[TowerTalk] Tower grounding connections and foundation

Jim Lux jimlux at earthlink.net
Sat Aug 8 17:50:18 EDT 2015


On 8/8/15 10:32 AM, JVarney wrote:
> Grant Saviers wrote: >>I would like to be enlightened why #00 wire is
> needed to attach to two ground rods which are likely several ohms
> resistance to the "remote earth"? Especially when #6 was ok in the last rev
> of the standard and when from an I^2*R and fusing current it would do the
> job.<<
>
> I'm only quoting the TIA-222-G code; I'm not aware of the facts or reasons
> that motivated the committee to upgrade the grounding standards from
> 222-F.  However, I will say that because the committee is composed of
> experts and engineers from leading tower designers, tower erectors,
> broadcasters and cell owners, I generally accept their judgment.  Just as
> the committee toughened standards for ice loading in response to tower
> failures, I presume they toughened the grounding standards in response to
> lightning-induced problems.
>
> Beyond that, my personal speculation is that going with a big #00
> essentially makes the radial wire act as a horizontal ground rod, thus
> increasing the total volume of earth available to lightning to dissipate to
> and of course lowering resistance of the system as well.
>
> Also, I misquoted TIA-222-G: #00 is not specifically required, it says
> connections between tower and rods shall be "not smaller in surface area
> than 2/0 solid." I interpret that to mean that the spec could be met with
> multiple strands of #4 or #6 in parallel.
>

I'm curious about why "surface area".  That's important for skin 
resistance, and would be real important if you were concerned about your 
AM transmitter antenna efficiency.


>
>
>>> Further, it is a bit of a mystery why Ufer grounding is not included
> ...This forum has opined extensively about why Ufer's beat ground rods. <<
>
> Again, I'm not aware of the committee's reasoning.  Just my personal
> opinion: I don't see how the math checks out for Ufer grounds for towers. A
> multiple radial and ground rod system might offer on the order of 10,000
> cubic feet of earth for lightning to dissipate to. With an Ufer on a tower
> with a 6x6x6 footing, that's only 216 cubic feet of volume available to
> dissipate one trillion joules of energy. If I'm building a tower I'd rather
> direct the lightning energy away from the tower versus concentrating it in
> the footing, IMHO.
>

No where near a Terajoule of energy.  Most of the energy in a lightning 
stroke is dissipated in the air (somewhere between 10 and 100kJ/meter). 
Typical peak stroke current is on the order of tens of kiloamps, and 
with a ground resistance of, say, 10 ohms, that's a high peak power: 
30kA@ 10 ohms is 9 GW. However, it only lasts for a few tens of 
microseconds, so the energy is pretty small: around a megajoule.  That's 
big, but not huge (a car battery is 3-4 MJ).. about the same as 1/4 lb 
of high explosive.


And most of the energy dissipation of a Ufer ground occurs in the soil 
surrounding the concrete, just as the dissipation in a driven rod system 
is in the soil. The important thing in all this dissipation stuff is the 
current density: you want that as low as possible for a variety of reasons.

So a 6x6x6 footing has 180 square feet (about 26,000 square inches) of 
surface area, through which 100 kA might flow (in a really big stroke): 
That's a current density of 3-4 Amps/square inch, which isn't very high, 
so you're not going to be getting boiling water, melting sand, burning 
peat, etc.

Or, another way, shifting to metric units because it's easier.. if you 
assumed that the concrete dissipates nothing, and all the energy is 
dissipated in a thin layer 1cm thick around the footing: You're 
dissipating a megajoule in about 170,000 cc of soil.  If we assume that 
soil is the density and specific heat of water, that's about 6 
Joule/gram, or 1.4 calorie/gram, which would heat about 1.4 degrees C.

In reality, the energy is dissipated over a MUCH larger volume, both in 
the concrete and around it, so the temperature rise from a single stroke 
is pretty small.




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