BACKGROUND OF THE INVENTION

[0001] This invention relates to a cable for use in a detonating system.

[0002] Electronic detonators in a detonating system can be interconnected using wireless techniques, or by using appropriate cables. At present wireless techniques have not found widespread acceptance. Reliable connections can however be made using suitable cables but, at a blast site, the cables can fail due to a variety of factors.

[0003] A cable which is placed in a borehole can break due to impact damage which occurs when the borehole is filled with explosive or stemming. Stretching and consequent breakage of the cable can be caused by slumping of the material in the borehole. If insulation on the cable is compromised by robust treatment conductors in the cable can be short-circuited, particularly if exposed to water or other fluids, or open-circuited.

[0004] Arduous conditions can prevail at a blasting site and cable damage can thus arise from a number of causes. For example vehicle traffic, and chemicals such as diesel, can stress insulation on a cable and increase the probability of breakage of the insulation, or of water ingress through the insulation. Environmental effects such as strong ambient temperature variations, which can range from -40 ^° C to +80 ^° C, can rapidly degrade the insulation in a cable.

[0005] Typically a cable has a steel or copper core encased in a suitable insulator. The tensile strength of a steel cable is greater than the tensile strength of a copper cable. Depending on the composition of the metal used, in each case, a copper cable could be elongated by up to 40% before breaking whereas a steel cable would normally break if elongated to a substantially lesser extent. Copper is however more expensive than steel.

[0006] It should also be borne in mind that, at a blast site, the prospect of recovering copper or steel after blasting has taken place is negligible as, inevitably, the cables are effectively destroyed by the effects of blasting.

[0007] An object of the invention is to provide a cable which addresses at least some of these issues.

SUMMARY OF THE INVENTION [0008] The invention provides a cable which includes an insulating stretchable sheath, a conductive core of a first length encased within the sheath, the core including a plurality of conductive elongate elements, at least some of the elements being shorter than the first length, the elements being assembled in longitudinally extending, side-by-side conductive contact with one another over the first length. [0009] In one form of the invention the core is formed from a plurality of discontinuous conductive elements which are positioned in a longitudinally extending configuration wherein some elements are displaced from other elements along the length of the configuration and wherein the elements are positioned to provide a continuous electrically conductive path along the length of the core. [0010] Each element may have a predetermined length which may be significantly shorter than the length of the core i.e. the first length which is of an indeterminate dimension. For example, each element may have a length of from 10cm to 10m. This is exemplary only. As indicated the elements are arranged so that the elements are located at spaced apart positions along the length of the cable. Thus each of the ends are displaced, relative to one another, in a longitudinal direction of the core.

[00 1] Elements within the core which contact one another may be twisted together. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The invention is further described by way of example with reference to the accompanying drawings in which -

Figure 1 illustrates a plurality of spun copper elements or strands,

Figure 2 illustrates an optional intermediate process in which the spun elements of Figure 1 are twisted together,

Figure 3 illustrates a length of a cable according to the invention, and

Figure 4 illustrates an alternative way of making a cable in accordance with the principles of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0013] Copper is a highly conductive material but it unfortunately does not have a meaningful tensile strength. Copper can possibly be elongated by up to 40% before breaking. The invention is directed to increasing the extent to which a copper cable can be elongated without causing electrical conductivity to be compromised. [0014] In a broad sense the invention provides a cable which includes an insulating sheath which encases a core comprising a plurality of copper elements, each of a relatively short length, which are configured so that they can slide relative to each other when the cable is stretched. The elements are positioned side by side, in an overlapping relationship with one another, along the full length of the cable, so that a continuous conductive path is provided by the assembly of elements. The sheath is made from a suitable insulating polymer material which is capable of accommodating a substantial degree of stretch.

[0015] Figure 1 of the accompanying drawings illustrates a number of copper elements 10. Each element 10 is formed from one or more copper filaments with a diameter of 0,07mm. The filaments are positioned in a longitudinal path overlapping each other. The degree of overlap may vary according to requirement. In one example, each element 10 has a length 12 of 10cm and a following element is longitudinally displaced from the preceding element by a distance 14 of about 1 cm.

[0016] Abutting elements in the overlapped configuration (Figure 2) are then twisted together about a longitudinally extending axis to form a core 16 which is thereafter encapsulated in an insulating polymer sheath 18 to produce a finished cable product 20 (Figure 3).

[0017] The cable product 20 has a conductive core 16 which has a resistance below a maximum target value of 1 ,5 ohms/m. The degree of stretch which the cable product 20 can withstand, without losing conductivity, depends on the lengths of the individual elements and the degrees of overlap between side by side elements in the assembly of elements in the core, subject to the polymer sheath 18 being capable of stretching without breaking. For example, a sample made from 3m long strands with a 90% overlap could be extended by over 100%.

[0018] Figure 4 illustrates an alternative way of making a cable 24 according to the invention. A core 26 is formed in a conventional manner by twisting continuous elongate strands 28 of copper wire around a longitudinal axis 30 as is known in the art. The core 26 is then cut to a predetermined depth 32 at each of a plurality of spaced locations 34. The depth of each cut is sufficient to ensure that one or more, but not all, of the copper stands are severed. On the other hand the wires in the core are kept in close electrical contact with each other. [0019] Thereafter the processed core 26A is passed through an extruding machine 36 which applies a conductive sheath 38 of an appropriate polymer to the core. The sheath 38 ensures that the strands 28 in the core 26A are kept together and provides an insulating function as is known in the art. The polymer has appropriate properties which make it suitable for use at a blast site and additionally is chosen to ensure that it can elongate to a substantial extent without breaking.

[0020] When the cable 24 is stretched various shorter lengths of wire formed by the cutting process can part and slide, as appropriate, relative to one another while being kept in close electrical contact with adjacent longitudinally extending wires due to the fact that the wires are held in a twisted configuration, inside the sheath, and also due to the binding effect of the sheath which can stretch and so ensure that the wires are kept in a side-by-side abutting relationship thereby maintaining electrical conductivity.