GB1564224A

GB1564224A - Excess voltage arresters

Info

Publication number: GB1564224A
Application number: GB2760676A
Authority: GB
Original assignee: MO Valve Co Ltd
Current assignee: MO Valve Co Ltd
Priority date: 1976-07-02
Filing date: 1976-07-02
Publication date: 1980-04-02
Also published as: DE2729463A1; FR2357055B3; FR2357055A1

Description

(54) IMPROVEMENTS IN OR RELATING TO EXCESS VOLTAGE ARRESTERS (71) We, THE M-O VALVE COM PANY LIMITED, of Brook Green Works, Hammersmith, London W6 7PE, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to excess voltage arresters.

The invention relates particularly to excess voltage arresters of the kind comprising 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 respective 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, and each electrode carrying a sleeve of electrically insulating material which extends outwardly from a point near the inner end of that electrode. Such an arrester is hereinafter referred to as an excess voltage arrester of the kind specified.One example of such an arrester is described and claimed in the specification of United Kingdom Patent No. 1,411,491.

It is an object of the present invention to provide an improved form of such an excess voltage arrester.

According to the present invention in an excess voltage arrester of the kind specified each said sleeve is in the form of a coating adhering to the surface of the electrode.

One advantage of an arrester in accordance with the present invention is that it is cheaper to manufacture than a corresponding arrester with sleeves in the form of ceramic tubular members such as are used in the embodiment described in U.K. Patent Specification No. 1,411,492.

However, in addition, an arrester in accordance with the present invention exhibits unexpected improved performance if a discharge occurs in the arrester over a prolonged period, as is further explained below.

Said coatings are conveniently formed by covering the relevant parts of the electrodes with films of liquid containing particles of an electrically insulating material in suspension, e.g. by spraying, and heating the films.

The insulating material is suitably magnesia or alumina.

Each said coating preferably extends outwardly to a point beyond the inner end of the insulating portion of the enclosure through which the electrode carrying that coating extends.

In one such arrangement each electrode is provided at its outer end with a flange sealed to the outer end of the insulating portion of the enclosure through which that electrode extends, and the outer end of each coating lies adjacent the flange of the associated electrode.

In an arrester in accordance with the invention each said coating can be arranged to be effective to increase the breakdown voltage between any point on the surface of the electrode carrying that coating and a conductive deposit 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.

In one such arrangement each said coating extends either side of 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 coating appreciably larger than the gap between the inner end of that electrode and the adjacent part of the central portion of the enclosure. In such an arrangement the portions of the electrodes projecting inwardly beyond the coatings are suitably of larger diameter than the adjacent portions of the electrodes.

Two arresters in accordance with the invention will now be described, by way of example, with reference to the accompanying drawing in which Figures 1 and 2 are respectively sectional views of the two arresters.

Referring to Figure 1 of the drawing, the first arrester to be described includes a hermetically sealed, hollow cylindrical, gasfilled 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, and at the other end into opposite ends of the central enclosure member 3.

Within the gas-filled enclosure are housed a pair of axially spaced, one-piece, cylindrical, copper electrodes 6 and 7. Each of the electrodes 6 and 7 has an outwardly extending flange 8 at one end and a portion 9 of reduced diameter near its other end. An axial bore 10 extends a short way into the electrode 6 from its flanged end. the inner end of the bore 10 communicating with a radial bore 11. and the bore 10 being enlarged over its outer portion to accommodate a pumping stem 12 which is pinched-off in conventional manner after the required gas-filling has been inserted in the enclosure.

The electrode 6 is positioned so as to extend coaxially through the ceramic member 4 with its flange 8 sealed between the base of the end cap 1 and the adjacent end of the ceramic member 4, the dimensions of the electrode 6 and the ceramic member 4 being such that the portion 9 of reduced diameter of the electrode 6 lies adjacent the end of the ceramic member 4 remote from the cap 1. The other electrode 7 is similarly positioned with respect to the ceramic member 5 with its flange 8 sealed between the member 5 and the base of the end cap 2.

The gap 13 between the electrodes, which lies within the central member 3 of the enclosure. is made to have the same d.c.

ignition voltage as the gaps 14 between the member 3 and the parts 15 of the electrodes inward of the portions 9 of reduced diameter.

Between its part 15 and flange 8, the external surface of each of the electrodes 6 and 7 is provided with a coating 16 of an insulating material. such as magnesia or alumina. Each coating 16 is suitably formed by spraying with a liquid containing particles of the insulating material in suspension, and then heating the electrode to evaporate the liquid and leave a coating of the insulating material adhering to the surface of the electrode.

In use of the arrester, the end caps 1 and 2 are respectively 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 between either one of the lines and the other line, or ground, a discharge occurs between one or both electrodes 6 and 7 and the metal member 3 of the enclosure. It will be appreciated that while initially the discharge may occur between the two electrodes 6 and 7 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 13 and 14.

