GB2181887A - Electrode of surge arrester - Google Patents

Abstract

In order to produce a low glow-to-arc transition current between the electrodes of a surge arrester, the electrodes are coated with the halide of metal selected from the group of Barium, Rubidium, Sodium, Caesium and Potassium. A preferred coating is of Barium Iodide. As shown, the gas-filled enclosure houses electrodes 8,9 whose spaced ends 18 are enclosed by a conductive liner 6 and each have a grooved head portion 11 provided with a coating 19 of barium iodide. <IMAGE>

Description

SPECIFICATION Improvements in or relating to electrodes This invention relates to electrodes, and particularly though not exclusively, to electrodes for use in excess voltage arresters.

A well known type of excess voltage arresters comprises two electrodes housed in a gas-filled enclosure to define between them a gap across which an electric discharge occurs in operation if the voltage between the electrodes exceeds a predetermined value.

In such an arrester a low glow-to-arc transition current is desirable to reduce heat dissipation whilst anchoring the arc helps to prevent the deposition of sputtered material onto insulating surfaces of the arrester with a consequent reduction in the operating voltage and/or insulation resistance of the arrester. Attempts to achieve this have been made by coating at least one of the electrodes with barium, strontium and calcium oxides. However, these materials tend to have poor adhesion to the material of the electrode, and their high resistance when cold can result in rupturing of the coating when a high current discharge is passed.

It is an object of the present invention to reduce these difficulties in coated electrodes.

According to the present invention, an electrode for an excess voltage arrester of the kind specified is coated with a coating consisting at least partly of a halide of an alkali metal.

The arrester in which one or more electrodes according to the invention may be incorporated may comprise a hollow cylindrical body which houses two electrodes which extend towards one another from opposite ends of the enclosure to define between their ends a gap which lies within an electrically conductive central portion of the enclosure which is electrically insulated from said electrodes, each said electrode having on its inner end a coating consisting at least partly of a halide of an alkali metal.

Such an arrester is suitable for use in protecting equipment connected to a pair of telephone lines from lightning strikes, the two electrodes being respectively connected in use to the two lines, and the central enclosure portion being grounded.

One arrester including a pair of electrodes coated in accordance with the invention will now be described, by way of example, with reference to the accompanying drawing which is a sectional view of the arrester.

Referring to the drawing, the arrester includes a hermetically sealed, hollow cylindrical, gas-filled enclosure comprising two cupshaped metal end caps 1 and 2, a tubular metal central member 3 and two tubular ceramic members 4 and 5. The ceramic members 4 and 5 are sealed at one end into the end caps 1 and 2, and at the other end into opposite ends of the central enclosure member 3.

A tubular metal liner 6, which suitably consists of a copper-nickel alloy, is disposed inside the central enclosure member 3 between the ceramic members 4 and 5. The liner 6 is secured in position by indents 7 in the central enclosure member adjacent the ends of the liner 6.

Within the gas-filled enclosure are housed a pair of axially spaced, one-piece, cylindrical, copper electrodes 8 and 9. Each of the electrodes 8 and 9 has an outwardly extending flange 10 at its outer end and a head portion 11 at its inner end which is joined to the remainder of the electrode by a portion 12 of reduced diameter. An axial bore 13 extends a short way into the electrode 8 from its flanged end, the inner end of the bore 13 communicating with a radial bore 14, and the bore 13 being enlarged over its outer portion to accommodate a pumping stem 15 which in pinched-off in conventional manner after the required gas-filling has been inserted in the enclosure.

The electrode 8 is positioned so as to extend coaxially through the ceramic member 4 with its flange 10 sealed between the base of the end cap 1 and the adjacent end of the ceramic member 4, the dimensions of the electrode 8 and the ceramic member 4 being such that the portion 12 of reduced diameter of the electrode 8 lies adjacent the end of the ceramic member 4 remote from the cap 1.

The other electrode 9 is similarly positioned with respect to the ceramic member 5 with its flange 10 sealed between the member 5 and the base of the end cap 2.

The gap 16 between the electrodes 8 and 9, which lies within the liner 7, is made to have substantially the same d.c. ignition voltage as the gaps 17 between the liner 7 and the curved surfaces of the head portions 11 of the electrodes.

