GB2083945A

GB2083945A - Excess Voltage Arresters

Info

Publication number: GB2083945A
Application number: GB8128234A
Authority: GB
Original assignee: MO Valve Co Ltd
Current assignee: MO Valve Co Ltd
Priority date: 1980-09-19
Filing date: 1981-09-18
Publication date: 1982-03-31
Also published as: GB2083945B

Abstract

An excess voltage arrester of the kind comprising a pair of electrodes defining a discharge gap housed in a hermetically sealed, gas-filled enclosure (1, 3, 5) wherein the electrodes (1 and 7) both extend between terminals for external connection of the arrester at each end of the enclosure so that the arrester can be connected in a pair of lines connected with equipment to be protected so as to form effectively part of the path constituted by the lines. By suitable choice of the dimensions and materials of its components, the arrester can be made to have a characteristic impedance similar to that of the lines and thus present only a small or negligible mismatch to the lines in its non- operated condition. The pair of lines may comprise a coaxial cable and the arrester may be so constructed with electrode 7 coaxially within tubular electrode 1 to connect directly at each end with a conventional coaxial cable socket on the cable. In a modification (Fig. 2) the inner electrode may be cranked. Graphite stripes on the reduced portion 9 of end plugs 3, 5 stabilizes the strike voltage. <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 hermetically sealed, gas-filled enclosure and a pair of electrodes housed within the enclosure which define between them a discharge gap.

In use, an arrester of the kind specified is normally connected to provide a shunt path across the terminals of an equipment to be protected, between lines connected with the equipment to be protected, or between one such terminal or line and ground.

In such an arrangement using arresters of known form the stray capacitance of the arrester presents an impedance mismatch to the line and/or equipment in its non-operated condition which, where the lines carry high frequency signals, can lead to unacceptable signal loss.

It is an object of the present invention to provide a form of excess voltage arrester which overcomes this problem.

According to the present invention there is provided an excess voltage arrester comprising a hermetically sealed, gas-filled enclosure and a pair of electrodes housed within the enclosure which define between them a discharge gap wherein the two electrodes extend between two different locations on the enclosure wall at each of which are provided terminals for making external connection to the electrodes.

An arrester in accordance with the invention is adapted for connection in series in a pair of lines connected with the equipment to be protected so as to form effectively a section of the signal path constituted by the lines. By suitable choice of the dimensions and material of its components the arrester can be made to have a characteristic impedance similar to that of the lines in which it is connected, and thus present only a small or negligible mismatch to the lines in its non-operated condition.

In one particular arrangement in accordance with the invention one electrode is of tubular form and the other electrode extends axially along the interior of said one electrode. In such an arrangement said one electrode is suitably constituted by at least part of the internal surface of a tubular member which forms part of the wall of the enclosure, and the other electrode is constituted by part of the surface of a further member which is sealed through members of electrically insulating material sealed into opposite ends of said tubular member to complete the enclosure.

In such an arrangement said further member suitably extends beyond the electrically insulating member at at least one end of the enclosure to provide a terminal outside the enclosure to permit external connection to said other electrode by means of a socket.

An arrester of such a form is adapted for use in the plug part of a two-part connector.

Two excess voltage arresters in accordance with the invention will now be described by way of example with reference to the accompanying drawing in which: Figure 1 is a sectional side view of one arrester; and Figure 2 is a sectional side view of the other arrester.

Referring to Figure 1, the first arrester to be described includes a hermetically sealed, hollow, cylindrical, gas-filled enclosure. The enclosure comprises a nickel-iron tube 1 into each end of which is sealed a ceramic plug 3 or 5. A nickeliron pin 7 is sealed through apertures formed centrally in the two plugs and extends axially along the whole length of the tube 1, the pin extending beyond the plugs at each end of the enclosure.

In use the arrester is connected so as to form part of a length of a coaxial cable connected with an equipment to be protected against excess voltage. The arrester is suitably connected to the cable at each end by way of a conventional form coaxial cable socket, the part of the pin outside the arrester enclosure at each end of the arrester and the adjacent part of the tube 1 respectively forming the inner and outer terminals of a coaxial plug which fits into the socket.

The dimensions of the tube 1, pin 7 and ceramic plugs 3 and 5 and the material of the plugs 3 and 5 are chosen so that the arrester has a characteristic impedance substantially the same as the coaxial cable.

