GB2103018A

GB2103018A - Electrical switchgear

Info

Publication number: GB2103018A
Application number: GB08218996A
Authority: GB
Inventors: Harro Boinski; John Parry
Original assignee: South Wales Switchgear Ltd; International Standard Electric Corp
Current assignee: Hawker Siddeley Switchgear Ltd; International Standard Electric Corp
Priority date: 1981-07-02
Filing date: 1982-07-01
Publication date: 1983-02-09
Also published as: GB2103018B

Abstract

In a contacts closed position of the switchgear, the main contact 20 of a movable contact assembly 10 engages fixed contact fingers 14 while an arcing contact 22 thereof engages the inner periphery of a tubular arcing electrode 18 which is electrically connected to a blow out coil 19. The arcing contact 22 is mounted on the main contact 20 by means of a pivot 23 whose axis extends transversely of a central axis A of the coil 19. During movement of the contacts to an open position, the main contact 20 disengages from the fingers 14 while the arcing contact 22 remains in contact with the electrode 18, so that current then flows through the coil 19 to generate a magnetic field. When the arcing contact 22 subsequently disengages from the electrode 18, an arc is drawn therebetween and the already generated magnetic field causes the arc to rotate and become extinguished. A pad or sector of arc- resistant material 38 can be provided on the arcing electrode 18 at least in the region where the latter is contacted by the arcing contact 22. <IMAGE>

Description

SPECIFICATION Electrical switchgear This invention relates to electrical switchgear of the type comprising first and second contacts which are movable between open and closed positions, a tubular arcing electrode with which the first contact forming an arc during movement of the contacts from their closed position to their open position, and a field coil connected to the arcing electrode such that the arcing current flows through the field coil to create a magnetic field which causes the arc to rotate and become extinguished.

Generally speaking, in conventional electrical switchgear of this type an initial arc is drawn between the first and second contacts upon their disengagement, and the root of the arc subsequently transfers from the second contact to the arcing electrode, whereupon rotation of the arc can commence. Before and immediately after such transfer, the arc is relatively immobile and is outside the zone in which rotation subsequently takes place. The arc energy thus produced can impose a limit to the interrupting capacity of the switchgear, partly by exposing major components and insulators to the direct effect of the arc. In order to overcome this problem, various techniques have been proposed to improve the transfer of the arc to the electrode.

One such proposal is disclosed in UK Patent Application No.8103269 and European Patent Application No. 82300368.6, wherein the initial arc drawn between the contacts is transferred rapidly to the arcing electrode by electromagnetic effects, and is positioned within its future rotation zone immediately upon such transfer so that rotation can commence straight away. Although giving much improved transfer characteristics, this proposal is slightly disadvantageous in that it is still necessary to initiate a finite arc which is not subject to rotation prior to transfer of the arc root, and thermal effects from this arc are experienced outside the physical protection afforded by the tubular arcing electrode. An alternative proposal which overcomes these particular disadvantages is given in published UK Patent Application No. 2052160.In this proposal, the first contact has a part which engages the arcing electrode before and for some time after the first and second contacts disengage so that, apart from minor arcing upon such disengagement of the contacts, the intial arc is drawn directly between said part of the first contact and the arcing electrode.

It is an object of the present invention to provide an improvement in the latter proposal.

According to a first aspect of the present invention, there is provided electrical switchgear comprising first and second contacts which are movable between a closed position in which they are mutually engaged and an open position in which they are mutually separated, a tubular arcing electrode with which the first contact forms an arc during movement of the contacts from their closed position to their open position and a field coil connected to the arcing electrode such that the arcing current flow through the field coil to create a magnetic field which causes the arc to rotate and become extringuished, the first contact including a first part which engages the second contact when the contacts are in their closed position and a second part which is in engagement with the arcing electrode for some time before and after the first part of the first contact disengages from the second contact, the second part being mounted on the first part for pivotal movement relative thereto about a pivot axis transverse to the field coil axis.

Thus, when the first part of the first contact disengages from the second contact, current flows through the contact point between the second part of the first contact and the arcing electrode, and hence through the field coil.

Consequentiy, the arc-rotating magnetic field is already established when the second part of the first contact subsequently disengages from the arcing electrode. This, coupled with the fact that the arc is drawn directly between the first contact and the arcing electrode, means that the arc can commence its rotation immediately these parts separate.

