INCREASING USE OF PLASTIC COVERINGS

Press, Volume CI, Issue 29754, 22 February 1962, Page 22

INCREASING USE OF PLASTIC COVERINGS

Various types of plasticinsulated and phattic-eheathed telephme cable to meet all the Dominion requirements are made by the new cable factory of Austral Standard. These are in addition to the main lines of production of paper-4nsulated cable.

Two primary insulants are used in plastic-covered cables —polyvinyl cMoride, com. monly known as P.V.C. and polyethylene—and they are used in various colours to provide the necessary pair and single wire identification.

The type and thickness of the insulating compound extruded over the wire are determined by the electrical properties of the cable. It is

sometimes necessary for different conductors within a cable to have different thicknesses of insulation.

In the manufacture of plastic-insulated and sheathed cables, the independent conductors are first insulated to form a single in one of several colours on a horizontal Archimedean screw extrusion machine where the wire is run through and simultaneously coated with extruded plastic at high temperatures, and at speeds of up to 1800 ft to 2000 ft a minute.

After leaving the extrusion head, the covered wire is eooled by passing through a water trough before being reeled up on a take-up spool.

The next move is to “twin” two wires together to form a pair or speech channel before they go to a stranding machine. The stranded conductors are lapped with a P.V.C. or polyethylene tape and the assembled core is sheathed. Over-all cable sheathing is done on a similar but larger extrusion machine which forms a continuous pipe of plastic of the same dimension as the cable core. P.V.C. sheathing is normally made from a soft semiflexible compound and unless otherwise specified is coloured grey.

Resistance Because it has greater resistance to ageing than other

colours, black is usually used for sheathing outside installations.

Polyethylene sheathing is made from a compound specially prepared to resist oxidation and cracking caused by exposure to strong sunlight. When it is necessary to sheath plastic insulated conductors with lead, the stranded conductors are often first lapped with a proofed cotton tape. For certain outdoor applications where it is desired to use lead sheathing, additional protection against corrosion may be obtained by the use of a P.V.C. or polyethylene covering over the lead.

The use of plastic insulated wires 'and cables is extending and is being adopted in almost every phase of telephone practice from single wires to multi-conductor cables, for in many instances this type of cable offers advantages.

Signal Strength In some fields, particularly those concerned with very high frequencies, transmission of signals over wires could be achieved only at the expense of great loss of signal strength over even short lengths of cable were it not for the properties of plastics, notably polyethylene, which in some b»»» is used in expanded or cellular form. At the lower frequencies, plastic wires and cables have the advantages of lower price, positive and permanent identification and reduction in the size of multi-way cables.

In local type cables for external installation, the plastic type also provide* a much lighter cable, with relative freedom from service interruption* caused by water damage.

The Port Office use* plastic insulated wires for jumper wire, the wiring of telephone inrtrument* and automatic switchbank*, the wiring cf automatic switchrack* and manual switchboard*, and for some cabling in automatic and manual exchanges.

Plastic insulated and sheathed cables are in some cases also used for internal cabling of telephone subscribers' premises.