What is being proposed is a common grounding point within the shack,
which is a good idea. THIS IS SEPARATE from the single point common
ground where everything comes into the house.
If your shack is right next to the SPG, well good. If your shack is at
a distance, like on an upper floor or in the "bonus room" over the
garage, then ground everything (this includes phone lines, cat5, etc)
in the shack to a common point in the shack, including a healthy
safety ground run to the ground at the SPG, and whatever you may be
using to effect an RF ground.
What the second (shack) common grounding point does is to keep all the
potentials in the shack to a common level if a strike occurs, and gets
inside somehow. A close strike can saturate the ground field and then
things start to rise. You want them all to rise together.
You want to avoid lightning coming into your shack on one conductor
and using some piece of equipment as a path to another conductor to
leave your shack and go to something less saturated with charge.
In these situations, phone lines and such can be the enemy because
they are a way OUT of the house to a less saturated place.
About the disconnects, listen long enough, and you will hear stories
of lightning jumping across open spaces of some several feet to
continue its path to neutral ground.
A disconnect point not grounded on the lightning side of the
disconnect can be just about useless. It takes BOTH. And the ground
has to be a good one.
73
----- Original Message -----
From: "KC2TN" <kc2tn@comcast.net>
To: "'Jon Ogden'" <na9d@speakeasy.net>; "'Tower Talk (mail list)'"
<towertalk@contesting.com>
Sent: Thursday, September 19, 2002 6:03 PM
Subject: RE: [Towertalk] Box forSPG and Lightning protectors
> Not to disagree with Bill because he's been a BIG help to a lot of
us here
> on this reflector (ME included) regarding station protection.
>
> BUT...
>
> I was confused about where to put my SPGP also ..My original design
had it
> right outside the shack in an enclosure
> but I changed the design after reading the Ron and Roger Block QST
> "Lightning Protection" series "What Your Mother Never told you!.
>
> In that article there was this excerpt:
>
> "...As we'll detail later on, the primary ground system is
represented
> by a set of copper-clad ground rods, interconnected below grade,
with bare
> copper radials. Also fundamental to a good protection scheme is the
creation
> of a single point ground WITHIN the ham shack. This single point
ground will
> be used to MOUNT ALL of the I/O protection equipment and to provide
a ground
> for all of the equipment cabinets at the station's operating
position. This
> INTERIOR single point ground is CONNECTED to the external ground
system
> (composed of those radials and ground rods) by the lowest impedance
copper
> strap that you can manage. The tower ground system outside and the
single
> point ground system inside must be solidly interconnected with a low
> impedance metallic strap, so that your coax cable's shield is not
the only
> interconnection conductor between these two ground points. Keeping
as much
> of the lightning's energy off of the coax shield as possible is
essential to
> minimizing damage from a direct hit. For larger strikes, it's best
to
> incorporate a grounding kit prior to the protector, to save your
expensive
> coax connectors from arcing damage. An effective (good quality)
coaxial
> in-line protector can then be used to handle smaller strike currents
that
> may be tempted to travel down the cable itself."
>
> After conferring with Ron over lunch (he lives close by) we decided
it was
> best to move the SPGP inside the shack
> to allow easier bonding of all the protection components, including
an AC
> protector, to one plate. The plate was then
> connected to the EXTERIOR ground system via 2 6" copper straps. The
reason
> for the two straps was to equal the total
> cross section of cable(s) entering (11") with the ground leaving
(12") to
> attach to the exterior ground system.
> This keeps all voltages rising and falling together with zero
current flow.
>
> My exterior ground system consists of 2" strap cadwelded to 4 legs
of
> radials off the tower consisting of
> 4 8' 5/8" ground rods 16' apart on each leg and also a perimeter
ground
> connected to the AC panel all tied
> to the SPGP via the 2 6" straps.
>
> I should mention that my tower is only 10 feet from the shack and
> Electromagnetic Pulse protection is near impossible
> given the proximity of the tower. Unless I wanted to encase my shack
in
> copper $$$$.
>
> BTW...This whole system did not come cheap! But the idea is to keep
the
> energy in the ground field and most of it
> outside the house. Also, Ron's SPGP is in his closet next to his
station.
>
>
> Joe - KC2TN
>
> -----Original Message-----
> From: towertalk-admin@contesting.com
> [mailto:towertalk-admin@contesting.com]On Behalf Of Jon Ogden
> Sent: Thursday, September 19, 2002 4:25 PM
> To: Bill Hider (N3RR); Tower Talk (mail list)
> Subject: Re: [Towertalk] Box forSPG and Lightning protectors
>
>
> on 9/19/02 2:23 PM, Bill Hider (N3RR) at n3rr@erols.com wrote:
>
> > As I read some of the comments/questions about covering the
lightning
> > protection boxes, I'm wondering if consideration was taken into
account
> for
> > a Single Point Ground (SPG)??
> >
> > If the lightning protectors are just laying around on the ground,
> presumably
> > near the ground rod, and then coax or control cables are just run
into the
> > house without a SPG, what's wrong with this picture? If that's
the
> > scenario, you're missing a couple layers of protection.
>
> But if the lightning protectors are all tied into the same ground
rod, isn't
> that the same as the SPG?
> >
> > First of all, a SPG box provides the common ground point ensuring
no
> > difference in potential even in the face of lightning.
> > Secondly, a SPG box made of metal, rather than plastic, provides a
Faraday
> > shield for all wires passing through the metal SPG box. A plastic
box
> does
> > not.
> > Third, a steel box, rather than aluminum, provides a magnetic
shield
> against
> > magnetic fields (produced by lightning strikes) inducing currents
into
> your
> > cables then entering the house.
> >
> Yes, but the box only shields the stuff inside the box. What about
all of
> the coax that is run inside the house that is not shielded by any
metal? I
> fail to see how a metal box that encloses other metal boxes is going
to do
> much.
>
> 73,
>
> Jon
> NA9D
>
> -------------------------------------
> Jon Ogden
> NA9D (ex: KE9NA)
>
> Life Member: ARRL, NRA
> Member: AMSAT, DXCC
>
> http://www.qsl.net/ke9na
>
> "A life lived in fear is a life half lived."
>
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