When should you use an ungrounded outlet?

Answer 1

If the wiring system into which you are installing an outlet has no ground available, use an ungrounded outlet. In an ungrounded system, an outlet with a ground contact would allow the outlet user to mistakenly, and perhaps dangerously, assume that a ground was present. A suitable ground may be available as a ground wire accompanying the hot and neutral wires in the cable, or a ground may be available via conductive conduit and a metal outlet box. In any case, use a tester to confirm the integrity of the assumed ground. A voltage test from the hot wire to the ground should show the same voltage as between hot and neutral (the black and white wires respectively). If you are replacing an ungrounded outlet, you need not assume there is no ground present. You may find, in the box, ground wires that were not connected to the outlet. You may come across grounded outlets that have no ground wire attached because they rely on grounding via the mounting screws through the outlet ears to the metal box. This is a less reliable grounding method. It is better to buy a ground-wire "pigtail," fasten the wire directly to a hole in the metal box with the supplied screw, and attach the other end of the ground wire to the outlet via the outlet's ground screw.