GB2078007A - Switch operating mechanism - Google Patents

Abstract

An earth switch in which an electric motor (64) operable in one sense only drives an endless screw (68) along which a follower (74) is reciprocable. The follower drives a pair of abutments (76). Two coil compression springs (84) are arranged between that pair of abutments (76) and a second pair (86) mounted on the earthing contact (24). As the follower (74) runs along the screw (68) the springs (84) are compressed. The abutments (76) driven by the screw (68) engage fulcrum pins (90, 92) near the ends of their travel and as the screw (68) continues to drive the abutments in the same direction the spring pass over-centre and the thrust on the the abutments (86) is reversed in direction to overcome static friction and initial dynamic friction resisting expansion of the springs, which then drive the earthing contact in the opposite direction to the movement of the screw follower. <IMAGE>

Description

SPECIFICATION Earth switch This invention relates to earth switches particularly, though not exclusively earth switches for use with metalclad power switch-gear.

According to the invention, an earth switch com- prises an earthing contact advanceable and retractable by a motor arranged to drive an endless screw in one direction only so as to move a member to apply load to spring means, the earthing contact being advanceable and retractable by dissipation of stress in the loades spring means, advance and retraction of the earthing contact being subject to the damping action of dashpot means.

Such an earth switch will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a simplified diagrammatic longitudinal section through the switch; Figure 2 is a transverse section through the switch shown in Figure 1 on the line II - II in Figure 1; and Figures 3 and 4 which are, respectively, a horizontal section and an elevation partly in vertical section showing part of the switch shown in Figures 1 and 2, and showing more details and some modifications.

Figure 3 shows the closed position of the switch and Figure 4 shows the open position.

Figure 1 shows a cylindrical enclosure 10 of metalclad power switchgear, for example 300/420 kilovolt gear, containing sulphur hexafluoride (SF6) gas as insulating medium. A primary conductor 12 extends within the enclosure 10 and carries a housing 14 containing a fixed copper contact cluster 15, which is positioned accurately by a single screw 16, which can be adjusted by the use of a suitable tool (not shown). The conductor 12 is positioned within a cylindrical shroud 18 within the enclosure 10, the shroud 18 having an aperture 20 aligned with the mouth of the housing 14.

The enclosure 10 has an aperture 22 opposite the aperture 20 and a hollow cylindrical earthing contact 24 of copper extends through the aperture 22 and is engageable at its right-hand end with the contact cluster 15. The contact 24 is slidingly engaged by a contact arrangement 26 including a hollow bush 28 located in the aperture 22 and which is secured by an earth palm 30 electrically connected to the contact arrangement 26. The palm 30 is secured to the enclosure 10 by bolts 32 and is electrically connected to the main earth bars (not shown) of the installation.

A cylindrical housing 40 has at its right-hand end an annular closure plate 42 secured to the palm 30 and has at its left-hand end another closure in the form of a gear housing 44.

Seals are provided at 46, 48, 50 and 52 to ensure gas-tightness for the housing 40, which contains SF6 gas.

The contact 24 is guided by a rod 60 which is secured at its left-hand end to the housing 44 and which engages a bearing carried by the contact 24 (Figure 3). The plate 42 and the housing 44 are interconnected by pillars 62.

The gear housing 44 carries a fractional horsepower electric motor 64 and a terminal block 66, both being outside the gas-filled housing 40. The motor 64 and the terminal block 66 are protected by a weather-resistant cowl 67.

The motor 64 drives a lead screw 68 rotatably mounted at one end in the plate 42 and at the other in the gear housing 44, a dynamic gas seal 70 being provided at that end. The motor drive is applied to the lead screw 68 through reduction gearing 72 within the gear housing 44.

The lead screw 68 has an endless track engaged by a follower (not shown) of a reciprocatable block 74.

The block 74 and the left-hand end of the contact 24 are interconnected by two compression spring assemblies. Each assembly consists of a tubular guide 76 pivotably connected at 78 to the block 74; a guide rod 80 pivotably connected at 82 to the contact 24 and slidable in the tubular guide 76; and a helical compression spring 84 surrounding the guide rod 80 and interposed between an abutment (not shown) within the guide tube 76 and an abutment in the form of a rectangular stop portion 86 of the guide rod 80. The spring 84 is under a pre-determined compression in the position shown.

The closure plate 42 and the gear housing 44 carry respective pairs 90,92 of fulcrum pins projecting inwardly into the housing 40, the function of which is explained below.

The housing 40 also contains limit switches 100, 102 actuable by the block 74 at the ends of its travel.

Dashpot means (Figure 4) are provided in the hollow contact 24, the damping medium being the high-density SF6 gas in which the mechanism is submerged in the housing 40. The dashpot means damps and controls the motion of the contact 24 and in particular prevents the contact 24 from rebounding after it has been advanced into contact with the cluster 15.