Depending upon the rating of the electrical switchgear, the arcing electrode can be provided with arc-resistant material at least in a region where it is engaged by the second part of the first contact. Desirably, the arc-resistant material is provided in a localised region on the internal surface of the arcing electrode, so that the arc is encouraged to leave the same and travel in a spiral fashion along the arcing electrode under the influence inter alia of electromagnetic loop forces in the plane of the arcing electrode axis. Most preferably, the arc-resistant material is provided as a sector extending only part-way around the interior of the arcing electrode, and the ends of the sector may be chamfered to assist detachment of the arc therefrom.

Preferably, the first part of the first contact is pivotable about an axis transverse to the field coil axis and generally parallel to the pivot axis of said second part.

Conveniently, resilient means act between the first and second parts of the first contact to urge said second part towards the arcing electrode.

The resilient means thus applied a force at the engaging parts of said second part of the first contact and the arcing electrode which tends to open the contacts, and this not only supplements the force applied by springs in an external operating mechanism for the contacts but also assists release of the second part of the first contact in the event that it becomes welded to the arcing electrode.

Pivotal movement of the second part owt the first contact relative to the first part thereon in a direction towards the arcing electrode can be limited by means of an adjustable stop. By suitably adjusting the stop, the point during movement of the contacts at which the second part of the first contact separates from the arcing electrode can be accurately controlled.

Advantageously, the electrical switchgear further comprises an operating mechanism for moving the contacts between their open and closed positions, the operating mechanism including an operating link which is pivotally connected to the first part of the first contact at the pivot axis of the second part thereof.

In one arrangement, the first part of the first contact is of generally U-shaped cross-section, and the second part of the first contact is received between the limbs of said U-shape. Alternatively, the second part of the first contact can include a portion of generally U-shaped cross-section with the limbs of said U-shape being disposed on opposite sides of the first part of the first contact.

Most desirably, the electrical switchgear employs a highly insulating gas (such as sulphur hexafluoride) to assist in arc extinction.

According to a second aspect of the present invention, there is provided electrical switchgear comprising first and second contacts which are movable between a closed position in which they are mutually engaged and an open position in which they are mutually separated, a tubular arcing electrode with which the first contact forms an arc during movement of the contacts from their closed position to their open position, and a field coil connected to the arcing electrode such that the arcing current flows through the field coil to create a magnetic field which causes the arc to rotate and become extinguished, a part of the first contact being in engagement with a point on the internal periphery of the tubular arcing electrode for some time before and after the first contact disengages from the second contact during movement of the contacts from their closed position to their open position, the arcing electrode being provided with arc-resistant material at least in a region where it is engaged by said part of the first contact.

The invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic side view of a first embodiment of electrical switchgear according to the present invention, the switchgear being shown in a contacts closed position; Figure 2 is a similar view to Figure 1 but showing the switchgear with its contacts partially open; Figure 3 is a similar view to Figures 1 and 2 but showing the switchgear with its contacts fully open; Figure 4 is a sectional view taken along the line X-X in Figure 3, and Figures 5, 6 and 7 are schematic side views of a second embodiment of electrical switchgear according to the present invention, the switchgear being shown with its contacts closed, partially open, and fully open, respectively;; Figure 8 is a schematic side view of a third embodiment of electrical switchgear according to the present invention; and Figure 9 is a view of part of the switchgear of Figure 8, as viewed in the direction of arrow IX.

Referring first to Figures 1 to 4, the electrical switchgear shown therein comprises a first contact 10 which is mounted on a conductive support 11, the support 11 being in turn electrically connected to a conductor 12 which passes through an insulating bushing 13. A second contact formed by contact fingers 14 is mounted on a conductive support 15, the support 1 5 in turn being electrically connected to a conductor 1 6 which passes through an insulating bushing 1 7. Also mounted on the support 1 5 are a tubular arcing electrode 1 8 and a field coil 19 which is wound co-axially around the external surface of the electrode 18, the field coil 19 being electrically connected between the electrode 1 8 and the conductor 1 6. The whole assembly is enclosed within a tank or housing (not shown) which contains the highly insulating gas sulphur hexafluoride, the bushings 13 and 1 7 together with their respective conductors 12 and 1 6 extending to the exterior of the housing.

