This invention relates to medium voltage electrical switching apparatus and relates particularly, but not exclusively, to switches and circuit breakers as used in an electrical ring main unit for medium voltage electrical supply distribution.

One known type of three phase ring main unit is disclosed in European patent application No 0283189, and comprises a lower chamber in which are mounted three input contacts, one for each phase, and three output contacts associated therewith. Three lower switching contacts are each movable between different switching positions, one switching position of the lower switching contacts effecting electrical connection between the input contacts and the associated output contacts. The ring main unit also comprises an upper chamber within which are mounted three tee-off contacts, one for each phase, and three upper switching contacts, each movable between different switching positions, one switching position of the upper switching contacts effecting electrical connection between the output contacts of the lower chamber and the corresponding tee- off contacts.

During movement of the upper or lower switching contacts between switching positions of such a ring main unit in the medium voltage region, arcing can occur across gaps between the switching contacts and the output and / or tee-off contacts. Such arcing is highly hazardous, and it is necessary to minimise its occurrence. With either load current or fault current the switching contacts must be moved at or above a minimum speed, and with fault current specific arc extinguishing technology is necessary. ^'

UK patent application No 2202089 discloses an electrical switch drive mechanism suitable for effecting rapid movement of the switching contacts of a medium voltage ring main unit. Movement of the switching contacts is achieved by charging a spring arrangement using a spring charging lever and, at the end of the travel of the spring charging lever, releasing a trigger to allow springs to rotate a drive shaft, which in turn drives the switching contacts at or above the reguired minimum

speed.

Although this arrangement enables the necessary rapid movement of the switching contacts, the spring charging arrangement is both complex in construction and bulky.

UK patent application No 2141295 discloses an electrical assembly for use in low voltage supply distribution in which a fuse carrier is mounted for rectilinear sliding movement between a first position and a second position wherein movable contacts on the carrier engage fixed contacts to close a circuit therebetween. A drive unit is provided for mechanically driving a selected fuse carrier between the two positions, the drive unit being a self-contained mechanism which can be detachably coupled to the required carrier.

By providing the mechanism for moving the fuse carrier in a separable drive unit, the bulk of the electrical assembly is reduced. The drive unit also shields the operator from any arcing which takes place during movement of the fuse carrier. However, this low voltage arrangement is not directly applicable to medium voltage switching.

The present invention seeks to overcome the disadvantages of the prior art by providing a medium voltage electrical switching apparatus that is simpler and cheaper than existing apparatus, that is of reduced bulk and that ensures sufficient speed of movement of switching contacts of the apparatus.

According to a first aspect of the invention, there is provided a medium voltage electrical switching apparatus comprising a switching compartment, first and second fixed contacts at spaced apart locations within the switching compartment, a switching contact movable within the switching compartment between a first position connecting the fixed contacts and a second position wherein said connection is broken, and drive means for effecting movement of the switching contact in either sense between its two positions, in which the drive means comprises a drive shaft extending through a wall of the switching compartment, locking means for locking the drive shaft against rotation when the switching contact is in the first or the second position, lock releasing means responsive

to torque applied to the drive shaft to release the locking means when the applied torque reaches a certain minimum value and so allow the shaft to drive the switching contact from one position to the other, and coupling means for allowing torque to be applied to the drive shaft from extemally of the switching compartment.

The provision of locking means for locking the drive shaft against rotation when the switching contact is in the first or the second position, in conjunction with lock releasing means which releases the locking means when torque applied to the drive shaft reaches a certain minimum value has the advantage of ensuring that a sufficiently large driving torque is applied to the drive shaft during rotation thereof to permit such movement of the switching contact to occur at or above the necessary minimum speed. The arrangement also reduces the bulk of the electrical assembly compared with the case of a spring arrangement which is continually charged during movement of the switching contact.

The apparatus preferably further comprises a sealed chamber defining said switching compartment, wherein said drive shaft extends through a seal in a wall of the chamber. Alternatively, the apparatus may be provided with a vacuum interrupter, or with more than one sealed chamber.

Preferably, the switching contact is pivotally movable between said first and second positions

Preferably, the apparatus further comprises a third fixed contact within the chamber at a location spaced apart from the first and second contacts wherein the switching contact is also movable in an opposite sense to movement between said first and second positions between said second position and a third position connecting the first fixed contact with the third fixed contact, and selector means operable only in the second position to selectively allow either movement between the first and second positions or between the second and third positions.

