US3626237A

US3626237A - Line electrical surge arrestor

Info

Publication number: US3626237A
Application number: US783238A
Authority: US
Inventors: Norman A Bolton; John H Auer Jr
Original assignee: General Signal Corp
Current assignee: SPX Corp
Priority date: 1968-12-12
Filing date: 1968-12-12
Publication date: 1971-12-07
Anticipated expiration: 1988-12-07

Abstract

An electrical surge arrestor in which a line electrode is serially connected in the line and a second electrode of predetermined diameter spaced from the line electrode forms a discharge gap.

Description

United States Patent Inventors App]. No. Filed Patented Assignee LINE ELECTRICAL SURGE ARRESTOR 8 Claims, 5 Drawing Figs.

US. Cl. 313/325, 317/61, 317/69, 315/35, 313/217, 313/307, 313/356, 313/357 Primary Examiner-John W. Huckert Assistant Examiner-Wi11iam D. Larkins Attorney-Harold S. Wynn ht. Cl H01! H14, H01t 3/00 Field 01 Search 313/325, AB TRACT: An electrical surge arrestor in which a line e1ec- 214, 217, 35, 36, 306, 307, 231.1, 315, 356, 357; trode is serially connected in the line and a second electrode 317/62, 70, 61 of predetermined diameter spaced from the line electrode forms a discharge gap.

PATENTEB nu: Han 3,626,237

SHEET 1 BF 2 FIG. 1 l5 l6 1 I4 I 7 W dll l lllll ll FIG. 2

INVENTORS N. A. BQLTON J. H. AUER, JR.

HIS ATTORNEY LINE ELECTRICAL SURGE ARRESTOR BACKGROUND INVENTION The present invention relates to an electrical line surge arrestor and more particularly to an arrestor having an electrode serially connected in the line.

The ever increasing utilization of electrical apparatus and equipment requires the development and implementation of protective devices of greater reliability and dependability. Of primary importanceis the protection of electrical equipment from surges which appear on the power lines. Surgesessentially comprise transient signal variations having extremely high current values and capable of transmitting relatively high amounts of power. These surges in their most deleterious aspect may result in damaging and often times in destroying the electrical equipment. In addition such transients may cause distortion in data as well as improper operation.

To adequately protect a device from electrical surges, it is necessary that the arrestor quickly and reliably divert the current and flow-through power of the surge from the electrical line. A number of arrestors of varying designs have been and are being used to achieve this desired end. In the most widely utilized surge protector an electrode connected to the line forms with a second electrode, a discharge gap, the gap is usually filled with an atmosphere of gas which in combination.

with the gap spacing determines the voltage at which an arc breakdown across the gap occurs. The second electrode is returned to an electrical plane of potential common to the surge source, normally ground. These arrestors display a rapidity of operation and predictability of performance; however, after repeated subjugation to surges, the gap electrodes erode due to sputtering, balling, etc., causing a resultant widening of the gap and concomitant increase in gap breakdown voltage. Hence the breakdown voltage eventually reaches a higher value than the electrical equipment is designed to withstand.

The foregoing limitation inherent in the prior art devices requires implementation of rigid maintenance procedures in order to obviate undesirable consequences. Such procedures necessarily adversely reflect upon the use of these arrestors and further, most importantly, fail to prevent the possible occurrence of surge damage.

It is therefore an object of this invention to provide an improved electrical line surge protector.

It is another object of this invention to provide a more reliable and economical electrical line surge arrestor.

It is another object of this invention to provide an electrical line surge protector which will prevent the conduction of excessively high surges.

It is another object of this invention to provide a surge arrestor having an increased life.

Yet another object of this invention is to provide a fail-safe electric line surge arrestor.

SUMMARY OF THE INVENTION In accordance with this invention there is provided a failsafe electric line surge arrestor. A line electrode of predetermined dimension is serially connected in the line, and a second electrode is spaced a predetermined distance from the first electrode forming a discharge gap. The dimensions of the line electrode are selected so as to cause rupture of the line electrode prior to the gap spacing exceeding a certain maximum distance.

