[Boatanchors] new Ground system
Glenn Little
[email protected]
Thu, 14 Mar 2002 23:46:52 -0500
Can I join in? This is another Glenn.
I also have been branded as an "expert" in grounding. The company that I
work for makes tower site monitoring equipment. We started this venture
with a red light controller and it evolved to a whole site monitor. It has
been my job to visit sites that continue to be hit by lightning. It is
amazing how much money a company will spend in replacing blown up equipment
and still not follow suggestions for proper grounding. My references come
from experience, the Motorola R-56 manual and MIL-HDBK 419. If a tower and
the site are properly grounded, the electrostatic potential (lightning)
will be equalized before strike potential is reached. The key to a
properly grounded site is bonding everything together, keeping ground leads
as short as possible and no sharp bends. Nothing that you can do will
protect you fron a direct strike. You can equalize the charge, but if the
charge builds faster than you can equalize it, you get a strike.
The FAA uses a charge dissipator that mounts on the tower legs and looks
somewhat like a fox tail. The R-56 manual says to place an aerospike at
the top of the tower and ground it with (I think) #2 solid or stranded
copper. All primary grounds (those that tie the ground rods together)
should be #2 copper. This should be buried (in our area) 18" and cadwelded
to the rod. Usually a "rainbird" cap is placed over the rod so that the
weld can be verified. The building should have a ground ring around it
with rods spaced about 15 feet apart. The tower should have a ring around
it with at least three rods. These two rings should be bonded together
along with the power and telco ground. If the equipment is properly bonded
to this ground system, even if a strike occures, minimul damage should
occur as every thing elevates above ground by the same amount and there is
very little potential difference between equipments.
I have seen "professionals" that used a single nylon tywrap to attach an
equipment ground to the ground ring. I have been told that I was full of
s**t and taht I did not know what I was talking about when I evaluated a
new site that was being struck by lightning. After the strobes had to be
replaced the second time (there was not enough left of the fixtures to
rebuild) and still no users at the site, the owner followed my
suggestions. There has not been a major lightning event at that site since.
Lightning can be tamed. It just takes a little common sense and doing
things correctly. Never run a ground wire through a slab or tower base
where the wire is in contact with the concrete. You use a piece of plastic
conduit around the wire. If you do not, and you take a strike, the heat
generated in the wire due to the I squared R losses will flash the moisture
in the concrete into steam and explode the concrete. Never use braid in a
lightning ground system. Lightning does not like to make sharp
turns. Braid is continous sharp bends. If you must use compression
fittings for a lightning ground you must ensure that the fittings stay tight.
Generally, you should use more than one ground rod. The rods should be an
absolute minimum of three feet apart. Any closer and you wasted your money
buying the rod and your time driving it into the ground. The idea is to
get the metal surface area fairly large and in contact with moist
earth. The larger this area the lower the ground resistance will
be. Motorola specifies a maximum of 5 Ohms for an acceptable groung. I
have seen very few that were less that 10 Ohms.
Enough rambling for now
73
Glenn
WB4UIV
At 11:03 PM 3/14/02 -0500, Gary Schafer wrote:
>Glen, are you saying that because of your grounding system that your
>tower has never been hit by lightning? Or that you have never had any
>damage from a lightning strike.
>
>73
>Gary K4FMX
>
>
>Glen Zook wrote:
>
> > My "main" tower went up in early May 1972 and has
> > NEVER been hit by lightning. The house across the
> > street has lost 3 trees and a chimney to lightning.
> > The "pole pig" (electric company transformer) at the
> > back corner of my lot has been hit by lightning. It
> > is about 50 feet from my tower and about 40 feet below
> > the top of my antenna. Other strikes have occured
> > around my neighborhood including other amateur towers.
> >
> >
> >
> > Glen, K9STH
> >
>
>_______________________________________________
>Boatanchors mailing list
>[email protected]
>http://mailman.qth.net/mailman/listinfo/boatanchors
---------------------------------------------------------------------------
Glenn Little [email protected] QCWA LM 28417
Amateur Callsign: WB4UIV [email protected] AMSAT LM 2178
QTH: Goose Creek, SC USA (EM92xx) ARRL TAPR
---------------------------------------------------------------------------