US3518483A - Fused pressure relief means for overvoltage protective device - Google Patents

Description

June 30, 1970 w. H. EASON ETAL 3,

FUSED PRESSURE RELIEF MEANS FOR OVERVOLTAGE PROTECTIVE DEVICE Filed March 27, 1968 2 Sheets-Sheet 1 ii fl 29' la a [rim/5m fizfi ewe Ufa/6150119 ////'am H2550, m. K

June 30, 1970 w. H. EAsoN ETAL 3,518,483

FUSED PRESSURE RELIEF MEANS FOR OVERVOLTAGE PROTECTIVE DEVICE Filed March 2'7, 1968 2 Sheets-Sheet :3

f5 f/ifi/IZM Unitfid Patent 3,518,483 Patented June 30, 1970 3,518,483 FUSED PRESSURE RELIEF MEANS FOR OVER- VOLTAGE PRQTECTIVE DEVICE William H. Eason, Pittslield, and Eugene C. Sakshaug,

Lanesboro, Mass, assignors to General Electric Company, a corporation of New York Filed Mar. 27, 1968, Ser. No. 716,628 Int. Cl. H01t 5/00; H0211 9/06 US. Cl. 31536 11 Claims ABSTRACT OF THE DISCLOSURE This invention relates to overvoltage protective devices and, in particularly, to reliable, fast-acting pressure relief means of the burn-through type for lightning arrester housings.

Lightning arresters of the nonlinear element, or valve type, consist essentially of one or more spark gap devices mounted in a series arrangement with a plurality of nonlinear resistance valve elements within an insulating housing. The spark gaps are designed to normally isolate the resistance valve elements from the conductor line voltage of a system on which the arrester is installed. When an overvoltage surge of predetermined magnitude occurs on the line conductor, the spark gaps break down to permit the surge voltage to be discharged to ground through the nonlinear resistance valve elements, which have low resistance under such surge voltage conditions. Due to their inherent valve characteristics, the nonlinear resistance elements help reduce the power-follow current to a small value that usually can be readily interrupted by the spark gaps after the surge voltage has been discharged.

In order to protect the functional elements of such lightning arresters from the detrimental effects of exposure to moisture and other contaminants the lightning arrester housings are usually sealed. Although such sealed arrangements are necessary to assure proper and reliable functioning of a lightning arrester, they form a gas trap that may be dangerous if the arrester fails to operate properly and interrupt power-follow current as it should. When such arresters are placed in operation on a power system having a relatively low available fault current, or if failure of the arrester does not result in flashover of the arresters internal components, the heating caused by the fault current through the arrester develops gases which build up the internal pressure of the arrester at a relatively slow rate. In such cases, the pressure build up is usually vented to the ambient atmosphere by causing a pressure relief diaphragm to rupture before the internal pressure attains a level that might shatter the porcelain housing of the arrester. On those protected systems where the available fault current at the location of protective lightning arresters is relatively high, the internal arrester elements may are over during failure of the arrester and this arcing condition within the arrester housing creates a very rapid rise in the internal pressure of the arrester. In order to avoid explosive shattering of the arrester porcelain under :such conditions, it is necessary to provide fast-acting venting means to relieve the internal pressure before it can develop a pressure level that will shatter the porcelain housing.

Purely mechanical methods of relieving internal excess pressure in lightning arrester housings, such as those that rely solely upon pressure relief diaphragms for this function, are inherently relatively slow. Therefore, more rapid venting has been accomplished in prior art lightning arresters by providing sealing diaphragms that are adapted to be burned through by the arcing current which flows during an arcover of the internal lightning arrester elements. Such a burn through pressure relief arrange ment is described in US. Pat. No. 2,422,987Olsen, issued June 24, 1947, and assigned to the assignee of the present invention. The diaphragm elements utilized in such burn through protective arrangements may be pressure sensitive, or rupturable in response to a relatively low over-pressure, or they may be formed of a relatively pressure resistant material that is punctured by the burn through effect of an arc terminating on its surface. To perform effectively, such diaphragrns need only be formed of a thin layer of material that will burn through rapidly when an arc terminates on it. Although lightning arresters with burn through pressure venting diaphragms normally vent excess pressure very rapidly and, thus, prevent damage to the lightning arrester housing, it has been found that these arresters do not consistently vent at the same rate of speed under comparable conditions. Such erratic operating characteristics are attributed to the fact that when the internal components of the arrester arcover, the are may not terminate directly on the burn through diaphragm but instead may terminate on springs or other metal components employed in the internal structure of the arrester assembly.