The provision of selector means operable only in the second position has the advantage that the switching contact is incapable of being driven directly between the second and third

fixed contacts. This is particularly important for safety reasons in the case that the second fixed contact is a live terminal and the third fixed contact is an earth terminal.

The apparatus preferably further comprises a circuit breaker including additional drive means connected to the drive shaft for effecting movement of the switching contact from the first position to the second position only, a fault detector, means responsive thereto to operate the additional drive means, and arc extinction means within the switching compartment to extinguish any arc struck between the first and second contacts.

This is particularly advantageous for safety reasons in the medium voltage region.

The apparatus may be incorporated in an electrical ring unit.

The apparatus is preferably incorporated in a three phase electrical ring main unit comprising a first set of three input contacts, one for each phase, three output contacts, one for each phase, and three switching contacts arranged on a common drive shaft, a second set of three input contacts, one for each phase, three output contacts, one for each phase, and three switching contacts arranged on a common drive shaft, and a set of three auxiliary contacts, one for each phase, three tee-off contacts, one for each phase, and three switching contacts arranged on a common drive shaft, wherein each drive shaft has an associated locking means for locking the drive shaft against rotation when the corresponding switching contacts are in the first or second position, and an associated lock releasing means responsive to torque applied to the drive shaft to release the locking means when the applied torque reaches a certain minimum value and to allow the shaft to drive the corresponding switching contacts from one position to the other, and wherein each drive shaft has substantially similar coupling means for allowing torque to be applied to the drive shaft by means of a common handle to be selectively engaged from externally of the chamber.

The or each locking means preferably comprises a

substantially flat plate rigidly mounted to the drive shaft and a respective detent arrangement for engaging the or each plate when the corresponding switching contact is in said respective first or said second position.

The construction of the or each locking means in this way permits a particularly compact construction of the apparatus.

Preferably, the detent arrangement is a ball bearing and aperture detent arrangement in which a ball bearing is biassed to engage an aperture and / or notch when the switching contact is in said first or second position.

The apparatus preferably further comprises a housing accommodating the switching compartment therein, wherein the or each substantially flat plate is mounted within the housing and the or each detent arrangement acts between the housing and the corresponding substantially flat plate.

The or each substantially flat plate may be arranged externally of the switching compartment. Alternatively, the or each substantially flat plate may be arranged within the switching compartment.

By providing the or each flat plate externally of the switching compartment, any necessary maintenance of the drive means can be carried out without the necessity of opening the switching compartment.

Preferably, the or each selector means comprises a pin for selectively engaging one of two arcuate apertures or recesses in the associated substantially flat plate for constraining the plate to selectively pivot between first and second positions and second and third positions, and wherein the arcuate apertures or recesses are separated by a radial aperture or recess which is engaged by the pin in the second position.

The or each detent arrangement advantageously comprises biassing means having an adjustable biassing force for biassing the detent arrangement into engagement with the corresponding substantially flat plate.

The provision of biasing means having an adjustable biasing force has the advantage of enabling the minimum value

of torque applied to the drive shaft to release the locking means to be adjusted to suit the particular circumstances of the apparatus.

The apparatus is preferably adapted for application of torque to the drive shaft from externally of the switching compartment by means of a manually actuable handle and / or a remotely operated motor.

According to another aspect of the invention, there is provided a combination of a medium voltage electrical switching apparatus as defined above and a separable handle for allowing torque to be applied to the drive shaft from externally of the switching compartment, the handle comprising a shaft, engaging means for engaging the drive shaft, and spring charging means acting between the shaft and the engaging means for delivering energy stored in the spring charging means to the drive shaft when the applied torque reaches a certain minimum value.

The provision of a separable handle enables the apparatus to be made operable only by authorised personnel equipped with such handles.

The spring charging means may be a torsion spring charging means.

Preferably, the spring charging means further comprises a ratchet for only allowing torque to be delivered from the handle in one sense.

This provides the advantage of preventing hazardous movement of the switching contact in one sense immediately after movement of the switching contact in the other sense.

Preferably, the handle further comprises additional engaging means for allowing torque to be applied to the drive shaft in an opposite sense to that applied via the engaging means.

By providing additional engaging means in this way, torque can be applied to the drive shaft in either sense by means of a single handle, but the necessity of using the additional engaging means prevents hazardous movement of the switching contact in one sense immediately after movement of the switching contact in the other sense.

