On 1/9/16 10:52 AM, StellarCAT wrote:
good point... maybe make the spacing from the tower closer to 6 or even
8'. I don't know if you can overkill it as long as you take into account
the possibility of current saturation in the soil
what do you mean by current saturation? DO you mean that the water in
the soil boils (called smoking or steaming rods in some of the literature)?
But, in general, soil is fairly linear.
As two rods are placed closer together, the fields from the two rods
tend to combine, so the total resistance rises, ultimately becoming the
same as one rod, when the two rods are right next to each other. There
are standard equations for estimating the ground resistance from one or
more rods in a variety of geometries, but a good rule of thumb is to
space the rods by two rod lengths.
If you were driving your rods next to a big block of concrete, I'd make
the first rod at least two rod lengths away: the concrete is a bigger,
lower resistance electrode than a rod. Driving a rod closer might even
increase the field in the soil in a localized place (I haven't run the
math..)
The goal is to "spread the current out"
For lightning impulses, with 1 microsecond rise times, the inductance of
the wire dominates over the resistance, as far as voltage rise goes. So
running a wire over the top of the soil to a distant rod doesn't
actually change the voltage distribution all that much. What it does do
it make the current density different, which reduces soil heating.
- for the cost of 3
ground rods and some #3 or #4 wire I'd rather chance the overkill then not.
I just did a Google search and it appears the thicker coatings on the
copper (10 mils is standard) outlasts the 3.9mil coating on galvanized
~40 years to 10-15. So seems copper is the way to go (also they're
higher tensile strength - ~58K vs > 90K for the copper).
Gary
-----Original Message----- From: Kelly Taylor
Sent: Saturday, January 9, 2016 1:47 PM
To: StellarCAT
Cc: tower
Subject: Re: [TowerTalk] copper or galvanized ground rods in red SC clay
Here's an interesting question: if the concrete base is an effective
ground connection, do you get more value by placing the rods a rod
length away from the base?
If the idea of separation is to prevent saturation during a strike,
isn't the base and rod combo at risk of saturation if the rods are too
close to the base?
73, Kelly
ve4xt
Sent from my iPhone
On Jan 9, 2016, at 12:41 PM, StellarCAT <rxdesign@ssvecnet.com> wrote:
Just curious if anyone has experience using either of these over a
number of years – whether they’re ‘eaten away’ one any faster than the
other.
Also the conductivity issue – not sure if it matters all that much for
lightning protection. Don’t know if you can ‘weld’ copper wire to the
galvanized ones using the welding devices (can’t remember what they’re
called at the moment).
Finally: with 3 on a tower – one on each leg – the rule is to separate
them by their length correct? So if I have 3 each on about 5’ of wire
from the tower that would mean each would be over 8’ (8.66’) from
each other .... this is correct – spaced at least equal to their
length (depth)?
Gary
K9RX
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