EP0011972B1

EP0011972B1 - Electrical switchgear

Info

Publication number: EP0011972B1
Application number: EP19790302617
Authority: EP
Inventors: John Parry
Original assignee: South Wales Switchgear Ltd
Current assignee: Hawker Siddeley Switchgear Ltd
Priority date: 1978-11-28
Filing date: 1979-11-19
Publication date: 1984-01-18
Also published as: EP0011972A3; EP0011972A2; DE2966563D1; EP0011972B2

Description

This invention relates to electrical switchgear, the term "switchgear" being used to embrace circuit breakers and other electrical switches.
A common form of distribution switchgear for voltages up to 36 kilovolts incorporates circuit breakers of the oil-filled type isolated from fixed units by vertical withdrawal. This range of switchgear also incorporates non-automatic load break switches and, especially for voltages of 12 kilovolts and lower, ring main equipment incorporating at least three switch functions to control, for example, a transformer and two ring main cables.
Although this type of equipment has been used satisfactorily for many years, in recent times circuit breakers have been developed which make use of the highly insulating gas sulphur hexafluoride to extinguish an arc drawn between contacts and subsequently made to rotate. It would be desirable to apply rotating arc sulphur hexafluoride techniques to the above-described switchgear to gain the advantages of higher interrupting performance with a corresponding reduction in the frequency of contact maintenance and freedom from fire hazard. It is particularly desirable for this class of equipment because of the small mechanical energy requirements resulting from the relatively short contact stroke and the fact that a mechanical compression device or puffer is not required. However, difficulties are experienced in applying rotating-arc sulphur hexafluoride techniques to circuit breakers and switches of the size associated with distribution switchgear up to 36 kilovolts. These difficulties include the need to ensure that the arc can be made to rotate reliably at all values of breaking current, and the need to provide a compact and economical arrangement which is not at a disadvantage in size or requirements of mechanical operating energy with respect to oil-filled equipment.
A switchgear construction which is capable in principle of satisfying the above-described needs is described in the article entitled "Unter- suchungen am rotierenden Schaltlichtbogen in Schwefelhexafluorid" by Markusch in ELEKTRIE No. 10, 1967, Berlin. In this construction, during opening of the switchgear contacts an arc is formed between a first contact and an arcing electrode, and the arcing current flows through a field coil connected electrically in series with the arcing electrode to produce a magnetic field which causes the arc to rotate with one root thereof maintained on the first contact and to become extinguished. The first contact is pivotable about an axis transverse to the axis of the field coil, and has an end portion which moves transversely of and towards the field coil axis when the contacts are moved to an open position.
The use of a pivotable first contact has the advantage that the pivot point controls the geometry of movement of the contact, no additional guides being required for this purpose. In addition, although not specifically described in the above-mentioned article, movement of the first contact between the contacts open and contacts closed positions can be performed by means of a lay shaft through the intermediary of a simple crank arm. These factors enable a compact construction to be obtained.
The switchgear disclosed in the Markusch article does, however, suffer from the major drawback that the field coil is connected in series with the arcing electrode and the first contact, so that it remains in circuit in the contacts closed position. Thus, the load current will flow through the field coil under no-fault conditions, and this gives rise to problems of heat generation which can impose a limit on the degree to which the size of the switchgear can be reduced. One possible answer is to provide a set of main contacts which are connected electrically in parallel with the field coil, the arcing electrode and the first contact, but this will greatly increase the bulk of the switchgear.
Published German Patent Application No. 2224082 discloses switchgear of the rotating arc type wherein coils which produce the arc-rotating magnetic field are kept out of circuit in the contacts closed position. In this switchgear, a pivotable first contact engages a fixed second contact in the contacts closed position, such that an arc is drawn therebetween during initial opening of the contacts. The roots of this arc are subsequently transferred to respective arc runners, thereby bringing into circuit the field coils (which are connected electrically in series with the arc runners), such that the magnetic field which is then generated causes the arc to rotate along a barrel-shaped path on the arc runners. During initial opening of the contacts, there will be no current flow through the field coils, and hence the arc runners will be at the same potential as the first and second contacts, respectively. Accordingly, there will be little likelihood of the arc transferring its roots as aforesaid (particularly the root on the first contact) until the gap between the first and second contacts has become somewhat larger than that between the arc runners. A significant delay will therefore occur before the field coils are brought into circuit, during which time the arc drawn between the first and second contacts will be highly unstable. This delay will impose a serious limitation on the current interrupting capability of the switchgear.
It is an object of the present invention to provide switchgear of the general type disclosed in the Markusch article, but wherein the field coil is kept out of circuit in the contacts closed position without the problems and disadvantages mentioned above.
The invention achieves this object by arranging for the first contact to engage a second contact disposed externally of the field coil in the contacts closed position of the switchgear such that an initial arc is drawn across a pole face of the field coil between the first and second contacts during movement of the contacts to their open position, and by arranging during further movement of the contacts to their open position for the arc to transfer its root from the second contact to the arcing electrode by the action of the end portion of the first contact passing within a short distance from the arcing electrode.
This arrangement has the particular advantage that transfer of the arc root to the arcing electrode can be obtained easily even when low fault currents (for example capacitive currents in unloaded cables and the small inductive currents associated with unloaded transformers and rotating machines) are to be interrupted.
Attention is hereby drawn to the fact that the embodiments of figs. 6, 7, and 10 are also disclosed in our European Application EP-A-0 012 522, published 25.06.1980, claiming a priority date of 28.11.1978, which claims electrical switchgear employing an electrically insulating fluid for arc extinction and comprising a pair of switches each having first and second contact means which are relatively movable between a closed position in which they are mutually engaged and an open position in which they are mutually separated, and an arcing electrode arrangement and shared field coil for both switches, movement of each switch to its open position causing an arc to be produced between the first contact means and the arcing electrode arrangement such that the arcing current flows through the shared field coil to create an arc-rotating magnetic field to extinguish the arc, characterised in that the second contact means of both switches are electrically connected to a common point, the shared field coil is electrically connected between said common point and the arcing electrode arrangement, and interlock means is provided to prevent simultaneous opening of the two switches.
Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:-

