such as lightning arresters. When the arrester is subjected to a current surge beyond its capabilities to extinguish, or is damaged internally from some other abnormal condition; an explosive element in the disconnector is detonated to remove the ground lead from the arrester while some system backup device is clearing the circuit.
Continuing efforts are made to design disconnectors that are simpler, easier to make, hence less expensive, without detracting from their electrical functions. One example of the prior art on which the present invention improves is copending application Ser. No. 816,538, filed Apr. 16, 1969, now Pat. No. 3,588,733, by C. H. Corothers and assigned to the present assignee.
The explosive element in a disconnector is detonated by reason of current crossing a spark gap between the metal explosive-containing case and another element. It has been recognized previously that there are advantages in having another electrical path parallel to the spark gap of some substantial resistance, for example 500 ohms, to carry the normal arrester grading current and to carry external leakage currents that may be created by contaminants on the housing so as to avoid radio noise occurring by minor discharges at the spark gap and to assist in the controlled functioning of the lightning arrester and disconnector.
The above-mentioned application provided a leakage-current resistor in the form of a simple carbon resistor secured across the spark gap. Such a resistor offers the advantage of economy. However, such carbon resistors are limited in power dissipation and, hence, in the amount of leakage current which they can sustain. Other types of resistors have also been used in disconnectors yet not without the difficulties of providing an optimum combination of economy and functional capability.
SUMMARY OF THE INVENTION The present invention provides a structurally simplified disconnector that permits use of an economical resistor element that provides all of the required electrical characteristics. Briefly, a disconnector in accordance with the present invention comprises a cup-shaped breakable housing having a rim for securing to an excess-voltage protective device and a center bore through which a ground terminal member extends. A support member of insulating material is disposed within the housing and has a recess accommodating an explosive-containing metal case in a manner that the metal case is in direct electrical contact with the end portion of the ground terminal and also provides a predetermined spacing between the metal case and another electrode to define the operative spark gap of the device. Connected in parallel across the spark gap is a resistor that comprises a flexible insulating core on which are wound a plurality of turns of resistive wire, such resistor encircling the support member for a substantial portion of the circumference. Such a resistor, commercially available and known as a flexible grading resistor, provides not only the desired electrical characteristics in terms of ohmic value and power dissipation but also permits a convenient arrangement wherein the insulating support member may be substantially concentrically arranged within the cup-shaped housing with a gap therebetween in which the grading resistor is disposed.
Among other advantages provided by preferred forms of the present invention are that the insulating support member, explosive-containing metal case, resistor and other elements may be formed in a sub-assembly separate from the cupshaped housing to facilitate overall assembly operations and permit resistor continuity and gap spacing electrical tests to be made prior to completion of the unit. Furthermore, in accordance with this invention, an additional advantage is provided in that the spark gap to the metal case is to the shank portion proximate to the center of mass of the explosive contained therein resulting in faster operating time and higher reliablity in firing the explosive. Also, since the metal mass is greater in the shank than in the head of the cartridge there is considerably less erosion due to arcing in normal arrester operations.
THE DRAWING FIG. 1 is a sectional view of a disconnector device in accordance with the present invention;
FIG. 2 is a plan elevation view, partly broken away, looking upward, of a portion of the device shown in FIG. 1; and
FIG. 3 is a plan elevational view looking down on a portion of the device of FIG. 1.
DESCRIPTION Referring now to the drawing, particularly FIG. 1 thereof, a disconnector in accordance with this invention is shown that includes a cup-shaped breakable housing 10, such as of some fracturable insulating material. The housing has a rim 12 for securing to an excess voltage protective device 14 such as a lightning arrester. The lightning arrester 14 includes a conductive end member 15 directly joined to the rim [2 of the housing. At the bottom of the housing 10 is a ground terminal member 16 which has a threaded stud portion 17 extending through a center bore 11 of the housing and also has an end portion 18 that is generally flat and is disposed within a recess 13 in the bottom of the housing interior. A nut 20 on the threaded stud 17 provides secure attachment of the ground terminal and the housing in a manner that avoids contaminants inside the housing such as by a gasket seal 22 occurring at the edge of the center bore 11 between a shoulder of the housing and the end portion 18 of the ground terminal.
FIG. 3 shows a view looking into the housing 10 from its upper rim l2 and showing the ground terminal 16 in its position, but without any other elements in the housing. The ground terminal 16 has some structural configuration on top of the otherwise flat surface of end portion 18 as will be subsequently described for support of additional elements of the device.
Referring back to FIG. 1, within the housing 10 and supported on the end portion 18 of the ground terminal 16 is an insulating support member 24 that may be of some molded insulating material and has within its lower face, also shown in FIG. 2, a recess 26 accommodating an explosive isolator device, 28, that is, an explosive containing metal case or cartridge. The insulating support member 24 preferably has an outer surface 27 that is substantially cylindrical and concentric with a substantially cylindrical inner surface 37 of the housing 10 with some appreciable space therebetween. The insulating support member 24 has its recess 26 configured to fit over an embossed portion 19 of the end of the ground terminal so that the cartridge 28 rests in a depression 30 in the embossed portion and its head portion 29 is disposed within a recess 31 in the end of the ground terminal.
The cartridge 28 is thus in direct electrical contact with the ground terminal 16. The insulating support 24 also includes a portion 32 (FIG. 1)that defines a predetermined spacing for a spark gap 33 between the cartridge 28 and an electrode 34, that may be in the form of a rivet as shown, that is secured into the upper portion of the insulating support. The rivet electrode 34 makes electrical contact with the end 15 of the lightning arrester by some sort of spring contact 36 which may be a leaf spring as shown or a coil spring.
