US3885203A

US3885203A - Excess voltage arresters

Info

Publication number: US3885203A
Application number: US453322A
Authority: US
Inventors: Basil Offor Baker; John William Ellis Ross
Original assignee: MO Valve Co Ltd
Current assignee: DUBILIER INTERNATIONAL PLC DUBILIER HOUSE RADLEY RD ABINGDON OXFORDSHIRE OX14 3XA ENGLAND A BRITISH Co; McGraw Edison Co
Priority date: 1973-03-23
Filing date: 1974-03-21
Publication date: 1975-05-20
Anticipated expiration: 1992-05-20
Also published as: GB1411492A

Abstract

An excess voltage arrester comprising a pair of spaced electrodes housed in a gas-filled enclosure wherein the electrodes carry sleeves of electrically insulating material to prevent reduction of the d.c. ignition voltage of the arrester due to the formation of conductive coatings on the internal surfaces of insulating portions of the envelope by sputtering from the discharge occurring in the arrester in operation.

Description

United States Patent [1 1 Baker et al.

[ 1 EXCESS VOLTAGE ARRESTERS [75] Inventors: Basil Offor Baker, Richmansworth;

John William Ellis Ross, Sidcup, both of England [73] Assignee: The M-O Valve Company Limited, London. England [22] Filed: Mar. 21, 1974 [2]] Appl. No.: 453,322

[30] Foreign Application Priority Data Mar. 23, 1973 United Kingdom [4138/73 [52] US. Cl 317/61; 313/231.1 [51] Int. Cl. H02h 9/04 [58] Field of Search 317/61, 61.5, 62, 66, 68; 313/214, 220, 231.1 5 6] References Cited UNITED STATES PATENTS 3,289,027 11/1966 Jones 317/61 X [11} 3,885,203 51 May 20, 1975 Primary Examiner-G. Harris Attorney, Agent, or FirmKirschstein, Kirschstein, Ottinger & Frank [57] ABSTRACT An excess voltage arrester comprising a pair of spaced electrodes housed in a gas-filled enclosure wherein the 5 Claims, 1 Drawing Figure PATENTEB MAY 2 0 I975 EXCESS VOLTAGE ARRESTERS This invention relates to excess voltage arresters.

A known form of excess voltage arrester comprises: a pair of spaced electrodes housed in a hollow cylindrical, gas-filled enclosure comprising: electrically conductive end portions and an electrically conductive tubular central portion which is sealed to and spaced from the end portions by respective tubular electrically insulating portions, the end portions constituting re spective terminals for the two electrodes, the two electrodes extending towards each other through the insulating portions so that the gap between the electrodes lies within the central portion. Such an excess voltage arrester is hereinafter referred to as an arrester of the kind specified. One such arrester is described in UK. Patent Specification No. 1,021,782.

In normal use of an arrester of the kind specified the two electrodes are respectively connected via their terminals to two lines, and the central portion of the enclosure is grounded. When an excess voltage appears between either one of the lines and the other line, or ground, a discharge occurs in the gas-filled enclosure, thus effectively grounding the lines.

In an arrester of the kind specified a discharge is intended to occur between the inner end of one or both the electrodes and the central portion of the enclosure, and the gap between these elements is chosen in conjunction with the pressure and composition of the gas filling to confer a desired operating voltage on the arrester.

It has been found that with known arresters of the kind specified, after a large number of discharges have occurred, subsequent discharges tend to occur at an intermediate point along the length of one or both electrodes, at a dc. ignition voltage lower than the designed voltage.

It is an object of the present invention to provide an arrester of the kind specified wherein this difficulty is overcome.

According to the present invention in an arrester of the kind specified each electrode carries a sleeve of electrically insulating material which extends from a point near the inner end of that electrode to a point within the insulating portion of the enclosure through which that electrode extends.

In a preferred arrangement in accordance with the invention the outer end of each said sleeve lies adjacent a section of the associated electrode which is a sliding fit in the associated insulating portion of the enclosure.

The invention resides in the appreciation by the inventors that the failure of prior art arresters results from the formation by sputtering from the discharge of conductive coatings on the internal surfaces of the insulating portions of the enclosure. The formation of such films is enhanced by a tendency for the cathode spot of the discharge occurring in operation to run along the surface of the electrode on which it is formed towards the end of the arrester.

In an arrester in accordance with the invention the sleeves reduce the likelihood of the formation of conductive coatings on the internal surfaces of the insulating portions of the enclosure.

In a particular arrangement in accordance with the invention each said sleeve is effective to increase the breakdown voltage between any point on the surface of the electrode carrying that sleeve and a conductive coating which may be produced in operation on the internal surface of the adjacent insulating portion of the enclosure to a value above the dc ignition voltage across the gap between the inner end of that electrode and the adjacent part of the central portion of the enclosure.