Figure 1 is a schematic diagram of a first embodiment of electrical switchgear according to the present invention, in the form of a circuit breaker for a single-phase electrical supply or one phase of a circuit breaker for a three-phase supply;
Figure 2 is a view in the direction of arrow II in Figure 1 of part of the electrical switchgear shown therein;
Figure 3 is a side view, partly in section, of a second embodiment of electrical switchgear according to the present invention;
Figure 4 is a front view, partly in section, of the electrical switchgear shown in Figure 3;
Figure 5 is schematic plan view of the electrical switchgear shown in Figures 3 and 4;
Figure 6 is a schematic diagram of a third embodiment of electrical switchgear according to the present invention, for use with ring main equipment;
Figure 7 shows a number of modifications which can also be applied to any of the above embodiments;
Figure 8 shows a further modification which can also be applied to any of the above embodiments;
Figure 9 is a sectional view taken along the line IX-IX in Figure 8; and
Figure 10 is a schematic diagram of a fourth embodiment of electrical switchgear according to the present invention, also for use with ring main equipment.

Referring to Figure 1, a circuit breaker is shown suitable for replacing an existing 12 or 36 kilovolt oil-filled circuit breaker in an electrical distribution system. The circuit breaker comprises a switch 1 contained in a gas-tight metal housing 2 on which terminal bushings 3 and 4 are mounted. The housing 2 and terminal bushings 3 and 4 correspond respectively to the tank and bushings of a conventional oil-filled circuit breaker. The interior of the housing 2 does not, however, contain oil but the well-known, highly insulating gas sulphur hexafluoride for the purpose of arc quenching. The gas is present preferably at a pressure of 3.17 kg/cm² (45 psi), and is supplied through a valve (not shown) in a wall of the housing 2. The mechanism of the circuit breaker is so constructed and arranged as to enable sulphur hexafluoride arc quenching to be applied to the breaking of currents occurring in an electrical distribution system within the space limitation imposed by making the circuit breaker a replacement for an existing oil-filled circuit breaker.
A conductor 5 passes through the bushing 3 and carries on its end within the housing 2 a transverse contact support arm 6 which carries resilient contact fingers 7, and a support member 8 which carries a field coil 9. A conductor 10 passes through the bushing 4 and carries on its end within the housing 2 a mounting 11 on which a movable contact arm 12 of circular cross-section is mounted for angular movement about a pivot 13. A flexible, electrically conductive strap 14 connects the contact arm 12 to the conductor 10 for the passage of most of the load current therethrough, the strap 14 being connected to the conductor 10 by way of an L-shaped copper bracket 15 and being bolted to the contact arm 12. As an alternative to the provision of the strap 14, the contact arm 12 can be mounted on the end of the conductor 10 by means of a spring loaded pivot through which the load current passes in use.
An operating shaft 16 is rotatable by means of an operating mechanism (not shown) disposed externally of the housing 2 and carries an arm 17 which is pivotally connected to one end of a linkage comprising a pair of parallel, spaced links 18 (only one shown) made of insulating material, such as PERMALI (Registered Trade Mark) which is a densified resin beech. The other end of the linkage is pivotally connected to the contact arm 12 at or near the centre of the latter, such that rotation of the shaft 16 causes the contact arm 12 to move angularly about the pivot 13 between a position in which an end portion 19 thereof is engaged with the contact fingers 7 (as shown in chain-dotted lines) and a position in which the end portion 19 is disengaged from the fingers 7 and is disposed on the axis of the field coil 9 (as shown in full lines). In the latter position of the contact arm 12 the axis of the operating shaft 16, the pivotal connection between the arm 17 and the linkage 18 and the pivotal connection between the linkage 18 and the contact arm 12 are substantially in a common plane. Therefore, any slight movement of the arm 17 due, for example, to play between the various parts or oscillation of the parts due to the absorbing of shocks upon opening of the switch will result in only a very small movement of the contact arm 12, and thus the end portion 19 thereof will remain substantially on the axis of the field coil 9.
