With home electrical wiring, why should the wires oxidise?

Normally, the current passing through the wires in your house does not generate enough heat in order to make them react with oxygen. Most wiring is copper, and you need to get copper to quite a high temperature to make it react with oxygen and produce copper oxide. This shouldn't happen unless the wiring actually catches fire.

However if the wires get wet dissolved oxygen can react with the copper much more easily, so they will oxidise even at normal temperatures.