Although most homes today are wired using copper wiring, some older homes were wired using aluminum wiring. Aluminum wiring cannot be used with standard wiring devices. Aluminum wire tends to oxidize over time, and the oxide layer does not conduct electricity. This results in a poor connection with the wiring device terminal screws, particularly if they are steel. The connections gradually heat up to the point where fires are started inside the walls.
For this reason, all standard wiring devices contain the warning, “For use with copper or copper-clad wire only”.
You can only use aluminum wiring with CO/ALR rated wiring devices. Leviton offers a selection of CO/ALR rated wiring devices for use with aluminum wires. In CO/ALR devices, the terminal screws are plated with the element Indium. Indium is an extremely soft metal that forms a gas-sealed connection with the aluminum wire, eliminating the problems caused by oxidation.