On 9/19/2012 2:24 PM, Dale Svetanoff wrote:
David,
All I can say about my post is this: IT WORKS.
YES, the power ground "bounces" up and down to the tune of lotsa' kV, no
doubt about it, during a strike. However, ALL of the grounds are bouncing
at once, and yes there will be some time (phase) differential involved
because of path length. A variety of sources I've reviewed over the years
say that typical products used in residential electrical power distribution
have an inherent flashover of about 600 V. Well, if I was seeing many kV
of differential between my grounds and the power feeders, I should have
plenty of arc tracks inside my breaker panel, outlets, and so forth. None
observed so far.
Remember, the radiated field from the strike envelopes the house wiring in
one huge common mode coupling of energy. (My main tower is within 40 feet
of the shack corner of the house, plenty close to get lots of coupled
energy.) Therefore, the actual hots and neutrals are bouncing around with
common mode induced (especially on that Romex), plus whatever energy had
coupled onto them via the protectors, rigs, and so forth. Let us not
forget that the poor old pole pig is sitting there (75 feet away on a pole)
getting its secondary side bounced all over, but it doesn't flash over
because its grounded case is tied to the bouncing ground bus (with power
company approval). The bouncing is in relative unison, thus minimizing
differential potentials..
I strongly suggest that you go on-line and read MIL-HDBK-419A, which
addresses facility grounding. You may also wish to consult
MIL-STD-188-125-1, which concerns nuclear EMP hardening for fixed site
facilities and to which I was an invited industry representative
contributor. Those documents, combined with the QST series of articles by
Roger Block (of Polyphaser fame), provide a pretty good summary of accepted
(and proven) techniques for lightning protection of radio and other
facilities. I also consulted with engineers from other aerospace companies
on the lightning transient issues for the F-35 Joint Strike fighter.
In short, I don't think I'm protected - I KNOW I'm protected because when
the lightning struck, I did not become a carbon block in my chair and none
of my equipment anywhere in the house was damaged. In fact, I was able to
keep transmitting with the stub that was left of my Cushcraft Ringo dual
bander after the strike vaporized the top 1/3 of the antenna. Note: I was
not intentionally trying to get zapped. The fact that my station got hit
as I was conducting emergency traffic related to severe storms was
coincidental. I must caution you, and all others: do not try this on your
own. Leave "live fire testing" to professionals (whether they intended to
do so or not). The only difference between the first strike (in 2008) and
the second one (in 2009) was that I was not on the air during the second
strike, but my equipment was fired up and running. Again, ZERO damage,
except for another vaporized dual band antenna atop the tower.
The issues you cite in your clip (regarding distance between parts of the
system) from my post are, quite frankly, ones that I had concerns about
when building the station and its protection scheme. Guess what? I don't
worry about them anymore.
73,
Dale Svetanoff, WA9ENA
Sr EMC Engineer
E-N-A Systems, LLC
Specializing in shielding applications, system grounding, and lightning
protection
[Original Message]
From: David Robbins <k1ttt@verizon.net>
To: <rfi@contesting.com>
Date: 9/19/2012 11:23:09
Subject: Re: [RFI] ISOBAR
Sep 19, 2012 11:55:43 AM, svetanoff@earthlink.net wrote:
"
I have 2 aluminum entrance panels: a small one on the south end of
the house and a large one on the north end, 54 feet apart. They are both
bonded to the perimeter ground. The perimeter ground is bonded to the
electric power ground right adjacent to the power entrance, on the south
side of the house. My shack is in the NE corner of the basement and is
bonded to the north entrance panel. ALL cable I/Os pass thru protectors in
one of the two entrance boxes.
People don't realize that even with a properly grounded home, a single
point ground and entrance panel where *everything* comes in that due to
the Dv/Dt the voltage differential between rooms on one end of the house
to those on the other end may be thousands of volts.
Then to complicate matters the routing of telephone lines and cable to
various parts of the home can make things unpredictable as the
instantaneous voltages on the branches will vectorarilly(sp?) add at any
specific spot. The result depending in the location of the strike as
those locations closer to the strike will rise first.
I gave up and hoped for the best on the CAT5e and CAT6 as I not only
have the house wired but an additional 3 runs from about mid house to
the shop that are 130 feet long. The network in both the house and shop
connects to equipment that also has antennas and power connected AND the
house and shop are on different electrical feeds.
"
And this is a perfect example of how people who understand impedance, but
not lightning transients, think they are protected, but they aren't really.
That 54' gives enough time for a lightning surge coming from the shack end
of the ground ring to raise the voltage on the shack equipment cases before
the lightning protection at the power entrance sees the surge and equalizes
the power conductors with it to protect the equipment. remember, the power
conductors coming from their entrance panel are at their normal 120v level
until the surge current travels all the way to the panel, 54', through the
surge arrester there, and back another 54' to the radio room. that is
something more than .33usec which can give even a moderate lightning stroke
time to get the ground voltage up to 10's if not 100's of kv above the
power ground. that gives more than enough time for a flashover from the
ground to the power line through the radio to do damage.
And a really close strike can induce substantial voltages in
interconnecting wires. A lightning strike a mile away can induce well
over a 1000 volts per meter of wire so think what a close one can do.
How much wire is just in the walls of the house?
73
Roger (K8RI)
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