A ring main unit embodying the invention will now be described in more detail, by way of example only, with reference to the accompanying drawings in which:-

Figure 1 is a front view of a casing of a ring main unit incorporating an aspect of the invention;

Figure 2 is a side view of part of the casing of the ring main unit of Figure 1 ;

Figure 3 is an end view of the interior of the ring main unit of Figures 1 and 2;

Figure 4 is a front view to reduced scale of the unit of Figure 3;

Figure 5 is a partially cut away view of a handle and mechanism for applying torque to a drive means of the ring main unit of Figures 1 to 4;

Figure 6 is a cross-sectional view of a detent arrangement shown in Figure 5;

Figure 7 is an exploded view of a drive means of the ring main unit of Figures 1 to 6 for use with a circuit breaker arrangement; and

Figure 8 is an assembled view of the drive means of Figure 7.

Referring first to Figures 1 and 2, these show a casing 1 of a three-phase medium voltage ring main unit for use with voltages within the range of 33,00 V to 24,000 V, the front of the casing being closable by a door 2 that can be locked in the closed position. The casing is designed to be weather-proof and vandal resistant, and can be fitted with external fixings that can not be released by conventional tools. Sealing means are associated with the door so that weather-proofing is maintained even in the door area. Supported in the upper part of the casing is a tee-off circuit breaker / earth switch housed in the upper module of a two-module cast resin chamber 3, and two ring switches housed in the lower module of the chamber. The ring switches and circuit breaker are fault- making, load-break switches, and the earth switches are fault- making switches. A control console of the casing incorporates a mimic diagram 4 and has switch control panels 5 and 6 for

operating the respective ring switches and a switch control panel 7 for controlling the tee-off circuit breaker / earth switch.

The panels each incorporate means for selecting either a main or an earth switch, and for moving the selected switch between on and off positions. They are shown in the condition wherein the main switches (M) are selected for operation on panels 5 and 7, the earth switch (E) is selected for operation on panel 6, and all three selected switches are in the on position. This information is shown on the mimic diagram below the control panels.

The casing also includes drive means (whose function is described in further detail below with reference to Figures 5 and 6) for the ring switches and an operating mechanism 206 for the circuit breaker, arranged along the front of the chamber 3, and which is described in further detail below with reference to Figures 7 and 8. The drive means are controlled from the panels 5 to 7, and the circuit breaker is also under the control of a mechanism responsive to detection of a fault on the circuit. Many different types of fault detector are known, and can be used to trip the circuit breaker as necessary. Reset means will generally also be provided to reset the circuit breaker after a fault has been remedied.

Figures 3 and 4 show the tee-off circuit breaker / earth switch and ring switch units in more detail. Further description is given in our European patent application No 0283189. The chamber 3 is formed by a lower chamber module 30 and an upper chamber module 31 , the two modules being mounted one on top of the other and being secured together by nut and stud arrangements such as 32. Each module is cast from or moulded in a suitable insulating resin, and the exposed outer surfaces of the modules are sprayed with a conductive coating so as to provide electrical screening. Suitable sealing means are located between the two modules, and the whole of the free space within each module is filled by an electrically insulating medium, usually SF ₆ gas. A base 55 of the upper module is formed integrally with the wall structure of that

module, and closes the open top of the lower module.

The lower module has a first set of three tapered input bushes such as 33a moulded integrally therewith, one for each phase of a first three-phase electrical supply. Conductors 34a, 34b, 34c of the supply pass through respective ones of the bushes to terminate within the lower chamber. The axes of the bushes and conductors are all parallel. Each of the conductors 34 has an input contact such as 35 secured to its inner end by bolts 36, and each input contact pivotally supports a switching contact 37a, 37b, 37c respectively. The axes of the input contacts lie in a plane A-A, parallel to the plane in which the axes of the bushes and input conductor lie.

Each of the switching contacts 37a, 37b, 37c is connected by a respective link 38a, 38b, 38c, each formed by two parallel arms of insulating material, and a respective lever arm such as 39a, to an operating shaft 40, all three switching contacts thus being driven from the same operating shaft. The shaft is supported by suitable bearings 41, 42 in the module 30, and sealing means 43 are located around the shaft where it leaves the module.

