US3162741A - Overvoltage arrestor having a light dispersion of fine metallic dust on its inside walls - Google Patents

Description

B. G. A. LINDGREN Dec. 22, 1964 OVERVOLTAGE ARRESTOR HAVING A LIGHT DISPERSION OF FINE METALLIC DUST 0N ITS INSIDE WALLS Filed March 1, 1961 ALUMINUM DU$T United States Patent f 3,162,741 OVERVOLTAGE ARRESTOR HAVING A LIGHT DESPERSIQN OF FINE METALLIC DUST ON ITS INSIDE WALLS B0 Gustav August Lindgren, Johanneshov, gweden, assignor to Telefonaktiebolaget L M Ericsson, Stockhelm, Sweden, a corporation of Sweden Filed Mar. 1, 1961, Ser. No. 92,570 Claims priority, application Sweden Mar. 11, 1960 Claims. (Cl. 200-1135) The present invention refers to an electric overvoltage protector in the form of a gas discharge tube.

With gas discharge tubes in general it is earlier known to provide a bridge or similar device consisting of some convenient electrically conducting material, in parallel to the discharge gap in order to reduce the ignition delay, said bridge being preferably located on the tube envelope. Furthermore this bridge is generally electrically connected to one of the tube electrodes, whereby a discharge appears between the bridge and the one electrode not electrically connected thereto, at the moment the tube is connected, whereafter the main discharge gap is ignited.

With gas discharge tubes of the type used as overvoltage protectors it is of vital importance that the tube has an ignition delay as short as possible in order to react very quickly to sudden shock voltages. To this end it has earlier been proposed to provide an auxiliary electrode in the form of a bridge in parallel to the discharge gap, and this bridge is preferably provided on the inside of the tube envelope in the form of a coating as in other types of discharge tubes, except that the coating must not be conducting, in any case not at lower voltages, and furthermore is preferably electrically insulated from the main electrodes. Such bridges have been prepared by metallic evaporation, so that a thin conducting layer is formed which subsequently is exposed to a heavy current shock and thereby divided into a multitude of insulated islands, so that a number of microscopic sparking gaps are obtained, whereby the electrons and ions, liberated at discharges in these gaps, cause a pre-ionizing of the main discharge gap. Another bridge type earlier proposed involves a purely mechanical mixture of at least two components, one of which is a conductor, the other being an insulator, which mixture gives a non-linear, voltage dependent resistance characteristic. This last mentioned type of bridge type has considerably better properties compared to the first-mentioned type, but has nevertheless certain practical disadvantages specifically in connection with the pure mechanical structure of the tube and the application of the bridge to the tube envelope.

These disadvantages are eliminated by the present invention by providing a new mechanical structure of the tube and an entirely new means for obtaining the desired reduction of the ignition delay. The invention resides basically in an overvoltage protector or arrestor in the form of a gas discharge tube including two generally parallel bimetallic contacts. These contacts define a discharge gap therebetween which is shortcircuited when the contacts are heated. The contacts are coated with a light radioactive coating on surface areas facing each other and a light dispersion of a fine dust of a light metal such as aluminum is provided on the inside walls of the discharge tube. The radioactive coating on the contacts causes an ionization of the gap space between the coated areas of the contacts so that a discharge will occur between said areas only, in response to a suddenly appearing excess voltage. The coating affords the further advantage that it protects the metal surfaces of the contacts during the discharge. The discharge current may momentarily reach a magnitude of several thousand amperes. A discharge of such magnitude, if occurring between uncoated metal sur- 3,162,741 ?atented Dec. 22, 1964 faces may tear off metal particles from such surfaces and from a metal coating on the inside walls of the discharge tube, known as the Edison effect. The result would be such a lowering of the insulating resistance between the electrodes that the tube is no longer usable.

The light metal dust may be introduced into the tube in any suitable manner. The individual particles, due to the insignificant weight and mass thereof will adhere themselves to the inside walls of the tube by the inherently present force of attraction. The particles form a multitude of minute spark gaps which will cause an additional ionization of the main discharge gap between the contact electrodes.

The invention will be further described with reference to the single figure of the attached drawing, said figure showing an embodiment of an overvoltage protector in accordance with the invention.

