GB2328567A

GB2328567A - High voltage/lightning arresters

Info

Publication number: GB2328567A
Application number: GB9818231A
Authority: GB
Inventors: Rodney Meredith Doone; Patrick George Short
Original assignee: Bowthorpe Industries Ltd
Current assignee: Bowthorpe Industries Ltd
Priority date: 1997-08-21
Filing date: 1998-08-20
Publication date: 1999-02-24
Also published as: GB9818231D0; WO1999010897A1; GB9717817D0; AU8817598A; WO1999010897A9; WO1999010897A8

Abstract

In response to failure mode short circuit current through a high voltage surge arrester 60, a disconnect device 65 operates to disconnect the surge arrester 60 by releasing one end of a connection lead 64, and the short circuit current through the arrester 60 is transferred to an arc gap between arcing horns 68, 69. This overcomes the problem that failure of the arrester can be a fire hazard if sparks or incandescent particles are emitted by the failed arrester. Disconnect device 65 may be an explosive type so that ionised gas emitted when it operates increases the conductivity between arcing horns 68, 69 to facilitate transfer of the short circuit current to the arc gap. The falling end of lead 64 tends to drag the ionised gases into the arc gap to assist in this process. Alternatively or additionally, transfer of current to the arc gap may be assisted by moving horns 68, 69 closer together - eg. upper horn 69 may be carried by a spring or gravity actuated plunger released by disconnect device 65. Alternative arrangements have a disconnect device (7, Figs.1A,1B; 23, Fig.2) which releases a spring biased rod (9) or a spring member (27) to short out the arrester (1 or 21) which may be connected in series with a spark gap (13,14,15). Another arrangement has an arrester (40, Fig. 3) connected in series with a closely coupled high voltage HRC fuse (44) which disconnects the arrester in response to a failure current therein before the arrester produces sparks and incandescent particles. In a further arrangement an arrester (50, Fig.4) is connected in series with a fuse (51) and a spark gap (52), the fuse and spark gap normally being shunted by a disconnect device (53) which responds to failure current in the arrester by removing the shunt so that the fault current then flows through the fuse and spark gap causing the fuse to disconnect the arrester.

Description

IMPROVEMENTS RELATING TO HIGH VOLTAGE ELECTRIC 1NSTALLATTONS Field of the Invention: This invention concerns improvements relating to high voltage electrical installations, and more particularly electrical installations including an electrical surge arrester, also known as a surge diverter, connected between a high voltage and ground potential. As is well known, surge arresters are used in high voltage electrical installations for providing a path to ground for surge overvoltages occasioned for example by lightning strikes and as switching transients. The surge arrester normally represents a very high impedance between the high voltage and ground potential, but has the capability to provide a low impedance path on the occurrence of an overvoltage transient, thereby allowing the transient access to ground, and to reset into its high impedance state once the transient has passed.

Todays surge arresters commonly employ ceramic Zinc Oxide varistor blocks as variable impedance elements, whereas spark (arc) gaps and electromagnetic spark rotation arrangements were previously employed.

Background ofthe Invention: As is described in GB-A-2 188 199, explosive shattering of the porcelain housings of surge arresters under failure conditions and the resultant showering of the surrounding area with hot fragments has been considered to be a cause of fires.

The bush fires which have ravaged areas of Australia may have resulted from such an arrester failure. The gapless, solid state surge arrester of GB-A-2 188 199 was developed inter alia to combat this problem. Rather than the gas filled porcelain housed arrester that was commonly used before the surge arrester of GB-A-2 188199 was developed, a surge arrester construction was proposed in which a solid, rigid, high strength core was formed by stacking a plurality of Zinc Oxide varistor blocks in face to face contact between a pair of end terminations, wrapping the varistor block stack and the end terminations in a glass fibre wrapping impregnated with uncured epoxy resin, and curing the wrapping under mould pressure so as to ensure absence of voids and gaseous inclusions. An external shedded housing is then provided on the thus-formed core, preferably by use of heat shrink techniques, though an elastomer housing could alternatively be released onto the core or a synthetic polymer housing could be moulded directly onto the core.