The switching contacts 37a, 37b, 37c are shown in the off position in solid lines in Figure 4, and they can be driven by rotation of shaft 40 either downwardly into connection with respective earth contacts 48a, 48b, 48c or upwardly into contact with respective output contacts 49a, 49b, 49c. The earth contacts are moulded in situ in the lower module and extend through a lower part thereof to leave exposed conductive sections 50a, 50b, 50c which can be connected to a star point earth contact or, if appropriate interlocks are embodied, to a test circuit as required.

A second set of three tapered input bushes 51a, 51b, 51c and appropriate conductors, one for each phase of a second three-phrase electrical supply, is associated with the lower module at the opposite side thereof, the arrangement being substantially a mirror image of the switching arrangement associated with input conductors 34a, 34b,34c. Thus, each conductor has an associated switching contact such as 52 driven

from an operating shaft 52a so that it can adopt a central off position, a lower on position in contact with a respective one of three earth contacts 53a, 53b, 53c or an upper on position in contact with a respective one of three output contacts 54a, 54b, 54c. The lower chamber thus houses two three-phase ring switches each capable of connecting a respective set of three input contacts to a respective set of three output contacts. Each switch is driven from a separate shaft 40 or 52a which is in turn driven by a drive means (see Figures 5 and 6) under control of the respective switch control panel 5 or 6.

Each set of earth contacts is enclosed by a star point cover 200, 200a (Figure 1 ) pivotally mounted on the casing. Star point interlocks of any suitable form control opening of the covers to allow removal of the contact. One way of controlling a suitable interlock is to apply to the front of the casing a cover plate 310, shown in dotted outline in Figure 1 , the plate having an axially captive key engagable in a keyhole 311 in the casing. When properly positioned, the key may be turned to lock the plate in position and release the interlock for the star point cover. The plate will cover the appropriate point of the mimic diagram and display a test symbol.

An insulating barrier plate 201 may optionally be secured in the module 30 between the switching contacts 37 and 52, and insulating phase barriers such as 202, 203 may optionally be secured in the housing between switching contacts of adjacent phases of the same switch.

The output contacts 49 and 54 are all moulded in situ in the base 55 of the upper module 31 , and they are joined by busbars 56, 57, 58 to three busbar contacts 59, 60, 61 projecting upwardly from the upper surface of the base 55, and secured by bolts such as 62 to the respective busbars. It will be seen that the axes of the output contacts 49 are parallel and lie in a plane B-B that is perpendicular to the plane A-A, and also that the axes of the busbar contacts 59 to 61 are parallel and lie in a third plane C-C that is perpendicular the plane B-B and lies at an angle (as shown about 80°) to plane A-

A. Similarly, the plane B-B of the axes of the output contacts 54 is parallel to the plane B-B, and so perpendicular to the planes A-A and C-C. The busbar contacts 59 to 61 are located in line in the upper chamber in an optimum position for efficient mounting and location of a three-phase tee-off circuit breaker / earth switch assembly in the upper chamber.

The tee-off circuit breaker / earth switches of the three phases are indicated generally at 71 to 73 in Figure 4, and only switch 71 will be described in detail, it being understood that the other two switches are similar, although they operate between different ones of the busbar contacts 59 to 61 and different outlet conductors. Switch 71 (Figure 3) is capable of connecting busbar contact 59 to an outlet conductor 74 passing through a tapered bushing 75 moulded integrally with the upper module. The busbar contact 59 is connected to an arcing electrode 76 having a substantially circular periphery. The arcing electrode is located within, and coaxial with, a conductive ring 77 around which is located a coil 78 held on an insulating support 79. The support is secured in position within the housing by bolts 80 fixed to appropriate brackets 81 within the housing. One end of the coil winding is electrically connected to the ring 77, and the other end of the coil winding is electrically connected to an auxiliary contact 82, spaced apart from the busbar contact 59.

It will be particularly noted that the coil 78 and its mounting is a self-contained assembly without any permanent electrical connection. Individual coil arrangements can thus be pre-assembled, or a sub-assembly including coils for all three phases can be pre-formed. Coil construction and fitting can thus be simplified

A switching contact 83 is pivotally mounted at 84 on a tee-off contact 85 secured by bolts 86 to the outlet conductor 74. The switching contact can be driven from an operating shaft 87, supported by bearings 87a, by a lever arm 88 and linkage 89 between an off position where the switching contact connects to auxiliary contact 82 and the tee-off contact 85, and an on position wherein the switching contact connects the

busbar contact 59 and the tee-off contact 85. In addition, the switching contact may be driven in the opposite direction from the auxiliary contact 82 into engagement with an earth contact 90, so that the switching contact then connects the earth contact and the tee-off contact 85. The switching contacts 83 of all three phases are driven in unison from a common shaft 87, which is in turn driven by a drive means under control of the switch panel 7, or by tripping in response to a detected fault.

