US2276638A

US2276638A - Shielded protector tube

Info

Publication number: US2276638A
Application number: US270814A
Authority: US
Inventors: Ackermann Otto
Original assignee: Westinghouse Electric and Manufacturing Co
Current assignee: CBS Corp
Priority date: 1939-04-29
Filing date: 1939-04-29
Publication date: 1942-03-17
Anticipated expiration: 1959-03-17

Description

March 17, 1942. o. ACKERMANN 2,276,638

SHIELDED 'PROTECTOR TUBE Filed April 29, i939 l yTNEssEs; V INVENTOR ATTORNEY Patentecl Mar. 17, 1942 UNTED S'i'iwifi @FICE Si-HELDED PROTECTOR TUBE Sylvania Application April 29, 1939, Serial No. 270,814

(Cl. F75- 30) 3 Ciaims.

My invention relates to shielded expulsion discharge-tubes such as are utilized for protecting electrical apparatus against excessive voltages, and more particularly my invention relates to such a tube having arc-responsive gas evolving material therein for assisting in extinguishing the discharge-arc.

An object of my invention is to provide such a protector-tube having two electrodes which extend into the opposite ends of an insulating tube in contact with the bore thereof, said tube being provided with an external shield. which is insulated from both electrodes, whereby an optimum operating performance wil1 be obtained, as will be hereinafter described.

A further object of my invention is to utilize a shield consisting oi a coating of aluminum paint which is sprayed onto the outer surface of the insulating tube; or the shield may be some other equivalent shielding-means consisting of a plurality of layers oi a plurality of overlapping at conducting akes or bodies which overlap suiciently to produce a practically continuous capacitor-effect, while being insulated from each other suiiiciently to malte the shield incapable of conducting current from end to end thereof under the voltage str-ess imposed by the normal linevoltage, and probably also under the dischargeconditions of the device.

With the foregoing and other objects in View, my invention consists in the structures, elements, combinations and relative dispositions of parts as hereinafter described and claimed, and illustrated in the accompanying drawing, wherein- Figure l is a sectional view of a simple shielded tube utilizing my invention,

Fig. 2 is an enlarged diagrammatic view illus-` trative of the functional operation of the shield, and

Fig. 3 is a sectional view of a more elaborate tube embodying my invention.

In Figure 1, my invention is applied to a prom tector-tube comprising an insulating tube l of iibre or other arc-responsive gas-evolving material which has the property of evolving a large quantity of substantially non-ionized gas, or equivalent small-particulate matter, in the presu ence of an arc. The tube shown in Fig. 1 is symmetrical, both ends being alike, and both ends being furnished with metal electrodes each consisting of an outer ferrule 2 and an inner tubular portion 3 which projects into the insulating tube l. In accordance with my invention, the tube of Fig. l is provided with an outer shield 4 in the form of a coating of aluminum paint which is sprayed onto the central portion of the cuter surface of the tube, so as to form a closely adhering cylindrical sleeve, the ends of which are spaced from both of the outer ierrules 2 of the terminal electrodes.

The essential or most important feature of the aluminum-paint shield is illustrated in Fig. 2, which diagraminatically represents an enlargement or a portion of the shield, showing it to consist of a plurality of layers of overlapping and unconnected metal or conducting flakes 5, the overlapping ends of which are close enough together to have a very strong capacitor-effect be tween the overlapping iialres, so that the shield operates, electrostatically, very much as if it were a continuous conducting member, whereas, from a conducting standpoint, the separate overlapping ales or conducting elements are sufliciently well insulated from each other, in general, so that the shield is incapable of actually conducting current frorn end to end thereof. In other words, when a low voltage is applied to the shield, from end to end thereof, or even when a fairly high surge-voltage is applied thereto, no current is conducted from end to end of the tube, the only current obtained being of practically the same order as the current which is obtained when voltage is applied to an insulator. rihis insulating property of the shield is extremely useful in increasing the ability of the tube to avoid external current-leakage or flashover.

My shielded tube, in common with all other successful internal-discharge tubes, must have an external i'iashover-voltage which is higher than the internal flashover-voltage, so that a flash-- over or arc always occurs on the inside, where it may readily be extinguished. In many designs, notably tubes which are not utilized in conjunction with an external series gap device, it is quite desirable for the external creepage-distance, along the surface of the insulation, or from the outer ferrule 2 of one electrode to the outer ferrule Z of the other electrode, in Fig. l, to be greater than the internal discharge-spacing between the inwardly projecting portions 3 of the respective electrodes. In order to afford an adequate factor of safety, I prefer, in such designs, to cause the shield 4 to be separated from both of the electrode-ferrules 2 by creepage-distances, the sum of which is greater than the spacing separating the electrodes 3 within the bore of the tube.