A plate 20 of arc-resistant material is provided adjacent the contact fingers 7 to protect the support member 8 and the field coil 9 from the effects of arcing. The arc-resistant material of which the plate 20 is made can be either conducting or insulating. If it is conducting, it must be ensured that the plate cannot short out the fieldcoil 59. This can be arranged by fixing the plate 20 at an angle to the support member 8 so that it is normal to the end portion 19 of the contact arm 12 when the latter engages the contact fingers 7 and is directed away from the outer windings of the field coil and the support member 8. If necessary, for certain applications of the switchgear, the end portion 19 of the contact arm 12 can have a region 21 which is also protected by conducted arc-resistant material.
The support member 8 is made of mild steel such that it serves to concentrate the magnetic field produced by the field coil 9 and screens the coil from the effects of adjacent metalwork or current-carrying conductors. The support member comprises a portion 22 defining part of a cylinder (as shown to advantage in Figure 2) carried on integral mounting lugs 23. The field coil 9 comprises a spiral metal strip of the same width as the portion 22 and consists of, for example, twenty turns of sheet metal 0.5 mm thick. The turns are equally spaced from each other, insulation between the turns being provided by means of an insulating coating or an inter-wound insulating strip. An inner end of the field coil 9 is attached to and assists in supporting a tubular arcing electrode 24 made of nonferrous metal which projects beyond the ends of the field coil and its support member. A suitable means of attaching the inner end of the field coil to the electrode is by rivetting and/or brazing or soldering. An outer end of the field coil is bolted between one of the lugs 23 and the support arm 6, as can be seen in Figure 2.
The above-described circuit breaker operates as follows. In a closed position thereof, the end portion 19 of the contact arm 12 is engaged with the contact fingers 7 so that current can flow through the circuit breaker by way of the conductor 10, the strap 14, contact arm 12, contact fingers 7 and the conductor 5. Opening of the circuit breaker is performed by rotating the operating shaft 16 by way of the aforementioned operating mechanism to pivot the contact arm 12 out of engagement with the contact fingers 7. During such movement of the contact arm 12, the end portion 19 thereof moves transversely relative to the end of the field coil 9 to draw an arc from the contact fingers 7 radially across the pole face of the coil. This arc subsequently transfers itself from the contact fingers 7 to the electrode 24, so that the field coil 9 (previously out of circuit) now forms part of the current flow path through the circuit breaker. The current flowing through the coil 9 creates a magnetic field which causes the arc to rotate in a known manner and become extinguished.
A porthole 25 is provided in side wall of the housing 2 so that a visual inspection can be made of the internal mechanisms. The porthole also permits photography of the rotating arcs to be taken.
The above arrangement can, if desired, be applied to a mere switch rather than to a circuit breaker.
The circuit breaker described above is intended to control one phase of a three phase electrical supply, similar circuit breakers being provided for the other two phases. The circuit breaker is, however, also suitable for controlling a single phase electrical supply.
The switchgear illustrated in Figures 3 to 5 is in the form of a circuit breaker for use with a three phase electrical supply, and comprises three switches 101 a, 101 b and 101 c (one for each phase) contained in a common housing 102 filled with sulphur hexafluoride gas. Each of the switches is similar to that described above with reference to Figures 1 and 2, similar parts being denoted by the same reference numerals but with 100 added. A common operating shaft 116 is used to operate all three of the switches, and passes through a gas-tight bearing 126 in a side wall of the housing 102.
The three switches are disposed generally on a common axis 127. In order to optimise the electrical clearances and magnetic separations of the switches, the field coils 109 thereof are mutually staggered transversely of the axis 127. In the particular arrangement shown, this means that the coils 109 are disposed in a triangular array, as can be seen to advantage in Figure 5. The screening effect performed by the support members 108 is now of particular importance, since each support member shields its respective coil 109 from the effects of the other phases of the electrical supply.
