[Boatanchors] new Ground system

[Gary Schafer]

Glenn Little wrote:

> 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.

That doesn't happen.


> 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.

You can not equalize the charge.


>
>
> 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.

The tower generally needs more than 3 ground rods. A "ring around the tower" is a
waste of wire. It is better to use that wire to make radials out from the ground
rods and tower.


> 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.

Actually using the concrete tower base makes an excellent ground. Of course the
conductors within the concrete need to be heavy enough so as not to fuse open.
Bringing up a connection to the rebar in the concrete with a welded piece of
rebar makes a good ground. Additional ground rods should still be installed.
And don't forget those guy anchors. They need a good ground system also as they
share part of the strike current with the tower.


> 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.

Braided wire will work just as well as solid wire. The current does not follow
the twists in the braid. There are some who think that the skin effect of braided
wire is better also because of the multi strands. If you think about it, all
those strands make contact with one another and short out each other. It looks
like one solid piece of wire.
The only negative in using braided or stranded wire outside is that it will
corrode. Once it corrodes it is impossible to get it clean again to make a proper
connection. So for that reason solid conductors are recommended outside.


>
>
> 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.

Ground rod spacing should be approximately the distance that is the sum of the
two lengths. So for 8 foot rods that would be 16 feet separation. What happens is
during a strike the ground around each rod gets saturated with energy. A given
amount of earth can only absorb so much energy in a given amount of time. Once
the ground around the rod saturates it will not accept more energy. The area that
is effected is roughly a diameter that is equal to the length of the rod.
A ground system consist not only of a resistive element but is also dependent on
the capacitive coupling too.  There is a time constant involved in the
dissipation.

73
Gary  K4FMX

>
>
> 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
> > >
> >
> >_______________________________________________
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> >Boatanchors@mailman.qth.net
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>
> ---------------------------------------------------------------------------
> Glenn Little                         glittle@awod.com   QCWA  LM 28417
> Amateur Callsign:  WB4UIV            wb4uiv@amsat.org   AMSAT LM 2178
> QTH:  Goose Creek, SC USA (EM92xx)                      ARRL  TAPR
> ---------------------------------------------------------------------------