>
>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.