Figure 6 illustrates switchgear for use with ring main equipment and comprises a pair of switches 201 a and 201 b for controlling respective ring main cables and a third switch 201 c for a tee-off circuit. The switch 201 c can provide automatic circuit breaking and/or can be associated with an externally-mounted high- capacity fuse: where three phases are provided, blowing of one such fuse can be arranged to cause the tee-off switches of all three phases to open.
Each of the switches 201 a, 201b and 201 c is generally similar to the switch 1 described - above in relation to Figures 1 and 2, similar parts being accorded the same reference numerals but with 200 added. However, the link mechanism which connects the operating shaft 216 of each switch to the respective contact arm 212 differs slightly from the arrangement depicted in Figure 1, in that triangular plates 230 are provided on the contact arm and the linkage 218 is pivotally connected to these plates, rather than being connected directly to the contact arm.
The ring main switches 201 a and 201 b are disposed adjacent one another and share a common field coil 209, support member 208 and arcing electrode 224. The contact arms 212 of the two switches are disposed at opposite ends of the field coil 209, and an electrically insulating member 231 extends transversely across the centre of the electrode 224 to help isolate the contact arms from each other when the switches are both in their open positions. Because the field coil 209 is spirally wound, it is symmetrical about a transverse plane through its centre: the coil 209 can, therefore, be relied upon to provide the same operating characteristics for each of the two switches 201 a and 201 b. A mechanical interlock (not shown) of known type is provided to prevent simultaneous opening of the switches 201 a and 201 b although consecutive opening (after the arc in one circuit has been extinguished) is permitted.
The field coil 209, support member 208 and arcing electrode 224 which are common to the switches 201 a and 201 b, and the corresponding parts of the tee-off switch 201 c are carried by a common insulating support 232 mounted on the housing 202. Moreover, the contact fingers 207 of all three switches are carried by a common support arm 206 which is in turn supported by the support 232. Again, the screening effect of the support members 208 is of particular importance since the coils 209 are shielded thereby against the effects of adjacent current-carrying conductors.
If desired, a fourth switch can be provided which shares the field coil and arcing electrode of the tee-off switch 201 c in the same manner as described above in relation to the ring main switches 201 a and 201 b. Again, a mechanical interlock will be used to prevent simultaneous opening of the switches. Reference 233 shows in broken line the manner in which a conductor and bushing for the fourth switch would be arranged on the housing 202.
Figure 7 illustrates a number of modifications which can be applied, singly or in combination, to any of the embodiments described above. Those components or elements which correspond to the parts of the switchgear embodiments already described are denoted by the same reference numerals as used in Figures 1 and 2 but with 300 added, and will not in general be described again.
In Figure 7, a cranked contact arm 312 is used instead of a straight one, the arm being pivoted at a point spaced from the axis of the field coil 309 so that in the open position of the switch the end portion 319 of the contact arm not only lies along the axis of the field coil but also extends into the adjacent end of the arcing electrode 324. This arrangement helps in transferring the arc from the contact fingers 307 to the electrode 324, and brings the arc within the coil where the magnetic field is more concentrated.
The arcing electrode 324 has a radial flange 340 at an end thereof which faces the contact arm 312 and is also provided with an internal annular insert 341 of bulged cross-section. The insert forms a so-called arc runner along which the arc tracks during its rotation, so that the arc can be made to rotate in a predetermined plane which is chosen with regard to the magnetic field generated by the field coil. The arrangement as illustrated is not suited to being shared between two switches: however, the provision of a flange and an annular insert at the other end of the electrode to give a symmetrical construction and the addition of a central insulating member similar to that referenced 231 in Figure 6 will enable the arrangement to be made common to two switches.
