3GRACE TECHNICAL NOTE
The short answer
A GFCI continuously compares current flowing through the intended circuit path. When the difference reaches its designed trip threshold, the device interrupts the circuit. UL describes Class A GFCIs under UL 943 and CSA C22.2 No. 144.1 as personal-protection devices.
Where GFCIs are commonly used
Public-safety guidance emphasizes potentially damp areas such as bathrooms, kitchens, garages, basements, exterior locations and areas near sinks or utility tubs. Exact requirements depend on the locally adopted electrical code and the authority having jurisdiction.
What TEST and RESET do
TEST initiates a functional check; RESET restores the receptacle after the protection mechanism has operated and the condition is safe to reset. The U.S. CPSC recommends testing installed GFCIs monthly.
Before selecting a product
Confirm current rating, receptacle configuration, TR or WR features, intended environment and the exact model-specific certification record. General guidance is not a substitute for product instructions or qualified installation.