Simply put, AFCI prevent fires from arcing in the walls. The arc is not enough to trip 15 and 20 amp breakers, but can start a fire. Arcs can be either in series with the wiring or in parallel with a wiring hence they are dual function. The breaker is also dual function.
GFCI prevent electrocution, they monitor the current going in one wire and coming back on the other and they should be the same amperage. If they differ by more than 6 mA the circuit shuts off. It takes about 20 mA to kill a human being. They are necessary wherever a human may be in contact with the Earth, such as outside in the yard, swimming pools, and on concrete floors as in your garage. Code requires one part of your electrical circuit be connected to the Earth. That is the groundED wire which is white and is part of the circuit. The groundING wire is green or bare and is connected to all metal that a human might come in contact with while standing on the Earth.
A regular circuit breaker is designed to protect the wiring. Also, no circuit breaker may be loaded more than 80% of its rating if the load is continuous. According to the code, I’ll continuous load as any load that may be on for three hours or more.
Hope all that helps clarify a complex subject. I taught almost every subject involving electrical, electronics, & computers, for 37 years at Ohio university. I trained thousands of engineers and electricians. Big I also hold a journeyman wireman’s card out of local 134 in Chicago. Good luck!
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