GB1599443A

GB1599443A - Over-voltage discharger

Info

Publication number: GB1599443A
Application number: GB3503/78A
Authority: GB
Original assignee: Cerberus AG
Current assignee: Cerberus AG
Priority date: 1977-01-27
Filing date: 1978-01-27
Publication date: 1981-10-07
Also published as: IT7812411A0; FI61973C; FI61973B; FR2379156B1; CH600630A5; NO780302L; FI773877A; SE7800923L; ATA928977A; NL170345B; SE426635B; IT1103172B; NL7800723A; DE2757337A1; FR2379156A1; AT363141B

Description

(54) OVER-VOLTAGE DISCHARGER (71) We, CERBERUS AG, of alte Landstrasse 411, Mannedorf, Switzerland., a Swiss company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: - - This invention relates to an over-voltage discharger including a tubular insulating housing with two electrodes sealed in a gas-tight manner to its ends with their inner surfaces opposed to each other and separated by a gas-gap, as well as to a method for the manufacture of such dischargers.

In over-voltage dischargers of this kind over-voltage applied between the electrodes are limited because at a predetermined firing voltage a gas discharge takes place in the gap between the electrodes. In order to obtain a definite, low firing voltage for rapid voltage rises, known as the dynamic firing voltage, which is particularly necessary when the over-voltage discharger is employed for the protection of lowvoltage equipment, it is known to employ auxiliary firing promoting means of various kinds. The proposed employment of radioactive materials for this purpose, however, often runs into difficulties owing to possible danger to health.

It has therefore been proposed in overvoltage dischargers of this kind to apply conductive strips to the inside of the insulating housing, the strips lying along the generatrices of the housing, which strips are connected alternately with one and insulated from the other of the two electrodes and extend at their free ends somewhat beyond the gas gap. However, at every operation and every firing of the over-voltage discharger, some electrode material is evaporated, which reduces the insulation resistance between these firing strips and between the strips and the respective counter-electrodes, and rapidly makes the over-voltage discharger unusable. In addition the electric field is greatly distorted by the presence of these firing strips and it is found to be very difficult to obtain an accurate and reproducible firing voltage for rapid voltage rises.

It has been proposed to improve the electrical field and the firing characteristics by arranging that the conductive layers applied to the inside of the housing have an irregular geometrical form, e.g. they may be elliptical, square or triangular regions of which some are connected with the electrodes while some are insulated therefrom. The distribution of the conductive locations is then extremely irregular, so that the dynamic firing voltage exactly reproduced. The danger of of such dischargers likewise cannot be reducing the insulation resistance between the conductive regions and the electrodes is also not completely abolished. In addition the manufacture of such devices is very complicated and therefore not very economical.

It has already been attempted to introduce additional auxiliary electrodes through the housing from outside in the vicinity of the discharge path between the main electrodes, so that the firing can be influnced by a voltage applied to the auxiliary electrodes. On the one hand the manufacture of such modified devices is difficult, unreliable and expensive because of the other hand such over-voltage dischargers cannot be connected directly between the leads to be protected, since an auxiliary voltage is necessary which in general is not available at the location to be protected and must first of all be generated by an additional circuit. Over-voltage dischargers with auxiliary electrodes are therefore applicable only in special cases.

The object of the invention is to abolish or reduce the disadvantages explained above and in particular to provide an overvoltage discharger which is very generally applicable, has a clearly defined firing voltage for rapid voltage rises and has a long working life without suffering a deterioration of insulation resistance even when repeatedly fired, and which may be reliably, accurately and economically manufactured.

According to the present invention there is provided an over-voltage discharger having a tubular insulating housing with two electrodes sealed in a gas tight manner to its ends, the inner surfaces of the two electrodes being opposed to one another and separated by a gas-filled gap, wherein there is applied to the internal wall of the housing in the region surrounding the gasfilled gap a continuous cylindrical layer of conductive material which is different from the material of the said electrodes and is either graphite or a transition metal, said layer being insulated from each of the two electrodes and having a width which exceeds the width of the gap between the electrodes, each edge of the layer extending axially of the casing beyond the working faces of the electrodes.

