I agree with most of Eric's comments. But
The Megger(TM) and equivalent devices test at around
100~400 Hz so the values they get are valid for power company
concerns, but not necessarily for lightning ground concerns.
Nevertheless, if the values gotten are 10-Ohms or less (and
preferably 5-Ohms or less), for all intents and purposes, you
have a good ground system. Realize that the Megger(TM)
system can be used both for analyzing a ground system (or
a rod within that system) as well as for how good (conductive)
the earth in that region is prior to creating the ground system.
In addition to GEM (rahter pricey), you can try coke breeze. This
is a by-product of burning coke. I know of one source in Utah
(Christensen Bros [forgot the town]), but am sure there are others.
Fluor-Daniel analyuzed several conductivity enhancement schemes
and judged coke breeze as provifding the best bang for the buck. I
am not sure I would recommend a Ufer ground over a good ground
system; for sure, a Ufer ground should not be your sole ground.
Have fun.
Bob AA0CY
----------
From: Eric Gustafson[SMTP:n7cl@mmsi.com]
Sent: Thursday, December 09, 1999 1:29 PM
To: towertalk@contesting.com; eDoc@netzero.net
Subject: Re: [TowerTalk] Three Lightning Protection Clarifications Please?
Hi Dave,
See my comments in the message text below.
>From: "DavidC" <eDoc@netzero.net>
To: <towertalk@contesting.com>
>Date: Wed, 8 Dec 1999 21:36:19 -0500
>
>I think I know the answers to these three questions from my
>perusal of the TT archives I saved ... but better safe than
>sorry!
>
>1. Ground rods:
>
> I am supplementing the existing service panel ground rod by
> adding one more rod off to the north and three to the south
> at 16 foot separations. A local phone company guy who is
> supposed to know these things told me that since the service
> panel rod had been "megged out" that I could use galvanized
> just as well as copper for the extra rods. True?
I have four comments here.
First, if the ground has been measured (I presume this is what he
meant by "megged out".) and found to be of adequately low
impedance, why does it need supplementing?
Second, I would not place a great deal of confidence in the
pronouncements of any phone company employee about lightning
safety grounding issues after seeing what is routinely done by
the phone company in this area.
Third, whether galvanized rod is appropriate in your application
depends entirely on what the other conductive ground system
components are made of. If there is a significant surface area
of buried copper in your ground system, connecting in a
galvanized steel rod will cause a galvanic reaction which will
result in the (not very) eventual destruction of the galvanized
rod.
Fourth, if you do need to lower the earth connection impedance of
your service entry panel ground, and you are certain that there
is actually already a ground rod at that point, it may be easier
to put in an "after the fact" UFER ground than to drive and
connect to more ground rods.
To make the "after the fact" UFER ground, you trench 18 inches
deep 6 inches wide adjacent to the side of the structure (2 to 3
feet out) for twenty feet in both directions away from the
service entry point. Then backfil the trench 6 inches deep with
Ground Enhancement Material (GEM - buy premixed or make your own)
and place the copper ground conductor (use 2/0 or flat strap for
this) in the center of the trench on top of the backfill. Then
backfill with another 6 inches of GEM. Water in the GEM and
backfill the remaining trench with dirt. This will give you two
conductors to tie into your existing service entry ground panel.
But the tie in will be above grade so a clamped connection can be
used. Just make sure both conductors are equal length and that
they run in opposing directions.
>2. Ground wire between rods:
>
> I understand that 0/0 is the best choice, though one needs to
> be Hercules to work with it. I have some of what I believe
> to be 0/2 or 0/3 available that I'd just need the local metal
> recycler to run through his stripper. Is that acceptable?
> (What is the thickness of 0/2 and 0/3? Mine is only
> identified 600v.) Is it OK to use two lengths of wire if the
> ends are joined at a CADweld?
You can parallel all you want.
2/0 or #00 is a pretty standard conductor size for this purpose.
When they installed the PCS site and tower on my property, they
used a tinned solid AWG #2 copper conductor for the ground system
subgrade interconnects. But there were many of them and the
lightning surge would use most of them effectively in parallel.
A lot depends on the design of the system as to what is really
required.
I don't know what your wire size designations mean. If your 0/2
is really AWG #2 wire, then the conductor would be a little less
than 3 tenths of an inch in diameter. The 600V rating refers to
the insulation. Which brings up another question. Is this a
single solid conductor or is it stranded wire? If it is a solid
conductor, and it is larger than #6 or so, then it is OK to bury
it without the insulation. If it is stranded, and the strands
are #14 or larger, it is OK to bury without the insulation. If
it is stranded and the individual strands are a lot smaller than
#14 (typical house wire conductor size), I would not bury without
its insulation or use it as a lightning conductor.
If you do use galvanized rods and copper interconnects, it would
be better not to remove the insulation from the copper. Then
cover the connection and ALL of the remaining exposed copper with
an effective and mechanically robust moisture barrier (roofing
tar, RTV silicone rubber, tool handle dip, etc.) before covering
with dirt. This assumes that the entire ground system will end
up below grade. You do not want both copper and steel or zinc
(galvanizing coating) electrically connected together and buried
in the same electrolitic media.
73, Eric N7CL
--
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
--
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
|