UNDERGROUND CABLES

Wairarapa Times-Age, 2 September 1938, Page 3

UNDERGROUND CABLES

“VAGABOND CURRENTS” & TRAFFIC VIBRATION. SOME ENGINEERING PROBLEMS. Over seventy per cent of New Zealand's telephone wires are carried underground in cables made up of scores of pairs of fine wires, insulated with paper, securely sheathed in lead. This modern development, which has effectively reduced the spiderwork of overhead wires in our towns, has taken place contemporaneously with other features of progress, heavy motor traffic and the increasing use of electricity for transport, both of which provide an occasional problem for the telephone engineers.

The telephone cables are carried underground in conduits of earthenware or metal pipes. At frequent intervals sets of cables branch off from the main one to serve groups of telephone subscribers. The branch leads up from a junction box to the pole carrying aerial wires. The whole equipment is secure against the flooding of winter time, but heavy motor traffic is a cause of failure at rare intervals, and. when this occurs it is well advertised through the loss of communication by a dozen or more telephones. The vibration of our roads, it has been found, slightly breaks down the lead covering of telephone cables at the point where they branch off from the main cable, as the different sizes of cable do not vibrate uniformly. What the engineers call “vagabond currents” is another threat to the efficient insulation of the underground telephone wire system. The current used by tramcars returns to its source through the rails, which are electrically bonded to increase their conductivity. However, these currents occasionally stray and take a shorter path. They find the lead covering of the underground telephone system a better conductor than the tramway rail, and travel along the sheath of the cable to the power house. The point at which the stray current leaves the telephone cable is carefully watched by the engineers, because the current takes away with it some of the lead covering. Extra heavy sheathing is provided to guard against this loss, and electrical tests are regularly carried out to trace the currents and provide against their damaging effects qn post office cables. Still another enemy of the telephone system has to be guarded against in some New Zealand towns, where chemicals in the soil, which are dissolved by water, set up erosion in the telephone ducts and eventually permit damp to enter, with bad results to the insulation. These processes of disintegration which have been described are difficult to detect because dampness can rbach the telephone wires through cracks of microscopic size. The engineers of the Post Office are constantly on their guard against these threats to the system of communication, and the result is that breakdowns are rare, and y when they do occur, are speedily corrected.