[TowerTalk] Fwd: Earthing a tower

[K8RI]

On 12/30/2012 6:06 PM, Hans Hammarquist wrote:

If there is anything predictable about lightning it's lightning's 
unpredictability.


>
>   Anecdote or not. If you push enough current though any conductor it will "explode". (This is actually use in one type of detonators.) I don't know how much current required to crack concrete and if a direct lightning strike can be enough, but I don't think we have to worry so much about that as such a "super" strike will cause more harm in other places around the tower hit.
>

It depends on the area interface of the conductor and concrete,  IE: 
The more conductor and the more volume of concrete the more current it 
takes to do damage. moisture, concrete makeup, and soil conductivity all 
play a part. The volume of concrete and size of the rebar cage are the 
most predictable.  So a large UFER ground is more predictable than just 
a lot of ground rods and wire.


> I have my present house designed to take a "super" strike. I am using 3/4" copper pipes in all four corners of the house. According to the engineer, helping with this design, my house is ready for one of these "super" strikes, observed on occasions. I believe it is a lightning with a peak current of 300 kA, maybe it was even more. Anyhow, after three confirmed, direct hits in the house, I feel relatively sure I did something right.
>

There is no guarantee against damage a "super strike". You are only 
playing the odds.  even with strikes that fall within the ordinary realm 
you are still playing the odds.

Each properly implemented precaution increases the odds in your favor, 
but they never reduce the chances of damage to zero.

My system consists of 32 or 33 8' ground rods, cad-welded(TM) to over 
600 feet of bare #2 copper in what now days could be considered an 
enhanced "Ring ground"

The "UFER ground" is considered to be one of the most reliable.
However the "Single point ground" where every thing enters the house is 
the most likely to prevent damage in the house.


A large rerod cage inside an even larger concrete block reduces the 
current density by increasing both the surface area and contact area to 
a point where the current is "unlikely" to do any damage.

I believe someone explained this in detail earlier. Broadcast stations 
and the military quite commonly use this method of grounding.  Detailed 
explanations are readily available on the net!

73,

Roger (K8RI)


> Happy New Year everyone,
>
> Hans - N2JFS
>
>
>
>
>
> -----Original Message-----
> From: Jim Lux <jimlux at earthlink.net>
> To: towertalk <towertalk at contesting.com>
> Sent: Sun, Dec 30, 2012 5:22 pm
> Subject: Re: [TowerTalk] Earthing a tower
>
>
> On 12/30/12 10:33 AM, Michael Mraz wrote:
>> My dear old SK friend Al Caplan, who used to manage the Hy-Gain amateur
>> division when it was part of Telex, was emphatic about proper grounding
>> when I bought my HG-52 crankup. He said that lightning could split the
>> concrete around the re-bar if the tower took a direct strike. It was a
>> long time ago, but my recollection was that this nasty event actually
>> occurred to some unfortunate ham who had to abandon the old foundation
>> and install a new one in a different location.
>>
>
> this anecdote and ones similar come up all the time.  The short answer
> is, "not if the system was properly constructed in the first place".
>
> If the rebar penetration of the concrete surface is incorrectly done,
> then it's a problem.  For instance, if the top of the concrete holds
> water at the steel/concrete join, it will eventually wind up inside a
> tiny crack, which then causes corrosion of the iron.  That in
> combination with water in the crack will cause spalling or cracking when
> lightning heats it up.
>
>
> And when discussing "well, what it the installation isn't done right,
> then disaster strikes"... I'd contend that the odds of getting a Ufer
> done right are a whole lot higher than some of the bolt on clamp and
> grounding rod schemes I've seen over the years.
>
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