The SPG you described is about as good as you can get. If not SPG, what
else would you call it?
When lightning strikes we can only manage the flow of electrons to a certain
extent. This is what a SPG is all about; put all of your equipment at the
same potential so there are only very low impedance paths between each unit.
Also, place as many outside dispersal points (e.g. copper straps leading to
other ground rods) as possible along the SPG connections to reduce the total
charge per path.
The electron flow itself is dependent on all possible paths for the free
electrons to find a resting place. An atom can hold just so many electrons,
so each free electron has to basically seek out a separate atom. Thus, the
discharge of a strike involves an arbitrarily large number of paths and
atoms. Each flow seeks the path of least impedance. Have you seen a high
speed photo taken of an actual strike? It will show literally thousands of
separate flow "fingers" surrounding the main strike point. These fingers
can travel over more than an acre of land.
If a flow makes it into the shack via a coax center conductor or rotor
control wire, it will go through some electronic components until it finds
the neutral of the AC cord for a final trip to the service entrance ground
rod, to be dispersed to the surrounding earth. In extreme cases the flow is
sufficient to literally blow tiles off of a wall that are near the romex.
This is why the coax/control cable should be tied to the SPG BEFORE it
enters the shack. If you were an electron, you would rather go to the
neutral ground rod via the SPG rather than through the longer path into the
shack. If you use Polyphasers, the shack path will be even less attractive.
IMO, if you do not have a SPG, and your equipment is not chassis grounded to
it, you should unplug the AC, coax and control cables to be completely safe.
However, with a properly implemented SPG, you should not have to unplug
anything (unless you are paranoid).
73, Keith NM5G
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of Jim Jarvis
Sent: Friday, October 07, 2005 2:00 PM
To: towertalk@contesting.com
Subject: [TowerTalk] FW: The PERILS of single point grounding
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@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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