Doesn't buy you much. Th concrete has a very low resistance, compared to the
surrounding soil(partly because concrete is hydrophilic, partly because of
ionic content) and large contact area.
On Apr 17, 2011, at 14:28, Bert Almemo <balmemo@sympatico.ca> wrote:
> I have a friend ready to put up a new tower and we're planning the grounding
> system.
>
> How about placing a big ground plate at the bottom of the hole for the tower
> foundation before pouring the concrete? Should the tower, ground plate and
> the rebar cage be connected together? Good or bad idea?
>
> 73 Bert, VE3NR
>
>
>
>
>
> -----Original Message-----
> From: towertalk-bounces@contesting.com
> [mailto:towertalk-bounces@contesting.com] On Behalf Of K8RI on TT
> Sent: Sunday, April 17, 2011 5:12 PM
> To: towertalk@contesting.com
> Subject: Re: [TowerTalk] grounding quesitons: Ufer, strap, exothermic welds
>
> On 4/17/2011 2:06 PM, David Gilbert wrote:
>> Again?? Seriously? That myth has been debunked so many times, both
>> here and elsewhere, that I can't believe people are still perpetuating
>> it. I've issued this challenge several times before and nobody has
>> ever responded ... please quote one single URL or technical document
>> that describes a verified instance where lighting passing through a
>> proper Ufer ground , or any conductive element inside a buried
>> concrete structure whatsoever, cracked the concrete. You'll find
>> instances where a direct lightning hit to the OUTSIDE of a block of
>> concrete caused damage (buildings, bridge abutments, etc), but not via
>> a Ufer system of sufficient size and construction.
>
> A while back I watched a training film from either NWS or NOAH that showed a
> lightning strike that had not just cracked, but blown part of a system apart
> BUT and and emphasize the but, stranded wire (either 0 or
> 00) had been used and it was in a wet location IIRC. Just what happened they
> weren't sure, but it was an extremely rare event and I think we need to keep
> in mind there is no 100 % guarantee against lightning, no matter what we do.
> That means we *always* end up playing the odds and approaching diminishing
> returns as we make our systems more robust and costly.
>
> OTOH every proper step taken increases the odds in our favor.
>
> Considering the odds I'd not worry a moment about tying the rebar in the
> concrete to the grounding system.
> I think it's a positive in our favor rather than a negative.
>
> I would make sure the rebar did not come within 3 to 6 inches of any surface
> of the concrete (code requirement I believe) and I would prefer solid copper
> instead of stranded, but even stranded can be sealed.
>
>
>
>> Check out the I.C.E. technical note on the subject if you don't
>> believe me. It's not difficult to find on their website ... it even
>> uses the word "myth" in the subject title.
>
> I think they are wrong to call it a myth, but it's such a rare event AND the
> procedure actually increases the odds in our favor so, it would not be a
> concern for me.
>
>> Besides ... just think about it for a second. There are thousands of
>> tower installations with a tower base buried in the concrete. What
>> would be the difference, other than beneficial spreading of the
>> current, if the tower was also properly connected to the rebar cage?
>
> Mine has taken at least 17 direct hits and there's not even a chip in the
> concrete. It was getting to be a regular thing, but nary a strike (that any
> one has seen) in the last 3 years. This will be the 4th summer. Who knows
> what it will bring.
>
>> Lastly, connecting the tower to the rebar cage is REQUIRED by most
>> zoning regulations.
>>
>> I'm honestly curious why certain urban legends, like this one, seem to
>> have such staying power in ham radio.
>
> Probably because those who have seen it were so impressed they let the
> rarity of the event outweigh the effectiveness of the procedure.
>
> 73
>
> Roger (K8RI)
>
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