Accordingly, an object of the invention is to provide an overvoltage protection device having a pressure relief diaphragm and means for rapidly and dependably burning through the diaphragm when an internal arcover occurs within the device.

Another object of the invention is to provide a valve type lightning arrester having maens for assuring the termination of an internal are occurring across the arrester elements on a burn through diaphragm thereby to immediately initiate puncture of the diaphragm when an arcover occurs within the arrester.

A further object of the invention is to provide a valve type lightning arrester having both pressure sensitive protective means and burn through protective means therein and including means to assure the puncture of the burn through means by terminating an are upon it immediately when an internal arcover occurs.

Other objects and advantages of the invention will be apparent from the following detailed description taken in connection with the accompanying drawings in which:

FIG. 1 is a side elevation, partly in cross section, showing a fragmentary view of a preferred embodiment of the invention, as it is utilized with respect to a valve type lightning arrester.

FIG. 2 is a side elevation, showing a cross sectional view of a fuse arrangement utilized in the preferred embodiment of our invention shown in FIG. 1.

FIG. 3 is a side elevation, partly in cross section, showing the arrangement of the spark gap assembly and nonlinear resistance structure used in the embodiment of our invention shown in FIG. 1.

Referring to FIG. 1 of the drawing, there is shown a lightning arrester 1 of the type that is suitable for protecting the line conductors and system components of a power transmission system. The arrester 1 includes a gen erally cylindrical porcelain housing 2 having a plurality of integral petticoats 3 which serve to lengthen the surface creepage distance between the upper and lower ends of the arrester 1. The housing 2 defines a cylindrical internal compartment 4 that is sealed on its upper end by a rup-turable lead diaphragm 5 disposed across the end of the housing 2 and clamped to a sealing gasket 6 by the metallic end cap 7 and an annular aluminum support plate 5. In like manner, the lower end of housing 2 is closed by a burn through diaphragm 8 that is clamped in position on sealing gasket 6 by end cap 7'. The respective end cap-s 7 and 7' are bonded to the cylindrical housing 2 with a suitable cement 9. In order to vent gas from the compartment 4 when either of the diaphragms 5 or 8 rupture, the end caps 7 and 7' are provided with gas venting passageways 10 and 10' respectively, which exhaust to the ambient atmosphere.

Within the compartment 4 of lightning arrester 1, there are disposed a plurality of spark gap asemblies 11 each stacked beside a plurality of nonlinear valve resistance structures 12 and electrically connected in series therewith. These components, 11 and 12 are depicted in outline form in FIG. 1 and to facilitate an understanding of the invention, they will be described in detail below. It will be understood that the number of spark gap assemblies 11 and non-linear resistant structures 12 utilized in any given lightning arrester will be determined by the design rating of the arrester, as it well known in the lightning arrester field. Accordingly, to describe our invention, in the arrester 1 hown in FIG. 1, only two sets of such units are depicted in the fragmentary view although such an arrester will usually contain many additional sets of these units in series. Of course, a greater or lesser number of sets of such units may be utilized in practicing the invention without departing from the scope of the invention. The respective spark gap assemblies 11 and associated nonlinear resistant structures 12 are secured in operating position within the compartment 4 by a support structure comprising rods 14 of insulating material, metallic plates 15 and 15, and the respective metallic nuts and washers 16 secured to the end of rods 14 in a suitable manner such that the coil springs 17 and 17' disposed between the respective sub-assemblies 11 and 12, are compressed to bias these subassemblies toward the ends of arrester housing 2. A pair of ceramic spacing discs 18 and 18', respectivley, are positioned between the top plate member 15 and the bottom plate member 15' and their associated diaphragms 5 and 8. Discs 18 and 18' are thick enough to prevent the nuts and washers 16 from contacting the diaphragms 5 and 8. In the preferred embodiment of the invention, the ceramic discs 18 and 18 are identical and accordingly their unique function will be described with reference to FIG. 2 wherein only the disc 18 is depicted, but it will be understood that the disc 18 and its associated fusible element function in a similar manner.

