On Tue, 25 Aug 1998, Guy Olinger, K2AV wrote:
> >If anyone knows of an authentic case of grounds through bases causing an
> >explosion, please post to the list, giving traceable info.
>
> Doesn't Polyphaser recommend tying the tower, the rebar, and the ground
> busses all together *before* the concrete is poured, emphasizing shortest,
> straightest path from the tower to the ground field, to keep inductance in
> the ground path to a minimum? I've got to find the book... It's up here
> someplace in the mess...
The problem is not the UFER ground, per se, but the distance from the
ground conductor(s) to the edge of the concrete. I do not have any
DIRECT evidence I can point to about towers falling down, but I have read
enough telling me NOT to do this that I would be very careful about the
process. At my place of employment, we install many towers. There are
two things we take as "gospel":
1) A UFER ground in and of itself is not adequate. We always place
ground rings around the tower, cadwelded together, made of 1/0 copper and
connected to a number of ground rods. We connect this to the tower and
to the ring around the associated radio vault/buildings;
2) We DO connect to the steel in the foundation, but the conductor comes
up through the concrete via a non-conducting PVC conduit the last six
inches or so.
The phenomenon I have seen quoted, time and time again, is that the
current conducted in a ground lead will be so intense the wire will heat
a great deal. This heat will, in turn, cause the everpresent moisture in
a piece of concrete to turn into steam, cracking and/or otherwise
damaging that concrete as it expands. The "rule of thumb" is to not have
any grounding conductor within three inches of any face of a piece of
concrete, touching that concrete. Insulate it from the concrete with
the PVC conduit and things are well and good.
Of course, this whole argument falls apart when you consider that the
tower itself is a metal conducting path directly into the concrete and
isn't insulated! But, for the price of a short piece of conduit, I
wouldn't temp fate!
73, Jim N6IG
--
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.htm
|