In , w_tom wrote:
Unfortunately Frank Gilliland has exaggerated his numbers
due to insufficient experience and too much time listening to
myths. His numbers will be exposed as fiction.
Field experience says repeatedly that antenna and radio can
suffer direct strikes without damage. That is proven about 25
times every year atop Empire State Building since the 1930s.
According to Frank, they must suffer damage 25 times per
year.
CB radio antennas are not commercial station towers. The latter are verticals
that have a direct connection to ground and the ground radials. Actually, an AM
broadcast tower is almost a perfect lightning rod by design because it not only
shunts the lightning directly to ground, but also distributes the power from the
strike over the whole counterpoise field. So the tower stays at a relatively low
potential even during a direct strike. And what -does- manage to sneak onto the
line has to deal with some rather expensive protection devices. Antennas mounted
seperately on towers (FM/TV BC, cell, commercial, etc) have the same problems as
any other antenna, but those problems are usually minimized by the use of coax.
More below.
Let's start with his numbers.
Millions of volts? Yes. But same voltage does not appear
everywhere in a circuit - basic circuit theory. Those
millions of voltage are in the sky. Surge protection is about
making those millions of voltage appear elsewhere which is why
industry professionals discuss impedance. A low impedance
connection to earth means no millions of volts.
A low impedance ground is fine for AC line protection, but it doesn't guarantee
lightning protection. We have all heard that lightning takes the shortest path
to ground, but that's not really true since electricity will take EVERY path to
ground available. Lightning creates it's own conduit from the clouds, but once
it hits a conductor on the ground it behaves just like any other form of
electricity -- almost. The fact is that wire has resistance, and the resistance
of copper increases with temperature, which is what happens when it passes the
current from a lightning strike. When that happens it will continue it's path to
ground (assuming the wire doesn't fuse), but other paths will share more of the
load. And because there is a resistance, there will also be a voltage potential
across that resistance. If that voltage potential is high enough it will happily
arc over to another ground path, and frequently does. More below.
Millions of amps? Only in dreams. Most lightning is below
20,000 amps and of such short duration as to not be high
energy. Lightning typically so low energy at the strike
location (not to be confused with what is miles above) that
well over 90% of all trees struck leave no indication of that
strike.
Let's take your figure of 20,000.... no, let's go even lower. Let's say only
1000 amps @ 1,000,000 volts. And let's say this is an unusual strike in that it
only hits once, not multiple times like a normal strike. And let's say the
duration of the hit is 1/10 of a second. This will be a pathetic bolt of
lightning to be sure! Ok, so let's do some numbers:
1,000,000 Volts x 1000 Amps = 1,000,000,000 Watts
1,000,000,000 Watts x 0.1 sec = 1,000,000 Watt/sec
One million joules is "low energy"? Get a grip.
Trees struck by lightning usually -do- leave an indication of being struck, but
most people don't climb them to search for the point of contact, which is
typically nothing more than a spot about one or two cm in diameter that has been
charred. And while the reason trees are able to survive direct lightning strikes
is still the subject of debate, the reason they make good lightning rods
(efficiently conducting the strike to ground) shouldn't be so suprising when you
take a look at a cross-section of the root structure -- interesting how it
resembles an electrical discharge, isn't it?
Ok, back to your low impedance ground. A ground rod is used to make an
electrical connection to the earth. But the impedance of that connection can be
anywhere from a few ohms to a few hundred ohms, depending on the type of rod and
the conditions of the soil. Let's just say we have a ground with an unbelievable
impedance of 1 ohm (a solid-silver rod in a heavily mineralized salt-water marsh
that was recently used for dumping copper turnings from a very poorly run
machine shop).....
1000 amps x 1 ohm = 1000 volts
So with an almost impossibly good ground and a puny bolt of lightning you
-still- have 1000 volts at the top of your ground rod. So a more typical ground
impedance of 50 ohms (not coincidence) and a more typical lightning strike of
10,000 amps will put 500,000 volts on your grounding strap.....YIKES!!!!! This
is a fact, and it certainly doesn't seem to jibe with your statement that the
voltage at the bottom is insignificant!
How big need a wire be to shunt (earth) lightning? Even the
US Army training manual TM5-690 requires 10 AWG wire to
conduct the direct lightning strike without damage.
Ever hear the term "military intelligence"?
Same wire
found in 20 or 30 amp AC electric boxes because lightning is
not the millions of amps so often claimed in urban myths.
Unlike Frank, numbers are provided by multiple, reliable
sources.
The ground wire in house wiring is intended for fault protection, not lightning
strikes. For example, if the hot wire in your vintage all-metal Craftsman drill
suddenly comes loose and shorts to the case, since the case is grounded it will
shunt the majority of the current to ground through the ground wire, not through
the person using the drill. And if your breakers and wiring are up to code
(neutral grounded at the box), that current lasts only for a very short time,
limiting any damage to the person and the drill. Therefore, the ground wire in
your house doesn't need to be as thick as the main wires, and it isn't. Next
time you visit your local hardware store, look at the specs on a spool of house
wire -- hot and neutral may be #10 while ground will be #12. Another spool may
have a pair of #12 wires and #14 for ground. If this ground wire was intended
for lightning protection, wouldn't it all be the same size? Fact: the NEC
doesn't define ground wire size based on it's ability (or inability) to protect
against lightning.
Another who does this for a living:
From Colin Baliss "Transmission & Distribution Electrical
Engineering":
Although lightning strikes have impressive voltage and current values
(typically hundreds to thousands of kV and 10-100 kA) the energy
content of the discharge is relatively low ...
