At 23:21 19/09/99 -0400, you wrote:
>At 08:01 PM 9/19/99 -0700, you wrote:
>>Recent threads on connectors and soldering and such brought this to mind...
>>Heard this story from a Brit I know......
>>
>
>ROTFL!!
>
>Speaking of ground rods: does anyone have a good explanation of why they
>work? I have been trying to understand for the longest while how a 6' rod
>stuck into the ground can provide a suitable return. Soil conductivity,from
>what I could find on the web,doesn't get much better than 400
>mS/meter.So,if my calculations are right, even placing a 1 meter sphere
>totally under ground would still give you a resistance of about 1 ohm per
>meter (distance between the two ground points that are involved in the
>return path,assuming the other point was also a 1 meter sphere). So, if you
>were,say, 1km from your substation,GND would give you 1 1kohm resistance,
>still pretty high! Even if GND were as close as the nearest power
>pole,we're still talking tens of ohms.
>
>Sean
>

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.

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.

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.

Dennis

>
>|
>| Sean Breheny
>| Amateur Radio Callsign: KA3YXM
>| Electrical Engineering Student
>\--------------=----------------
>Save lives, please look at http://www.all.org
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>
>