Electric circuit interupter

Nov. 25, 1941. e. babler 2,263,752

ELECTRIC CIRCUIT INTERRUPTER

Filed April 17, 1940

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-A—,Patented Nov. 25,1941

2,263,752

UNITED STATES PATENT OFFICE

2,263,752

ELECTRIC CIRCUIT INTERRUPTER

Egon Babler, Brighton, England

Application April 17,1940, Serial No. 330,034 In Great Britain April 26, 1939

Claims.

This invention relates to electric circuit interrupters and refers more particularly to the type of circuit interrupter described in my prior specification Serial No. 228,083, which comprises a metallic layer which is preferably so thin that arcing destroys it at less than the current required to rupture it.

In electric circuit interrupters as described in said prior specification aforesaid, although the film is destroyed at a temperature which is below the melting point (due to the tension set up in the surface of the film) the following difficulty may in some cases arise: The necessary temperature rise for the initial destruction of the film is naturally imparted also to the support. The normally high resistivity of the support is thereby lowered, and it assumes a slight conductivity. As a matter of fact, if for instance ordinary glass is used for the support (having a high negative temperature resistance co-efficient) it may happen that the interrupter does not clear, because as the metallic layer is destroyed, the surface supporting it becomes increasingly conductive. Naturally the rate of increase of the current to be interrupted is a big determining factor. If this current rises very steeply, no such difficulties are encountered, because there is no time to impart any dangerous amount of heat to the support.

This difficulty was overcome in certain embodiments of said prior specification aforesaid, by introducing a restriction into the conductive layer, thus limiting the locality of thermal destruction and correspondingly limiting the area of increased conductivity of the support. In addition to this a material was chosen for the support with a low temperature co-efficient for example, alumina or mica. These two principles make it possible to produce an Interrupter, Which works with a considerable margin of safety, even if the fault current rises very slowly, say for several minutes.

Another difficulty, however, was experienced with circuit interrupters as described in said prior specification, viz. that the rupturing speed, as in the case of all thermally operated devices, is relatively slow, whereas high rupturing speed is of the greatest importance, particularly when the fault current rises very quickly, because it is then desirable to interrupt the circuit before the current assumes a dangerous magnitude.

The object of the Invention is the provision of an improved circuit interrupter of the kind set forth in which the above disadvantages will be obviated and the rupturing speed increased.

(Cl. 200—131) ' In order to increase this rupturing speed and as a further help to overcome the aforementioned difficulty of the support becoming conductive, I have now developed the following method: The 5 substantially straight path of the conducting layer is interrupted by a small gap and this gap is provided with a by-path, preferably of relatively high resistance. The normally small current flows naturally through the by-path because the 10 voltage drop thereacross is insufficient to break down the gap. If the current rises, the potential drop across the by-path rises too, in some cases more so, because the temperature of the by-path rises. The by-path may or may not become dels stroyed by the heat. If the voltage drop becomes sufficient, the gap is broken down and the film on the cool part of the support is rapidly destroyed by arcing. The breakdown across the small gap is not only favoured by the voltage drop in the 20 by-path, but also by the natural tendency of an electric current to pursue a straight path. The electro-magnetic force (set up in the by-path) increases with the current and helps thus to bring about a breakdown across the gap, some25 times without the loss of time required for the thermal destruction of the restriction. This is especially so when the current rises very quickly, that is when a speedy interruption is most desirable. The operating time may be cut down 30 further by introducing more than one gap and by-path. There are thus provided several centres where the ionic destruction of the film commences, until such a total length of the film is destroyed that the discharge extinguishes.

The by-path may take the form of a conductive layer similar to that of the straight path and a restriction may be, but need not necessarily be, provided in said by-path to give a high resistance.

In order that the invention may be the more clearly understood, a number of circuit breakers· in accordance therewith will now be described, reference being made to the accompanying drawing wherein:

Figure 1 is a sectional side elevation of a circuit breaker in accordance with the invention illustrating the general structure thereof,

Figure 2 is a plan of the circuit breaker proper and the support therefor,

Figure 3 is a view to an enlarged scale of the part of said circuit breaker proper at which breakdown occurs,

Figure 4 is a similar view to Figure 3 of a modified form' of the part of the circuit breaker proper ⁶⁵ at which breakdown occurs,

2,263,762

Figure 5 is a similar view to Figure 3 of another modified form of the part of the circuit breaker proper at which break down occurs,

Figure 6 is a similar view to Figure 3 of another modified form of the part of the circuit breaker proper at which break down occurs,

Figure 7 is a plan of the portion of the circuit breaker proper at which break down occurs together with a portion of the support and the container illustrating a different form of bypass,

Figure 8 is a similar view to Figure 7 of a slight modification,

Figure 9 is a similar view to Figure 7 of a further slight modification, and

Figure 10 is a similar view to Figure 7 of a further modification.

