[TowerTalk] Static, Lightening, and protection
tower at rogerhalstead.com
Sun Mar 21 20:10:10 EST 2004
I've seen a lot of opinions, and different approaches to lightening and
lightening protection over the past few days.
I first want to point out that the effectiveness of lightening rods and
static wicks are as hotly debated within engineering circles as without.
So, you are going to find, so called experts on both sides of the fence. I
am in no way supporting or denying some specific device works or does not
work, or does not work as advertised. I will state that I have never worked
for a chemical company that didn't have lightening rods on top of every
tower and high spot within the fence line and I've worked for several large
We hear a lot about static electricity, lightening, and protection.
First, Lightening is a discharge of static electricity, just like the spark
from your finger to the door knob after walking across a carpet. They are
pretty much developed by the same kind of process which is friction removing
electrons from atoms.
The lightening strike occurs when a low enough resistance path is
established between two points be they cloud to ground or between two
One thing to remember is the lightening strike is between two *localized*
charge differentials. Generally the charge on the ground is spread of a
relatively wide area, but not nearly as wide as some believe. It can be as
small as a few hundred yards, or a path a couple miles long and only a few
hundred yards wide.
Any object in this area that "sticks up" such as trees, towers, and even
people are at a much greater risk of being struck than the over all area.
Some times only a few feet can make a big difference.
It can be, and has been shown using a Tesla coil, and Vandegraff generator
that some pretty hefty sparks can be developed. Add a couple Lenhide Jars
(sp?) and they and give you a poke you won't soon forget. It has also been
shown that a single *sharp* protrusion added to the static ball on top can
completely eliminate the *big* spark. A coronal discharge will develop
around the sharp point bleeding off the energy.
This is the premise behind lightening rods, static balls, and static wicks.
How well it works in real life is the question.
It should be noted that "Static wicks" are a requirement on most aircraft
certified in the last 20 or 30 years. Flying in a storm and having all your
nav radios go dead is an interesting experience to say the least. I had it
happen twice on the same flight. I had to shut off the electrical main and
then recycle everything. When my transponder code disappeared ATC was a bit
When you are getting the snot beat out of you due to turbulence and suddenly
lose all contact with the outside world except for the artificial horizon
and directional gyro it does raise your level of awareness.
OK, so we know lightening is nothing more than the release of a very
powerful "spark" of static electricity between two charged points which
equalizes the charge (more or less). Actually it is a repeated spark that
travels alternately back and fourth between the objects until it can no
longer maintain the potential to bridge the gap.
Protection comes in two phases. The first is the effort to eliminate, or
minimize the likelihood of an object being struck by lightening and the
other is how to handle (minimize) the results of a strike when it does
Lightening rods (single sharp point), or static balls (lots of sharp points)
will bleed off charge as a corona. It can be seen. The corona and spark do
not aid the development of a feeder. Nor is a tower with a lightening rod,
or static ball more likely to take a powerful hit than one without.
I'll leave the debate as to whether they actually diminish the likely hood
of a strike to the engineers who can't seem to agree as to whether they do
or they don't.
That moves us to the protection phase where we try to do something to
mitigate the damage when the tower is struck.
As I said earlier, lightening is a simple phenomena by itself, but its
actions are quite unpredictable and far reaching.
The strike itself raises some very complex issues. By its nature,
lightening has very steep rise and fall times giving some huge dv/dt and
di/dt figures. In plane language, the voltage and current change rapidly
The rapid voltage change develops a wide range of frequencies (static
crashes over a wide spectrum) while the rapid current change develops
tremendous magnetic fields. With the latter, virtually any wire in the area
becomes part of a huge transformer with strikes a mile away developing as
much as a 1000 volts per meter in a piece of wire.
So, we ground the tower, but that grounding system may be called on to
dissipate a huge current in a very short time.
A single ground rod, or even three at the base of a tower may give a good
electrical ground for safety, but not for lightening. Ground protection for
lightening needs to be spread out over a wide area. A large network of
ground rods and bare copper wire of sufficient gage to handle the current
can provide a very efficient path to ground for the lightening. A rule of
thumb would be to run the ground cables out as far as the guy anchors with
ground rods every 8 to 16 feet. I use the "every 16 feet", but I don't
believe there is a "magic figure".
The huge current spike through the tower can develop thousands of volts per
meter in near by wiring, conduit, pipes, or any metal objects such as
telephone and electrical lines. Hence the need to add transient protection
to wiring coming into the house. The same is true for the coax cables from
the tower. They may be grounded and the antennas my operate at DC ground,
but the cable itself may end up with thousands of volts to ground induced
due to the strike.
Sometimes the rise time for the current is so fast that the inductance of
the tower (which is a straight conductor) quenches the arc and the
lightening will jump off part way down. I've seen strikes arc out from the
side of a grounded tower several hundred feet above ground and not even
bother follow the guy lines.
So we move on to lightening arrestors such spark gap devices and gas
discharge tubes to take care of any small voltages that get into the cables.
We also add grounding and bypasses where the coax enters the home.
Most damage from lightening strikes is not from direct strikes, but from
induced voltages that come in on the phone lines or electrical service.
For the majority of stations the old adage about unplugging everything and
throwing the coax out the window is probably the safest, but it is not
always a viable solution for some stations. Even with the equipment
unplugged, a "super strike" within a few hundred feet could induce enough
voltages and currents within the equipment and interconnecting cables to
still cause damage very much like the Star Wars EMP.
As an added note, my tower gets hit roughly three times a year. Last time
(last Summer) my neighbor was standing in his back doorway looking right at
the tower when it took a direct hit.
He asked later how much damage I had. It told him none. He remarked, "so
you disconnect everything?" and I replied that most of the stuff was not
only hooked up but still turned on when the strike occurred. (I still
recommend the disconnect and throw the coax out the window approach though
if it's possible)
Roger Halstead (K8RI, EN73 & ARRL Life Member)
N833R, World's Oldest Debonair (S# CD-2)
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