The first contact 10 is composed of a first contact arm 20 which is pivotally mounted at one end thereof on the support 11 by means of a pivot pin 21, and a second contact arm 22 which is pivotally mounted on an intermediate portion of the contact arm 20 by means of a pivot pin 23.

The pivot axes of the pins 21 and 23 are mutually parallel, and both are perpendicular to a central axis Ax of the field coil 1 9. In the region of the pivot pin 23, the contact arm 20 is of inverted Ushaped cross-section, with a flattened portion of the contact arm 22 being received between the limbs of the said U-shape, as can be seen to advantage in Figure 4. In the contacts closed position of the switchgear (shown in Figure 1) an end portion 24 of the contact arm 20 is engaged between the fingers 1 4 of the second contact, while an end portion 25 of the contact arm 22 engages the internal surface of the arcing electrode 1 8 adjacent one end thereof. Preferably, the end portion 25 is equipped with a special arcresistant material, which may be welded or brazed into position.

The flattened portion of the contact arm 22 has a U-shaped bracket 26 secured thereto. A head 27 on one end of a pin 28 is pivotally received between the limbs of the bracket 26, while the opposite end of the pin 28 is slidably received through an aperture in a flange 29 secured to the contact arm 20. A compression spring 30 surrounds the pin 28 and acts between the head 27 and the flange 29 to bias the contact arm 22 anticlockwise relative to the contact arm 20, thereby urging the portion 25 of the contact arm 22 against the arcing electrode 18. A flexible conductive strap 31 (which may be in the form of a loop) interconnects the contact arms 20 and 22 for the passage of current therethrough, while a similar strap (not shown) connects the contact arm 20 to the support 11 for the passage of load and short-circuit currents. Opening and closing of the switchgear is effected by means of an operating mechanism including a rotatable shaft 32 which passes sealingly through a side wall of the aforementioned housing, a crank arm 33 which is rotatable with the shaft 32, and an insulating operating link 34 which is pivotally connected at one end thereof to the crank arm 33 and at the other end thereof to the contact arm 20. In fact, the pivot pin 23 which mounts the contact arm 22 on the contact arm 20 also serves to connect the operating link 34 to the latter. For the sake of convenience, the crank arm 33 and the link 34 are shown schematically by chaindotted lines.

When the contacts are in their closed position, as depicted in Figure 1, the crank arm 33 occupies a position A. In order to open the contacts, the shaft 32 is rotataed anti-clockwise so that the operating link 34 pivots the contact arm 20 clockwise about the pivot pin 21 , thereby disengaging the end portion 24 of the arm 20 from the contact fingers 14. At this time, however, the end portion 25 of the contact arm 22 is still biassed by the spring 30 about its pivot pin 23 into contact with the arcing electrode 18.

Since the current path through the switchgear is still maintained by this contact, when the contact arm 20 disengages from the contact fingers 14 only minor arcing occurs therebetween due to the voltage drop across the field coil 1 9 and the items 18,25 and 31,etc.

When the first contact 10 reaches the position shown in Figure 2, the flattened portion of the contact arm 22 comes into abutment with the base of the U-shaped part of the contact arm 20.

Upon further pivotal movement of the contact arm 20, this abutment prevents the spring 30 from maintaining the contact arm 22 in engagement with the arcing electrode 18, so that separation occurs between the end portion 25 and the electrode 18 thereby causing an arc to be drawn therebetween. At this time, current is already flowing through the field coil 19, so that a magnetic field is already established when the end portion 25 separates from the electrode 1 8.

This magnetic field causes the arc to rotate around the end portion 25 of the contact arm 22 and become extinguished, such extinction being assisted by the sulphur hexafluoride gas in the housing. At the moment when the contact arm 22 disengages from the electrode 18, the crank arm 33 is disposed in a position C as depicted in Figure 2. The fully open position of the switchgear is shown in Figure 3, wherein the end portion 25 lies along the axis Ax of the field coil 19 and the crank arm 33 is disposed in a position B.

In the switchgear shown in Figures 1 to 4, the arcing electrode 1 8 and the field coil 1 9 are inclined to the vertical so that the axis Ax is nonhorizontal. Figures 5 to 7 show an alternative arrangement wherein these parts are disposed vertically, and wherein in addition the construction of the first contact 10 is modified.