A helical groove 18 is cut in the curved surface of the head portion 11 of each of the electrodes 8 and 9, and each grooved head portion 11 is provided with a coating 19 of barium iodide.

In use of the arrester, the end caps 1 and 2 are respectively connected to a pair of lines e.g. telephone lines connected with the equipment which it is desired to protect against excess voltage, and the metal enclosure member 3, and hence the liner 6, is grounded. On the occurrence of an excess voltage between either one of the lines and the other line, or ground, a discharge occurs between one or both electrodes 8 and 9 and the liner 6. It will be appreciated that while initially the discharge may occur between the two electrodes 8 and 9 the discharge will rapidly transfer to the gap 1 7 between one or both electrodes and the grounded liner 6.A desired d.c. ignition voltage for the arrester, that is the voltage at which a discharge is initiated, is obtained by appropriate choice of the pressure and composition of the gas-filling and the sizes of the gaps 16 and 17.

The presence of the coatings reduces heat dissipation in the arrester when a discharge forms. The reason for this is as follows.

Gas-filled excess voltage arresters operate for part of the time in a so-called 'glow' mode, when the voltage drop across the arrester is in the region of 200 volts and heat dissipation is relatively high, and for part of the time in a so-called 'arc' mode, when the arrester voltage drop is in the region of 30 volts and the heat dissipation in the arrester is relatively low. The coatings 19 reduce the glow-to-arc transition current with the result that the arrester runs in the glow mode for a shorter time and less heat is dissipated in the arrester in operation. The reason for the reduction in transition current is a combination of the increase in glow current density due to the coatings, and an electron-emissive effect at the edges of the coatings. The latter effect is enhanced by the helical form of the coatings.

In addition to reducing heat dissipation the coatings 19 also serve to anchor the arc discharge in operation to the head portions 11 of the electrodes. This is desirable because sputtered material from the the discharge forms electrically conductive deposits on the internal surfaces of the liner 6 and the ceramic members 4 and 5 which are electrically connected to ground via the metal member 3. Consequently, if such deposits extend along the inner curved surfaces of the ceramic members 4 and 5 towards the end caps 1 and 2, the operating voltage of the arrester will eventually be significantly reduced, and ultimately the arrester may even short circuit the lines to which it is connected.

By anchoring the discharge the formation of such conductive deposits is substantially restricted to the exposed end faces of the ceramic members 4 and 5 and the operating voltage of the arrester is not significantly affected.

The use of a halide of an alkali metal such as barium iodide for the coatings instead of conventionally used materials such as barium, strontium and caicium oxides has the advantage that the coatings exhibit high electrical conductivity and are therefore not subject to rupture when a high discharge current is passed. Whilst Barium lodide has proved to be highly satisfactory in performance, Rubidium, Sodium, Caesium and Potassium lodide, together with the Chlorides of all the previously mentioned alkali metals also provide satisfactory results.

The coatings are suitably formed by painting the relevant electrode surfaces with a saturated solution of the halide in an organic solvent which can be industrial methylated spirits or even water. The coatings are then sintered at a temperature of 800"C to cause the mixture to decompose and produce the halide coating.

Sintering of the whole arrester is normally required as a step in its manufacture to seal the various parts of the arrester together. The sintering of the coatings can thus conveniently be carried out at the same time.

A particular advantage derived from coating the electrodes as just described is that during load testing, for example when incorporated in an arrester, the confinement of the arc as a result of the coating, substantially reduces sputtering.

Another advantage is that the coatings can be easily and rapidly applied to any type and shape of electrode.

Claims (5)

1. An electrode for an excess voltage arrester and coated with a coating consisting at least partly of a halide of an alkali metal.

2. An excess voltage arrester comprising a hermetically sealed, hollow cylindrical gas-filled body, a pair of electrodes mounted within said body and defining a gap, an electrically conductive enclosure surrounding the adjacent ends of the electrodes and electrically insulated therefrom, said enclosure in use of the arrester being connected to ground, and a coating of a halide of an element selected from the group of Barium, Rubidium, Sodium, Caesium and Potassium deposited on said adjacent ends of said electrodes.

3. A surge arrester as claimed in Claim 2, wherein said coating is Barium lodide.

4. A surge arrester as claimed in Claim 3, wherein each electrode is formed with a helical groove.

5. A surge arrester substantially as hereinbefore described with reference to the accompanying drawings.