The field coil 309 is helically, rather than spirally, wound. If the coil is to be shared between two switches, it is to be appreciated that the inherent asymmetry of the helical coil may result in some difference in operating characteristics between the two switches. Because the helical coil 309 is not self-supporting, a separate mechanical support is provided for the arcing electrode 324. This support can be in the form of an electrically-insulating member 342 as shown, or the coil can be cast onto the electrode using, for example, an epoxy resin.
An electrically conductive finger 343 is provided on the supprt arm 306 adjacent the contact fingers 307, the initial arc being drawn from this finger rather than from the contact fingers 307 when the contact arm 312 moves away from the latter. The finger 343 can thus be made of arc-resistant material, whereas this may not be desirable for the contact fingers 307.
Figures 8 and 9 show two modifications (usable singly or in combination) to the switchgear of Figures 1 and 2 but which can likewise be applied to the switchgear embodiments of Figures 3 to 6 and which can be used in combination with modifications shown in Figure 7. Components or elements shown in Figures 8 and 9 which correspond to parts described already are given the same reference numerals as used in Figures 1 and 2 but with 400 added, and will not in general be described again.
In Figures 8 and 9, an insulated supporting cup 450 is provided within the arcing electrode 424 and has mounted therein a ferromagnetic ring 451. The cup 450 shields the ring 451 from the arc, and the ring concentrates the magnetic field produced by the field coil 409 to aid arc extinction. The action of the ring is of particular benefit when breaking relatively low currents. For some applications of the switchgear, it may be desirable to permit a flow of gas axially through the electrode 424, and for this reason, the supporting cup 450 can be made of annular configuration as indicated in broken line in Figure 8.
A ferromagnetic yoke 452 is provided to concentrate the magnetic field to encourage the initial arc to stay at the end of the contact arm 412 to facilitate transfer to the electrode 424. If desired, the yoke 452 can be covered in insulating material (for example, epoxy resin) to enable it to be placed close to the initial arc. The yoke enhances the action of the electromagnetic loop defined by the contacts and the arc.
Figure 10 shows schematically how the features shown in Figures 6 and 7 can be combined to produce ring main switchgear of compact form. A metal housing 500 filled with sulphur hexafluoride gas has mounted therein two ring main switches 501 and 502 which share a common field coil assembly 503 and a tee-off circuit breaking or load break switch 504 (which has a similar function to the switch 201 c in the embodiment of Figure 6) which has an associated field coil assembly 505. The field coil assemblies 503 and 505 and fixed contact assemblies 506 for the various switches are all carried by a common insulating support 507. An insulating member 507' is provided transversely of the shared coil assembly 503 to isolate the contact arms of the ring main switches 501 and 502 from one another when in their open positions. If desired, a fourth switch whose bushing is indicated in broken line at 508 can also be provided to share the field coil assembly 505 with the switch 504. The conductor bushings for the switches 501, 502 and 504 can be arranged radially of the housing 500 as shown in full lines, or tangentially of the housing as indicated in broken lines.
If desired, features shown in Figures 8 and 9 can also be provided in this arrangement.
As an alternative to the use of circular cross-section components, the contact arms on the embodiments of Figures 1 to 10 can be of rectangular cross-section, and the field coil and arcing electrode can be of oval cross-section.
The use of a rectangular cross-section arm is advantageous in that any burning caused by the arc upon opening of the switch under fault conditions occurs at the corners of the contact arm, the side surfaces of the contact arm which engage the fixed contact fingers in the closed position of the switch being substantially unaffected by such burning.
The invention has other applications besides the distribution switchgear described above. It is applicable to the control of industrial circuits and to distribution and transmission circuits at higher voltages. It can also be supplied to circuit breakers and switches having an insulated enclosure.

