Comment by pram

1 year ago

Another thing I learned recently is above-ground telephone lines on poles are pressurized. Those long black tubes on the cables are splices that also allow the pressure to be read by sensors. The central offices had compressors that fed all the cables.

http://cityinfrastructure.com/single.php?d=RuralOutsidePlant...

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pram

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tyre  1 year ago

For those curious why, it’s to prevent water from leaking in. Instead, air from the pressurized interior will push out. This forces humid air to circulate with desiccants at various points, which will pull moisture from the enclosure.

Lastly, the sensors identify pressure dips, which tells technicians roughly where the leak is. Then they know which portion of cable to repair or replace.

myself248  1 year ago

Underground lines, too. The same cable might be underground and aerial and underground again, several times over its length. It would be installed in segments (1200-pair cable is heavy and you don't get much on a spool anyway) and spliced, but the splice cases are pressure vessels with gaskets at each cable entry and exit, so there's air continuity to the next segments. The cable might branch at several points, but again, sealed splices mean the whole branching structure is pressurized.

The cycling of the compressor, and the periodic click-hiss of the dual-column desiccant dryer, was an ever-present noise in the basement of every CO. Somewhere over near the air plant, would be a rack full of panels of pressure and flow gauges, and a bundle of tubes taking them over to the individual sealed splice closures where the air was injected into the cables.

A cable pressurization logbook sat on a shelf in the gauge rack, and one of the CO tech's duties was to periodically (weekly?) write down the readings from all the gauges, and compare them to last week and last year. Significant increases were handed off to the outside-plant crews.

At some point, typically where the branched cables ended at cross-connect boxes, the end of the cable would be sealed (air blocked by epoxy potting around the end), and the small lines leaving the crossbox would be icky-PIC instead of pressurized.