The surge arrester of GB-A-2 188 199 has achieved considerable commercial success, primarily on account of its robust, high-strength construction.

Furthermore, it maintains its structural integrity under failure mode to a significant degree. Even so, a requirement has arisen that, to minimize the fire hazard of a failed surge arrester, it should be taken out of commission so that it does not continue to pass a combustion sustaining fault current and emit sparks or hot particles which could cause fires.

In order to meet this requirement, we have considered (but not publicly disclosed) two possible arrangements in which operation of a circuit disconnect device close coupled in series with a surge arrester releases a conductive member which then moves into a position where it bridges and shorts out the failed surge arrester. Figures 1A and 1B and Figure 2 of the accompanying drawings illustrate these arrangements.

Referring to Figures 1A and 1B an electrical surge arrester 1, for example a surge arrester as described in GB-A-2 188 199, is mounted on an electrically conductive mounting bracket 2 by means of an electrically insulating clamp arrangement comprising a cup 3 which receives the lower end of the arrester 1 on the upper side of the bracket 2, and a cap 4 which fits onto a portion of the cup projecting through an opening in the bracket on the underside of the bracket, the clamp arrangement being secured by means of the nut and bolt lower end tertrsution 5 ofthe arrester. A latch bracket 6 is secured to the bottom end ofthe arrester and a disconnect device 7 is mounted on the latch bracket 6 and carries at its right-hand end (as shown in Figure 1A) a latch 8 serving to retain a spring-biassed rod electrode 9 against its anti-clockwise spring bias. The rod electrode 9 is mounted on an extension 10 of the bracket 2 and is electrically coupled thereto by means of a conductor strap 11. A second, catcher electrode 12 is provided on the upper end of the arrester 1. Electrodes 13 and 14 mounted, respectively, on the brackets 2 and 6 define a spark (arc) gap 15.

In normal operation, the surge arrester 1 passes surge transients via the spark gap 15 and the mounting bracket 2 to ground, and the disconnect device 7 is not responsive to this level of current. If the surge arrester fails, however, a much greater through current flows through the arrester and causes the disconnect device 7, which may for example be a thermomechanical device incorporating a small explosive charge, to function. This releases the latch 8 which causes rod electrode 9 to spring counter-clockwise into contact with the catcher electrode thereby shorting out the failed arrester 1.

Figure 2 shows an alternative arrangement wherein a surge arrester 21 is mounted by means of an electrically insulating mounting bracket 22 and has a disconnect device 23 secured to the lower end termination 24 of the arrester. A first ground lead 25 is coupled to the disconnect device 23. An electrically insulating mounting bracket 26 provides a mounting for one end of a phosphor bronze spring member 27 which is bent as shown, against its spring pressure, back on itself and retained by a frangible member 28. An end portion 29 ofthe spring 27 bears against the external surface of the disconnect device 23. A second ground lead 30 is coupled to the fixed end of the spring 27 and a catcher electrode 31 is provided at the top end of the surge arrester.

In operation of the arrangement of Figure 2, failure of the surge arrester 21 with consequent overload current flow in the disconnect device 23 will cause the disconnect device to operate. This causes the member 28 to break, thus releasing the spring 27 which resiles into contact with catcher electrode 31 so taking the surge arrester 21 out of circuit.

The devices of Figures 1A and 1B and Figure 2 both worked well under test, but the former involves moving parts which could fail over an extended time period and the latter relies upon the spring member 27 retaining sufficient resilience over a long time period. The search for a better solution thus continued.

A different approach is taken in the device that is disclosed in WO 95/03643. Whereas the arrangements proposed in Figures 1A and 1B and in Figure 2 aimed to take the failed arrester out of circuit so that combustion of the arrester is not sustained by fault current, the arrangement of WO 95/03643 seeks to contain the arc across the failed arrester until such time as system circuit breakers or fuses operate to interrupt the current. In accordance with the teachings of WO 95/03643, open loop conductor elements are provided at opposite ends of a surge arrester and are formed such that an arc struck between the conductor elements will be rotated electromagnetically around the arrester. The problem with this arrangement is that, in outdoor situations, it provides an ideal nesting location for the Oozalum bird, a species known to the inventors of the present invention for its prodigious nest building activities and voluminous defecation. The presence of combustible material in the conductor elements of WO 95/03643, or either of them, clearly would be a fire hazard. Furthermore, tests that we have conducted have indicated that the device of W095/03643 does not work with surge arresters other than those supplied by the proprietors of W095/03643. Since there are many of our own surge arresters and surge arresters of yet other manufacturers in the field, the inability of the device of W095/03643 to be retrofitted to such arresters is a substantial disadvantage.