Claims

1. Electrical switchgear employing an electrically insulating fluid for arc extinction in which during opening of the switchgear contacts an arc is formed between a first contact (12) and an arcing electrode (24), the arcing current flowing through a field coil (9) connected electrically in series with the arcing electrode (24) to produce a magnetic field which causes the arc to rotate with on root thereof maintained on the first contact (12) and to become extinguished, the first contact being pivotable about an axis transverse to the axis of the field coil (9) and having an end portion (19) which moves transversely of and towards the field coil axis when the contacts are moved to an open position, characterised in that the first contact (12) engages a second contact (7) disposed externally of the field coil (9) in the contacts closed position of the switchgear such that an initial arc is drawn across a pole face of the field coil (9) between the first and second contacts (12, 7) during movement of the contacts to their open position, and during further movement of the contacts towards said open position the arc is caused to transfer its other root from the second contact (7) to the arcing electrode (24) by the action of the end portion (19) of the first contact (12) passing within a short distance from the arcing electrode (24).

2. Electrical switchgear according to Claim 1, wherein the end portion (19) of the first contact (12) is elongate and lies along the field coil axis when the switchgear is fully open.

3. Electrical switchgear according to Claim 1 or 2, wherein the end portion (319) of the first contact (312) extends into the field coil (309) when the switchgear is fully open.

4. Electrical switchgear according to Claim 1, 2 or 3, wherein the first contact (312) is pivoted at a point (313) spaced from the field coil axis. 5. Electrical switchgear according to any preceding claim, wherein the first contact (312) is a cranked arm which is pivoted at an end thereof remote from said end portion (319).

6. Electrical switchgear according to any preceding claim, further comprising a preferably annular ferromagnetic member (451) disposed at least partly within the field coil (409) to concentrate the magnetic field produced by the latter.

7. Electrical switchgear according to Claim 6, wherein the arcing electrode (424) is tubular and has the ferromagnetic member (451) disposed therein.

8. Electrical switchgear according to any preceding claim, wherein a ferromagnetic yoke (452) is associated with the second contact (407) to assist in positioning the initial arc on the end portion (419) of the first contact (412).

9. Electrical switchgear according to any one of Claims 1 to 4 and 6 to 8, wherein the field coil (9) is composed of a self-supporting strip of conducting material arranged in a spiral, an outer end of the spiral being attached to mounting means (8) and an inner end thereof being attached to the arcing electrode (24).

10. Electrical switchgear according to any preceding claim, wherein the arcing electrode (24) is in the form of a tubular member about the outside of which the turns of the field coil (9) run.

11. Electrical switchgear according to Claim 10, wherein the tubular member has a flange (340) which faces the first contact (312).

12. Electrical switchgear according to Claim 10 or 11, wherein the tubular member has an internal annular projection (341) along which the rotating arc runs.

13. Electrical switchgear according to any preceding claim, including two switches (201 a, 201 b) each of which has respective first and second contacts (212, 207), the switches sharing a common field coil (209) on opposite sides of which the first contacts are respectively disposed.

14. Electrical switchgear according to Claim 13, wherein the arcing electrode (224) is a tubular member common to both switches (201a, 201b).

15. Electrical switchgear according to Claim 13 or 14, wherein an insulating member (231 ) is arranged transversely within the arcing electrode (224).

16. Electrical switchgear according to any preceding claim, including a plurality of switches (201a, 201b, 201 c) each having respective first and second contacts (212, 207), the second contacts (207) being mounted on a common insulating support (232).

17. Electrical switchgear according to Claim 16, including three switches (201a, 201b, 201 c) and two field coils (209), one of the field coils being shared between two of the switches (201 a, 2016).

18. Electrical switchgear according to Claim 16, including four switches and two field coils, each field coil being common to a respective pair of the switches.

19. Electrical switchgear according to any one of Claims 1 to 12, including a plurality of switches (101a, 101b, 101c) each having a respective field coil (109) and a respective arcing electrode (124), the switches being disposed substantially on a common axis (127) with the field coils (109) mutually staggered transversely of said axis.

20. Electrical switchgear according to Claim 19, including three switches (101a, 101b, 101c) whose associated field coils (109) are disposed in a triangular array.

21. Electrical switchgear according to any preceding claim, wherein the electrically insulating fluid is a highly insulating gas, preferably sulphur hexafluoride.