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

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

Tony P. (kd1s@nospamplease.cox.reallynospam.net)
Thu, 27 Jan 2005 21:48:30 -0500

In article <telecom24.38.9@telecom-digest.org>, nmclain@annsgarden.com
says:

> 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

Well -- whatever suffered the suckout blacked out the entire city of
Providence.

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