US2795725A - Electrical overload protector - Google Patents

Description

June 11, 1957 H. sTRB K 2,795,725

ELECVTRICAL OVERLOAD PROTECTQR i Filed march 1e, 1954 2 sheets-sheet 1 ooooo-o oo-ooooo ooooooooo l ocvoooooo coocoooo HERMANN sfR/ June 1l, 17957 H. STRB Filed March 16, 1954 ELECTRICAL OVERLDAD4 PROTECTOR 2 Sheets-Sheet 2 ELECTRICAL OVERLOAD PROTECTOR Hermann Strb, Boll, Germany, assignor to Robert Bosch G. m. b. H., Stuttgart, Germany Application March 16, 1954, Serial No. 416,615

Claims priority, application Germany March 31, 1953 3 Claims. (Cl. 313-325) This invention relates to an electrical overload protector, and more particularly to an improved arrangement for protection against surge or other excessive voltages.

In the past there have been employed excessive voltage protectors which comprise metal points spaced a fixed distance apart between a potential source and ground, commonly known as spark gaps. Such devices ordinarily are used with antennas, for example, when it is desired to protect the equipment connected to the antenna from lightning. They are also used with switches and many other equipments. Upon the occurrence of a voltage surge, a spark is formed between the two points and the high voltage energy is grounded out. These devices are reliable only for relatively high voltages.

It is an object of the present invention to provide a new type of surge or other excessive voltage protection arrangement.

it is a further object of the invention to provide an arrangement of the type described above which is especially adaptable for use with relatively low voltages, that is, voltages as low as 1000 or 500 volts or even less.

In accordance with the invention there is provided an overload protection arrangement comprising a layer of insulating material having opposite faces and formed with a plurality of apertures through the insulating material opening at opposite ends at said opposite faces. A pair of thin metallic layers are superimposed upon the opposite faces of the insulating material so that portions of each layer cover the opposite ends of said apertures. At least some of the portions of at least one of the metallic layers are adapted to be destroyed when subjected to a short duration electric are of relatively low voltage extending between the metallic layers through the apertures covered by the respective metallic layer portions.

In a preferred embodiment of the invention the layers of insulating and metallic material are built up to forma laminated structure preferably in the shape of a roll of material somewhat like a condenser. One preferred metallic layer which may be employed is zinc.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings, in which:

Fig. 1 is a cross-section of a portion of an overload protector in accordance with the invention;

Fig. 2 is a plan view of a layer of insulating material shown in cross section in Fig. l;

Fig. 3 is a cross-section of a second embodiment of the invention;

Fig. 4 is a plan view of an insulator and metallic electrode shown in crosssection in Fig. 3; and

Fig. 5 is a graph of the performance of one embodiment of the invention.

Referring now to the drawing and more particularly tate Patent to Fig. 1 there is shown insulating layers 1, 2, 3 which are respectively formed with metallic layers 4 5, 6 7, and 8-9 on opposite sides thereof. The insulating layers may be paper and the metallic layers zinc. The metallic layers are provided by any means known in the art as, for example, by causing the vaporized metal to plate out on the paper. Between members 1, 2 and 3 and their metal coatings are insulating layers 11 which are provided with a plurality of apertures 1i) therethrough opening on opposite faces of member 11. These insulating layers may be made from paper, too, or from an articial material such as polystyrol, triacetate cellulose or polyethylene. The metallic layers on insulators 1 and 3 extend from contact terminal 13 almost to contact terminal 12 and similarly the metal layers on insulator 2 extend from contact terminal 12 almost to contact terminal 13. The contact terminals are ordinarily connected between ground (not shown) and the source of potential (not shown).