It then occurred to us that spark free surge arrester operation could possibly be achieved by close coupling to the surge arrester a non-mechanical device capable of discriminating between normal surge arrester duty and a failure mode condition, the device letting through all surge currents required for the surge arrester to perform its normal function and, in the event of the surge arrester being damaged by excessive lightning or switching duty, serving to disconnect the arrester from the power system before the failed arrester can produce sparks and/or incandescent particles that could ignite combustible material. It occurred to us that certain types of current limiting fusels, well known for other applications, might satisfir the requirements of a discriminator device as set forth above. We pursued this line of investigation and found, much to our surprise, that current limiting fuse links to British Standards dimensions and also fuses to DIN "S" range type complying with the requirements of EC 282-1-1974 rated 12 kV and 24 kV demonstrated the capability, providing a correctly rated current is selected, to clear a tuulty distnbution class surge arrester before there is any significant spark generation.

For example, we placed an HV HRC fuse of Cooper Bussman type BDGHC rated to pass 50 amps and withstand 12 kV in series with a prepared surge arrester as described in GB-A-2 188 199 and rated 12 kV. With a short circuit prospective current set at 6 kA with an applied voltage of 12 kV rms, the fisse cleared the faulty surge arrester in less than 8.5 milliseconds and no sparks were observed from the arrester. Similarly a SIBA HV HRC fuse rated 80 amps at 24 kV was placed in a circuit with a prospective current of 800 amps rms with a 2.2 kA peak; the fuse cleared the circuit in less than 0.1 seconds.

We therefore thought that our search for a solution to the aforementioned problem would be satisfied if we were to mount a suitable fuse in close coupled relationship with a surge arrester. For example, the fuse could be mechanically connected to the line terminal or earth (ground) terminal of the surge arrester, or the fuse housing could be mechanically combined with the body of the surge arrester such that the HV terminal of the surge arrester becomes the HV terminal of the fuse or, alternatively, the earth terminal of the arrester becomes the earth terminal of the fuse.

It had of course been known to provide Hv HRC fuses in electrical installations incorporating surge arresters. For example, in electrical distribution systems surge arresters are commonly provided to protect overhead power lines against the effects of lightning strikes and the like, the surge arresters being provided at the locations of power line support poles and the like, and in such systems it is well known to provide fuses at the ends of the lines to protect equipment connected thereto. However, it had not to our knowledge previously been known to provide fuses in such installations at the surge arrester locations and, more particularly, it had not been known to close couple the fuses to the surge arresters. Additionally it had not been known to select the fuse ratings with the express purpose of avoiding sparking of an arrester under failure conditions.

The fuses could be arranged so as to be permanently connected electrically in series with the surge arresters; namely in an electrical power distribution system, for example, a fuse mounted on top of a surge arrester would have its upper end connected to the HV line and the lower terrnination of the arrester would be connected to earth. However, in such an arrangement, the fuse would be required to withstand the through current developed in the surge arrester in normal operation of the arrester to pass an overvoltage surge to ground. Consequently, large sized heavy duty fuses would have to be employed. To avoid this requirement for high duty fuses, we thought to shunt the fuse with a disconnect device so that in normal surge arrester operation the surge duty was through the disconnect device and the fuse was effectively isolated, but in the case of arrester failure the disconnect device operated and the system fault current was diverted through the fuse. In this latter arrangement, the fuse would not be required to carry the normal operating current of the surge arrester, this being shunted past the fuse by the disconnect device, and a much reduced fuse rating could be employed.

