[TowerTalk] Braid as an Effective Grounding Material

[Roderick M. Fitz-Randolph]

>HI Rod...I've never thought braid was good for a ground, and now that
>I understand the mechanics (OK< the electromechanics) of the situation
>my gut feel was right.  You need the ground wire to conduct gobs of current,
>and it needs to be THICK because of skin effect.  Braid, even tinned, still
>doesn't have near the effective thickness of good old solid copper wire.
>The energy in lightning (last measurements I saw from a research team in
>the late 70's) goes way up into the VHF region, but there's plenty of
>energy below that to sizzle your braid.
>
>Just to give you an idea...in nuclear stations they use copper BAR (abt
>1 in by 3 in as I remember) for grounding.
>
>No, no, no...do not use braid for a ground.  You'll have plenty of people
>say "I use it, and nothing happens."  I say:  "You haven't seen 'real'
>lightning yet."
___________________________________________________________________________
I considered the braid because it has less inductance than copper.  My
particular situation here is not quite the same as for most situations in
that I have a 130 foot tower with a 20 foot mast (7 down and 13 up) and a
lightning rod that extends to 150 feet above the ground.  Each leg of the
Rohn 25G is connnected to two 8 foot lightning rods, 16 feet apart... for a
total of 6 lightning rods.  The coax cable that connects the antennas to the
ham shack is buried for 250 feet from the base of the tower to the hamshack.
Some of the coax is 3/4" CATV aluminum shield cable that is buried direct (we
have acidic soil here so direct burial of CATV aluminum jacket coax is
acceptable).

Only a small amount of voltage has reached the ham shack during a lightning
strike.  Two of the three strikes did not hurt the Alinco VHF transceivers
that remain connected to my PacketCluster node 24 hours a day.  I have
installed PolyPhaser IS-50US-C1 devices in both of those coax lines now and
intended to run the 3/4" braid 6 feet (from the 1/8" thick aluminum plate upon
which the PolyPhaser devices are mounted) to ground rod system at the ouside
wall of the hamshack (which is a different ground system than that of the
tower because of the distance involved and PolyPhaser's statement that if
the tower is more than 100-150 feet away from the hamshack, a common point
ground is not necessary; rather two separate, but adequate grounding systems
should be used).

There are two prime concerns:  (1) The braid must have low enough DC
resistance to effectively short the current to ground and (2) the braid
(or whatever is used) must be of small enough inductance to assure that
L di/dt does not cause the sort of build up over its length that allows
a 100 foot well grounded tower (that has only 0.1 ohm DC resistance from
top to bottom) to achieve > 200,000 volts from top to bottom, instantaneous
at the moment of the typical 18,000 Ampere strike.  The only two viable
choices are a copper strip 6" to 9" wide running from the common tie point
at the station console to the grounding system 8 feet away (and through the
wall via a 3" PVC) on the outside of the house.

Copper strip is out of the question (even though it exhibits much less
inductance than either a wire or braid) because of the small aperture of the
PVC.  This leaves only large copper braid or tinned copper braid as a viable
choice.  Based on the excellent protection the 7 feet of 3/4" copper braid
afforded at the digipeater site at which every other piece of radio/electric/
communications equipment was destroyed, the electrical distrubution box
was blown to pieces, the repeater telephone line input box was nearly
vaporized, the ground lug on my power supply was almost arced in half, etc.,
yet my equipment protected by the 3/4" braid to two ground rods and the
PolyPhaser device inline with the coax, leaves me to the inescapable
conclusion that the small charge remaining at 250 feet from the well-grounded
tower will be successfully "bled" or grounded by braid that is even more
massive than that which successfully protected the equipment from the effects
of a direct strike at the digipeater site.

My question is: Is the tinned copper braid to be avoided because of any
oxidation problems with the tin?.... i.e., should I only use large copper
braid as opposed to large tinned copper braid?  Does the tin on the outside
of the copper pose a problem in light of the well connected and Penetrox A
protected joints at either end?

Comments gratefully accepted.

Rod, N5HV



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