Howdy, TowerTalkians --
Here's some interesting lightning info from one of our very own TT'ers -
Dave, K1TTT.
Cheers,
Steve K7LXC
>> Unfortunately most of the specific data is still considered proprietary.
We had been planning to publish more of it in the future, though our
spin-of from the parent non-profit to a for-profit subsidiary will
probably slow that down as priorities have changed. Below are some
general comments and observations, most of this can be gleaned from
published sources in the ieee, cigre, or iclp proceedings... ieee and
iclp have also published info on the static brushes.
The biggest thing is the understanding that lightning currents in the
ground do not just uniformly dissipate. The ground is not just one big
resistor when you pump that much current into it. What happens around
ground rods, buried radial wires and other underground structures is
that the air gaps ionize and create underground streamers that carry
much of the current away. The higher the current, the longer the
streamers, and the bigger the underground ionized area. Note that this
ionization is not carried by ions, it is more of a corona and air gap
breakdown phenomena than conduction... as such it is highly non-linear.
A typical single ground rod that may measure, say 25 ohms at low
frequency, may show an effective resistance of only 5 to 10 ohms as the
peak of the lightning flows into it. Visible evidence of these can be
seen where lightning that hits sandy soil creates hollow glass channels
that branch and wind around in the ground, also this is commonly seen
where lightning hits a tree and creates mole like tracks where it flows
under the surface for some distance from the tree in a branching
pattern. When the current is carried deeper into the ground and spread
out by a conductor the streamers are harder to see but have been
detected and measured in various experiments done at our lab and other
places.
1. chemical grounds. While good for low frequency grounding their
effects on lightning grounds are almost nil.
2. a common utility company practice of trenching in a single ground
connecting the bases of their towers along a line is not as good (and
often more expensive) than several rods or shorter radials close to each
structure. This goes along with the observation that a few long radials
are not as good as several shorter ones.
3. adding successively larger rings of rods around a tower may reduce
the effectiveness of the ground. The outer rings have longer (read
higher inductance) connections and reduce the effectiveness of the close
in rods by reducing the ionization around them... at some point more
rods are not better.
4. another common bad practice is relying on rebar as a ground...
besides the possibility of puncturing the foundation (not blowing it up,
just blowing holes in it that allow ground water to reach the rebar),
these grounds may be good for low frequency current but do not do well
for lightning currents.
5. a similar problem is seen in what is called 'butt wrap' poles, where
the outside of a concrete or wood pole is wrapped with wire or sheet
metal as a ground. The fat surface makes a nice low frequency ground
but does not help much with lightning as the big surface area doesn't
allow initiation of underground streamers that dissipate a lot of the
lightning current.
I haven't commented much on the grounding problem in the reflector
because most of the amateur type installations and advice I have seen is
probably 'good enough' for the typical ham. The general advice to
follow the nec should be enough for most. And the other advice about
tying grounds together, keeping leads short and straight to the rods
around a tower, even the recommendations to add some buried radials is
all reasonably good stuff.
Another unfortunate thing is that most of my work is limited to what
happens on the tower end of the stroke, not how to protect low voltage
equipment inside a structure. If you want to know more about how to
prevent flashovers of large power line insulators or how zinc-oxide
arresters rated up to 500kv work I can help with that, but I don't get
to work on the 120/240v or rf stuff except at home.
David Robbins K1TTT
e-mail: mailto:k1ttt@arrl.net
web: http://www.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net >>
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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