You wrote:
>The problem with connecting the AC green wire to the radio room ground and
then connecting my bulkhead "SPG" to a new ground rod or even a perimeter
ground connection is that my radio installation becomes part of the AC
protective ground system for the house. That is, any house fault or other
current will partly be routed through my radio ground system. Is that what
we want?
When you say "house fault" I assume you are referencing a ground fault
circuit interrupter (GFCI) system. Such a system interacts with the white
neutral ground, not the green safety ground. So the answer is yes, we do
want the safety ground connected.
>If I install a new ground rod system near the radio room, it is unlikely it
will be as good as the water pipe ground (about 35-40 ft
distant) that is used by the house AC.
Close may be better than good. If you don't have a lot of soil to work with
there are other ways to get decent conductivity. There is plenty of info on
the polyphaser site.
>If all wiring and equipment are protected by and bonded to this plane, why
do we care if there is a good earth connection at all?
All of those electrons have to go/come-from somewhere. They can be routed
through the slab or the earth. Take your pick.
As long as all equipment grounds are at the same potential as all other
ground systems, little to no current will flow between them, which is what
you want to prevent lightning damage.
Keith NM5G
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of Martin AA6E
Sent: Friday, February 25, 2005 10:06 AM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Ground system design, RF vs AC
I appreciate the good counsel to date. I am trying to zero in on a few
issues.
1. The problem with connecting the AC green wire to the radio room ground
and then connecting my bulkhead "SPG" to a new ground rod or even a
perimeter ground connection is that my radio installation becomes part of
the AC protective ground system for the house. That is, any house fault or
other current will partly be routed through my radio ground system. Is that
what we want? I believe the electrical code would permit the installation
of an isolation transformer where the primary is grounded back through the
house ground and the secondary is grounded locally. That would be a serious
expense for a possibly minor benefit - but it would ensure that house ground
currents would not flow through the radio system. I suppose it's more of an
issue for industrial installations or recording studios...
2. If I install a new ground rod system near the radio room, it is unlikely
it will be as good as the water pipe ground (about 35-40 ft
distant) that is used by the house AC. Especially since I can't drive any
deep rods due to rock ledge. How do I decide if it's better to run a
low-inductance strap to the water pipe or to make as good a local connection
as I can (horizontally buried cable or rod, probably)? Of course, some say
we should do both. (More work!)
3. What's wrong with the following picture? When we develop a local
"single point ground" system, we are really defining a local ground
reference plane. If all wiring and equipment are protected by and bonded to
this plane, why do we care if there is a good earth connection at all?
(It's like living in a Faraday cage, sort of.) Even the best ground is
likely to have several ohms of DC resistance and maybe 50 ohms of reactance
at 1 MHz. Your random lightning bolt might give your system a 1 kA surge,
raising your SPG to 1-10 kV relative to
(distant) earth. The earth ground system gives the current a controlled way
to earth, but doesn't keep your SPG at zero volts. As long as your gear is
well bonded and surge protected, it may survive.
(Don't ask about the op!) Thinking this way, we conclude that the questions
about earth connections are about fire prevention & building safety
(boring!), more than saving your equipment (important!). Or are we missing
something?
73, Martin AA6E
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
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_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
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