GB2203286A - Surge arrester - Google Patents

Abstract

A surge arrester comprising a ceramic tube 12, electrodes 10, 11, set with a relatively wide gap 15 and a resistive pellet, 14 positioned to give a narrower gap 17, for example 30 microns wide. The electrode 10 has a cylindrical boss 16 whose outside surface faces the interior cylindrical surface of a recess 13 formed in the electrode 11, thus defining an annular gap 15, wider than the narrow gap. A voltage surge across the electrodes will cause breakdown first of the narrow gap then of the wider gap 15 which has a higher current carrying capacity. The breakdown across the narrow gap causes a voltage drop across the resistive pellet 14. In another embodiment (Fig 2) the narrower gap (30) is formed by a resistive coating (25) on the inside of the ceramic tube (22), the coating being connected to one electrode (20) but spaced from the other electrode (21) by a washer (24). <IMAGE>

Description

"SURGE ARRESTER" The present invention concerns surge arresters and particularly, though not exclusively, surge arresters of the kind which can be incorporated in telephone transmission lines to protect telephone equipment against sudden power surges in the lines.

The essential elements of such arresters are well known and consist of a pair of electrodes in a gas-filled housing. Should a power surge occur the gas breaks down to allow conduction to an earth electrode. Many arresters of this basic type have been used in recent years.

The present invention has for an object to provide an improved form of surge arrester.

In accordance with the invention there is provided a surge arrester comprising a housing, a pair of electrodes mounted on said housing, said electrodes being positioned to define therebetween a first relatively wide gap, and a resistance element electrically connected to one of said electrodes and positioned so as to be spaced from the other of said electrodes by a second gap narrower than the first whereby, upon a voltage surge being applied across the electrodes, said second gap breaks down first.

The two electrodes may be substantially identical in shape and may be plane-parallel electrodes.

One electrode may be in contact with a resistive coating on the main body which provides the resistive pathway, whilst the other electrode is spaced from the film to provide the narrower of the two gaps.

In an embodiment of the invention, said resistive element comprises a pellet of resistive material which is attached to said first electrode.

In an alternative embodiment, said resistive element takes the form of a resistive coating applied to an inside surface of said housing.

In order that the invention may be more readily understood, two embodiments thereof will now be described by way of example, and with reference to the accompanying drawings, in which: Figure 1 is an axial section through a first embodiment of a surge arrester according to the present invention; and Figure 2 is a similar view of a second embodiment.

Referring n6w to Figure 1 of the drawings, this shows a surge arrester comprising a pair of coppernickel alloy electrodes 10, 11 brazed into the ends of a housing comprising an electrically insulating ceramic tube 12. The electrode 11 is formed with a recess 13 whilst electrode 10 carries a resistive pellet 14 which lies symmetrically within recess 13. The paraxial gap 17 between the pellet 14 and the base of recess 13 is of the order of a few tens of microns, for example 30 microns, whilst the radial gap 15 between the central boss 16 of electrode 10 and the walls of recess 13 can be in the region of ten times larger than the paraxial gap.

For a given gas pressure and gas mixture the smaller, paraxial, gap 17 will break down first when a rapidly rising voltage is applied across electrodes 10 and 11. After such a breakdown the current flowing across the gap increases the voltage drop across that part of the resistive pellet 14 which is proud of the boss 16 of electrode 10. Eventually the voltage between the two radial walls of the electrodes 10 and 11 reaches the breakdown voltage for the wider gap 15 and triggered breakdown of the second gap occurs.

Since the surface areas surrounding gap 15 are much greater than those of the narrow paraxial gap 17, the majority of the current flowing through the surge arrester will flow in the radial direction. The surfaces of electrodes 10 and 11 adjacent to the wider gap may be coated with materials designed to minimise the effects of electrode sputtering and melting which would otherwise detrimentally affect the insulation and dynamic breakdown.

