US1509493A - Lightning arrester - Google Patents

Description

Sept. 23 1924. 1,509,493

J. SLEPIAN LIGHTNING ARRESTER Filed Feb. 11 1922 Fly. 1. E7. 2.

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ATTORNEY Patented Sept. 23, 1924.

UNITED STATES PATENT OFFICE.

JOSEPH SLEPIAN, OF SWISSVALE, PENNSYLVANIA, ASSIGNOB TO WESTINGHOUSE ELECTRIC & MANUFACTURING COMPANY, A CORPORATION 01' PENNSYLVANIA.

LIGHTNING ARBESTER.

Application filed February 11, 1922. Serial No. 535,810.

To all whom it may concern:

Be it known that I, Josnrn SLEPIAN, a citizen of the United States, and a resident of Swissvale, inthe county of Allegheny 5 and State of Pennsylvania, have invented a new and useful Improvement in Lightning Ari-esters, of which the following is a specification.

This invention relates to lightning arresters, more particularly to a simple form of lightning arrester of the spark ga type having properties superior to those 0 prior lightning arresters.

Up to the present time, lightning ar- 1 resters were (generally of two ty s; first, spark gap an second, chemical. park gap lightning arresters generally consisted of cylinders, plates or disks of material, such as -brass, graphite, and the like separated by airspaces and insulated from each other. In some cases the air gaps were shunted by resistors, and in some instances instead of air gaps a dielectric, such as mica, was placed between the disks. These various types of lightning arresters had many disadvantages, it being difiicult to obtain structures which operated successfully under all conditions.

There has also been proposed a lightning arrester consisting essentially of alternate blocks of film-forming metal and graphite or other material which did not fuse to the metal. Spacers of sheet mica were placed between the blocks. Generally the metal used was aluminum, thereby providing an oxidizable metal which formed a non-conducting coating on the surface thereof which was intended to be ruptured upon the passage of an abnormal voltage after which 40 oxidation of the metal healed the rupture.

The action of this arrester is chemical.

Unlike the chemical arresters, the sparkgap arresters do not merely discharge the abnormal voltage on a power line, but when they function they also constitute a shortcircuit or low resistance path for the normal or dynamic voltage of the power line. The

non-arcingproperties of the gap electrodes is depended upon for clearing this short circuit at the end of the half cycle. On

systems of large capacity, however, so much power is expended in this short circuit that the gaps are unable to clear, and in order to prevent destruction of the arrester, it is necessary to put limiting resistance in series. This, however, also limits the ability to discharge the abnormal voltage so that the protective power is impaired.

The electrolytic lightning arrester, which was the most widely used of the chemical arresters, consisted generally of plates or cups of aluminum or similar meta-l placed within each other, the spaces therebetween being filled with an electrolyte, such as a borax solution. The entire structure was generally immersed in a vessel filled with an insulatin oil. The action of the arrester was (ependent primarily upon the thin film of aluminum oxide on the plate which was broken down by the abnormal voltage of a lightning discharge.

This arrester had many desirable proper- "M ties, being admirably fitted for use as a lightning arrester, but was subject to the disadvantage of the necessity of constant maintenance thereof. The film of oxide on the aluminum was very thin and deterio-- rated during the period when the arrester was inactive, and re uired periodical reforming or charging to restore the same. Since lightning arresters are placed on the line at various points which are more or less inaccessible, the necessity for periodically charging the electrolytlc arrester became asource of considerable. expense and trouble.

It is generally agreed that the value of the electrolytic arrester lay in its voltampere characteristic. Below a certain critical voltage, V,, the arrester had a high resistance and passed little current. For voltages above the critical voltage the arrester passed current freely, the current being calculated approximately by the equation where R is a constant called the discharge resistance of the arrester, V, the voltage on characteristic operating advantages and efficiency of the electrolytic lightning arrester and obviate the disadvantage thereof of high maintenance cost.

It is a further object of the present invention to provide such a lightning arrester ,which shall be simple in construction, be

capable of manufacture in large quantities at a relatively low cost, which shall not require the services of expert workmen and which shall be efficient in operation.