The invention also provides a method for the manufacture of an over-voltage discharger as defined in the preceding paragraph, wherein the conductive material used to form the said layer is initially applied to at least one of the two electrodes, the electrodes are sealed to the housing and the conductive material is then sputtered and deposited as a continuous cylindrical layer upon a part only of the inner wall of the housing by the passage of electric curent between the electrodes.

The invention will now be described with reference to the accompanying drawings, in which: Figure 1 shows a cross-sectional elevation of one embodiment of over-voltage discharger in accordance with the invention; and Figure 2 is a diagram illustrating a method of manufacturing an over-voltage discharger.

The embodiment of over-voltage discharger represented in Figure 1 comprises a tubular housing 1 which is constructed of insulating material, for example of glass or preferably of ceramic material, in the two ends of which are inserted metal electrodes 2 and 3. The sealing of the electrodes to the housing may in known manner be effected by a metal-ceramic seal. The inner faces 4 and 5 of the two electrodes are opposed to one another so that a gap 6 is formed between these surfaces 4 and 5. The interior of the over voltage discharger, including the gap between the electrodes 2 and 3, is filled with a suitable gas, in accordance with the deesired electrical characteristics. The gas gap 6 forms the actual discharge path of the over-voltage discharger.

On the inner surface of the tubular housing 1, substantially at the position of the gas-gap 6, a layer 7 of conductive material is arranged in a cylindrical annular zone. This layer 7 substantially covers the zone of the housing inner wall adjacent and may possibly extend a little therebeyond, provided that the layer 7 is completely insulated from each of the two electrodes 2 and 3.

Through this arrangement of a layer 7 of conductive material acting as a firing promoter, upon the inner wall of the housing at the position of the electrode gap 6, a definite and relatively uniform variation of field strength is achieved, as soon as an electrical potential is applied to the electrodes 2 and 3 and the layer 7 sets itself to a coresponding potential. It is found that the described arrangement of a firing-promoting layer on the inner wall of the housing allows the firing voltage to be more accurately and definitely set than with the previously known firing strips or firing promoting layers. This must obviously be attributed to the advantageous and uniform distribution of the electrical field strength which is obtained by use of the invention, a point which was obviously not taken into account in the previously known constructions.

It has been found to be particularly advantageous for the thickness of the layer 7 to be greatest at the level of the centre of the gap, and to diminish continuously in the direction twoards the two electrodes.

It must be assumed that he particularly advantageous characteristics of this construction are to be attributed to the fact that the increased field strength at the margins of the electrode surfaces 4 and 5 is at least partly compensated by the maximum layer thickness in the centre between the two electrode surfaces and possibly uncontrollable maxima of the field strength are reduced, so that a more accurate and definite adjustment of the dynamic firing voltage is possible.

The material of the firing-promoting layer 7 may be a transition metal or, in particular, it may be of graphite. However, the poor adhesion of some of these materials to the glass or ceramic inner wall surface must be taken into account.

Investigations have shown that certain transition metals, e.g. metals of the iron group or noble metals, are relatively advantageous for this purpose. For overvoltage dischargers with ceramic housings, however, those metals which readily form compounds with aluminimum oxide, for example titanium and zirconium and certain other transition metals, such as for example manganese, have shown out standing characteristics as regards ease of application and good adhesion.

The application of the firing promoting layer to the inner wall of the housing may likewise result in known manner, so as to produce the required effect, for example by spreading on a graphite layer of spreading on a paste which is subsequently burnt on. Although such methods of manufacture can be carried out successfully by manual operations, they are however difficult to perform automatically and therefore not very economical for mass-production.

