Subject: Cable Pressurization
Date: 13 Aug 89 10:29:08 EDT (Sun)
From: Larry Lippman <kitty!larry@uunet.uu.net>

> 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?"