Even if a system is properly grounded, minor faults in a circuit can cause dangerous shock to a person using an electrical appliance in a damp location or near water. The GFCI senses the flow of electricity through a circuit. When you push the test button, the reset button should pop out, shutting off the circuit. If it doesn't, the breaker is not working properly. If you don't test them once a month, the breakers have a tendency to stick, and may not protect you when needed.
Further protection is provided in the electrical system by the use of polarized plugs and outlets.
These ensure that the hot and neutral wires of an appliance connect to the hot and neutral wires inside the receptacle. Polarized plugs have one prong wider than the other, so that the plug can only be inserted the correct way. Three prong plugs are automatically polarized because the hot and neutral prongs are properly positioned in the outlet by the grounding plug. Most electrical wire is made of copper. Nevertheless, some houses built or remodeled between the early 1950's and 1979 were wired with aluminum.