Figures 5 and 6 show a drive means 500 for use with either of the ring switches and a hammer-shaped handle 501 having a mechanism for applying torque to a drive shaft 502 of the drive means to move the switching contacts 37 or 52 between switching positions. The handle 501 is constituted by an elongate torque applying shaft 503, to one end of which is attached a cylindrical housing 504 arranged with its axis perpendicular to the axis of the shaft 503. The housing 504 contains a cylindrical fixed ratchet sprocket 505 fixed to shaft 506 such that sprocket 505 is rotatably mounted in the housing and shaft 506 protrudes from both ends of the housing 504. A torsion spring 507 is arranged around the fixed ratchet sprocket 505 and has one of its ends attached to one end of the housing 504 and the other of its ends in engagement with movable ratchet sprocket 508 mounted on the sprocket 505. Sprocket 508 is freely rotatable relative to sprocket 505 in one sense only. Toothed engaging means 509, 510 are arranged at the end of the sprocket 505 coaxially therewith, and protrude through suitable apertures in the ends of the housing 504.

The drive shaft 502, which is connected to shaft 42 or 529 has a toothed socket 511 at its end protruding from the casing 1 for receiving either of the engaging means 509, 510. The socket 511 is so shaped as to be unsuitable for engagement by conventional tools.

A flat circular plate 512 is rigidly mounted to the drive shaft 502 coaxially therewith between support plates 513 through which the drive shaft is rotatably mounted. The

circular plate 512 is provided with two arcuate slots 514, 515 therethrough which are connected by a radial slot 516 having generally parallel sides. The circular plate 512 is also pivoted with a pair of circular apertures 517 therethrough which are so sized that when the switching contacts are in a switching position, one of the apertures 517 is engaged and gripped by a pair of ball bearings 518, each of which is supported by one of the support plates 513 and is urged towards the circular plate 512 by means of a compression spring 519. In this way, the gripping force of the ball bearings 518 on the plate is greater when in a switching position than between switching positions, so that when sufficient torque is applied to the drive shaft 502 to overcome the detent force of the ball bearings 518 gripping the aperture to rotate the plate 512, the gripping force is then reduced when the apparatus 517 moves free of the ball bearings 518 so that the plate 512 can rotate rapidly. This arrangement is shown in greater detail in Figure 6. The gripping force of the ball bearings 518 may be adjusted by adjusting the spring force of compression springs 519.

The support plates 513 are each provided with a radial slot 520 for receiving a pin 521 of rectangular cross-section which also passes through the arcuate slots 514, 515 in the circular plate 512. The slots 520 are aligned relative to each other so that the pin 521 is only movable along the slots 520 when the radial slot 516 in the circular plate 512 is aligned with the slots 520 in the support plates 513. The pin 521 is connected to the corresponding mains switch (M) and earth switch (E) on control panel 5 or 6. The slot 516 is only aligned with slots 520 when the associated switching contacts 37 or 52 are in the off position, and movement of the pin 521 along one of the slots 514, 515 corresponds to pivoting of the plate 512 to enable switching of the associated switching contacts 37 or 52 between the off and earth position, while movement of the pin 521 along the other slot 514, 515 corresponds to switching of the contacts between the off and on position. In this way, it will be seen that movement of the pin 521 to enable switching between on and off and off and

earth (or vice versa) is only possible when the associated contacts 37 or 52 are in the off position.

Operation of the ring main unit will readily be understood. In the on condition, either or both of the three- phase inputs is or are connected by its switching contacts 37 and / or 52 in the lower chamber to the respective set of output contacts 49 or 54, and so to the busbar contacts 59 to 61. The switching contacts 83 in the upper chamber engage the busbar contacts 59 to 61 to connect them to respective tee-off contacts 85 and current flows through the unit. That current may be switched off by returning appropriate switching contacts 37, 51 to the off positions shown in solid lines in Figure 4. Either set of three-phase inputs may, from the off position, be connected to earth by downward movement of the switching contacts 37 or 51 , and thereafter, following operation of suitable interlocks, may be connected to appropriate test circuitry connected to the earth contacts.

