The GFCI system is a system designed to detect when there is a "ground fault". The GFCI system is used both for breakers and for outlets. What exactly is a ground fault? Let's look at a simple two wire system. Power plants provide voltage by maintaining one "cold" side (the white wire in most proper household wiring) at a constant voltage ... close to zero most of the time. The other "hot" side (the black wire) swings from +170 to - 170 VAC as discussed in the AC power module. Under normal operating conditions, the current that flows in one wire is exactly equal to current in the other. Current flows into/out of one wire, through the electrical device, and then out of/into the other wire. But if there is a path for current to flow from the hot side directly into the local ground, then you will have a ground fault. The current flowing in the cold wire will not be equal to that flowing in the hot wire. The GFCI system detects that the current is not the same in both wires and breaks the circuit.
Consider a case in which the insulation on the hot wire coming into your toaster becomes worn and makes contact with the outer metal casing. If no one is touching the toaster casing, this might not be a problem. But if someone touches the toaster while touching a metal faucet, this may provide a path to the ground outside. Current will flow through the person and into the ground. The toaster may or may not be operating! (Note that this is not really a ideal complete circuit. But the ground can absorb a lot of electrons before giving up any, so a dangerously high current is possible.) The GFCI outlet will detect that the current flowing through the hot side is not the same as that flowing through the cold side and will break the circuit.
Here are a few suggestions for questions to address in your investigation into the GFCI system.
Below are a few suggested resources to get you started. You should be able to find more information with a general internet search.
Standard search engines for your own search.
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