GB2125626A - Arc interruption in air break switch - Google Patents

Description

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GB 2 125 626 A

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SPECIFICATION Air break switch

5 This invention relates to an air break switch, and to a method of reducing shock wave generation in operation of such switch.

Previous air break switches have suffered from the disadvantage that when separate contacts of the 10 switch are opened, an electrical arc can occur within the switch casing, these arcs generating (air) shock fronts in within the switch, often causing considerable damage to the switch.

It is an aim of this invention to provide an air break 15 switch in which such shock fronts are prevented from attaining sufficient strength to damage the switch.

According to a first aspect of the invention I provide an air break switch comprising two electrical 20 contacts (as herein defined) which can undergo relative motion between open and closed positions and a coil through which at least part of the current undergoing interruption flows and which provides a magnetic field in the vicinity of at least one of said 25 contacts, and which is of such configuration as to hinder or prevent the generation of any air shock front.

The word "contact" is used herein in a generic sense to include not only to a component of the 30 switch which specifically provides a contact surface which at the closed position abuts some other component having a cooperative contact surface providing for the flow of said current, but also to refer to any current carrying part of said switch 35 which and which may provide a site for the establishment of an electrical arc when the contact is moved relatively from its closed to its open position.

Said coil may be connected in series with said contacts.

40 Alternatively or additionally said coil may be powered by an energy source other than the switched current, and derived from or controlled by other contacts of a circuit and said switch operating in timed relation to those between which it is desired to 45 reduce shock wave generation.

Current flow in the electrical arc which occurs between said contacts has a magnetic effect which tends to aid development of any shock front associated with such an arc. Thus an air break switch in 50 accordance with the invention will work at its best when the magnetic field provided by the coil is sufficiently strong and so directed so as to have an effect which significantly counteracts or even surpasses the magnetic effect of said current flow in the 55 arc.

The magnetic field provided by said coil may be sufficiently strong, at least in said vicinity of at least one of said contacts, to cause the Alfven speed (at least in said vicinity) in plasma associated with the 60 arc to be significantly in excess of the airspeed attainable by any air shock front.

Additionally, with an Alfven speed as described above in said vicinity, the direction of said field (at least in said vicinity) may be such as to permit a 65 significant fraction of energy released by any such arc to be dispersed mainly by oscillations of said plasma rather than mainly by one or more potentially damaging air shock fronts.

Said switch may further comprise at least one 70 magnetic flux director means made of magnetically susceptible material for directing and/or enhancing the effect of said magnetic field.

Said coil, or said coil and flux director means in combination, may be so arranged that, in said 75 vicinity, said field is directed transversely of the direction of relative separating motion of said contacts.

In any such arrangement said field may be directed transversely of the shortest path between 80 said contacts when they are separated.

Said field may be directed essentially normal to any electrical arc which may occur between said contacts when relative movement from the closed position towards the open position takes place. 85 Further, said switch may have an arc chamber for containing said arc while the energy thereof is dispersed.

Said arc chamber may be provided with chutes and/or runners which operate to sub-divide the arc 90 and help to direct said shock fronts and/or help to disperse energy carried by said shock fronts which has been released by said arc or arcs.

Air-occupied space encompassed by part or all of said coil may provide at least a part of said arc 95 chamber, thus ensuring the presence of said field within said arc chamber.

Alternatively or additionally, at least a part of said coil may lie around at least one switch component.

According to a further aspect of the invention, I 100 provide a method of at least reducing the generation of a shock wave at the site of an arc established between separable contacts of a switch as these contacts open, comprising causing the current flowing between the contacts to flow in a configuration 105 establishing a magnetic field at the site of the arc, such field being of a form and strength to oppose Shockwave generation.

Two embodiments of the invention will now be described, by way of example only, with reference to 110 the accompanying drawings wherein:-

Figure 1 is a diagrammatic side elevation of a first air break switch embodying the invention;

Figure 2 is a diagrammatic enlarged sectional side elevation of a portion of the switch shown in side 115 elevation in Figure 1; and.

Figure 5 is a diagrammatic partially sectional side elevation of a portion of a further air break switch embodying the invention.

Figure 1 of the drawings shows an air break switch 120 10 comprising a pair of electrical contacts 11 and 15 and a further pair of electrical contacts 12 and 16. An operating means for making and breaking an electrical connection between the two contacts of each pair is indicated at 13.

125 In orderto permit current flow through the switch, operating means 13 is used to move an electrically conductive connecting member 14 which carries or incorporates contacts 15and16so that contacts 15 and 16 conductively abut fixed contacts 11 and 12 130 respectively, to permit a current flow through the

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switch via member 14 and both pairs of contacts.

