GFCI vs AFCI vs dual-function: where code wants each one and why

GFCI (ground fault) trips at 5 mA leaking to ground, protects you from shock. Required anywhere near water: kitchens, baths, garages, outdoors, basements, laundry.

AFCI (arc fault) detects the signature of a sparking connection and protects the house from fire. Required in bedrooms, living areas, and in new code nearly everywhere.

Dual-function breakers do both. GFCI receptacles protect themselves and everything downstream on their LOAD terminals, which is why one GFCI in the garage can trip outlets outside.

A GFCI that trips with nothing plugged in has either a downstream fault (usually a wet outdoor box) or is worn out; they do wear out, test monthly.