[TowerTalk] Fwd: Fwd: tower grounding
HansLG at aol.com
HansLG at aol.com
Mon Sep 1 19:37:43 EDT 2008
I am not an expert in this but find the comment about concrete interesting.
From: jimlux at earthlink.net
To: HansLG at aol.com
CC: towertalk at contesting.com
Sent: 9/1/2008 6:15:46 P.M. Eastern Daylight Time
Subj: Re: [TowerTalk] Fwd: tower grounding
HansLG at aol.com wrote:
> Hi Dragonsong,
> I believe the rebar cage will be totally isolated from ground as it is
> surrounded by concrete. The concrete is a relative good electric insulator
> will therefor not be good for grounding the tower. You have to add
> rods outside the lump of cement holding your tower in place.
This is untrue. Concrete (unless is specially made to have high
resistivity, as for electric railroads) has higher conductivity than the
surrounding soil. Lots of ions, it's hydrophilic, etc.
The relatively high conductivity of concrete is the basis for the
concrete encased grounding electrode (aka Ufer ground) currently
required by many jurisdictions as the primary grounding means. Whether
you like it or not, the rebar cage is electrically connected to the
block of concrete it's imbedded in (unless you've taken specific
precautions against it, which almost nobody does..)
The efficacy and durability of the Ufer ground has, by now, been
established by over 50 years of field experience, numerous controlled
laboratory tests, etc.
Ensuring. I have never heard about anyone having problems with corrosion of
the rebar unless it gets exposed. I am a little consern about the
electrolytic corrosion still though.
> There is plenty of information of the "best way" to ground the tower. Two,
> three grounding rod driven 8 feet into ground is, I believe, considered
> acceptable grounding for lightning strikes.
Acceptable is determined by many factors, not the least of which might
be your local jurisdiction's rules (at least if you want insurance on it).
>From an engineering standpoint, you need to decide what the purpose of
the lightning ground is (i.e. what sort of impedance are you willing to
I still would consider some grounding rods being better. Maybe I am wrong
and will correct my mind if so.
When it comes to RF grounding the field
> is open for suggestions. Little depending on the surrounding ground I,
> myself, believe in radials stretch just above or on the surface of ground.
> folks believe the radials have to be buried or left several feet above
> The length and amount of radial is a science by itself and I think the
> advice is:" The more the better", up to some point that varies between 15
I would say more a craft or art, than a science. For most situations,
the soil properties are not well known enough to actually model the
behavior in detail. So you keep adding wire until the performance
doesn't improve, or you run out of money, or you get tired of trenching.
**************It's only a deal if it's where you want to go. Find your travel
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