A GFCI is really a pretty simple device. There is a small transformer in there (sensing coil) with the "hot" wound one way(top pair of green wires) and the "neutral" wound the other way (bottom pair of green wires. That is the primary. If the current is exactly the same they cancel, there is no flux in the core and the secondary winding has zero volts. Any difference shows up as a secondary voltage and the device trips.

[Linked Image from gfretwell.com]

Last edited by gfretwell; 10/13/10 02:31 AM.
Greg Fretwell