During movement of member 14, at stages when there is only a small air gap between contact 11 and contact 15 and/or there is only a small air gap 5 between contact 12 and contact 16, it is possible that an electrical arc will occur in any such gap, said arc ionising the air along its path to form a plasma. The arc thus effectively continues to form a conductive link between the contacts. Such an arc generates 10 intense heat and tends to generate shock fronts in the surrounding air.

In order to limit the development of such shock fronts, an electrically conductive coil 20 which may be of an elongate essentially rectangular form, as 15 viewed in plan, is provided around the perimeter of an arc chamber 17 and is seen in Figure 1 from a direction perpendicular to one of its longer sides. If desired, said coil may surround more of said than is the case in Figures 1 and 2 and said coil may, for 20 example, extend further in a direction towards the operating means 13 up to or beyond said contacts so that a part of said coil lies around said contacts.

One end 20a of said coil is conductively connected to contact 12 by connector 21, and the other end 206 25 of said coil is connected to a terminal 22. Contact 11 is conductively connected to a further terminal 23 via a solenoid 24 and a bi-metallic strip 25. In use, the air break switch is connected into an external circuit using terminals 22 and 23.

30 Coil 20 is thus in series with contacts 11 and 15, member 14, and contacts 12 and 16, and carries all the current which flows through the switch either via only the contacts and member 14 or via the contacts and member 14 and an arc or arcs between contacts 35 of either pair. As a result of carrying said current, coil 20 generates a magnetic field which occupies said chamber 17 and extends into the vicinities of contacts 11,12,15, and 16.

A biasing spring 26 which may be made of a 40 magnetically susceptible material, such as steel for example, and consequently serves a dual purpose of urging contacts 15 and 16 on member 14to make good electrical contact with contacts 11 and 12 respectively, when required, and of acting as a flux 45 director which causes said field to pass through a gap between the two contacts of each pair when current flows through the switch due to the establishment of an arc between those contacts.

All of the contacts 11,12,15 and 16 in the 50 embodiment illustrated in Figures 1 and 2 lie between said biasing spring 26 and said coil 20.

Additionally, flux directors 27 are provided specifically in order to direct said flux between said contacts located between flux directors 27 and coil 55 20 in the embodiment illustrated in Figures 1 and 2. Flux directors 27, as illustrated, are each in the form of a plate, but each may alternatively be in the form of a rod or may have any other suitable form.

Figure 2 shows the lines offeree said magnetic 60 field in a part of said arc chamber and in the vicinities of the contacts, member 14, connector 21 (of negligible magnetic susceptibility), spring 26 and one of the flux directors 27. Dashed lines 28 and 26 are typical of these field lines, and all of these lines are 65 substantially parallel until they emerge from chamber 17.

A possible path along which an arc could occur between contact 12 and contact 16 is indicated at 30.

Dashed lines 31 and 32 and similar lines represent 70 successive positions which may be occupied by a shock front (generated by an arc along path 30) as it travels towards and into chamber 17. The flux directors have sizes, shapes and positions such that said field is approximately normal to path 30 and 75 subsequent shock front positions, and such that said field is enhanced in the vicinity of said contacts.

Energy released by said arc would, in prior art switches, have been carried away from said arc mainly be (air) shock fronts in the air within the 80 switch. However, energy can also be carried away from the arc via oscillations of the plasma of ionised air produced by the arc.

In the air break switch in accordance with the invention illustrated in Figures 1 and 2, the magnetic 85 flux is so strong in the region of path 30 that the Alfven speed (i.e. the speed at which plasma oscillations perpendicular to the field propogate along the field lines) in so much in excess of any speed of travel attainable by an air shock front, that the effect 90 of the flux is to bring about dispersal of said energy by way of oscillations rather than by a unidirectional-ly travelling shock front. Said energy is thus very rapidly conveyed by waves of plasma oscillations along said field lines at the Alfven speed into 95 chamber 17, where the field lines are more widely spaced than in said region, and very little energy remains in the vicinity of the arc path at 30 for the generation of any air shock front. Most of an arc's energy is in fact dispersed by waves of plasma 100 oscillations before enough time has elapsed for such a shock front to be generated.

Thus the generation of air shock fronts which could damage said switch is severely hindered or prevented in this air break switch.

105 Energy conveyed into said arc chamber by said waves of plasma oscillations (and by any now relatively insignificant air shock front which may still form) passes down chutes 18 between runners 19 in said arc chamber parallel to the magnetic field in 110 said arc chamber. Said runners help to conduct away said energy, and each runner comprises a plate of thermally conductive/absorptive material arranged so that its plane is approximately parallel to the magneticfield in the arc chamber. The runners 19 115 are not shown in Figure 2, but have end portions which are indicated in Figure 1.

The magnetic field produced by the flow of said current in coil 20 is arranged to be sufficiently strong to have a greater effect on the plasma than that due 120 to the field set up by the current flowing inthearc itself.

