Do outlets within cabinets

[LarryFine]

i don't think it would be possible as current could be equal on both legs or some could be returning on neutral.

GFCIs pass all circuit conductors through a ring. As long as no current is leaking out of the circuit, no voltage is induced in the ring. If any current does leak out, a voltage is developed and detected.

A single-pole GFCI works the same way as a multi-pole unit does. In fact the load circuit need not even have a neutral conductor at all. The main requirement is that the supply has a grounded conductor.

 

[kwired]

GFCIs pass all circuit conductors through a ring. As long as no current is leaking out of the circuit, no voltage is induced in the ring. If any current does leak out, a voltage is developed and detected.

A single-pole GFCI works the same way as a multi-pole unit does. In fact the load circuit need not even have a neutral conductor at all. The main requirement is that the supply has a grounded conductor.

Grounded/grounding conductor not required for a GFCI to function. Though if not a grounded system there won't really be any leakage current to ground either, at least not until there is more than one fault on the system.