I have discovered that a. volt-ampere characteristic similar to that of the electrolytic arrester may be. obtained by utilizing small air gaps between electrodes of material having a relatively high specific resistance. This causes the current density in the discharge to be kept relatively small, usually a few amperes per square centimeter, so that no local heating occurs. The voltageof the discharge is relatively large, making the discharge voltage of the same order as the break-down voltage of the. gap. Such a condition is practically possible at atmosphericpressure only with minute gaps and I obtain a device capable of passing large timeter cube.

currents by providing a sufficiently large area in the electrodes forming the gaps. In practising my invention, I prov1de layers or bodies of a suitable materlal having a relatively .highspecific reslstance, generally in the form of disks which may consists of any suitable material. A composition which I have found to be effective in carrying out my invention consists of amixture of carborundum, lampblack and kaolin which has been compressed and then baked in the form of a rod which is then cut to form disks. The specific resistance of the material is about 100 ohms per cen- The faces of the disks are made as smooth as possible and are treated with a very thincoat of plaster of Paris to prevent small particles of the resistance material from becoming loose.

The disks are assembled, being actually in contact at only a few points and being, over the major portion of the contacting surfaces, separated by minute gaps. With ordinary workmanship, the gaps will vary from fractions of a mil to several mils. No portion of a gap should exceed about 8 mils, as then that portion becomes less effective in carrying discharge.

When a low voltage is applied, current is carried only at the few contact points and of a lightning arrester made in accordance with the present invention;

Fig. 2 is an enlarged view of two of the bodies forming a section of my lightning arrester, the contact surfaces of said bodies and the gaps therebetween being exagger ated for the sake of clearness and illustrating the passage of current therethrough under low voltage;

-Fig. 3 is a similar view showing the passage of current through the bodies at a voltage above the critical or break-down voltage of the arrester;

Fig. 4 is a curve showing the relation between the break-down voltage of agap and the length thereof;

ig. 5 is a curve showing the character of discharge of a gap under different conditions of voltage and current, and

Fig. 6 is a view showing the various phenomena in a glow discharge between two electrodes. 7

Referring particularly to Fig. 1, to any point in a line 1 is connected a conductor 2 having a spark gap 3 of any suitable type in series therewith and connected to a lightning arrester 4. The lightning arrester comprises a tubular casing 5 of porcelain or other insulating material having an inwardly projecting annular flange 6 at the bottom thereof. A- metal plate 7 having a conductor 8 securedtheretmconnected to ground 9, is placed on flange 6.

Disks. 10 of material having a high specific resistance are placed on metal plate 7, a suitable number being assembled toprovide an arrester having the desired breakdown voltage. A metal plate 11 is placed on the uppermost disk 10 and a conductor 12 secured thereto is connected to conductor 2. A cover member 13 generally of insulating material having an opening therein to allow the conductor 12 to pass therethrough is secured to the top of casing 5 and a spring 14, interposed between metal plate 11 and cover 13, holds the disks 10 in fixed relation to each other. Disks 10 are ground smooth but nevertheless show minute elevations and depressions so that, when assembled, the disks touch at a relatively few points 16 forming gaps 17 of relatively large area, as shown in Fig. 2. When the voltage on the disks is low, the leakage current can pass between disks only at the points of contact 16, which are few in number and small in area. Therefore, the resistance of the arrester to the passage of current at a low voltage is very high and no appreciable current will pass under such conditions.

However, as illustrated in Fig. 3, when break-down occurs at one or more points between the disks 10 owing to an abnormal voltage being applied thereto, the breakdown extends over the entire surface of the disks, allowing current to pass therethrough practically uniformly over the entire surface of the arrester. The current density at any point is kept small because the resistance of the disks opposes any concentration of the discharge and, therefore, no local heating occurs, thus preventing the changing of the discharge from a form called the glow discharge which requires high voltage to sustain it to the form called the arc, which requires only a low voltage to be maintained.

As illustrated in Fig. 4, the break-down voltage for small gaps for most electrode materials, is a minimum when the length of gap is about .001 centimeters, being equal to about mil. Therefore, the gap necessary for break-down should be about mil and, after break-down has occurred at such points between the disks, this discharge will pass to points having a greater separation. In practice, with the materials used, I find that about 8 mils is the maximum effective separation which can be used practically, still maintaining a glow discharge.

