All of the cables in our subdivision are underground but a lightning strike
on a tree by the lake on the "farm" behind us about 100 yards from my house
took out my cable modem, router, two switches and three nic cards. Induced
voltages, YES. The closest it would have been to any of the cables would
have been 50 yards and that in the house two houses up from me.
True war stories, 73, Jim
----- Original Message -----
From: "Phil Camera" <firstname.lastname@example.org>
Sent: Wednesday, September 12, 2007 7:42 AM
Subject: [TowerTalk] It can be Done!
> "Simple, but wrong."
> Nope simple and it works. The arrestor manufacturers have done lot of
testing and calculations and real world testing of their products. The
proof? Do any of the commercial radio stations or your fire and police
departments disconnect and stop operating when there's a storm?
> They don't, right? Know why? They are using the same arrestors and
design as we're talking about here. They're not replacing equipment all the
time so a properly designed and installed grounding system does indeed work.
> "There is no such thing as "no voltage difference" when you are talking
> a million amps or so."
> As stated, it's more like kilo amps but it does sure seem like a million!!
> And remember we are more concerned with nearby strikes than a direct
strike. A strike a mile away is said to be able to induce enough energy in
your system to possibly zap your electronics. I know from personal
experience that a direct strike that blew up my neighbor's tree, 300 feet
from my barn, 400 feet from my little tower and yagi, and 700 feet from my
VHF/UHF antennas, induced more than 30 amps of current in the barn
electrical. None of my equipment was affected and the VHF/UHF is on 24/7.
> "Let's say your tower takes a direct hit. Lightning wants to go to
> ground. It doesn't want to go to your house, but it will if there is a
> voltage difference and a path. You may think the base of your tower
> has a zero impedance path to ground, but at a million amps, I
> guarantee the impedance is small, but not zero. Even a perfectly
> straight tower leg or piece of ground wire has some inductance and
> resistance. Because your coax and rotator lines come off the tower at
> some point above earth ground, even if only a few inches, they are
> going to see a potential difference, and it can be quite significant
> depending on your particular configuration. This potential difference
> is going to seek its own path to ground, and that path is through the
> coax/rotor wires running off to your house. At the point where those
> wires enter the house, you need to provide a second path to ground
> lest the voltage pulse get into your house and equipment and find its
> own path to ground."
> You are totally correct and at the house entrance is what is considered
the SPG, which may be more than one rod. It's just a term, SPG, and it can
be confusing. That little bit of energy heading down the coaxes/cables will
be shunted to ground via the arrestors located on those lines at the SPG.
> "And this is why the concept of a "Single Point Ground" makes no sense
> to me. In the case above, there must be at least two grounds or there
> will be trouble."
> A properly designed grounding system, to me, consists of 5 elements:
> 1) Tower/Antenna/Mast Ground (Tower legs each should have 75 feet of
grounding radials with rods every 2X height.)
> 2) SPG (Low impedance/resistance ground where all the cables collect
before entering the house.)
> 3) Arrestors (On all cables and tied via low impedance/resistance to the
SPG. Also, don't forget arrestors on phone/TV/internet lines and those may
not be at the SPG but you may want to establish their own SPG and then bond
that, via an outdoor path, to the radio SPG.)
> 4) Shack Ground (All internal equipment grounds are tied together and
routed, via low impedance/resistance ground back outside to the SPG.)
> 5) Service Entrance Ground (This is bonded via an outdoor path, to the
> There you have it, 5 easy pieces, to a properly designed grounding system.
> It really works.
> Phil - KB9CRY
> Lockport, IL
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