Disconnect devices are known in the art and have been widely used with distribution class surge arresters. A convenient form of disconnect device is manufactured by Bowthorpe EMP limited of Stevenson Road, Brighton, Sussex (GB) and incorporates a small explosive charge which is arranged to be detonated to effect disconnection when an excessive current flows through the device. Other disconnect devices are known which incorporate springs retained by fusible material with the spring arranged to release and effect disconnection when the fusible material melts under ground fault current.

Figure 3 of the accompanying drawings shows an arrangement as discussed above which we believed compared favourably with the arrangements of Figures 1A and IB and Figure 2 and with the proposal described in WO 95/03643.

Referring to Figure 3, a surge arrester 40 constructed for example according to the teachings of GB-A-2 188 199 is mounted on an electrically insulating bracket 42 and carries a discriminator device 44 at its upper end. The upper end of the discriminator device 44 is shown connected to an overhead line 46 and the lower end of the surge arrester 40 is connected to system earth (ground). As described previously herein, the discriminator device 44 had to be capable of passing all surge currents encountered in normal operation of the surge arrester 40, but in the event of the arrester 40 being damaged by excessive lightning or switching duty had to disconnect the arrester 40 from the overhead power line 46 before sufficient energy had caused the failed arrester to produce sparks and incandescent particles that could ignite combustible material. We had in mind that the Bussmann DIN "S" range of high voltage current-limiting fuses manufactured by the Bussnn Division of Cooper (UK) Limited, of Burton-on-the-Wolds, Leicestershire (GB) would be suitable for this purpose, as would be the equivalent powder-filled fusels manufactured by SIBA Sicherungen-Bau Gmby of Borker Strasse 22, D44534 Lunen, Germany (I)E).

We also thought to provide a spark gap in series with the fuse and to provide a disconnect device across the series connected spark gap and fuse combination, and Figure 4 of the accompanying drawings is a circuit diagram of such an arrangement. As shown, a surge arrester 50 is series connected with a fuse 51 and a spark gap 52 between ground and an HV line and a disconnect device 53 is connected in parallel with the series-connected fuse 51 and spark gap 52. As previously explained herein, we considered that the disconnect device 53 would shunt normal surge arrester operating current around the fuse 51, and only in the failure mode of the surge arrester 50, when the disconnect device 53 operated, would the fuse 51 be subjected to failure mode current which would cause the fuse rapidly to operate. Under test conditions, however, we found that this did not happen and that the fuse 51 flashed over externally. A study of this situation has revealed an interesting combination of events which has led us to the present invention.

Obiects and Summarv ofthe Invention: It is the principal object of the present invention to provide a surge arrester arrangement which is spark free under conditions of surge arrester failure.

Another object of the present invention is to provide a solution to the spark free surge arrester failure problem which enables existing surge arresters to be retrofitted, unlike the proposal of W095/03643.

When we analysed the failure of the Figure 4 arrangement, we concluded that responsibility for the failure resided with the large amount of ionised gas that was produced when the disconnect device operated, so that the air surrounding the disconnect device became electrically conductive. Before the fuse operated, the voltage across the outside of the fuse was zero. When the fuse attempted to clear a short circuit current through the surge arrester by melting of its internal fuse component(s), a transient arc voltage developed across the fuse. Under normal air conditions the fuse would then interrupt the current and there would be no flashover, but in the presence of ionised gas from the disconnect device the fault current was able to flash over the external insulation of the fuse without any interruption. It occurred to us that the ionised gas output of the disconnect device could be turned to advantage if it could be used to transfer the surge arrester short circuit failure mode current from the arrester to materials that would not produce sparks or incandescent particles.

According to one aspect of the present invention, therefore, there is provided in or for a high voltage electrical installation, the combination of an electrical surge arrester for providing a path to ground for high voltage surges encountered in operation of the installation, a disconnect device responsive to failure mode short circuit current through the surge arrester for disconnecting the same and an arc gap, the arrangement being such that operation of the disconnect device so affects the arc gap as to cause the short circuit current through the surge arrester to be transferred to the arc gap.

