How does a GFCI work:
GFCIs typically test for the following condition:
* A Hot to Ground (safety/earth) fault. Current flows from the Hot wire to Ground bypassing the Neutral. This is the test that is most critical for safety.
* A Grounded Neutral fault. Due to miswiring or a short circuit, the N and G wires are connected by a low resistance path downstream of the GFCI. In this case, the GFCI will trip as soon as power is applied even if nothing is connected to its protected (load) circuit.
To detect a Hot to Ground fault, both current carrying wires pass through the core of a sense coil (transformer). When the currents are equal and opposite, there is no output from its multiturn sense voltage winding. When an imbalance occurs, an output signal is produced. When this exceeds a threshold, a circuit breaker inside the GFCI is tripped.
To detect a Neutral to Ground fault there is a second transformer (left toroid in the illustration below) placed upstream of the H-G sense transformer (in the illustration above). A small drive signal is injected via the 200 T winding which induces equal voltages on the H and N wires passing through its core.
Neutral to Ground Fault Detection
* If N and G are separate downstream (as they should be), no current will be flow in either wire and the GFCI will not trip. (No current will flow in the H wire as a result of this stimulus because the voltage induced on both H and N is equal and cancels.)
* If there is a N-G short downstream, a current will flow through the N wire, to the G wire via the short, and back to the N wire via the normal connection at the service panel. Since there will be NO similar current in the H wire, this represents a current unbalance and will trip the GFCI in the same manner as the usual H-G short.
* If there is a H-H
[Incidently, a type A GFCI will detect a "hotted hot" <G> as well as a gounded neutral. If there is a parallel path path from the load side hot back to line side hot, it will trip via the same mechanism as the load grounded neutral trip. So, a GFCI won't work on a "double ended" circuit.
It works pretty simply when you study that circuit you pointed us to to. The second coil has as it's primary the ufiltered output of the full-wave rectifier. If a closed loop condition exists between any of the two wires going through the coil, this will induce a ~120Hz current in that closed loop. Ingenious!...