At 09:57 AM 6/7/2007, Dick Green wrote:
>Excellent advice on conduits, Bill. I agree with almost everything. Here are
>some comments:
>
>1. Water in conduit
>
>I have read many, many posts here about drilling holes in conduit to let the
>water escape. My conduits do not have holes drilled in them, do not have
>gravel drainage under the terminations, and I have never, ever had a drop of
>moisture in them during the 10 years since they've been installed.
>
>I believe the drainage issue depends on your location, the conduit material,
>how the conduit sections are connected, the type of termination, and/or how
>deeply the conduit is buried.
<snip>
>Finally, the termination makes a difference. If your conduit just rises out
>of the ground into a 180-degree curve with cables exiting downward, and your
>location frequently has high humidity, then I think you're going to get a
>lot of condensation in the pipe over time. It's certainly possible that
>moisture can enter the pipe from inside the house. In my case, the basement
>is quite dry -- especially when we run heat and air conditioning. I'd be
>real concerned about open conduit entry/exit in a place like Barbados!
>However, you can seal up those points with silicone sealant, at the cost of
>making it a lot more difficult to add/subtract cables.
And here you hit it on the head. In most places (assuming you
installed the conduit correctly) it's not leakage that gets the water
in there, it's condensation. As the atmospheric pressure changes,
air moves in and out of the conduit. If the dewpoint/wet bulb
temperature of the air being sucked in is less than the soil
temperature, the water condenses out. When the outside pressure
drops, the (saturated) air moves out, leaving the water
behind. Then, when the pressure rises again, new damp air goes in,
gets cooled, condenses, etc.
Obviously, whether you get condensation or not depends a LOT on the
temperature and humidity of the air vs the temperature of the ground,
and whether there is any mechanism to push air in and out.
Oddly, if air freely circulates the length, persistent condensation
is less likely (except if you live in a place with 90% RH all the
time, you poor devil).. If the RH is even as high as 60%, it will
probably pick up moisture that's inside the pipe, rather than leaving
it behind.
In most places, the RH might get real high in the night, but it's
also cooler then, so you don't have the combination of warm moist
outside air getting into cooler conduit and condensing.
One could also get the moisture out by heating the wires in the
conduit (even a slight temperature rise will probably get most of the
moisture out). Here you go... why you absolutely, positively must
have that kilowatt amp... to keep moisture from accumulating in the
conduit by the losses in the coax.
>My advice is to check out how professional contractors bury electrical
>cables in your area, and try to mimic exactly what they do.
One might want to only look at what contractors who have been in
business for a while do<grin>..
>2. Messenger line
>
>I like this idea a lot. I have a messenger line, but I pulled it with the
>cables, so it's probably twisted up with them and probably useless. I'll
>probably have to pull all the cables and re-pull in order to add any cables.
>Luckily, I installed a bunch of spare cables just in case. My messenger line
>isn't twice as long as the conduit. I figured I could splice another line
>onto either end.
>
>Only thing about the messenger line is that I don't see how it can help you
>more than once. If you use the messenger line to pull more cables and more
>messenger line, the latter will get twisted up with the new cables.
Use the messenger to pull two messenger lines, being careful to not
let them twist too much as they go into the pipe. Use one of the
lines to pull the new cable, leaving the other one in place. the time
required to pull two messengers is pretty short (because they're
small and flexible.. not like laying out hundreds of feet of wire and
cable) Sometimes it works, sometimes it doesn't. If it doesn't work,
you're no worse off. If it does work, you've saved yourself a bunch of time.
BTW, you really want to use "messenger tape" instead of twisted rope
or mason's twine or something like that. I used to use 1/4" or 3/8"
poly rope as a pull line, but after I found out about tape, I became
a believer. It doesn't twist by itself, and it seems to not twist
the wires as much when you pull it. The twisted poly rope always
seems to twist the wires you're pulling (probably because putting
tension on it causes it to slightly untwist).
Jim, W6RMK
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