Hi Bob,
I have no serious disagreement with your post. But I think it
points up the necessity for me to clarify some of my statements.
And I do have a comment or two. See below.
73, Eric N7CL
>From: Bob Wanderer <aa0cy@nwrain.com>
To: <towertalk@contesting.com>
>Date: Thu, 9 Dec 1999 14:39:47 -0800
>
>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.
Actually, the "megger" or earth resistance tester measurement is
to determine the quality of the earth electrode connection to
_Earth_ at the electrode / Earth interface for both power system,
telco, and lightning fault purposes. The RF or time domain
considerations associated with a lightning fault are mostly a
concern for the other components (and the design) of the
grounding system.
The only important consideration about what frequency is used to
make this measurement is that it not be an integer harmonic of
the local power line frequency and that the measurement frequency
is low enough to avoid reactance effects in the long leads
required to make the measurement properly. If not for the
electrolytic nature of the earth material, it could be done at
DC. My tester runs at somewere near 800 Hz. The exact frequency
was selected to avoid interference from both 50 and 60 Hz
powerline frequencies.
The earth connection resistance is what it is. Talking about its
bandwidth is somewhat like talking about the bandwidth of the
bulk resistive material in a dummy load. Its bandwidth is more
than adequate to take care of sinking the required charge in the
required amount of time. Interconnection inductances in the
remainder of the system and the topology of the interconnect
scheme determines how successfully the system deals with the
short time scale (or high frequency) components of a lightning
fault event.
>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.
Yup. But I like to get ground systems for structures that I
_KNOW_ are going to be struck (hilltop towers, etc.) down into
the one to two ohm region. Five ohms maximum is pretty much the
accepted standard.
>
>In addition to GEM (rather pricey),
I don't find that GEM is all that pricey. I think the last batch
I priced was well under a buck a pound in 50 pound bags. This
was for a very small quantity of product.
>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.
My experience with coke breeze as a substitute for GEM leads me
to believe that F-D was more concerned with buck than bang when
they made that determination.
>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.
Nor would I.
I was specifically suggesting its use only as a means to further
reduce the earth connection resistance of an existing service
entry point ground which already had a ground rod at the entry
point. Even if the UFER is used, it would presumably _not_ be
the only component of the lightning protection ground system.
Even if a UFER ground was properly planned for and installed in
the concrete pour for the structure at the time of construction,
it could not (at least _should_ not) be used without an external
ground rod.
But for the rest of the grounding scheme to have much of a chance
of working properly to protect the station, it is important for
the service entry ground connection to earth to be of high
quality. Particularly if it is to be used (correctly) as the
single point ground for all of the conductive entry into the
structure.
>
>Have fun.
AMEN!
>
>Bob AA0CY
>
>----------
>From: Eric Gustafson[SMTP:n7cl@mmsi.com]
>Sent: Thursday, December 09, 1999 1:29 PM
>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
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