A bi-metallic strip 25 is arranged for use in conjunction with solenoid 24 to stop current flow in said switch when the current becomes too high (the 125 current flows through the bi-metallic strip, which is heated by the current to a temperature - and consequently to a deflection - dependent upon the magnitude of that current). Solenoid 24 operates a plunger 33 which in turn acts on operating means 13. 130 A manually operable release lever 34 is provided

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to allow the switch to be switched off and/or on manually.

A speed-controlling spring 35 is used to exert, on operating means 13, a force which ultimately acts on 5 member 14 to cause the connection between contacts 11 and 15 and the connection between contacts 12 and 16 to be broken rapidly so as to minimise the duration of any arcs which may occur.

Figure 3 shows a portion of a further air break 10 switch 110 in accordance with the invention.

Switch 110 comprises a fixed contact 108 and a movable contact 106 on a movable conductive member 101. Contact 108 is conductively connected via a conductive element 140 to one end 120a of an 15 electrically conductive coil 120, and the other end 1206 of coil 120 is conductively connected via a further conductive element 141 to a further fixed contact 109 and thence to a terminal 122.

Suitable insulating portions 142,143 serve to 20 insulate conductive elements 140,141 and the coil ends 120a, 1206 from each other.

Conductive member 101 is T-shaped, with contact 106 located on its central stem portion 104 (seen from one side in Figure 3) and a further movable 25 contact 107, fixed with respect to movable contact 106, on its cross-member 105 (seen end-on in Figure 3), and is conductively connected to another terminal 100. Terminals 100 and 122 are used for connecting switch 110 into an external circuit, and the switch 30 110, when thus connected, is used to control current flow between those terminals.

Member 101 is mounted, via a spring 103, on a movable arm 102 which is in turn mounted on a pivot, said pivot being located at an end (not shown) 35 of arm 102 distant from spring 103. Figure 3 shows the contacts at their open positions.

In order to move member 101 to its closed position so as to permit current flow between terminals 100 and 122, arm 102 is lowered so as to lower member 40 101 in the direction indicated by arrow A. Member 101 is mounted and/or balanced in such a way that contacts 106 and 108 conductively abut each other and establish current flow before contacts 107 and 109 abut each other. During opening of the switch no 45 arc occurs between contacts 107 and 109 since contacts 106 and 108 are still in abutting relation and provide a short circuit across contacts 107,109.

When contacts 106 and 108 begin to separate and are only a short distance apart, an electrical arc will 50 occur through the air between them. The coil 120 is effectively in series between contacts 109 and 108 and will carry the arc current thereby generating a magnetic field in the vicinity of said arc. This is sufficiently strong, and suitably directed (i.e. appro-55 ximately normal to any shock front that may be generated by said arc) to hinder or prevent development of potentially damaging air shock fronts by said arc, said magnetic field having this effect for the same physical reasons as those given in the descrip-60 tion of the switch 10 of Figures 1 and 2.

Energy released by any arc in switch 110 is dispersed (by plasma oscillations) along chutes 118 situated between the base portions 1196 (seen in cross-section in Figure 3) of runners 119 which are 65 U-shaped as viewed in plan. Each said runner 119

comprises a base portion 1196 and two side limbs 119a connected to opposite ends of base portion 1196.

Coil 120 lies around runner base portions 119b, 70 said limbs having their lengths extending longitudinally of the coil axis and hence approximately parallel to said magnetic field generated by coil 120.

Central stem portion 104 of member 101 moves within a part of an arc chamber 117 located between 75 the side limbs 119a of runners 119.

Claims (14)

1. An air break switch comprising two electrical 80 contacts (as herein defined) which can undergo relative motion between open and closed positions, and a coil through which at least part of the current undergoing interruption flows, which provides a magnetic field in the vicinity of at least one of said 85 contacts, and which is of such configuration to hinder or prevent the generation of any air shock front.

2. An air break switch according to claim 1 wherein said coil is connected in series with said

90 contacts.

3. An air break switch according to either of the preceding claims further comprising at least one magnetic flux director means of magnetically susceptible material for directing and/or enhancing said

95 magnetic field.

4. An air break switch according to any one of the preceding claims wherein said coil, or coil and flux directing means in combination is so arranged that, in said vicinity, said field is directed transversely of

100 the direction of relative separating motion of said contacts.

5. An air break switch according to Claim 4 wherein said coil, or coil and flux directing means in combination, is so arranged that, in said vicinity,

105 said field is directed transversely of the shortest path between said contacts when they are separated.

6. An air break switch according to any one of the preceding claims wherein, in said vicinity, said field is directed essentially normal to any electrical arc

110 which may occur between said contacts.

7. An air break switch according to any one of the preceding claims wherein, in use, said field is sufficiently strong at least in said vicinity to cause the Alfven speed at least in said vicinity in plasma

115 associated with any such arc to be significantly in excess of the air speed attainable by any air shock front.

