Response inline....
______________________
Clay Autery, KY5G
MONTAC Enterprises
(318) 518-1389
On 9/6/2017 1:42 PM, jimlux wrote:
> 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.
My statement was directed at the ASSUMED small footprint of the
foundation as opposed to most Ufer grounds in much larger residential or
commercial foundation footprints. Think a) metal to concrete contact
area, and b) dissipation area of foundation external dimensions to earth.
>
>> * 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.
It would appear that you are assuming that the tower will transmit the
entire strike and that the section to concrete interface will take the
whole charge. It will not. The charge WILL be split in some fashion
between the tower and the conductors exiting the tower.
You are assuming a SINGLE, one-way stroke event.
And a bunch of other stuff I have neither the time or inclination to
address/discuss/argue...
Facts:
1) In this case, the tower section was painted with a substance which
will at best reduce the conductivity between the tower section and the
concrete (not taking into consideration what was on the tower section
before the rattle can treatment).
2) In this case, the concrete has already been poured. There is jack
that can be done to alter the results now.... so why worry about it?
3) "I" would absolutely under NO CIRCUMSTANCES rely on the tower section
to foundation interface to contribute measurably to my lightning
protection of persons or equipment IN THIS CASE. And no rational
engineer would either considering the inability to actually quantify it
in any meaningful way. THUS, as I said, the OP should simply NOT WORRY
about
I was NOT arguing against the use of Ufer grounds... I was simply
stating that IN THIS CASE, the OP should NOT worry about it OR consider
it in his protection planning due to the unreliable and UN-quantifiable
contribution it MIGHT make to the overall system.
Have a nice day! <smile>
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