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. In some very early cases, the aluminum wiring was in a steel metal jacket, and therefore must be carefully examined to verify that it is aluminum. Aluminum wiring can be hazardous because of its tendency to oxidize and its incompatibility with fittings designed for other metals used in the electrical system. Improper connections can create electrical resistance, which may in turn cause overheating and fire. In older homes, the conductors are sometimes not enclosed in jackets, but run singly, supported on porcelain fittings or behind special wood molding.