Relative to what?
or Martin A Uman in All About Lightning
Most of the energy available to the lightning is converted along
the lightning channel to thunder, heat, light, and radio waves,
leaving only a fraction available at the channel base for
immediate use or storage.
Then I guess all the people that have been killed by lightning didn't die from
the power in the lightning, did they? And all the damage to electrical equipment
caused by lightning wasn't from the lightning at all, was it? And that pro
golfer that was knocked flat on the links by a nearby strike must have been hit
in the head with a ball at the exact same time, huh? No, no and no.....
The power of a bolt of lightning isn't the big issue here since it doesn't take
much power to cause damage. The issue is how well you are protected from
whatever amount of power that -does- make it to the surface.
In short, Frank Gilliland's numbers are classic myths.
Pre WWII ham radio operators demonstrated what was required
for protection. First they would disconnect antenna and still
suffer damage. Then placed antenna lead in a mason jar, and
still suffered damage. But when antenna was connected to
earth ground, then no damage. Neither a mason jar nor "one
of those big blade switches" sufficiently blocks destructive
transients. Of course not. Lightning was not blocked by
miles of air. Is a mason jar or knife switch to do what miles
of air could not?
You bet it will!
Suppose you use one of those basic air-gap devices. Ok, you have 10,000 amps
passing through a gap that now consists of plasma. But even plasma has
resistance and will develop a considerable voltage across it. Plus, there is the
brief voltage potential that exists across the gap immediately before the plasma
is ignited, as well as after it is extinguished. These are the voltages that
will be developed across the conductors of the transmission line going into the
shack. How high is that voltage? It can peak at several thousand volts, and the
arc itself can develop several hundred volts across the points. That's enough to
fry a radio. Even if the impedance is high coming into the shack, it will still
destroy any overload protection in the radio, making it vulnerable to more
serious damage from nothing more than a surge or static discharge.
The coax doesn't offer much protection by itself, but it can limit the maximum
voltage entering the shack (or coming down the tower from the antenna). Every
type of coax has a core insulation that is rated for a certain breakdown, or
'pucture' voltage. RG-58 is rated for 1900 volts RMS, or roughly 2800 volts
peak. That means the coax is going to permit 2800 volts into the shack before it
fails and starts arcing internally. That's still enough to fry a radio. And
that's assuming the lightning hits the antenna and not the coax. If it hits the
coax, that's a different game altogether.
There are other factors that can put large voltages on the coax, since inductive
and capacitive reactances of a ground system, while small, can become very
significant when the current is on the order of 10,000 amps. We can take that
route too if you want.
The fact is that simply sinking a ground rod is NOT ENOUGH. A large blade switch
will easily block any voltage that can make it through the coax. Additionally,
it will also protect the coax by shorting it, thereby preventing a voltage high
enough to cause the insulation to fail. This is a method that has been proven
time and time again since the beginning of radio.
Of course not. For no damage, provide the
destructive transient what it wants - earth ground.
zeeeeeeee's antenna installation is demonstrated by a
figure in TN CR 002 The Need for Coordinated Protection
(corrected URL)
http://www.erico.com/public/library/...es/tncr002.pdf
"Equipotential Earth Bonding" = ground loop = bad news when using unbalanced
transmission line (coax). Whoever made that design has no experience with radio
communication systems, which is especially evident because most commercial
stations bury their coax, affording a level of protection far superior to the
design in your reference (....gawd I hate pdf's!).
Need anyone suffer damage from direct lightning? Of course
not. Such damage is considered a human failure because proper
earthing is so effective and so inexpensive. Another
professional who makes that point in direct contradiction to
posted myths:
http://www.harvardrepeater.org/news/lightning.html
Well I assert, from personal and broadcast experience spanning
30 years, that you can design a system that will handle *direct
lightning strikes* on a routine basis. It takes some planning
and careful layout, but it's not hard, nor is it overly
expensive. At WXIA-TV, my other job, we take direct lightning
strikes nearly every time there's a thunderstorm. Our downtime
from such strikes is almost non-existant. The last time we went
down from a strike, it was due to a strike on the power company's
lines knocking *them* out, ...
Since my disasterous strike, I've been campaigning vigorously to
educate amateurs that you *can* avoid damage from direct strikes.
The belief that there's no protection from direct strike damage
is *myth*. ...
The keys to effective lightning protection are surprisingly
simple, and surprisingly less than obvious. Of course you *must*
have a single point ground system that eliminates all ground
loops. And you must present a low *impedance* path for the
energy to go. That's most generally a low *inductance* path
rather than just a low ohm DC path.
Important point. This professional did not say
'resistance'. He said 'impedance' which is why wire length is
so critical. 'Impedance' is why an incoming wire (antenna,
CATV, telephone) must first drop down to make a short
connection to earth before rising up to enter a building.
Just one of the "careful layout" techniques learned from
underlying theory tempered by decades of experience.
Although your statement is inconsistent with your pdf reference, the reason coax
is sometimes physically located at ground level at the entrance to the shack is
because of the differences between RF ground and DC ground, -not- because of the
effect of inductive and/or capacitive reactance in a lightning strike. If that
were the case then the antenna system would need to be 'tuned' for lightning,
which is nearly impossible since lightning has no fixed frequency.
zeeeeeeee's tower requires earthing to meet human safety
requirements of National Electrical Code AND to provide
transistor safety. Earthing required twice over. Once
properly earthed, then even unplugging for protection would be
unnecessary - as has been demonstrated too many times at too
many locations since before WWII.
It's obvious that you have no experience in the real world with lightning
damage. Get some.
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