8. An air break switch according to claim 7 wherein the direction of said field in said vicinity

120 permits a significant fraction of energy released by any such arc to be dispersed mainly by oscillations of said plasma rather than mainly by one or more potentially damaging air shock fronts.

9. An air break switch according to any one of the

125 preceding claims wherein said switch has an arc chamber for containing said arc while the energy thereof is dispersed.

10. An air break switch according to claim 9 wherein part or all of said coil provides at least a part

130 of said arc chamber.

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11. An air break switch according to claim 9 or claim 10 wherein at least a part of said coil lies around at least one switch component.

12. A method of at least reducing the generation 5 of a shock wave at the site of an arc established between separable contacts of a switch as these contacts open, comprising causing the current flowing between the contact to flow in a configuration establishing a magnetic field at the site of the arc, 10 such field being of a form and strength to oppose Shockwave generation.

13. An air break switch substantially as hereinbefore described and/or as illustrated in the accompanying drawings.

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14. Any novel feature of novel combination of features disclosed herein and/or illustrated in the accompanying drawings.

Printed for Her Majesty's Stationery Office, by Croydon Printing Company Limited, Croydon, Surrey, 1984.

Published by The Patent Office. 25 Southampton Buildings, London, WC2A1 AY, from which copies may be obtained.

13. A method of reducing the generation of a Shockwave at the site of an arc substantially as hereinbefore described with reference to and as

15 illustrated in the accompanying drawings.

14. An air break switch substantially as hereinbefore described and/or as illustrated in the accompanying drawings.

15. Any novel feature or novel combination of 20 features disclosed herein and/or illustrated in the accompanying drawings.

New claims filed on 30th June 1983 Superseded claim 1 to 15 25 New or amended claims:-

1. An air break switch comprising two electrical contacts {as herein defined) which can undergo relative motion between open and closd positions,

30 and a coil through which at least part of the current undergoing interruption flows, which, in conjunction with magnetically susceptible material, provides a magnetic field in the vicinity of at least one of said contacts, and which is substantially normal to a 35 shock front which tends to be set up by establishment of an arc between the contacts and has a value such as to cause the Alfven speed, at least in said vicinity in plasma associated with the arc, to be significantly in excess of the air speed attainable by 40 such shock front.

2. An air break switch according to claim 1 wherein said coil is connected in series with said contacts.

3. An air break switch according to any one of the 45 preceding claims wherein the direction and strength of said field in said vicinity permits a significant fraction of energy released by the arc to be dispersed mainly by oscillations of said plasma rather than mainly by one or more potentially damaging air 50 shock fronts.

4. An air break switch according to any one of the preceding claims wherein said switch has an arc chamber for contaiing the arc while the energy thereof is dispersed, and wherein in said vicinity,

55 said field is directed transversely to the length of the arc between said contacts and lengthwise of a path of arc travel extending from said vicinity into said chamber.

5. An air break switch according to claim 4

60 wherein part or all of said coil provides at least a part of said arc chamber.

6. An air break switch according to claim 4 or claim 5 wherein at least a part of said coil lies around at least one switch component defining the arc

65 chamber.

7. An air break switch according to any one of the preceding claims wherein the switch includes chutes or runners for sub-dividing the arc and wherein the magnetic field set up by the coil extends lengthwise

70 of the chutes or runners.

8. An air break switch according to claim 7 wherein the coil is wound around the chutes or runners with its axis parallel, or approximately parallel, to the lengths of the chutes or runners.

75 9. An air break switch according to any one of the preceding claims wherein the contacts comprise a first cooperative pair connected in parallel through said coil with a second cooperative pair, which latter are arranged to open after the first pair has opened,

80 the arc being established between the second pair.

10. An air break switch according to claim 9 wherein one of the contacts of the second pair is carried on the stem of a T-shaped contact member, the cross piece of which acts as one of the contacts

85 ofthe first pair, the other contact of the second pair and of the first pair being spaced from each other so that the pairs of contacts open in the required sequence and being electrically connected to each other through said coil.

90 11. A method of at least reducing the generation of a shock wave at the site of an arc established between separable contacts of a switch as these contacts open, comprising causing the arc current to flow in a configuration establishing, in conjunction

95 with magnetically susceptible material, a magnetic field at the site ofthe arc, such field being substantially normal to a shock front which tends to beset up by the establishment ofthe arc and which is of a form and strength to cause the Alfven speed at least

100 in the vicinity of said site and in plasma associated with the arc to be significantly in excess ofthe air speed attainable by such shock front.

12. A method of reducing the generation of a Shockwave at the site of an arc substantially as

105 hereinbefore described with reference to and as illustrated in the accompanying drawings.