In one embodiment of the invention the various layers of metal and insulating material may be at. If it is desired to increase the surface area of the overload protector and still keep the protector of small size the Various bands of material may be wound upon themselves so as to form a roll. For example, members 1, 2 and 3 along with their metal coatings and the layers 11 of insulating material may be wound as a unit about an insulated member having an axis indicated by the dashed line in Fig. l so that the resulting structure forms a roll. After winding the protector, the product can be completed in a manner similar to that used for manufacturing condensers. For example, the product may rst be dried by baking in order to eliminate all water vapor and then the product may be impregnated with an impregnating agent such as paraiin and Vaseline or oil. As is well known, the dielectric strength of impregnated paper is much higher even than that of the im'pregnating` agent alone and such impregnation prevents the destruction of the insulating layers due to excessive voltages.

The contact terminals 12, 13 are normally secured in place before the baking and impregnating process described above. The metal may be sprayed onto the ends of the structure or vaporized on to the ends of the structure. These terminals are normally made relatively thick so that they will be unaffected by heavy electrical loads and so that they form a supporting arrangement for the protector. If desired, the finished protector may be coated with an insulating substance.

Fig. 2 is a plan View of the insulating layer 11 shown in Fig. l. As can be seen, in a preferred embodiment of the invention the apertures 10 are uniformly distributed over the entire area of the insulating layer. The cross-sectional area of the apertures and the strength of the dielectric material determine the voltage at which a spark will be formed between the metal layers on either side of member 10 through one or more of the apertures in layers 11. The dimensions and materials are so chosen, of course, that the breakdown potential is above the operating potential of the equipment it is desired to protect.

In operation, when an overload occurs, that is, when the voltage is excessive, a spark is formed between layers 5 6 and/ or layers 7 8 through one or more of the apertures in insulating layers 11. (This is the case when the various layers are ilat, however, it should be appreciated that in the general case, for example when the layers are wound upon themselves, the arcing may occur between any of the metal layers conductively connected to contact terminal 13 and the next adjacent metal layer conductively connected to contact terminal 12.) When the arc occurs the excessive voltage is bypassed to ground.

When an arc occurs, it starts at a metal layer connected to one of the contact terminals and passes through one or more apertures dependent on the severity of the load and terminates at the metal layer connected to the other contact terminal.

The intense heat generated due to arcing causes the portionk of the metal layers adjacent the aperture or apertures through which the arc or arcs form to melt and then vaporize and disappear. The portion of the layers adjacent said aperture or apertures is, in effect, destroyed. The greater the number of arcs which have occurred the less metal remaining in the metal layers and the less the capacitance of the overload protector. Accordingly, one may determine the number of arcovers which have occurred and therefore the number of stand-by apertures remaining through which further arcs may occur by measuring the capacitance of the protector.

The performance of the overload protector can be somewhat improved if the metal layers are vaporized onto an insulating layer which has a very smooth surface, for example, a surface which has been covered with a layer of lacquer prior to the plating-out thereon of the metal.

Tremendous loads can be handled by protectors of the type described. It has been found that in a protector designed for 500 volts, energies on the order of 5 joules can readily be dissipated. Currents on the order of 1000 amps, for a duration of about 10-5 seconds have been measured during the arcing period.

In a protector actually constructed suitable for 500 volts, metal layers 4-9 were zinc and were made millimeters thick (0.05 micron). Layers 1, 3, and were paper and insulating layers 11 were formed of polystyrol sheets about 11 to 20 microns thick. Apertures 10 in the polystyrol sheets were about l millimeter in crosssectional diameter and were iilled with paraffin. The paraffin was also used to impregnato the entire overload protector.

Fig. 5 illustrates the performance `of the 500 volt overload protector. The Y axis indicates the potential Iequired to cause an arc and the X axis indicates the number of arcs which have already occurred. It will be noticed that as the number of breakthroughs increase, the voltage required to cause an arc also increased. However, the curve is relatively flat over a fairly long interval.

