[TenTec] Another grounding question

Tue Aug 12 13:41:08 EDT 2003

I regret to write openly on the reflector about the following post but due
to the error in the statement, I feel compelled to do so.  The opening
statement is absolutely wrong!!  It is very dangerous to use this approach
to grounding.  The writer stated that he cut the cut the link with a bolt
cutter.  Keep in mind that the alleged lightning stroke that hit the tower
has already traveled through a few thousand feet of air.  A few inches or
even a few feet of open link is of no extended value for further protection.
It is however an invitation to big trouble.

If every ground system is tied together the voltage across the ground points
is nearly zero volts due to the low resistance, typically less than 0.1
ohms, provided by the common bond between the points.  On the other hand, if
grounds are not connected, the resistance of the earth between any two
points is several ohms to several hundred ohms.  It must be understood that
lightning is a current source of extremely wide frequency coverage.
Actually it is a series of pulses in a single stroke.  These pulses have a
very sharp rise time and a slow decay time thus the wide RF spectrum of
coverage.  The values of ground system inductance, ideally very low thus the
reason for large flat connectors, and resistance of low R materials, of a
ground system contribute to the overall efficiency of the system.

Case in point.  Take two values of R being a low value typical of ground
points bonded together and two values of R such as two or more driven
grounds in the earth.  Pick a value of current in amps and calculate the
voltage drop across the values of R in both cases.  Now decide if you want
two grounds bonded together or two separate grounds.  The voltage between
the two example systems should tell the whole story.  Low voltage, little
damage.  High voltage, lots of damage.

Now, about direct strikes.  It is virtually impossible to protect from a
direct lightning strike.  The total effort with regard to lightning
protection is focused on protection from "nearby strikes" and prevention of
direct stroke build-up.  These are the ones that cause more than 90% or more
of the damage and these are the ones that the population is least protected
from.

A better understanding of lightning and the effects are best found on the
"Lightning Safety Institute" website.

For additional reading, please see the following:
http://www.amwindow.org/tech/htm/lightning.htm

Also please see the references at the end of the paper.

73
Bob, K4TAX

----- Original Message -----
From: "K Van Horn" <W7SV at sbcglobal.net>
To: <tentec at contesting.com>
Sent: Tuesday, August 12, 2003 10:59 AM
Subject: Re: [TenTec] Another grounding question

> Dear Ed and Janet,
>
> Do not EVER tie them together! When lightning strikes your tower, and it
> WILL, you will have upwards of 100000 volts destroying every electrical
> device in your home from the ground lead of your electrical system during
> the 15 -50 milliseconds that it takes for the voltage to bleed off into
the
> ground system!! Many textbooks recommend tieing these together, but they
are
> in ERROR!! Save yourself a lot of money and more trouble than you can
> imagine!
>
> If you tie them together and you take a strike, the following is a list of
> what you can expect to replace:
> (even if they are turned off at the time of the strike because the voltage
> is so high that it jumps the switches)
>
> Air Conditioner (inside and outside unit)
> Refrigerator
> Dishwasher
> Oven
> Microwave
> Television sets
> Amplifiers
> Radios
> Computers
> Can Openers
> Clock Radios
> Light Bulbs
> Doorbells
> Stereo Amplifier and Receiver
> Garage door opener
> Security System
>
> All in all, over $20,000 in damage on two strikes 6 days apart. Whereupon
I
> realized the error and cut the link out with a boltcutter and solved the
> problem!
>
> Sincerely,
>
> Kirby
> ----- Original Message -----
> From: "Ed and Janet" <janed at burgoyne.com>
> To: <tentec at contesting.com>
> Sent: Monday, August 11, 2003 12:37 PM
> Subject: [TenTec] Another grounding question
>
>
> What is the thinking on grounding the tower ground to the ground coming
into
> the house at the power, 220 service entrance?  Tie them together or not?
Ed
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