This invention relates to so-called impedance bonds.

Impedance bonds find use in electric railway systems using direct current power for which return connection is through both running rails of each track of the system, which running rails are further used, via electrically isolated sections, thereof to carry alternating current signals for detecting presence of running stock whose axles short circuit those signals. Impedance bonds provide both of a low-resistance high- current d.c. path between running rails of each track and a high impedance normally low-current a.c. connection therebetween, and further provide for interconnection of impedance bonds of successive isolated track sections to afford d.c. continuity. In meeting those requirements, impedance bonds need to be operationally immune to any possible false signalling effects arising from inevitably high transient currents associated with drawing traction power and/or associated with foreseeable fault conditions.

Those requirements of impedance bonds are well- known, as are basic electro-magnetic structures to achieve them. Thus, it has become conventional for

impedance bond structures to provide magnetic coupling between two equal and opposite heavy duty windings extending from a common central tapping to connections for each of a pair of running rails. They afford d.c. connections, using the central tap to interconnect track sections for d.c. continuity. A relatively light duty a.c. winding also traverses the same magnetic coupling. Meeting performance requirements is obviously important, particularly doing so in an efficient and cost effective way, and we have directed particular attention to improving magnetic coupling.

In doing so, we have confirmed that laminated E- cores and closure bars are less efficient, even with easily saturable material between them, for impedance bonds, than laminated C-cores cut from forming rings and paired up again to encircle d.c. and a.c. windings. That is to be expected from effective continuity of grain structure of metal laminations at least in each C- core, thereby substantially reducing at least eddy- current effects.

Use of C-cores is generally well-known, and an impedance bond using them is described in British Patent No 1601352. That Patent is primarily concerned with providing a low-height structure that can be installed

between railway line sleepers without intruding into track bed material. The resulting structure has d.c. windings in an elongated, quite narrow rectangular loop configuration, both of the long sides of which pass through magnetic couplings that are effectively "tunnels" of side-by-side confronting C-core pairs, one "tunnel" along each of the long sides. Each of the paired C-cores has one core on its back below the other. The d.c. windings are installed along lower cores in two layers side-by-side, and with the a.c. winding further at the inner side of the d.c. windings. The upper cores are then fitted and the intended low-height structure is achieved.

However, with such a low-height structure, the two "tunnels" of paired C-cores leave substantial lengths of windings, at corners and along shorter sides of the rectangular loop configuration, outside the magnetic coupling afforded along long sides of the loop configuration. In fact, for an overall rectangular loop configuration with an aspect ratio of about 2:1, less than half of the d.c. winding length is within the tunnels of paired C-cores. The corresponding figure is about 70% for the a.c. winding where that is innermost of the combination of a.c. and d.c. windings, but, of course, also with uncoupled run lengths in two parts

each amounting 15% or more of the total winding extent.

It is an object of this invention to provide an impedance bond with improved magnetic coupling and/or lower intrinsic materials content thus manufacturing cost.

According to one aspect of this invention, magnetic coupling means is more evenly distributed about d.c. and a.c. windings of an impedance bond, preferably with more than half the total runs of all, further preferably each, of the windings (excluding excursions to end and centre tap connections) within the magnetic coupling means.

According to another aspect of this invention, magnetic coupling elements leave a lesser proportion of d.c and a.c. winding runs uncoupled by those elements, preferably coupling to more than 50% of the runs of each of the windings, further preferably with no run length for any winding free of magnetic coupling that exceeds 20% preferably 12%.

Either, indeed both, of those aspects is or are achievable for a substantially square loop configuration, i.e. rectangular with an aspect ratio of

about 1:1, and with magnetic couplings to each of its sides, i.e. omitting coupling mainly if not only at corners.

For circular loop configuration of d.c. and a.c. windings, and even distribution of magnetic coupling elements, run lengths of windings free of the coupling elements can be substantially equal and less than 4%, preferably about 3% or less.

With the aforementioned low-height elongate winding impedance bond, centre tap provision is made at one of the short sides and end connections provided from the other of the short sides, neither of which provisions contributes to compactness of construction.

According to another aspect of this invention, at least centre tap connection of d.c. windings is made in free space about which loops of the winding configuration are disposed.

For preferred embodiments hereof, the two d.c. windings are one above the other, so occupying more height than those of the aforementioned low-height elongate winding structure, and that contributes to minimising run lengths of windings free of magnetic coupling. Preferably magnetic cores used are of greater

height than lateral extent, preferably one deep, effectively U shaped, and as the lower element (into which the windings can be readily emplaced) , and the other shallow, effectively C-shaped. Paired core elements are, of course, then asymmetric and generally unequal.

Specific implementation of this invention will now be described by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a diagrammatic plan view of one impedance bond of circular windings configuration;

Figure 2 is a diagrammatic plan view of windings and cores alone;

Figure 3 is a diagrammatic sectional view on line A-A of Figure 2;

Figure 4 is a section of an impedance bond installed in ballast between running rails of railway track;

Figure 5 is a side view thereof;

Figures 6A and 6B show winding connection details

for the bond of Figures 5 and 6; and

Figures 7 and 8 show rectangular winding schemes.

