Two categories of protective device appear in modern panels, and they are frequently confused because they occupy the same space and cost about the same.
GFCI — ground fault circuit interrupter
Protects people. It compares the current going out on the hot with the current returning on the neutral. A difference means current is leaving the circuit by some other path — through a person, for instance — and the device opens in milliseconds at a very small imbalance.
Required in wet and damp locations and where contact with ground is likely: bathrooms, kitchens, garages, unfinished basements, crawl spaces, outdoors, laundry areas, near sinks, and for certain equipment.
The list has expanded with each Code cycle, and current editions require it for a wider range of locations and appliances than a decade ago.
AFCI — arc fault circuit interrupter
Protects the building. It detects the electrical signature of an arcing fault — a loose connection, a damaged cable, a nail through a wire — which produces heat without necessarily drawing enough current to trip a conventional breaker.
Arcing faults are a recognised ignition source in electrical fires, and that is the hazard AFCI addresses.
Required for most circuits supplying living areas of a dwelling: bedrooms, living rooms, dining rooms, hallways, kitchens, laundry areas and similar.
Dual-function devices
Where both are required — a kitchen counter circuit, for instance — a dual-function AFCI/GFCI breaker provides both in one device.
They are more expensive than either alone and they are what a modern kitchen or laundry circuit generally uses.
Where the protection sits
At the breaker, protecting the whole circuit including the wiring.
Or at the receptacle, protecting that outlet and everything downstream of it, but not the wiring between the panel and it.
A GFCI receptacle is cheaper. A GFCI or AFCI breaker protects more. For AFCI in particular the breaker is the usual choice, since the hazard is in the wiring.
Nuisance tripping
Both device types trip on conditions that are not always a fault, and the complaints are well known.
GFCI: appliances with motors or heating elements that have small inherent leakage — old freezers, sump pumps, some outdoor equipment — and long circuits with capacitive leakage. Moisture in an outdoor receptacle is the most common cause and it is a genuine fault.
AFCI: certain motor loads, some LED drivers and dimmers, and shared neutrals wired as multi-wire branch circuits without the right breaker.
Repeated tripping is a diagnosis, not a defect. A device tripping regularly is either detecting something real or is mismatched to the load, and both are worth resolving rather than working around.
What not to do
Do not replace an AFCI or GFCI breaker with a standard one to stop the tripping. That removes the protection and leaves whatever caused it.
Do not bypass a GFCI receptacle by moving the load to the line side terminals.
Both are found in houses and both defeat the purpose entirely.
Retrofit
Existing circuits are generally not required to be brought up to current requirements simply because the Code changed.
They are when the circuit is extended, modified or reconnected — which is why a panel replacement typically brings AFCI and GFCI requirements with it, and why the breakers are a large part of the cost of that job.
Adding them voluntarily to an older house is inexpensive relative to the hazard, and a bedroom circuit in a house with decades-old wiring is exactly the case AFCI was introduced for.
Testing
Both device types have a test button, and they should be tested periodically — the usual advice is monthly.
A device that does not trip on its test button has failed and is providing no protection, which is the one failure mode that is invisible without pressing it.