An important feature of my invention is that the terminal electrodes 3, which extend into the respective ends of the insulating tube I, terminate at a point or points in contact with the bore Ii of the tube, as indicated at I in Fig. 1. This feature has a double advantage in that it brings the tip of the electrode, at 'I, as close as possible to the adjacent end of the shield 4, since both the shield and the electrode are in contact with the respective walls of the insulating tube I. In addition to this advantage, the fact that the inwardly projecting electrode 3 terminates in contact with he inner surface of the insulating tube I (and it is suflicient if this contact is a very light contact, not necessarily anything approach* ing a perfect contact), contributes materially to the electrical performance oi the tube, by reducing the internal breakdown-voltage thereof, because .so-called surface discharges start more easily, as at the contacting surfaces at I, than discharges which must pass through a free air space.

In general, the length of my shield il, as measured along the length of the tube, is less than the internal arcing-distance or separation between the inwardly projecting terminal-electrodes 3. This feature, again, has several advantages, because, after the shield has been lengthened so as to extend as far, on the outside of the tube, as the internal arcing-distance on the inside, any further extension of the shield, so as to overlap a portion of the inwardly projecting electrode at either end, will have little or no advantage from the standpoint of reducing the internal flashovervoltage,. while it will have a very distinct disadvantage in increasing the danger' of obtaining an external flashover rather than an internal flashover, that is, an external flashover at a breakdown-voltage which is lower than the internal breakdown-voltage. And aside from the external flashover-voltage, it is extremely desirable to keep down the external surface-creepage or lealiage distance, as previously explained.

The operative effect of my shield is to cause practically all of the applied voltage, prior to a breakdown or internal flashover of the tube, to be concentrated in the short spaces, measured through the walls of the tube, between the respective ends 'l of the electrodes and the adjacent ends of the shield li, so that the voltage-gradient, or stress per centimeter, which appears on the arcing-tips i of the electrodes, is much greater than it would have been if the shield il had been omitted from the tube.

3 shows a more elaborate form of embodiment of my invention, in which the fibre tube I is made in the form of a plurality of layers 8, Si and 2U of telescoping fibre or other insulating tubes of various lengths. The upper electrode in Fig. 3 is in the form of a hollow metal shell I2, which is inserted in the upper end of the outer insulating tube S, and which terminates in an arcing-tip i3 which is flush with the inner surface or bore of the inner insulating tube or spacer Iii. The upper electrode I2 is connected, by means of a threaded steel stud I4, to an upper terminal-member i5 which is connected to a line-conductor it. The bottom electrode in Fig. 3 is a massive metal casting I'I, which has a tubular portion yIii projecting up into the insulating tube and similarly terminating flush with the inner bore thereof. The bottom portion of the lower electrode I is also provided with a lateral vent or opening I8 which communicates with the bore of the tubular electrode-portion I8, so as to provide a means for venting the tube. The bottom electrode I1 is also provided with a terminal-member 2l which engages a ground-conductor 22. The outer insulating layer 8 of the insulating tube I is provided with a shield 4 similar to that which has been described in connection with Fig. 1.

The particular tube which is illustrated in Fig. 3 has a more elaborate internal structure and a more elaborate external structure than the simple tube which is shown in Fig. 1. The internal structure consists of a plurality of slotted diffuser elements or

plugs

24, 25 and 2S, which are spaced by the inner spacers or layers I0 of the composite insulating-tube structure, these so-called

diffuser plugs

24, 25 and 26 being built up of pieces of fibre which are slotted, as indicated by dotted lines 28, so as to provide a plurality of arc-discharge paths which are in very close or intimate contact with the walls of the gas-evolving fibre, as described and claimed more explicitly in a patent of Hodnette et al., No. 2,147,440, granted February 14, 1939, and assigned to the Westinghouse Electric & Manufacturing Company.

An external porcelain housing-member 30 is also provided, for the tube which is shown in Fig. 3, said porcelain housing-member being provided with a supporting-clamp 3| which terminates in a supporting-bracket 32, so that the shield I4 is well insulated from the supporting bracket 32, and the entire tubular member I is well pi'otected against the weather.

The operation of the tube shown in Fig. 3 is very similar to that which has been described in connection with Fig. 1.

In both of the embodiments of my invention, shown in Figs. l and 3, respectively, the entire outer surface of the insulating tube I, after the shielding coating 4 has been applied, is usually covered with a protective coating of an insulating paint, which covers both the shield and the tube, and which further serves to insulatingly protect the shield, while at the same time improving the physical appearance of the device. This outer paint-coating is too thin to be shown on the drawing, other than as the outer surfaceline which is indicated at 34. In fact, the aluminum-paint coating 4, which constitutes the shield, is also too thin to be properly shown on the drawing, its thickness being grossly exaggerated for the sake of illustration.