Referring now to FIG. 2, it will be seen that the disc 18 has a suitable fusible element 19, which in the preferred form of the invention is an aluminum ribbon, mounted on it between the diaphragm 5 and the upper plate member 15. The fusible element 19 is secured in position on the disc 18 by a rivet 20 of insulating material which extends through the plate 15 and the disc 18 to firmly secure these elements in a predetermined relation with respect to one another. It will be understood that other suitable means might be employed in lieu of the insulating rivet 20 for securing these respective components in their operating relationship.

Referring to FIG. 1, the ceramic disc 18 positioned between burn through diaphragm 8 and the bottom of the lowermost plate member 15' has a fusible element 19', which is identical to element 19, mounted on it to perform the same unique function afforded by the fusible element 19 on disc 18. A characteristic feature of our invention is that the fusible elements 19 and 19' comprise a necessary link to complete the electrical series circuit between the respective end cap terminals 7 and 7 when the spark gap assemblies 11 conduct current, or when an arcover occurs across any portion of the operating components. Therefore, when an arcover current flows through the compartment 4 between the end caps 7 and 7 it must either establish an are directly between the diaphragms 5 and 8, or it must flow through at least one of the fusible elements 19 and 19'. Accordingly, since such an arcover current through either of the fusible elements 19 and 10 would fuse them to thereby terminate an are on the diaphragm adjacent the fused element, this novel arrangement guarantees that an arc will terminate directly on the diaphragms 5 and 8 when the internal assembly flashes over, or virtually immediately when an excessive are current flows through a portion of the support structure, eg. through a nut and washer 16, then through one of the fusible elements 19 and 19 causing it to fuse and terminate an are on one of the diaphragms 5 or 8. Of course, the fusible elements 19 and 19 are designed to withstand normal operating surge currents through the arrester 1 such as those created by the discharge of lightning or switching surges through the arrester. However, these fusible elements must be designed to fail immediately if the spark gap assemblies 11 and nonlinear resistance structures 12 should for some reason fail to limit the power follow current through the arrester 1, so that this current continues to flow after the surge voltage is discharged to ground.

Ordinarily, when a lightning arrester fails a succession of relatively high current surges pass through the spark gap assemblies 11 and the non-linear valve elements 12 to the grounded terminal 7, before the arrester completely breaks down and arcs over across the outside of the nonlinear resistance structures 12. Accordingly, it is possible that the fusible elements 19 and 19' may be fused during this pre-arcing portion of a failure cycle. However, when one or more of the nonlinear resistance elements in the structures 12 fail, a complete flashover from the top to the bottom of compartment 4 is most likely to result before current flowing through the spark gap assemblies 11 and the series nonlinear resistance structures 12 causes the fusible elements 19 and 19 to rupture. When such a flashover occurs, the arc may terminate directly on the respective diaphragms 5 and 8 and, thus, immediately burn one or both of these diaphragms through, thereby venting the compartment 4 to the atmosphere through the respective passageways 10 and 10. However, it is equally possible that such an arc may terminate, at least momentarily, i.e., for an interval varying from a fraction of a cycle to several cycles, on some other conductive element or component in the compartment 4, such as on the outermost plates 15 or 15' at the ends of the assembly. In such instances, where an arc in fact terminates on components other than the diaphragms 5 and 8, a very high gas pressure might be rapidly built up within the compartment 4 except for the operation of our invention. With the fusible elements 19 and 19 of the invention disposed around the insulating discs 18 and 18 respectively, these elements are immediately forced to carry the entire arc current even when an arc fails to terminate directly on the diaphragms 5 and 8. Accordingly, these fusible elements 19 and 19' will immediately vaporize, thereby causing an arc to be formed, or terminated on the diaphragms 5 and 8, to immediately initiate the puncturing or burn through action of these diaphragms. Therefore, the diaphragms 5 and 8 are rapidly burned through before a dangerously explosive pressure level can be. developed by the gases generated by the arc in compartment 4 and the porcelain housing 2 is protected from being violently shattered due to the build up of such excessive pressures therein. It will be understood that while I have shown a lightning arrester having two fusible elements 19 and 19 at the opposite ends thereof, it is possible to practice our invention by using a single fusible element in contact with a burn through diaphragm, such as the element 19' and the diaphragm 8 respectively, Without providing a second fusible element in each arrester. In such a construction of our invention, it would only be necessary to provide means, such as a current conducting strap, to complete the operating circuit between the uppermost plate member and the other unfused sealing diaphragm of the arrester, such as the pressure sensitive diaphragm '5 in the embodiment depicted in FIG. 1.

