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re: antenna rig and Lightning



>    To John and group,
>I have been following the lightning discussion with interest. I would like to know just
>how to go about the ground resistance measurements?
>                                                                   Thanks,
>                                                                    Tom, N9RPQ


It takes special equipment to do this.  The two earth resistivity testers I am
familiar with are the Vibroground from Associated Research and the Megger Earth
Tester from Biddle Instruments.  Both these companies supply test equipment
to the power industry.

These are classified as four terminal test sets and have two current leads and
two potential leads.

A typical test for an earth electrode would be to run a current and a potential lead
to the earth electrode (C1 and P1) and run a current lead (C2) to a test probe
75 to 100 ft away.  The distance of C2 is governed by the size of the earth
electrode, C2 must outside the sphere of influence of the earth electrode, a point
where any added earth does effect the resistance of the earth electrode.
A potential lead (P2) is hooked to a probe driven at 62% of the C2 distance.

The test equipment has a vibrator that switches the polarity of a 12 volt
battery across the primary of a step up transformer producing a 90 hertz
square wave which is applied to the test setup above.  The device measures
the current flowing into the ground on C2 and the voltage drop to the P2 probe and
calculates the resistivity of the ground electrode.

The four point system ensures that the resistance of the test leads are not included
in the measurement.  The use of 90 cycle square wave reduces interference
from 60 cycle leakage in the ground.

You might find more info at:  http://www.avointl.com/products/groundtster/index.html
The book they have for $5 "Getting Down To Earth" includes a broad overview of 
grounding in general and pretty good explanations of different test methods used in
the utilities industry.  I received a few of these at an AVO training seminar and passed
them around our shop, it helped our guys get a better grip on what our test equipment
actually does.

-Mike McCarty, KB8YHV
 Yigo, Guam
 


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