[TowerTalk] FW: The PERILS of single point grounding

[Martin, AA6E]

Jim,

I agree with Gary.  One way to look at this is to imagine that your
shack is enclosed by solid metal ground walls.  All your cables,
including power, telephone, coaxes, etc., are surge protected as they
pass through the shield wall.  The shield wall is connected to earth
as well as possible.  The point is that whatever external surge occurs
on your antennas, power lines, etc. is conducted to earth on the
outside of the shield wall.  Inside the shield, everything is calm  --
all the "grounds" stay at the same voltage.  This seems to be how
commercial cell phone equipment buildings are built.

In reality, you don't need a complete enclosing shield (unless you
expect lightning to strike the room itself!).  It is enough to have a
sturdy grounded plate on which you mount all the surge devices.  But
every single wire the comes in or goes out, including telephone,
Ethernet, power, rotator control, has to be grounded and/or protected
with respect to that single point ground.

Even if your SPG has a good earth connection (maybe a couple of ohms),
the SPG's voltage in a high-current event may surge to kilovolts above
"true" ground.  If everything is tied to the SPG, it won't matter. 
Some examples at my blog
http://blog.aa6e.net/2005/03/in-shack-ground-system.html .

73 Martin AA6E

On 10/7/05, Gary Schafer <garyschafer at comcast.net> wrote:
> Hi Jim,
>
> What you have described is not a single point ground system as many may
> believe. It is a multiple point ground system.
>
> To be a true single point ground system the power and coax lines must be
> referenced to exactly the same point. That is a power line must be run
> over to the point / panel that the coax lines are terminated on. The
> power must have protectors at that point to make sure the power is
> referenced to that exact panel. All equipment grounds must be run to
> that exact same panel.
>
> Now everything is referenced to only one point.
>  From that one reference point you may run as many ground leads as
> desired to the earth over as many paths as desired.
>
> 73
> Gary  K4FMX
>
>
> Jim Jarvis wrote:
> >
> > We've beaten to death the notion of 'common' and 'gnd' being tied
> > together at one point only...the service entrance.  NEC is clear
> > on that.
> >
> > What we haven't done is carefully examine what happens with our RF
> > SPG, as it relates to the power ground, and the chassis grounding
> > of our equipment.
> >
> > Picture this installation... we install a perimeter ground around the
> > house using strap.  We bond to 8' ground rods at each corner of the house,
> > and extend strap radials outward from the perimeter, as we can.  Then,
> > we make ONE connection to the electrical system ground, at the service
> > entrance, and add another 8' rod at that point.  So far so good.
> >
> > Then...we run all our coaxes and control lines through a grounded panel,
> > which may or may not be adjacent to the electrical service entrance.
> > Then...we plug our equipment into the power system in the shack.
> >
> > In the event of a lightning strike,  the surge impedances of the various
> > paths to the equipment via RF gnd and power gnd will be different.  Maybe
> > a little, maybe a lot.  A potential difference will be generated between the
> > grounds, and across some aspect of the equipment.
> >
> > Can you really HAVE a single point ground?  Even if the RF and power service
> > entrances were immediately adjacent, and tied together with 3" strap...the
> > drop circuits from each will follow different paths, and may have induced
> > current on them from the primary strike...with different potentials due to
> > their geometries.  I still disconnect everything in the main shack.
> >
> > In a 24/7 broadcast installation, or amateur repeater, you clearly can't do
> > that.  You take reasonable precautions, and hope for the best.
> >
> > Thoughts/comments?
> >
> > n2ea
> > jimjarvis at ieee.org
> >
> >
> > _______________________________________________
> >
> > 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.
>
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--
martin.ewing at gmail.com
http://blog.aa6e.net