A method has proved to be particularly valuable for large scale manufacture, in which the conductive, firing-promoting layer is deposited by sputtering the initial material from the electrodes on to the inner wall of the housing. Such a method is explained with reference to Figure 2, in which parts similar to those of Figure 1 are denoted by the same reference numerals as in that Figure.

In this method, there is first of all deposited in the recesses of electrodes 2 and 3, which are here formed as re-entrant electrodes, an emission-promoting material 8, 9 of a kind which is known in itself.

After this the material 10 to be deposited on the inner wall of the cylinder by sputtering is deposited upon the electrodes pre-treated as described. The material 10 may be applied over the whole of the interior surface of the electrodes 2 and 3 or, as is illustrated in Figure 2, in the form of an annular outer zone on the margin of the active area of the electrode. Finally the two electrodes 2 and 3 are secured to the ends of the ceramic tubular housing 1 by gas-tight bonds and the interior of the housing is filled with a suitable gas. After this final assembly operation one or more pulses of curent are produced within the tube by the application of pulsatory voltages to the electrodes 2 and 3, through which the material 10 is sputtered and is deposited in an annular cylindrical zone 7 on the inner wall of the housing.In so doing it is particularly advantageous for the part of the zone midway between the electrodes to receive material from both of the rings 10, so that a thickness of deposited material is automatically produced there which is greater than that at the margins of the zone 7.

The deposition of the material on the inner wall of the housing may in fact be accelerated by mechanical forces, e.g.

centrifugal forces. This may be effected by setting the over-voltage discharger into rapid rotation about its axis of symmetry, during the operation of depositing the wall coating. In mass production, however, this has proved to be too difficult and complicated to be economical. On the other hand the material 10 may very conveniently be guided to the required positions by the application of suitable electric or magnetic fields during the current or discharge pulses, so that a conductive layer 7 with the desired distribution of thickness is produced exactly at the required zone of the inner wall of the housing. This method has proved to be particularly advantageous in the manufacture of of an over-voltage discharge with an exactly adjustable and definite value of dynamic firing voltage.

Another advantageous method is the application of titanium hydride in suspension to the appropriate portions of the inner surface of the housing and subsequent burning-on in a protective atmosphere or in vacuum, for example at a temperature of some 1000"C.

WHAT WE CLAIM IS: 1. An over-voltage discharger having a tubular, insulating housing with two electrodes sealed in a gas-tight manner to its ends, the inner surfaces of the two electrodes being opposed to one another and separated by a gas-filled gap, wherein there is applied to the internal wall of the housing in the region surounding the gasfilled gap a continuous cylindrical layer of conductive material which is different from the material of said electrodes and is either graphite or a transition metal, said layer being insulated from each of the two electrodes and having a width which exceeds the width of the gap between the electrodes, each edge of the layer extending axially of the casing beyond the working faces of the electrodes.

2. An over voltage discharger in accordance with claim 1, wherein the layer of conductive material is a noble metal.

3. An over-voltage discharger in accordance with claim 1, wherein the layer of conductive material is titanium, zirconium or manganese.

4. An over-voltage discharger in accordance with any of the preceding claims, wherein the thickness of the layer of conductive material is greatest opposite the centre of the inter-electrode gap and diminishes continuously towards either margin.

5. A method for the manufacture of an over-voltage discharger in accordance with claim 1, wherein the conductive material used to form the said layer is initially applied to at least one of the two electrodes, the electrodes are sealed to the housing and the conductive material is then sputtered and deposited as a continuous cylindrical layer upon a part only of the inner wall of the housing by the passage of electric current between the electrodes.

6. A method in accordance with claim 5, wherein the passage of said electric current is produced by applying a pulsatory voltage between the electrodes.

