[time-nuts] Lightning and grounds...

[phil]

----- 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 
>> only
>> 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 
> dish
> 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 
lightening
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 
cables.

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.

Phil