A better understanding of the present invention together with other and further objects thereof will be achieved upon a reading of the following description taken in connection with the accompanying drawings.

A BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic embodiment of the invention as utilized in typical line application.

FIG. 2 illustrates an embodiment of the invention in an air gap surge arrestor.

FIG. 3 illustrates an hermetically sealed arrestor in a holder.

FIG. 4A illustrates the application of the invention to a multiple gap embodiment.

FIG. 4B is a section view of the embodiment of FIG. 4A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The arrestor of this invention is primarily intended for the protection of electrical equipment from lightning surges which are conducted on the power supply lines. It is obviously equally adaptable to surges of any nature which exceed certain safe limits of voltage.

In FIG. I is shown a diagrammatic configuration of a typical application utilizing an arrestor embodying the present invention. The power supply 12 comprises any source, whether AC or DC for supplying energy to an electrical load 13, and the

lines

14 and 15 represent the power conductors. The arrestor comprises a line electrode 10 serially connected between the power source conductor 14 and the load'conductor 17; a second electrode 11 spaced a predetermined distance d" from the first electrode 10 forms a discharge gap. The second electrode II is conducted to the common of the source of the surges or transients. The connections between source conductor I5 and line conductor 16 are not shown but such lines may be directly connected, or include a surge arrestor similar to that in the first line.

When an electrical transient occurs on line 14 there is immediately present across the first electrode 10 and second electrode 1 I a potential difference commensurate to the level of the surge. The spacing d" is chosen so that the dielectric atmosphere between the electrodes breaks down when the electrical stress exceeds a predetermined value. Thus if the surge potential reaches a voltage level exceeding the maximum maintainable dielectric stress, then the atmosphere breaks down and discharges the surge across the gap.

A discharge is accomplished mainly through the medium of an electric are forming between the two electrodes. When the surge voltageexceeds the breakdown threshold, the are begins with extremely small values of current. Within a short time streamers appear with current at a somewhat higher level, these streamers cross the gap and form avalanche areas. In the avalanche period the current increases rapidly causing a spreading conductive path. With the increase of current, the atmosphere becomes highly ionized and the temperature increases sharply, injecting vaporized metal particles from the electrodes into the conductive region, resultingv in sharply increased conduction and eventually a fully conducting arc. Since an arc displays a negative impedance characteristic it is capable of conducting as much current as the source of surge potential can supply, and as long as the surge voltage remains above the arc extinguishing level, conduction continues.

As the foregoing indicates, with each occurrence of an arc, portions of the electrodes are vaporized and injected into the conducting path. This sputtering and balling of the electrode metal causes an erosion of the electrodes, so widening the gap as to requirea breakdown potential greater than that for which the load 13 is designed. When this point is reached the prior art arrestors no longer provide protection and if not changed, resultant damage occurs. The dimension of line electrode 10, d of this embodiment however, is selected so the erosion of the electrode material causes rupture of the electrode prior to the gap reaching maximum dimension commensurate with an excessive breakdown threshold. With the rupture of the electrode 10 the power source is immediately cut off from the load 13 and no surges may then pass. It further immediately gives indication that the lightning arrestor must be changed. Thus the arrestor of this embodiment presents a fail-safe device by opening of the line prior to the discharge gap exceeding a certain maximum spacing.

Application of this invention to an air gap surge arrestor is now described with reference to the embodiment of FIG. 2. A line electrode 25 is electromechanically connected in the line by means of line terminal 20 and load terminal 20A. The second electrode comprising semiconductor member 23 and ground terminal 21 is concentrically coaxially aligned with the line electrode 25, forming the discharge gap therewith. An insulator member 22 having a number of radially arrayed apertures connects the line electrode 25 to the second electrode 21 and forms a chamber inclosing the discharge gap. A spacer 24 and an insulator 29 properly align the line electrode 25 and the second electrode and further serves to electrically isolate them. Locking

members

26 and 27 mounted on the terminals and 20A of the line electrode 25, provide forces holding the entire assembly together.