Figure 5 shows the arrangement of the drive means when the switching contacts 37 or 51 are in the off position so that the pin 521 can move along the radial slot 516 in the circular plate 512 to selectively follow either of the arcuate slots 514, 515 to allow the contacts 37 or 51 to be pivoted upwards into connection with the output contacts 49 or downwards into connection with the earth contacts. In either of these positions, the ball bearings 519 engage one of the circular apertures 517 through the circular plate 512.

The arrangement shown in Figure 5 allows the switching contacts 37 or 51 to be pivoted upwards by means of anti¬ clockwise rotation of the circular plate 512. This is achieved by turning the shaft 502 of the handle 501 anti-clockwise when the engaging means 510 is fitted in the end of the drive shaft 502, so that the torsion spring 507 is charged until the torque applied to the circular plate 512 is sufficient to overcome the clamping force of the associated aperture 517 between the ball bearings 518. When this occurs, the circular aperture 517 moves out of engagement with the ball bearings 518, which results in rapid release of the clamping force of the ball

bearings 518, and simultaneous transfer of the energy stored in the torsion spring to the drive shaft 502. As a result, the drive shaft 502 is rapidly turned anti-clockwise until the switching contacts 37 or 51 come into contact with the output contacts 49. When this occurs, the ball bearings 518 come into clamping engagement with one of the other circular apertures 517 in the circular plate 512.

If an attempt is now made to immediately reverse the pivoting movement of the switching contacts 37 or 51 by immediate clockwise rotation of the circular plate 512, torque applied to the shaft 502 of the handle 501 in the clockwise sense is not transmitted through the housing 504 to the engaging means 510 because of the ratchet 505, 508. In order to effect clockwise rotation of the drive shaft 502, the housing 504 must be reversed relative to the drive shaft 502 so that the ratchet operates in the opposite sense i.e. so that the other engaging means 509 engages with the end 511 of the drive shaft 502. The delay necessarily involved with this procedure prevents hazardous clockwise rotation of the drive shaft 502 immediately after anti-clockwise rotation thereof. It will be appreciated that rotation of the circular plate 512 in the opposite sense to that described above takes place in an analogous manner.

Reverting to the on condition, however, if fault current is experienced on one of the three phases, then a fault current sensing device will operate and will cause tripping of a mechanism attached to shaft 87 so that such shaft is rotated clockwise . from the on position of each switching contact 83, shown in broken lines in Figure 3, to the off position wherein contact 83 engages the auxiliary contact 82. During such movement, an arc is struck between the movable switching contact 83 and the busbar contact 59, one arc root being located on a tungsten-copper plate 94 extending towards the arcing electrode 76, and the other arc root being located on tungsten copper inserts at the tips of the switching contact 83. The effect of magnetic loop forces on the arc will cause the arc root on the plate 94 to migrate along the plate and

onto the electrode 76, while the other root of the arc migrates onto the inner surface of the conductive ring 77. With the arc in this position, arcing current passes through the coil 77, and the magnetic field induced by the coil causes high speed rotation of the arc around the arcing electrode 76. The arc is cooled, loses energy, and is extinguished in the SF ₆ gas with which the chamber is filled.

A similar action occurs when the switching contacts are moved to the off condition in response to appropriate operation of the switch control panel 7.

Figures 7 and 8 show a modified drive means 600 and the operating mechanism 206 for use with the switching contacts 83. For manual switching of the switching contacts 83 by means of a handle 501 , the mechanism and its operation is in many respects similar to that already described with reference to Figures 5 and 6, and for convenience identical parts are indicated by the same reference numerals used in Figures 5 and 6, but increased by 100.

The drive means 600 comprises a base plate 613 having a pair of spaced apart circular apertures 617 therethrough and arranged adjacent to one end thereof, and a rectangular slot 620. The plate 613 also has a further circular aperture 630, larger than the two circular apertures 617, for pivotally mounting the plate 613 about a drive shaft 87 (see Figure 3):

The base plate 613 is also provided adjacent to the side thereof remote from the circular apertures 617 and the rectangular slot 620 with a generally Y-shaped aperture 631. A roller track 632 is pivotally mounted to the edge of the base plate 613 adjacent to the Y-shaped aperture 631 and is held in position by engagement with a trip catch 633 so as to form a curved slot 634 in the upper region of the plate 613 such that a roller 635 can roll along the curved slot 634. A hold on catch 636 is pivotally mounted to an edge of the base plate 613 and can come into engagement with the roller 635 in the curved slot 634.