To assure a complete understanding of the normal series circuit formed between the terminals 7 and 7' of the lightning arrester 1, reference is made to FIG. 3 of the drawing where there is shown a cross sectional view of one of the spark gap assemblies 11 and nonlinear resistance structure arrangements 12 of the arrester shown in FIG. 1. It will be seen that the nonlinear resistance structure 12 comprises a plurality of ninlinear resistance structure 12 comprises a plurality of nonlinear resistance blocks 12A, 12B, 12C, 12D and 12E, stacked on plate 15 beside a plurality of spark gap units 11A, 11B, 11C, 11D and 11E. The spark gap units 11A-E may be of any suitable conventional form, but in the preferred embodiment of the invention, each of these units is similar to the spark gap assembly disclosed and claimed in US. Pat. No. 3,151,273Stetson et al., issued Sept. 29, 1964 and assigned to the assignee of the instant invention. A plurality of metal straps 21 of any suitable conductive material are utilized to electrically connect the bottoms of the non-linear resistance elements 12A-E to the tops of the spark gap units 11A-E, respectively; while a second plurality of conductive metal straps 22 are connected between the respective bottom-most contacts of spark gap units 1'1AD to the tops respectively of nonlinear elements 12B through 12E. Cup shaped insulating members 23 are disposed between the nonlinear resistance elements 12A-E and the respective spark gap units 11A- E to prevent an electrical circuit from being formed therebetween. Accordingly, when a high voltage is impressed across the top and bottom plate members 15 and 15, an electric circuit is formed from plate member '15 through nonlinear resistance 12A to conductive strap 21 and thence through sparked over gaps of spark gap unit 11A to conductive strap 22, back to nonlinear resistance element 11B and so on through the remainder of the components to spark gap unit 11E, which is in contact With the lower plate member 15'. Thus, it will be seen that in normal operation a high voltage surge current flows in series through a nonlinear resistance element, then through a spark gap unit, then through another nonlinear resistance element, etc., to complete its serpentine path through these series connected devices.

Referring now to FIG. 2, it will be seen that the series circuit between the plate member 15 covering the uppermost set of spark gap assemblies 11A-E and associated nonlinear resistance elements 12A-E, is completed to the diaphragm 5 by the fusible element 19. In order to assure a relatively low resistance conductive path between the respective subassemblies, 11 and 12, separated by the springs 17 and 17'; shown in FIG. 1, conductive straps 24, which shunt the springs 17 and 17, are secured at their opposite ends by axial spring pressure to plates 15 and 15' abutting these springs.

It will be understood by those skilled in the art that the current carrying capacity of the fusible elements 19 and 19' of our invention will be determined primarily by their respective dimensi0ns,.which in turn will be determined by the rating parameters of the lightning arrester 1 in which they are assembled. In addition, it will be appreciated that various materials may be used to form these fusible elements 19 and 19'. However, by way of example, in order to further facilitate an understanding of our invention, it has been found that for lightning arresters designed to operate at a voltage rating of 144 'kilovolts, a strip of commercially pure aluminum onehalf inch wide by 15 mils thick when employed as fusible elements 19 and 19' of our invention, provides suitable current capacity for normal operation of the arrester while affording fast-acting over current protection, in

the manner described above, if the arrester fails and arcs over internally.

While we have shown and described a specific embodiment of the invention, we do not desire to be limited to the illustrated embodiment, and we intend by the appended claims to cover all modifications that fall within the true spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. In combination, a lightning arrester including a plurality of lightning arrester elements connected in series within an insulating housing that is sealed by a diaphragm, a fusible element disposed within said housing and electrically connected between said diaphragm and one of said arrester elements, said fusible element being adapted to fuse and form an arc that rapidly burns through said diaphragm when an excessive current flows through the fusible element for a predetermined length of time.

