Cool ap, Mike.

I suggest looking at the Hall effect app notes from Allegro.  I think you
should make a hall sensor to do the GFI.  Picture a split toroid core, with
the hall sensor inbetween (probably the analog out version).  Wind the 48VDC
source and return around the core in OPPOSITE directions.  The flux will
then cancel itself out at any current.  You will also achieve galvanic
isolation.  Trim up the analog output, add a zero offset (it will not be
zeroed on initial assembly).  Make sure you add some sort of time delay to
the device, I am sure you might get false trips if you look for any
instantaneous output value.

Montaigne, Mike wrote:

> We are doing a project using a PIC for temperature control at the diver
> location and another PIC at the surface for status.  I would like to add