One arrangement in accordance with the invention will now be described, by way of example, with refer ence to the accompanying drawing which is a sectional view of a gas-filled excess voltage arrester. Referring to the drawing, the arrester includes a hermetically sealed, hollow cylindrical, gas-filled enclosure comprising two cup-shaped 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, locating on chamfers 6, and at the other end into opposite ends of the central enclosure member 3. To this end, the outer surface of each of the ceramic members 4 and 5 is metallised in two

bands

8 and 9, annular seals being established between the inter-fitting parts by brazing during assembly.

Within the gas-filled enclosure are housed a pair of axially spaced

electrodes

10 and 11. The electrode 10 is formed in two parts 12 and 13 which are brazed together, end on. The outer part 12 lies mainly within the ceramic envelope member 4 and comprises three

sec tions

12a, 12b and 12c. The central major section 12!) is a sliding fit in the ceramic member 4. The relatively short outer section 12a is of reduced external diameter and is brazed to the end cap 1. The inner section 12c is of similarly reduced external diameter, has a length about half that of the central section 12b, and at its inner end is brazed to the other part 13 of electrode 10. The part 13 also comprises three

sections

13a, 13b and 13c, the section 130 adjacent section 12c of part 12 being of smaller diameter than section and of relatively short length. The central major section 1312 is of the same external diameter as section 12c and has a length such that the inner section 130, which is of relatively large external diameter, lies near the centre of the metal member 3 of the enclosure. The parts 12 and 13 have communicating

axial bores

12d and 13d, the bore 12d being enlarged at its outer end to accommodate a pumping stem 23 which is pinched-off in conventional manner after the required gas-filling has been inserted in the enclosure.

The other electrode 11 is brazed to the end cap 2 at one end and extends through the ceramic member 5 so that, its other end lies near to, but spaced from, the corresponding end of the electrode 10, the electrode 11 being otherwise identical with the electrode 10 except that it is not provided with a pumping stem.

In the drawing the two parts of the electrode 11 are given reference numerals 14 and 15 with suffixes a, b and c, as for electrode 10.

The gap 16 between the electrodes 10 and ll is made to have the same d.c. ignition voltage as the gaps l7 and 18 between the central enclosure member 3 and the innermost sections 13c and 15c of the electrodes.

A ceramic sleeve 19 fits closely around sections 120 and 13b of electrode 10, with its outer end abutting section 12b, and a corresponding sleeve 20 fits closely around sections 14c and 15b of electrode 11.

In manufacture of the arrester each end cap 1 or 2 is first brazed to its associated electrode 10 or II (with

ceramic sleeve

19 or 20 in position) and to ceramic member 4 or S to form a sub-assembly, the brazing being carried out with the electrode axis vertical and the end cap lowermost, and with rings of a suitable brazing material placed around the

sections

12a and 13a of electrode 10 and the corresponding sections of electrode 11. The two sub-assemblies are then sealed into opposite ends of the central member 3 of the en' closure. and the required gas filling is inserted via the pumping stem 23.

In use of the arrester, the end caps 1 and 2 are re spectively connected to a pair of lines connected with the equipment which it is desired to protect against excess voltage. and the metal enclosure member 3 is grounded. On the occurrence of an excess voltage be tween either one of the lines and the other line, or ground, a discharge occurs between one or both

electrodes

10 and 11 and the metal member 3 of the enclosure. It will be appreciated that while initially the discharge may occur between the two

electrodes

10 and 11 the discharge will rapidly transfer to the gap between one or both electrodes and the grounded member 3. 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 l6, l7 and 18.

When a discharge takes place in the arrester, sputtered material from the discharge forms electrically conductive coatings on the internal surfaces of the metal member 3 and the ceramic members 4 and 5 of the enclosure. The formation of these coatings is enhanced due to a tendency of the cathode spot of the discharge to run along the surface of the electrode on which it is formed towards the nearer end cap 1 or 2. It will be appreciated that the abovementioned coatings are electrically connected to ground in operation via the metal member 3. Hence, if coatings are formed which extend along the inner curved surfaces of the ceramic members 4 and 5 towards the

electrode sections

12b and 14b, the operating voltage of the arrester will eventually be very significantly reduced, and ultimately the arrester will short circuit the lines with which it is connected.

In accordance with the invention, the sleeves l9 and 20 substantially restrict the formation of such conductive coatings to the end faces 21 and 22 of the ceramic members 4 and 5. This is achieved partly by virtue of the gap between the

sleeves

19 and 20 and the ceramic members 4 and 5 being too small to allow easy access of the sputtered material to the internal curved surfaces of members 4 and 5, and partly by virtue of the

sleeves

19 and 20 preventing the cathode spot of the discharge from moving very far towards an end cap 1 or 2 in operation.