2. In combination, a lightning arrester including a plurality of lightning arrester elements connected in series within an insulating housing that is sealed by a plurality of diaphragms, a plurality of fusible elements each electrically connected respectively between one or more of said arrester elements and one of said diaphragms, each of said fusible elements being adapted to fuse and form an are that rapidly burns through the diaphragm to which the fusible element is connected when an excessive current flows through said fusible element for a predetermined length of time.

3. A lightning arrester comprising a tubular insulating housing, means forming a spark gap assembly within said housing, a nonlinear valve resistor mounted in spaced relation to said spark gap assembly within said housing and electrically connected in series with spark gaps in said assembly, a diaphragm mounted over one end of said housing to seal said one end, sealing means disposed across the other end of said housing for sealing said oher end, said diaphragm being adapted to be burned through by an are terminated on it thereby to vent said housing, insulating means for electrically insulating said spark gap assembly and said valve resistor from the diaphragm, and a fusible element electrically shunting said insulating means, said fusible element being adapted to fuse when a predetermined magnitude of current flows through it thereby to establish an arc that terminates on the diaphragm causing it to rapidly burn through and vent said housing.

4. A lightning arrester as defined in claim 3 wherein said insulating means comprises a porcelain block disposed between said diaphragm and said spark gap assembly, and said fusible element comprises a strip of metal having a predetermined current carrying capacity.

5. A lightning arrester having an insulating housing provided with a sealed compartment in which are disposed in radially spaced relation to the inner walls of said compartment a nonlinear valve resistor and a plurality of electrodes defining a spark gap assembly, means electrically connecting said resistor in series 'with the spark gap assembly, spring means mounted within said compartment to resiliently bias said valve resistor and said spark gap assembly together with said connecting means to maintain electrical contact therebetween, means defining an aperture through a wall of said compartment, a diaphragm mounted over said aperture to seal it, and a fusible element electrically connected between said diaphragm and said spark gap assembly, said fusible element being adapted to fuse when a predetermined magnitude of current flows through it thereby to establish an are that terminates on the diaphragm causing it to rapidly burn through and vent said compartment.

6. A lightning arrester having an insulating housing provided with a sealed compartment in which are disposed a spark gap assembly and a valve resistor held in operating relationship by a support structure, at least a portion of said structure being electrically conductive, means defining an opening through a wall of said compartment, a diaphragm mounted over said opening to seal it, and a fusible element electrically connected between said diaphragm and said structure, said fusible element being adapted to fuse when a predetermined magnitude of current flows through it thereby to establish an are that terminates on the diaphragm causing it to rapidly burn through and vent said compartment.

7. An overvoltage protective device comprising an insulating housing having a generally cylindrical sealed compartment therein, means defining an aperture through one end wall of said compartment, a diaphragm mounted over said aperture to seal it, and overvoltage protective means disposed in said compartment, insulating means for maintaining said protective means and said diaphragm in spaced-apart relation, and a fusible element electrically connected between said diaphragm and said protective means, said fusible element being adapted to fuse when a predetermined magnitude of current flows through it thereby to establish an arc that terminates on the diaphragm causing it to rapidly burn through and vent said compartment.

8. An overvoltage protective device as defined in claim 7 wherein said protective means comprises a nonlinear valve resistor electrically connected in series With a plurality of electrodes defining a spark gap assembly.

9. An overvoltage protective device as defined in claim 8 including spring means arranged within said compartment to resiliently bias said valve resistor and said gap structure toward the end walls of said housing thereby to maintain them in a predetermined operating relation.

10. An overvoltage protective device as defined in claim 7 wherein said insulating means comprises a block of insulating material disposed between said protective means and said diaphragm and said fusible element comprises a length of metallic eutectic material disposed at least partially around said block of insulating material.

11. In combination, a lightning arrester including a plurality of lightning arrester elements connected in a series circuit within an insulating housing that is sealed by a rupturable diaphragm, a fusible element electrically connected in said circuit, and means responsive to the fusion of said fusible element for rupturing said diaphragm, whereby rupture of the diaphragm is a function of the operating characteristics of said fusible element.

References Cited UNITED STATES PATENTS 2,422,978 6/1947 Olsen 313-174 2,571,814 10/1951 Beck 3l766 X 2,593,955 4/1952 Ackermann 3177(l X 3,290,547 12/1966 Sankey 31536 JAMES W. LAWRENCE, Primary Examiner C. R. CAMPBELL, Assistant Examiner US. Cl. X.R. 317-66,

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