A pivot plate 612 has a circular pivot aperture 639 generally at a central region thereof for rigidly mounting the

plate to the drive shaft such that rotation of the plate 612 causes rotation of the drive shaft, and two arcuate slots 614, 615 connected by a radial slot 616. The pivot plate 612 is also provided with a circular aperture 640 slightly smaller than the apertures 617 in the base plate 613, and carries a pin 641 at its end remote from the circular aperture 640. The pin

641 is pivotally connected to the end of an elongate push link

642 which carries the roller 635 as its other end. The pin 641 can slide in a longitudinal slot in a spring cage 643. The pin 641 and the corresponding end of the pivot plate 612 are arranged to abut against the end of a compression spring 644, the other end of which is connected to a fixed point in the housing of the ring main unit.

A generally triangular drive lever 645 is provided with a circular aperture 646 at one corner thereof for pivotally mounting the lever about the drive shaft, and is pivoted at another corner to an input drive link 647 (of which only part is shown in Figures 7 and 8) which is connected to a drive shaft arranged on switch control panel 7. A pull link 648 is pivoted to the third corner of the drive lever 645 remote from the circular aperture, the opposite end of which carries a pin 649 for rotatably mounting the link 648 to the roller 635 through one end of push link 642.

A triangular cover plate 650 has a circular aperture 651 at one of its corners for pivotally mounting the plate 650 about the drive shaft, and a circular aperture 652 corresponding to aperture 617 in the base plate 613 adjacent each of its other corners. The plate 650 is also provided with a horizontally extending slot 653 adjacent its upper edge corresponding to the slot 620 in the base plate.

As shown in greater detail in Figure 8, when the drive means 600 is assembled, the pivot plate 612 and drive lever 645 are mounted on the drive shaft between the plates 613, 650, which are also mounted on the drive shaft. At the same time, roller 635 is mounted to pin 649 through the end of link 642, and pin 641 passes through the slot in the spring cage 643, the upper part of which is attached to the upper edge of base plate

613. Slot 653 is aligned with slot 620 to receive a pin 621, in which position the apertures 652 will be aligned with corresponding apertures 617. Apertures 652, 617 are between ball bearings 618 under the force of compression springs 619, such that when aperture 640 in the pivot plate 612 is aligned with either of the pairs of apertures 617, 652, it is gripped between ball bearings 618 in a manner similar to that shown in Figures 5 and 6.

The pin 621 , which is connected to main switch (M) and earth switch (E) on the switch control panel 7, is only movable between slots 614, 615 when slots 620, 616 and 653 are aligned, which corresponds to the off position of the switching contacts 83. This will be readily appreciated by analogy with Figures 5 and 6.

In particular, to move switching contacts 83 from the off position shown in Figure 8 to the earth position, input drive link 647 is moved upwards to rotate drive lever 645 clockwise, which in turn causes clockwise rotation of pivot plate 612 via links 648 and 642 once the detent force of the ball bearings 619 has been overcome, to rotate the drive shaft. The roller 635 moves along slot 634 and the end of pivot plate 612 carrying pin 641 moves out of engagement with the end of compression spring 644.

Similarly, in order to move the contacts 83 from the off position to the on position, input drive link 647 is moved downwards to rotate lever 645 anticlockwise, which in turn rotates pivot plate 612 anticlockwise once the detent force of the ball bearings 619 has been overcome, and rotates the drive shaft. At the same time, pin 641 moves to the bottom of the slot in spring cage 643 against the force of compression spring 644, and roller 635 is moved to the left-hand end of slot 634 and is held in position by indent 638 in the end of the hold on catch 636.

If, in the on condition of contacts 83, a fault current is experienced on one of the three phases, then a fault current sensing device causes trip catch 633 to be displaced in the clockwise sense as shown in Figure 7. This in turn causes the

roller track 632 to fall in a clockwise sense to the bottom of Y-shaped aperture 631 so that the roller falls to the bottom of the Y-shaped aperture 631. As a result, the hold on catch 636 can no longer hold the roller 635 at the left-hand end of the slot 634 against spring 644. The compression spring 644 therefore extends to rotate the pivot plate 612 and drive shaft 87 clockwise to move the switching contacts 83 back into the off position.

It will be appreciated by persons skilled in the art that various modifications of the construction of the apparatus are possible without departing from the scope of the invention.