WO2017147628A1

WO2017147628A1 - Detonator cable

Info

Publication number: WO2017147628A1
Application number: PCT/ZA2017/050012
Authority: WO
Inventors: Johannes Petrus Kruger
Original assignee: Detnet South Africa (Pty) Ltd
Priority date: 2016-02-25
Filing date: 2017-02-23
Publication date: 2017-08-31
Also published as: AR107744A1; ZA201801216B

Abstract

A cable for use in a detonating system which includes at least two conductive cores, in side-by-side electrical contact with each other, which are encased in an insulating sheath, and which have different characteristics to ensure that when the cores are tensioned, one core breaks before the other.

Description

DETONATOR CABLE 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 to detonators 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 cable failure. Also, environmental factors such as large ambient temperature variations, which can range from -40°C to +80^CC. can rapidly degrade the insulation on a cable.

[0005] Typically a cable has a steel or copper core encased in a suitable insulating sheath. 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 INVENTION

[0008] The invention provides a cable which includes at least first and second elongate, flexible, electrically conductive cores which are in side-by-side electrically conductive contact with one another and which are enclosed in a sheath of an electrically insulating material, the first core having at least one physical property which differs from a corresponding physical property of the second core and wherein, the first core, if tensioned in a longitudinal direction above a predetermined level, breaks at a location along a length of the cores and thereby forms a non-conductive gap between opposed ends of the first core at the break, the second core remains electrically conductive over at least a portion of the second core which is adjacent the non-conductive gap, and wherein the first core remains in side-by-side electrically conductive contact with the second core.

[0009] Each conductive core may be made from a metal or metal alloy, such as copper or steel, or a combination of metals and metal alloys.

[0010] Preferably, the cable includes more than two conductive cores. It is envisaged that a preferred embodiment would include at least three conductive cores. [0011] Without being limiting, the differing physical properties may be selected from core hardness, tensile strength, cross-sectional area, core conductivity and core composition.

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 is a cross-sectional view from one end of a multi-core cable according to the invention;

Figure 2 is a perspective view of one end of the cable of Figure 1 , with an outer insulating layer removed; and.

Figure 3 is a perspective view, in cross section, of the cable of Figures 1 and 2, wherein two cores of the cable have snapped.

DESCRIPTION OF PREFERRED EMBODIMENT

[0013] Figures 1 and 2 show a cable 10, according to the invention, in cross section and in perspective, respectively. The cable 10 has a body 12 which includes an electrically insulating sheath 14 which encapsulates three cores 16, 18 and 20, which are in side-by-side electrically conductive contact with one another.

[0014] Each core 16, 18 and 20 is elongate and is made from a suitable electrically conductive material which is flexible at least to some extent.

[0015] Each core 16, 18 and 20 has at least one distinct physical property which is different from the corresponding physical property of the other cores. In this embodiment, each core is made from a conductive metal. The cores 16, 18 and 20 have diameters 22, 24 and 26, respectively which differ from one another. This is by way of example and is non-limiting. The cores may also differ in respect of other physical properties such as the type of material, e.g copper or steel, from which each core is manufactured. The cores thus have different tensile strengths.

[0016] Within the cable 10 the cores may be twisted or be in straight line electrical contact with one another.

[0017] The cable is used, generally in a known manner, in a blasting system to establish electrical connections between a plurality of detonators located in respective explosive- charged boreholes and surface conductors or a harness, on a blast site, leading to a blasting machine. [0018] Inevitably in use, the cable is exposed to stresses and strains which can have unpredictable effects on the cable. The cores 16, 18 and 20 which have different physical properties e.g. different diameters 22, 24 and 26 or are made from different materials, respond to the stresses and strains in different ways. For example, if the cable is tensioned in a longitudinal direction the cores are elongated and can break at respective pre-determined tensile stress levels. These levels could however be lowered if the cable has been bent or, kinked, or has suffered damage e.g. due to an impact blow. Thus it is possible that one core could break at a first position along the length 34 of the cable, and another core could break at a second position, displaced from the first position, along the length of the cable.

[0019] Figure 3 shows schematically how the cable remains conductive after two of the cores, namely the cores 18 and 20, have snapped. In this case the cores 18 and 20 are made from steel and are included primarily to give the cable tensile strength. The core 16 is made from a highly conductive and malleable copper and thus is capable of significant extension without breaking.

[0020] A gap 28, which is formed between two opposed ends 32 and 34 of the snapped core 18, is bridged by an electrically conductive portion 36 of the core 16 which also bridges a gap 38 formed between opposed ends 40 and 42 of the snapped core 20.

[0021] An advantage of the cable according to the invention is that, even if all the cores of the cable were to snap, in all likelihood the various breakages would occur at different locations along the length of the cable. Thus it is likely that portions of each snapped core would remain in electrically conductive contact with the other cores or portions thereof and electrical conductivity along the cable would be maintained.

Claims

1. A cable which includes at least first and second elongate, flexible, electrically conductive cores which are in side-by-side electrically conductive contact with one another and which are enclosed in a sheath of an electrically insulating material, the first core having at least one physical property which differs from a corresponding physical property of the second core and wherein, the first core, if tensioned in a longitudinal direction above a predetermined level, breaks at a location along a length of the cores and thereby forms a non- conductive gap between opposed ends of the first core at the break, the second core remains electrically conductive over at least a portion of the second core which is adjacent the non- conductive gap, and wherein the first core remains in side-by-side electrically conductive contact with the second core.

2. A cable according to claim 1 wherein each conductive core is made from a metal or metal alloy.

3. A cable according to claim 1 or 2 which includes three of the conductive cores.

4. A cable according to claim 1 , 2 or 3 wherein the physical property is selected from the following: the hardness of the core; the tensile strength of the core; the cross-sectional area of the core; the conductivity of the core; and the composition of the core.