More particularly, the contact arm 20 is of solid rectangular cross-section along the whole of its length, and the contact arm 22 includes a Ushaped portion 35 whose limbs embrace the contact arm 20 and are pivotally secured thereto by the pivot pin 23. The end portion 25 of the contact arm 20 is secured to the base of the Ushaped portion 35 between the limbs of the latter, with one end of the flexible strap 31 being sandwiched between these parts. The bracket 26 is now omitted, and the head 27 of the pin 28 is pivotally connected directly to the portion 35. The end portion 25 of the contact arm 22 disengages from the arcing electrode 1 8 to draw the initial arc when a chamfered surface 36 on the end portion 24 of the contact arm 20 abuts against a recessed surface 37 on the portion 25.Otherwise, the construction and operation of the switchgear shown in Figures 5 to 7 is similar or identical to that of the switchgear described above with reference to Figures 1 to 4. The embodiment of Figures 5 to 7 is intended to meet higher ratings of interrupting capacity and normal currents.

In both of the above-described embodiments, because the arc is drawn directly between the first contact 10 and the arcing electrode 18, the initial arc and at least its early rotation are contained within the protection of the tube formed by the electrode 1 8. Therefore, the problems experienced previously due to the thermal effects of the initial arc drawn between the first and second contacts 10 and 14 are avoided. Moreover, because the magnetic field is already established when the portion 25 of the contact arm 22 separates from the arcing electrode 18, arc rotation commences immediately upon such separation.Furthermore, electromagnetic loop forces in the plane of the Figures will cause the arc and its products to progress along the arcing electrode 18 away from the contacts 10 and 14, thereby minimising contamination produced by the arcing products and consequent restriking of the arc at this point.

Axial progression of the arc along the electrode 18 also gives rise to the advantages that the arc length is allowed to expand with consequential increased resistance, which aids interruption and minimises the production of over voltages, and the arc root can move over a comparatively large area with a resultant reduction in erosion of the electrode and greater extraction of heat energy from the arc. These factors combined enable a high interrupting capability to be obtained. A typical track of the arc root along the electrode 18 is indicated at T in Figures 3 and 7.

Another advantage of the switchgear described above is that the geometry of the various parts of the contact 10 produces a "rolling butt" contact between the end portion 25 of the contact arm 22 and the arcing electrode 18 which enables the arc to be initiated near the tip of portion 25 while allowing the portion 25 to roll or rock to a more favourable point for engagement with the arcing electrode 1 8 in the contacts closed position. The rolling butt contact also creates a self-cleaning action due to the slight sliding movement between the parts 25 and 18.

The compression spring 30 in urging the end portion 25 against the arcing electrode 1 8 exerts a force which tends to open the contacts, and thus supplements directly the opening force applied by tripping springs (not shown) in the operating mechanism, thereby adding to the tripping energy and providing an additional force to open the main contacts. In addition, if there is any tendency for the parts 25 and 1 8 to weld together, the release of these parts will be assisted by the force exerted by the spring 30, as well as by the above-mentioned rolling butt contact and the impulse applied to the contact arm 22 when it comes into abutment with the base of the U-shaped part of the contact arm 20 as depicted in Figure 2, or when the chamfered surface 36 comes into engagement with the recessed surface 37 as depicted in Figure 6.

When drawn, the initial arc is acted upon by the magnetic field already esablished by the current flowing through the field coil 19, which magnetic field causes the arc to rotate, and by the above-mentioned loop forces which cause the arc to move axially along the arcing electrode 18.

Both of these effects tend to move the arc root quickly from the initiating point on the surface of the electrode, so that erosion of the electrode 1 8 is minimised. Therefore, for certain ratings it may not be necessary to provide special arc-resistant material on the electrode 1 8. For higher ratings, however, it is possible to add a special arcresistant material 38 to the electrode 1 8 to protect the latter against erosion resulting from the arc continually being initiated at the same point. The loop forces which tend to move the path of the rotating arc and the arc products along the electrode 1 8 reduce the tendency for the arc to re-strike at the material 38.

The arc-resistant material 38 can be provided as a localised button or pad where the arcing electrode 1 8 is contacted by the end portion 25 of the contact arm 22, or may be in the form of a continuous annulus on the internal surface of the electrode 1 8. In the latter case, however, there is a danger that the electromagnetic loop forces may not be strong enough to move the arc root from the annulus, resulting in the arc rotating on the annulus of the arc-resistant material and not progressing along the electrode 1 8 as aforesaid.

