> Rather, the device detects them coming through a second wire, and so it stands to reason that you need to close the circuit into that second wire for it to ob

> Rather, the device detects them coming through a second wire, and so it stands to reason that you need to close the circuit into that second wire for it to observe the fault... GFCIs measure the difference in current between the hot and neutral lines, and trip if the difference is above some threshold. This way, they can detect current flowing from hot to ground through _any_ path, not just the ground terminal on the same outlet. If they only measured current through the outlet's ground terminal, they wouldn't be able to detect e.g. a (usually 2-prong) hair dryer falling into a bathtub (which would typically be grounded through the metal plumbing of the building.)