Re: [IRCA] Measuring "AC resistance" of ground rods to earth

[nhp <Nick_Hall-Patch@xxxxxxxxx>]

At 21:04 17/07/2007, you wrote:

>  -------------- Original message ----------------------
>From: Nick Hall-Patch <nhp@xxxxxxxx>
>.  Most of the traditional
> > methods use power line frequencies; however, I've used the three
> > point method using an AC generator at various frequencies up to 5
> > MHz, and ended up getting really confused with the results
> > (decreasing conductivity with frequency....).   Ran out of time, and
> > never went back to it.
> >
>Nick
>
>Did you mean decreasing conductivity with increasing frequency?  If 
>so, that's the way it's supposed to work.  Otherwise, something 
>weird happened if conductivity decreased at lower frequencies.

Umm, bad phrasing there on my part, which implied the measurement of 
ground conductivity.

If I recall correctly, what the three point method measures is the 
resistance between a ground rod and the earth, and with more 
conductive soil, that resistance would be lower.  Conductivity and 
resistance are DC measurements, and the results using the three point 
method and a DC source were about as expected (multiple rods gave a 
low resistance, a single 8" rod gave high resistance)

Using AC you'd actually be measuring impedance I would guess.   I 
believe there's capacitive reactance in parallel with that resistance 
between the rod and the earth, and I'd expect that reactance to 
become lower as the frequency went up, and the overall impedance would drop.

Instead, I saw the "impedance" from rod to earth go up with 
increasing frequency, so I must be misunderstanding something, or 
calculating incorrectly.

If you know of any published results for a test like this, please 
refer me to it, so I can see where things went wrong (if they 
did).   Now to find the spare time to follow up on it.

best wishes,

Nick





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