The line terminal 20 and load terminal 20A comprise two threaded sections for making electromechanical connections, the center portion of line electrode is frusto-conically shaped reducing to a predetermined diameter consistent with design and protection requirements. The line electrode 25 is normally formed in three sections, viz, the line terminal 20 and the load terminal 20A, joined by a wire of the predetermined diameter concentrically and axially extending through the both terminals. Other constructions and shapes may of course be utilized if incorporating a portion of predetermined dimension for providing the fail-safe feature. The second electrode, formed by

member

21 and 23, has an interior surface substantially complementary to the line electrode 25. Tl-le complementary and coextensive surface of the electrodes constitute the arc carrying portions and establish the desired gap spacing. The semiconductor member 23 and the ground member 21 of the second electrode are in conductive relationship; the interior surface of the semiconductor member 23 is a frusto-conical section, the apex portion of which establishes the minimum gap spacing and commensurately the primary point of breakdown. The interior surface of member 21 is contiguous with the base of the conical surface of member 23 and possesses an interior angle less than both the conical section of member 23 and the complementary conical section of the first electrode 25, thereby forming an expanding volume proceeding from the point of primary breakdown to the chamber formed by the insulator member 22.

The spacer 24 holds the semiconductor member 23 against second electrode member 21 and isolates it from the load terminal 20A. The generally cylindrical coaxial insulator member 29 spaces and electrically isolates an inner surface of the second electrode member 21 from the load terminal 20A. The outer surface of member 21 has a threaded portion for making common connections and is inwardly turned or swaged so as to lock the insulator member 29 between the second electrode member 21 and the load terminal 20A. The insulator member 22 is mounted on integrally formed shoulders of the second electrode member 21 and the line electrode 25. Nuts or locking

members

27 and 26 threaded on the line terminal 20 and load terminal 20A respectively apply compressive forces to the various structural members locking them in the desired alignment; a spacer 28 is located between the locking nut 27 and the insulator member 29.

In operation the line terminal 20 and load terminal 20A are serially connected in the electrical line between the power source and the load. With appearance of a surge on the line, breakdown occurs in the gap at the point of minimum spacing, i.e., the apex of semiconductor member 23 and the predetermined diameter portion of the first electrode 25. The expanding gaseous material proceeds from the portion of the chamber having a minimum volume, i.e., a portion surrounding the initial or primary discharge gap to the chamber formed by the insulator member 22. This blowout of the discharge are causes it to shift to the portion of the gap formed between the complementary frusto-conical sections of the first electrode 25 and the interior section of the second electrode member 21; these are heavier sections and allow conduction of greater current without excessive heating and thermal deterioration of the arrestor. The propagation and maintenance of the arc in this manner greatly reduces electrode erosion and significantly adds to the useful life of the arrestor. This arc carrying portion is referred to as the secondary gap. A further advantage derived from the arc blowout to the secondary gap is,

that the balling or sputtering of electrode materials does not extend across and short the gap, due to the substantially increased gap spacing. As previously described, the increased spacing is determined by the relative dimensions and angles of the conical section of the semiconductor member 23 and the predetermined diameter of the line electrode 25, and the conical section of the second electrode member 21 and the conical section of the line electrode 25 When the arrestor has been subjected to a large number of discharges, the predetermined diameter of the first electrode 25 becomes substantially eroded. With this erosion the breakdown threshold voltage of the gap rises. However, because of the shared function of the first electrode 25, i.e., both surge arc and line current carrying functions, the erosion cannot proceed past a point of danger before the first electrode 25 is eroded through and ruptures, thus disconnecting the load from the line in a fail-safe manner. The semiconductor material member 23 due to its positive thermal coefficient of re sistivity provides added protection to the primary discharge gap in that its increased resistance after ignition of the arc causes the arc to be conducted to the secondary gap area and further aids in its extinguishment. Various semiconductor materials exhibiting the desired characteristics may be used for this purpose, in the illustrative embodiment, either carborundum, germanium or silicon among others may be selected. It is also pointed out that arrestors may be designed without the incorporation of such semiconductor material and not substantially depreciate performance. A further feature of using a refractory semiconductor such as carborundum is its resistance to balling and sputtering which prevents erosion and shorting of the gap.