Referring first to Figure 1, the circuit breaker comprises a thin layer i of metal mounted on an insulating support 2 in the manner described in said prior specification Serial No. 228,083. As described in said prior specification, said layer I and support 2 are enclosed in a container comprising a glass tube 3 having its ends closed by means of metal caps 4 through which the external connections are established. . The connections between the ends of said layer I and said end caps 4 are established through wires 5 connected, by means of solder 6, to thicker layers 7 deposited over the ends of said layer I, said wires being also connected to said end caps in any suitable way. Said wires 5 also form the means for retaining the support 2 in place, the mechanical strength of the connection of said wires to said support being increased by means of solder 8, all as described in said prior specification aforesaid. The container, as in said prior specification, is packed with sand 8α.

In the arrangement shown in Figure 2, the gap and by-path are produced very simply in the following way. The normally straight and parallel sided path which the conducting layer I follows between the points a and b is enlarged by a short branch 9 of double width or more extending laterally to one side only. The whole path I and branch 9 is then parted at 10 by scratching the finished metallic layer with a needle, the scratch completely traversing the normal path I and continuing up to nearly the end of the branch 9, the extremity being left intact to form a restriction 11 on the by-path. When the normal small current flows through the circuit breaker, it flows through the by-path constituted by this restriction 11 which is preferably so narrow as to be of fairly high resistance. If the current rises, the potential drop across the by-path will rise also, in some cases at a higher ratio owing to the increase in temperature of the by-pass. When the current rises to the rupturing value there may or may not be time for the by-path at II to be destroyed by the heat. If it is destroyed by the heat, arcing will instantly take place across the gap formed by the scratch 10 and the film I will be destroyed on the cool part of the support 2. If there is not time for the by-path to be destroyed by the heat, the potential drop thereacross will become so great that, once more, arcing will take place across the gap and the film I will be destroyed with gre^t rapidity at the cooi part of the support 2. Of course, the restriction could be so designed that it is never intended to be ruptured by the heat, the first breakdown always taking place across the gap. In any case the gap en10 sures rapidity of rupture at a cool part of the support.

It will be appreciated that the break down across the gap is favoured, not only by the voltage drop in the by-path, but also by the natural tendency of an electric current to pursue a straight path. The electro-magnetic· force set up in the by-path increases with the current and thus helps to bring about a break down across the gap sometimes, as stated without thermal destruction of the restriction. Naturally the width of the restriction 11 determines the rupturing capacity. In this way the restriction becomes very short and the heat energy dissipated very small. The dimensions can be chosen ad libitum, but the gap 10 across the straight path should be small enough to be broken down readily by small voltages. I have obtained very favourable results with a gap of 0.004 inch, but I by no means limit myself to this dimension. The above design is only a simple example; there are many ways to produce an interrupter according to the present invention.

For example, in the arrangement described above, in order to speed up further the rupturing, two further scratches 12 may be provided in each end of the by-pass, as shown in Figures 4 and 5, said scratches cutting into the branch 9, at its point of junction with the main path I and extending nearly as far as the original scratch 10. Said further scratches 12 may be either parallel to the main path as in Figure 4 or at an angle cutting into the main path I as shown in Figure 5. By virtue of these scratches 12, access to the by-pass becomes sooner interrupted. The two additional restrictions 13 consequent upon these further scratches 12 should be (but need not necessarily be) wider than the original restriction 11 resulting from the original scratch.

A further modification in accordance with the invention is as follows: It will be appreciated that, in the arrangements heretofore described, the two limbs of the by-path on either side of the main scratch 10 constitute a condenser. This condenser is shunted by the resistance constituted by the restriction but nevertheless the energy stored up in this condenser will help to break down the gap across the main path especially after the restriction has been thermally destroyed. In some cases therefore it is desirable to increase the capacity of this condenser by specially designing the by-path to this end.

