42 Technical Information Mining 42 Cable breakdowns are neither mysterious nor unaccountable and almost without exception can be traced to one or more of the following causes: 1. Excessive tension 2. Mechanical damage 3. Current overload 4. Improper splicing and termination techniques Excessive Tension Many cable failures are the direct result of excessive tension. A cable that has been “stretched” no longer has the balanced construction that is so vital to long life. Tension on the conductors subjects the individual wires in the strand to compression and shear. These thin wires are damaged and will break more easily when bent or flexed. Tension also elongates the conductor insulation. The elongated insulation is then vulnerable to compression cutting. It will rupture more easily when it is crushed against the stranded conductor during runovers. The insulation will also have a tendency to creep over the conductor at a splice. Jackets under tension lose a considerable part of their resistance to mechanical damage. A jacket under tension is much more likely to be cut or torn. Stretching also causes the copper conductors to take a permanent set. Of course, the insulation and jacket are stretched as well, but they will return to their original length when the tension is removed. This difference in the properties of rubber and copper when subjected to tension will cause the conductors to be wavy and fail prematurely. To reduce tension on the cable: 1. Avoid backspooling, if possible. 2. If backspooling is unavoidable, locate the tie point as far back from the haulageway as possible. 3. Tram slowly when passing the tie point. 4. Set hydraulic tension on the cable reel so that approximately 12-15 feet of cable is picked up off the mine bottom when starting to tram. Mechanical Damage This is one of the most prevalent sources of trailing cable failures. Factors initiating mechanical damage include cutting, compression (crushing), punctures and abrasion. In extreme cases of mechanical damage, the failure is instant, and the cause can be assigned on the spot. Many times, however, the cable components are merely “injured” and become latent failures. At that point, it may be more difficult to pinpoint the exact cause and to take remedial action. Why and How Mining Cables Fail