GB1591150A - Gas discharge surge arresters - Google Patents

Abstract

In a gas-discharge overvoltage arrester with a gas-filled case which contains a radioactive substance for the preionisation of its gas filling, it is proposed that the radioactive substance, contained in water glass (9), is attached either to an inside wall part of the tubular insulating body (1) or to one or more electrodes (2, 3) and/or as electrode activation compound or its component to the electrodes (2, 3). This type of construction prevents the workplaces from being contaminated during the production of the overvoltage arrester. <IMAGE>

Description

(54) IMPROVEMENTS IN OR RELATING TO GAS DISCHARGE SURGE ARRESTERS (71) We, SIEMENS AKTIENGESELL SCHAFT, a German Company of Berlin and Munich, German Federal Republic, 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:- The present invention relates to gas discharge surge arresters comprising a gas-filled housing, preferably filled with an inert gas, and a pair of electrodes covered at least partially with an electrode-activation compound having surfaces arranged opposite to one another within the housing, the electrodes being held in spaced relationship by a tubular insulating body, and the housing containing a radioactive material serving to produce an initial ionisation of the gas filling the housing.

Gas-filled surge arresters are gas-discharge components which become conductive when an ignition voltage is exceeded and then divert the current resulting from the excess voltage from other devices were it might cause damage.

The value of the voltage at which protection of these other devices commences is governed by the response voltage of the surge arresters.

The ignition of a surge arrester when the ignition voltage is exceeded is always subject to a delay caused by the need to produce free charge carriers for the ionisation of the gas discharge path to produce ignition. When a voltage which changes with time is connected both to the surge arrester and to the device to be protected, such a delay in ignition also involves a higher response voltage which could endanger the device to be protected. In general, the response voltage of surge arresters rises when the voltage connected to the gas discharge path increases with time.

It is known that the electron concentration in the gas discharge path can be increased by the use of a radioactive substance which emits P-rays to such an extent that the ignition delay caused by the necessity of producing free charge carriers from the electrodes is virtually eliminated (see, for example, German Patent Specification No. 2 445 063). For measuring the response d.c. voltage, the d.c. voltage connected to the surge arrester is increased by about 100 V/s until the surge arrester ignites.

With adequate preliminary ionisation of the gas filling, no fluctuation occurs in the production of free electrons from the cathode so that, with uniform manufacture, a reproducible value is obtained for the response d.c. voltage, provided the other production parameters remain the same. The reproducibility of the response d.c. voltage is of particular significance so far as compliance with the lower level at which protection is provided, and thus as regards the reliability of the protection afforded to the device.

In gas discharge surge arresters, where large time-related voltage changes are concerned, it has been proposed to reduce the ignition delay and thus the response surge voltage by the use of strips of electrically conductive material arranged on the inner wall of the insulating body (so-called ignition strips or ignition lines) from which electrons are freed at high concentration by field electron emission produced by the change in voltage (see, for example, German Patent Specification No. 2 346 174).

The radioactive doping of the discharge chamber of a gas discharge surge arrester, which strongly influences the response d.c.

voltage, has hitherto been effected using glass powder having a low melting point, to the surface of which promethium chloride or promethium hydroxide was attached by adhesion.

For this purpose, the activated glass powder is suspended in an alcoholic liquid and spots of the suspended glass powder deposited in the liquid form are baked. Since adhesion-bonded promethium chloride or promethium hydroxide is not firmly bound to the glass powder, even after fusion of the glass powder, subsequent atomisation of the radioactive substance from the glass powder must always be accepted. Consequently, the carrying out of this process necessitates a high outlay in protective equipment, such as reduced pressure enclosures, suction devices, and work areas completely equipped for radioactive work.

It is an object of the present invention to provide a gas discharge surge arrester which is subject to only a very slight ignition delay and thus has a low response voltage, and in which atomisation of radioactive substance is reduced.

According to the invention, there is provided a gas discharge surge arrester comprising a gas-filled housing and having a pair of electrodes with opposed active surfaces within said housing, at least a part of the active surfaces of said electrodes being covered by an electrode-activating material said electrodes being held in spaced relationship by a tubular insulating body forming part of said housing; wherein to promote the ionisation of the gas filling the housing, a radioactive material contained in a water glass (as hereinbefore defined) is applied to a part of the inner wall of said insulating body, and/or is applied to at least one of the electrodes, The radioactive material contained in a water-glass may be applied to the electrode or electrodes in addition to the electrode-activating material, or it may also form the electrode-activating material, or a component thereof.The radioactive substance is preferably promethium chloride.

By the term "water glass" as used herein is meant an alkali metal silicate in aqueous solution. Sodium silicate or potassium silicate are preferably used for this purpose. Sodium silicate is water-soluble and will take up an aqueous solution of promethium chloride.