Since the successful operation of my new arrester depends on keeping the discharge in the form of a glow, and preventing the formation of an are, it is well to consider the conditions under which glow discharge takes place. Fig. 5 shows the volt-ampere characteristic of a. gap for small currents. It will be seen for very small currenlts for which there is little heating of the cathode, the glow discharge takes place, and requires in excess of 350 volts to be maintained. Upon the electrodes heating, the discharge changes abruptly from the glow to the arc, which requires a relatively small voltage, usually under 50 volts, to maintain.

It is my purpose in this invention to avoid the formation of an are after break-down because if an arc were formed, the voltage necessary to maintain the same would be relatively small, therefore resulting in the normal line voltage being capable of main'taining the arc. By causing the discharge to be distributed over the entire surface of the disks, I avoid heating thereof and am,therefore, enabled to keep the glow discharge at all times. This distribution of the current is effected by using electrodes of relatively high specific resistance, for evidently then any tendency on the part of the discharge to concentrate is opposed by the ohmic resistance of the small section path offered to the discharge if it should concentrate.

To explain further why small gaps are desirable if concentration of discharge is to be prevented, it is necessary to consider the voltage-current density characteristics of the various parts making up a glow discharge. Fig. 6 shows the structure of a glow discharge considerably magnified. At the electrode 18, which is the cathode, there is produced a very short dark space 20, known as the cathode dark space, corresponding in length to the minimum break-down gap shown in Fig. 4. Next to this is the cathode glow 21 also being of relatively small length, about four or five mils. Then comes the Faraday'dark space 22 and the positive column 23 of considerably grealter length.

A large part of the voltage necessary to maintain the glow is concentrated at and around the cathode dark space 20, which voltage, upon increase of current density, tendstorise, that is, the voltage-current density curve is a rising one. The rest of the volt-age is consumed in the positive column 23, which increases or decreases in length as the electrodes are separated or caused to approach each other.

The voltage in the positive column varies in an inverse manner with the current density so that the voltage-current density curve is a falling one. It is evident that a form of discharge with a falling voltagecurrent density characteristic tends to concentrate in section, because in so doing the voltage for a given current is lowered. As it is my purpose to reduce all effects which tend to concentrate the discharge, I make the positive column as short as pomible by as close a spacing of the electrodes as is practicable. It will be seen, therefore, that by the use of a small gap and electrodes of considerable specific resistance, the discharge is kept in the glow form requiring several hundred volts for its maintenance. Such a discharge may be initiated at about the same voltage necessary to maintain the glou discharge.

In my new type of arrester, with ordinary workmanship, the distance between adjacent faces of disks will vary from zero at actual contacts to a milror so at the points of greatest separaltion. There will always be, therefore, portions of the opposing disk faces which will be separated by the distance corresponding to the minimum sparking potential. Hence, the break-down potential of the gaps between the disks will always be the minimum sparking potential for air, and this is also nearly equal to the voltage required to'maintain a short glow discharge.

The volt-ampere characteristic for a discharge across a gap having the minimum sparkover potential and with electrodes maintained cool is then as follows: Until the break-down potential, which is also the voltage necessary to sustain a glow discharge, is reached, no current passes. Large currents pass for voltages only slightly above the break-down potential. This is a characteristic similar to that of the aluminum electrolytic arrester.

I am aware that it has been proposed to construct a lightning arrester of two plates of high resistance material spaced apart with an air gap therebetween. In order to carry out this idea with usual air gaps, the material used would necessarily have a very high specific resistance to overcomefthe concent-rating tendency of the long positive column. The specific resistance of the ma.- terials utilized must be so high that the dis charge capacity of the arrester is seriously limited. If insufliciently high resistance material is utilized, the concentration of current and voltage at some point or points of the plates burns and punctures the material rendering it useless. Furthermore, the equality between voltage necessary to initiate the discharge and that necessary to sustain the same is lost: unless the resistance is made so high as to keep the current density at an excessively low value.

