On Fri, 23 Jun 1995, M Walter wrote:

> At 03:16 AM 06/23/98 -0400, you wrote:
>
> >  This seems a little complex to me. I would sample the voltage off both
> >pos and neg to earth and if the voltage is not equal (or roughly so) you
> >have an earth or partial earth. You would have to determine how much
> >leakage is acceptable. If you are in saltwater any leaky insulation will
> >show up real quick!
> >
> >        Barry
> Barry: I think you have a much more fundimental(?) problem, and that is the
> idea of leakage to ground period. The reason the AC line voltage GFCI's
> were developed is because one of the the current carrying path is the
> ground. The AC system ties the return lead to ground. As a result, if a
> grounded person should touch a conductor at line potential he/she will
> receive a shock shince their body is across the line potential.  Some
> (small) amount of current flows out of the line thru the person and then to
> ground. This is refered to as a ground fault, and is the whole reason for
> GFCI circuits. The GFCI measures the current goint from one conductor
> (line) to the load and compares it to the current comming back from the
> load into the neutral line.
>
> If a system did not have a grounded line then there would be no problem
> with a person (grounded) souching the line. This is one technique used to
> prevent shock hazards: isolate the system from ground by using a
> transformer to remove the ground connection. Of course a person still
> cannot grab one conductor in one hand and touch the other. Your DC system
> seems to fit into this category: there is no connection to ground (or water
> in this case) so a GFCI won't work.

I have another one of my "maybe there is something that I don't see here,
BUT" questions <G>

If a person standing on land (forget the diver's situation for a minute)
with rubber (or synthetic) soled shoes on touches a wire which is at 120V
relative to ground AND TOUCHES NOTHING  ELSE, shouldn't the shoes
insulate them so that only a negligible amount of current would flow?
(unless there was some sweat or other solution bridging the sole of the shoe)

The real problem seems to me to be the situation where someone is also
touching the case or some other grounded object at the same time as they
touch the hot wire. I realize that this situation is quite prevalent.

Am I wrong so far?

Another quick question: what is the resistivity of typical soil? I never
bothered to really figure it out or ask anyone before, but how well do
ground rods really allow a ground based current path? For example, If I
put a 6foot rod into the ground at one spot and another some distance
away and measure the resistance between them, what range could I expect?
Where I live, the ground is very rocky, so I have had trouble
actually trying anything like this :)

Thanks in advance for anyone's answer and putting up with this basic
question!

Sean


>
>
> Mark Walter
>