Tl-le discharge gap spacing and the size of the line electrode may be selected in accordance with criteria well known to those skilled in the art. Such criteria involves the current load capabilities of various size conductors and the prediction of dielectric breakdown dependent on electrode shaping and spacing as well as atmospheric characteristics. Of course it must be recognized that final determination of desired parameters is dependent upon empirical data.

The use of air gap arrestors is severely limited in surge ranges and does not normally display the predictability of threshold settings as is needed in many applications. The most widely used arrestors are of the hermetically sealed gas filled spark gap type, i.e., units in which the chamber surrounding the discharge gap is hermetically sealed, and into which a gas of fixed and known dielectric constant is injected.

FIG. 3 illustrates a surge arrestor of this type, mounted in a holder. The line electrode 60, similar to the air gap type, comprises a conically shaped line terminal 30 section reducing to a predetermined diameter section. The second electrode 33 provides a discharge path to a common potential and displays an interior surface substantially complementary with the shape of the first electrode which coextensive portions form the desired discharge gap. A frangible seal 34 serving a similar purpose as that of the insulator member 22 contained in the illustrative embodiment of FIG. 2 seals the first electrode line terminal 30 to the second electrode 33 and forms a sealed cavity inclosing the discharge gap. A second sealing member 35 completes an hermetically sealed chamber by joining the load terminal 32 portion of the line electrode 60 to the second electrode 33. The predetermined diameter of the line electrode 60 is determined by the size of a wire 31 inserted coaxially through the line terminal 30 and the load terminal 32 portions of the line electrode 60. The insertion passage through the load terminal 32 portion for the predetermined diameter wire 31 is of slightly greater diameter providing a port for evacuating and back filling the hermetically sealed chamber. A sealing material 37 caps this passage thereby preventing leakage.

When serially connected in the line the primary breakdown occurs on that portion of the predetermined diameter wire 31 having closest spacing to the second electrode 33. This point occurs at the apex of a frusto-conical section formed by the interior surface of the second electrode 33. A secondary gap or are conducting portion is formed between the frusto-conical section of the line terminal portion 30 and its complementary surface on thesecond electrode 33. Once the arc is initiated by a surge exceeding the threshold limit, it is blown from the primary gap portion to the secondary gap portion due to the difference in volume in the primary gap area as compared to that surrounding the secondary gap area. This ofi'ers the same advantages of preventing gap shorting by sputtering electrode material and permits the heavier portion of the electrodes to conduct the high current surges. As in the air gap device with continuing erosion the diameter of the wire 31 is reduced until it can no longer carry the line and surge currents and a failsafe rupture results.

Since an hermetically sealed chamber is provided, any gap displaying the desired range of breakdown voltage may be used, thereby providing a wide range of application. Since the chamber is sealed, the atmosphere contained therein is not affected by any ambient conditions other than temperature, and provides a relatively stable level of breakdown. In addition to utilizing gases displaying different characteristics, the electrode surfaces may be radioactively doped or a gaseous radioactive isotope may be employed adding further control to the breakdown potential of the gap. Obviously the introduction of ions by the action or decay of the radioactive material effectively alters the dielectric stress to which a gas may be subjected. Various materials such as americium, radium, plutonium, carbon 14 and tritium may be used for this purpose. Gases providing the atmosphere for the discharge gap may be selected from a wide range of inert fluids, e.g., nitrogen, argon, neon or the radioactive isotope krypton 85. The hermetically sealed unit gives added protection and warning against extremely high surges in that the frangible seal 34 is blown out if the arc exceeds acceptable limits. The frangible seal 34 may comprise glass, porcelain, ceramic or a number of other insu lative frangible materials.

