Date: Tue, 4 Jan 2011 21:40:58 -0500
From: Charles Coldwell <coldwell@gmail.com>
Subject: Re: [TowerTalk] WHOLE HOUSE SURGE/LIGHTNING PROTECTION
On Tue, Jan 4, 2011 at 3:43 PM, Stephen Davis <sdavis@davisrf.com> wrote:
> Hi Chip,
>
> ?Ref your input, ?"
>
>> ...., IMHO, just running a long THHN wire between grounds is
>> not enough to prevent equipment damage. ?"
>
> ?I'm not sure how to interpret that remark, but just want to be sure that you
> or anyone else does not think that my ?ref. to the THHN tie between grounds
> was in and of itself a panacea for damage prevention. ? In addition to many
> OTHER prevention measures, it is academic that a separate station ground be
> employed and tied back to the entrance panel ground. ?I'm pretty sure you
> agree, based on your other astute input.
Yes I basically agree, but I wanted to give my example of how a
reasonable person can make a serious error in designing a grounding
system. In my house there were two grounds: the electrical panel in
the front of the house tied to a water pipe and the antenna ground
tied to a rod driven into the back yard. The two grounds were tied to
each other by the long wire running across the full width of the
basement.
In principle, the wire itself does no harm because it parallels the
conductive path between the pipe and the rod through the soil itself.
The theory of the wire (required by the NEC), is that if lightning
struck the antenna and the current is conducted to the ground through
the rod, poor soil conductivity would mean that during the strike the
ground rod could rise to a higher potential than the water pipe. By
paralleling the poor conductor (soil) with a good conductor (copper
wire), we lower the impedance (increase the conductivity) between
them.
But not nearly enough! The inductance of a straight conductor is on
the order of 1 uH/m, so call it 10 uH to run across my basement. The
rise time of a return lightning stroke is about 0.2 us, call it 5 MHz,
so it sees 50 ohms in that wire (neglecting some factors of 2 pi just
to get the right order of magnitude). With a typical peak current of
20 kA, we have a megavolt across that wire. Now, admittedly not all
of that current is at 5 MHz, so the real voltage is smaller, but this
back-of-the-envelope calculation shows that the long wire running
between the two grounds is completely inadequate to the task of
maintaining an equal potential between them during a lightning strike.
Trying to keep all the grounds at the zero potential during a strike
is probably hopeless. As was said upthread, the better strategy is to
try to have them all rise and fall together so as not to put any
stress on equipment that is connected to more than one. That gets us
back to the single point ground.
### Ok, my 200A main panel is at the front of the house. The copper pipe
that went
to the water meter gave way..and was replaced with buried 1" plastic pex pipe.
Plan B was to
bond the 200A main panel over to the SPG... which is actually a lot closer to
the 200A main panel
than the original CU water pipe. The SPG is just a large AL plate in the
basement, 18" x 24" x 1/4" thick.
SPG plate is also bonded to outside grnd rod via 2 ga wire [ cadwelded]. The
tower in back yard has 3 x grnd
rods, also cadwelded. The 3 x grnd rods at base of tower are 65' away from
the SPG. I used 65' of
bare 2 ga wire... to bond the 3 x rods at base of tower to 4th rod that's
just outside the basement spg plate.
## OF tower goty hit by lightning, I beleive the lightning strike is tryoing to
find it's way to dirt grnd.... via the
3 x 8' cadwelded rods at base of tower. If any of the fault current wants to
flow through the 65' of bare 2 ga
wire back to the 4th grnd rod... [ then into the basement via 2 ga wire to the
spg plate] the entire 65' of 2 ga bare cu
is buried several inch's into the soft dirt in the garden bed... that runs at
least 63' of that 65' feet. IF lightning is
trying to find it's way to dirt grnd.... well it's got a dandy 65' to travel
through..and all through dirt. IMO, tying the
spg to the tower grnd via heavy ga bare wire is ok.... and is better if that
wire resides in dirt... outside of the house.
## I don't think I'd want that 65' of 2 ga wire taking a short cut..and
travelling it's entire length..through the inside
of the basement. With frost, and freezing in the winter time, it should
probably be down much deeper into the soil,
for it's entire length..but that's a project for next summer.
later... Jim VE7RF
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