February 14, 1913. THE COLLIERY GUARDIAN. 33 L The compound with which the paper dielectric of this class of cable is impregnated varies with different makers. For mining work, where temperatures are often high, and more especially for shaft work, it is obviously desirable that the physical consistency of the compound under working conditions of the cable should be such that it has no tendency to flow or creep away from the paper. Hence it is fairly common to specify that the compound shall show no tendency to drain away from the cable, while some engineers specify a “ weeping ” test, which consists of suspending a length of the cable vertically at a given temperature for 24 hours. Though the results of injury to paper cables are usually serious, it is remarkable how much mechanical stress they will withstand. At Hulton, the writer believes he is correct in saying that not a single cable was brought to bank after the recent disaster, all being now in commission. They lay under falls, the weights of which have probably never been exceeded, many being crushed into abnormal shapes. Paper Leadless Cables.—These are, as their name implies, insulated with paper and sheathed with a non- metallic material, which is almost invariably vulcanised bitumen. They have certain advantages in the way of dissipation of heat due to overload, and also as regards internal solidity, because of the comparative incom- pressibility of the paper insulation. In the event of damage, however, they have the same disadvantages as paper lead-covered cables, and for shaft work, at any rate, the author considers this type of cable unsuitable. Bitumen Cables.—These are, in the writer’s opinion, by far the most suitable for average colliery work. They are certainly the most suitable for shaft work. Until quite recently great difficulty was experienced in getting the bitumen insulation to such a consistency that in moderately high temperatures it would not decentralise, while in low ones it would not become brittle. As the result of research and manufacturing improvements, it is now possible to avoid these troubles over a range of temperature which covers all ordinary working con- ditions—-say, 28 to 120 degrees Fahr. Further, whereas it was formerly necessary to introduce, in the construc- tion of the cable, mechanical reinforcements such as tapes, braids, &c., the improvements in physical proper- ties of the bitumen and methods of manufacture now enable cables to be made without these hygroscopic, and therefore electrically undesirable, components. For instance, the “ solid ” three-core bitumen cable which is familiar to most colliery engineers is so constructed as to contain no hygroscopic material whatever inside its sheath, so that, in effect, the conductors are embedded in a solid cylinder of vulcanised bitumen. A more recent form of construction, called “ Oracore ” (fig. 2), consists in the laying up of the separately pressures exceeding “low pressure,” it is, of course, now necessary that the conductivity of the metallic coverings must be at least equal to 50 per cent, of that of the largest conductor which they enclose, and in no case of less cross-sectional area than that equivalent to 0 022 square inch of copper. This last-named point would appear to apply to even the smallest size of trailing cables. The following diagram (fig. 3), for which the writer is indebted to Mr. C. J. Beaver, shows Fig. 1. Hl Fig. 2. could only be met in this case by running separate and additional conductors at earth potential down the shaft in parallel with the sheathing of the H.T. feeder. The importance of efficiently protecting the armouring from corrosion can hardly be exaggerated. Both inner and outer servings of jute, as well as the steel wire armour, should be heavily and separately compounded with water-resisting mediums. A factor which is often overlooked as a cause of corrosion is leakage current, and consequently a pit water may get credited with corrosive properties which it does not really possess. On the other hand, mines are almost always wet enough and the water impure enough to provide a sufficiently good electrolyte for leakage currents to do an almost unlimited amount of damage. Electrolytic action is far less readily set up on alternating systems than on continuous, but in either case it may be said that leakage currents can never be entirely eliminated. This being so, the obvious thing is to provide their paths (generally the cable sheathings) with such sound mechanical and electrical connections to earth that corrosion cannot take place. Trailing Cables The chief essentials in the construction of these cables are those relating to flexibility and resistance to external abrasion and ill-usage. The former is attained chiefly by making the conductors of a large number of fine wires, and the latter by special coverings, usually of a non-metallic character, such as rope lappings, leather or whipcord braidings, rubber sheathings left bare, &c. Metallic armourings are now considered undesirable for portable flexible cables. The latest construction consists of rubber-insulated conductors laid up with an earth conductor having a conductivity of not less than 50 per cent, of that of the largest main conductor, with a minimum conductivity equivalent to 0*022 square inch of copper. These cores are usually sheathed after laying up with a vulcanised bitumen or rubber sheath. While having the obvious advantage of excluding moisture from the cores, the mechanical support afforded by the sheath also tends to w Fig. 3. . it i'ljr.iH It ingii iiii Fig. 4. prevent kinking of the cable. The various forms of mechanical protection mentioned above are then applied as desired. Installing. The space between generating station and headgear is perhaps the most prolific field of any for cable break- downs, and too much attention cannot be given to the handling of cable in this area. To lay any cable in the ground, either on the solid or direct systems, is to hide what the irresponsible navvy soon finds with pick or bar, and mechanical injury of such nature is a very serious matter in the case of fibrous cable on account of the moisture which it will soon pick up. On the other insulated cores on a central cradle of vulcanised bitumen the approximate relation between the conductivity of hand, the steam-laden soil of most colliery yards is an shaped to receive them. The whole is then sheathed the “largest conductor” and metallic sheaths (made effective ban to the use of bitumen so laid. over all with a tube of vulcanised bitumen which is according to the Engineering Standards Committee’s | The best means of grappling with this difficulty are, cylindrical on the exterior, but shaped internally so as tables) in several types of cable. From this it will be either to build a well-ventilated surface culvert (open or to fit the interstices of the laid-up cores. This con- seen how very slightly the sheathings of any but large-1 covered with chequer .plate) in which the cables can be struction, by distributing the mechanical pressure sized cables have had to be reinforced under the new carried on brackets; or to sling them overhead from a between the cores over a large area, enables even twin rules by manufacturers. ’ catenary wire. Old haulage rope can generally be used bitumen cables to be made without the inclusion of j As in most great reforms, however, the new rules are for this purpose, made taut with capels and tightening hygroscopic substances anywhere within the boundary | likely to lead to some remarkable situations—none more screws. A useful type of sling much used by the of the vulcanised bitumen sheath. In these types of so than in this very question of armour conductivity. I writer for this purpose is made on the wedge principle, cable the conductors should always have the strand! Imagine an H.T. system with transformers at shaft with a former of sheet lead, braided with compounded interstices filled with compound of such a character | bottom. The H.T. shaft feeder takes, say, one-tenth of that it will not melt or become displaced under the ’ the current of the L.T. roadway cable or cables. What influence of heating effects set up by excessive currents must the section of the H.T. armouring be ? Logically, in the conductors. five times the conductivity of the H.T. conductors if Bitumen cables should not be installed immediately there is only one L.T. distributor, or half the conduc- after direct exposure to frost or very keen winds. tivity of the largest L.T. cable if more than one In all of the above-mentioned types, if used for * distributor. It appears that the spirit of the rules yarn. If the shaft is some distance from the power-house, paper lead-covered cable laid solid is worth considering. In such a case a disconnecting box should certainly be inserted at the pit top. For even greater distances— especially on E.H.T. lines—bare overhead conductors are frequently used.