TELECOM Digest OnLine - Sorted: Cox CATV Outage (was Did it Snow Over the Weekend?)

Cox CATV Outage (was Did it Snow Over the Weekend?)

Neal McLain (nmclain@annsgarden.com)
Wed, 26 Jan 2005 21:30:33 -0600

Tony P. <kd1s@nospamplease.cox.reallynospam.net> [TD V24 #32] wrote:

> It's hard to tell just how much snow we've gotten in Providence
> because of the damned drifting. ... Walking back I noticed three
> Cox trucks humming down the road. When I walked in the door the
> cable was on again - they must have lost a head end or at least
> power to it.

I doubt that it was the headend. If they'd lost the headend, the
entire system would be down, and the techs wouldn't be humming down
the road; they'd be back at the headend desperately trying to fix
things.

The most likely cause for a cold-weather outage is what cable guys
affectionately call a "suckout" -- a situation where a CATV
distribution cable is literally pulled out of a tap or an amplifier.

CATV distribution cable is mostly aluminum (copper-clad aluminum
center conductor; solid aluminum sheath; plastic dielectric, typically
foamed polyethylene). The Coefficient of Linear Thermal Expansion for
aluminum is about 12.7E-6 foot/foot/degree F. [1]

CATV distribution cable is mechanically supported by steel "strand,"
typically 1/4- or 3/8-inch stranded steel cable. The strand is placed
under tension (typically a few hundred pounds) to prevent sag. The
Coefficient of Linear Thermal Expansion for high-strength steel is
about 6.7E-6 foot/foot/degree F. [1,2]

In other words, when the temperature falls, aluminum shrinks about
twice as much as steel.

Now let's consider what happens when a cable company installs a
1000-foot run of strand-supported CATV cable on a nice warm summer
day, and then measures it on a cold winter night a few months later:

AMBIENT ALUMINUM STEEL
AIR CATV CABLE STRAND
SITUATION TEMP (FEET) (FEET)

Summer day 80F 1000.0 1000.0
Winter night 0F 999.0 999.5

When the temperature drops to zero, the CATV cable is about 6" shorter
than the strand. Result: something breaks. Usually, the CATV cable
is pulled out of some component: suckout.

In order to prevent suckouts, every CATV cable has a "loop" at almost
every pole. Theoretically, the loop is supposed to provide enough
slack to absorb the thermal shrinkage. It usually works, although
suckouts sometimes occur anyway if there's ice buildup on the cable
and/or wind whipping it from side to side.

Of course, there's another reason for suckouts: errant vehicles
colliding with utility poles. This could occur in any weather, but
for obvious reasons it happens more frequently in snowy weather.

[1] Comparisons of Materials: Coefficient of Thermal
Expansion. "Materials Selector", Reinhold Publishing Co., Penton/IPC.
http://bobmay.astronomy.net/misc/tempcoef.htm

[2] An illustration of strand-supported CATV cable is at
http://www.annsgarden.com/poles/poles.htm#strand

Neal McLain

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