Basic Electrical Safety Question

[whit3rd]

Protected against "dangerous third-prong ground" how?

Well, it's called a "ground fault interruptor" because it
protects against FAULTY grounding. The fault can be a
crossed wire (excessive current in the ground wire), or
loose connection (faulty ground connection), or can
be something as simple as a water pipe coming into
the house from the north, while the electric service
and its grounding stake is located to the south.
Two different potentials, but both 'ground'.

As for HOW it protects, it uses a sensor that disconnects
the AC current in your wiring if and only if the hot (black)
and the neutral (white) wire don't carry equal and opposite
current. The excess of current would mean that a third
conductor (you, or leakage to the protective ground wire)
is involved, in an unintended way.

 

[krw]

Well, it's called a "ground fault interruptor" because it
protects against FAULTY grounding.

No, it protects against a fault *to* ground. The grounding may be
perfect.

The fault can be a
crossed wire (excessive current in the ground wire)

Nope. It doesn't detect current in the ground wire at all.

or
loose connection (faulty ground connection), or can
be something as simple as a water pipe coming into
the house from the north, while the electric service
and its grounding stake is located to the south.
Two different potentials, but both 'ground'.

Absolutely wrong. GFCIs detect an imbalance in the hot and
neutral. Any difference indicates a fault *to* ground and shuts
the circuit down.

As for HOW it protects, it uses a sensor that disconnects
the AC current in your wiring if and only if the hot (black)
and the neutral (white) wire don't carry equal and opposite
current. The excess of current would mean that a third
conductor (you, or leakage to the protective ground wire)
is involved, in an unintended way.

Right. It has nothing to do with the ground currents.