When a discharge takes place in the arrester, sputtered material from the discharge forms electrically conductive deposits on the internal surfaces of the metal member 3 and the ceramic members 4 and 5 of the enclosure. The formation of these deposits 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 deposits are electrically connected to ground in operation via the metal member 3. Hence, if deposits are formed which extend along the inner curved surfaces of the ceramic members 4 and 5 towards the end caps 1 and 2 respectively, 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 the arrester being described the formation of such conductive deposits is substantially restricted to the exposed end faces 17 of the ceramic members 4 and 5. This is achieved partly by virtue of the gap between the electrodes 6 and 7 and the ceramic members 4 and 5 being too small to allow easy access of the sputtered material to be internal curved surfaces of members 4 and 5, and partly by virtue of the insulating coatings 16 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 coatings 16 makes the shortest gap (via the gas filling) between a conducting deposit on an end face 17 and an exposed part of the adjacent electrode 6 or 7 larger than the gaps 13 and 14. Thus, the presence of the coatings 16 increases the breakdown voltage between a conducting deposit on an end face 17 and the adjacent electrode 6 or 7 to a value above the d.c. ignition voltage set by the gaps 13 and 14, thus preventing the deposit reducing the d.c. ignition voltage of the arrester.

Hence, the coatings 16 prevent the arrester breaking down at a d.c. ignition voltage lower than the designed voltage. Thus the coatings 16 perform the same function as the sleeves in the form of tubular ceramic members in the arrangement described in the specification of United Kingdom Patent No. 1,411,492. However, the coatings 16 are appreciably cheaper than ceramic tubular members.

A further advantage of coatings over ceramic tubular members is that, in the event of excessive current loading of the arrester, the parts 15 of the electrodes 6 and 7 can be relied upon to melt partially and move into contact with the central enclosure member 3, so providing a short-circuit to ground. With the arrangement described in U.K. Patent Specification 1,411,492 there is a tendency for the ceramic tubular members to prevent the partially melted ends of the electrodes from moving into contact with the member 3.

An unexpected further advantage of an arrester in accordance with the invention over prior art arresters is as follows. In use of prior art arresters, if the arrester conducts for a prolonged period, the temperature of an end cap sometimes rises rapidly while the central envelope member remains relatively cool. In an arrester in accordance with the invention the degree to which this can occur is much reduced.

This is of particular importance when the arrester is used in a mount of the kind wherein the temperature of the central enclosure member of the arrester is sensed and in the event of the central enclosure member temperature exceeding a predetermined value the end caps of the arrester and hence the lines connected thereto are grounded by closure of a switch incorporated in the mount. By this means the lines connected to the arrester are permanently grounded if the arrester conducts current for a prolonged period, thereby providing a safeguard against failure of the arrester due to overheating. One such mount is described in the specification of United States Patent No. 3,254,181 issued on 31st May 1966 and assigned to The Bell Telephone Company of Canada.

In use of an arrester in a mount of this kind, if the temperature of an end cap rises rapidly while the central conductor member remains relatively cool, that end cap may reach a temperature high enough to consitute a fire hazard before the switch in the mount operates in response to a rise in the temperature of the central enclosure member. The reduced tendency for the temperature of an end cap to rise rapidly in an arrester according to the invention avoids this problem.

The reason for the reduced tendency for the temperature of an end cap to rise rapidly in an arrester according to the present invention is thought to be as follows.

Gas-filled excess voltage arresters operate for part of the time in a so-called 'glow' mode, when the voltage drop across the arrester is in the region of 200 volts and heat dissipation in the arrester is relatively high and for part of the time in a so-called 'arc' mode, when the arrester voltage drop is in the region of 30 volts and heat dissipation in the arrester is relatively low. In a typical prior art arrester the transistion from glow to arc mode takes place when the current exceeds about 1.2 amperes, but the phenomenon is very variable and with a particular electrode structure the transition current may vary from 0.8 to 2.0 amperes. When a discharge occurs between two electrodes most of the heat dissipation occurs at the electrode serving as the cathode for the discharge.Thus rapid heating of an end cap whilst the central enclosure member remains relatively cool is a result of the electrode associated with that end cap serving as the cathode for a large part of the time that the flow discharge is running.

In an arrester in accordance with the invention the coatings reduce the glow to arc mode transition current with the result that the arrester runs in the glow mode for a shorter time and less heat is dissipated in the arrester in operation. Hence, even if a particular electrode serves as cathode for the discharge for a large part of the time that the discharge is running the associated end cap and is unlikely to reach a dangerously high temperature before the switch in the mount operates. The reason for the reduction in transition current is thought to be a combination of the reduced emitting areas of the electrodes by reason of the presence of the coatings, which effectively increases the glow current density, and an emissive effect at the edges of the coatings on the electrodes.

