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October 22, 1937
Pyrotenax Cables
A New Insulating Medium.
It has long been a commonplace that one of the chief difficulties of the Special Areas has been a too strict adherence in the past to the large basic industries. Consequently, in any ebb tide of prosperity they were the first to be left high and dry, while other districts, blessed with more diversified industries, could keep afloat, if not with ease, at least without excessive difficulty. It is therefore gratifying to see some of these smaller industries settling down in places which have hitherto been considered the strongholds of the staples.
One of the latest of these is a new factory on Tyneside sponsored by A. Reyrolle and Co., Ltd., which is devoted to the manufacture of "Pyrotenax" cable, a French patent. This factory was opened officially on October 12 by the Rt. Hon. Viscount Ridley of Blagdon, and it is interesting to note that all the plant installed is of British manufacture and that electricity forms the sole source of power.
The insulating material used in Pyrotenax cable is perhaps the most unusual feature of its construction. It was devised to avoid, in a measure at least, some of the drawbacks inherent in the more usual insulating materials, such as shortness of life, inflammability, susceptibility to the ingress of moisture, etc. and it was finally decided to use dehydrated magnesia in a metal sheathed cable. The following is a brief description of the processes of manufacturing standard 600 volt single and multi-core Pyrotenax cable :—
Magnesia is formed into small blocks in an insulation press. These blocks are compressed out complete with holes for the conductor rods, under a pressure of 25 tons. The blocks are then heated in a rotary hearth electric furnace until all physically and chemically combined water is driven out. The tubes of copper which form the sheath of the cable are cleaned and dried internally before being moved over to the insulation furnace for filling. One end of the tube is "swaged" to allow it later to pass through the drawing die and to be gripped by the draw-vice.
The blocks, hot from the furnace, are fed into the tubes while neutral gas flows through the tube to prevent oxidisation of the copper due to the heat given off by the blocks. The conductors, which are originally bought in coils, have to be straightened before insertion into the cable and a machine is provided for this purpose. After the insertion of the conductors, the filled tubes or "work-piece" is drawn out to two to three times its original length on the straight drawbench, which has two-speed gear drive. As the work-piece becomes hardened with drawing, it is passed from time to time through an annealing furnace. The furnace is kept filled with neutral gas to prevent the formation of scale on the sheath. At the outlet end the furnace muffle is water-cooled, to ensure that the workpieces emerge below oxidation temperature and cool enough to be handled immediately.
By this time the workpiece is sufficiently reduced in diameter to enable it to be coiled, and further drawings take place on the 6 ft. diameter bull-block. A two-speed gearbox is incorporated in the motor drive.
The bell furnace for coil annealing is in the right-hand corner of the factory, and consists of three containers and one electric bell furnace, which can be placed over one container while the other two are being cooled or are being recharged. Again the annealing takes place in a neutral gas, which is stored in a gasometer nearby: as can be readily understood, large quantities of gas are required immediately for each recharge, as distinct from the steady supply needed by the continuous furnace.
Cables below 0.5 in. over-all diameter are finally drawn on a 3 ft. drum; three-speed motor drive is incorporated in this case.
Whether taken from the large or the small bull-block, all coils are annealed and rendered soft for easy erection before passing to the test room, where tests in accordance with the B.E.S.A. specification are carried out before the cable is finally dispatched to the storeroom. Finally, the cable is dispatched in coils, protected by bessian wrapping, whether for use in Great Britain or overseas, and is not wound on drums.
On the occasion of our visit to these works a number of demonstrations was carried out to show the various special features of these cables.
Current Carrying Capacity. — A current of 80 amp. was passed through a rubber-insulated 3/029 cable and a Pyrotenax cable of equal conductor area in series. The rubber sheath quickly melted and dropped off the former, while the Pyrotenax was not heated enough to singe a piece of paper. The head of a match could be safely rested against the Pyrotenax, while if this were done, to the other the insulation burst into flames.
Mechanical Strength. — A two-core table was subjected to 50 to 60 blows from a hammer until it was flattened to about the thickness of a sixpence or a shilling. A lamp and fuse were connected in the circuit so that when the live wire was sufficiently near the return wire or the sheath to break down the insulation, the fuse blew and the lamp was extinguished. On cutting open the cable it was found that the actual conductor wires were flattened in the same proportion as the sheath.
Resistance to Fire. — A length of two-core Pyrotenax cable, together with lengths of cab-tyre-sheathed, V.I.R. cable in conduit, and asbestos braided cable in conduit, were exposed to the flames of a furnace sufficiently fierce to render the conduit red-hot. Within one or two minutes of applying the match to the fuel, one type of cable usually succumbed, and the remainder, excepting the Pyrotenax (which was used for successive demonstrations), quickly followed as the fire developed.
A lamp and fuse were installed in each circuit, so that it would be noted when the live wire in each pair was short-circuited to the return wire or to the conduit. Doubt having been expressed as to which was the hottest part of the fire, the Pyrotenax cable was looped round the others. The dense acrid smoke emitted by the rubber-insulated cables could be seen at the sides of the furnace; this smoke is a feature which adds to the terror of an electrical fire and which impedes fire-fighting.
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