According to another, more particular aspect ofthe present invention, there is provided, in or for a high voltage electrical installation, the combination of an electrical surge arrester for providing a path to ground for high voltage surges encountered in operation of the installation, a disconnect device connected at one side to one terminal end of the surge arrester, and first and second spaced-apart arc supporting elements electrically connected one to the other side of the disconnect device and the other to the other terminal end ofthe surge arrester, the arrangement being such that, in the event of a short circuit arrester failure and consequent operation of the disconnect device with release of ionised gas, the electrical conductivity of the air gap between said arc supporting elements increases to the point where the short circuit current through the arrester is transferred to the arc supporting elements and away from the arrester, and the arc supporting elements comprising material that does not produce sparks or incandescent particles when subject to the short circuit current.

The above and further features of the present invention are set forth in the appended claims and, together with advantages thereof, will become clear from consideration of the following detailed description given with reference to the accompanying drawings.

Description ofthe Drawings: Figures 1 A and 1B show side and end elevation views of a first possibility that occurred to us for ensuring spark free surge arrester failure; Figure 2 is a side elevation view of a second possibility that occurred to us for ensuring spark free surge arrester failure; Figure 3 shows the close coupled series combination of a surge arrester with a discriminator (fuse) device, according to a third possibility that occurred to us; Figure 4 shows the equivalent circuit of an arrangement according to Figure 3 which further includes an arc (spark) gap and a disconnect device; and Figure 5 shows a surge arrester installation according to the present invention.

Detailed Description ofthe Embodiment: Referring to Figure 5 of the accompanying drawings, the arrangement shown comprises a surge arrester 60, for example a surge arrester constructed according to the teachings of GB-A-2 188 199 aforementioned, having one end 61 connected to ground via an earth connection lead 62 and having its other end 63 connected by means of an electrically insulated flexible connecting lead 64 to one end of a disconnect device 65, the other end of the disconnect device 65 being connected to line voltage by means of line connection lead 66. The grounded end 61 has an electrically conductive support 67 bolted thereto and a lower brass arcing horn 68 is bolted to the end of support 67 spaced apart from the surge arrester 60.

At the other end 63 of the surge arrester 60 an electrically insulating support 68 is bolted to the arrester and carries at its distal end a second or upper brass arcing horn 69 bolted thereto. The lower and upper arcing horns 68 and 69 thus define a location for an arc between but laterally spaced from the ends of the surge arrester 60. An electrically conductive connecting bar 70 connects upper arcing horn 69 to that end of disconnect device 65 that is connected to line voltage. An arc shield 71 may be mounted on support 68 for directing ionised gases produced by disconnect device 65 away from the line terminal end 63 of surge arrester 60, but is not essential and may be omitted.

The arcing horns 68 and 69 are preferably formed of brass on account of the fact that brass is unlikely, when subjected to a power arc, to emit long range molten or incandescent particles or sparks such as to give rise to a fire risk, and preferably carry a thin layer of electrically insulating material on their cylindrical side surfaces, as shown at 100, but not on their hemispherical tips. The lower conducting support 67 and the connecting bar 70 may likewise be formed of brass or may be formed of another metal and preferably are covered with a thin layer of electrical insulation to prevent arc damage. Indeed it is preferred that all external metal work should be insulated by means of a thin layer of electrically insulating material. Furthermore, the tips of the arcing horns 68 and 69 may be shrouded with electrically insulating material to discourage local wildlife from bridging the gap and causing a flashover.

In the event of a short circuit failure of the surge arrester 60 the short circuit current through the arrester will cause the disconnect device 65 to operate.

This will occur within a few milliseconds and the flexible conductor 64 will then begin to fall away towards the lower brass arcing horn 68. As the body of the disconnect device separates due to the explosive action of its internal mechanism, the disconnect device preferably being of the kind aforementioned which incorporates a small explosive charge, a cloud of ionised gas will permeate the space between the arcing horns 68 and 69, assisted by the body part of the disconnect device 65 that remains attached to lead 64 and falls under gravity. This causes the conductivity of the air gap between the arcing horns to rise (its resistance falls) and causes an arc to strike between the arcing horns 68 and 69 and the short circuit current through the surge arrester 60 "jumps" from the arrester and "attaches" to the arcing homs. In this way, the power arc is held between two points, namely the arcing horns, which are configured to minimize the output of incandescent material so that sparks and other combustible material are prevented from reaching the ground.

