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Re: [TowerTalk] SPG & Service Entrance Grounds.

To: Bill Turner <>
Subject: Re: [TowerTalk] SPG & Service Entrance Grounds.
From: Jim Lux <>
Date: Thu, 13 Sep 2007 17:00:38 -0700
List-post: <>
Bill Turner wrote:
> On Thu, 13 Sep 2007 08:58:15 -0500, W0UN -- John Brosnahan
> <> 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 

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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