[jeroenhd]

Old homes also often have been wired to use radiator/water pipes as ground. This causes a lot of issues now that old, metal pipes are being replaced with modern plastics, which turn a stupidly-grounded socket into a potential death trap.

Also, when there's a light circuit involved, there may be a black wire. In very old wiring situations, you may also encounter red/white/green wires, with green being the live wire rather than the ground!

[jacquesm]

That practice is highly illegal and reason for getting cut off from service.

Good point about the other color codes, I should have definitely mentioned that. There are three different kinds that you can find still, the white isn't white though, it is gray, and then, in even older houses but usually only in Groningen and Zeeland if they have not been renovated you can still find cotton/rubber insulation which is super dangerous. If you find any of the older systems codes you should basically just rip it all out and rewire.