Tests that we have conducted with short circuit currents of 65 amps for 10 seconds, 800 amps for 2 seconds and 6 kA for 0.9 seconds have established that arc transfer from the surge arrester 60 to the arcing horns 68 and 69 is very rapid and such that the short circuit current through the surge arrester does little or no damage to the arrester body.

In the field, the addition to a surge arrester installation of the components illustrated in Figure 5, other than the arrester 60 itself, would thus provide the arrester with a spark free failure mode. The components are readily retrofitted to existing surge arrester installations and will operate with any kind of surge arrester.

The current transferred to the arcing horns will be sensed by appropriate means connected to the line and will terminate when the line is taken out of commission.

Having thus described the present invention by reference to a specific embodiment, it will be appreciated that modifications and variations will occur to those skilled in the art without departure from the scope of the invention as set forth in the appended claims, and that the described embodiment is exemplary only.

In the described embodiment, the use of an explosive type of disconnect device is advantageous, since the ionised gas emitted when the device operates is instrumental in increasing the electrical conductivity of the gap between the arcing horns 68 and 69 so that the short circuit current through the arrester transfers to the arcing horns. Furthermore, the arrangement of the lead 64 connecting the bottom end of the disconnect device 65 to the top end 63 of the surge arrester 60 is significant in that the lead 64 constrains the falling body of the disconnect to swing down between the arcing horns and, in effect, to drag the ionised gases emitted by the disconnect device down into the gap between the arcing horns. The arc shield 71, when provided, can assist in this process and can even be shaped so as to be more effective in diverting ionised gas downwards between the arcing horns 68 and 69, though its principal function is to divert ionised gas away from the line terminal 63 of the surge arrester 60. Non-explosive type disconnect devices could possibly be utilised, but in the absence of an emission of ionised gas, a mechanical arrangement would have to be employed to cause the short circuit current through the arrester to transfer to the arcing horns 68 and 69, for example by effectively moving them closer together. The upper arcing horn 69 could, for example, be carried by a spring loaded or gravity actuated plunger which was normally held in retracted position but was arranged to be released when the disconnect device operated. Such an arrangement would be feasible, but is not preferred, just as the arrangements of Figures 1A and 1B and Figure 2 are not preferred, on account of its reliance upon mechanical parts which may be subject to jamming or other performance inhibiting influences. Of course, an explosive type disconnect device such as to liberate ionised gas into the gap between the arcing horns could also be employed in combination with a mechanical arrangement in a belt and braces" solution.

Furthermore, whereas the surge arrester that is incorporated into the embodiments is said to be constructed in accordance with the teachings of GB-A-2 188 199, it could be alternatively constructed and, in particular, it could be constructed according to the teachings of our British Patent Application No.

9803998.5 which discloses and claims an electrical surge arrester comprising a stack of varistor elements retained between end terminations by means of an electrically insulating rod passing through through-holes in the varistor elements and secured to the end terminations, the through-holes being larger than the rod cross-section, and a moulded plastics material such as a silicone rubber material for example filling the void that otherwise would exist between the rod and the varistor elements and extending around the external surfaces of the stacked varistor blocks, the varistor elements being formed without passivation coatings on their internal and external surfaces, and said moulded plastics material being selected to provide a passivation function.

It is to be noted that the arrangements described herein with reference to Figures 1A and IB, Figure 2, Figure 3 and Figure 4 have not been publicly disclosed. The appended claims include claims directed to these arrangements, notwithstanding that they are not presently preferred.

Claims

CLAIMS:

1. In or for a high voltage electrical installation, the combination of an electrical surge arrester for providing a path to ground for high voltage surges encountered in operation of the installation, a disconnect device responsive to failure mode short circuit current through the surge arrester for disconnecting the same and an arc gap, the arrangement being such that operation of the disconnect device so affects the arc gap as to cause the short circuit current through the surge arrester to be transferred to the arc gap.

