On Tue, Jan 4, 2011 at 11:45 AM, Stephen Davis <firstname.lastname@example.org> wrote:
> and I find that often when a single point ground was used, close to the
> station, what was missing was the return from that back to the entrance panel
> ground. This is such a simple undertaking....usually a # 6 or # 4 THHN
I recently bought a house that has the main electrical panel at the
front (grounded to a water pipe) and had (past tense) both OTA and
satellite TV coax coming from the antenna/dish through the back of the
house. The 75 ohm coax runs were grounded with a rod driven into the
back of the house, and there was a long long #6 or #4 THHN wire
running from that grounding block at the back of the house to the
panel at the front.
What's wrong with this picture? Well, that long run of #6 or #4 has
an inductance. That inductance becomes an impedance at high
frequencies during a lightning strike, which means that the voltage
potential at the panel ground and the coaxial ground could differ as
K4FMX said, "Sometimes by many thousands of volts!"
That voltage difference between the coaxial ground and the panel
ground will appear across the electronics that are connected to both
grounds: i.e. the satellite receiver and television that are both
plugged into the wall and connected to the antennas. Voom!
Once again, as K4FMX said, "This means that ALL equipment and lines
connected to that ground system must rise together to avoid damage."
That is why, IMHO, just running a long THHN wire between grounds is
not enough to prevent equipment damage. Yes, it is a short circuit at
DC, but don't let that fool you. Lightning is an impulse with all
kinds of high frequency components!
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
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