Since the radioactivity of promethium chloride is extremely high (1 mg having a radioactivity of about 1 Ci), the amount of promethium chloride added to the aqueous sodium silicate solution is so small that there is no danger of the promethium silicate formed being precipitated. Usually, a suitable dispensing device is used to deposit on the insulating body or on the electrodes of the gas discharge surge arrester, a spot of the radioactively doped water glass having a radioactivity of 1 to 7 ,umCi.

The firmly adhering spots of water glass allow the insulating bodies or electrodes to be processed to form complete surge arresters without atomised radioactive material being emitted. The unpleasant contamination of the work areas observed in the case of doping with radioactive glass powder is thus avoided in a reliable way. During the subsequent fusion of the electrode coating, the previously dried water glass loses its remaining water and adheres strongly to the inner wall of the insulating body or to the electrodes.

Since sodium silicate and potassium silicate can also be used as electrode-activating materials, i.e. in order to facilitate electron emission therefrom, the radioactive doping material can be added to the electrode activation means, preferably using potassium silicate as the activation material, the water present being completely eliminated prior to the fusion of the coating. The doped potassium silicate can be reliably secured to the electrode by suitable shaping of its active surface, e.g. by giving it a honeycomb formation, the potassium silicate being secured in the interstices of the honeycomb.

Finally, it is possible with advantage to use an electrode-activation material doped with promethium chloride in water glass in embodiments in which the electrodes are of the "hollow cathode" type, e.g. in "button" arresters employing such electrodes. In this case, the doped activation material may be lodged on the base of the "hollow cathode" within a cathode ring welded to the base.

The invention will now be further described with reference to the drawing, in which: Figure 1 is a schematic side-sectional view of a first form of gas discharge surge arrester in accordance with the invention; Figure 2 is a schematic side-sectional view of a second form of gas discharge surge arrester in accordance with the invention; and Figure 3 is a schematic side-sectional view of a third form of gas discharge surge arrester in accordance with the invention.

The gas discharge surge arrester shown in Figure 1, because of its shape which permits arresters of very small dimensions to be produced, is often referred to as a "button" arrester. This button arrester has two frustoconical electrodes 2 and 3 which are inserted in gas-tight fashion into a tubular electrically insulating body 1 with their flat end portions facing towards one another. The housing formed by the tubular body 1 and the electrodes 2 and 3 is filled with gas, preferably an inert gas.

A layer 4 made of a material having a high thermal electron emission capacity is applied to the opposed active surfaces of the electrodes 2 and 3, the material being secured in recesses 7 formed in these active surfaces. In this exemplary embodiment a strip 8 of electrically conductive material (a so-called ignition line) is provided on the inner wall of the insulating body 1 and extends in the direction from one electrode towards the other. A spot 9 of water glass containing a radioactive material is applied to the inner wall of the insulating body 1 at the level of the discharge path between the active surfaces of the electrodes.

In the gas discharge surge arrester illustrated in Figure 2, frusto-conical electrodes 2 and 3 are again inserted in gas-tight fashion into the ends of a tubular insulating body 1 and are secured thereto by glass seals 6, so that the end faces of the electrodes are opposed and form the active surfaces. The housing formed by the electrodes 2 and 3 and the insulating body 1 is filled with gas, preferably an inert gas. In this embodiment, the electrodes 2 and 3 are each provided on their facing active surfaces with a metal ring 5 which converts the electrodes 2 and 3 into so-called "hollow cathode" electrodes Each ring 5 preferably consists of iron and is welded to its electrode. A layer 4 consisting of a material having a high electron emission capacity is applied to the electrodes 2 and 3 within the rings 5 so that it is lodged in the cavity formed between the electrode surface and the ring 5. Because of the size of this cavity, the activation material of the layer 4 can be applied in a relatively large amount, whilst simultaneously ensuring good adhesion.

Consequently, the electrodes 2 and 3 provided with the ring 5 act like supply cathodes of electron tubes. The radioactive substance contained in the water glass is again applied in the form of a spot 9 to the inner wall of the insulating body 1. However, in this case, it can advantageously also or alternatively be contained in the activation layer 4, or can itself form this layer. This also applies to the gas discharge surge arrester illustrated in Figure 3.

The gas discharge surge arrester illustrated in Figure 3 comprises two concentrically arranged electrodes 2 and 3. In this embodiment, the electrode 2 is in the form of a solid cylinder and projects into the electrode 3 which is in the form of a hollow cylinder. The electrodes 2 and 3 are secured to metal caps 11 and 10 respectively, which serve as electrical terminals and are electriclaly insulated from one another by a tubular insulating body 1.

