The way I understand it is that all three conductors pass through the coil, and the current should balance to zero under normal operation. When a fault occurs, there is an imbalance that is detected by the coil and opens the line.
I was thinking before that it would take one coil for each pair (Line "A" to Ground, Line "B" to Ground, Line "A" to Line "B") but I'm mistaken.
As far as getting deeper into how they work... Better let Scott handle it!
-Virgil Residential/Commercial Inspector 5 Star Inspections Member IAEI
But I am not clear on whether a symmetrical fault to earth would be detected.
Such a fault is improbable in a hot tub. But I want to know whether it would be detected, if, for instance, a 240 V floor outlet were filled with Pepsi causing a symmetrical fault to ground on both poles.