On Tue, 4 May 2004 11:32:42 EDT, Cqtestk4xs@aol.com wrote:
>Could it be that the low resistance required is to
>allow the tower to bleed off the charge easily and thereby reducing the chance
>of a direct hit? Is there anybody out there who is more of a tech than I am,
>who would know for sure?
Research published by IEEE says that the energy content in lightning has a
broad peak around the middle of the AM
broadcast band. Simple circuit analysis says that the highest priority should
be given to achieving a low INDUCTANCE
path AND a low resistance for that energy to be discharged as quickly as
possible so that the voltage on your system
wiring doesn't build any higher than need be. Inductance is reduced by
providing multiple, parallel, paths to earth, and
keeping all of them as short as possible. Resistance is mostly reduced by
getting better contact with mother earth.
To understand this, remember that one way lightning protection works is to
offer a good discharge path for energy at
moderate levels before it can achieve the higher levels that does the damage.
And also remember that there is lots of resistance between ground electrodes
driven into the earth at different points.
As others have noted, it is also wise to bond those multiple paths together
OUTSIDE your premises. The last thing you
want is high potential differences across wiring within your building!
I concur with W8JI that when possible, all grounds, including the power system
ground, should be bonded at a
common point outside the building.
Also remember that with all of this, we are simply playing the percentages and
trying to put them as much in our favor as
possible by good engineering. Because of the huge potential energy in a
ligthing strike, any given storm condition can
cause a lightning strike with damage, despite our best implementation of good
practice. That good engineering can
raise the odds in our favor that we survive without damage, or experience less
It is far too simplistic to say that a lightning strike has enough energy to
jump around whatever obstacle is in its path.
We can reduce the energy remaining in a strike by giving it a better and faster
means of discharge than through our
equipment (or house). It's all Kirchoff's law (with XL in the equation) for the
division of current in parallel circuits, and
Ohm's law for the voltage across an impedance due to current flow.
Jim Brown K9YC
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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