Line Construction

Part of Telephony

Building and installing telephone line plant — poles, aerial wire, underground cable — to connect subscribers to an exchange.

Why This Matters

The telephone line plant is the most visible and most expensive part of any telephone network. Miles of wire strung between poles or buried underground represent years of labor and materials investment. Getting line construction right from the start prevents costly rework, service outages, and safety hazards. Getting it wrong means sagging wires that break in ice storms, underground cables that flood at splice points, and pole lines that blow down in moderate winds.

Line construction knowledge covers two distinct environments: aerial plant (wire on poles) and underground (buried cable or conduit). Each has specific requirements for materials, installation technique, and long-term maintenance. Knowing both lets you choose the appropriate method for each segment of a network based on terrain, expected hazards, and available materials.

For a post-collapse community telephone system, line construction will be the main labor bottleneck. A small team can wire a dozen instruments in a building in a day; the same team may take weeks to string wire between buildings one kilometer apart. Understanding the principles helps you organize work efficiently and prioritize the highest-impact construction first.

Pole Line Planning

Aerial telephone lines on wooden poles are the most common telephone plant in rural areas worldwide. Poles are spaced 40-60 meters apart (40 meters in areas with heavy ice or wind loads, up to 60 meters in sheltered areas with light wire). Shorter spans reduce wire sag but require more poles; longer spans increase sag and mechanical stress.

The horizontal loading from wind, ice, and wire tension determines pole size and embedment depth. For light rural telephone plant (one or two pairs of 0.4-0.64 mm copper wire), a 7-meter pole with 1.2 meters burial depth is adequate in most conditions. In areas with reliable heavy ice accumulation, use 8-meter poles with the same burial depth.

Pole material preference: freshly cut hardwood (oak, locust, cedar) resists ground rot longer than softwood. Treat buried sections with creosote, coal tar, or copper naphthenate preservative to extend life in wet soils. An untreated softwood pole in wet ground may rot through in 7-10 years. A treated pole of any species typically lasts 30-50 years.

At corners and line termination points, the pole must resist the net pull of wire tension in multiple directions. Brace corner poles with a diagonal strut from the top of the pole to a buried anchor 2-3 meters out from the base in the direction of the wire pull. Or use a down-guy — a galvanized steel wire from the pole top to a buried screw anchor — for the same purpose. Unguyed corner poles eventually lean, then fall.

Aerial Wire Installation

Standard telephone aerial wire is a single conductor with weatherproof insulation, mechanically supported by a messenger wire (steel strand) or by the conductor’s own insulation in the case of self-supporting cable.

For the simplest rural construction, bare copper wire on glass or ceramic insulators works and requires no special cable. The insulators screw onto wood pins nailed to the pole crossarm. Thread the wire through the insulator ties and tie it off at each end span. The wire must be tensioned to the correct sag — measured as the drop at mid-span below the level of the attachment points.

Correct sag matters. Too tight and the wire breaks in cold weather when metal contracts. Too loose and the wire sways and contacts adjacent wires or vegetation in wind. The standard sag formula considers wire weight, span length, and mechanical tension. For typical rural telephone plant with 40-meter spans, a mid-span sag of 0.6-0.9 meters (about 1.5-2% of span length) is appropriate at 15°C installation temperature. Adjust for temperature at time of installation — strings installed in summer at low tension will sag even more in winter wind; strings installed in winter at high tension may break in extreme cold.

Underground Cable Installation

Underground cable avoids weather exposure and vandalism but requires trenching and is harder to repair when damaged. For urban and suburban areas where aerial plant is prohibited or impractical, underground is the only option.

Direct-buried cable (no conduit) is the simplest underground installation. Bury cable at 0.6-1.0 meter depth in areas subject to frost (below the frost line) or 0.4-0.6 meter in frost-free areas. Use cable with polyethylene or PVC jacket rated for direct burial. Place a layer of sand around the cable at the bottom of the trench to prevent rocks from abrading the jacket. Lay a warning tape 0.2-0.3 meter above the cable to alert future excavators.

Mark all cable routes precisely on a property map and maintain these records permanently. Undocumented underground cables are severed regularly by excavation work. Include stake markers at regular intervals on routes crossing open land.

Testing Before Service

Test every new line segment before connecting instruments. Measure loop resistance (short the far end and read resistance from your end — should match the calculated value for the wire type and length). Measure insulation resistance to ground (should be hundreds of megaohms for new aerial cable on clean insulators, at least tens of megaohms for underground cable). Test for pair balance (the resistance of each conductor in the pair should be within 2% of each other — imbalanced pairs cause noise from induced interference).

Document all test results at installation and repeat measurements annually. Trending resistance and insulation values over time reveals aging before failure, allowing scheduled maintenance instead of emergency repair. A cable whose insulation resistance has dropped from 1,000 megaohms to 50 megaohms over three years is heading for failure — schedule replacement before service disruption.