If an electrical system works correctly, power travels safely through insulated wires to fixtures and appliances. However if a receptacle is damaged or a wire comes loose, power may flow to unwanted places such as a metal electrical box or the metal housing of an appliance. Because the human body is a fairly good conductor of electricity, a person touching these objects receives a painful shock. Grounding and polarization reduce this danger.
You can quickly find out whether a receptacle is grounded and polarized by using a receptacle analyzer.
Electricity always travels the path of least resistance, either in a circuit back to its place of origin or toward the earth. A grounded electrical system provides an alternate path for the power to follow in case something goes wrong.
Many homes built before World War II have 120-volt circuits with only two wires, 240-volt circuits with only three wires, and no metal sheathing or conduit to act as a ground. These homes are not grounded, and receptacles in them should have only two slot-shaped holes.
Most modern homes are grounded. They have a third (or fourth) wire, usually bare copper or green insulated, called a ground wire; or their wires may run through metal conduit or flexible metal sheathing that can be used as a path for ground. A ground wire or metal sheathing carries misguided electricity harmlessly to the earth. Receptacles in these homes should have a third, rounded hole, which connects to the ground wire or sheathing.
Most receptacles -- grounded or ungrounded -- have one slot that is longer than the other, so that a plug that has one prong wider than the other can be inserted only one way. These receptacles and plugs are polarized. If a polarized receptacle is wired correctly, the narrow slot connects to the hot wire (delivering power to the receptacle) and the wide slot connects to the neutral wire (carrying power back to the service panel). When you plug a light or appliance into the receptacle, its switch controls the hot wire.
If the receptacle or plug is not polarized or if a receptacle is wired incorrectly, the switch controls the neutral wire. This means that power is still present in the wires inside the light or appliance when you turn it off, posing a safety hazard.