[TowerTalk] another subject: coax and cables buried or elevated above ground?

[jimlux]

On 1/28/16 6:17 AM, Al Kozakiewicz wrote:
> QST is not peer reviewed.  All kinds of nonsense can make its way
> into print.
>
> The notion of copious condensation in buried conduit has always
> seemed like ham urban legend to me.
>
> First off, there are literally millions of miles of buried electrical
> conduit in the world filled with wire not rated for immersion. The
> failure rate for this infrastructure does not seem to be anything
> exceptional.

I don't know that I'd say that.  I think that a lot of this is just not 
publicised, except in notorious cases like dogs and drunks getting 
shocks when urinating on a cover that happens to be live because of 
moisture providing the conducting path.

It's also true that most wire these days *is* reasonably ok in wet 
environments.

 From a personal standpoint, I've pulled wire from conduit in my yard 
and it was wet or showed signs of having been wet in the past. And I 
live in Southern California, not noted for high water table, high 
rainfall, or high humidity. (over sprinkling, sure...)

Did the wet adversely affect the AWG12 THHN/THWN?  not that I can tell.



>
> More important, condensation requires a constant supply of moist air.
> Where is this supply coming from? Not convection, unless someone
> changed the laws of physics while I wasn't looking. If there really
> is enough air flow from the pressure difference between the ends of
> the conduit sufficient to bring in the required moisture (unlikely),
> then seal the ends. Simple.

Pressure difference can create a surprising amount of flow but air 
doesn't carry all that much water.

COnsider a 100 foot long tube 2" in diameter.  That's (3.14/144*100) 
about 2 cubic feet.
A pressure difference of 0.5" of mercury out of 30" is a 1/60th change 
in the contents of the pipe (by mass or volume), so 2/60ths of a cubic 
foot move in or out.

a cu ft of air holds 0.59E-3 pounds of water at 50F (10C) and 1.9E-3 
pounds at 86F (30C)

So you could conceivably dump the difference.. about 1.3E-3 pounds of 
water per pressure cycle (if outside is 30C and humid). That's about 
1/50th of an ounce.

Over a couple months, (assuming a daily cycle) you'd be up to a whole 
ounce of water.


I suspect a more likely case for condensation would be where there's a 
static pressure differential between the two ends of the conduit (e.g. 
one is inside the house and the other isn't).  If your house has 
slightly positive pressure, then you're continuously flowing air through 
the pipe, and the pipe is almost certain to be colder than your house.

ANother source of continuous flow would be wind.  It's easy to get a 
differential pressure of a few inches of water from wind blowing.  (2 
feet of water is about 1 psi).  If one end of the conduit is on the high 
pressure side of the house and the other is in the lee of a building, 
you could be set up to have continuous slow flow.

Either of these could accumulate substantial water.






>
> Al AB2ZY
>
> ________________________________________ From: TowerTalk
> <towertalk-bounces at contesting.com> on behalf of StellarCAT
> <rxdesign at ssvecnet.com> Sent: Thursday, January 28, 2016 8:22 AM To:
> tower Subject: [TowerTalk] another subject: coax and cables buried or
> elevated        above ground?
>
> My ongoing tower/antenna projects... hopefully I’m not boring others
> out there...
>
> I had planned on burying all lines to the towers (2, one at 150’ out
> and one at 300’ out) ... at first I thought in solid joined (sealed)
> PVC which I can get for $9/10’ ... and I’ve also thought about direct
> burial which for the coax and control cables has the added benefit of
> increased capacitance to ground and thus a reduction of energy
> (lightening event) that reaches the shack... but control cables
> aren’t normally rated for direct burial... or at least I don’t think
> so (researching that now)...
>
> Then this latest QST article on coax says DON’T bury in any type of
> conduit be it plastic or otherwise. It says condensation WILL get
> inside and ruin (flood) the cables.
>
> So .... if the cables are underground – below the frost line – say
> minimum 1’ down (upstate SC) ... how will condensation develop? Or
> will it? I’m an engineer (HW/SW development) and reasonably
> intelligent but find it difficult to wrap my head around this
> issue... will it or won’t it – flood that is? if it were always under
> ground – including both ends then I’d think not.... but it isn’t
> always under as the ends come up to and above the surface... does
> that introduce a temperature variant enough to induce condensation?
> And I don’t like the idea of holes in the plastic – that just means
> mud will indeed eventually work its way inside.
>
> Maybe use corrugated tubing (again – no cuts or openings) ... this
> would allow whatever moisture there might be, and I can’t imagine its
> going to be a great deal, to settle in the lower points of the
> corrugation.
>
> Comments from others that have been through this - in the south with
> similar conditions would be appreciated.
>
> Alternatively I could use a leader line and go from say 15’ on the
> first tower to the second tower and then that same level to the shack
> with supporting 4x4 poles along the way... but this seems iffy as
> well as first the cables are ‘leaving’ the tower above ground
> potential so that might mean a higher voltage (common mode –
> probably?) on the lines (again: lightening type event)... and the
> first tower is a rotating tower ... although thinking about it that
> might work quite well to go from the tower with a ‘swing-arm’ of
> hanging coax/cables to allow rotation.
>
> thoughts?
>
> Gary K9RX _______________________________________________
>
>
>
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