GB2046009A

GB2046009A - Excess voltage arrester

Info

Publication number: GB2046009A
Application number: GB8009316A
Authority: GB
Original assignee: MO Valve Co Ltd
Current assignee: MO Valve Co Ltd
Priority date: 1979-03-19
Filing date: 1980-03-19
Publication date: 1980-11-05
Also published as: GB2046009B

Abstract

in an excess voltage arrester of the kind comprising a gas-filled enclosure (1, 3, 5) housing a pair of electrodes (7, 9) which define between them a discharge gap, and a coating (13 or 15) of insulating material on at least one region of the surface of at least one of the electrodes which serves to reduce the glow-to-arc transition current of the arrester, the discharge gap is defined between the internal surface of a substantially cup- shaped portion of one electrode (7) and the surface of a cylindrical portion of the other electrode (9) which extends substantially axially into said cup-shaped portion. The construction is found to reduce rise in d.c. strike voltage at high peak currents. <IMAGE>

Description

SPECIFICATION Excess voltage arresters This invention relates to excess voltage arresters.

The invention relates particularly to excess voltage arresters of the kind comprising a gasfilled enclosure housing a pair of electrodes which define between them a discharge gap.

In order to improve the performance of such arresters one or more regions of the surface of the or each electrode are sometimes provided with a coating of insulating material. The coating or coatings serve to reduce the glow-to-arc transition current of the arrester, and consequently reduce heat dissipation in operation. An especially suitable insulating material is borosilicate glass, but other materials such as magnesia and alumina may be used. Typically coatings covering a number of small spaced regions on each electrode surface are provided. This maximises the ratio of edge length to area of the coatings and thereby improves performance since the reduction in glow-to-arc transition current appears to result in part from an emissive effect at the edges of the coatings.

It has been found that excess voltage arresters of the kind specified provided with such coatings exhibit an unacceptably high rise in d.c. strike voltage at high peak currents and it is an object of the present invention to provide an excess voltage arrester of the kind specified wherein this problem is overcome.

According to the present invention in an excess voltage arrester comprising a gas-filled enclosure housing a pair of electrodes which define between them a discharge gap, and a coating of insulating material on at least one region of the surface of at least one of the electrodes which serves to reduce the glow-to-arc transition current of the arrester, the discharge gap is defined between the internal surface of a substantially cup-shaped portion of one electrode and the surface of a cylindrical portion of the other electrode which extends substantially axially into said cup-shaped portion.

Preferably the enclosure is of hollow cylindrical form and said cup-shaped and cylindrical portions of the electrodes are disposed substantially coaxially within the enclosure.

One excess voltage arrester 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 cup-shaped metal end caps 1 and 3 and a tubular ceramic member 5, the caps 1 and 3 being sealed onto opposite ends of the ceramic member 5.

Within the gas-filled enclosure are housed a pair of electrodes which are respectively electrically connected with the end caps 1 and 3.

Thus, one electrode comprises a cup-shaped metal member 7 disposed coaxially within the enclosure with its base sealed to the base of the end cap 1. The other electrode comprises a solid cylindrical metal member 9 disposed coaxially within the enclosure and sealed at one end to the centre of the base of a cup-shaped metal member 11 which is sealed around its mouth to be the base of the end cap 3.

At its free end the member 9 projects within the member 7, and the part of member 9 within the cup is provided with a helical surface coating 13 of glass. The internal surface of the member 7 is also provided with a glass coating 1 5.

In use the arrester is connected across the possible excess voltage. Typically, the end caps are connected to a pair of lines connected with an equipment which it is desired to protect against excess voltage.

On the occurrence of a voltage between the end caps 1 and 3 in excess of the arrester d.c.

strike voltage, a discharge occurs between the electrodes 7 and 9. The desired d.c. strike voltage is obtained by appropriate choice of the pressure and composition of the gas-filling, and the geometry of the electrode structure.

The rise in d.c. strike voltage at high peak discharge currents e.g. 10,000 amps with an 8/20 micro-second current pulse, is found to be quite acceptable. In one particuiar arrester of the form shown in the drawing with a nominal d.c. strike voltage of 300 volts, the rise was typically from zero to 100 volts. In comparable prior art arresters having electrodes of cup-shaped form defining a discharge gap between the outer surfaces of their bases, rises from 300 volts to 700 volts typically occur under these conditions.

The reason for this improvement with an arrester according to the invention is not fully understood. However, it is thought that the rise in d.c. strike voltage with prior art arresters is due to gas evolved from the insulating coatings at the very high temperatures occurring at high peak discharge currents. The improvement with an arrester according to the invention is consequently thought to be due to the fact that the electrode geometry minimises heating of the coatings, and thereby reduces gas evolution. In this connection it is pointed out that insulating materials such as borosilicate glasses which most effectively reduce glow-to-arc transition current tend to evolve relatively large amounts of gas.

A further advantage exhibited by an arrester according to the invention is that under overload conditions both electrodes will melt and move into electrical contact with the other. The arrester thus exhibits reliable fail-short characteristics.

It will be understood that while the arrester described by way of example has end caps for external connection, other arresters in accordance with the invention may be provided with other means for external connection such as lead wires or tags.