In addition, the presence of the

sleeves

19 and 20 makes the shortest gap (via the gas filling) between a conducting coating on an

end face

21 or 22 and the

adjacent electrode

10 or 11 larger than the gaps l7 and 18. Thus, the presence of the

sleeves

19 and 20 increases the breakdown voltage between a conducting coating on an

end face

21 or 22 and the

adjacent electrode

10 or 11 to a value above the d.c. ignition voltage set by the gaps 17 and 18, thus preventing the coating reducing the dc ignition voltage of the arrester.

ln one particular arrester of the form shown in the drawing having a dc. ignition voltage of 400 volts, the gas filling consists of a 10% hydrogen argon mixture at a pressure of 50 torr, the radial spacing between the external surfaces of the

electrodes sections 12c

13b, 14c and 15b and the internal surfaces of ceramic members 4 and 5 is 0.95 millimetres and the

sleeves

19 and 20 have internal and external diameters of 2.92 millimetres and 3.98 millimetres respectively and consist of 98% alumina.

In the arrester shown in the drawing the adjacent sections 13c and 15c of the

electrodes

10 and 11 are of relatively large external diameter, but this need not be the case in an arrangement in accordance with the invention. Thus in alternative arrangements the portions of the electrodes projecting inwardly beyond the sleeves l9 and 20 may be of the same diameter as the portions within the sleeves.

It is pointed out that while the particular arrester shown in the drawing is designed for connection in an external circuit by means of clips or the like contacting the end caps 1 and 2 and the enclosure metal member 3, other arresters in accordance with the invention may be provided with lead wires for external connection.

We claim:

1. An excess voltage arrester comprising: a hollow cylindrical gas filled enclosure comprising a tubular electrically conductive central portion, a pair of electrically conductive end portions, and a pair of tubular electrically insulating portions by means of which said end portions are respectively sealed to and spaced from said central portion; a pair of spaced electrodes housed within the enclosure for which said end portions serve as respective terminals, said electrodes extending towards one another through said insulating portion of the enclosure so that the gap between the electrodes lies within the central portion of the enclosure; and a pair of sleeves of electrically insulating material carried on said electrodes respectively. each sleeve extending from a point near the inner end of the associated electrode to a point within the insulating portion of the enclosure through which the associated electrode extends.

2. An arrester according to claim 1 wherein the outer end of each said sleeve lies adjacent a section of the associated electrode which is a sliding fit in the associated insulating portion of the enclosure.

3. An arrester according to claim 1 wherein each said sleeve is effective to increase the breakdown voltage between any point on the surface of the electrode carrying that sleeve and a conductive coating which may be produced in operation on the internal surface of the adjacent insulating portion of the enclosure to a value above the dc. ignition voltage across the gap between the inner end of that electrode and the adjacent part of the central portion of the enclosure.

4. an arrester according to claim 3 wherein each said sleeve projects inwardly beyond the inner end of the adjacent insulating portion of the enclosure by an amount sufficient to make the shortest gap, via the gasfilling, between said end and the electrode carrying that sleeve appreciably larger than the gap between the inner end of that electrode and the adjacent part of the central portion of the enclosure.

5. An arrester according to claim 4 wherein the por tions of the electrodes projecting inwardly beyond the sleeves are of larger diameter than the portions of the electrodes within said sleeves.

Claims

1. An excess voltage arrester comprising: a hollow cylindrical gas filled enclosure comprising a tubular electrically conductive central portion, a pair of electrically conductive end portions, and a pair of tubular electrically insulating portions by means of which said end portions are respectively sealed to and spaced from said central portion; a pair of spaced electrodes housed within the enclosure for which said end portions serve as respective terminals, said electrodes extending towards one another through said insulating portion of the enclosure so that the gap between the electrodes lies within the central portion of the enclosure; and a pair of sleeves of electrically insulating material carried on said electrodes respectively, each sleeve extending from a point near the inner end of the associated electrode to a point within the insulating portion of the enclosure through which the associated electrode extends.

3. An arrester according to claim 1 wherein each said sleeve is effective to increase the breakdown voltage between any point on the surface of the electrode carrying that sleeve and a conductive coating which may be produced in operation on the internal surface of the adjacent insulating portion of the enclosure to a value above the d.c. ignition voltage across the gap between the inner end of that electrode and the adjacent part of the central portion of the enclosure.

4. an arrester according to claim 3 wherein each said sleeve projects inwardly beyond the inner end of the adjacent insulating portion of the enclosure by an amount sufficient to make the shortest gap, via the gas-filling, between said end and the electrode carrying that sleeve appreciably larger than the gap between the inner end of that electrode and the adjacent part of the central portion of the enclosure.

5. An arrester according to claim 4 wherein the portions of the electrodes projecting inwardly beyond the sleeves are of larger diameter than the portions of the electrodes within said sleeves.