Means are provided to form two electrical paths between the face 15 of the lightning arrester and the ground terminal 16. A first electrical path in this embodiment is one that passes from the lightning arrester through the spring contact 36, the
rivet electrode 34, across the spark gap 33 to the shank of the explosive cartridge 28 and through the ground terminal l6. The explosive cartridge 28 is an elongated member as is normally used in disconnectors but in this invention it is disposed with its long dimension parallel to the base of the disconnector rather than extending in line with the ground stud. This arrangement makes the spark occur directly to the portion of the cartridge associated with the greater mass of explosive material to result in prompt, reliable firing. Other arrangements in which the spark gap is made to the head of the cartridge have been found to operate in a longer time and generally more unreliably, one reason being because of greater erosion of the metal from the edge of the head of the cartridge as compared with the shank portion, as well as the greater distance from the center of mass of the explosive.
The second electrical path in the device is provided through a resistor 38 that is electrically in parallel with the spark gap 33. The resistor 38 is connected by spring contacts 39 and 40 at its ends. One spring contact 39 is joined to one end of the resistor at the side of the upper portion of the insulating housing and is in contact with contact 36. The other spring contact 40 extends under the edge of the insulating support member 24 and makes contact to the head 29 of the explosive cartridge 28.
The resistor 38 is, in accordance with one aspect of this invention, a flexible grading resistor, that is, one that comprises a flexible insulating core 42 (FIG. 2) on which are wound a plurality of turns 44 of fine resistive wire such as of Nichrome alloy. After the resistor 38 is wound, there may be an insulating coating 46 applied over the turns for protection thereof. Such flexible grading resistors are known per se and have found application of the past in automotive voltage regulators, for example. Here advantage is taken of the fact that the flexibility of the resistor 38 permits it to be wound around the insulating support, substantially encircling it. For whatever portion of its length is necessary, a flexible conductor 48 may be provided. Thus the length of the resistor 38 does not impose undue space requirements on the disconnector device. In fact, if an equivalent resistor were to be provided by rigid material it would require substantially more size. Furthermore, the flexible grading resistor 38 is one that has greater power dissipating capability than. a carbon resistor, as in the above-mentioned copending application, and can thereby withstand a 100 milliampere leakage current which could be encountered in areas with severe atmospheric contamination. There are commercially available such resistors of suitable ohmic value, such as 500 ohms, suitable flexibility and power dissipation of about 8 watts as compared to only one-half watt with carbon resistors of similar price. The flexible resistor 38 can be easily formed to fit the contour of the molded cup, thereby occupying a minimum of space and necessitating little increase in the size of the disconnector cup. In fact, as compared with the embodiment of the described copending application, the outside diameter of the cup has been maintained. Thus the method of attaching the disconnector to the arrester is the same as before. The use of a rigid resistor of the required resistance and power dissipating capability would necessitate a much larger cup and would cost about five times the price of the flexible resistor in accordance with this invention. Attachment of the resistor 38 to the spring contacts 39 and 40 such as by a spot weld provides sufficient mechanical strength to allow construction of a sub-assembly of all the internal parts on the insulating support member, as shown in FIG. 2, separate from the cup 10 and ground terminal 16. Such a sub-assembly fabricated outside the molded cup facilitates assembly and allows resistor continuity and gap spacing electrical tests to be made prior to completion of the unit.
The cylindrical wall of the housing provides increased torque strength. For example, in embodiments that have been made torque strength has been found to be in a range of 40 to 45 ft. lbs. as compared to 30 to 35 ft. lbs. for similar sized items in accordance with thecopending application. Also the stud supporting surface is inside the cup eliminating the possibility of the stud being pulled loose from the disconnector. The screw-on nut 20 insures a weather-tight seal and adds strength to the bottom portion of the molded cup by placing the material in compression.
1. An explosive circuit-opening device comprising: a cupshaped breakable housing having a rim for securing to an excess-voltage protective device and a center bore through which a ground terminal member extends; said ground terminal member having an end portion within said housing, a unitary support member of insulating material having a recess accommodating an explosive-containing metal case, said insulating support member being disposed on said end portion of said ground terminal with said metal case being in direct electrical contact with said end portion; an electrode member within a portion of said insulating support member and spaced a predetermined distance from said metal case and defining the operative spark gap of said device therebetween.
2. The subject matter of claim 1 wherein: a resistor is connected to provide an electrical path in parallel with said spark gap, said r'esistor comprising a flexible insulating core on which are wound a plurality of turns of resistive wire, said resistor encircling said support member for a substantial portion of its circumference.
3. The subject matter of claim 2, wherein: said housing has a substantially cylindrical inner surface; said insulating support member has a substantially cylindrical outer surface that is substantially concentric with said inner surface of said housing with a space therebetween.
4. The subject matter of claim 3 wherein a first spring contact makes electrical contact between one end of said resistor and said metal case and a second spring contact makes electrical contact between said electrode and theother end of said resistor.
5. The subject matter of claim 1 wherein: said end portion of said ground terminal is a generally flat portion resting within a recess in the bottom surface of the inside of said housing.
6. The subject matter of claim 5 wherein: said explosivecontaining metal case has an elongated configuration with its long dimension substantially parallel with said end portion of said ground terminal.