M Walter wrote:
>
> >
> >Early GFI circuits used to use TWO toroid coils with the current carrying
> >conductors run thru the cores.  One coil had a high frequency oscillator
> >driving it, the other was the pickup coil.  An inbalance in the current
> >carrying conductors coupled energy from one toroid to the other.  As far as
> >I know, that system worked down to DC.
> >
> >From what I remember the GFI circuits utilize AC only. One of the toroids
> is double wound: one winding is the hot lead, the other is the neutral. The
> windings are set to cancel out each other: if the same current is flowing
> in each line then there is no magnetic field. A third winding on the toroid
> senses the inbalance, and generates a signal only if there is a net
> maganetic field. So if there is voltage on the sense winding it means that
> there is a magnetic field in the toroid produced by an inbalance in the
> currents. This current inbalance is due to differances in the hot and
> neutral line currents caused by a ground fault.
>
> The second toroid is there to detect a I believe a neutral to ground short.
>
> Look into a old Raytheon data book; they make a number of GFCI chips and
> have a nice write up. I believe that a different method will have to be
> used in the diver suit GFCI since it is DC.

  Possibility:  Does anything stop you from using 48VDC through a couple
chokes & then addding in an AC Bias, so you can detect the GFI leakage
of the AC component only?  Obviously, if you have a 1 VAC Bias voltage,
you'll only have 1/48 of the leakage be AC.  Still possible, though.

  Or could you PWM the 48 VDC, possibly?

  Mark