Barry Cooper said: (much skipped)

 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.
>

Good point. People are talking here about systems that are "referenced to
earth ground" or "isolated from earth ground" as if those were absolutes.
Given that it takes only a few milliamperes of current to cause a fatal
shock, it should be recognized that power sources are ALWAYS referenced to
earth ground, even if it's not intentional and not significant from the
standpoint of power usage. The best you could do, if the impedance of the
system was balanced, would be to have each conductor at half the system
potential with respect to earth ground. But that says nothing about what
the impedance to earth ground from each of those points might be, and for
almost any imaginable real-world hardware, it's probably quite low. Let's
say your 48VDC system is balanced about earth ground; the max voltage will
be 24VDC. Is the impedance from either conductor to earth low enough to
pump several milliamperes into a diver? A diver wearing a wet-suit already
has a full-body connection to earth ground. So the current will be limited
mostly by voltage and the afore-mentioned impedance.

Maybe one of the new 8-pin PICs with A/D could be the basis of an
inexpensive GFI unit?

Reg Neale