I don't have to re-read it -- I *wrote* it!
A GFCI is nothing more than a tiny transformer with "sense electronics" as its "load". The primary to the transformer is the circuit being monitored. *BOTH* conductors pass through the transformer.
Since an alternating current generates a magnetic field, that field couples *through* the transformer to the
*secondary* of the transformer -- which is the "sense electronics". The field generated by the primary is a function of the *net* current flowing through the primary "winding" (winding can often be confusing in this context as it is usually just a "single turn" -- as such, it doesn't even go completely *around* the transformer's core!).All of the current flowing *to* the INTENDED load (remember, the GFCI can also see an *unintended* load!) goes through the supply/hot lead, through the transformer's primary. All of the current *returning* from the load passes through the neutral/return conductor *also* through the transformer's primary.
If any of the supply current has "leaked" away via some other path (to "ground") -- like through a person's body -- then the current to and current from will not be equal. As such, the magnetic fields generated by each conductor won't
*perfectly* cancel out. As a result, some energy will be coupled across the transformer's core to its secondary.You can have a *lot* of gain across the transformer since the secondary doesn't need much power to function. As such, you can look for very small "leaks" even in circuits carrying very *large* currents! I.e., aside from the physical size of the transformer and the primary conductors passing through it, a GFCI for a 1000A circuit is essentially the same as one for a 20A "household" circuit.
(N.B. this would not be the case if you tried to *directly* measure the individual currents -- e.g., resistively -- and form the difference -- i.e., comparison -- algebraically)
If you've ever examined a GFCI circuit breaker, you will note that it isnt the simple "two terminal" device of a regular (non GFCI) breaker. This is because the GFCI breaker has to have an additional "ground" connection (which a regular breaker doesn't need) usually implemented with a short pigtail (that you mechanically fasten to the panel's ground).
GFCI *outlets* are dogs. They work the same way but are usually built of lesser quality components. Also, they are subject to more abuse (each time an appliance is plugged/unplugged). And, are often exposed to more environmental extremes than a GFCI breaker in a panel box.
Also, note that there is no way a (typical) GFCI can "test itself" to determine that it is (likely) operational. So, a failure in the sense electronics can cripple the protection feature in a way that is not obvious to the user (i.e., the circuit still supplies "unprotected power").
Note that a GFCI offers no protection against an unintended load directly across the hot + neutral. I.e., if you wear rubber soled shoes (which is advisable when working with electricity) and accidentally touch hot *and* neutral, the circuit will gladly deliver its full rated capacity
*through* your body -- as if you were a light bulb! :>OTOH, if you did NOT have rubber soled shoes on (or had some *other* path to "earth"), the GFCI *would* protect. (i.e., don't fall victim to the false sense of security that a GFCI protected circuit is somehow *safer* than a regular circuit!)
Anything else you need to know? Wanna check my spelling? Or, my grammar??