Bill Turner wrote:
> ORIGINAL MESSAGE:
>
> On Thu, 13 Sep 2007 08:58:15 -0500, W0UN -- John Brosnahan
> <shr@swtexas.net> wrote:
>
>> Not only did you NOT use the term "single-point ground" neither you nor
>> Bill have made any suggestion for a more accurate term that is simple,
>> while remaining descriptive.
>>
>> I would suggest that since we are speaking about some sort of grounding
>> concept that the term "ground" needs to appear in the term. And it will
>> need
>> a one-word or two-word modifier to distinguish this concept from
>> other grounding
>> concepts -- so make a suggestion. If you can come up with something that
>> is better, more accurate, and no more cumbersome, then I can support that.
>
> ------------ REPLY FOLLOWS ------------
>
> I did make a suggestion - actually three - in another email, but I
> will repeat them here.
>
> 1. Single Point House Ground
> 2. Single Point Tower Ground
> 3. Single Point AC Mains Ground
>
> #1 could also be called a "Single Point Cable Entrance Ground" or some
> variation thereof. Either way communicates the concept better than
> "Single Point Ground" by itself.
>
> Bill W6WRT
WHen considering grounding, as a general thing, you have three basic
flavors:
1) Grounds used for electrical safety (that's the green wire ground).
2) Grounds used in connection with an antenna as an RF return (e.g. the
radials under a vertical)
3) Grounds used to dissipate lightning
The "common return point" or "equipotential grid" you're talking about
actually doesn't anything to do with "ground" in the "earth connection"
sense. You're just tying everything together in a way so that voltage
differences between conductors are minimized.
For example, say your shack was in a very tall tree. You'd bring all
your feedlines and power lines to the shack, and connect all the wires
all togther at that point (or use some form of overvoltage clamp where
you can't connect directly... e.g. the power line or coax center
conductor). Then, if something happens and current flows from some part
of the system outside to some other part of the system outside (say,
lightning hits a power line), at least the stuff in your tree house all
is at the same voltage (pretty much.. differential mode voltages are
limited by your clamps).
You could, for instance, have your shack hanging at the center of a
dipole made with very sturdy wire. (Think Arecibo)
Now, as a practical matter, the electrical code requires that:
a) "things you can touch" have to be connected to the same earth ground
as the greenwire ground (i.e. the case has to be connected to the
electrical safety ground)
b) And, at one, and only one, place in the AC power system, one of the
AC line terminals has to be connected to that same ground (except in
some industrial installations).
c) And, things which are subject to being inadvertently shorted to
external power lines (e.g. antennas, masts, dish antennas, etc.) have to
be connected to the electrical safety ground too.
nothing in the code says that this has to be a low RF impedance
connection.. It's purely to limit shock and fire hazards from line
frequency (50/60 Hz) power. the only real driver is that the series
resistance be low enough that if there's a line to case short, the
breaker will trip, which means that you wind up using various sized
wires, depending on the details, such as whether the conductor is Copper
or Aluminum).
It's the (A) requirement above that leads to the need to interconnect
all these various grounding networks in some fashion.
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