All choices in life are driven by the evaluation of the criteria at hand.
Some of the choices people make seem eccentric to others. Non-hams wonder
why we erect towers and antennas.
As an engineer, this is an exercise in economics vs. probability.
The grounding discussion here seems to rotate around diverting a direct
lightning strike to ground.
Emotionally, we all want our families, ourselves and our "stuff" to be safe.
Unfortunately, the reality is that we can't engineer a solution to many
things, including the dissipation of the energy of a direct hit from
lightning. There is some validity to almost every post here, but you must
ask yourself what you can do in your situation to come to reasonable
accommodation of risk and keep yourself and your household safe.
A direct hit = you're going to get stuff fried, maybe wires are going to
vaporize anyway. I have seen aluminum service drops - 5/8" aluminum
conductors - vaporized and leave only shards of insulation. Goodbye ham
radio station, computers, televisions, just about anything connected to AC
power if your breakers don't trip immediately!
The object (and original discussion) was how to provide a better path from
your communications ground to the SPG than through the household appliances
- from one point on earth to another.
Every electrical outlet in every house with single phase service is
connected to the SPG through the ground lug on the outlet, both of which
should be bonded to neutral at the SPG. When you connect your ground wire
from your ground rod to your rig, you then are connecting your ground rod to
the SPG through your rig's cabinet.
The bonding REQUIRED by the NEC provides that multiple ground systems be
bonded directly for a number of safety reasons as well as a direct lightning
strike. I'll allow the group to ponder this - I can think of one case
related to measuring 100 Ohms to ground from an outlet!
For almost all cases, a #6 solid wire from the SPG ground to your tower
ground rod system will be fine to keep your home safe. Certainly it is
compliant with NEC. Compliance with code is my reasonable accommodation of
risk. It is what is required of a contractor or engineer, legally.
You can build a Faraday cage around your house and ground it with 512 ground
rods if you like, but it just doesn't make economic, technical or legal
sense!
Spend your money as you like, but let's not insult each other with
half-truths and old wives's tales about lightning.
Jim's statement about lightning having "its energy centered around the AM
broadcast band" is factually incorrect. Even if it were correct, nothing
passive has a lower impedance than a large, straight conductor in a line
between two points.
If you want to know what electrical characteristics of lightning have been
measured, here is some actual science:
http://www.lightning. <http://www.lightning.ece.ufl.edu/PDF/cotisonrtl.PDF>
ece
<http://www.lightning.ece.ufl.edu/PDF/cotisonrtl.PDF>.<http://www.lightning.ece.ufl.edu/PDF/cotisonrtl.PDF>
ufl
<http://www.lightning.ece.ufl.edu/PDF/cotisonrtl.PDF>.<http://www.lightning.ece.ufl.edu/PDF/cotisonrtl.PDF>
edu
<http://www.lightning.ece.ufl.edu/PDF/cotisonrtl.PDF>/<http://www.lightning.ece.ufl.edu/PDF/cotisonrtl.PDF>
PDF
<http://www.lightning.ece.ufl.edu/PDF/cotisonrtl.PDF>/<http://www.lightning.ece.ufl.edu/PDF/cotisonrtl.PDF>
cotisonrtl
<http://www.lightning.ece.ufl.edu/PDF/cotisonrtl.PDF>.<http://www.lightning.ece.ufl.edu/PDF/cotisonrtl.PDF>
PDF <http://www.lightning.ece.ufl.edu/PDF/cotisonrtl.PDF>
Follow the NEC and bond! If you want to install a 4" copper pipe to do so,
good for you - but it isn't necessary to be safe and it could cost more than
a house full of ham gear.
My interest in this topic...
On July 20, 2000, I was in my back yard when lightning hit a tree about 15'
away. It split the tree and eventually killed it. Energy through the (dry,
solid) ground injured my leg muscles - I spent the next 5 days in the
hospital and have had 4 surgeries on the leg.
As a result, I became a bit obsessed by the topic of lightning. As an
electrical engineer (who now works for a storage technology company) I have
found, read and understand most of the literature. I believe I could make a
living as a lightning protection engineer, but fortunately, I'm having a
good time doing something else that I know even better.
Mickey, N4MB
On Thu, Jun 17, 2010 at 12:09 PM, Jim Brown <jim@audiosystemsgroup.com>wrote:
> On Wed, 16 Jun 2010 22:08:24 -0400, Mickey Baker wrote:
>
> >K7DD is right. Bond to your SPG with as big a wire as you feel you can
> >afford.
>
> Right.
>
> >Longer path = bigger wire.
>
> WRONG! Lightning is NOT a DC event. It is a TRANSIENT, with its energy
> centered around the AM broadcast band and extending for several octaves
> either side (that is, from about 200 kHz to about 5 MHz). INDUCTANCE is
> what matters, and that is proportional to length. The only benefit of a
> bigger conductor is that the conductor can discharge more of the event
> before it vaporizes. :)
>
> 73, Jim Brown K9YC
>
>
> _______________________________________________
>
>
>
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>
--
Mickey Baker
Fort Lauderdale, FL
“Tell me, and I will listen. Show me, and I will understand. Involve me, and
I will learn.” Teton Lakota, American Indian Saying.
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