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.
BTW, my numbers in the paragraph above come from Ronald B. Standler's
book "Protection of Electronic Circuits from Overvoltages"
(recommended reading) Table 2-1 page 14 (lightning rise time and
current) and page 202 (parasitic inductance of a straight conductor).
Also good reading is Martin Uman's "Lightning" or his more recent "The
Art and Science of Lightning Protection" now available in paperback.
Professor Uman is on the faculty of the University of Florida, where
they know a thing or two about lightning!
--
Charles M. Coldwell, W1CMC
"Turn on, log in, tune out"
Belmont, Massachusetts, New England (FN42jj)
GPG ID: 852E052F
GPG FPR: 77E5 2B51 4907 F08A 7E92 DE80 AFA9 9A8F 852E 052F
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|