GRAEME SMITH                         email: grysmith@freenet.edmonton.ab.ca
YMCA Edmonton

Address has changed with little warning!
(I moved across the hall! :) )

Email will remain constant... at least for now.


On Sun, 19 Sep 1999, Sean H. Breheny 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
>
Most grounds today, are taken, (at least in Canada) from the copper Water
pipes that feed the hotwater heater.

This creates a massive ground plane, in that the pipe, the water, and any
soil that comes in contact with the pipe, become the ground mass.

The, I think they recommend 30 ft long copper bar, used for grounding, is
only a second best bet. I think the idea here, is to burry the end so
deep, that it gets imersed in the ground water, which increases the ground
mass significantly. Remember the only reason for a ground to exist at all,
is to have a reference voltage against which to measure other voltages.
When we talk about the ground or earth connection we are really talking
about a connection to a source that is normalized to the ground, and
therefore has a voltage of 0 volts with respect to ground.

We use this for safety, under the mistaken assumption that any connection
that has a 0 volt value with respect to ground, will also have zero
current unless connected to a larger voltage differential source. This of
course begs the question of whether a 0 volt with respect to ground value,
at one end of a long wire is the same as a 0 volt with respect to ground
value at the other end of the wire. (Which is one reason why we use signal
grounds, that are not the same as chassis grounds.)

Anyway, the primary way that a GROUND or EARTH connection works, is to
provide as low as possible resistance to ambient voltage with respect to
the earth surface at the point of measurement. Whether we do this by
hooking up to a common water pipe, or by sinking a long copper connector
into the ground, doesn't really matter, except for the voltage
differential that MIGHT exist between the real ground, and the wire
because of the resistance.

                                GREY