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@arrl.net
web: http://wiki.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://k1ttt.net
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