Hi Dennis,

Thanks for the answer:

At 02:57 PM 9/20/99 +1000, you wrote:
>If you look at the ground stake you will find that the resistnace created
>is hemispherical is shape, thus a 6" stake provides you with a reference
>point that is 6" deep and 12" in diameter. Conduction will vary appon
>constuction of the surrounding area and other metalic objects in the
>ground. You will find that taking an earth measurment, is actually like
>taking a hemispherical sample.

I don't quite understand - How can resistance have shape? Do you mean that
is is equivalent to a 12 foot diameter conductive hemisphere? The way I did
my (rough) calculations was to assume that the ground path consisted of two
ground connections,which formed a lossy capacitor with the soil as the
(resistive) dielectric. By integrating the product of conductivity times E
field over a sphere surrounding one of the two ground points, I got the
total current that comes from it. The applied voltage divided by this
current gave me the resistance. I used a value of about 300 mS/meter for my
conductivity(I took this from a soil profile on a web page,it was the peak
conductivity value)

>
>The substation earth is very different in construction, you will find that
>it will have a resistance of less than 0.01 ohms with respect to a remote
>earth (Nominaly tested 100 meters away in more than one direction from the
>source). You seem to be speaking of a reference earth, nominaly not ment to
>carry current, just provide a dump point in case of a fault condition. The
>single wire earth retun circuits are rare due to the climatic changes that
>effect the earth point.

Are you saying that the resistance between ground points (rods or whatever
they are) is independent of the distance between them? If so, my concept of
the conductivity must be incorrect. In addition, it seems to me that the
resistance from ground point to ground point depends on the structure of
the grounds at BOTH locations. If one is a tiny rod,making the other huge
won't make all that much difference.

I was thinking of a typical 6' saftey grounding rod,yes. However, I think
my argument is still valid. The ground rod at my house is supposed to give
a good enough ground to trip a circuit breaker in the event that something
shorts to a grounded case,right? This would meant no higher than a 120/15 =
8 ohm total path resistance from it to the substation or other relevant
ground point.

>
>In the point of telephones it is different again. Anyone know why ring is
>90VRMS? Well it is because of earth, the idea is that ring will always get
>to the phone and provide an indication to the subscriber that a call is
>emminent, even if the reistance is so high that off hook can not be detected.

Yes,this makes a good deal of sense.

>
>Dennis
>

Sean

|
| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
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