Operation In the position of the switch shown in Figures 1 and 2, the contact 24 is in engagement with the cluster 15 so as to earth the conductor 12.

The pre-loaded springs 84 act on the contact 24 to resist spurious retraction movement of the contact 24 away from the cluster 15.

The earth condition of the conductor 12 is removed by energisation of the motor 64, which rotates the endless screw 68 to drive the block 74 to the right as shown in Figure 1. The springs 84 are thus further compressed. During the major part of the rightward travel of the block 74, the springs 84 continue to act on the contact 24 to urge it into engagement with the cluster 15.

Ultimately, by the rightward travel of the block 74, the guide tubes 76 are brought into engagement with the ends of the respective pins 90. The tubes 76 at this point are almost vertical. Continued rightward movement of the block 74 moves the tube 76 "over-centre" that is, past their vertical positions.

The two tubes 76, each with its respective guide rod 80 now act as levers applying force in a leftward direction to the contact 24. This lever force overcomes the static frictional resistance and initial dynamic frictional resistances to leftward movement of the contact 24.

After a small amount of leftward movement of the contact 24, the high compression loads in the springs 84 become effective to accelerate the contact 24 to the left. This ensures a rapid separation of the contact 24 from the cluster 15. Leftward motion of the contact 24 is arrested by the dashpot means mentioned above (using the SF6 gas as damping medium) and the contact 24 comes to rest in a fully retracted leftward position (not shown) in which the springs 84, the tubes 76 and the rods 80 occupy positions inclined similarly but oppositely to the position shown in Figure 1.

The rightward travel of the block 74 during the sequence just described is arrested when the block actuates the limit switch 102, which de-energisesthe motor 64.

The springs 84 act on the contact 24 to resist spurious rightward movement of the contact.

To re-make the earthed condition the motor 64 is energised so as to drive the block 74 to the left by means of the endless screw 68 now acting on the block 74 through the screwthread in the opposite sense to that which was effective during the "break" sequence already described. The springs 84 continue to urge the contact 24 to the left until the tubes 76 engage the respective pins 92 and reach their vertical positions. Continued leftward movement of the block 74 causes the tube 76 and their rods 80 to act as levers, in a manner similar to that already described, to move the contact 24 to the right.

Continued leftward movement of the block 74 causes the springs 84 to act to accelerate the contact 24to the right to move rapidly into engagement with the cluster 15. Thereafter, the springs 84 act to resist spurious leftward movement of the contact Leftward movement of the block 74 ceases when the block actuates the limit switch 100 to de-energise the motor 64.

Figure 3 shows the dashpot means referred to above. The rod 60 engages linear ball bearing assemblies at 101 which are located in a tubular extension 102 secured to the end of the contact 24.

A portion of the extension 103 is secured within the contact 24 at 104 and the internal diameter of that portion at 106 is relatively enlarged to form a first dashpot cylinder.

A second dashpot cylinder consists of a cylindrical cup-shaped member 108, which is fitted into the bore of the contact 24 and secured against a shoulder 110 therein. The number 108 has an annular end wall having small through-apertures 112 around a central aperture through which the stem of a valve member slides. The valve member has a head 114 which can obturate the apertures 112.

The rod 60 carries an annular piston 1 16which in the position shown in Figure 3 is a close running fit in the first dashpot cylinder at 106. As the contact 24 approached the fully closed position shown, the piston 116 would have relatively entered the dashpot cylinder at 106, whereupon SF6 gas trapped in the dashpot cylinder would be forced to escape only through the very small clearance between the periphery of the piston 116 and the dashpot cylinder surface 106, producing a high degree of damping of the motion of the contact 24. The effective area of the piston 116 is the annular area surrounding the rod 60, so that the amount of energy dissipated is relatively high in the direction of closing of the contact.

During closing movement the valve head 114 is in the position shown, leaving the apertures 112 open so that SF6 gas can flow through the apertures 112 as the contact 24 moves rightwardly relatively to the piston 116.

During the greater part of the travel of the contact 24 the helically-coiled annularwire contacts at26 roll on the contact 24. During the last stage of movement the contacts 26 are in the positions shown and the contact 24 slides past the contacts 26. This produces an additional frictional damping of the motion and enhances electricai contact in the region in which arc formation between the contact 24 and the contact cluster 15 is most likely.

When the contact 24 is withdrawn, the contacts 26 roll leftwards during the greater part of the motion, then they reach the leftward end of the bush 28 and stop. The contact 24 then slides relatively to the contacts 26, so that additional frictional damping is again provided.

The piston 116 enters the second dashpot cylinder 108 during the final stage of the leftward motion of the contact 24. The inertia of SF6 gas in the contact 24 has already caused the valve head 114 to close againstthe end wall of the cylinder 108 to obturate the apertures 112. When the piston 116 enters the dashpot cylinder 108, SF6 trapped therein can escape only through the very small running clearance between the piston 116 and the dashpot cylinder 108, so that damping action is produced, the full diameter of the piston 116 now being effective.

