43 Mining Technical Information 43 Current Overload The temperatures of the conductors, insulation and jacket are, of course, elevated when cables are subjected to an electrical load. The resistance of the copper is increased, voltage drop in the cable is increased, and therefore, a reduced voltage is supplied to the machine. As a result, the machine calls for more current, which adds further to cable heating. A trailing cable’s insulation and jacket materials exhibit maximum resistance to physical abuse at the rated conductor temperature of 90°C or less. The ability of these components to withstand damage decreases as the temperature increases. Conditions which normally cause few cable failures suddenly become a problem. At elevated temperatures, the jacket has lost much of its resistance to cutting, crushing, tearing and abrasion. The section of the cable that remains on the reel is most likely to be damaged by electrical overload. Layering on the reel hinders ventilation and heat dissipation. Continued exposure to elevated temperatures will age the jacket, making it hard and brittle and causing crazing or cracking upon subsequent reeling. Improper Splicing and Termination Techniques Over the years, much work has been done to improve both splicing materials and techniques. The following items have been found to be primarily responsible for unsatisfactory splice service: 1. Ending up with a grounding or ground-check conductor which is shorter than the power conductors 2. Semi-conducting residue on the insulation surface was not removed 3. Gaps, voids or soft spots in insulating tape build-up 4. Improper termination of shielding system, leaving inward-pointing projections 5. Damage to factory insulation by improper removal of shielding systems 6. Excessive slack in one or more individual conductors 7. Splice has low tensile strength and is easily pulled in two 8. Individual wires are damaged during application of connector 9. Splice is too bulky — will not pass through cable guides or over sheaves 10. Improper application of the outer covering, allowing water to enter the cable interior By choosing a cable with an adequate current rating, avoiding excessive tension and mechanical damage, and using proper splicing techniques, it is not unreasonable to reduce cable-related downtime by 50 percent or more. This will, of course, translate into increased production and profits. Why and How Mining Cables Fail