----------
From: Scott Bullock[SMTP:twoway@scoden.ma.ultranet.com]
Sent: Tuesday, September 23, 1997 2:21 AM
To: Scott Neader
Cc: towertalk@contesting.com
Subject: [TowerTalk] Re: Are "Ufer" grounding BEFORE pouring base cement?
>As some of you may know, I am finally putting up a tower (72 ft Trylon
>self-supporter, C4XL on top) -- more on that in another message, later.
>
>KA1CLX's message about building the rebar cage interests me, of course,
>since I'll be doing the same thing very shortly. He mentioned tying the
>rebar together with a twirley tool, but not welding them. This is
>interesting because I have in front of me two nice articles about "Ufer"
>grounds (no, I won't fax them to you for free)... one is from December 1992
>CQ and the other is from PolyPhaser.
>
>Basically, in a Ufer ground system, you use either copper wires inside the
>concrete tower base, or the rebar cage if you have one, as part of your
>ground system. The tower is tied to the cage, and the cage is tied to a
>soil ground radial/rod system.
I always thought this was a big no no, because the rebar would then have a
potential difference on it, and that would cause the water molecules to
heat up during a lightning strike thereby causing the concrete to explode.
Actually NOT using the Ufer could cause this problem to occur. Usually
if anything adverse occurs, the concrete cracks rather than explodes. See
another posting regarding some caveats about Ufers and rebars.
>
>Both articles seem to indicate definite advantages to using the "ufer"
>concrete base grounding system to augment the radial (with ground rods if
>possible) grounding system, if/when the tower is hit by lightning.
>
>The CQ article by VE3OLN explains the actual Ufer installation better, and
>says...
>
> "Cadwelding is the most effective way of achieving both a good
>mechanical and electrical bond between lengths of rod. In lieu of
>cadwelding, arcwelding seems to be an acceptable alternative, as long as
>all rods are thoroughly welded on all sides, not just tacked. Thorough
>welding will enhance electrical continuity and current-carrying capacity.
>
Yikes! You would have to be a millionaire to cadweld all the rods in this
base! Rohn specifically says not to weld the rebar at all in their specs.
Last time I saw before I bought the SSV, Trylon didn't even have
specs-that's what made me nervous about buying their tower.
Once you buy the mold (1 time expense), the shot is the only replenishable
item. Yes, it's probably expensive, and it would be interesting for someone
to do a cost vs benefits study.
> "Avoid brazing or simply tying the sections of rod together. Brazing
>tends to break down in high current (and high heat) conditions and simply
>tying the sections of rod together neither mechanically nor electrically
>bonds the sections of rod. With a breakdown of or lack of good electrical
>continuity comes arcing, and it is this arcing that can cause extremely
>high heat conditions within the concrete block. This extreme heat is what
>can cause extensive damage and deterioration within the concrete block or
>foundation"
>
>OK... now the author is saying not only that it's a good idea, but if you
>don't, it could cause damage to the concrete block!
>
>Thoughts?????????
>
I'll stick to the tried and true methods of using external rods with heavy
strap to each tower leg. Once you pour the base, it's there for life! If it
ever turned out later for this way of grounding that it was wrong, i'd feel
nervous when every electrical storm approached.
Thanks,
Be nervous, Scott, as one or a few rods to the tower legs does not a
lightning ground make. You need a radial system to disperse the current
so it can be absorbed by the earth. One ground rod per leg would saturate
(electrons speaking) the earth.
73, Bob AA0CY
Scott
>Scott KA9FOX
>QCAO* Future Inductee - 2004
>
>(* Quarter Century Appliance Operator)
>
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search
|