[time-nuts] Lightning and grounds...
fortime at bellsouth.net
Mon Oct 5 00:27:42 UTC 2009
----- Original Message -----
From: "Chuck Harris" <cfharris at erols.com>
To: "Discussion of precise time and frequency measurement"
<time-nuts at febo.com>
Sent: Sunday, October 04, 2009 7:24 PM
Subject: Re: [time-nuts] Lightning and grounds...
> Don Latham wrote:
>> Use of "brush" or radioactive lightning rods, as has been pointed out, is
>> pointless and even dangerous. If you feel you have to experiment with
>> such, at least just use a sort of ball of barbed wire rather than paying
>> someone a small fortune for junk. Any "brush discharge" lightning rod
>> serves to protect itself, and not even that in the presence of wind.
> Curious, a quick tour of Johns Hopkins Applied Physics Lab's satellite
> farm finds many instances of brush style lightning rods...
> They must all be wrong too!
> -Chuck Harris
A prior poster said:
"A number of the brushes are used to protect an area. If you want to attract
replace the bottle brush with a metal sphere."
That's a wives tale. Some use that analogy to sell their wares but it doesn't
hold water. It has been said, if it's going to strike, it's going to strike
so just be ready for it. Cloud to ground lightning usually starts with an
upward leading streamer, which can originate from a sharp blade of moist
grass, a point on a leaf, or a "sharp metal object". You would increase the
likelihood of lightning striking with your sharp points as opposed to a
rounded tip lightning rod.
I get direct hits on average of 3 or 6 times a year and have no damage other
than pitted lightning rods; we even have a strike counter on the metalwork
verifying strikes. Allot of the people on this board have a fairly good
grasp while some with excellent knowledge on lightning protection. In
reality, the physics of lightning protection is rather simple, yet I think
some of the PhD's that write on the subject still think lightning comes from
the Greek god "Zeus" as some of the papers contain allot of myths.
Strikes can deliver current anywhere from 30 to 300 thousand amps with
perhaps 90 plus percent, in the 30 to 60 thousand-amp range. I say that
because the size of wire/cable/metalwork that carries that charge around
your building/equipment from lightning rod to ground rod needs to be large
enough not to explode should you get that mother of all strikes. The
absolute most important thing in protection is a single point ground. The
ground rods or buried copper is important to lightning protection, but not
as much to the protection of your equipment as single point grounding. IF
you keep your sensors, building, and all equipment at the same potential, it
has the same effect as a Faraday cage. Everything will elevate in voltage
during a strike, but also falls in voltage together.
Look at the simple ohms law to calculate differences in voltages on a single
ground wire. Most suggest a #4 copper wire for grounding. With 100 feet
having a "dc" resistance of 0.02533 and a 60,000-amp strike, that's a
difference of 1560 volts from one end to the other. Even using a 2/0 copper
for 100 feet with that same strike gives you 462 volts difference. Now
elevate that strike to say 150,000 amps and that same drop/difference on the
#4 is 3800 volts and the #2/0 is 1155 volts. You defiantly don't want that
potential/difference between a grounded chassis to a data input cable.
Granted, the resistance varies with frequency but that huge raw current is
the killer. Typically your ground rods will lave a better lower impedance
ground than dc ground.
Cumulonimbus clouds can reach 40,000 to 60,000 feet, should you get "a bolt
out of the blue", one originating from the upper parts of that cloud, you
can expect the higher currents exceeding 100,000 amps. That's also the bolt
that destroys/explodes most in it's path.
The other main "got-ya" is the buildings interface to the utilities, power,
phone, and possibly wires/cables to outbuildings, including underground
Electromagnetic fields caused by direct or even nearby lightning strikes can
also induce surge voltages in the antenna and above ground cables as well as
underground cables. This EMF can damage electronics in the antennas or other
or electronics in the building. These voltages can be high, but are of such
short duration that diodes and gas tubes can virtually eliminate that damage
too. This protection needs to be applied not only to antenna cables, but all
wires entering or leaving the building above and below ground.
If a building/object/instrument is not first protected from the excessive
current that can literally blow something up like a stick of dynamite,
concern over EMF is rather immaterial, that is addressed only after you
solve the high current issue. Lightning protection is a system, no single
cause or cure. With proper design, and use of various suppression devices,
lightning damage can be virtually eliminated.
The original poster made reference to his GPS antenna being mounted on top
of his TV antenna. That sounds like it is the highest part of his
installation and could take the brunt of a possible strike. The GPS antenna
doesn't need to be elevated unless something locally obstructs its view of
the sky. Most manufactures recommend the antenna be mounted away from
lightning rods, not be mounted on the same mast that lightning may strike,
yet under the "zone" of protection of the lightning rod.
My HP/Symmetricom gps antennas are some 50 feet from the base of the various
masts that hold other antennas/sensors/lightning rods and mounted only 3
feet above a flat roof. With 5 years of strikes, everything is still going
strong. I'm located 50 foot away from a mountain ridge some 10-foot less in
elevation; however, various antennas and weather sensors are forty feet high
or still 30 foot higher than the ridge.
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