Although, superficially, my new lightning arrester bears some resemblance to the carbon pile resistor, it differs therefrom in several material respects. I utilize high re sistance material whereas the pile resistor is made of low resistance material. In my device, the pressure on the plates is kept constant whereas in the pile resist-or the pressure is varied. My arrestor operates by sparking at the gaps while in the pile resistor sparking is avoided. Further, there is no analogy between a lightning arrestor and a carbon pile resistor which operate on entirely different principles and for different purposes.

It will be apparent from the above description of my invention that I have provided a lightning arrester which is extremely simple in construction and reliable in operation. The maintenance cost of the arrester is very low, as is the cost of production.

Although I have described my invention, giving a specific example of the construction thereof, it is obvious that various changes may be made in details of my invention without departing from the scope and spirit thereof. For instance, although I prefer to use gaps between adjacent disks as small as practical, it is apparent that various sizes of gaps may be utilized, and, although I have mentioned 8 mils as the largest gap which is practical with the materials thus far used, this figure is not limiting but is merely illustrative of the gap lengths which I may use.

The resistance material stated in my specification has 'a specific resistance of about 100 ohms per centimeter cube but I may use material having a lower resistance, say about 25 ohms per centimeter cube provided that the heat conductivity thereof is sufficient to prevent local heating of the surfaces, and, therefore, prevent the formation of an are.

I may use metals in place of non-metallic materials, it being merely necessary that the metal or other material have a high heat conductivity and a sufficient specific resist-' ance to prevent the formation of an arc and so that the counter electromotive force de-' veloped under abnormal voltage is not less than the voltage necessary to maintain a glow discharge.

I claim as my invention:

1. A lightning arrester comprising a plurality of plates of high resistance material disposed in contact with each other.

2. A lightning arrester comprising a plurality of plane bodies of high resistance material disposed in contact with each other.

3. A lightning arrester comprising a plurality of plates of non-metallic conducting material disposed in contact with each other.

4. A lightning arrester comprising a plurality of plates of high resistance material disposed in contact with each other at a relatively small number of points.

5. A lightning arrester comprising a plurality of plates of high resistance material disposed in contact with each other at a relatively small number of points and being separated by small gaps at the remainder of the contact surfaces.

6. A li htning arrester comprising a plural'ity of odies of high resistance material disposed in contact with each other at a rel atively small number of points and being separated by small gaps at the remainder of the contact surfaces, the voltage necessary to break down said gaps being less than 500 volts per gap.

7. A lightning arrester comprising a plurality of bodies of high resistance material disposed in contact with each other at a relatively small number of points and being separated by small gaps at the remainder of the contact surfaces, the voltage necessary "to break down said gaps being approximately 350 volts per gap.

8. A lightning arrester comprising a plurality of bodies of high resistance material disposed in contact with each other at a relatively small number of points and being separated by small gaps at the remainder of the contact surfaces, said gaps being less than 8 mils in length.

9. A li htning arrestor comprising a plurality of odies of high resistance material disposed in contact with each other at a relatively small number of points and belng separated by small gaps at the remainder of the contact surfaces, said gaps varying from to mils in length.

'10. A lightning arrester comprising a plurality of bodies of high resistance material disposed 'in contact with each other at a relatively small number of points and being separated by small gaps at the remainder of the contact surfaces, the gas pressure in said gaps being substantially atmospheric.

11. A lightning arrester comprising a plurality of plates of high resistance material disposed in contact with each other, said bodies being maintained at a relatively low temperature during a discharge.

12. A lightning arrester comprising a plurality of plates of high resistance material disposed in contact with each other, said bodies being maintained at atmospheric temperature during a discharge.

13. A lightning arrester comprising a plurality of bodies of high resistance material disposed in contact with each other at a relatively small number of points and being separated by short gaps at the remainder of the contact surfaces, said bodies being maintained at relatively low temperature during a dischar e.

14. A ightning "arrester comprising a plurality of bodies of high resistance material dis osed in contactwith each other at a relatively small number of points and being separated by small gaps at the remainder of the contact surfaces, said bodies being maintained at atmospheric temperature during a discharge.