In order to avoid this problem, it is preferred that the arc-resistant material 38 is provided in a sector extending only part-way around the electrode 18, as shown in the drawings. In this case, the arc root may become detached from the sector 38 during its initial rotation since the magnetic field produced by the field coil 1 9 and causing the arc to rotate will be stronger than the field causing the loop forces. During continued rotation, the arc will be free to move axially along the electrode 1 8 as described above. In order to assist detachment of the arc still further, particularly in the case of small currents, the ends of the sector 38 may be chamfered to approximately a crescent shape.The mobility of the arc root will, of course, generally be better on the basic material of the electrode 18 (e.g. copper or copper alloy) than on the special arcing material (e.g. copper tungsten).

In the above-described constructions, separation of the portion 25 from the arcing electrode 1 8 occurs when the contact arm 22 comes into contact with the base of the U-shaped part of the contact arm 20 in the case of Figure 1 to 4, and when the chamfered surface 36 on the contact arm 20 comes into abutment with the recessed surface 37 of the contact arm 22 in the case of Figures 5 to 7. An adjustable stop can however be provided between these parts, for example as shown in Figure 8.The switchgear shown in this Figure is generally similar to that illustrated in Figures 5 to 7 except that the portion 25 of the contact arm 22 is now replaced by a construction including a strap-like part 40 which is received between the limbs of the U-shaped portion 35, an adjusting bolt 41 being threadedly engaged with a hole in one end of the part 40 and bearing against the underside of the contact arm 20. At its opposite end, the part 40 has a cylindrical contact element 42 secured thereto which is positioned for engagement with the arcing electrodes 18. The bolt 41 limits pivotal movement of the contact arm 22 relative to the contact arm 20.

By suitably turning the bolt, the point during pivotal movement of the contact arm 20 at which the contact element 42 separates from the electrode 1 8 can be accurately set. In addition, the bolt 41 may be adjusted to centralise the contact element 42 within the electrode 18 in the contacts open position.

Figure 8 also shows the previously mentioned loop form of the conductive strap 31 which interconnects the contact arms 20 and 22, and further illustrates at 43 the aforesaid flexible conductive straps which are connected between the contact arm 20 and the support 11 (the latter not being shown in Figure 9). An alternative, diagonal mounting arrangement of the arcing electrode 18 and the field coil 19 on the support 1 5 is also shown, which arrangement avoids the need to cantilever these parts, for example in the manner indicated in Figure 6. In Figure 8, the end portion 24 of the contact arm 20 has a relatively sharp apex 44 between its upper and end surfaces: in an alternative arrangement, this apex may be radiussed in the manner indicated in Figures 5 to 7.

Also in Figure 8, the ends of the sector 38 of arcresistant material on the electrode 1 8 are radiussed to improve the shape of the sector and to prevent ,-e-strike of the arc back onto the arcresistant material. Figure 9 shows in particular the sector 38 when viewed axially of the arcing electrode 18.

In addition to the advantages mentioned above, the switchgear of the invention also has the following favourable effect. It is well known that electrical switchgear employing sulphur hexafluoride for arc extinction suffers much less contact erosion than an equivalent oil-filled circuit breaker, for example. The switchgear is, therefore, capable of performing an increased number of operations in service before maintenance is required. Indeed, it can be envisaged that on-site maintenance of the contacts may not be necessary at all during the effective life of the switchgear, thereby obviating the need to open the gas-filled tanks on site and avoiding the attendant need to handle the toxic products of arcing and to provide special gas-handling equipment.

In the embodiments described above, contact erosion can be assessed on site by determining the amount of rotation of the shaft 32 which is required to bring the contact arm 22 into engagement with the arcing electrode 18, for example with the use of a simple angular scale on the exterior of the housing at the shaft bearing.

Engagement of the arm 22 with the electrode 1 8 can be detected by connecting a special bell circuit to the conductors 12 and 1 6 externally of the tank after isolating the circuit breaker from its primary circuit. Where the switchgear is designed for three-phase operation, the bell circuit can be connected to each phase in turn. If measurements are repeated after a period of service, an accurate indication of contact erosion can be established and the further service life of the contacts can be estimated accordingly.