The exemplified arrestor comprises a tube 1 having a base 2. In the base there are provided two contact tags 3 and 4 which are electrically connected to two bimetallic electrodes 5 and d. The contact tags and also the bimetallic electrodes are secured in limit means in the form of a ceramic bushing 7 which is provided inside the tube in connection with the tube base. The bimetallic electrodes 5 and 6 are at their free ends provided with molybdenum bars 8 and 9, respectively, which are arranged perpendicularly to each other on the respective electrodes and the purpose of which is to establish contact between the two electrodes when the same are heated by a prolonged discharge current. Due to the high melting point of molybdenum the risk of the sticking of the two electrodes because of the heat development, when the temperature approaches the melting point of the bimetallic components, is eliminated. The limitation imposed by bushing 7 upon movements of the bimetallic electrodes 5 and 6 prevents an increase of the air gap. This is important as an increase of the air gap would change the ignition voltage of the tube. It has been found that the contact electrodes, when separating after having been in contact with each other due to overheating, tend to return into positions in which the air gap is increased. Such return beyond the initial position is prevented by the bushing. In order to reduce the ignition delay of the tubes, the electrodes are coated with some weakly radioactive activating means 10, ll, but this alone is not sufiieient to obtain the desired functioning of the tube. To attain the desired result, a light metal powder, for instance aluminum powder 12 is blown into the tube before mounting the electrodes. This powder, due to the attraction force and the eventually existing electrostatic charges, is drawn to and deposited on the inside wall of the tube. Due to the tendency of the individual particles to adhere themselves to the inside wall of the tube in a dispersed relationship and due to the automatically forming surface oxidation of the aluminum particles, aluminum oxide being a non-conductor, the particles will be, generally speaking, insulated from each other against the electrical potentials which are normally always present between lines in operation. As a result, minute sparking gaps between the particles will be formed in which discharges will appear when the voltage difference between the electrodes is suddenly increased above its normal value. Due to the random distribution of the particles on the inside wall of the tube envelope, one or several of these discharges will always appear in a convenient location relatively to the main discharge gap between the tube electrodes, thus effectively contributing to the ionization of said discharge gap and, consequently, to a reduction of the ignition delay of the tube.

During the manufacture of an overvoltage protector or arrestor fine aluminum powder is blown into the finished tube envelope which at one end is connected to a gas pumping tube, whereby the different particles because of their insignificant weight and mass are drawn to the envelope and adhered to the inner wall thereof without the use of any special binding means. Thereafter the internal assembly of the tube, consisting of the ceramic bushing wherein the electrodes and the conductors are secured, is inserted, and the soldering of the tube end is carried out in such a way that said conductors pass through the base of the tube to serve as contact tags. The tube is subsequently evacuated, whereafter the inert gas filling is provided, whereupon the final soldering is carried out.

The invention is not limited to the example given in the description, several modifications being possible within the scope of the invention.

I claim: 7

1. An overvoltage arrestor comprising a sealed glass tube, two lead-in wires sealed through the base of the tube, two bimetal contact strips extending from the lead-in wires substantially in parallel relationship inside the tube, a gaseous filling in said tube, and a light dispersion of a fine metal dust loosely adhering to the inside walls of the tube to form a multitude of small air gaps between the dust particles.

2. An overvoltage arrestor according to claim 1, wherein the bimetal contact strips are provided with an activating, weakly radioactive coating on areas facing each other.

3. An overvoltage arrestor according to claim 1, comprising limit means for limiting movement of the bimetal strips, said limit means being in the form of a substantially cylindrical insulation bushing closely encompassing the lower part of said strips.

4. An overvoltage arrestor according to claim 3, where in the bimetal contact strips are provided with an activating, veakly radioactive coating on areas facing each other.

5. An overvoltage arrestor according to claim 1, wherein said metal dust consists of discrete particles of oxidized aluminun References tCited in the file of this patent UNITED STATES PATENTS 2,608,545 Dobke et al July 16, 1935 2,062,663 Kautter et a1 Dec. 1, 1936 2,259,111 Laidig Oct. 14, 1941 2,457,487 Peacock et al. Dec. 28, 1948 2,741,680 Lemaigre-Voreaux Apr. 10, 1956

Claims (1)

1. AN OVERVOLTAGE ARRESTOR COMPRISING A SEALED GLASS TUBE, TWO LEAD-IN WIRES SEALED THROUGH THE BASE OF THE TUBE, TWO BIMETAL CONTACT STRIPS EXTENDING FROM THE LEAD-IN WIRES SUBSTANTIALLY IN PARALLEL RELATIONSHIP INSIDE THE TUBE, A GASEOUS FILLING IN SAID TUBE, AND A LIGHT DISPERSION OF A FINE METAL DUST LOOSELY ADHERING TO THE INSIDE WALLS OF THE

Images