For example, as shown in Figure 6, the branch or enlargement on one side of the main path I may take the form of a rectangular enlargement 9α extending a considerable distance alongside the main path and having a breadth say two or three times that of said main path. The scratch 10α traversing the main path I will now extend a little way into the enlargement 9α where it will join at right angles one limb of an elongated Ushaped scratch 14 whose two extremities approach one end of the enlargement 9α and whose crutch portion approaches the other end. A third scratch 15 runs mid-way between the limbs of the U from a point very near the crutch portion completely to the far end of the enlargement 9α, and scratches 12α separate the enlargement 9α from the main path I right up to points close to the main stratch 10α. With this arrangement the small clearance Ila between the end of the scratch 15 and the crutch portion of the U-shaped scratch 14 constitutes the restriction in the bypath and it will be seen that the by-path itself follows a tortuous path and the capacity between

2,263,752 its two limbs is greatly Increased. The operation is as before: When, with or without the thermal destruction of the restriction at I la, the voltage drop across said restriction becomes too high, arcing takes place across Ila and the film is rapidly destroyed at a cool part of the support.

It will be appreciated that there are many ways by which the increased capacity between the limbs of the by-path may be attained.

Figures 7, 8, 9 and 10 illustrate arrangements in which the by-path does not consist of a thin metallic layer at all but is made as a separate circuit. Thus in Figure 7 the by-path is constituted by a resistance 16. The main path i is formed with a scratch I Ob of about the same thickness as the previous scratches 10 and 10α and which completely separates said main path. On the' two sides of this scratch the path is formed with backwardly inclined branches IT and the ends of these branches are connected to the ends of the resistance 16 by thicker layers 18 and solder 19 in the same way as the resistances 5 are connected to the ends of the layer I as shown in Figures 1 and 2. In the case of this figure of course there will never be any thermal destruction of the circuit interrupter but the initial destruction will always take place by arcing across the gap I Ob in the main path I, the support 2 remaining cool. If desired the resistance 16 of the by-path could be made variable as 16α in Figure 8 and the resistance material chosen could have a high temperature co-efflcient in order further to assist the break down across the gap.

In Figures 7 and 8, the resistances (6 and 16α respectively are shown as being outside the tube 3 forming the container for the circuit breaker. If desired the resistance may be inside said container as 16b in Figure 9. In all these Figures 7, 8 and 9, scratches 12α corresponding to the scratches 12 of Figure 5 are shown.

In Figure 10 an arrangement is shown in which the by-path is constituted by a condenser. 20 in place of the resistance 16 of Figure 7. A condenser can of course only be employed where the circuit breaker is to be used with alternating current. . Otherwise, the operation is exactly the same as in Figures 7, 8 and 9.

In short the by-path may be constituted by any circuit which brings about the break down across the gap at the required current value.

It will be apppreciated that, if desired a plurality of gaps and by-paths may be provided at different positions along the main layer I. This as before stated will further cut down the operating time by providing several centres at which the ionic destruction of the film commences.

It will be observed in Figure 2 that the main film I pursues a zig-zag or tortuous path. This is simply in order that its resistance shall be high without the overall length of the circuit breaker as a whole being unduly great.

It will be seen that according to the invention it is essential to interrupt the more or less straight conducting layer I in one or more places by a, gap 10, 10α or 10b and to bridge this gap by a suitable by-path. The by-path need not be in the same plane as the original conductor, and if it is not so this will further increase the tendency of the break down to occur, particularly with a rapidly rising current. For example in the case where the by-path takes the form of a thin layer like the main path (as in Figures 1 to 6) the base may take the form of a cylindrical rod so that the by-path will be out of the plane of the main path.

Another condition is that, as in the prior specification aforesaid, the conductor layer I should preferably be thin enough to be destroyed by the arcing of a current smaller than the current which is safely carried if the layer were nowhere interrupted. Thus, if the film I is accidentally ruptured by corrosion or mechanical means such as vibration or shock, any arcing across such accidental interruption will instantly destroy the film instead of slowly generating heat and possibly causing dangerous explosions.

I have stated in my said prior specification that no organic materials must be associated with the layer I. I have found that, with an arrangement in accordance with the present invention, the layer I may be protected by coating it with a varnish. The varnish coat is interposed between the metallic film I and the sand 8α surrounding it, but no varnish should be between film and support 2, and the support itself should preferably be of inorganic material as described in said prior specification.

The layer I may be produced in the same way as in my prior specification aforesaid and in other respects the arrangement may be the same as in said prior specification.