15. A lightning arrester comprising a plurality of bodies of high resistance material disposed in contact with each other, the size and specific resistance of said bodies being such that when a discharge passes therethrough, the current density is kept sufficientlylow to prevent the formation of an arc.

16. A lightning arrester comprising a plurality of bodies of high resistance material disposed in contact with each other, the size and specific resistance of said bodies being such that when a discharge passes therethrough, the current density is a relatively few amperes per square centimeter.

17. A lightning arrester comprising a plurality of stacked bodies of high resistance material disposed in contact with each other in a gaseous medium at a pressure on the order of atmospheric pressure.

18. A lightning arrester comprising a plurality of stacked bodies of high resistance material disposed in contact with each other, the contact surfaces-of said bodies being substantially plane.

19. lightning arrester comprising a plurality of stacked disks of high resistance material disposed in contact with each other.

20. lightning arrester comprising a plural ty o stacked disks of high resistance material disposed in contact with each other, the contact surfaces of said disks being substant ally plane.

21. lightning arrester comprising a plurality of bodies of high resistance material d sposed in contact with-each other, the specific resistance of said material being of the order of 100 ohms per centimeter cube.

22. lightning arrester comprising a plurality of bodies of high resistance material d s sed in contact with each other, the speci c resistance of said material being sufficient to prevent the formation of an are between said bodies under a discharge.

23..A hghtning arrester comprising a plurality of bodies of high resistance material disfiposed in contact with each other, the speci c resistance of said material being such that the counter electromotive force developed in a discharge is not less than the voltage necessary to initiate the discharge between said bodies.

24. lightning arrester comprising a plurality of bodies of high resistance material d1 s sed in contact with each other, the spool c resistance of said material being such that the counter electromotive force developed per gap in a discharge is not less than 250 volts.

25. lightning arrester comprising a plurality of bodies of high resistance mater1al d1s sed in contact with each other, the speci c resistance of said material being such that the counter electromotive force necessary to maintain the discharge between said bodies is of the same order as that necessaryto initiate the discharge.

26, A lightning arrester comprising a plurality o bodies of material having a discharge ga'p therebetween, the specific resistance thereof being sutliciently great to cause a discharge to be distributed over the discharge surfaces of said bodies, portions of said surfaces being separated a distance such that the breakdown voltage is less than 500 volts per gap.

27 lig tning arrester comprisin a plurali y of bodies of material, the lengti of the gap therebetween being sufficiently small to cause a discharge to be distributed over the surfaces of said bodies.

28. A lightning arrester comprising a plurality of bodies of material, the specific resistance thereof being sufficiently great to cause a discharge to take the glow form.

29. A lightning arrester comprising a lurality of bodies of resistance material, the length of the gap therebetween being sufficiently small to cause a discharge to take the glow form. t

30. A series of gaps, means for causing said gaps to break down at a voltage ap proximately equal to the voltage necessary to maintain. a discharge.

31. A series of ga the number being such that the value 0 the voltage at which discharge begins divided by the number of gaps is equal to a few hundred volts.

32. The combination with a circuit to be protected, of an excess-voltage discharge de vice comprising electrodes having a glow discharge therebetween at gaseous pressures substantially atmospheric.

33. A lightning arrester consisting of a series of discharge members, successive members of which being at some points spaced apart a distance substantially equal to that having the minimum sparking potential.

34. A lightning'arrester comprislng a plurality of plates of material,- the specific resistance thereof being over 20 ohms per centimeter cube.

35. A lightning arrester comprising a plurality of plates of material, the specific resistance thereof being over 20 but under 150 ohms per centimeter cube.

36. A lightning arrester comprising a pin:

rality of bodies of material, the spacing therebetween bein at one point at least, such that the brea down voltage thereof is substantially equal to the voltage drop in the cathode dark space of a glow discharge.

37. A lightning arrester comprising a plurality of bodies of material in air, the spacing therebetween being, at one point at least, such that the breakdown voltage thereof is substantially equal to the voltage drop in the cathode dark space of a glow discharge in air.

38. A lightning arrester comprising a plurality of bodies of material, the spacing therebetween being, at one point at least, such that the breakdown voltage thereof is substantially equal to the voltage drop in the cathode dark space of a glow dischar e in a gaseous medium at a pressure of t e order of atmospheric pressure.

