At 06:43 PM 23.6.95 -0400, you 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.

  In the systems I work with (navy shipboard electrical) we use what are
called above ground systems. The frames of all the metal electrical
equipment are bonded to a common ground. NONE of the current conducters are
connected to ground. The power system isolation is tested by connecting
lights to ground. If one of the lights goes out or is dimmer to the 2 on
the other phases you have a ground fault. (in the Canadian navy called an
earth) You then have to track it down before another phases goes to ground.
(usually accompanied by a loud bang and the smell of smoke.(g)

>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.
>

  If you measure the voltage on both legs to ground at the same time you
will find they each drop half the line voltage. If one side goes to ground
the potential between it and ground disappears. Ergo you have a ground
fault. (works on 24 volt should work on 48 volt).

  BTW, most heavy industrial electrical systems, at least in Canada,
operate with an above ground system. This allows you to keep operating the
equipment even with a fault. You can track the fault and repair it without
most of the accompanying smoke and fire. (I love the smell of electrical
varnish in the morning). It does leave you with a potential problem. Your
entire plant is now at line potential to the other 2 phases, which is why
residential systems have one grounded leg. You KNOW which on is at ground
potential and which one is live. It won't booby trap you.

        Barry