In normal practical application, the arrestor shown in FIG. 3 is used in conjunction with a holder for ease of installation and added security. The holder in which the arrestor of FIG. 3 is mounted forms a complete unit with the arrestor displaying added features and advantages over the prior art. A generally cup-shaped insulator member 41 provides a seat for the line terminal portion 30 which is electrically connected through a spring 44 to a threaded line terminal 40 extending through the base of the cup shaped insulator member 41. Shoulder 36 on the line terminal 30 provides a mounting seat for the electrical spring 44. The insulator member 41 further contains apertures passing therethrough for the venting of gases upon rupture of the frangible seal 34. Fastened to the open end of the insulator member 41 on an outside threaded section, is outside ground terminal 39 of generally cylindrical shaped and in conductive relationship with the second electrode 33. A peripheral extension of the secondary electrode 33 is clamped between the end of insulator member 41 and an internal integral shoulder of ground terminal 39. A second cup shaped insulator member 45 into which the load terminal portion 37 of the line electrode 60 is inserted, holds the load terminalportion 37 in electrical conductive relationship with a threaded load terminal 38 extending through the base of the cup. Although FIG. 3 shows a clip-type holder into which the line terminal 32 is inserted, any suitable device may be used. The second insulator member 45 seats on an internal integral shoulder and is held by a swaged end of the ground terminal 39. All members are coaxially aligned with the line electrode 60 of the arrestor.

In a typical installation the ground terminal 39 is fastened to the wall of a housing 43 trough which the holder is inserted. The ground terminal 39 is clamped by a locking device 42 threaded on the outside ground terminal 39 bearing an integral outside shoulder of the ground terminal 39 against the opposite face of housing 43.

The operation of this holder-arrestor combination is essentially the same as that described for the arrestor of FIG. 3, with added features and security. Should the line be subjected to an extremely high surge, as previously indicated the frangible seal 34 breaks but now the force provided by the helical spring 44 acts against the first electrode 60 line terminal 30 bringing it into contact with the second electrode, shorting it into contact with the second electrode, shorting the entire line to ground. Should it not be desirable that the load be shorted to ground, a series fuse may be included with the arrestor causing the line to open as soon as the first electrode 60 contacts the second electrode 33. Further the helical spring 44 effectively comprises a serially connected electrical unit displaying an inductive reactance characteristic which alters the surge by reducing the steepness of its wave front. This reduction in wave front provides both the load and arrestor with an unbalanced degree of protection against the current overshoots normally characteristic of a line transient. In the event of extremely high surges which are statistically possible, another safety feature is added to the arrestor as mounted in its holding structure. The first insulator member 41 is so designed that upon occurrence of an extremely high or explosive surge its walls or threaded portion will contract thereby disengaging this member from the outside ground terminal 39 and allowing the entire structure to blow apart. In actual implementation of this invention, to gas filled discharge gap arrestors, it has been found reasonable to expect breakdown voltages in the 200 to 400 volt range with machine tolerances in the primary gap which can beheld without'undue difficulty.

In many applications it is desirable to not only provide surge protection between each line and ground, but also between the lines themselves, i.e., to provide for discharge whenever the potential across the line exceeds a certain threshold value. FIGS. 4A and 48 display a triple gap version of such a surge arrestor. Referring to FIG. 4A there is shown two

line terminals

50 and 51 each connecting through portions of predetermined diameter wire to load tenninals 55 and 56 respectively, thus forming serially connected first electrodes for both lines. A ground terminal 43 is coaxially mounted around the

predetermined diameter portions

52 and 53 of these electrodes.

