[TowerTalk] Grounding

[K8RI]

On 11/17/2012 7:40 AM, K1TTT wrote:
>> Dale - WD4IFR
>> In any lightning strike, even a 2" solid piece of copper takes on the
>> characteristics of a light bulb filament.
>
> It is simple statements like this that make the rest of what you say
> questionable.  The fact is that MOST lightning strokes hitting even small
> conductors will not significantly heat them.  To get any significant warmth
> in 2" copper is probably even beyond even the largest measured mega strokes.
>
>> Let me go on record that if "the finger of God"
(Known as the "Fickle finger of fate")
decides it your turn
>> to be touched, nothing, I repeat, NOTHING will stop damage from a
>> strike.  I work in the broadcast and wireless industries where
>

What we really do is lightning damage mitigation. IE, we reduce our odds 
of taking damage from a strike, but it is never reduced to zero, even if 
you throw all the antenna leads out the window.


> If you were seriously in the industry you would know that radio and tv
> towers get hit every day and the station doesn't go off the air.

I lived for sometime on a hill that also had a tower at the peak of the 
hill. Our house was bout 400 yards from the tower. I used to do the 
maintenance on the Farwell repeater antenna mounted on the tower. I 
spent quite a bit of time in the car parked on the hill along guys with 
fire trucks and EMTS doing tornado watches.  When that tower would take 
a hit all I can say is it sure was loud! We'd often see the lightning 
jump from the tower, over us, and hit out in the field.

   The same
> with cell towers and power lines, they are designed to take the stroke and
> not suffer any damage, and a properly designed ham station is more than
> capable of surviving direct hits.
>
> You are at least accurate with your ground field resistance description, aim
> for 25 ohms and that should be good enough if you do the proper single point
> ground entrance panel... however:

In the spring mine is lower than that, but late in the season I'm lucky 
to maintain 25 ohms.

>
>> BTW -- You do not have to use exothermic welds exclusively for your
>> ground connections.  One of the companies that make crimp on grounding
>> connectors is "Burndy".  These clamps are designed to get away from
>> Cadwelding but still require the use of a 12 ton crimper to bond the
>
> I have never cadwelded a ground connection, most commercial electricians I
> know would probably look at you with complete confusion if you asked them to
> do that.  The basic brass clamp sold in every hardware store is what
> probably 99% of the ground connections in this country are connected with is
> just fine.  After all a discharge that has just jumped a mile to the ground
> isn't going to be stopped by a few microns of oxide in a connection between
> two conductors.  Even lightning rod installers use easily hand crimped
> splices and connectors.

They are a whole lot cheaper, quicker, and require much less prep time.

>
>
>> Remember now, that lightning will take the lowest resistance path to
>> ground....make sure that, if your tower is the target, then the ground
>
> No, it doesn't... sorry, this is an old myth.

One strike ignored my tower, hit a transformer about 100 feet to the N 
up the street, and wiped out most of the electrical stuff in their house 
including the well with a submersible pump.  It also hit a pine tree 
directly across the road from me and to the back of that lot.  That tree 
literally exploded.

  If this were true then it
> would be easy to predict where lightning would go... it isn't.  lightning
> hitting a tower and running down encounters a cable going off the side, at
> that point the wavefront doesn't know what is at the end of that cable is a
> good ground, the energy divides between the tower and the cable regardless
> of what is at the bottom of the tower... it always has and always will,
> that's the laws of physics.  Now when the wave progresses to the ground and
> finds a nice low impedance that information (in the form of a reflected
> wave) travels back up the tower to where the cable left the side and THEN
> can reduce the available current going down that other cable, but what has
> started down the cable ain't turning back, its going for your shack and
> nothing is going to stop it!
>
> Another typical example... lightning hits tower, just above the great ground
> system at the base of the tower it jumps the air gap (a VERY high resistance
> path) over to a building next to it, goes through a glass window(a good
> insulator), and hits equipment inside that isn't even connected to
> power(sitting on an insulated work bench).
>
> Yes, there is lots of science behind lightning protection, but you also have
> to be aware that at the voltages and currents involved there are all sorts
> of unexpected things that can happen.  The better you plan for them the more
> likely you are to get through a hit unscathed.
>
>
>> In my set up, I have 14 ground rods driven into the ground 8 ft deep
>> under the concrete base of my tower.

My grounding network consists of over 600 feet of bare #2, cadwelded to 
32 or 33 ground rods forming a network from the front of the house to 
near the rear property line.  The shop, the house, and two towers tie 
into this system. The lines extend out toward each guy anchor post from 
the towers by about 80 feet. They also follow the conduit containing the 
coax to the entrance SPG with rods about every 16 feet.

>
> If I am to literally believe that you put 14 rods under the base of the
> tower that means you wasted at least 10 of them unless the base of your
> tower is HUGE!  Under most tower bases it would be impossible to get 14 rods
> far enough apart so that they are all being effective.
>
>
> Oh, one more while I'm at it:
>
>> copper braid that are cadwelded (exothermic) to the tower legs.  From
>
> Braid?? Cadwelded??  I always thought this was a no-no.  and who runs braid
> for a lightning or safety ground?  Don't all codes specify solid conductors?
> I wouldn't trust braid for an outdoor application like that anywhere, unless
> maybe you are talking about braid that uses something  like 10ga strands.
>

An effective tower leg connection is fanning out the strands in stranded 
cable and laying them parallel on the leg. Then clamping them to the leg 
with SS hose clamps and a little copper sheet.  Some use thin SS sheet 
along with a bit of Noalox, or copper based Never seize.(SP?)  I just 
used plain old compression connections.

I'd never want to use anything that would heat a tower leg hot enough to 
fuse it with anything.

73

Roger (K8RI)
>
> David Robbins K1TTT
> e-mail: mailto:k1ttt at arrl.net
> web: http://wiki.k1ttt.net
> AR-Cluster node: 145.69MHz or telnet://k1ttt.net
>
>
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