What Is a GFCI Outlet?
That trip threshold is roughly 1/2000th of the current draw of a household hair dryer, and it's fast enough to act before a lethal heart arrhythmia can set in.
How the sensing actually works
Inside every GFCI is a small current transformer that watches both conductors at once. Under normal operation, exactly the same amount of current leaves on the hot wire and returns on the neutral — the two cancel out to zero in the sensor.
The instant a portion of that current escapes through an unintended path — a wet appliance casing, a person, a damaged cord — the sensor detects the imbalance and trips the internal relay, cutting power to the receptacle almost instantly.
This is a fundamentally different job than a circuit breaker's. A standard breaker protects the wiring and the building from overload and short circuits; a GFCI protects a person from shock, and it can trip at current levels far too small to ever bother a breaker.
Where the National Electrical Code requires GFCI protection
Bathrooms, kitchens (near counters and sinks), garages, unfinished basements, crawl spaces, and all outdoor receptacles have required GFCI protection for years under the NEC, and the list of covered locations has only expanded with newer code cycles.
Protection can be delivered three different ways that all satisfy the same code intent: a GFCI receptacle at the outlet itself, a GFCI circuit breaker in the panel protecting the whole circuit, or a portable in-line GFCI device.
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