GB2084393A

GB2084393A - Excess Voltage Arresters

Info

Publication number: GB2084393A
Application number: GB8121040A
Authority: GB
Original assignee: MO Valve Co Ltd
Current assignee: MO Valve Co Ltd
Priority date: 1980-09-19
Filing date: 1981-07-08
Publication date: 1982-04-07
Also published as: GB2084393B

Abstract

An excess voltage arrester of the kind comprising a gas-filled enclosure having an electrode structure 1, 11, 13 within the enclosure which defines at least one discharge gap. A portion of at least one electrode 13 is adapted to melt an overheating of the arrester so as to take up a generally spherical form 19 under the action of surface tension and thereby contact another electrode 1. Preferably the electrodes are designed so that the radius r, of electrode 13, the internal radius r2 of electrode 1 and the axial overlap x of the two electrodes 1 and 13 satisfy the relationship:- <IMAGE> Both electrodes 11 and 13 may be similarly constructed and preferably r2-r1</=0.5 mm.

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 and an electrode structure within the enclosure which defines at least one discharge gap.

In use an arrester of the kind specified is connected across an equipment it is desired to protect against excess voltage, the discharge gap breaking down on the occurrence of excess voltage. In the event that a discharge should occur in the arrester over a prolonged period such that the arrester overheats, it is important that the arrester should reliably fail in a safe manner so as not to leave the equipment unprotected.

It is an object of the present invention to provide an excess voltage of the kind specified which on over-heating fails in such a manner.

According to the present invention in an excess voltage arrester of the kind specified a portion of at least one electrode is adapted to melt on overheating of the arrester so as to take up a generally spherical form under the action of surface tension and thereby contact another electrode.

In one particular arrangement in accordance with the invention said one electrode is in the form of a metal rod and the other electrode is in the form of a tube surrounding one end of the rod.

In such an arrangement where as is normally the case said other electrode coaxially surrounds said one electrode, then, in accordance with the invention, the arrester is constructed so that 4 7err12 (x+r2) > - r23 3 where r, is the radius of said one electrode r2 is the internal radius of said other electrode; and x is the axial overlap of the two electrodes.

One excess voltage arrester 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 the arrester; and Figure 2 is the same view of the arrester when overheated.

Referring to Figure 1 , the arrester includes a hermetically sealed, hollow, cylindrical, gas-filled enclosure. The enclosure comprises a central tubular metal portion 1 having a portion of increased internal diameter at each end which is sealed coaxially, in overlapping relation, to a tubular ceramic member 3 or 5. A pair of metal end caps 7 and 9 respectively close the ends of the ceramic members 3 and 5.

Housed coaxially within the enclosure are a pair of axially spaced, one-piece, cylindrical copper electrodes 11 and 13, each electrode having an outwardly extended flange 15 or 1 7 at its outer end which is sealed between the base of the adjacent end cap 7 or 9 and the end of the adjacent ceramic member 3 or 5.

The gap between the inner ends of the electrodes 11 and 13 lies centrally with the central member 1 of the enclosure the central part of whose inner surface serves as a third electrode of the arrester. In use of the arrester the end caps 7 and 9 are typically respectively connected.to a pair of lines connected with an equipment which it is desired to protect against excess voltage, and the metal tubular member 1 is grounded. On the occurrence of a voltage between the two electrodes 11 and 1 3, or between either electrode 11 or 13 and ground, in excess of the arrester d.c. strike voltage, a discharge occurs in the enclosure, thereby protecting the equipment from excess voltage.

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.

If the discharge is maintained for a prolonged period, e.g. 4 to 5 seconds, the heat generated by the discharge causes one end of one or both of the electrodes 11 and 1 3 adjacent the discharge to melt. Under the action of surface tension the Imolten material takes up a spherical shape 19 and contacts the member 1, as shown in Figure 2 in respect of electrode 1 3. The electrode is thus permanently shorted to ground terminating the discharge and preventing further generation of heat in the arrester which might lead to dangerous overheating and fire.

It will be appreciated that for reliable operation the radius of the sphere of molten material 19 must reach a value greater than the internal radius of the central portion of the member 1 before the electrode has melted back into the associated ceramic member 3 or 5. To a first approximation this requires that 4 ,rr12(x+r2) > 3 err23 3 where r1 is the radius of the electrode r2 is the internal radius of the central part of the member 1; and x is the axial overlap of the electrode and the member 1.

It will be understood that in order for the molten material to form into a sphere rather than fall under the action of gravity, the electrode dimensions will be relatively small, and by virtue of the above required relation between r1, r2 and x, the difference between r1 and r2 will be quite small if x is not to be impracticably large. Typically r2-r1 is 0.5 millimetres or less.

A further advantage of the invention is that reliable fail short operation on overheating is obtained whether the arrester is mounted horizontally or vertically, and when mounted vertically, whether the discharge is restricted to the top or bottom half of the arrester.

It will be understood ti-lat the invention is applicable to arresters having a different electrode geometry to that of the arrester described by way of example, and in particular is applicable to arresters having only two electrodes or more than three electrodes

Claims

1. An excess voltage arrester of the kind specified in which a portion of at least one electrode is adapted to melt on overheating of the arrester so as to take up a generally spherical form under the action of surface tension and thereby contact another electrode.

2. The excess voltage arrester according to Claim 1 wherein said portion of said one electrode consists of copper.

3. An excess voltage arrester according to Claim 1 or Claim 2 in which said one electrode is in the form of a rod, and said other electrode is in the form of a tube surrounding one end of the rod.

4. An excess voltage arrester according to Claim 3 in which said other electrode coaxially surrounds one end of said one electrode, and the arrester is constructed so that 7rr12(X+r2) > 4/37rr23, where r1 is the radius of said one electrode, r2 is the internal radius of said other electrode and,( is the axial overlap of the two electrodes.

5. An excess voltage arrester according to Claim 2 or Claim 3 including a third electrode in the form of a rod disposed in axial spaced relation with said one electrode, and said other electrode surrounds the ends of both said one and said third electrode and the gap therebetween, and a portion of said third electrode is also adopted to melt an overheating of the arrester so as to take up a generally spherical form under the action of surface tension and thereby contact said other electrode.

6. An excess voltage arrester substantially as hereinbefore described with reference to the accompanying drawings.