Referring now to Figure 2, in a modification of the arrester shbwn in Figure 1 a tubular metal liner 18, suitably consisting of a copper-nickel alloy, is disposed inside the central enclosure member 3 between the insulating members 4 and 5 to increase effectively the wall thickness and hence the thermal mass of the central part of the arrester enclosure adjacent the inner ends of the electrodes 6 and 7. In addition, the innermost part 19 of each of the electrodes 6 and 7 is made to have a somewhat smaller diameter than the corresponding part 15 of the arrester of Figure 1, and the gaps between the parts 19 and between each part 19 and the internal surface of the liner 18 are made somewhat smaller than the corresponding gaps 13 and 14 in the arrester of Figure 1. To obtain the same d.c. ignition voltage, the pressure of the gas-filling is adjusted.Otherwise the arrester of Figure 2 is identical with the arrester of Figure 1, corresponding parts in the two arresters being given the same reference numerals.

As a result of these changes the inner ends 19 of the electrodes 6 and 7 melt and move into contact with the central part 3, 18 of the arrester enclosure more rapidly in the event of excessive current loading. Furthermore, due to its increased thermal mass, the possibility of the central part 3, 18 of the arrester enclosure reaching a sufficiently high temperature to constitute a fire hazard is appreciably reduced.

It will be appreciated that the presence of the portions 9 of reduced diameter of the electrodes 6 and 7 assist in ensuring that the parts 15 (or 19) of the electrodes 6 and 7 contact the central part 3 (or 3, 18) of the enclosure in the event of overloading.

However, it will be understood that a portion of reduced diameter is not essential in an arrester according to the present invention.

Similarly, whilst copper is a suitable material for the electrodes 6 and 7, other metals such as steel, nickel or a nickel-iron alloy may be used.

It is further pointed out that while the particular arresters shown in the drawings are 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.

WHAT WE CLAIM IS: 1. An excess voltage arrester of the kind specified wherein each said sleeve is in the form of a coating adhering to the surface of the electrode.

2. An arrester according to Claim 1 wherein said coatings are formed by coating the relevant parts of the electrodes with films of liquid containing particles of an electrically insulating material in suspension and heating the films.

3. An arrester according to Claim 1 or Claim 2 wherein said insulating material is magnesia or alumina.

4. An arrester according to any one of the preceding claims wherein each said coating extends outwardly to a point beyond the inner end of the insulating portion of the enclosure through which the electrode carrying that coating extends.

5. An arrester according to Claim 4 wherein each electrode is provided at its outer end with a flange sealed to the outer end of the insulating portion of the enclosure through which that electrode extends, and the outer end of each coating lies adjacent the flange of the associated electrode.

6. An arrester according to any one of the preceding claims wherein the part of said central portion of the enclosure adjacent the inner ends of the electrodes is of increased wall thickness.

7. An arrester according to Claim 6 wherein said central portion of the enclosure comprises a tubular member of substantially uniform wall thickness provided with an internal tubular liner between said insulating portions of the enclosure.

8. An arrester according to any one of the preceding claims wherein each said coating is arranged to be effective to increase the breakdown voltage between any point on the surface of the electrode carrying that coating and a conductive deposit 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.

9. An arrester according to Claim 8 wherein each said coating extends either side of 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 coating appreciably larger than the gap between the inner end of that electrode and the adjacent part of the central portion of the enclosure.

10. An arrester according to Claim 9 wherein the portions of the electrodes projecting inwardly beyond the coatings are of larger diameter than the adjacent portions of the electrodes.

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

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. innermost part 19 of each of the electrodes 6 and 7 is made to have a somewhat smaller diameter than the corresponding part 15 of the arrester of Figure 1, and the gaps between the parts 19 and between each part 19 and the internal surface of the liner 18 are made somewhat smaller than the corresponding gaps 13 and 14 in the arrester of Figure 1. To obtain the same d.c. ignition voltage, the pressure of the gas-filling is adjusted. Otherwise the arrester of Figure 2 is identical with the arrester of Figure 1, corresponding parts in the two arresters being given the same reference numerals. As a result of these changes the inner ends 19 of the electrodes 6 and 7 melt and move into contact with the central part 3, 18 of the arrester enclosure more rapidly in the event of excessive current loading. Furthermore, due to its increased thermal mass, the possibility of the central part 3, 18 of the arrester enclosure reaching a sufficiently high temperature to constitute a fire hazard is appreciably reduced. It will be appreciated that the presence of the portions 9 of reduced diameter of the electrodes 6 and 7 assist in ensuring that the parts 15 (or 19) of the electrodes 6 and 7 contact the central part 3 (or 3, 18) of the enclosure in the event of overloading. However, it will be understood that a portion of reduced diameter is not essential in an arrester according to the present invention. Similarly, whilst copper is a suitable material for the electrodes 6 and 7, other metals such as steel, nickel or a nickel-iron alloy may be used. It is further pointed out that while the particular arresters shown in the drawings are 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. WHAT WE CLAIM IS:

1. An excess voltage arrester of the kind specified wherein each said sleeve is in the form of a coating adhering to the surface of the electrode.