Receptacles (electrical outlets) that have only two slots are called polarized receptacles. These have a smaller "hot" slot and a larger neutral slot, ensuring that the electrical currents flow along the appropriate wires in the circuit; hot along hot, neutral along neutral. All modern plugs have corresponding wide and narrow prongs, or blades so that you can plug into the receptacle only one way.
This is all part of a safety system that's been in wide use since the 1920s. Receptacles today have three slots--hot, neutral, and ground--and accept three-prong grounded plugs. When properly wired, this provides a reliable ground system that is safer than the older systems with polarized receptacles.
The Importance of Grounding
Grounding is a safety system that provides a path for electrical current to flow to a safe destination if there is a problem with a circuit. For example, if a wire comes loose from a receptacle and touches the side of a metal electrical box, the electricity can flow through the box and along the ground wire attached to the box, ultimately dissipating safely into the earth outside the home. The same thing can happen with a loose wire inside a plugged-in appliance or another device. Because polarized receptacles do not have a ground slot, they cannot provide a ground path between a plugged-in electrical device and the circuit ground.
In fact, polarized receptacles in many homes are not grounded at all because the system does not have a true ground.
Polarized receptacles in older homes may or may not be connected to grounded circuits. If the receptacle is housed in a metal box that is connected to a metal conduit (rigid or flexible), the receptacle may be grounded through the box and conduit (but again, it cannot ground a plugged-in device).
With this type of system, there would be only black and white circuit wires. There is no ground wire because the metal box and conduit provide the ground path. However, the ground system must be continuous back to the home's service panel (breaker box) for the ground system to be intact. In other systems, if the receptacle box is not metal or if there is no metal conduit, there is no ground in the circuit. Many homes built in the 1950s, for example, have metal boxes and 2-wire nonmetallic cable with no ground wire. These systems have no ground.
Replacing Polarized Receptacles
If your circuit wiring includes a ground, simply replacing old polarized receptacles with new 3-slot grounded receptacles will give you grounded receptacles. If the circuits do not have grounds, installing grounded receptacles will not provide a ground. It will allow you to plug in 3-prong plugs, but there will be no ground protection at the outlet. The best way to provide a ground is to rewire the circuit with grounded cable and install new grounded receptacles. One alternative that adds a measure of safety but does provide a ground is to replace a polarized receptacle with a GFCI receptacle. A GFCI will detect a ground fault, such as a short in an appliance, and shut off power at the receptacle, de-energizing the faulty appliance.
However, it does not add a ground to the receptacle or the circuit.