While I have illustrated my shield as consisting of a sprayed-on coat of aluminum-paint, I intend that this showing, and in particular the enlarged illustration in Fig. 2, shall be regarded as being symbolic or representative of any substantially functionally equivalent shieldingmember which may consist either of small metallic flakes, as in metal paints, or of overlapping larger shielding-elements of much larger overlapping and insulated conducting flakes, sheets or pieces, or other means for causing the shield to have a capacitor-effect substantially as if it were a solid conducting sheet, whereas it is actually incapable of conducting current from end to end thereof.

I desire my illustrations of my invention to be construed as being merely illustrations and as not indicating that my invention is specifically limited to any precise structural combination other than is necessarily inferred from the language of the appended claims, when read in the light of the prior art.

I claim as my invention:

1. A shielded expulsion discharge tube comprising a tubular insulating housing-structure, two terminal electrodes at the opposite ends of the tube, respectively, at least one of said electrodes projecting into its end of said tubular insulating housing-structure and terminating at a point or points in contact with the bore of said tubular insulating housing-structure, at least one of said electrodes being vented to the outside of the tube, the internal discharge-space or spaces between the two electrodes within the bore of the tubular insulating housing-structure including arc-responsive gas-evolving insulating material, a shield in contact with the outside surface of said tubular insulating housing-structure, said shield comprising a plurality of layers of a plurality of overlapping flat conducting bodies which are suciently well insulated from each other to make the shield incapable of conducting current from end to end thereof, said shield being disposed in non-overlapping relation with respect to both of the electrodes and extending from a point near the arcing-tip of one electrode to a point near the arcing-tip of the other electrode, the extent of the projection of the respective electrode or electrodes into the tubular insulating housing-structure being sufliciently great so that the shield is insulatedly separated from both oi said electrodes, externally of the tube, by creepage-distances the sum of which is greater than the eective spacing separating the arcing-tips of the electrodes within the bore of the tubular insulating housing-structure, and means for so supporting the tube that said shield is electrically insulated from ground.

2. A shielded expulsion discharge tube cornprising a tubular insulating housing-structure, two terminal electrodes at the opposite ends of the tube, respectively, at least one of said electrodes projecting into its end of said tubular insulating housing-structure and terminating at a point or points in contact with the bore of said tubular insulating housing-structure, at least one of said electrodes being vented to the outside of the tube, the internal discharge-space or spaces between the two electrodes within the bore of the tubular insulating housing-structure including arc-responsive gas-evolving insulating material, a shield in contact with the outside surface of said tubular insulating housing-structure, said shield comprising a composite sheet of a plurality of materials of such nature and composition that the shield has a capacitor-effect substantially as if it were a solid conducting sheet, whereas it is actually incapable of conducting current from end to end thereof, said shield being disposed in non-overlapping relation with respect to both of the electrodes and extending from a point near the arcing-tip of one electrode to a point near the arcing-tip of the other electrode, the extent of the projection of the respective electrode or electrodes into the tubular insulating housing-structure being suiiciently great so that the shield is insulatedly separated from both of said electrodes, externally of the tube, by creepage-distances the sum of which is greater than the effective spacing separating the arcing-tips of the electrodes within the bore of the tubular insulating housing-structure, and means for so supporting the tube that said shield is electrically insulated from ground.

3. A shielded expulsion discharge tube comprising a tubular insulating housing-structure, two terminal electrodes at the opposite ends of the tube, respectively, at least one of said electrodes projecting into its end of said tubular insulating housing-structure and terminating at a point or points in contact with the bore of said tubular insulating housing-structure, at least one of said electrodes being Vented to the outside of the tube, the internal discharge-space or spaces between the two electrodes within the bore of the tubular insulating housing-structure including arc-responsive gas-evolving insulating material, a shield in contact with the outside surface of said tubular insulating housing-structure, said shield comprising a coating of paint comprising a large number of metal flakes arranged in at least partially overlapping layers, with the akes suiiiciently well insulated from each other to make the shield incapable of conducting current from end to end thereof, said shield being disposed in non-overlapping relation with respect to both of the electrodes and extending from a point near the arcing-tip of one electrode to a point near the arcing-tip of the other electrode, the extent of the projection of the respective electrode or electrodes into the tubular insulating housing-structure being suiiciently great so that the shield is insulatedly separated from both of said electrodes, externally of the tube, by creepage-distances the sum of which is greater than the effective spacing separating the arcing-tips of the electrodes within the bore of the tubular insulating housing-structure, and means for so supporting the tube that said shield is electrically insulated from ground.

OTTO ACKERMANN.