It is also possible using the principles already outlined to construct overload protectors suitable for voltages above and below 500' volts. For example, an overload protector designed for 1000 volts requires insulating layers 11 about 40 microns thick and apertures 10 in layers 11 about l to 2 millimeters in cross-sectional diameter. A protector designed for operation at 250 volts requires polystyrol sheets 11 of about 8 to l0 microns thick with apertures not greater than l millimeter in crosssectional diameter.

Fig. 3 illustrates a second embodiment of the invention. It comprises an insulating layer 15 similar to the insulating layers 1, 2, 3 shown in Fig. 1 and a pair of metal layers 16, 17 formed on opposite sides of the insulator layer in the same manner as previously described. Adjacent the metal layers are insulator layers 20, 21 formed with apertures 18, 19 respectively. These last insulator layers, as in theA case of the structure described in Fig. 1, are formed of paper or polystyrol or similar material. On one surface of the protector are located spaced metal foils 22, 23 and on the opposite surface of the protector spaced metal foils 24, 25. These, of course', can be substituted by metal layers which are platedonto the protector in the manner already described. The terminal members described are strong and are capable of carrying extremely large currents, and also,` quickly conduct heat away from Within the overload protector. This last feature prevents the protector fronrbeing destroyed when excessively heated by internal arcs.. v

As in the case of the embodiment of Fig. Vl', the species illustrated in Fig. 3 can either be manufactured as a at member or as a member rolled upon itself to form a rolllike structure. The terminals of the protector are members 22, 24 and members 23, 25, respectively, members Z2, 24 normally being grounded and 23, 25 connected to a potential source, or vice-versa.

Fig. 4 illustrates ay preferred method of forming the metal electrodes 16, 17 upon the paper or other insulating layer 1S. These are formed as a plurality of spaced sections 26, 27, 28 and, as already described, may be manufactured by causing a vaporized cloud of a metal such as zinc -to plate out onto the insulator layer. This type of lconstruction may be used with both embodiments of the invention. Itis especially suitable in the embodiment of Fig. 3 for it is another means for preventing mechanical destruction of the overload protector due to excessive current. This is because the breakthrough energy is to a certain extent proportional to the area of the section of metal layer where the breakthrough occurs and can therefore be lessened by dividing the layer into various sections. This phenomenon is in part explainable in terms of the electrical decoupling between the separate metal layer sections.

The overload protector is usually protected against mechanical damage, especially when formed in a roll, by covering the outer surface of the roll with paper windings. Paper windings may also be used for the core of the roll so that the final product has paper inside and outside and consequently, even though containing extremely thin metal layers, the protector is not easily damaged by mechanical shock.

Although in the description above, only zince is mentioned as a suitable electrode material, it is to be understood that other metals may advantageously be employed for Vthe same purpose. For example, such metals have been employed as cadmium, aluminum, silver and nickel. In addition, it is possible to use many other types of metals provided that the layer of metal is sufliciently thin so that when subjected to an arc it rst vaporizes and then disappears from the arc area. Also, the dielectric material employed must be of such a nature that the spark which Yforms during an overload does not damage the dielectric, for example, by carbonizing the same or by mechanically destroying the same.

In the voperation of the overload protector, upon the occurrence of an arc a portion of the metal layer adjacent the aperture or apertures through which the arc or arcs form is vaporized and subsequently the vapor is converted into small particles of metal oxide. These particles and also small carbon particles are blown laterally by Ithe gases formed during the are and are probably deposited adjacent the aperture yor apertures through whichthe arc or arcs are formed. In any case, after vthe occurrence of an arc, there remains an area free of metal immediately adjacent the aperture through which the arc was formed. Successive arcs result in the disappearance or destruction of more and more of the meta-l layer or layers. l