Figures 3 and 4 show a modification over Figures 1 and 2 in that the pins 90, 92 each consists of a fixed stem 120 and a head 122 (Figure 4) slidable on the stem 120 with a compression spring 124 acting on the head and surrounding the stem. After engagement of the heads 122 of a pair of such pins by the guide tubes 76 continued movement of the guide tubes in the same sense causes the compression springs 124 on the pins to compress. Ultimately, the compression load in those springs 124 increases to a point where further compression is impossible. The guide tubes 76 continue briefly to fulcrum about the heads 122 of the pins after which the tubes 76 move "over-centre" and the contact 24 is moved as already described. The compression springs 124 on the pins then re-extend to their normal positions.

Figure 3 and 4 also show modified limit switch arrangements. The contact 24 is connected to two4 profiled tubular nylon members 130 slidable on respective pillars 132 similar to the pillars 62 arranged equidistantly from the line of travel of the contact. The members 130 engage operating members 134 of limit switches 136 which take the place of the switches 100,102 shown in Figure 1.

The members 130 are shown in Figure 3 connected to opposite extreme ends of respective stub shafts 140, which extend laterally away from the extension 103. The stub shafts 140 provide the pivotal connections at 82 for the guide rods 80 referred to in relation to Figures 1 and 2.

Figure 4 also shows a further detail over Figure 1.

The telescopic assemblies inter-connecting the con tact 24 and the block 74 are here shown as each comprising a guide tube 76, an intermediate tubular guide member 150, about which a first helically coiled compression spring 152 extends and which has an integral collar 154 intermediate its ends. The spring 152 extends into the tube 76 and is trapped between the collar 154 and the upper end of the tube 76. The member 150 can move within the tube 76 as the spring 152 changes length. The telescopic assembly further comprises a guide rod 156 which is slidable within the member 150 and is connected to the block 86. A second helically coiled compression spring 158 surrounds the rod 156 and part of the member 150 and is trapped between the block 86 and the collar 154.

The action of such assemblies is the same as that of the corresponding assemblies described in rela tion to Figures 1 and 2.

The contact 24 is indicated in ghost outline in Figure 4, its central longitudinal axis being indicated at 160.

In another modification (not shown) the dashpot means may be provided outside the contact 24. The contact would in that case be secured to a dashpot piston or cylinder means which would cooperate with a fixed dashpot cylinder or piston means outside the contact.

The earth switch described above is applicable particularly though not exclusively to metalclad power switchgear and is capable of operation to cause the earth contact 24 to engage the primary conductor 12 while the latter is carrying system voltage and of conducting fault current to earth in accordance with International Electrotechnical Com mission recommendations.

Claims (8)

1. An earth switch comprising an earthing con tact advanceable and retractable by a motor arranged to drive an endless screw in one direction only so as to move a member to apply load to spring means, the earthing contact being advanceable and retractable by dissipation of stress in the loaded spring means, advance and retraction of the earthing contact being subjected to the damping action of dashpot means.

2. An earth switch according to claim 1, in which the earthing contact is guided for longitudinal move ment parallel to but spaced from the axis of rotation of the screw and in which the spring means is arranged between first and second abutments, the first abutment being pivotably secured about an axis transverse to the axis of rotation of the screw to said member and the second abutment being pivotably secured to the earthing contact, the arrangement being such that when the earthing contact is in either its advanced or its retracted position, movement of the first abutment towards the second stresses the spring means and brings it to a position of maximum stress after which stress in the spring means is dissipated to move the second abutment away from the first.

3. An earth switch according to claim 1 or claim 2, in which said spring means is guided be a telescopic guide assembly extending between said abutments.

4. An earth switch according to claim 3, in which fulcrum means is provided which is engageable by part of said guide assembly when the spring means near its position of maximum stress, further movement of said member in the same sense, causing said part of said guide assembly to turn about said fulcrum means to transmit thrust through said assembly from said member to the contact.

5. An earth switch according to claim 4, in which said fulcrum means comprises an element engageable by said part of said assembly and thereafter movable by said further movement of said member to stress a spring element until the stress in said element equals the force applied by said part, continued movement of said member in the same sense causing said stress to be dissipated as said part of said assembly turns about said fulcrum means to transmit thrust to the contact.

6. A earth switch according to any preceding claim in which the sring means comprise two helical compression spring means one on either side of the screw, the spring means having respective guide parts extending therethrough which guide parts include parts guided by respective hollow members forming a telescopic arrangement secured between respective first and second abutments.

7. An earth switch according to claim 1, substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings.

8. An earth switch according to claim 1, substantially as hereinbefore described with reference to Figures 1 to 4 of the accompanying drawings.