7. A method in accordance with claim

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. likewise result in known manner, so as to produce the required effect, for example by spreading on a graphite layer of spreading on a paste which is subsequently burnt on. Although such methods of manufacture can be carried out successfully by manual operations, they are however difficult to perform automatically and therefore not very economical for mass-production. A method has proved to be particularly valuable for large scale manufacture, in which the conductive, firing-promoting layer is deposited by sputtering the initial material from the electrodes on to the inner wall of the housing. Such a method is explained with reference to Figure 2, in which parts similar to those of Figure 1 are denoted by the same reference numerals as in that Figure. In this method, there is first of all deposited in the recesses of electrodes 2 and 3, which are here formed as re-entrant electrodes, an emission-promoting material 8, 9 of a kind which is known in itself. After this the material 10 to be deposited on the inner wall of the cylinder by sputtering is deposited upon the electrodes pre-treated as described. The material 10 may be applied over the whole of the interior surface of the electrodes 2 and 3 or, as is illustrated in Figure 2, in the form of an annular outer zone on the margin of the active area of the electrode. Finally the two electrodes 2 and 3 are secured to the ends of the ceramic tubular housing 1 by gas-tight bonds and the interior of the housing is filled with a suitable gas. After this final assembly operation one or more pulses of curent are produced within the tube by the application of pulsatory voltages to the electrodes 2 and 3, through which the material 10 is sputtered and is deposited in an annular cylindrical zone 7 on the inner wall of the housing.In so doing it is particularly advantageous for the part of the zone midway between the electrodes to receive material from both of the rings 10, so that a thickness of deposited material is automatically produced there which is greater than that at the margins of the zone 7. The deposition of the material on the inner wall of the housing may in fact be accelerated by mechanical forces, e.g. centrifugal forces. This may be effected by setting the over-voltage discharger into rapid rotation about its axis of symmetry, during the operation of depositing the wall coating. In mass production, however, this has proved to be too difficult and complicated to be economical. On the other hand the material 10 may very conveniently be guided to the required positions by the application of suitable electric or magnetic fields during the current or discharge pulses, so that a conductive layer 7 with the desired distribution of thickness is produced exactly at the required zone of the inner wall of the housing. This method has proved to be particularly advantageous in the manufacture of of an over-voltage discharge with an exactly adjustable and definite value of dynamic firing voltage. Another advantageous method is the application of titanium hydride in suspension to the appropriate portions of the inner surface of the housing and subsequent burning-on in a protective atmosphere or in vacuum, for example at a temperature of some 1000"C. WHAT WE CLAIM IS:

1. An over-voltage discharger having a tubular, insulating housing with two electrodes sealed in a gas-tight manner to its ends, the inner surfaces of the two electrodes being opposed to one another and separated by a gas-filled gap, wherein there is applied to the internal wall of the housing in the region surounding the gasfilled gap a continuous cylindrical layer of conductive material which is different from the material of said electrodes and is either graphite or a transition metal, said layer being insulated from each of the two electrodes and having a width which exceeds the width of the gap between the electrodes, each edge of the layer extending axially of the casing beyond the working faces of the electrodes.

7. A method in accordance with claim

5 or claim 6, wherein said electrodes are provided with emission-promoting layers before the application thereto of said layerforming conductive material.

8. A method in accordance with claim 5, 6 or 7 wherein the over-voltage discharger is caused to rotate about its axis of symmetry during the passage of said electric current.

9. A method in accordance with claim 5, 6 or 7 wherein the deposition of said conductive material upon the inner wall of the housing is guided by the application of an electric field.

10. A method in accordance with claim 5, 6 or 7, wherein the deposition of said conductive material upon the inner wall of the housing is guided by the application of a magnetic field.

11. A method in accordance with any one of claims 5 to 10, wherein the conductive material used to form said layer is initially applied to both said electrode in the form of annular rings surrounding the working surface of the electrodes.

12. A method of manufacturing an overvoltage discharger in accordance with claim 5 and substantially as herein described with reference to Figure 2 of the accompanying drawing.

13. An over-voltage discharger substantially as herein described with reference to Figure 1 of the accompanying drawing.

14. An over-voltage discharger made by the method of any one of claims 5 to 11.