1. An excess voltage arrester comprising a gasfilled enclosure housing a pair of electrodes which define between them a discharge gap, and a

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Excess voltage arresters This invention relates to excess voltage arresters. The invention relates particularly to excess voltage arresters of the kind comprising a gasfilled enclosure housing a pair of electrodes which define between them a discharge gap. In order to improve the performance of such arresters one or more regions of the surface of the or each electrode are sometimes provided with a coating of insulating material. The coating or coatings serve to reduce the glow-to-arc transition current of the arrester, and consequently reduce heat dissipation in operation. An especially suitable insulating material is borosilicate glass, but other materials such as magnesia and alumina may be used. Typically coatings covering a number of small spaced regions on each electrode surface are provided. This maximises the ratio of edge length to area of the coatings and thereby improves performance since the reduction in glow-to-arc transition current appears to result in part from an emissive effect at the edges of the coatings. It has been found that excess voltage arresters of the kind specified provided with such coatings exhibit an unacceptably high rise in d.c. strike voltage at high peak currents and it is an object of the present invention to provide an excess voltage arrester of the kind specified wherein this problem is overcome. According to the present invention in an excess voltage arrester comprising a gas-filled enclosure housing a pair of electrodes which define between them a discharge gap, and a coating of insulating material on at least one region of the surface of at least one of the electrodes which serves to reduce the glow-to-arc transition current of the arrester, the discharge gap is defined between the internal surface of a substantially cup-shaped portion of one electrode and the surface of a cylindrical portion of the other electrode which extends substantially axially into said cup-shaped portion. Preferably the enclosure is of hollow cylindrical form and said cup-shaped and cylindrical portions of the electrodes are disposed substantially coaxially within the enclosure. One excess voltage arrester 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 cup-shaped metal end caps 1 and 3 and a tubular ceramic member 5, the caps 1 and 3 being sealed onto opposite ends of the ceramic member 5. Within the gas-filled enclosure are housed a pair of electrodes which are respectively electrically connected with the end caps 1 and 3. Thus, one electrode comprises a cup-shaped metal member 7 disposed coaxially within the enclosure with its base sealed to the base of the end cap 1. The other electrode comprises a solid cylindrical metal member 9 disposed coaxially within the enclosure and sealed at one end to the centre of the base of a cup-shaped metal member 11 which is sealed around its mouth to be the base of the end cap 3. At its free end the member 9 projects within the member 7, and the part of member 9 within the cup is provided with a helical surface coating 13 of glass. The internal surface of the member 7 is also provided with a glass coating 1 5. In use the arrester is connected across the possible excess voltage. Typically, the end caps are connected to a pair of lines connected with an equipment which it is desired to protect against excess voltage. On the occurrence of a voltage between the end caps 1 and 3 in excess of the arrester d.c. strike voltage, a discharge occurs between the electrodes 7 and 9. The desired d.c. strike voltage is obtained by appropriate choice of the pressure and composition of the gas-filling, and the geometry of the electrode structure. The rise in d.c. strike voltage at high peak discharge currents e.g. 10,000 amps with an 8/20 micro-second current pulse, is found to be quite acceptable. In one particuiar arrester of the form shown in the drawing with a nominal d.c. strike voltage of 300 volts, the rise was typically from zero to 100 volts. In comparable prior art arresters having electrodes of cup-shaped form defining a discharge gap between the outer surfaces of their bases, rises from 300 volts to 700 volts typically occur under these conditions. The reason for this improvement with an arrester according to the invention is not fully understood. However, it is thought that the rise in d.c. strike voltage with prior art arresters is due to gas evolved from the insulating coatings at the very high temperatures occurring at high peak discharge currents. The improvement with an arrester according to the invention is consequently thought to be due to the fact that the electrode geometry minimises heating of the coatings, and thereby reduces gas evolution. In this connection it is pointed out that insulating materials such as borosilicate glasses which most effectively reduce glow-to-arc transition current tend to evolve relatively large amounts of gas. A further advantage exhibited by an arrester according to the invention is that under overload conditions both electrodes will melt and move into electrical contact with the other. The arrester thus exhibits reliable fail-short characteristics. It will be understood that while the arrester described by way of example has end caps for external connection, other arresters in accordance with the invention may be provided with other means for external connection such as lead wires or tags. CLAIMS

1. An excess voltage arrester comprising a gasfilled enclosure housing a pair of electrodes which define between them a discharge gap, and a coating of insulating material on at least one of the electrodes which serves to reduce the glowto-arc transition current of the arrester, and wherein the discharge gap is defined between the internal surface of a substantially cup-shaped portion of one electrode and the surface of a cylindrical portion of the other electrode which extends substantially axially into said cup-shaped portion.

2. An arrester according to Claim 1 wherein the enclosure is of hollow cylindrical form and said cup-shaped and cylindrical portions of the electrodes are disposed substantially coaxially within the enclosure.

3. An arrester according to Claim 1 or Claim 2 wherein said enclosure comprises a tubular member of electrically insulating material with end caps sealed across its ends, and said one electrode comprises a cup-shaped member with its base sealed to one end cap and said other electrode comprises a solid cylindrical member sealed at its end remote from said one electrode to the base of a cup-shaped member which is sealed around its mouth to the other end cap.

4. An arrester according to any one of the preceding claims wherein the surface of said cylindrical portion of said other electrode is provided with a helical coating of insulating material.

5. An arrester according to any one of the preceding claims wherein said internal surface of the substantially cup-shaped portion of said one electrode is provided with a coating of insulating material.

6. An arrester according to any one of the preceding claims wherein said insulating material is a borosilicate glass.

7. An excess voltage arrester substantially as hereinbefore described with reference to the accompanying drawing.