I LOVE the organization of that “problem” and your solution with a clear and
detailed step-by-step explanation. Reminds me of the very best way to learn
and understand a complex concept – by thinking about the problem, setting it up
properly and then solving it stepwise while “showing your work”. :-)
Thank you for addressing the key concepts conceptually and mathematically Jim.
It makes it all so much easier to understand!
73
Bob KQ2M
From: jimlux
Sent: Wednesday, September 06, 2017 2:42 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] getting the ufer ground effect with a burried painted
section
On 9/6/17 10:54 AM, Clay Autery wrote:
> I am certainly no expert, have no detail information of your environment
> or tower/foundation design/materials, and I suspect you will get many
> more competent opinions than mine here, but here is my 2 cents for free:
>
> * Considering the numbers involved in your average lightning strike, the
> dissipation potential from even an "ideal Ufer" ground setup in the
> average tower foundation will likely be of negligible benefit overall.
In fact, the Ufer ground was specifically designed to dissipate the
energy from a direct strike in a safe and non-destructive manner.
>
> * An "adequate" lightning ground SYSTEM must dissipate an enormous
> quantity of electrons over the largest area possible in the shortest
> time period possible. What is "adequate" is subject to discussion and
> mathematics.
Actually, the charge transfer isn't huge - a few coulombs at most. It's
the rate of transfer that's the dicey part - 10-100 kiloamps for 50
microseconds.
(note that 100kA peak, which is a big strike, for 50 microseconds, is
only 5 coulombs, 5 Amps for a second - and the *average* current in a
strike is much less than the peak)
The important part of dissipating the strike energy is:
1) spreading the heating out so nothing melts
2) keeping the voltage difference over a short distance reasonable so
people aren't electrocuted from the "step potential" and equipment
that's not too far apart doesn't see radically different voltages.
So, let's look at some numbers - say you're spreading 100kA out over
the surface of a block of concrete that's 3 feet on a side - the total
surface area is 45 square feet, so the current density is about 15
amps/square inch. That's not very big... a AWG 14 house wire about
0.003 square inches, and nobody worries much about sending 15 amps
through that.
SO let's look at the voltage rise at that kind of current density
through soil or concrete.
Concrete has a resistivity of 3000 ohm/cm - that means if you have a
cube of concrete that's 1cm on a side, it's 3000 ohms.
We just figured that we have a current density around 15 A/square inch
and there's about 6 square cm/square inch, so we're seeing 2-3 A/square cm.
Across that 3000 ohm resistance, you're looking at around 10 kV/cm
Yep, better not be standing with your feet too far apart when lightning
strikes.
And how hot will it get?
Let's assume the 3 Amps lasts for 50 microseconds. The power is
3*3*3000, or 27kW for 50 microseconds - 1.35 joules - that's not so huge.
A cubic centimeter of concrete weighs about 3 grams, and the specific
heat is very close to 1 Joule/gram/degree, maybe a bit less. So dumping
a couple joules into 3 grams of concrete raises the temperature 5-6
degrees C.
>
> * Regardless with what you choose to coat the embedded tower section, or
> indeed if you decided to blast it to bright metal, in my opinion, you
> should not rely on the tower section or the tower foundation to
> measurably increase the effectiveness of your lightning protection.
>
> ++++ In short: don't worry about it. Devote your attention toward the
> REST of your lightning protection system. <smile>
>
> 73,
>
> ______________________
> Clay Autery, KY5G
>
> On 9/6/2017 11:55 AM, Jeff AC0C wrote:
>> I’m pouring concrete for a tower base using a bottom section standing on
>> gravel in the hole. I had assumed that would prove some UFER ground benefit
>> but wanted to ask because before placing the section into the hole it was
>> painted well with the Rustoleum cold galvanizing paint.
>>
>> I hope that I have not voided my ufer ground effect with the application of
>> the paint. Because here in the Midwest, lightning is definitely going to be
>> a problem faced sooner than tower corrosion.
>>
>> 73/jeff/ac0c
>> www.ac0c.com
>> alpha-charlie-zero-charlie
>
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>
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>
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