2. In or for a high voltage electrical installation, the combination of an electrical surge arrester for providing a path to ground for high voltage surges encountered in operation of the installation, a disconnect device electrically connected at one side to one terminal end ofthe surge arrester, and first and second spaced-apart arc supporting elements electrically connected one to the other side of the disconnect device and the other to the other terminal end of the surge arrester, the arrangement being such that, in the event of a short circuit failure of the surge arrester and consequent operation of the disconnect device, the electrical conductivity of the air gap between said arc supporting elements is increased to the point where the short circuit current through the surge arrester is transferred to the arc supporting elements and away from the surge arrester.

3. The combination claimed in claim 2 wherein the electrical conductivity of the air gap between the arc supporting elements is increased by virtue of the disconnect device releasing an arrangement such as to shorten the length ofthe gap when the disconnect device operates.

4. The combination claimed in claim 2 or 3 wherein the electrical conductivity of the air gap between the arc supporting elements is increased by virtue of the disconnect device being of a type incorporating an explosive charge which liberates ionised gas into the said air gap when it operates.

5. The combination claimed in claim 4 wherein the disconnect device is connected to said one terminal end of the surge arrester by means of a flexible conductor arranged so that when the disconnect device operates, a part thereof moves through the air gap between the arc supporting elements and positively introduces ionised gases into said air gap.

6. The combination claimed in claim 5 wherein the arrangement is such that gravity is sufficient to effect such movement.

7. The combination claimed in any of claims 4 to 6 wherein an arc shield is provided to inhibit ionised gas from the disconnect device from accessing the surge arrester.

8. The combination claimed in any of claims 2 to 7 wherein said arc supporting elements comprise material such as not to emit particles giving rise to a fire risk when an arc is struck therebetween.

9. The combination claimed in any of claims 2 to 8 wherein the surge arrester comprises a plurality of stacked varistor blocks formed into a rigid core and an extemal shedded housing provided on the core.

10. A surge arrester installation substantially as herein described with reference to Figure 5 ofthe accompanying drawings.

11. In or for a high voltage electrical installation the combination of an electrical surge arrester for providing a path to ground for high voltage surges encountered in operation of the installation, a disconnect device responsive to failure mode short circuit current through the surge arrester for disconnecting the same, and a mechanical device arranged to be triggered when the disconnect device operates and to place a conductor across the terminals of the surge arrester.

12. The combination claimed in claim 11 wherein said conductor comprises a flexible spring blade which is normally held by the disconnect device in a looped configuration and is released when the disconnect device operates.

13. The combination claimed in claim 11 wherein said conductor comprises a spring biassed rod which is normally retained by the disconnect device in an inoperative position and is released when the disconnect device operates.

14. The combination claimed in claim 11 or 12 or 13 including an arc gap in series with the surge arrester.

15. In or for a high voltage electrical installation, the combination of an electrical surge arrester for providing a path to ground for high voltage surges encountered in operation of the installation and a device capable of discriminating between normal surge arrester duty and failure mode condition, said device passing surge arrester currents encountered in normal operation of the surge arrester and, in the event of the surge arrester being damaged by excessive duty, serving to decouple the arrester from the installation before it can produce sparks and/or incandescent particles capable of igniting combustible material.

16. The combination claimed in claim 15 wherein said discriminating device comprises a current limiting fuse mounted in close-coupled relationship with the surge arrester.

17. The combination claimed in claim 15 or 16 further comprising a disconnect device shunting the fuse so that in normal surge arrester operation the surge duty is through the disconnect device and not through the fuse, but in the event of failure of the surge arrester the operation of the disconnect device causes system fault current to be diverted through the fuse.

18. The combination claimed in claim 17 wherein a spark gap is provided in parallel with the disconnect device.

19. The combination claimed in claim 18 wherein the spark gap is provided in series with the fuse and the disconnect device is connected across the series-connected spark gap and fuse.

20. A method of protecting a high voltage electrical installation against overvoltage surges, said method comprising providing one or more surge arresters in said installation to provide a path to ground for such surges and, to reduce the risk of emission of sparks and/or incandescent particles from the arrester(s) under failure mode condition, connecting to the or each said surge arrester an arrangement which is inoperative in normal surge arrester operation and, in the event of failure of the surge arrester, serves to decouple the surge arrester from the installation whilst continuing to provide a current path to ground.