Date: Wed, 14 Oct 1992 00:14:42 -0400
From: Don.Kimberlin@f730.n250.z1.FIDONET.ORG (Don Kimberlin)
Newsgroups: comp.dcom.telecom
Subject: Fibre Cables Not That Immortal
Organization: TELECOM Digest
X-Submissions-To: telecom@eecs.nwu.edu
X-Administrivia-To: telecom-request@eecs.nwu.edu
X-Telecom-Digest: Volume 12, Issue 780, Message 1 of 6

[originally posted in Fidonet BROADCAST conference, with some spelling
corrections]

Here's a just a bit of "real world" about fiber optic cable and those
overblown statements about the apparent invincibility of fiber optic
cable ... those things that trumpet phrases like, "Impervious to harm
and interruption ... non-metallic and interference-proof!"... and so
on.

In fact, practical fiber optic cable of the type planted in the ground
for intercity use has at least two important natural enemies, both of
which have caused, and continue to cause failures of those cables,
gophers and lightning.  Both have been significant causes of failure
of preceding cable technologies in the twisted copper pairs and
coaxial cables of the earlier analog era.

A recent edition of "Lightguide Digest" published by AT&T technologies
tells quite openly how AT&T's RL series of fiber optic cable is
constructed to attempt to avoid both kinds of natural enemies.

First addressing the gopher threat, the article describes it as a
"gnawing problem" that "is growing."  It says, "the fact is well
established that rodents such as gophers will gnaw through unprotected
buried cables.  Information collected in the 1950s indicated that
gopehrs infested more than 60 percent of the continental U.S." An
accompanying figure shows a distribution of gopher areas to include
almost all the area west of the Mississippi River plys most of
Florida, Georgia and about a third of Alabama.  The article goes on,
"To grind down incisors which grow approximately 12 inches per year,
gophers spend 10 percent of their time, which means more than 90,000
bites per week, gnawing various items including fiber optic cable."
The bite pressure exerted by a gopher on fiber optic cable is said to
reach as high as 18,000 pounds per square inch.

The habits of the gopher are described as a creature that keeps a home
territory of several hundred square feet, saying that cables buried
through gopher territories are the victims of repeated gnawing
attacks. Reports have been made of gophers completely severing
all-dielectric fiber cables in Ohio, an area not previously thought to
be in the "gopher zone," and Colorado reports indicate gophers there
are a widespread threat to unprotected fiber optic lines.

Two methods are the only ones known to be effective against gopher
attack:

One is to place the nominal 1/2-inch diameter fiber cable in a 2-1/2
to 4 inch diameter rigid PVC conduit, an expense communications
companies would rather avoid.  The other is to use a cable with a
stainless steel sheath around its fiber optic contents ... which makes
the cable conductive, increasing the risk of lightning striking the
cable and literally blowing the cable in pieces. Indeed, the article
reports that one Mountain Bell fiber cut was caused by gnawing through
a two-inch outer conduit, anyway.  Larger cement or ceramic conduit
might be used, but that would increase costs of placing cable evern
more.

To attempt gopher protection, AT&T has two calbe designs in its RL
series. The first, for direct burial in rural areas, is called Primary
Rodent- Lightning (Primary RL) Sheath. It consists of a corrugated
stainless steel armor layer bonded to an outer polyethelyne jacket
that contains a surrounding layer of wire strength members. The visual
representation of this cable winds up looking very much like
transoceanic deep-sea cables that have evolved over more than 130
years of submarine telegraphy around the world, the only difference
being that the center of the cable is hollow, providing for a variety
of forms of fibers inside. The other type, called the Lightguide
Express Entry Rodent-Lightning LXE-RL Sheath, has but two strength-
member wires in its jacket and a layer of "Water-Blocking/Lightning
Tape" located under its stainless steel jacket.

The article goes on to describe how such cable types are actually
tested and rated for gopher resistance.  A sample of cable is buried
in a closed-off area (underground barricades to contain the gophers
within a confined area) with ten gophers that have nothing else to
gnaw on. Ratings of one to five indicate whether any given gopher
managed to inflict damage ranging from cutting the outer jacket to
completely biting the cable in two.  The final rating of a given cable
is the averaage of the scores of the ten different gophers placed in
the area to attack the cable.

The gophers used for the test are provided by the Denver Wildlife
Research Center of the U.S. Department of Agriculture, and the gopher
most often supplied is the common plains "pocket gopher" called Geomys
Bursarius, a small animal only 7 ot 9 inches in length that rarely
weighs more than half a pound.

The other hazard that arises to intercity fiber optic cables, since
metallic sheaths must be used to prevent gopher damage, and wires
along the cable must be added to provide "strength" so the cable can
be pulled and handled, is lightning.

One of the lesser-understood factors about buried metallic conductors
is that since they are often more conductive than the surrounding soil
they are buried in, they are better, more attractive conductors to
lightning than the dirt that surrounds them.  This means that
lightning can, and is, actually attracted to even buried cable, and
that momentary lightning currents in the metallic members of those
cable can rise high enough to actually melt a hole in the cable.

Indeed, buried fiber optic cable can be struck in open rural areas,
resulting in failure of the cable until the damaged section is
replaces.  One notable such case happened to MCI in a buried cable
west of Jacksonville, Florida, where a lightning bolt that seemed to
appear out of nowhere stabbed down into a buried fiber cable in an
open field. Even more dramatic is the piece of damaged RL cable on
display in the Charlotte, NC office of AT&T, which can be seen to have
split open from the forces exerted on it when struck by lightning.
That piece of damaged cable was struck with such force when it was
buried four feet under an open soybean field near Davidson, NC. The
cableman who repaired it stated that on the surface, a hole about the
size of a golf ball had been made in the dirt, with spherical balls of
dirt thrown back up around the hole where the lightning struck the
ground on its way down to reach the metallic elements of the fiber
cable.

So, now you have some facts to bear in mind the next time you hear
some salesman touting the apparently impervious, uniterruptible nature
of fiber optic cable plant. It has the same sort of failure potentials
that previous cables have had for decades!


Don Kimberlin - via FidoNet node 1:250/98
INTERNET: Don.Kimberlin@f730.n250.z1.FIDONET.ORG