As already mentioned in connection with the graph of Fig. 5, .as the number of breakthroughs increase, the overload voltage required also increases. This is probably due tothe fact that there are small constructional and spatial d ilferences between dilerent sections of the overload protector. For example, the apertures in the insulating material are not all of precisely uniform crosssectional conligura'tion. Also, the dielectric layer is not ofthe samev dielectric strength throughout nor is it of precisely the same thickness throughout. Also, during the impregnating process certainareas of the impregnated dielectric material are not impregnated as thoroughly as other areas. Therefore `these are not of the same dielectric strength as `other areas. All ofthe differences above are slight, however, they do result in* a curve of the type illustrated. As can be seen from Fig. 5, it requires only a `very short period of time for major inaccuracies in the overload protector to disappear. Thus, after twenty arcs any weak spots in the dielectric which allowed for arcing to occur in the area from 300 to 500 volts are eliminated and subsequent arcing must therefore occur in the desired region, that is, the region over 500 volts.

In the preceding discussion, when an overload protector for a given voltage is specified it is to be understood that this voltage is the operating voltage of the equipment it is desired to protect. Thus, the graph of Fig. is designed to protect against overload voltages in excess of 500 volts. As can be seen from the graph, the overload protector is usable for a relatively long period of time, that is, for the period between about 25 breakthroughs until about 250 or so breakthroughs. The protector, of course, eventually operates at too high voltage to be useful and when this occurs it is replaced with a new protector.

It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of overload protectors differing from the types described above.

While the invention has been illustrated and described as embodied in an overload protector employing metal electrodes made of thin layers of zinc, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be secured by Letters Patent is:

1. An overload protection device comprising a rst layer of insulating material having opposite faces; a pair of thin metallic layers superimposed upon said faces of said layer of insulating material respectively; a pair of second layers of insulating material having opposite faces and formed with a plurality of apertures therethrough opening at opposite ends in said faces, said second layers of insulating material respectively superimposed upon said thin metallic layers; a first pair of aligned terminal members respectively secured to the respective free faces of said second pair of layers of insulating material, opposite ends of some of said apertures lying between each terminal member and the metallic layer closest thereto; a second pair of aligned terminal members spaced from said rst pair of terminal members and respectively secured to the respective free faces of said second pair of layers of insulating material, said second pair of terminal members and thin metallic layers being so arranged that opposite ends of other of said apertures lying between each one of said second pair of metallic terminal members and the metallic layer closest thereto, whereby when an excessive voltage is developed between said first pair of terminal members and said second pair of terminal members, an arc is formed through at least one aperture between one of said rst pair of terminal members and the thin metallic layer closest thereto and a second arc is formed through at least another aperture between one of said second pair of terminal members and the thin metallic layer closest thereto. i

2. An overload protection arrangement as set forth in claim l wherein said terminal members, metallic layers and insulating layers form a laminated structure which is wound upon itself as a unit forming a roll.

3. An overload protection device comprising a layer of insulating material having opposite faces and formed with a plurality of apertures therethrough opening at opposite `ends in said faces; a thin metallic layer superimposed on one of said faces and having metallic layer portions covering one end of said apertures, said portions of said metallic layers being adapted to be destroyed when subjected to a short duration electric arc of relatively low voltage extending through the apertures covered by the respective metallic layer portions; a first terminal member formed of metal foil superimposed upon the other of said faces of said layer of insulating material and covering the other end of some of said apertures; a second terminal member formed of metal foil spaced from said rst terminal member and superimposed upon said other of said faces of said layer of insulating material and covering said other end of other of said apertures, whereby when an excessive voltage is developed across said two terminal members a rst arc is formed at least between one of said terminal members and said metallic layer and a second arc is formed between said metallic layer and said other terminal member, said arcs destroying the portions of said metallic layer adjacent the apertures through which said arcs are formed.

References Cited inthe file of this patent UNITED STATES PATENTS Re. 2,043 Stearns Aug. 1, 1865 476,988 Edison June 14, 1892 532,354 Wurts Jan. 8, 1895 869,031 Stocking Oct. 22, 1907 982,551 Vaughn Jan. 24, 1911 2,288,428 Babler June 30, 1942

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