In both of the above-described embodiments, the contact arm 20 is mounted on the support 11 by means of a pivot pin 21, supplemented by a flexible connection, such that it is pivotable about an axis perpendicular to the field coil axis Ax.

However, other forms of mounting can be employed as long as the end portion 25 of the contact arm 22 still moves inwardly of the field coil axis as the contacts move towards their fully open position. For example, the pivot pin 21 can be replaced by a flexible connection. In addition, the flexible connection may be replaced by other means for carrying current between the support 11 and the contact arm 20, such as a rotating contact arrangement.

Claims

1. Electrical switchgear comprising first and second contacts which are movable between a closed position in which they are mutually engaged and an open position in which they are mutually separated, a tubular arcing electrode with which the first contact forms an arc during movement of the contacts from their closed position to their open position, and a field coil connected to the arcing electrode such that the arcing current flows through the field coil to create a magnetic field which causes the arc to rotate and become extinguished, the first contact including a first part which engages the second contact when the contacts are in their closed position and a second part which is in engagement with the arcing electrode for some time before and after the first part of the first contact disengages from the second contact, the second part being mounted on the first part for pivotal movement relative thereto about a pivotal axis transverse to the field coil axis.

2. Electrical switchgear as claimed in claim 1, wherein the arcing electrode is provided with arcresistant material at least in a region where it is engaged by the second part of the first contact.

3. Electrical switchgear as claimed in claim 2, wherein the arc-resistant material is provided in a localised region on the internal surface of the arcing electrode.

4. Electrical switchgear as claimed in claim 2, wherein the arc-resistant material is provided as a sector extending only part-way around the interior of the arcing electrode.

5. Electrical switchgear as claimed in claim 4, wherein the ends of the sector are chamfered.

6. Electrical switchgear as claimed in any preceding claim, wherein the first part of the first contact is pivotable about an axis transverse to the field coil axis and generally parallel to the pivot axis of said second part.

7. Electrical switchgear as claimed in any preceding claim, wherein resilient means acts between the first and second parts of the first contact to urge said second part towards the arcing electrode.

8. Electrical switchgear as claimed in any preceding claim, wherein pivotal movement of the second part of the first contact relative to the first part thereof in a direction towards the arcing electrode is limited by an adjustable stop.

9. Electrical switchgear as claimed in any preceding claim, further comprising an operating mechanism for moving the contacts between their open and closed positions, the operating mechanism including an operating link which is pivotally connected to the first part of the first contact at the pivot axis of the second part thereof.

10. Electrical switchgear as claimed in any preceding claim, wherein the first part of the first contact is of generally U-shaped cross-section, and the second part of the first contact is received between the limbs of said U-shape.

11. Electrical switchgear as claimed in any one of claims 1 to 9, wherein the second part of the first contact includes a portion of generally Ushaped cross-section with the limbs of said Ushape being disposed on opposite sides of the first part of the first contact.

12. Electrical switchgear as claimed in any preceding claim, wherein a highly insulating gas is employed to assist in arc extinction.

1 3. Electrical switchgear as claimed in claim 12, wherein the highly insulating gas is sulphur hexafluoride.

14. Electrical switchgear comprising first and second contacts which are movable between a closed position in which they are mutually engaged and an open position in which they are mutually separated, a tubular arcing electrode with which the first contact forms an arc during movement of the contacts from their closed position to their open position, and a field coil connected to the arcing electrode such that the arcing current flows through the field coil to create a magnetic field which causes the arc to rotate and become extinguished, a part of the first contact being in engagement with a point on the internal periphery of the tubular arcing electrode for some time before and after the first contact disengages from the second contact during movement of the contacts from their closed position to their open position, the arcing electrode being provided with arc-resistant material at least in a region where it is engaged by said part of the first contact.

1 5. Electrical switchgear as claimed in claim 14, wherein the arc-resistant material is provided in a localised region on the internal surface of the arcing electrode.

1 6. Electrical switchgear as claimed in claim 14, wherein the arc-resistant material is provided as a sector extending only part-way around the interior of the arcing electrode.

1 7. Electrical switchgear as claimed in claim 16, wherein the ends of the sector are chamfered.

1 8. Electrical switchgear substantially as hereinbefore described with reference to Figures 1 to 4 or Figures 5 to 9 of the accompanying drawings.