Referring now to Figure 2 of the drawings this shows two plane-parallel surge arrester electrodes 20, 21 brazed to the ends of a hollow ceramic tube 22 so that a gap 23 of approximately 0.4 mm exists between their adjacent ends. The base of electrode 21 is held a short distance from the end surface of ceramic tube 22 by a metallic or insulating washer 24 which is approximately 50 microns thick. This forms a narrow gap 30. The inner walls of the ceramic tube are coated by a resistive layer or film 25 which at one end is in direct electrical contact with electrode 20 and at the other stops just short of the insulating washer 24. In this way a very small gap is maintained between the edge of resistive film 25 and the base of electrode 21.The electrode-washer-ceramic arrangement is fixed into position by the use of metallic end caps 26, 27 which form hermetic seals to the ceramic body on the outer surface rather than at the end of the arrester.

On the application of a very rapidly rising voltage pulse across the ends of the device the narrow gap 30 will break down before the main electrode gap 23. As the current flowing through the gap 30 increases so does the voltage drop across that part of the resistive film on the walls of the arrester between the small gap 30 and the base of the electrode at the opposite end of the tube 22. Eventually the voltage across the wide gap 23 reaches a level sufficient to cause breakdown across this larger gap.

The surfaces of the main electrodes 20, 21 can be coated with materials designed to reduce electrode damage at large arc currents and to confine the arc discharge to regions of the device where insulation loss due to electrode surface sputtering is kept to a minimum. Such materials could not effectively be incorporated into the small gap without causing bridging of the gap during high current loading either by the coating or by the material of the electrode itself.

Both these applications have various advantages over existing devices. For example it has not proved possible to construct an inter-electrode gap which is both narrow enough to ensure rapid breakdown of the arrester at high rates of rise of applied voltage and rugged enough to remain intact after high current loading. The surfaces of the bulk resistive pellet can be finished to a higher degree of flatness than would normally be possible with a metallic low-cost electrode and will therefore enable the problem of intermittent initial shorts to be minimised. Photoetched metallic washers can be accurately and cheaply manufactured to far greater accuracy than the equivalent high tolerance surge arrester electrode. These components are usually of uniform thickness having accurately reproducible inside and outside diameters and thus represent a high tolerance component which is easy and economical to make.

By varying the chemical makeup of the composite pellet and the surface film the current level at which the narrow gap triggers the main gap can be accurately controlled. Hence a realistic compromise between current damage to the surface of the narrow gap electrodes and burn-out of the resistive pathway can be achieved.

Claims (12)

1. A surge arrester comprising a housing, a pair of electrodes mounted on said housing, said electrodes being positioned to define therebetween a first relatively wide gap, and a resistance element electrically connected to one of said electrodes and positioned so as to be spaced from the other of said electrodes by a second gap narrower than the first whereby, upon a voltage surge being applied across the electrodes, said second gap breaks down first.

2. A surge arrester as claimed in claim 1 wherein said resistive element comprises a pellet of resistive material which is attached to said first electrode.

3. A surge arrester as claimed in claim 2 wherein the pellet is shaped to define a surface which faces a corresponding surface of said second electrode across said second gap.

4. A surge arrester as claimed in any one of the preceding claims wherein those surfaces of said electrodes which face one another across said first gap are generally cylindrical in shape, the cylindrical facing surfaces being coaxial such that said first gap is annular in section.

5. A surge arrester as claimed in claim 1 wherein said resistive element takes the form of a resistive coating applied to an inside surface of said housing.

6. A surge arrester as claimed in claim 5 wherein said inside surface of the housing is generally cylindrical in shape to define a cylindrical resistive coating, wherein said one electrode is connected to the resistive coating at one end thereof, and wherein the other electrode is spaced from the other end of the cylindrical coating.

7. A surge arrester as claimed in claim 6 wherein the second electrode is spaced from the end of the cylindrical coating by means of a washer.

8. A surge arrester as claimed in claim 1 or any one of claims 5 to 7 wherein those surfaces of said electrodes which face one another across said first gap are generally planar in shape.

9. A surge arrester as claimed in any one of the preceding claims wherein said first gap is about ten times larger than said second gap.

10. A surge arrester as claimed in any one of the preceding claims wherein said second gap is of the order of a few tens of microns wide.

11. A surge arrester as claimed in claim 10 wherein said second gap is approximately 30 microns wide.

12. A surge arrester as claimed in any one of the preceding claims wherein said first gap is approximately 0.4 mm wide.