Together with the two metal caps 10 and 11, the insulating body 1 forms a gas-tight housing for the gas discharge surge arrester which is filled with a gas, preferably an inert gas. The gas-tight connections between the insulating body 1 and the metal caps 10 and 11 may be established, for example, by means of a glass solder layer 6. A strip 8 consisting of electrically conductive material (a so-called ignition strip or ignition line) is applied to the upper inner surface of the insulating body 1. The electrodes 2 and 3 are each provided with a layer 4 of electrode-activation material on their facing active surfaces. The radioactive material contained in water glass is applied in the form of a spot 9 to the insulating body 1, and/or to the electrode cap 10.

WHAT WE CLAIM IS: 1. A gas discharge surge arrester comprising a gas-filled housing and having a pair of electrodes with opposed active surfaces within said.

housing, at least a part of the active surfaces of said electrodes being covered by an electrode activating material; said electrodes being held in spaced relationship by a tubular insulating body forming part of said housing; wherein to promote the ionisation of the gass filling the housing, a radioactive material contained in a water glass (as hereinbefore defined) is applied to a part of the inner wall of said insulating body, and/or is applied to at least one of the electrodes.

2. A gas discharge surge arrester as claimed in Claim 1, wherein said radioactive material contained in a water glass is applied to said electrode or electrodes additionally to said electrode-activating material.

3. A gas discharge surge arrester as claimed in Claim 1, wherein said radioactive material contained in a water glass also forms said electrode-activating material, or a component thereof.

4. A gas discharge surge arrester as claimed in any one of Claims 1 to 3, wherein the water glass is sodium silicate or potassium silicate.

5. A gas discharge surge arrester as claimed in any one of the preceding Claims, wherein the radioactive substance is promethium chloride.

6. A gas discharge surge arrester as claimed in any one of the preceding Claims, wherein said insulating body is provided with a strip of electrically conductive material extending over at least a part of the inner wall thereof.

7. A gas discharge surge arrester substantially as hereinbefore described with reference to and as illustrated in Figure I, or Figure 2, or Figure 3, of the drawing.

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

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. can be applied in a relatively large amount, whilst simultaneously ensuring good adhesion. Consequently, the electrodes 2 and 3 provided with the ring 5 act like supply cathodes of electron tubes. The radioactive substance contained in the water glass is again applied in the form of a spot 9 to the inner wall of the insulating body 1. However, in this case, it can advantageously also or alternatively be contained in the activation layer 4, or can itself form this layer. This also applies to the gas discharge surge arrester illustrated in Figure 3. The gas discharge surge arrester illustrated in Figure 3 comprises two concentrically arranged electrodes 2 and 3. In this embodiment, the electrode 2 is in the form of a solid cylinder and projects into the electrode 3 which is in the form of a hollow cylinder. The electrodes 2 and 3 are secured to metal caps 11 and 10 respectively, which serve as electrical terminals and are electriclaly insulated from one another by a tubular insulating body 1. Together with the two metal caps 10 and 11, the insulating body 1 forms a gas-tight housing for the gas discharge surge arrester which is filled with a gas, preferably an inert gas. The gas-tight connections between the insulating body 1 and the metal caps 10 and 11 may be established, for example, by means of a glass solder layer 6. A strip 8 consisting of electrically conductive material (a so-called ignition strip or ignition line) is applied to the upper inner surface of the insulating body 1. The electrodes 2 and 3 are each provided with a layer 4 of electrode-activation material on their facing active surfaces. The radioactive material contained in water glass is applied in the form of a spot 9 to the insulating body 1, and/or to the electrode cap 10. WHAT WE CLAIM IS:

1. A gas discharge surge arrester comprising a gas-filled housing and having a pair of electrodes with opposed active surfaces within said.

housing, at least a part of the active surfaces of said electrodes being covered by an electrode activating material; said electrodes being held in spaced relationship by a tubular insulating body forming part of said housing; wherein to promote the ionisation of the gass filling the housing, a radioactive material contained in a water glass (as hereinbefore defined) is applied to a part of the inner wall of said insulating body, and/or is applied to at least one of the electrodes.

2. A gas discharge surge arrester as claimed in Claim 1, wherein said radioactive material contained in a water glass is applied to said electrode or electrodes additionally to said electrode-activating material.

3. A gas discharge surge arrester as claimed in Claim 1, wherein said radioactive material contained in a water glass also forms said electrode-activating material, or a component thereof.

4. A gas discharge surge arrester as claimed in any one of Claims 1 to 3, wherein the water glass is sodium silicate or potassium silicate.

5. A gas discharge surge arrester as claimed in any one of the preceding Claims, wherein the radioactive substance is promethium chloride.

6. A gas discharge surge arrester as claimed in any one of the preceding Claims, wherein said insulating body is provided with a strip of electrically conductive material extending over at least a part of the inner wall thereof.

7. A gas discharge surge arrester substantially as hereinbefore described with reference to and as illustrated in Figure I, or Figure 2, or Figure 3, of the drawing.