With the occurrence of a surge appearing on either first electrode, a discharge occurs between the

predetennined diameter portion

52 or 53 and the ground terminal 54 thereby effectively suppressing the surge characteristics. In addition if a surge appears across the lines, a third discharge gap between the

predetermined diameter portions

52 and 53 breaks down, shunting the electrical load. Under normal conditions, the possibility of a surge potential being created between the lines and not from line to ground is entirely possible and probably. FIG. 4B isa sectional view of the triple-gap arrestor structure and shows the spacing which may be utilized to form the desired breakdown gaps. The triple-gap arrestor may be constructed as an air gap or an hermetically sealed gas type and inclosed in a suitable housing displaying features similar to those detailed in the discussion of the arrestors of FIGS. 2 and 3.

The foregoing description in conjunction with the drawings describe improved electrical line surge fail-safe arrestors of either the air gap or hermetically sealed gas type. These ar restors comprise a line electrode of predetermined dimensions serially connected in the line and spaced a predetermined distance from a second electrode forming a discharge gap. With the occurrence of each gap breakdown are initiated by electrical surges, the line electrode erodes until a point is reached where the line electrode is no longer capable of carrying the line and electrical surge currents and ruptures. At the point of rupture the line opens and the arrestor fails in a safe manner.

While there have been described what are at present considered to be the preferred embodiments of the invention, it is pointed out that such are intended to be illustrative of the invention and it is realized various changes and modifications may be obvious to those skilled in the art without departing from the spirit and scope of the present invention.

We claim:

1. A line surge arrestor adapted to be connected in series between a load and a line circuit comprising:

a. a first electrode having a wire connecting spaced coaxial line circuit and load terminals respectively,

b. a second electrode adapted to be connected to ground and having a semiconductor portion,

c. the semiconductor portion having an opening uniformly tapered throughout its length and coaxial with the first electrode and cooperating with the wire to form a primary breakdown gap,

d. means for energizing the load comprising connection of the load fully in series with the wire of the first electrode, whereby energization of the load checks continuity of a series circuit through the wire of the first electrode of the arrestor.

2. A line surge arrestor according to claim 1 wherein the first and second electrodes have cooperating complementary coaxial conical portions forming a secondary breakdown gap axially adjoining the primary breakdown gap.

3. A line surge arrestor according to claim 2 wherein the secondary breakdown gap is greater than the primary breakdown gap.

4. A line surge arrestor according to claim 2 wherein the primary and secondary gaps are confined within relatively small and large spaces respectively whereby expanding gases in the region of the primary gap move in the direction of the secondary gap.

5. A line surge arrestor according to claim 4 wherein means is provided for venting the large space to atmosphere.

6. A line surge arrestor according to claim 1 wherein the semiconductor portion of the second electrode is a semiconductor material selected from the group consisting of silicon carbide, germanium and silicon.

7. A line surge arrestor according to claim 2 wherein the portions of the electrodes forming the primary and secondary breakdown gaps are contained in a sealed capsule.

8. A line surge arrestor according to claim 7 wherein a housing is provided for the sealed capsule and means is provided for quickly detachably securing the capsule within the housmg.

Claims

1. A line surge arrestor adapted to be connected in series between a load and a line circuit comprising: a. a first electrode having a wire connecting spaced coaxial line circuit and load terminals respectively, b. a second electrode adapted to be connected to ground and having a semiconductor portion, c. the semiconductor portion having an opening uniformly tapered throughout its length and coaxial with the first electrode and cooperating with the wire to form a primary breakdown gap, d. means for energizing the load comprising connection of the load fully in series with the wire of the first electrode, whereby energization of the load checks continuity of a series circuit through the wire of the first electrode of the arrestor.

8. A line surge arrestor according to claim 7 wherein a housing is provided for the sealed capsule and means is provided for quickly detachably securing the capsule within the housing.