larry@uunet.uu.net (Larry Lippman) (08/13/89)
> I've seen compressed gas cylynders near poles and I've always wondered what > purpose they are used for? Could anyone shed some light on this? The compressed gas cylinders contain dry nitrogen, and are used as a source of gas to purge a cable of moisture following construction or repair work. Such nitrogen cylinders are not used as a permanent source of pressurization, but only as a temporary source. Pressurized cables are normally fed with compressed air whose moisture content has been removed by an air dryer. The distribution apparatus for cable pressure apparatus is usually located in the central office or in repeater huts and manholes for say, the L5 coaxial cable system. The distribution apparatus contains an array of needle valves and flowmeters to monitor the flow rate to each individual cable. The effectiveness of pressurization is directly measured through pressure sensors located at certain strategic points in the outside cable plant area. The pressure sensors transmit data (usually just pressure okay/not-okay) to a pressurization alarm system located in the CO. Some cable pressurization sensors are clever in that they "borrow" a regular subscriber pair and signal the pressure sensor output by placing a balanced ground of say, 200,000 ohms across the subscriber loop (which will not interfere with normal telephone service). While virtually all toll cables have pressurization, by no means is pressurization found on all exchange area cable. Pressurization is only used on major exchange area cables, and those combined toll/exchange area cables which carry N carrier (still around!), T carrier and wideband data circuits. It is important to understand that polyethylene insulated cable (PIC) is still susceptible to the effects of moisture, although the primary effects are only noticed at frequencies above 100 kHz. The presence of moisture attentuates higher frequency signals and increases conductor capacitance and conductance to ground. When pressurized cables pass through cross connection points that are exposed to atmosphere, a "pressure dam" is made using an epoxy sealing compound and a pneumatic tube continues the pressurization to the next section of cable. Pulp-insulated (i.e., paper) cable is still used, and opening such a cable on hot, humid day will cause the cable to suck up moisture from the air like a sponge, while transmission quality at high frequencies sinks like a lead balloon. <> Larry Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp. <> UUCP {allegra|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry <> TEL 716/688-1231 | 716/773-1700 {hplabs|utzoo|uunet}!/ \uniquex!larry <> FAX 716/741-9635 | 716/773-2488 "Have you hugged your cat today?"