39. A lightning arrester comprising a plurality of bodies of material in a gaseous medium at substantially atmospheric pressure, the maximum spacing between effective surfaces thereof being approximately equal to the combined length of the cathode dark space, cathode glow and Faraday dark space of a glow discharge.

40. A lightning arrester comprising a plurality of bodies'of material in a gaseous medium at substantially atmospheric pressure, the specific resistance thereof being sufiiciently great and the length of gap therebetween being sufficiently small to cause a glow discharge having substantially no positive column to be distributed over the sur- 42. A lightning arrester comprising a pair of bodies in a gaseous medium at substantially atmospheric pressure, the specific resistance of at least one of said bodies being of the order of 100 ohms per centimeter cube.

43. A resistor comprising a pluralit of plates of high-resistance material, sai resistor being capable of allowing a relatively heavy current to pass therethrough at a predetermined voltage and preventing any substantial amount of current from passing therethrough below said voltage.

44. A device including an electrical element and a resistor comprising plates of in shunt thereto,

high-resistance material said resistor being capable of allowing. a relatively heavy current to pass therethrough at a predetermined voltage and preventing any substantial amount of current from passing therethrough below said volt age. v

45. A space-current discharge device comprising at least one having a dischar e gap therebetween, the average spacing being between and 8 mils, whereby the space-current flow between the lplates always takes the form of a glowdisc arge at gaseous pressure of the order of atmospheric pressure.

46. A space;current discharge device comprising a pair of electrode plates having j uxtaposed surfaces, the major portions of said surfaces being spaced distances between the length of the cathode dark space of a glow discharge and the combined lengths of the cathode dark space, cathode glow and Faraday7dslk'k space of a glow discharge.

4 prising a pair of electrode plates having juxtaposed surfaces, the major portions of said surfaces being spaced distances between the length of the cathode dark space of a glow discharge and the combined lengths of the cathode dark space, cathode glow and Faraday dark space of a glow discharge, at least one portion of the juxtaposed surfaces being spaced a distance corresponding to the minimum breakdown voltage.

48. A space-current discharge device comprising a pair of electrode plates having juxtaposed surfaces, at least one portion pair of electrode plates space-current discharge device com-.

of the juxtaposed surfaces being spaced a distance corresponding to the minimum breakdown voltage, the plates being in a gaseous medium at substantially atmospheric pressure.

49. A space-current discharge device comprising a pair of electrode plates having juxconcentration of the discharge into an are under normal conditions of discharge, the plates being in a gaseous medium at substantially atmospheric pressure.

50. A space-current discharge device comprising a pair of electrode plates having juxtaposed surfaces, the major portions of said surfaces being spaced distances between the length of the cathode'dark space of a low discharge and the combined lengths o the cathode dark space, cathode glow and Faraday dark space of a glow dischar e, at least one portion of the juxtaposed sur aces being spaced a distance corresponding to the minimum breakdown voltage, the resistivity of at least one of the electrode plates being such as to insure against the concentration of the discharge into an are under normal conditions of discharge, the plates being in a gaseous medium at substantially atmospheric pressure.

51. A space-current discharge device comprising a pair of electrode plates having juxtaposed surfaces, at least one portion of the juxtaposed surfaces being spaced a distance corresponding to the minimum breakdown voltage, the plates being in a gaseous medium at substantially atmospheric pressure, the resistivity of at least one of the electrode plates being such as to insure against the concentration of the discharge into an are under normal conditions of discharge, the plates beingin a gaseous medium at substantially atmospheric pressure.

52. A space-current discharge device comprising a pair of electrode plates having juxtaposed surfaces, the spacing between said juxtaposed surfaces being such that the breakdown voltage is less than 500 volts, the resistivity of at least one of the electrode plates being such as to insure against the concentration of the discharge into-an are under normal conditions of discharge, the plates being in a gaseous medium at substantially atmospheric pressure.

In testimony whereof, I have hereunto subscribed my name this 10th day of February, 1922.

JOSEPH SLEPIAN.

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