Telephone Cable

a low-frequency balanced cable used in local (city and rural) telephone networks chiefly for conveying subscriber loops. A telephone cable comprises a large (up to 3,600 and more) number of pairs of insulated copper conductors having diameters from 0.3 to 0.9 mm that are twisted into groups of two (two-pair stranding) or four (four-pair stranding). There is at present a tendency in a number of countries to replace the copper conductors with aluminum (or aluminum-alloy) conductors of somewhat larger (approximately 1.3 times) diameter.

Two methods are used in the manufacture of telephone cable. With the first, the groups of two or four pairs are wound in coaxial layers. With the second, which is the preferred method, the pairs are first grouped into smaller (10–25 pairs) or larger (50–100 pairs) units. The thickness of the insulation surrounding each conductor is generally 0.4–0.6 times the conductor’s diameter in the case of air-and-paper and polyethylene insulation and 0.25 times the diameter in the case of polyethylene foam. Industry produces telephone cables with various types of sheaths, chiefly metallic, such as those of lead (extruded) or thin-walled steel (welded, corrugated), and plastic. The plastic, usually polyethylene, is applied over an electrical shield of aluminum foil. Cables sheathed in metalloplast (usually as a plastic tube with a metallized interior), which have better moisture resistance, are becoming increasingly common, as are hermetically sealed cables in which the space between the insulated conductors is filled with a viscous hydrophobic compound. The outer diameter of a telephone cable does not exceed 75 mm, and the cables are usually placed in conduits having a standard diameter of 100 mm.

Since telephone cables are used only for single-channel communication, there is a single pair of conductors running from the distribution terminal to each subscriber’s telephone set. For every 1,000 telephone sets there are, on the average, 2,000–4,000 km of telephone circuits (pairs), or 40–80 km of conventional (50-pair) telephone cable. World production of telephone cable (1974) exceeds 1 million km per year.