On occurrence of a voltage between the inner and outer of the coaxial cable, and hence between the pin 7 and the tube 1, in excess of the d.c.

strike voltage of the discharge gap defined by the part of the internal surface of the tube between the ceramic plugs 3 and 5 and the corresponding part of the pin 7, a discharge occurs in the enclosure, thereby protecting the equipment from the excess voltage. The desired d.c. strike voltage is obtained by appropriate choice of the pressure and composition of the gas filling, and the radial spacing between the tube 1 and the pin 7.

The surge strike voltage of the arrester may be stabilised in known manner by a conductive stripe, e.g. of graphite, on a part of the surface of one or both of the ceramic plugs within the enclosure, the stripe suitably extending parallel to the enclosure axis on a reduced diameter inner portion 9 of the plug 3 or 5. The d.c. strike voltage of the arrester may be stabilised in conventional manner by providing a small quantity of radioactive beta emitter in the enclosure, either as a component of the gas filling e.g. tritium, or as a component of the conductive stripe material.

in one particular embodiment of the arrester of Figure 1 suitable for lightning protection and having a characteristic impedance of 50 ohms, dimensional details of the arrester are as follows: millimetres Overall enclosure length 10 Tube 1; internal diameter 2.0 Tube 1; external diameter 2.4 Pin 7; diameter 0.5 Pin 7; external projections 5.0 Discharge gap length 4.0 Referring now to Figure 2, in a modification of the arrester of Figure 1 for use in protecting equipment against electromagnetic wave energy pulses having rapid rise times a centrally cranked pin 11 is used to define with the central part of the internal surface of the tube 1 a relatively small discharge gap e.g. of 0.1 millimetres.It will be appreciated that characteristic impedance of the part of the arrester in which the cranked part of pin 11 is housed can be made substantially the same as the characteristic impedance of the other parts of the arrester.

A number of arresters of the form shown in Figure 1 or Figure 2 may be utilised in a multiway two part connector to provide excess voltage protection in each of the lines passing through the connector. In such an arrangement, the arresters are suitably incorporated in the plug part of the connector, the socket part having an earthed plane within which the outer conductors of the arresters make contact, and a separate socket for each line into which one end of the inner conductor of the associated arrester is inserted when the two parts of the connector are coupled.

It will be appreciated that whilst in the arresters described above by way of example, one electrode is tubular and the other electrode extends axially within said one electrode, this is not necessarily the case in an arrester according to the invention. Thus in an alternative form of arrester in accordance with the invention the two electrodes may, for example, comprise parallel spaced apart conductors extending through a hollow cylindrical envelope of electrically insulating material.

Claims

1. An excess voltage arrester comprising a hermetically sealed, gas-filled enclosure and a pair of electrodes housed within the enclosure which define between them a discharge gap wherein the two electrodes extend between two different locations on the enclosure wall at each of which are provided terminals for making external connection to the electrodes.

2. An arrester according to Claim 1 wherein one electrode is of tubular form and the other electrode extends axially along the interior of said one electrode.

3. An arrester according to Claim 2 wherein said one electrode is constituted by at least part of the internal surface of a tubular member which forms part of the wall of the enclosure, and the other electrode is constituted by part of the surface of a further member which is sealed through members of electrically insulating material sealed into opposite ends of said tubular member to complete the enclosure.

4. An arrester according to Claim 3 wherein a conductive stripe is provided on a part of the internal surface of at least one of the members of electrically insulating material to stabilise the surge strike voltage of the arrester.

5. An arrester according to Claim 4 wherein said stripe extends substantially parallel to the enclosure axis on a reduced diameter inner portion of the member of electrically insulating material.

6. An arrester according to any one of Claims 3 to 5 wherein said tubular member which forms part of the wall of the enclosure is a metal tube whose external surface thus provides terminals for making external connection to said one electrode.

7. An arrester according to any one of Claims 3 to 6 wherein said further member extends beyond the electrically insulating member at at least one end of the enclosure to provide a terminal outside the enclosure to permit external connection to said other electrode by means of a socket.

8. An arrester according to any one of Claims 2 to 7 wherein said other electrode is constituted by the surface of a cranked portion of a member which otherwise extends coaxially within said one electrode.

9. An excess voltage arrester substantially as hereinbefore described with reference to Figure 1 or Figure 2 of the accompanying drawing.