Steve Bookout wrote:
>
> Hello all,
>
> I thought that a single point for grounding EVERYTHING was recommended. As
> I remember it, this would keep the difference in the quality of multiple
> grounds, in ohms, from causing a difference in potential along the multiple
> ground, ground buss.
>
> Difference in resistance = difference in voltage drop for the same REALLY
> high current = different voltage potential to ground along the bus =
> current flow.
>
> I am not an expert, but this is sorta how I remember it from a forum at
> some big convention.
>
> Also, I've seen what can happen to disconnected coax in the shack when it
> takes a direct hit.... PL-259 shrapnel, quite literally.
>
> 73 de Steve, NR4M
>
> At 10:21 AM 10/20/1999 , Dr. Gerald N. Johnson, P.E. wrote:
>
> >GROUNDING of the antennas SEPARATE from the house wiring ground and thus
> >the radio equipment ground is ESSENTIAL!!!
>
> ---------------------------------------------
> Steve Bookout, NR4M (ex NJ4F)
> Rapidan Data Systems
Single point grounding is a great theory but its NOT achievable. The
phone line is referenced to ground at the central office, miles away. On
it spark gap protectors won't allow the wires to get more than 300 volts
from LOCAL ground at each protector. Power lines are grounded all over
the place both local and distant and all those grounds are connected
together, but NOT with wires good for kiloamps. During normal operations
there can easily be a volt or two between your nearest ground rod and
the earth nearby, this is a real problem on dairy farms that is only
cured (to the comfort of the cows) by isolation. Adding grounds never
solves it at least with reasonable quantities of grounds. There'd be
less of a problem if the barn lot was paved with copper sheet.
And that single point ground would be great if all the connections to it
used conductors with very low inductance and with the capability of
carrying 10 kiloamps with only a few volts drop. Since that is rarely
practical connecting grounds with smaller conductors means they get
separated in voltage even if the conductors between them survive and
things like radios connected to antenna, power line, and phone line
grounds get fried because the radio ground paths can't carry a fraction
of 10 kiloamps.
Its a whole lot easier to proclaim a zillion grounds will protect than
to accept that isolation not based on tradition will protect better.
I prefer a tower to have multiple grounds, including the guy wires.
Insulated quy wires are not a good idea for lightning protection in my
view, and multiple grounds that are spaced closer than the length of the
ground rods are less effective. The large screw in guy wire anchors make
added grounds with continuous guy wires. And I prefer that the house
ground be ISOLATED from the antenna ground unless I have an RF problem
at the radio probably from feeding some wire against ground for an odd
frequency. Otherwise I keep the antenna system ground separated from the
power line ground, so when the antenna is hit, the lightning stays
mostly in the antenna, doesn't transfer to the power system in the
house.
I'm not in complete agreement with the codes because I think they aren't
correct, though they are simplistic.
I think of it as a filtering process. Depending on grounds is depending
only on a shunt element. Filters are FAR more effective when there are
multiple elements combining a shunt element, a series element, and
another shunt element. In this case, each shunt element is a ground
system, then the series element is the air gap of the disconnected coax
cable. When the grounds are interconnected and not all that effective,
signals (in this case really BIG signals) transfer around the filter.
Locally, the earth is not a great conductor, though it does fairly well
over the long distance, but in the vicinity of a lightning strike its
not very good, sometimes just standing with your feet apart can be
deadly because of the voltage drop across the surface.
73, Jerry, K0CQ
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