In the drawings, a lower casing 10 has a well 12, for accommodating a generally circular winding and magnetic core system 20, between shelf formations 14,16 to ends of the casing affording mounting lugs 18A,B,C, and winding connections at tabs 22A,22B for d.c. rail connections, and block 24 for a.c connections.

The windings and core system 20 is of substantially circular loop configuration through magnetic cores 26 disposed in a circle. Core corners to their innermost sides are shown closely adjacent, and the cores extend generally radially to their outermost sides, marked 26A,26B for one core 26X. There are two d.c. windings extending at 28 and 30 from the end connection tabs 22A and 22B, respectively. Each of those d.c. windings 28,30 traverses -the circle of magnetic cores 26 twice. In Figure 1, one d.c. winding 30 makes a counterclockwise outer traverse 30A through all but one of the cores 26, then a transition 30B through the "last" of the cores 26Z to an inner counterclockwise traverse 30C through the all but one cores 26, and out at 30D into the free space 32 within the circle of cores 26. The other d.c. winding 28 enters the circle of

cores 26 on the other side of the "last" one thereof (26Z) below the first loop 30A of the one d.c. winding 30, and makes a clockwise outer traverse of the all but one of the cores 26 below the loop 30A, then a transition 28B through the "last" core (26Z) to an inner clockwise loop under the loop 30C, and out of 28D into the free space 32.

The inward excursions 28D and 30D of the d.c. windings pass to each side of the "last" core 26Z, which, is accommodatingly spaced from adjacent ones of the other cores 26. Figure 1 shows those excursions 28D,30D bent up and back at 28E,30E to come out over the "last" core 26Z at 28F,30F to centre tap construction tabs 34A,34B. Alternatively, and as shown in Figure 2, inward excursions 28D,30D may be connected together in the space 32, see connnection 36, and a further suitable connection (not shown) made to a single centre tap connection (not shown) at any convenient location. A further alternative is for the d.c. windings to have outward excursions from their second turns, rather than the inward excursions 26D,30D.

Turning to Figure 3, the two oppositely wound pairs of loops 30A,30C and 28A,28B are shown above and below, respectively, on a.c. winding 40. Also, the core 26 is shown as two parts 26A,26B of which the lower part

26B is of deep U-shape into which the windings 28,40 and 30 are loaded successively at fabrication, and a much shallower C-shape upper part 26A. The two parts 26A,26B of each core 26 are indicated, diagrammatically in dashed lines only, with an external strap 42 and screw fastener 44 for holding them together.

Compared with the aforementioned low-height windings and core system, the asymetric component cores 26 hereof are stood upright, i.e. on their narrow dimension, and with the windings 28,30, also 40, stacked vertically, rather than on their sides for equal C- shaped components. That assists maximisation of overall length of loops of the d.c. windings 28,30 that are within the cores 26. However, a windings/cores system with such orientation, and composition of cores, also represents an operational improvement on the aforementioned low-height system by reason of greater and more even magnetic coupling. Moreover, there is a substantial saving in required lengths of heavy duty d.c. winding rod or bar (typically from about six to about four metres ) , in fact also of number of magnetic cores required (typically from fourteen to twelve) , for satisfactory electrical performance. In fact, for the same cross-section of conducting rod or bar, a substantial increase in current-carrying capacity results. Alternatively, of course, the same current-

carrying capability as hitherto can be provided using conducting rod or bar of correspondingly smaller section.

The preferred windings- and cores system with the cores 26 upright and the windings stacked vertically does not permit installation without some excavation of railway track ballast material between sleepers, i.e. to take the well 12 into which the stacked windings and surrounding cores extend, see at 50 in Figures 4 and 5 . for typical minimum incursion into ballast. However, it does assume that the larger dimension of the cores is upright, thus facilitating reduction of windings runs free of those cores, including for square or rectangular windings configurations with plural cores on each side, or at least one core on the shortest sides.

Figures 7 and 8 show such rectangular systems in principle for windings indicated globally by a thick solid line (20A, 20B) and cores as dashed boxes (26A, 26B).

Figures 4 and 5 also show a revised and particularly preferred winding stack with the auxiliary winding 40 at the bottom and shown separated from the main windings 28, 30 by a shielding insulating ring 52.

Figures 4, 5 and 6 further show connection of

those windings 28, 30 to be common by way of a central post formation 54 to which ends of the winding portions 28D, 30D extend and are secured, say by bolting or screwing. That can usefully further afford electrical connection through to a connector block 34C, conveniently through the body of the lower housing part 10 acting after the manner of a common earth or chassis connection when of electrically conducting material such as aluminium or aluminium alloy as is preferred. The part 54 is actually indicated as integrally cast with the part 10, as is the connector block 34C.

Significant performance improvements arise for preferred embodiments of this invention, normally together with cost savings, particularly in conjunction with the C-core etching teaching of our pending patent application No. 88 28976.4, particularly in terms of achieving impedance above 11 ohms and high current carrying capability.

However, a further or other improvement arises from addressing the problem of reduction of current carrying capability generally experienced for bent of fabricated windings, compared with theory or expectation based on straight rod or bar of the same cross-section. We find that the acts of bending and/or the facts of providing junctions clearly contribute, and that

casting of windings gives superior results. Accordingly, and at least for impedance bonds, casting of windings constitutes a further aspect of invention.